Geology of Tight Gas Reservoirs· in 

the Pinedale Anticline Area, Wyoming, 

and at the Multiwell Experiment 

Site, Colorado 

Prepared in cooperation with the U.S. Department of Energy 
Front Cover: CER Corporation, MWX-2 well, Piceance basin, 

Colorado. The Eocene Wasatch and Green River Formations form 

the Roan Cliffs in the background. (Photograph by Allan R. Sattler, 

Sandia National Laboratories, Albuquerque, New Mexico.) 

Back Cover: El Paso Natural Gas, Wagon Wheel no. 1 well, 

Green River basin, Wyoming. In the background are Precambrian 

rocks of the Wind River Mountains. (Photograph by Jack 

Shaughnessy, Consulting Geologist, Amarillo, Texas.) 

Cover designed by Loretta J. Ulibarri 
Geology of Tight Gas Reservoirs in the 

Pinedale Anticline Area, Wyoming, and at the 

Multiwell Experiment Site, Colorado 

Edited by B.E. LAW and C.W. SPENCER 

Prepared in cooperation with the U.S. Department of Energy 

U.S. GEOLOGICAL SURVEY BULLETIN 1886 
DEPARTMENT OF THE INTERIOR 

MANUEL LUJAN, JR., Secretary 

U.S. GEOLOGICAL SURVEY 

Dallas L. Peck, Director 

Any use of trade, product, or firm names in this publication is for descriptive 

purposes only and does not imply endorsement by the U.S. Government. 

UNITED STATES GOVERNMENT PRINTING OFFICE: 1989 

For sale by the 

Books and Open-File Reports Section 

U.S. Geological Survey 

Federal Center 

Box 25426 

Denver, CO 80225 

Library of Congress Cataloging-in-Publication Data 

Geology of tight gas reservoirs in the Pinedale anticline area, Wyoming, and at 

the Multiwell Experiment site, Colorado I edited by B. E. Law and C.W. Spencer. 

p. em.- (U.S. Geological Survey bulletin; 1886) 

"Prepared in cooperation with the U.S. Department of Energy." 

Bibliography: p. 

Supt. of Docs. no.: I 19.3: 1886 

1. Gas reservoirs-Colorado-Piceance Creek Watershed. 2. Gas 

reservoirs-Green River Watershed (Wyo.-Utah) 3. Gas, Natural-GeologyColorado-Piceance Creek Watershed. 4. Gas, Natural-Geology-Green 

River Watershed (Wyo.-Utah) I. Law, B. E. II. Spencer, Charles Winthrop, 

1930- Ill. United States. Dept. of Energy. IV. Title: Pinedale anticline 

area, Wyoming. V. Title: Multiwell Experiment Site, Colorado. VI. Series. 

QE75.B9no. 1886 

[TN880] 

557.3 s-dc20 

[553.2'85' 0978785] 89-600155 

CIP 
CONTENTS 

[Letters designate chapters] 

(A) Introduction, by B.E. Law and C.W. Spencer. 

(B) Structural and stratigraphic framework of the Pinedale anticline, Wyoming, and the Multiwell Experiment site, Colorado, by B.E. Law and 

R.C. Johnson. 

(C) Comparison of overpressuring at the Pinedale anticline area, Wyoming, 

and the Multi well Experiment site, Colorado, by Charles W. Spencer. 

(D) Mineral composition, petrography, and diagenetic modifications of lower 

Tertiary and Upper Cretaceous sandstones and shales, northern Green 

River basin, Wyoming, by Richard M. Pollastro. 

(E) Thermal history and provenance of rocks in the Wagon Wheel No. 1 well, 

Pinedale anticline, northern Green River basin-evidence from fissiontrack dating, by Nancy D. Naeser. 

(F) Analysis of vitrinite maturation and Tertiary burial history, northern 

Green River basin, Wyoming, by Warren W. Dickinson. 

(G) Thermal maturity patterns in the northern Green River basin, Wyoming, 

by M.R. Lickus, M.J. Pawlewicz, B.E. Law, and W.W. Dickinson. 

(H) Gamma-ray well-log anomaly in the northern Green River basin of 

Wyoming, by Stephen E. Prensky. 

(I) Quantitative model for overpressured gas resources of the Pinedale anticline, Wyoming, by R.R. Charpentier, B.E. Law, and S.E. Prensky. 

(J) Petrology and isotope geochemistry of mineralized fractures in Cretaceous 

rocks-evidence for cementation in a closed hydrologic system, by Janet 

K. Pitman and Warren W. Dickinson. 

(K) Reservoir sedimentology of rocks of the Mesaverde Group, Multiwell Experiment site and east-central Piceance basin, northwest Colorado, by 

John C. Lorenz. 

(L) Thermal history of selected coal beds in the Upper Cretaceous Mesaverde 

Group and Tertiary Wasatch Formation, Multiwell Experiment site, Colorado, in relation to hydrocarbon generation, by Vito F. Nuccio and 

Ronald C. Johnson. 

(M) Fluid inclusion evidence for paleotemperatures within the Mesaverde 

Group, Multiwell Experiment site, Piceance basin, Colorado, by Charles 

E. Barker. 

(N) Rock-Eval analysis of sediments and ultimate analysis of coal, Mesaverde 

Group, Multiwell Experiment site, Piceance basin, Colorado, by Charles 

E. Barker. 

(0) Azimuthal vertical seismic profiles at the Multiwell Experiment site, northwest Colorado, by Myung W. Lee. 

Volume Contents Ill 
CONVERSION FACTORS FOR SOME 51 METRIC AND U.S. UNITS OF MEASURE 

To convert from 

Feet (ft) 

Miles (mi) 

Pounds (lb) 

Degrees Fahrenheit ('F) 

IV Conversion Table 

To 

Meters (m) 

Kilometers (km) 

Kilograms (kg) 

Degrees Celsius ( °C) 

0.3048 

1.609 

0.4536 

Multiply by 

Temp OC=(temp 'F-32)/1.8 
Chapter A 

Introduction 

By B.E. LAW and C.W. SPENCER 

Prepared in cooperation with the U .5. Department of Energy 

U.S. GEOLOGICAL SURVEY BULLETIN 1886 

GEOLOGY OF TIGHT GAS RESERVOIRS IN THE PINEDALE ANTICLINE AREA, WYOMING, 

AND AT THE MULTIWELL EXPERIMENT SITE, COLORADO 
CONTENTS 

Pinedale anticline Al 

Multiwell Experiment AJ 

References cited A6 

FIGURES 

1. Map of central Rocky Mountain region of Utah, Wyoming, and Colorado 

showing location of Pinedale anticline and Multiwell Experiment site A2 

2. Map showing wells drilled in the Pinedale anticline area, northern part of 

Green River basin, Wyoming AJ 

3. Diagram showing relative location of MWX wells in the Piceance basin of 

Colorado AS 

TABLE 

1. Wells drilled in Pinedale anticline area, Sublette County, Wyoming A4 
Introduction 

By B.E. Law and C.W. Spencer 

Low-permeability (tight) gas reservoirs are rocks 

that have an estimated in situ permeability to gas of 0.1 

millidarcy (mD) or less. Within the United States, these 

reservoirs contain major resources of gas. The National 

Petroleum Council (NPC) (1980) estimated the in-place 

gas resource for tight reservoirs in the United States at 

about 924.0 trillion cubic feet (TCF). More recent 

estimates of the gas resource in the Greater Green River 

basin of Wyoming, Colorado, and Utah, and in the Piceance basin of Colorado indicate that the NPC estimates 

for these basins are very conservative. In the Greater 

Green River basin, estimates of the in-place gas resource 

for tight reservoirs range from 3,600 to 6,800 TCF, and 

the mean estimate is 5,063 TCF (Law and others, 1989); 

in the Piceance basin, estimates of the in-place gas 

resource range from 274.5 to 605.0 TCF, and the mean 

estimate is 419.5 TCF (Johnson and others, 1987). 

In 1977, the U.S. Geological Survey, in cooperation with the U.S. Department of Energy, initiated 

geologic investigations of low-permeability (tight) gas 

reservoirs in the Rocky Mountain region. The overall objectives of these efforts are to geologically characterize 

tight sandstone reservoirs and to estimate gas resources 

in selected basins. In order to accomplish these objectives, 

our efforts focused on small areas containing several wells 

or on individual wells containing large amounts of highquality data. Two such areas that have been extensively 

studied are the Pinedale anticline in the northern part of 

the Green River basin of Wyoming and the Multi well Experiment site in the Piceance basin of Colorado (fig. 1). 

Results of the geologic investigations indicate that 

the tight gas-bearing reservoirs in these areas have experienced similar depositional, structural, burial, and 

thermal histories. Some of the relationships between the 

occurrences of gas and geologic characteristics are 

dissimilar, however, because of slight differences in the 

timing, intensity, and duration of geologic processes in 

each area. Thus, the purpose of this publication is to provide both geologic data and interpretations of tight gas 

reservoirs for two well-studied areas that will contribute 

to an improved understanding of unconventional reservoirs in other areas. 

Manuscript approved for publication March 15, 1989 

PINEDALE ANTICLINE 

The Pinedale anticline is in the northern part of the 

Green River basin of Wyoming (figs. 1, 2). The first well 

drilled on the Pinedale anticline was completed in 1939 to 

a depth of 10,000 ft (3,048 m) by the California Company. 

Since that time, 19 additional wells have been drilled on 

the anticline (fig. 2). Specific locations and pertinent well 

data are shown in table 1. Most of the wells have been 

drilled to depths of 10,000-12,000 ft (3,058-3,658 m) in the 

Upper Cretaceous Lance Formation. The deepest well is 

the El Paso Natural Gas Company Wagon Wheel No. 1 

well, which was drilled to 19,000 ft (5,791 m) into the Upper Cretaceous Baxter Shale. All the wells have encountered 

extensive thicknesses of gas-saturated rocks, and data obtained from the Wagon Wheel well conclusively demonstrate the occurrence of a thick gas-bearing interval below 

a depth of about 8,000 ft (2,438 m). The indicated in-place 

gas resource contained in low-permeability reservoirs in the 

Pinedale anticline area is about 159.0 TCF (Charpentier 

and others, 1987; this volume). 

As a consequence of the extensive testing and coring 

conducted in the Wagon Wheel well, most of the geological interpretations in the Pinedale area are derived from 

data obtained from this well. Within the depth interval of 

5,000 to 18,000 ft (1,524-5,486 m), approximately 900 ft 

(274m) of rock was cored. The cored intervals are shown 

on figure 3 of Law and Johnson (this volume). Data derived from cores in the Wexpro and American Hunter wells 

(table 1) were also incorporated into these investigations. 

The Wagon Wheel well, completed in 1970, was proposed as part of a nuclear stimulation project. It was intended to be the fourth nuclear experiment following the 

Gasbuggy, Rulison, and Rio Blanco detonations in the 

San Juan basin of New Mexico and the Piceance basin 

of Colorado. Plans for detonation of a nuclear device in 

the Wagon Wheel well were abandoned, however, due to 

environmental considerations and disappointing test 

results obtained from the previous three experiments. This 

project, including drilling and testing results, is discussed 

by Martin and Shaughnessy (1969) and Shaughnessy and 

Butcher (1973, 1974). Several other geologic studies conducted in the Wagon Wheel area are discussed in Law 

(1984). 

Introduction A 1 
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25 50 75 MILES 

25 50 75 KILOMETERS 

Figure 1. Central Rocky Mountain region of Utah, Wyoming, and Colorado showing location of Pinedale anticline 

and Multiwell Experiment (MWX) site. 

Five wells currently are producing in the Pinedale 

anticline area and several other wells are shut-in (fig. 2, 

table 1). Gas production, with minor amounts of conA2 Geology of Tight Gas Reservoirs, Wyoming and Colorado 

densate, is from low-permeability lenticular sandstones 

in the Lance Formation below a depth of about 8,000 ft 

(2,438 m). The reservoirs require hydraulic-fracture 
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__. •.______.. __ Thrust fault; sawteeth on upthrown side 

--4-4 -1--- Anticline, showing direction 

of plunge 

Well; number refers to table 1 

~ Producing gas well 

~. ~ Shut-in gas well 

~ 0' -¢- Drilled and abandoned well 

10 20 MILES 

10 20 KILOMETERS 

Figure 2. Wells drilled in the Pinedale anticline area of the northern part of the Green River basin, Wyoming. 

Detailed location information for wells shown in table 1. 

stimulation for sustained economic production rates. Initial gas production rates are highly variable. Maximum 

rates of 1.5 million cubic feet per day (MMCFD) from 

single reservoirs and about 2.5 MMCFD from multiple 

reservoirs have been reported. 

MULTIWELL EXPERIMENT 

The U.S. Department of Energy (DOE) conducted 

a field-oriented, enhanced gas recovery research experiment in the southern part of the Piceance basin of 

Colorado. This project is called the Multiwell Experiment 

(MWX) and was undertaken as a major part of the 

Department's Fossil Energy Western Gas Sands Subprogram. The main purpose of the research is to provide 

new and improved technology for the identification of 

natural gas in tight-gas reservoirs and to develop 

economically viable and reliable methods to recover the 

gas (Northrop and others, 1984; Spencer, 1984). The 

MWX site is on the south side of the valley of the Colorado River (fig. 1) at a surface kelly-bushing elevation 

of about 5,355 ft (1 ,630 m). Three closely spaced 

(115-215 ft, 35-66 m), extensively cored and logged wells 

Introduction AJ 
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Table 1. Wells drilled in Pinedale anticline area, Sublette County, Wyoming 

[Locations of wells shown on figure 2. Kelly bushing and total depth in feet. Status: D&A, drilled and abandoned; Prod., producing well; SI, shut-in well. Leaders(---) indicate no 

data] 

Map Kelly Total Formation 

no. Well name Oeerator Location bushing deeth at total deeth Status 

1 Unit 1 Texaco NE1/4SW1/4 sec. 25, T. 29 N., R. 107 W. 7,134 11,008 Lance D&A 

2 New Fork 1 American Hunter SW1/4NE1!4 sec. 25, T. 30 N., R. 108 W. 7,338 10,989 Lance Prod. 

3 Pinedale 6 El Paso Natural Gas SEl/4NWl/4 sec. 21, T. 30 N., R. 108 W. 7,294 11,057 Lance D&A 

4 Pinedale 7 El Paso Natural Gas SEl/4NEl/4 sec. 15, T. 30 N., R. 108 W. 7,369 10,270 Lance SI 

5 Pinedale 1 El Paso Natural Gas NE1/4SE1/4 sec. 6, T. 30 N., R. 108 W. 7,196 10,550 Lance D&A 

6 Wagon Wheel 1 El Paso Natural Gas SE1/4NW1/4 sec. 5, T. 30 N., R. 108 W. 7,089 19,000 Baxter SI 

7 Pinedale 5 El Paso Natural Gas C SE1/4 sec. 5, T. 30 N., R. 108 W. 7,056 15,018 Rock Springs D&A 

8 New Fork2 American Hunter SW1/4NE1/4 sec. 2, T. 30 N., R. 108 W. 7,282 11,986 Lance D&A 

9 Pinedale 2 E1 Paso Natural Gas SE1/4 sec. 29, T. 31 N., R. 108 W. 7,034 10,694 Lance SI 

10 New Fork4 American Hunter NWl/4SEl/4 sec. 35, T. 31 N., R. 109 W. 6,957 11,550 Lance D&A 

11 Government 1 California Company SW1/4NE1/4 sec. 14, T. 31 N., R. 109 W. --- 10,000 Lance D&A 

12 Pinedale 3 El Paso Natural Gas C NW1/4 sec.13, T. 31 N., R. 109 W. 6,982 11,009 Lance D&A 

13 Jensen 1 Leonard Hay SW1/4NW1!4 sec. 11, T. 31 N., R. 109 W. 6,932 10,320 Lance Prod. 

14 Jensen 2 Leonard Hay SE1/4SE1/4 sec. 11, T. 31 N., R. 109 W. --- 10,310 Lance Prod. 

15 Pinedale4 El Paso Natural Gas C NW1/4 sec. 34, T. 32 N., R. 109 W. 7,204 11,123 Lance SI 

16 Mesa2 Wexpro SE1/4NW1/4 sec. 16, T. 32 N., R. 109 W. 7,474 12,190 Lance Prod. 

17 Mesa 1 Wexpro NE1/4NE1/4 sec. 7, T. 32 N., R. 109 W. 7,525 12,050 Lance Prod. 

18 Pinedale 8 Mountain Fuel NE1/4SW1/4 sec. 20, T. 33 N., R. 109 W. 7,448 10,500 Lance SI 

19 Unit 1 Stanolind NW1/4SE1/4 sec. 17, T. 33 N., R. 109 W. --- 7,797 Unnamed Tertiary D&A 

20 Baumgartner Fed. 21-24 Black Hawk Resources NE1/4NW1/4 sec. 24, T. 33 N., R. 110 W. 7,344 11,238 Lance D&A 
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10 20 30 40 50 FEET 

15 METERS 

Figure 3. Relative location of MWX wells in the Piceance basin of Colorado. Large circles adjacent to 

well labels represent surface locations; small circles represent locations of well bore at depth (in feet) 

indicated. Modified from Mann (1984, fig. 7). 

Introduction AS 
were drilled. They are designated CER Nos. 1, 2, and 3 

MWX and are all located in the northwest quarter of sec. 

34, T. 6 S., R. 94 W., Garfield County, Colorado. The 

wells were spudded in the upper part of the Tertiary 

Wasatch Formation and reached total depths of from 

7,565 to 8,350 ft (2,306-2,545 m), at or near the base of 

the Upper Cretaceous Mesaverde Group. Figure 3 shows 

the location of the three wells relative to each other at 

the surface and at various depths. As can be seen in this 

figure, the well bores deviate only slightly from vertical 

and have only minor lateral drift with depth. In the 

discussions and chapters that follow, these wells will be 

referred to as MWX-1, MWX-2, and MWX-3. Many 

of the geologic studies conducted at the MWX site are 

discussed in Spencer and Keighin (1984). 

One of the main reasons the MWX wells were 

drilled so close together was to facilitate well-interference 

production testing in tight sandstone reservoirs. The basic 

approach was to perforate and produce gas from one of 

the three wells and then observe the pressure response or 

pressure interference in the two adjacent (observation) 

wells. The close spacing and extensive well coring also 

provide data on the lateral and vertical variations of reservoir characteristics and on the distribution of natural fractures in ''blanket'' marine sandstone and lenticular fluvial 

reservoirs. The primary-objective reservoirs studied, 

however, are the lenticular sandstones in the fluvial part 

of the Mesaverde Group because: 

1. Low-permeability, gas-bearing reservoirs in Upper Cretaceous fluvial sandstone occur in sequences, often 

thicker than 5,000 ft (1,524 m), in the Piceance basin and 

many other Tertiary basins in the Rocky Mountain 

region. Furthermore, some Rocky Mountain basins, such 

as the Greater Green River basin, have more than 14,000 

ft (4,270 m) of strata containing mostly tight gas sandstones. 

2. Artificial stimulation (hydraulic fracturing) has 

been relatively unsuccessful in lenticular rock sequences 

and somewhat more successful in the so-called ''blanket'' 

marine reservoirs of Cretaceous age in the Rockies. 

Studies at MWX can be generally grouped into five 

disciplines: (1) geology, (2) geophysics, (3) rock mechanics, (4) borehole geophysical-log interpretation, and (5) 

well-testing and well-data diagnostics (Clark, 1983; Searls 

and others, 1983; Branagan and others, 1984; Northrup 

and others, 1984; Sattler, 1984; Spencer, 1984; Warpinski 

and others, 1984). There is considerable interrelationship 

and interaction among the various disciplines. The 

distribution of natural fractures vertically and areally, for 

example, is a very important aspect of the recovery of 

gas from tight reservoirs. Natural fracture analysis and 

the prediction of orientation of induced hydraulic fractures at the MWX site involves integration of regional 

tectonic information, surface fracture mapping, thermal 

and pressure history, rock mechanics, geophysics, core 

A6 Geology of Tight Gas Reservoirs, Wyoming and Colorado 

study, borehole well-log analysis, well-testing, and fracture diagnostics. Interpretation of porosity and water 

saturation values from borehole geophysical logs requires 

integration of laboratory-measured core porosity, grain 

density, and water saturation data and petrographic, 

petrologic, and stratigraphic data. 

REFERENCES CITED 

Branagan, P., Cotner, G., and Lee, S.J., 1984, Interference testing 

of naturally fractured Cozzette Sandstone; a case study at 

the DOE MWX site: Society of Petroleum Engineers/Gas 

Research Institute/Department of Energy Unconventional 

Gas Recovery Symposium, Dallas, Thx., Proceedings, 

p. 359-366. 

Charpentier, R.R., Law, RE., and Prensky, S.E., 1987, Quantitative 

model of overpressured gas resources of the Pinedale 

anticline, Wyoming: Society of Petroleum Engineers/ 

Department of Energy Joint Symposium on LowPermeability Reservoirs, Denver, Colo., 1987, Proceedings, 

p. 153-164. 

Clark, J.A., 1983, The prediction of hydraulic fracture azimuth 

through geological, core, and analytical studies: Society of 

Petroleum Engineers/Department of Energy Symposium 

on Low-Permeability Gas Reservoirs, Dallas, Thx., Proceedings, p. 107-114. 

Johnson, R.C., Crovelli, R.A., Spencer, CYV., and Mast, R.R, 1987, 

An assessment of gas resources in low-permeability sandstones of the Upper Cretaceous Mesaverde Group, Piceance 

basin, Colorado: U.S. Geological Survey Open-File Report 

87-357, 26 p. 

Law, B.E., ed., 1984, Geological characteristics of low-permeability 

Upper Cretaceous and lower Tertiary rocks in the Pinedale 

anticline area, Sublette County, Wyoming: U.S. Geological 

Survey Open-File Report 84-753, 107 p. 

Law, B.E., Spencer, CYV., Crovelli, R.A., Mast, R.F., Dolton, 

G.L., Charpentier, R.R., and Wandrey, C.J., 1989, Gas 

resource estimates of overpressured low-permeability 

Cretaceous and Thrtiary sandstone reservoirs in the Greater 

Green River basin, Wyoming, Colorado, and Utah: Wyoming Geological Association Field Conference Guidebook, 

p. 39-61. 

Mann, R.L., 1984, Multiwell Experiment Drilling Review, in 

Frohne, K-H., ed., Western Gas Sands Subprogram review: 

Morgantown, U.S. Department of Energy DOE/METC/ 

84-3, p. 155-164. 

Martin, W.B., and Shaughnessy, J., 1969, Project Wagon Wheel, 

in Symposium on Thrtiary rocks of Wyoming: Wyoming 

Geological Association Field Conference, 21st, Guidebook, 

p. 145-152. 

National Petroleum Council, 1980, Tight gas reservoirs, part I, 

in Unconventional gas resources: Washington, D.C., National Petroleum Council, 222 p., with appendices. 

Northrop, D.A., Sattler, A.R., Mann, R.L., and Frohne, K-H., 

1984, Current status of the Multiwell Experiment: Society 

· of Petroleum Engineers/Gas Research Institute/Department 

of Energy Unconventional Gas Recovery Symposium, 

Dallas, Tex., Proceedings, p. 351-358. 
Sattler, A.R., 1984, The Multiwell Experiment core program, 

II: Soceity of Petroleum Engineers/Gas Research Institute/Department of Energy Unconventional Gas Recovery Symposium, Dallas, Tex., Proceedings, p. 2235-2244. 

Searls, C.A., Lee, M.W., Miller, J.J., Albright, J.N., Fried, 

J., and Applegate, J.K., 1983, A coordinated seismic 

study of the Multiwell Experiment site: Society of 

Petroleum Engineers/Department of Energy Symposium 

on Low Permeability Gas Reservoirs, Dallas, Tex., Proceedings, p. 115-120. 

Shaughnessy, J., and Butcher, R.H., 1973, Geology of Project Wagon Wheel Nuclear Stimulation Project, in Fassett, 

J .E., ed., Cretaceous and Tertiary rocks of the southern 

Colorado Plateau: Four Corners Geological Society, 

p. 185-196. 

___ 1974, Geology of Wagon Wheel Nuclear Stimulation 

Project, Pinedale field, Wyoming: American Association 

of Petroleum Geologists Bulletin, v. 58, no. 11, 

p. 2250-2259. 

Spencer, C.W., 1984, Overview of U.S. Department of Energy 

Multiwell Experiment, Piceance Creek basin, Colorado, 

in Spencer, C.W., and Keighin, C.W., eds., Geologic 

studies in support of the U.S. Department of Energy 

Multiwell Experiment, Garfield County, Colorado: U.S. 

Geological Survey Open-File Report 84-757, p. 1-13. 

Spencer, C.W., and Keighin, C.W., eds., 1984, Geologic studies 

in support of the U.S. Department of Energy Multi well 

Experiment, Garfield County, Colorado: U.S. Geological 

Survey Open-File Report 84-757, 134 p. 

Warpinski, N.R., Branagan, P., and Wilmer, R., 1984, In situ 

stress measurements at DOE's Multiwell Experiment, in 

Frohne, K-H., ed., Western Gas Sands Subprogram 

review: Morgantown, U.S. Department of Energy, 

DOE/METC 84-3, p. 181-189. 

Introduction A7 

Chapter B 

Structural and Stratigraphic Framework of the 

Pinedale Anticline, Wyoming, and the 

Mu ltiwell Experiment Site, Colorado 

By B.E. LAW and R.C. JOHNSON 

Prepared in cooperation with the U.S. Department of Energy 

U.S. GEOLOGICAL SURVEY BULLETIN 1886 

GEOLOGY OF TIGHT GAS RESERVOIRS IN THE PINEDALE ANTICLINE AREA, WYOMING, 

AND AT THE MULTIWELL EXPERIMENT SITE, COLORADO 
CONTENTS 

Abstract Bl 

Introduction Bl 

Pinedale anticline Bl 

Structure Bl 

Stratigraphy B3 

Cretaceous rocks B3 

Tertiary rocks B6 

Multiwell Experiment site B6 

Structure B6 

Stratigraphy B6 

Cretaceous rocks B6 

Tertiary rocks B8 

References cited B9 

FIGURES 

1. Map showing structure contours of top of Cretaceous rocks, northern 

Green River basin Bl 

2. Cross section through northern Green River basin, extending from Wyoming thrust belt northeast to Wind River Mountains B2 

3-4. Charts showing: 

3. Stratigraphic sequence and cored intervals, El Paso Natural Gas Company Wagon Wheel No. 1 well, northern Green River basin B3 

4. Stratigraphic correlation for Pinedale anticline, MWX site, and intermediate areas B4 

5-6. Maps showing: 

5. Location of Greater Green River basin and Piceance basin and Western 

Interior epeiric seaway during Late Cretaceous time BS 

6. Structure contours of top of the Rollins and Trout Creek Sandstone 

Members of the lies Formation of the Mesaverde Group, Piceance 

basin B7 

7. Cross section through Piceance basin including MWX site B8 

8. Chart showing stratigraphic sequence and cored intervals, MWX-1 and 

MWX-2 wells, Piceance basin B9 
Structural and Stratigraphic Framework of the 

Pinedale Anticline, Wyoming, and the 

Multiwell Experiment Site, Colorado 

By B.E. Law and R.C. Johnson 

Abstract 

Both the Pinedale anticline and the U.S. Department of 

Energy Multiwell Experiment site are located in Rocky Mountain foreland basins that have experienced similar styles of 

deformation during the Laramide orogeny but have significantly 

different structural complexity. The Pinedale anticline is a large, 

thrust-rooted fold that formed in response to structural deformation associated with the evolution of the adjacent Wind River 

Mountains. The Multiwell site, by contrast, is in a structurally 

simple area near the synclinal axis of the Piceance basin. 

Gas-bearing reservoirs occur in marine and nonmarine 

clastic sequences that are about 4,200 ft (1 ,280m) thick at the 

Multiwell site and more than 10,000 ft (3,000 m) thick in the 

Pinedale anticline area. These rocks were deposited in and adjacent to the western margin of the Western Interior seaway 

during early Campanian to late Maestrichtian time. 

INTRODUCTION 

The structural and stratigraphic framework of the 

Pinedale anticline area and U.S. Department of Energy 

Multiwell Experiment (MWX) site is known with varying degrees of certainty. The structural and stratigraphic 

setting of the MWX area is fairly well known (Lorenz, 

1983, 1984; Johnson and Nuccio, 1984, 1986; Johnson, 

1987, 1989), but considerably less is known about the 

Pinedale area. The most significant previously published 

structural and stratigraphic studies relevant to the Pinedale area are by Martin and Shaughnessy (1969), 

Shaughnessy and Butcher (1973, 1974), Law and Nichols 

(1982), Shuster and Steidtmann (1983), Law (1984), and 

Hagen and others (1985). 

During the course of recent investigations in the 

Pinedale and MWX areas, data were obtained from well 

logs and cores, particularly from the Pinedale area, that 

necessitate modification of the existing geologic framework. For this reason and in order to place the other 

reports in this volume in the proper geologic setting, we 

will describe the structural and stratigraphic framework 

of each area. 

PINEDALE ANTICLINE 

Structure 

The Pinedale anticline is in the Rocky Mountain 

foreland in the northern part of the Green River basin, 

between the Idaho-Utah-Wyoming thrust belt to the west 

and the Wind River Mountains to the northeast (Law and 

Spencer, this volume, fig. 1). The northern part of the 

Green River basin formed during the Laramide orogeny 

in response to tectonic loading by thrusting events in the 

1-

_J 

~ \ 

1-

Cf) 

::::> 

a: 

I 

1-

10 MILES 1---,.......L-,----' 

0 5 10 KILOMETERS 

EXPLANATION 

Figure 1. Structure contours of the top of Cretaceous rocks, 

northern Green River basin, Wyoming. Contour interval, 100 

and 500 ft. Line of section A-A' (fig. 2) also shown. 

Structural and Stratigraphic Framework, Pinedale Anticline and Multiwell Site 81 
Idaho-Utah-Wyoming thrust belt and by uplift of the 

Wind River Mountains (Jordan, 1981; Hagen and others, 

1985). 

The Pinedale anticline is the largest structural 

feature in the northern Green River basin. It is about 

35 mi (56 km) long and 6 mi (10 km) wide. Its structural 

configuration is shown in the structure contour map of 

the northern Green River basin (fig. 1) and in cross section A-A' (fig. 2), which extends from the Wyoming 

thrust belt northeastward to the Wind River Mountains. 

The anticline is asymmetric in that its west flank is steeper 

than its east flank. Structural relief at the top of Cretaceous rocks is about 2,000 ft (610 m), with two areas of 

local closure. According to Shaughnessy and Butcher 

(1973, 1974), the west flank of the anticline is bounded 

by a buried, high-angle, east-dipping reverse or thrust 

fault (fig. 2) that flattens out at depth. Shaughnessy and 

Butcher indicated about 600ft (182m) of displacement 

across the fault and believe the fault intersects the El Paso 

Natural Gas Wagon Wheel No. 1 borehole at a depth of 

about 18,000 ft (5,486 m). Examination of the two deepest cores from the well (17 ,163-17,193, 17,959-17,996 ft) 

shows increasingly steeper dips and increasing fracture 

frequency, both of which indicate proximity to a fault. 

Surface expression of the anticline in the Eocene Wasatch 

Formation is subtle, and, dips of 1 °-2 ° are locally present on the flanks of the structure. 

The Pinedale anticline is a thrust-rooted detachment structure that probably formed in response to 

sw 

southwest-directed compression associated with structural 

deformation of the Wind River Mountains. The parallel 

trend of the fold axis, the asymmetry of the anticline, 

and the geometric similarities between the west-bounding 

fault and the Wind River thrust are consistent with this 

interpretation. Based on this interpretation, the structural 

evolution of the Pinedale anticline is inferred from structural and stratigraphic data to be related to uplift and 

erosion of the Wind River Mountains. 

Structural growth of the Wind River Mountains 

probably was episodic and may have begun as long ago 

as 90 Ma (Shuster, 1986; Shuster and Steidtmann, 1988), 

prior to the Laramide orogeny. Love (1960) and Berg 

(1961) both concluded that uplift of the Wind River 

Mountains began during Late Cretaceous time. Keefer 

(1965) observed that the Wind River Mountains probably 

began to rise at the beginning of Paleocene time and that, 

by middle to late Paleocene time, the Precambrian core 

of the range had been breached. These conclusions are 

supported by Shuster and Steidtmann (1988). Independent investigations by Prensky (1984, this volume) of a 

unique, regionally occurring gamma-ray response in 

Paleocene rocks of the northern Green River basin indicate that, by early to middle Paleocene time, the 

Precambrian crystalline core of the Wind River Mountains was exposed and shedding arkosic debris into the 

adjacent basin. 

Definitive evidence of the earliest syntectonic 

sedimentation related to uplift and erosion of the Wind 

NE 

A---------------- 55 MILES----------------A' 

FEET 

10,000 

5,000 

SEA LEVEL 

-5,000 

-10,000 

-15,000 

2 

II I I 

0 2 5 

I 

10 MILES 

I 

10 KILOMETERS 

El Paso Natural Gas 

Wagon Whssl No. 1 

Fort Union Formation 

METERS 

3,000 

2,000 

1,000 

SEA LEVEL 

-1,000 

-2,000 

-3,000 

-4,000 

-5,000 

-6,000 

Figure 2. Cross section through the northern Green River basin, extending from the Wyoming thrust belt northeast to the 

Wind River Mountains. High-angle reverse fault on west side of Pinedale anticline from Martin and Shaughnessy (1969) and 

Shaughnessy and Butcher (1973, 1974). TO, total depth of well. Line of section shown in figure 1. 

82 Geology of Tight Gas Reservoirs, Wyoming and Colorado 
River Mountains is lacking in the Pinedale anticline area, 

but mineralogical compositions of the Upper Cretaceous 

Lance Formation as determined by Pollastro (this 

volume) indicate that the Wind River Mountains probably 

began to supply sediments to the adjacent basin during 

deposition of the Lance Formation. Core-derived mineralogical data from wells in the Pinedale anticline show 

that the Lance contains detrital limestone and dolomite 

grains (Law and others, 1986; Pollastro, this volume). 

Although the source of the Lance cannot conclusively be 

demonstrated, its composition is compatible with a Wind 

River Mountains source. Thus, structural growth of the 

Pinedale anticline, which is assumed to be genetically 

linked to deformation associated with the Wind River 

Mountains, probably began during Late Cretaceous and 

continued through Eocene time. 

Stratigraphy 

Most of the stratigraphic data for the Pinedale anticline area are from the El Paso Natural Gas Company 

Wagon Wheel No. 1 well. This well was drilled to a total 

depth of 19,000 ft (5,791 m) and was extensively cored 

and logged; it is presented in this report as the stratigraphic reference well for the Pinedale area (fig. 3). The 

stratigraphic nomenclature used in the Pinedale anticline 

area for Cretaceous rocks is mostly from the area of the 

Rock Springs uplift and is based on stratigraphic correlations shown in figure 4. Stratigraphic correlations of 

Upper Cretaceous rocks in the Pinedale area, at Oak 

Creek in the southeastern part of the Sand Wash basin 

of Colorado, and at the MWX site in the Piceance basin 

of Colorado are also shown. 

Cretaceous Rocks 

About 11,500 ft (3,505 m) of Upper Cretaceous 

rocks was penetrated in the Wagon Wheel well and includes, in ascending order, the Hilliard Shale, Rock 

Springs Formation, Ericson Sandstone, and Lance Formation. With the exception of the Hilliard Shale and the 

lower 800-1,000 ft (244-305 m) of the Rock Springs Formation, the entire sequence was deposited in nonmarine 

environments adjacent to the western margin of the 

Western Interior seaway (fig. 5) in response to thrusting 

events in the Wyoming thrust belt and uplift of the adjacent Wind River Mountains (fig. 1). East and southeast 

of the Pinedale anticline, these rocks grade into and intertongue with marine, marginal-marine, and nonmarine 

rocks. 

Hilliard Shale.-The Hilliard Shale is composed 

mostly of gray shale that grades upward to silty and sandy 

shale. About 700 ft (213 m) of Hilliard Shale was 

.....J 

o<X: 

STRATIGRAPHIC ~ DEPTH 

UNIT 0 ~ (in feet) uz 

6000 

8000 

c 

0 

·.;::; 

C1l 

E 

0 10,000 LL 

Q) 

() 

c 

C1l 

_J 

12,000 

14,000 

16,000 

18,000 

TOTAL DEPTH 19,000 FT 

DOMINANT 

LITHOFACIES 

D 

Nonmarine 

sandstone and shale 

Marginal-marine 

sandstone and shale 

~ -

Marine siltstone 

and shale 

~ 

UNCONFORMITY 

Figure 3. Stratigraphic sequence and cored intervals, 

El Paso Natural Gas Company Wagon Wheel No. 1 

well, northern Green River basin, Wyoming. Location 

of well shown in figure 2. 

penetrated in the Wagon Wheel well, but wells west of 

the Pinedale anticline indicate that the total thickness of 

the Hilliard Shale is about 3,500 ft (1 ,067 m). The upper 

part of the Hilliard Shale was deposited in a shallow-water 

marine environment. The Hilliard Shale is, in part, 

equivalent to the Baxter Shale in the Rock Springs uplift 

and to the Mancos Shale in the Oak Creek and MWX 

areas (fig. 4). In the Wagon Wheel well, the top of the 

Hilliard is at a depth of about 18,270 ft (5,569) (fig. 3). 

Structural and Stratigraphic Framework, Pinedale Anticline and Multiwell Site 83 
= "'" 

~ 

~ 

j 

-< 

s. 

-1 

cjQ" 

=:-

~ 

Ill 

Ill 

Ill 

~ 

~ 

~ 

~· ~ 

~ 0 

3 

:;· 

CIQ 

Ill 

:::s 

Q. 

n 

0 

0 

~ Q. 

0 

UJ>:- ~ (/) 

<( UJ UJ WESTERN INTERIOR ROCK SPRINGS t9E a: ~ a: STAGE AMMONITE ZONES PINEDALE ANTICLINE UPLIFT OAK CREEK MWXSITE <(:.§.. UJ ~ UJ 

(/) (/) 

Discoscaphites nebrascensis, Lance Formation Hoploscaphites nicol/etti Lance Formation 

c: 

Hoploscaphites aff. H. nicolletti, 0 

Fox Hills Sandstone ·.;:: 

Sphenodiscus coahuilites E 

MAESTRICHTIAN Baculites clinolobatus Lance Formation Fox Hills Sandstone 0 Lewis Shale c. u.. 

70- :::J 

Baculites grandis ""' 0 

t5 ~ Q) (/) 

Baculites baculus Lewis Shale ""' "E E Q) "' 

oc: > :.= 

Baculites eliasi u..o 

c. E·~ Twentvmile Ss. Member ~ ~ ~ Baculites jenseni 6 mE 

Baculites reesidei Almond Formation ~ ~~ 

~~ Baculites cuneatus _.,.,. ~ ;:!_P~ _,..,. ~ Canyon Creek Mbr. Q; Cameo-Fairfield coal zone Baculites compressus ~ Didymoceras cheyennense Q) Trout Creek Ss. Mbr. c: Rollins Sandstone Member 

t----... Exiteloceras jennyi _,-- ~ § Oak Creek sandstone 1 0 Tongue of Mancos Shale (/).;:: (/) ~"'"""'\ Didymoceras stevensoni / a>"' ~~ Double Ledge ss. 1 ozzette ::>s. Member :::> =E =E Tongue of Mancos Shale 0 (/) UJ 

r----.Didymoceras nebrascenser- u.. 0 ~ Tow Creek Ss. Mbr. Corcoran Ss. Mbr. I 75- U:::> u Baculites scotti 

oo ~ I 

NUJ Baculites gregoryensis ou UJ 

(/)<( a: UJ~ u Baculites perplexus (late form) ~~ a: u UJ Q) c:~._--~-

a.. 

a.. Baculites gilberti 5l "E ~ :::> (.) Q) (.) 

~ > ~ Rusty zone Hunt Creek ss. 1 Baculites perp/exus (early form) (/) "' Lower Q) and ~ 

CAMPANIAN Baculites sp. (smooth) Trail member 

Bacu/ites asperiformis Devil's Grave ss. 1 

~· ,.... ,7, ~ .a. ...... Q) 

Baculites mclearni 

~~ (ij 

.r: Baculites obtusus (f) Unnamed member 

80- . . (/) Mancos Shale 

8 Boculites sp. (weak flank ribs) c: --

Rock Springs Formation ~ Baculites sp. (smooth) 

Scaphites hippocrepis lli Rock Springs Formation 

Blair Formation 

Scaphites hippocrepis lJ 

Scaphites hippocrepis I Baxter Shale 

SANTONIAN Hilliard Shale 

(part) Desmocaphites bassleri 

-

1 Unranked unit 

Figure 4. Stratigraphic correlation chart for Pinedale anticline, Green River basin, and MWX site, Piceance basin. Stratigraphy of intermediate 

areas also shown. Location of areas shown in Law and Spencer (this volume, fig. 1 ). 
Rock Springs Formation.-In surface exposures in 

the Rock Springs uplift and in core from the Wagon 

Wheel well, the Rock Springs Formation is composed of 

tan to light-gray, fine- to medium-grained sandstone, 

siltstone, shale, carbonaceous mudstone, and coal. Its 

lower contact intertongues with the underlying Hilliard 

Shale, and in well logs this contact is placed at the base 

of predominantly sandstone bearing rocks. The Rock 

Springs Formation is about 4,800 ft (1 ,463 m) thick and 

can be subdivided into three parts. The lower part is 

800-1,000 ft (244-305 m) thick and consists of intertonguing sandstone and shale deposited in shallow-water 

marine and marginal-marine environments. The middle 

part is about 2,450 ft (747 m) thick and consists of sandstone, siltstone, shale, and coal deposited in middle- to 

lower-delta-plain environments. The upper part is about 

1,550 ft (472 m) thick and consists of interbedded sandstone, siltstone, and mudstone deposited in alluvial-plain 

environments. The Rock Springs Formation in the Pinedale anticline area is equivalent to the upper part of the 

Baxter Shale, Blair Formation and Rock Springs Formation in the Rock Springs uplift area and to the middle 

part of the Mancos Shale in the Oak Creek and MWX 

areas (fig. 4). In the Wagon Wheel well, the depth to the 

top of the Rock Springs is about 13,465 ft (4,104 m) 

(fig. 3). 

Ericson Sandstone.-The Ericson Sandstone is 

composed mostly of sandstone and lesser amounts of 

siltstone and mudstone. In the Pinedale area, the Ericson is divided into two parts. The lower part is 290 ft 

(88 m) thick and is a sandstone that has interbeds of 

siltstone and shale deposited in meandering- and braidedstream systems. The nature of the contact with the 

underlying Rock Springs Formation is uncertain; in the 

western part of the Green River basin, the contact probably is unconformable; farther to the east, it probably 

is conformable. The lower part of the Ericson is 

equivalent to the Rusty zone and Trail Member of the 

Ericson Sandstone in the Rock Springs uplift area, to the 

intertonguing sandstones and shales in the upper part of 

the Mancos Shale in the Oak Creek area, and to the upper part of the Mancos Shale in the MWX area. In the 

Wagon Wheel well, the depth to the top of the lower 

Ericson is about 13,165 ft (4,013 m) (fig. 3). 

The upper part of the Ericson is about 100 ft (30 m) 

thick and is a conglomeratic sandstone. It is more quartz 

rich and contains less siltstone and shale than the lower 

part. The upper part is predominantly light to medium 

gray and consists of very fine to coarse grained quartzose sandstone that contains granules of chert, quartzite, 

and siltstone. Sedimentological characteristics of the 

upper part, as determined from both cores and outcrops, 

indicate deposition in a braided-stream system. Regional 

stratigraphic relationships (Law, 1979; Law and Nichols, 

1982; Law and others, 1983; Bucurel-White, 1983) 

Figure 5. Location of Greater Green River basin and Piceance 

basin and Western Interior epeiric seaway during Late 

Cretaceous time. Modified from Gill and Cobban (1973). 

suggest that the contact between the lower and upper 

parts is unconformable. The upper part of the Ericson 

is equivalent to the Canyon Creek zone of the Ericson 

Sandstone in the Rock Springs uplift area, to the lower 

part of the Williams Fork Formation in the Oak Creek 

area, and to part of the Cameo-Fairfield coal zone of the 

Williams Fork Formation in the MWX area (fig. 4). In 

the Wagon Wheel well, the depth to the top of the upper 

Ericson is about 13,065 ft (3,982 m) (fig. 3). 

Lance Formation.-In the Wagon Wheel well, the 

Lance Formation is about 5,500 ft (1,692 m) thick and 

is composed of tan to brown, fine- to medium-grained 

sandstone, siltstone, shale, and carbonaceous mudstone. The Lance is interpreted to have been deposited 

in an alluvial-plain environment. Its lower contact is 

conformable with the underlying Ericson Sandstone, 

and its upper contact is unconformable with overlying 

Tertiary rocks. The lower part of the Lance in the 

Pinedale area is temporally equivalent to the Almond Formation, Lewis Shale and Fox Hills Sandstone in the Rock 

Springs uplift area, to the upper part of the Williams Fork 

Formation, Lewis Shale, Fox Hills Sandstone and Lance 

Formation in the Oak Creek area, and to the Williams 

Fork Formation above the Cameo-Fairfield coal zone at 

the MWX site (fig. 4). In the Wagon Wheel well, the 

depth to the top of the Lance is about 7,520 ft (2,292 m) 

(fig. 3). 

Structural and Stratigraphic Framework, Pinedale Anticline and Multiwell Site 85 
Tertiary Rocks 

Lower Tertiary rocks in the Pinedale anticline area 

were deposited in dominantly fluvial environments. Depositional patterns in these rocks are more complex than those 

in Cretaceous rocks because of multiple source terranes, 

the emergence of adjacent highlands, and the development 

of internal basin drainage during the Tertiary. Our discussion of Tertiary rocks is restricted to the lower Tertiary inasmuch as younger Tertiary rocks are not gas bearing. 

Probably the most significant departure from 

previous stratigraphic studies in the Pinedale area is the 

identification of the Cretaceous-Tertiary boundary. In 

southwestern Wyoming, the base of the Tertiary is 

marked by a regional unconformity (Roehler, 1961; Ritzrna, 1965; Dorr and others, 1977; Beaumont, 1979), and 

throughout most of the region the oldest Tertiary unit 

is the Paleocene Fort Union Formation or its equivalent. 

In the subsurface of the northern Green River and Great 

Divide basins, however, an older unnamed Tertiary interval has been identified on the basis of well-log correlations and palynology (Law, 1979; Law and Nichols, 1982; 

Bucurel-White, 1983). In the Pinedale area, studies by 

Martin and Shaughnessy (1969) and Shaughnessy and 

Butcher (1973, 1974) indicate that the Cretaceous-Tertiary 

boundary is at a depth of about 10,000 ft (3,048 m) in 

the Wagon Wheel well; however, more recent stratigraphic and palynological studies by Law (1979) and Law 

and Nichols ( 1982) indicate that the boundary is at a 

depth of 7,520 ft (2,302 m). 

Unnamed unit.-The unnamed unit consists of interbedded conglomerate, sandstone, siltstone, and mudstone. In the Pinedale area, it unconformably overlies the 

Lance Formation and is 1,285-1,510 ft (391-460 m) thick. 

The siliciclastic sediments of this unit were derived from 

the Wind River Mountains and deposited in alluvial-plain 

and fluvial-deltaic environments. The age of the unit is 

uncertain but probably is early to middle Paleocene. In 

the Wagon Wheel well, the depth to the top of the unnamed unit is about 6,215 ft (1,894 m) (fig. 3). 

Fort Union Formation and younger rocks.-The 

Fort Union Formation and younger rocks consist of interbedded conglomerate, sandstone, siltstone, mudstone, 

and carbonaceous mudstone. Coal beds, commonly 

associated with the lower part of the Fort Union Formation in the Rock Springs uplift area and elsewhere, are 

not present in the Pinedale area, presumably because of 

facies changes related to the proximity of source terranes 

in the nearby Wind River Mountains. Regionally, the base 

of the Fort Union is unconformable, but in the vicinity 

of the Pinedale anticline the nature of the contact with 

the underlying unnamed unit is uncertain. The Fort Union 

Formation and younger rocks were deposited in 

dominantly fluvial environments, and the age of these 

rocks is middle Paleocene to Eocene. 

86 Geology of Tight Gas Reservoirs, Wyoming and Colorado 

MULTIWELL EXPERIMENT SITE 

Structure 

The Multiwell Experiment (MWX) site is in the 

Rulison gas field on the gently dipping southwest flank 

of the Piceance basin near its structural axis (Law and 

Spencer, this volume, fig. 1) (fig 6). The Piceance basin 

is a Laramide structural anc~ sedimentary basin that was 

subsiding from the latest Cretaceous through about the 

end of the Eocene. It is bounded on the northwest by the 

Uinta uplift, on the east by the White River uplift, on 

the southeast by the Sawatch Mountains uplift, on the 

south by the San Juan volcanic field, and on the southwest by the Uncompahgre uplift. 

The Piceance basin is highly asymmetric; it has 

gently dipping west and southwest flanks and a sharply 

upturned east flank. The structural axis of the basin is 

just west of the steeply dipping east flank. The east flank, 

called the Grand Hogback, is believed to be underlain 

by a deep-seated west-thrusting reverse or normal thrust 

fault (Gries, 1983). 

Stratigraphy 

Cretaceous Rocks 

The principal low-permeability gas-bearing interval at the MWX site is the Upper Cretaceous Mesaverde 

Group. At the MWX site, the Mesaverde is about 4,200 ft 

(1 ,280 m) thick and is approximately equivalent to the 

upper half of Upper Cretaceous low-permeability reservoirs in the Pinedale anticline area (fig. 4). The stratigraphic sequence penetrated at the MWX site is shown 

in figures 7 and 8 and includes, in ascending order, the 

Corcoran, Cozzette and Rollins Sandstone Members of 

the Iles Formation and the Williams Fork Formation. 

These Upper Cretaceous rocks were deposited in and adjacent to the Western Interior seaway during the Late 

Cretaceous (fig. 5). Rapid subsidence during the Late 

Cretaceous caused a major marine incursion, and an 

epeiric seaway covered much of the foreland basin, including the area of the Piceance and Green River basins. 

During this marine incursion, about 5,500 ft (1,680 m) 

of marine Mancos Shale was deposited in the MWX area. 

The Mancos Shale conformably underlies the Mesaverde 

throughout the Piceance basin. 

lies Formation of the Mesaverde Group.-At the 

MWX site, the Iles Formation of the Mesaverde Group 

extends upward from the underlying Mancos Shale 

through the marginal-marine Rollins Sandstone Member 

(equivalent to the Trout Creek Member at Oak Creek) 

and consists of a series of transgressive and regressive 

marine cycles. The Iles Formation was deposited during 
10 

I 1 I 

10 20 

I 

20 MILES 

I 

30 KILOMETERS 

RIVER 

UPLIFT 

oDelta 

Figure 6. Structure contours of the top of the Rollins and Trout Creek Sandstone Members of the lies Formation 

of the Mesaverde Group, Piceance basin. Contours in feet. Line of section 8-8' (fig. 7) also shown. 

late Campanian time, during which a series of clastic sediment wedges were deposited in response to orogenic activity in the Sevier orogenic belt farther to the west (Fouch 

and others, 1983). During deposition of the lies Formation, there were several marine transgressions and 

regressions. 

The three major regressive cycles recognized at the 

MWX site are, in ascending order, the Corcoran, 

Cozzette, and Rollins Sandstone Members of the lies Formation. Tongues of transgressive marine shale separate 

each of the three cycles. The Corcoran and Cozzette 

cycles prograded from northwest to southeast across the 

MWX area and undergo a facies change into Mancos 

Shale about 25-30 mi (40-48 km) southeast of the MWX 

site. The position of maximum regression of the Rollins 

or Trout Creek cycle is east of the Grand Hogback, and, 

although the orientation of the strandline is difficult to 

determine, it appears that the Rollins cycle prograded 

from west to east across the MWX area. 

At the MWX site, the Corcoran is about 135 ft 

(41 m) thick, the Cozzette about 165ft (50 m) thick, and 

the Rollins about 135ft (41 m) thick. Throughout most 

of the southern part of the Piceance basin, the Corcoran 

and Cozzette regressive cycles are very complex, and commonly each cycle contains more than one shoreface sandstone sequence, as well as lenticular channel sandstones 

Structural and Stratigraphic Framework, Pinedale Anticline and Multiwell Site 87 
--------------------------------66MILES--------------------------------

FEET 

10,000 

5,000 

SEA LEVEL 

-10,000 

NW 

B 

APPROXIMATE VERTICAL EXAGGERATION x 5 

a:: 

~ 

~ 

0 

Q 

~ 0 

-l 

0 

u 

Wasatch 

Fork 

Mancos Shale 

and 

DIVIDE CREEK GRAND HOGBACK 

ANTICLINE 

Tsnnsco 

No.1 

Cameo 

well 

SE 

B' 

METERS 

2000 

1000 

SEA LEVEL 

-1000 

-2000 

-3000 

5 10 MILES I I 

I I 

10 15 KILOMETERS 

Figure 7. Cross section through Piceance basin including MWX site. Line of section shown in figure 5. 

and some coal. At the MWX site, both the Corcoran and 

Cozzette contain two shoreface sequences (Lorenz, this 

volume). In contrast, the Rollins consists of a single 

coarsening-upward shoreface sandstone. The individual 

shoreface sandstones in the Corcoran and the Cozzette 

are probably blanket sandstones from a reservoir point 

of view but are regionally less persistent than the Rollins 

shoreface sandstone and can seldom be traced over large 

areas. 

Williams Fork Formation of the Mesaverde 

Group.-About 3,500 ft (1,066 m) of Upper Cretaceous 

Campanian and Maestrichtian nonmarine rocks of the 

Williams Fork Formation overlies the lies Formation in 

the MWX area. The lower 800 ft (244 m) of the Williams 

Fork consists mostly of carbonaceous shale and coal 

deposited in a coastal-plain or delta-plain (paludal) environment. Most of the thick coal beds are in the CameoFairfield coal zone, which makes up about the lower 

450 ft (137m) of this sequence. The Cameo-Fairfield is 

present in all but the northwesternmost part of the 

Piceance basin and is by far the most significant coal zone 

in the basin. At the MWX site, about 80 ft (24 m) of coal 

occurs in the Cameo-Fairfield coal zone. About 2,700 ft 

(823 m) of mostly fluvial point-bar and overbank deposits 

overlies the paludal sequence. Lorenz (1983, this volume) 

recognized a 550-ft-thick (168m) transition zone between 

the underlying paludal and overlying fluvial rocks that 

he referred to as the coastal zone, and he suggested that 

there is some marine influence in the uppermost 350ft 

(107 m) of the Williams Fork. 

88 Geology of Tight Gas Reservoirs, Wyoming and Colorado 

Tertiary Rocks 

A major regional unconformity is present at the 

base of Tertiary rocks throughout most, if not all, of the 

Piceance basin (Johnson and May, 1978, 1980). A considerable thickness of Mesaverde rocks was probably 

eroded during this period of beveling in many parts of 

the basin (Johnson, 1989). The thickness of section 

eroded at MWX is not known, but, because the MWX 

site is near the trough of the basin, the amount of beveling at the site is probably much less than that on the 

flanks (Johnson, 1989). 

The lithology of the lower Tertiary rocks overlying the unconformity at MWX has not been described 

in detail but is probably similar to that of lower Tertiary 

rocks in nearby wells (Johnson and others, 1979) and on 

outcrops at Rifle Gap (Donnell, 1969; Johnson, 1982). 

Wasatch Formation.-About 2,400 ft (732 m) of 

Paleocene-age Atwell Gulch Member of the Wasatch Formation overlies the Cretaceous-Tertiary unconformity at 

MWX (Donnell, 1969). The lower 770ft (235m) of the 

Atwell Gulch consists of interbedded gray mudstone and 

conglomeratic sandstone. Pebbles and cobbles in the basal 

conglomerate at MWX are mostly varicolored chert, 

whereas andesite and dacite pebbles and cobbles predominate throughout the rest of the interval. The conglomeratic interval thickens toward the southeastern 

margin of the basin and was probably derived from the 

Sa watch uplift to the southeast. The remaining 1, 730 ft 

(527 m) of Atwell Gulch consists of drab-colored mudstone and a few lenticular channel sandstones. 
LU 

z 

LU 

u 

0 

LU c 

Q 

rti 

E 

0 LU LL 

z .r:: 

LU ~ 

u cc 

<Jl 

0 cc 

LU s _J 

<! 

0... 

(j) 

:J 

0 

LU 

u 

<( 

tLU 

a: 

u 

LU 

t-

<! 

_J 

STRATIGRAPHIC UNIT 

Q; 

·~-E .r::Cll 

(/)~ 

.r:: 

~,.... 

:::J Cll <._?..0 

= E Cll Cll 

~~ 

<( Unnamed conglomeratic unit 

UNCONFORMITY 

c 

0 

·;:; 

cc 

E 

0 

LL 

.::t!. 

0 

LL 

<Jl 

E 

~ 

~ 

TOTAL DEPTH 8,300 FT 

_J 

51~ o:::O::: 

ow 

u ~ DEPTH 

(in feet) 

1,000 

2,000 

3,000 

4,000 

5,000 

6,000 

7,000 

8,000 

Figure 8. Stratigraphic sequence and cored intervals, 

MWX-1 and MWX-2 wells, Piceance basin, Colorado. 

Unlabeled units between Rollins and Cozzette and 

Cozzette and Corcoran are tongues of Mancos Shale. 

A thin remnant of the Molina Member of the 

Wasatch Formation overlies the Atwell Gulch Member 

at MWX (Donnell, 1969; Johnson, 1979). The Molina 

Member is the sandiest interval in the Wasatch and has 

been identified along both the east and west flanks of the 

basin, where it consists of interbedded conglomeratic 

sandstone and gray mudstone. Pebbles consist mostly of 

varicolored chert and quartzite. At MWX, only the very 

persistent basal sandstone of the Molina Member (depth 

of 1,485-1,505 ft in MWX-1) can be identified with certainty. Two sandstones above the basal sandstone in the 

MWX-1 well may also be Molina sandstones. Thus far, 

attempts to date the Molina Member using palynomorphs 

have been unsuccessful (Johnson and May, 1978). 

The remaining 1,100 ft (335 m) of lower Tertiary 

rocks at MWX is included in the Shire Member of the 

Wasatch Formation. The Shire Member consists of varicolored mudstone and sparse lenticular channel sandstones 

and has been dated as early Eocene in age (Wood, 1962; 

Donnell, 1969; Newman, 1974; Johnson and May, 1978). 

It is exposed at the surface at MWX, extends about 1,200 

ft (366 m) up the sides of nearby canyon walls, and is 

overlain by the Eocene Green River Formation. 

REFERENCES CITED 

Beaumont, E.A., 1979, Depositional environments of Fort 

Union sediments (Tertiary, northwest Colorado) and their 

relation to coal: American Association of Petroleum 

Geologists Bulletin, v. 63, no. 2, p. 194-217 . 

Berg, R.B., 1961, Laramide tectonics of the Wind River Mountains, in Wiloth, G.J., ed., Symposium on Late 

Cretaceous rocks, Wyoming and adjacent areas: Wyoming Geological Association Field Conference, 16th, 1961, 

Guidebook, p. 70-80. 

Bucurel-White, H., 1983, Section C-C', subsurface and surface correlations of some Upper Cretaceous and Tertiary 

rocks from the northwest flank of the Rock Springs uplift 

to the Rawlins uplift, Great Divide basin, Wyoming: U.S. 

Geological Survey Open-File Report 83-418, 2 sheets. 

Donnell, J.R., 1969, Paleocene and lower Eocene units in the 

southern part of the Piceance Creek basin, Colorado: U.S. 

Geological Survey Bulletin 1274-M, p. Ml-M18. 

Dorr, J.A., Jr., Spearing, D.R., and Steidtmann, J.R., 1977, 

Deformation and deposition between a foreland uplift and 

an impinging thrust belt, Hoback basin, Wyoming: 

Geological Society of American Special Paper 177, 82 p. 

Fouch, T.D., Lawton, T.F., Nichols, D.J., Cashion, W.B., and 

Cobban, W.A., 1983, Patterns and timing of synorogenic 

sedimentation in Upper Cretaceous rocks of central and 

northeast Utah, in Reynolds, M.W., and Dolly, E.D., eds, 

Mesozoic paleogeography of the West-Central United 

States: Society of Economic Paleontologists and 

Mineralogists Rocky Mountain Paleogeography Symposium, 2nd, Denver, Colo., 1983, p. 305-336. 

Gill, J.R., and Cobban, W.A., 1973, Stratigraphy and geologic 

history of the Montana Group and equivalent rocks, Montana, Wyoming, and North and South Dakota: U.S. 

Geological Survey Professional Paper 776, 37 p. 

Gries, Robbie, 1983, Oil and gas prospecting beneath Precambrian of foreland thrust plates in Rocky Mountains: 

American Association of Petroleum Geologists Bulletin, 

v. 67, no. 1, p. 1-28. 

Hagen, E.S., Shuster, M.W., and Furlong, K.P., 1985, Tectonic loading and subsidence of intermontane basins; 

Wyoming foreland province: Geology, v. 13, p. 585-588. 

Johnson, R.C., 1982, Measured section of Upper Cretaceous 

Mesaverde Formation and lower part of the lower Tertiary Wasatch Formation, Rifle Gap, Garfield County, 

Colorado: U.S. Geological Survey Open-File Report 

85-590, 11 p. 

___ 1987, Geologic history and hydrocarbon potential of 

Late Cretaceous-age low-permeability reservoirs, Piceance 

basin, western Colorado: U.S. Department of Energy, 

Fossil Energy, DOE/MC/20422-2337, 97 p. 

Structural and Stratigraphic Framework, Pinedale Anticline and Multiwell Site 89 
___ 1989, Geologic history and hydrocarbon potential of 

Late Cretaceous-age, low-permeability reservoirs, Piceance basin, western Colorado: U.S. Geological Survey 

Bulletin 1787-E, 51 p. 

Johnson, R.C., Granica, M.P., and Dessenberger, N.C., 1979, 

Cross section B-B' of Upper Cretaceous and lower Tertiary rocks, southern Piceance Creek basin, Colorado: 

U.S. Geological Survey Miscellaneous Field Investigations 

Map MF-1130-B, 2 sheets. 

Johnson, R.C., and May, F., 1978, Preliminary stratigraphic 

studies of the upper part of the Mesaverde Group, the 

Wasatch Formation, and the lower part of the Green River 

Formation, Debeque area, Colorado, including environments of deposition and investigation of palynomorph 

assemblages: U.S. Geological Survey Miscellaneous Field 

Studies Map MF-1050, 2 sheets. 

___ 1980, A study of the Cretaceous-Tertiary unconformity 

in the Piceance Creek basin, Colorado; the underlying 

Ohio Creek Formation (Upper Cretaceous defined as a 

member of the Hunter Canyon or Mesaverde Formation: 

U.S. Geological Survey Bulletin 1482-B, 27 p. 

Johnson, R.C., and Nuccio, V.F., 1984, Late Cretaceous 

through early Tertiary general stratigraphy and structural 

geology of the Piceance Creek basin, Colorado, in 

Spencer, C.W., and Keighin, C.W., eds., Geologic studies 

in support of the U.S. Department of Energy Multiwell 

Experiment, Garfield County, Colorado: U.S. Geological' 

Survey Open-File Report 84-757, p. 14-20. 

___ 1986, Structural and thermal history of the Piceance 

Creek basin, western Colorado, in relation to hydrocarbon occurrence in the Mesaverde Group, in Spencer, 

C.W., and Mast, R.F., eds., Geology of tight gas reservoirs: American Association of Petroleum Geologists 

Studies in Geology 24, p. 165-206. 

Jordan, T.E., 1981, Thrust loads and foreland basin evolution, 

Cretaceous, western United States: American Association 

of Petroleum Geologists Bulletin, v. 65, no. 12, 

p. 2506-2520. 

Keefer, W.R., 1965, Stratigraphy and geologic history of the 

uppermost Cretaceous, Paleocene, and lower Eocene 

rocks in the Wind River basin, Wyoming: U.S. Geological 

Survey Professional Paper 495-A, p. Al-A 77. 

Law, B.E., 1979, Section B-B', subsurface and surface correlations of some Upper Cretaceous and Tertiary rocks, 

northern Green River basin, Wyoming: U.S. Geological 

Survey Open-File Report 79-1689, 2 sheets. 

___ 1984, Structure and stratigraphy of the Pinedale anticline, Wyoming, in Law, B.E., ed., Geological 

characteristics of low-permeability Upper Cretaceous and 

lower Tertiary rocks in the Pinedale anticline area, 

Sublette County, Wyoming: U.S. Geological Survey 

Open-File Report 84-753, p. 6-15. 

Law, B.E., Bucurel-White, H., and Bader, J.W., 1983, 

Sedimentological aspects of stratigraphic correlations in 

the Upper Cretaceous Ericson Sandstone, Greater Green 

River basin, Wyoming, Colorado, and Utah: Geological 

Society of America Abstracts with Programs, v. 15, no. 

5, p. 333. 

Law, B.E., and Nichols, D.J., 1982, Subsurface stratigraphic 

correlations of some Upper Cretaceous and lower Tertiary 

810 Geology of Tight Gas Reservoirs, Wyoming and Colorado 

rocks, northern Green River basin, Wyoming [abs.], in 

Steidtmann, J.R., ed., Subsurface practices in geology and 

geophysics: Laramie, Wyo., University of Wyoming, 

Department of Geology and Geophysics, Earth Science 

Bulletin 15, p. 17. 

Law, B.E., Pollastro, R.M., and Keighin, C.W., 1986, Geologic 

characterization of low-permeability gas reservoirs in 

selected wells, Greater Green River basin, Wyoming, 

Colorado, and Utah, in Spencer, C.W., and Mast, R.F., 

eds., Geology of tight gas reservoirs: American Association of Petroleum Geologists Studies in Geology 24, 

p. 253-270. 

Lorenz, J.C., 1983, Reservoir sedimentology in Mesaverde rocks 

of the Multi-Well Experiment site: Sandia National Laboratories Report SAND83-1078, 36 p. 

___ 1984, Reservoir sedimentology of Mesaverde rocks at the 

MWX site, in Spencer, C.W., and Keighin, C.W., eds., 

Geologic studies in support of the U.S. Department of 

Energy Multiwell Experiment, Garfield County, Colorado: U.S. Geological Survey Open-File Report 84-757, 

p. 21-32. 

Love, J.D., 1960, Cenozoic sedimentation and crustal movement in Wyoming: American Journal of Science, v. 

258-A, p. 204-214. 

Martin, W.B., and Shaughnessy, J., 1969, Project Wagon 

Wheel, in Symposium on Tertiary rocks of Wyoming: 

Wyoming Geological Association Field Conference, 21st, 

Guidebook, p. 145-152. 

Newman, K.R., 1974, Palynomorph zones in early Tertiary formations of the Piceance Creek and Uinta basins, Colorado 

and Utah, in Murray, D.K., ed., Guidebook to the energy 

resources of the Piceance Creek basin, Colorado: Rocky 

Mountain Association of Geologists Field Conference 

Guidebook 25, p. 47-56. 

Prensky, S.E., 1984, A gamma-ray log anomaly associated with 

the Cretaceous-Tertiary boundary in the northern Green 

River basin, Wyoming, in Law, B.E., ed., Geological 

characteristics of low-permeability Upper Cretaceous and 

lower Tertiary rocks in the Pinedale anticline area, 

Sublette County, Wyoming: U.S. Geological Survey 

Open-File Report 84-753, p. 22-35. 

Ritzma, H.R., 1965, Fossil soil at base of Paleocene rocks, 

southern Rock Springs uplift, Wyoming, in Sedimentation of Late Cretaceous and Tertiary outcrops, Rock 

Springs uplift: Wyoming Geological Association Field 

Conference, 19th, 1965, Guidebook, p. 137-139. 

Roehler, H.W., 1961, The Late Cretaceous-Tertiary boundary 

in the Rock Springs uplift, Sweetwater County, Wyoming, 

in Symposium on Late Cretaceous rocks, Wyoming and 

adjacent areas: Wyoming Geological Association Field 

Conference, 16th, 1961, Guidebook, p. 96-100. 

Shaughnessy, J., and Butcher, R.H., 1973, Geology of Project Wagon Wheel nuclear stimulation project, in Fassett, 

J .E., ed., Cretaceous and Tertiary rocks of the southern 

Colorado Plateau: Four Corners Geological Society, 

p. 185-196. 

___ 1974, Geology of Wagon Wheel nuclear stimulation 

project, Pinedale field, Wyoming: American Association of Petroleum Geologists Bulletin, v. 58, no. 11, 

p. 2250-2259. 
Shuster, M. W., 1986, The origin and sedimentary evolution of 

the northern Green River basin, western Wyoming: 

Laramie, University of Wyoming, Ph.D. thesis, 323 p. 

Shuster, M.W., and Steidtmann, J.R., 1983, Origin and 

development of northern Green River basin; a 

stratigraphic and flexural study [abs.]: American Association of Petroleum Geologists Bulletin, v. 67, no. 8, 

p. 1356. 

___ 1988, Tectonic and sedimentary evolution of the northern Green River basin, western Wyoming, in Schmidt, 

C.J., and Perry, W.J., Jr., eds., Interaction of the Rocky 

Mountain foreland and the Cordilleran thrust belt: 

Geological Society of America Memoir 171, p. 515-529. 

Wood, A.B., 1962, The early Tertiary rodents of the Family 

Paramyidae: American Philosophical Society Transactions, v. 52, p. 261. 

Structural and Stratigraphic Framework, Pinedale Anticline and Multiwell Site 811 

Chapter C 

Comparison of Overpressuring at the 

Pinedale Anticline Area, Wyoming, and the 

Mu ltiwell Experiment Site, Colorado 

By CHARLES W. SPENCER 

Prepared in cooperation with the U.S. Department of Energy 

U.S. GEOLOGICAL SURVEY BULLETIN 1886 

GEOLOGY OF TIGHT GAS RESERVOIRS IN THE PINEDALE ANTICLINE AREA, WYOMING, 

AND AT THE MULTIWELL EXPERIMENT SITE, COLORADO 
CONTENTS 

Abstract Cl 

Introduction Cl 

Methods of study C2 

Pinedale area C3 

Multiwell Experiment site C4 

Comparison of overpressuring in the Pinedale and MWX areas C12 

Summary and conclusions C12 

References cited C14 

FIGURES 

1. Map showing distribution of overpressuring in Greater Green River basin, 

Wyoming, Utah, and Colorado C3 

2. Graphs showing subsurface pressure, temperature, and vitrinite reflectance, 

El Paso Natural Gas No. 1 Wagon Wheel well, on the crest of the 

Pinedale anticline C4 

3. Drill-stem test chart from test run in Upper Cretaceous Lance Formation, 

El Paso Natural Gas No. 1 Wagon Wheel, between depths of 10,978 and 

11,070 ft cs 

4. Cross plot of interpreted reservoir pressure and depth for six wells drilled 

on Pinedale anticline C6 

5. Interpreted pressure profile for MWX wells C7 

6-7. Temperature logs for MWX-1 well measured by Los Alamos National 

Laboratories: 

6. From high-resolution, and openhole geophysical logs and an openhole 

drill-stem test C8 

7. Six months after hole was cased C9 

8. Differential temperature gradient log for MWX-1 well ClO 

9. Graph showing vitrinite reflectance of kerogen extracts obtained from 

MWX wells as a function of depth Cll 

10. Profile through MWX area showing topographic relief in Colorado River 

valley C12 

11. Graph showing pressure gradients in Pinedale and MWX areas C13 
Comparison of Overpressuring at the 

Pinedale Anticline Area, Wyoming, and the 

Multiwell Experiment Site, Colorado 

By Charles W. Spencer 

Abstract 

Abnormally high reservoir pressures occur in many Rocky 

Mountain basins in deep, hot (>200 °F, 93 °C), low-permeability 

(tight) reservoir sequences. Although the pressuring fluid phase 

is usually natural gas, some overpressured oil and gas reservoirs 

are also present. The high pressure is interpreted to be caused 

by active or recently active generation of hydrocarbons at a rate 

that exceeds hydrocarbon loss. 

Comparison of overpressuring in Upper Cretaceous tight 

gas sandstone reservoirs at the Pinedale area, Sublette County, 

Wyo., and at the U.S. Department of Energy Multiwell Experiment site, Garfield County, Colo., indicates that although these 

two areas have experienced different paleothermal histories, 

many similarities exist between the areas. Both have been cooled 

below their maximum levels of paleoheating, and this cooling 

has resulted in the interpreted development of shallow zones 

of inactive or residual overpressuring. These shallow zones are 

intervals where gas is no longer being generated and is being 

lost by diffusion and gas-phase migration. Below these inactive 

zones, pressure-causing gas generation is actively occurring at 

temperatures higher than about 200 Of (93 °C). In the active 

generation zones, pressures are variable but higher than those 

in the inactive zones. Pressure variations in the active zones are 

related to rock permeability and organic richness and to the ability of source rocks to generate gas at a rate that exceeds gas loss. 

In both areas, maximum pressures are about equal to the 

fracture gradient of the sandstone reservoirs. High pore pressures 

may be a factor in the development of natural fractures. Limited 

studies of fracture-filling cements at the Multiwell site indicate 

that the fractures formed at elevated temperatures and at a time 

when reservoir waters were gas saturated. 

Inconclusive data from the deep, highly overpressured parts 

of the Multiwell Experiment wells suggest that some retardation 

of thermal maturation (vitrinite reflectance) may have occurred. 

Data from the Pinedale wells, however, do not suggest suppression of vitrinite reflectance. 

INTRODUCTION 

Overpressured low-permeability (tight) gas reservoirs are common in the deeper parts of many Rocky 

Mountain basins (Spencer 1983, 1987). Pore pressure or 

reservoir pressure, as it is more commonly described, is 

often expressed in terms of a pressure gradient in psi/ft 

(kPa/m). In freshwater reservoirs, normal pressure is 

about 0.43 psi/ft (9.73 kPa/m), and if reservoir pressures 

exceed 0.5 psi/ft (11.31 kPa/m), the reservoirs usually 

are considered to be overpressured (reservoirs containing 

highly saline water are exceptions to this generalization). 

Upper Cretaceous sandstone, shale, and coal strata are 

substantially overpressured at both the Pinedale anticline 

area in the Green River basin of southwestern Wyoming 

and the U.S. Department of Energy Multi well Experiment 

(MWX) site in the Piceance basin of northwestern 

Colorado (Law and Spencer, this volume, fig. 1); 

maximum reservoir pressures in both areas are about 

0.8 psi/ft (18.1 kPa/m). Both areas are in regionally 

overpressured basins that have been studied by the U.S. 

Geological Survey (USGS) in cooperation with the U.S. 

Department of Energy's Western Tight Gas Sands 

Subprogram. 

Although a variety of factors can cause overpressuring (Fertl, 1976), the generally accepted causes of most 

overpressuring are dewatering (compaction) of shales 

(Harkins and Baugher, 1969) or alteration of "montmorillonite" (smectite) to a less hydrated mixed-layer 

illite-smectite mineralogy (Powers, 1967; Burst, 1969). 

Both concepts assume that water is the pressuring 

mechanism, but in most tight reservoirs oil or gas is the 

dominant pressuring fluid phase. In fact, overpressured 

water is rarely encountered in Rocky Mountain reservoirs, 

and the active or recently active generation of hydrocarbons at a rate that exceeds loss by migration is believed 

to be the most likely cause of overpressuring in rock sequences where overpressured water is generally not encountered (Meissner, 1976, 1978a, b; Law and others, 

1980) and is the most likely cause of overpressuring in 

both the Pinedale (Law, 1984a; Spencer, 1984a) and 

MWX areas (Spencer, 1987). 

Regionally overpressured tight gas-bearing rock sequences in the Rocky Mountain basins have the following characteristics in common. 

Comparison of Overpressuring, Pinedale Anticline and Multiwell Site C1 
1. Hydrocarbons are almost always the mobile 

reservoir-pressuring fluid. 

2. Overpressuring mostly occurs in rock sequences 

having present-day temperatures of about 200± 10 op 

(93 ± 6 °C ) or higher. Some residual or ''inactive'' (Law, 

1984a) overpressuring occurs at lower temperatures; this 

phenomenon will be discussed later in the report. 

3. Overpressuring only occurs in tight rock 

sequences. 

4. Organic-rich source beds are still capable of 

generating hydrocarbons at rates that exceed loss by 

migration. 

5. Minimum vitrinite reflectance in oil (RcJ of the 

kerogen in these rock sequences is usually greater than 

0.6 percent in very rich oil-prone source beds and greater 

than 0.7 percent in very rich gas-prone source beds. In 

relatively organically lean oil-prone source beds, the 

minimum R0 is usually greater than 0.7 percent and in 

lean gas-prone sedimentary rocks greater than 0.8 percent. A relationship may exist, however, between thermal maturation and permeability (B.E. Law, written 

commun., 1986), and as noted previously overpressuring only occurs in tight rocks. If the rock sequence is 

either very organically lean or nonorganic, regional overpressuring will not occur. 

6. Local structure and stratigraphy are of secondary importance in the occurrence of overpressuring .. 

7. Reservoirs rarely have discrete gas-water or oilwater contacts and commonly produce no water. 

8. Natural fracturing is necessary for high gas 

production rates. 

9. Maximum pressure gradients in an area 

are about equal to the natural-fracture gradient of the 

rocks. 

10. Most classical hydrodynamic concepts and 

models developed for conventional blanket reservoirs do 

not apply to overpressured sequences, except that oil and 

gas in high-pressure sequences tend to migrate from high 

pressure to lower pressure. 

11. Overpressured strata are commonly in the 

center of the basin. 

12. Water-bearing reservoirs having normal to subnormal pressure are usually updip from overpressured, 

deep rocks. 

Acknowledgments.-The support of the U.S. 

Department of Energy Morgantown Energy Technology 

Center is gratefully acknowledged. R.A. Ullrich, El Paso 

Natural Gas Company provided core samples and well 

data from the Wagon Wheel No. 1 well. Paul Branagan 

supplied MWX pressure and other data. B.E. Law and 

R.R. Charpentier reviewed this paper and provided constructive comments. 

C2 Geology of Tight Gas Reservoirs, Wyoming and Colorado 

METHODS OF STUDY 

Reservoir pressures in the Pinedale anticline and 

MWX areas do not represent isolated data points but are 

parts of a study of regionally overpressured tight-gas 

basins. In conventional rocks, reservoir pressure is generally easily estimated or calculated because the rocks have 

good permeability. Reservoir pressures are calculated by 

extrapolation of drill-stem test (DST) shut-in pressures 

to an undisturbed original pressure or by long-term 

measurement of cased-hole, shut-in pressures. In tightgas reservoirs, in situ permeability to gas is very low 

(0.1-<0.0001 mD) and the usual methods of measuring 

reservoir pressure may not be adequate. In the MWX 

wells, numerous cased-hole pressure measurements were 

obtained during rock-stress tests and other well tests, but 

in the Pinedale area only a few extrapolatable DSTs are 

available. 

In gas reservoirs and especially in tight gas reservoirs, accurate static pressure calculations are impeded 

by the fact that gas is highly compressible. After a well 

is flowed (pressure drawdown), the well is shut in and 

gas recompression occurs in the hole (wellbore storage) 

and in the formation (afterflow). Shut-in times of many 

weeks or months may be required to estimate pressures 

in tight reservoirs because of the compressibility of gas 

and the low permeability. Drill-stem tests can indicate, 

therefore, minimum levels of high pressure but generally cannot be used to estimate true pressure. 

Fracture permeability greatly enhances both rockmatrix permeability and the rock-surface area evaluated 

by a DST, but probably no more than 10 percent of the 

DSTs run in tight reservoir sequences in the Rocky Mountain region evaluate rock sequences that are highly fractured at the well bore. Some DSTs in fractured reservoirs 

can be used to estimate true reservoir pressure; however, 

other methods to detect overpressuring should also be 

used, including correlation of data from drilling-mud 

weights, drilling-time charts (d-exponent), and mud-log 

gas shows and kicks. Estimation of pore pressure by using 

borehole geophysical logs of Tertiary and Cretaceous 

rocks in the Rocky Mountain region is not yet reliable 

(S.E. Prensky, oral commun., 1984). 

Mud weights and gas shows are the two most widely 

available sources of data to indicate overpressuring. With 

experience, the top of overpressuring can be accurately 

identified by using these data, but caution must be used 

when attempting to estimate maximum reservoir pressures 

within the overpressured interval. Various operators have 

their own approaches to drilling high-pressured, tight 

rocks. Some operators drill "underbalanced"; that is, 

they drill using mud weights known to be less than true 

reservoir pressure. Drilling underbalanced greatly 

enhances drill-bit penetration rates. Some operators will 

unknowingly drill underbalance~, but most try to use 
mud weights equivalent to the reservoir pressure of the 

section being penetrated. Balanced drilling permits good 

bit penetration rates, minimizes the potential for well 

blowouts, and reduces the chances of accidt!ntally fracturing the open part of the hole by mud weights that exceed the formation fracture gradient. 

PINEDALE AREA 

The Pinedale area (fig. 1) includes wells on and adjacent to the northwest-trending Pinedale anticline. The 

area is in the northwest part of the greater Green River 

basin and has been studied in detail because of the 

availability of deep well and core information (Law, 

1984b). 

Abnormally high pressures in the northwest part 

of the Greater Green River basin were noted by Rathbun 

(1968) and Rathbun and Dickey (1969). The northern 

Green River basin is between the southwest-thrusted 

Wind River Mountains uplift and the east-thrusted 

Wyoming thrust belt, and Rathbun and Dickey believed 

that the overpressuring was caused by tectonic compression. They also believed that borehole-geophysical logs 

could be used to identify a conductivity anomaly 

associated with overpressuring in the northern Green 

River basin, but most of their interpretation of abnormal pressures was based on mud-weight data. Computer 

processing of well logs by Prensky and Dickinson (1986) 

indicates that borehole-geophysical logs are not a reliable 

means to identify overpressuring in the well-consolidated 

rocks of the northern Green River basin. Overpressuring in the basin is regionally distributed and not confined 

to tectonically stressed localities (fig. 1). Limited data suggest that pressures are normal just in front of and beneath 

the thrust along most of the southwest side of the Wind 

River Mountains uplift. The occurrence of apparently 

normal pressures adjacent to the Wind River Mountains 

thrust is probably related to loss of gas caused by vertical fracturing in rocks adjacent to and under the thrust. 

TheEl Paso Natural Gas No.1 Wagon Wheel well 

was drilled in sec. 5, T. 30 N., R. 108 W., Sublette County, Wyo .. The well was drilled on the axis of the Pinedale 

anticline and was in the Upper Cretaceous Hilliard Shale 

at total depth of 19,000 ft (5,791 m). Figure 2A shows 

a plot of interpreted subsurface temperature and pressure 

gradients of the well, and figure 2B shows a plot of 

vitrinite reflectance data correlated with depth. The 

pressure interpretation is based mostly on mud weight and 

other data compiled from mud logs, well logs, and 

borehole mud weights determined from DST hydrostatic 

pressures. None of the DST shut-in pressure data was 

extrapolatable because of the low permeability of the 

reservoirs tested, but a few DST' s had sufficient pressure buildup during initial shut-in times such that the 

111" 

50 MILES 

I I I 

50 100 KILOMETERS 

1-

43° .....J 

UJ 

OJ 

1-

0 CJ) 

:::::> :c a: <C I 

9 1-

a: 

UJ 

> 0 

42° (.!) 

z 

~ 

0 

~ 

41° 

Figure 1. Generalized distribution of overpressuring (pattern) 

in the Greater Green River basin of Wyoming, Utah, and Colorado. Query (?) indicates overpressuring uncertain. 

overpressuring could be clearly verified even though its 

maximum amount could not be determined. Figure 3 

shows a DST chart from the well, run in the Upper 

Cretaceous Lance Formation between depths of 10,978 

and 11,070 ft (3,346.1-3,374.1 m). A visual interpretation of the initial shut-in curve and pressure data confirms the presence of abnormally high reservoir pressure. 

The initial shut-in pressure was 6,805 psi (46.9 MPa) at 

a gauge depth of 11,066 ft (3,372.9 m), and the pressure· 

was increasing at the time the shut-in period ended (point 

A). The true reservoir pressure is slightly higher than the: 

maximum hydrostatic pressure (point B) of 7,600 psi 

(52.4 MPa). Although the DST pressure data are not extrapolatable, I believe that the well was being drilled 

slightly underbalanced at the time the test was run, and 

therefore the true reservoir pressure is greater than 7,600 

psi (52.4 MPa). Consequently, the reservoir pressure gra-· 

dient is greater than 0.69 psi/ft (15.61 kPa/m). This DST 

yielded gas to the surface in 55 minutes at a rate too low 

to gauge and recovered 2,800 ft (853.4 m) of water 

cushion and 90 ft (27 .4 m) of slightly gas-cut mud. 

The pressure-depth plot (fig. 2A) shows several in·· 

flections in the pressure gradient with depth. The first 

inflection is at about 8,000 ft (2,438 m) in the Lance For-· 

mation and marks the top of the abnormally high pressun:: 

interval. Another inflection is at about 10,400 ft 

(3,170 m), and a third is at about 15,000 ft (4,572 m), 

adjacent to a coal zone in the middle of the Rock Springs 

Formation (Law, 1984a). Law described the interval be-· 

tween 8,030 ft (2,447.5 m) and 10,400 ft (3,170 m) as a 

Comparison of Overpressuring, Pinedale Anticline and Multiwell Site C21 
A B 

FT TEMPERATURE M FT M 

100 200 300 (° F) 

2 2 

100 150 (OC) 

EXPLANATION 

4 c Mud weight from mud log 4 >- a: • Mud weight from well logs 4: .& Mud weight from drill-stem test i= • Temperature a: 6 6 L.U 

PRESSURE GRADIENT 2 I-

\ 

8 8 

0 

0 

0 10 ...... 3 10 

X 

:r: 12 12 Ia. 

UJ 4 0 

14 14 

5 

16 

18 18 

20 6 20 

PSI 5 10 15 0.2 0.4 0.6 0.8 1.0 2.0 3.0 

(x 1000) PRESSURE VITRINITE REFLECTANCE (R0 ) 

Figure 2. Subsurface pressure, temperature, and vitrinite reflectance, El Paso Natural Gas No. 1 Wagon Wheel well, on the 

crest of the Pinedale anticline, sec. 5, T. 30 N., R. 108 W., Sublette County, Wyo. A, Pressure and uncorrected temperature. 

Modified from Law (1984a, fig. 12). Location of well shown in Law and Spencer (this volume, fig. 1). Temperature data from 

Wagon Wheel and other nearby wells. 8, Vitrinite reflectance (in percent) of kerogen extracts from core of Wagon Wheel well. 

zone of inactive gas generation and interprets the source 

beds in this zone to have been formerly at sufficiently high 

temperature (>190 "F, >88 OC) to have generated gas. 

He believed the source beds were overpressured at the time 

of gas generation and gas generation ceased when a cooling event caused the source beds to cool below the 

190-200 "F (88-93 OC) critical minimum temperature for 

gas generation. The uncorrected temperature at 8,000 ft 

(2,438 m) is now about 150 "F (66 °C) and the corrected 

temperature about 175 "F (79 OC). The inactive zone represents a residual overpressured interval, and a similar zone 

is interpreted to exist at the MWX site. 

The pressure-gradient increase at about 10,400 ft 

{3,170 m) in the No. 1 Wagon Wheel is at a present-day 

corrected subsurface temperature of approximately 200 "F 

(93 OC), interpreted to be the estimated minimum temperature necessary for significant gas generation from humicrich source beds still capable of gas generation at a rate 

that exceeds pressure loss. The vitrinite reflectance (RJ at 

the top of the inactive zone is about 0.74 percent (fig. 2B), 

and the highest vitrinite reflectance value measured in the 

well is 2.0 percent at about 18,000 ft (5,486 m). 

The No. 1 Wagon Wheel site was a proposed site for 

nuclear stimulation and, consequently, the well was drilled with considerable concern to keep reservoir pressures 

and mud weights as close to a balanced condition as 

C4 Geology of Tight Gas Reservoirs, Wyoming and Colorado 

possible. For this reason, the general pressures for the 

Pinedale anticline are best shown by this profile (fig. 2A); 

other wells in the area may show some differences in reservoir pressure Figure 4 shows mud-weight profiles for several 

other wells in the anticline area. This compilation shows 

that most of the mud weights were adjusted to the pressure 

variations in a similar manner. Somewhat lower mud 

weights were used to drill the El Paso Natural Gas No. 5 

Pinedale Unit well in the interval from about 10,000 ft to 

13,060 ft (3,050-3,980 m). This well is only about 0.5 mi 

(0.8 km) southeast of the No. 1 Wagon Wheel well. It is 

interpreted that the No. 5 Pinedale Unit well was drilled 

underbalanced to a depth of 13,060 ft {3,981 m) and the 

casing was set, at which point drilling was resumed in an 

apparently balanced condition to total depth. 

It is not unusual to encounter an apparently normally 

pressured well in an area where most wells are overpressured. The normally pressured well may be the result 

of loss of pressure caused by extensive vertical fracturing. 

MULTIWELL EXPERIMENT SITE 

The deeper parts of the Piceance basin of northwestern Colorado, including the area of the Multiwell Experiment (MWX) site, are interpreted to be regionally 
w 

a: 

::::> 

Cf'J 

Cf'J 

w 

a: 

a.. 

-------------TIME --------------31-. 

Figure 3. Drill-stem test chart from test run in Upper Cretaceous Lance Formation, El Paso Natural Gas No. 1 Wagon Wheel, 

between depths of 10,978 and 11,070 ft (3,346.1-3,374.1 m). Initial shut-in pressure (point A), 6,805 psi (46.9 MPa); maximum 

mud hydrostatic pressure (point 8), 7,600 psi (52.4 MPa). 

overpressured (Spencer, 1987). The MWX site is in the 

valley of the Colorado River, about 6.5 mi (10.5 km) southwest of Rifle, Colo. (Law and Spencer, this volume, fig. 

1). The U.S. Department of Energy supported the drilling 

of three closely spaced wells at this site in order to advance 

knowledge of well-log interpretation, artificial stimulation 

(hydraulic fracturing), and recovery of natural gas from 

tight gas reservoirs (Northrop and others, 1984; Warpinski and others, 1985). The MWX wells are the only wells 

drilled in the basin for which the operator has attempted 

to obtain good pressure and temperature data throughout 

a thick sequence of tight rocks. The three wells are located 

in a triangle and are quite close (115-215 ft, 35-66 m) to 

each other (Spencer, 1984b ). They are designated as the 

CER Nos. 1, 2, and 3 MWX and are in the NWl/4 sec. 

34, T. 6 S., R. 94 W., Garfield County, Colo. In this volume, 

the wells are referred to as the MWX-1, -2, and -3. The 

wells were spudded in the upper part of the Tertiary 

Wasatch Formation and reached total depths at or near 

the base of the Upper Cretaceous Mesaverde Group. Many 

geologic and engineering studies have been conducted at 

the MWX site (Spencer and Keighin, 1984). 

A series of experiments involving in situ stress testing and hydraulic fracturing were conducted (Branagan and 

others, 1984; Warpinski and others, 1984; Branagan and 

others, 1985). As a result of this research and other unpublished studies (Paul Branagan, written commun., 1986), 

a considerable amount of reservoir-pressure and fracturegradient data have been developed. Figure 5 shows the interpreted pressure profile for the MWX wells and the fracture gradients for various lithologies. 

The top of the overpressured interval is in gas-bearing 

sandstones in the fluvial part of the Mesaverde Group at 

a depth of about 5,600 ft (1,707 m) and a present-day 

temperature of 165 Of' (74 OC). It is interpreted that overpressuring from the 165 Of' (74 OC) line to the 200 Of' 

Comparison of Overpressuring, Pinedale Anticline and Multiwell Site C5 
Iw 

w 

u... 

u... 

0 

(/) 

0 

z 

<( 

(/) 

::> 

0 

I 

Iz 

::C 

Ia_ 

w 

0 

10 

15 

EXPLANATION 

No.1 Wagon Wheel 

- - ----- No.1 Mesa Unit 

- •-•- No.2 Pinedale Unit 

•••••• •• •• No.4 Pinedale Unit 

---- No.5 Pinedale Unit 

---- No.7 Pinedale Unit 

2 

(/) 

c: 

w 

Iw 

3 ~ 

0 

.....J 

~ 

z 

::C 

h: 

w 

0 

4 

5 

6 

2 4 6 8 

PRESSURE 

10 12 14 PSI (x 1000) 

Figure 4. Cross plot of interpreted reservoir pressure and depth for six wells drilled on Pinedale anticline. Data mostly 

compiled from drilling-mud weights and gas shows from mud logs. Modified from Spencer (1984a). Locations of the 

wells shown in Law and Spencer (this volume, fig. 1). 

(93 OC) line represents a zone of residual overpressuring 

and that little or no gas is being generated in this interval. 

The gas pressure in this interval probably is being depleted 

by migration along fractures and by diffusion. This lowtemperature overpressuring in thermally mature rocks is 

C& Geology of Tight Gas Reservoirs, Wyoming and Colorado 

similar to the "inactive zone" of overpressuring described 

by Law (1984a) in the Pinedale area in the Green River 

basin. The interpreted top of "active" present-day overpressuring at MWX is at a depth of 7,024 ft (2,141 m) and 

a temperature of 200 Of (93 OC). 
FT EXPLANATION 

' \ 8 Measured pore pressure 

\ 

' ' 

e Mud-weight pressure, MWX-1 

X Mud-weight pressure, MWX-3 

2 ' --Interpreted pressure gradient 

' ' 0 Fracture gradient in shale and mudstone 

' • Fracture gradient in sandstone 

' • Fracture gradient in coal 

0 

0 

0 

3 

4 

X -5 

I 

b: 

LU 

0 

6 

7 

8 

10 

LU 

z 

0 

N 

LU 

> 

6 

<( 

' ' ' \ 0.7 PERCENT R0 

' ' ' 

20 

' 0.8 PERCENT R0 

0 

60 MPa 

2 3 4 5 

PRESSURE 

6 7 8 9 PSI (x1 000) 

Figure 5. Interpreted pressure profile for MWX wells, Piceance basin, sec. 34, T. 6 5., R. 94 W., Garfield County, Colo. Measured 

pore pressure and fracture-gradient data from stress and other tests (Warpinski and others, 1984; Paul Branagan, written commun., 1986, 1987). Fracture gradients for mudstone and shale are higher than that for sandstone. Vitrinite reflectance values 

of 0.7 and 0.8 R0 from Bostick and Freeman (1984). 

The temperature data were derived from highresolution temperature logs run in the MWX-1 well by 

Los Alamos National Laboratory (LANL) six months 

after the hole was cased (Bostick and Freeman, 1984). 

Figure 6 shows a plot of the LANL data and boreholegeophysical log maximum bottom-hole temperatures 

from the MWX open-hole logs. The standard American 

Association of Petroleum Geologists (AAPG) bottomhole temperature correction for these log depths is also 

shown (Meissner, 1978a). This correction is necessary 

because the circulation of drilling mud cools the borehole. 

At depths of less than 7,000 ft (2,134 m), the AAPGcorrected temperatures are within 10 Of ( 6 OC) of the 

LANL temperatures and tend to be slightly higher than 

the LANL data. Below 7,000 ft (2, 134 m), there are a 

few points that suggest a spread of corrected temperature 

data higher and lower than the LANL temperatures. 

Many factors can affect the maximum log temperatures 

and the amount of correction needed. Although this study 

cannot resolve these differences, these discrepancies are 

recognized as a source of error in interpreting the exact 

temperature of the top of overpressuring when only openhole maximum log temperatures are available. 

The increase in temperature gradient at a depth of 

about 7,000 ft (2,130 m) is probably caused by the low 

thermal conductivity of a sequence of Mesaverde coal 

Comparison of Overpressuring, Pinedale Anticline and Multiwell Site C7 
FT M 

0 ( 

• EXPLANATION • 

• LANL temperature 6 months after • • casing set 

• 

• • Maximum log temperature 

• (open-hole geophy.) 

0.5 • A AAPG-corrected temperature 

• X DST temperature test, 3 MCFD 2 • recovered, 60 ft mud 

• 

• 

• 

• 

3 • 

• 

• 

• 

0 • 

0 

0 

4 ~ 

• X • -

I • 

1- • a.. 

UJ 5 

1.5 ~---A 

0 • 

--. • 

• 

• 

6 • 

• 

• 2 • 

7 ~. • 

•• ......... .a. 

• 

• 

8 • 

2.5 ........... • 

TEMPERATURE 

Figure 6. Temperature log for MWX-1 well. Temperatures 

from Los Alamos National Laboratories (LANL) high-resolution 

log, openhole geophysical logs, and an openhole drill-stem 

test (DST). Standard American Association of Petroleum 

Geologists (AAPG) (Meissner, 1979a) correction applied. 

beds from about 6,900 ft (2, 103 m) to 7,455 ft (2,272 m). 

This major coal zone is approximately correlative with 

the Cameo-Fairfield coal zone (Nuccio and Johnson, 

1983, 1984a). Several thin coals are present above this 

main coal zone. Figure 7 shows the temperature profile 

for the MWX-1 well and the changes in temperature gradients associated with the coaly interval above the Rollins 

Sandstone Member of the lies Formation. Figure 8 shows 

the actual variations in the differential temperature 

gradient caused by lithologic changes and, especially, 

the high gradients associated with the low thermal 

C8 Geology of Tight Gas Reservoirs, Wyoming and Colorado 

conductivity of the coals. The increase in subsurface 

temperatures below the coal zone must have persisted 

through geologic time and should have caused a corresponding increase in the gradient of vitrinite reflectance 

levels in the underlying source beds. 

Vitrinite reflectance values (fig. 9) (Bostick and 

Freeman, 1984) show that no such apparent increase in 

the trend of vitrinite reflectance values has occurred in 

the overpressured coal zone and underlying highly 

pressured rocks (fig. 5). (For an MWX vitrinite profile 

having more data points, see Nuccio and Johnson, this 

volume.) Vitrinite reflectance values below depths of 

about 7,000 ft (2,130 m) appear to be suppressed. This 

suppression may have significance in interpreting rates 

of thermal maturation in organic-rich rocks that have experienced long-term levels of hydrocarbon-caused, abnormally high pressure. Cecil and others (1977) conducted 

laboratory studies of coalification and proposed that thermal maturation can be retarded in closed thermodynamic 

(high-pressure) systems. Extrapolation of their laboratory 

tests to the MWX site suggests that pressure-caused 

vitrinite suppression may have occurred. Although the 

data are presently inconclusive, it is speculated that 

organic-rich source rocks (coals) are capable of generating 

hydrocarbons with consequent abnormally high pressures 

over a geologically long period of time, and that kerogen 

maturation of source rocks might be suppressed in a prolonged high-pressure environment in which the maturation products (gas and gas condensate) cannot easily leave 

the site of origin. This maturation suppression may be 

similar to other thermochemical reactions that can be intentionally suppressed by increasing the partial pressure 

of the evolved gas reaction products. For example, 

calcium carbonate calcinates at a higher temperature if 

the partial pressure of carbon dioxide is increased. 

Anomalously low vitrinite reflectance values also 

have been noted in lipid-rich (oil-prone) source beds (Hutton and Cook, 1980; Walker and others, 1983; Nuccio 

and Johnson, 1984b). The conditions that produce these 

low values may or may not be related to abnormally high 

paleopressures because suppression has been noted in lowtemperature, very low maturity lipid-rich oil shales (Nuccio and Johnson, 1984b) that probably have not been 

overpressured. At the MWX site, vitrinite from coaly 

matter in dominantly gas prone rocks was used. No obvious suppression of vitrinite reflectance values is seen 

in the Wagon Wheel No. 1 well in the Pinedale area 

(fig. 2B). 

The reservoir-pressure profile (fig. 5) shows the approximate depth of the 0. 7- and 0.8-percent vitrinite 

reflectance in oil (R0 ) levels projected by Bostick and 

Freeman (1984) by using core data. Additional vitrinite 

values for MWX samples are shown in Nuccio and 

Johnson (this volume). Tertiary rocks in the MWX wells 

are organically very lean; consequently, overpressuring 
FT 

c 

0 

·.;::; 

<0 

E 

2 0 

LL. 

..c 

~ Ctl 

<I) 

Ctl 

~ 

3 

0 4 

0 

0 

X 

I 

Ia.. 5 c UJ 0 

·.;::; 

0 Ctl 

E 

0 

u... 

c. ..>!. 

::J 0 0 u... 

6 

(5 <I) 

Ql E "0 

Q; ~ 

<I) 

> Ctl ~ Ql 

~ 

7 

8 

<I) 

0 Ql uc <0 Ctl ..c ~(f) 

50 

.A 

~ ;..o 

~ ~ 

~ ((\ 

~ (:;) 

Coal zone 

Tongue of Mancos Shale 

Cozzette and Corcoran Sandstone Members 

of lies Formation of Mesaverde Group 

20 40 60 80 

100 150 

I 

I 

I 

I 

I 

I 

_I 

~I 

~I 

81 Nl 

I 

I 

I 

I 

I 

I 

I 

I 

I 

I 

200 250 

M 

~ 0.5 

<( 

i= 

ex: 

UJ 

fCJ) 

=::>• 

0 

UJ 

u 

~ UJ 

ex: 

1.0 

1.5 

u 2.0 ex: 

UJ 

0... 

0... 

=> 

2.5 

TEMPERATURE 

Figure 7. Temperature log for MWX-1 well. Measured by Los Alamos National Laboratory 6 months after hole 

was cased. Temperature gradient change in coal zone of Mesaverde Group shown. 

as the result of active hydrocarbon generation probably 

never occurred in Tertiary rocks at MWX. The 

0.8-percent level is, however, at a present-day well depth 

of about 4,000 ft (1 ,220 m) in Mesaverde rocks that include dark-gray to black organic shales and mudstones. 

The top of the Mesaverde and the base of the Tertiary 

is at a present-day depth of 3,900 ft (1,189 m) based on 

correlations by R.C. Johnson (oral commun., 1982). 

Maximum burial probably occurred about 9-10 m.y. ago 

(Bostick and Freeman, 1984; Nuccio and Johnson, 

1984a}, and at that time the top of the overpressured 

zone probably was as shallow as the present 4,000-ft 

(1,220 m) depth in the MWX wells. After maximum 

burial, the Colorado River eroded 4,000-5,000 ft 

Comparison of Overpressuring, Pinedale Anticline and Multiwell Site C9 
FT~~~~~~~~~~;;~==========r===::::::::::::::~==============~M 

0.5 

2 

3 

1.0 

0 

0 

0 

Interbedded sandstone, shale, and mudstone 

..... 

X 

I b: 5 

UJ 

0 

6 

7 

1.5 

2.0 

Interbedded sandstone and shale 

8 

2.5 

9 

0 5 10 15 20 

GRADIENT, IN OC/100M 

Figure 8. Differential temperature gradient log for MWX-1 well showing strong gradient increases across coal 

zone in Mesaverde Group. Temperature log measured by Los Alamos National Laboratories shown in figure 7. 

(1 ,220-1,520 m) of rocks to the present elevation at the 

well site. This erosion caused considerable cooling of the 

rock sequence, and it is interpreted that as the rocks 

cooled gas generation ceased in the upper part of the 

Mesaverde Group. Reservoir pressure began to decrease in the MWX rocks because of gas diffusion and 

migration in the interval between the present 4,000-ft 

(1 ,220 m) depth to the top of active generation at about 

7,000 ft (2, 130 m). This pressure decay continued and the 

present-day top of interpreted inactive overpressure is at 

a depth of 5,600 ft (1,707 m). Pressures at depths less 

than 5,600 ft (1,707 m) are shown as normal in figure 

5; however, pressures in gas reservoirs shallower than 

about 5,000 ft actually may be slightly subnormal on the 

basis of studies by Meissner (1978b) and Law and Dickinson ( 1985) in other basins. 

The high topographic relief in the MWX area (fig. 

10) complicates the use of pressure gradients as a means 

C10 Geology of Tight Gas Reservoirs, Wyoming and Colorado 

to describe reservoir pressure. If a reservoir has the same 

pressure at two locations, one in a valley and the other 

on a mesa, then the pressure gradient of the well drilled 

in the valley will be higher than that in the well drilled 

on the mesa. In cases such as these, it is helpful to evaluate 

abnormal pressure in terms of a calculated potentiometric 

surface. For example, the calculated freshwater potentiometric surface of rocks at the base of the Mesaverde 

Group is 12,408 ft (3,782 m) above sea level. The nearest 

exposure of equivalent rocks is at an elevation of about 

6,000 ft (1 ,830 m) at Rifle Gap, 12 mi (19 km) northeast 

of the MWX site. It is unlikely, therefore, that abnormal pressures at the MWX site are caused by recharge 

by meteoric water. In theory, any fluid movement should 

be toward the outcrops at Rifle Gap. 

Overpressuring as a possible cause of natural fracturing has been proposed by Chaney (1949), Meissner 

(1978b), Hedberg (1980), and Law and others (1980). 
0 

0 

0 

0 

2 

3 

~ 4 X 

I 

t 

LlJ 

0 5 

6 

7 

8 

(") 

0 '<~" L!'l <D r--oomo 0 0 oooo.....: 0 

N 

VITRINITE REFLECTANCE (R0 ), IN PERCENT 

Figure 9. Vitrinite reflectance of kerogen extracts obtained from MWX wells. Modified from Bostick and Freeman 

(1984, fig. 3). 

0 

C"i 

Based on maximum pressures at MWX and other sites, 

it would seem that gas (and oil) migration in many lowpermeability reservoirs may be controlled by pressureinduced fractures in the sandstones and that the shales 

serve as confining seals and source beds. Fracturegradient data obtained from measurement of instantaneous shut-in pressures in the MWX wells, compiled by 

Warpinski and others (1984) and Paul Branagan (written 

commun., 1985), are shown on the pore-pressure profile (fig. 5) and were used to measure minimum rock 

stress; that is, the approximate pressure at which a 

natural or artificial fluid-driven fracture will propagate. 

These data show that the fracture gradient is lowest in 

sandstone and high but variable in shale and mudstone. 

The gradients were measured only in two coals, but both 

seem to be higher than most measured in sandstones. 

These data suggest that, if abnormal pressures can cause 

natural fractures, the limiting maximum pore pressure 

in a sandstone reservoir is controlled by the sandstone 

fracture gradient. It is also interpreted that pressure caused by hydrocarbon generation in coal, mudstone, and 

shale hydrocarbon-source beds is probably higher than 

that in adjacent sandstones. Therefore, source beds having a high fracture gradient can be pressurized by gas 

generation to a higher level than can adjacent sandstones. 

When the source bed-fracture gradient is exceeded, the 

pressure is probably rapidly released into the lower fracture gradient sandstone. This rapid release causes the 

sandstones to fracture and the gas and other fluids to 

migrate. 

There are many calcite-cemented and open natural 

fractures in the MWX core. Pitman and Sprunt (1984) 

studied Be and tso stable i1;otopes of the cements and 

concluded that carbon becomes slightly heavier with 

depth but isotopically light oxygen remains relatively constant. According to them, the isotope trends may reflect 

fracture cementation at different times but under somewhat similar conditions of depth and temperature. This 

conclusion is consistent with the concept that some or 

many of the natural fractures were caused by overpressuring initiated by large volumes of gas (and some water) 

being generated in the source beds as the rocks were progressively heated (buried). Pitman and Dickinson (this 

volume) discuss isotopic compositions of calcite fracturefilling cements in closed hydrologic systems at Pinedale 

and MWX. 

Fluid inclusions in the MWX calcite fracture-filling 

cements have been studied by C.E. Barker (oral 

commun., 1984). In this technique, rock thin sections are 

heated until the inclusion liquid expands and the associated gas bubble in the inclusion goes into solution. The 

temperature at which the gas completely dissolves is called 

the homogenization temperature and is inferred to be 

approximately the minimum temperature at which the 

cement, or mineral, was precipitated. All the inclusions 

in calcite-filled fractures from deep core samples from 

the MWX and Pinedale areas that Barker studied burst 

(decrepitated) or deformed prior to reaching the homogenization temperature. Barker concluded the gas had low 

solubility in the liquid and the internal pressure of the 

inclusion exceeded the strength of the crystal. He also 

concluded from crushing-stage studies that the inclusion 

gases are methane and that the calcite was precipitated 

from methane-saturated water under relatively high 

pressure and temperature. This last conclusion also somewhat supports the concept of fracture cementation from 

gas-saturated water at elevated temperature and pressure 

but does not necessarily prove the fractures were initiated 

by abnormal pressures. 

Comparison of Overpressuring, Pinedale Anticline and Multiwell Site C11 
NNW 

FT M 

12 

10 

0 8 0 

0 6 X 

4 

z 

0 2 

~ > 

0 

UJ -2 .....J 

UJ -4 

-6 

Ground surface, 9 m.y. ago 

MWX wells, total depth 

MWX Kelly-bushing elevation, 5,372 ft (1 ,637 m) 

Mesaverde Group 

SSE 

North Mamm 

Peak 

>- c:c 

<( 

f= 

c:c 

UJ 

f-

(/) 

::> 

0 

UJ 

u 

~ Mesaverde Group, tongues of Mancos Shale, and Mancos Shale UJ 

c:c 

u 

I 

3 MILES I 

4 KILOMETERS 

Figure 10. Profile through MWX area showing topographic relief in Colorado River valley. Modified from Bostick and Freeman 

(1984, fig. 4). 

COMPARISON OF OVERPRESSURING 

IN THE PINEDALE AND MWX AREAS 

As noted earlier, overpressuring in the Pinedale and 

MWX areas does not represent isolated occurrences but 

comprises better than normal data sites within regions of 

abnormally high pressure. Pressure gradients at the two 

locations can be compared by adjusting the gradients at 

both sites to the top of the 200 op (93 °C) subsurface 

temperature points. In this manner, pressure gradient 

changes can be evaluated irrespective of actual reservoir 

pressure and differences in drill depths and temperature 

gradients (fig. 11). Both locations have an interpreted 

shallow zone of inactive or residual overpressure. This 

zone is about 2,400 ft (730 m) thick in the Wagon Wheel 

No. 1 well and about 1,400 ft (430 m) thick at the MWX 

site. Assuming the factors controlling overpressuring are 

correctly understood, then the relative thicknesses of the 

inactive zones are, in part, related to pressure loss owing 

to gas-phase migration and diffusion after gas generation ceased. Because the stratigraphy and reservoir 

character of both areas are similar, pressure loss should 

be greater at the site where the generation had ceased for 

the longest period of time, and indeed this seems to be 

the case. Rocks at the MWX site started cooling about 

10 m.y. ago (Bostick and Freeman, 1984; Nuccio and 

Johnson 1984a), and the original top of high pressure is 

interpreted to have been as shallow as 4,000 ft (1 ,220m) 

(Spencer, 1987). The pressure between 4,000 and 5,600 ft 

(1,220-1,707 m) in the MWX wells appears to have decayed to normal or subnormal pressures, and active 

pressure maintenance is interpreted to be occurring at and 

beneath the 200 op (93 OC) present-day subsurface 

temperature level. 

C12 Geology of Tight Gas Reservoirs, Wyoming and Colorado 

The extent to which the MWX inactive zone is being 

charged by vertical gas migration from the active zone is 

not known. It is not likely that this effect occurs for more 

than a few hundred feet above the active zone because of 

the extremely low permeability of the horizontally bedded 

sealing shales and mudstones. Most of the pressure probably is being lost by slow diffusion through shale beds 

and by lateral gas migration in the sandstone reservoirs, 

which, although very tight, have slightly better matrix and 

fracture permeability than the argillaceous seals. 

The cooling event in the Pinedale area started more 

recently than that at the MWX site. On the basis of 

fission-track annealing studies, the rocks at the Wagon 

Wheel No. 1 started cooling only about 2-4 m.y. ago 

(Naeser, 1986), as compared with 10 m.y. ago for the 

MWX site. This more recent cooling seems to be a 

reasonable explanation for the somewhat thicker inactive zone in the Wagon Wheel No. 1 well, as compared 

with that in the MWX wells (fig. 11). The more recent 

cooling also helps explain why the maturation level (R0 

0. 74 percent) equivalent to the onset of significant gas 

generation (Law, 1984a) is about at the top of the inactive zone in the Wagon Wheel well. Conversely, the 0.7-

to 0.8-percent vitrinite reflectance levels in the MWX 

wells are well above the top of the inactive zone (fig. 11). 

SUMMARY AND CONCLUSIONS 

A comparison of overpressuring in the Pinedale and 

MWX areas shows many similarities in spite of some 

significant differences in depths to the top of the overpressuring zone, vitrinite reflectance levels, thermal and 

pressure gradients, thickness of the inactive zone, and 
FT r-M----------~-----------------------------------------------------M-. FT 

I 

I 

3 

I 

I 

I 

0.7 PERCENT R0 I 

1-----•1 

I 

I 

4 ~~ERCEN~~ 

I 

I 

5 

6 

2 

0.74 PERCENT R0 

,..------1--8 ? 

I 

I 

I \ 0.8 PERCENT R0 _J 

_J 

LU 

~ 6 _J 

LU 

s 

X 

s 

~ 

~ 7 0 

0 

0 

I 

1-

a.. 

LU 8 0 

9 

10 

11 

12 

----, 

I ~ I 

2 I 

-~ I 

tl I 

~ I 

2 ~I 

~I ---xl 

3 

0.3 

<lJ 

c 

0 

N 

<lJ 

> .B 

<r: 

s1 

::?: 

0.4 

,_ -............ 

.................... " 

''.!!:!vv--r ........ _11/e;;s -...... ........ 

' 

200 OF (93 °C) ' \ ------------~~----~--~\-------------

' ', 

~ w 

co 

0 

:::l 

~ -:7 

ro 

~ 

z 

~ 

0.5 0.6 0.7 

PRESSURE GRADIENT, IN PSI/FT 

' ) 

I 

" ' '\ 

I 

0.8 0.9 

0 

z 

Qj 

<lJ 

..c 

s 

c 

0 

01 

~ 

1.0 

9 s 

0 

z 

_J 

LU 

LU 

10 I 

s 

z 

0 

~ 

<( 

s 

11 0 

0 

0 

12 

13 

14 

15 

I 

1-

a.. 

LU 

0 

Figure 11. Pressure gradients in Pinedale and MWX areas. Gradients adjusted to common 200 °F (93 °C) 

subsurface temperature datum. 

geologic time since the latest cooling of the rocks began. 

All these differences can be readily explained if the concepts of pressure-causing gas generation are recognized 

and accepted as a working model. 

All the wells drilled on or near the Pinedale anticline 

encountered overpressured tight gas reservoirs to varying degrees. In the Wagon Wheel No. 1 well, the maximum reservoir pressure encountered (based on mud 

Comparison of Overpressuring, Pinedale Anticline and Multiwell Site C13 
weights) is about 14,800 psi (102 MPa) at 19,000 ft 

(5, 791.2 m). This pressure is equal to a gradient of almost 

0.8 psi!ft (18.1 kPa/m). Measured maximum gradients 

in the active gas generation zone at the MWX site are 

slightly higher than 0.8 psilft (18.1 kPa/m). These 

pressure gradients are about the same as those needed to 

artificially fracture tight sandstones in most Rocky Mountain Cretaceous-age rocks. No reliable fracture-gradient 

data exist for the Pinedale area, but much data exists for 

the MWX site (fig. 5). At MWX, fracture gradients in 

shale and mudstone are generally much higher than those 

in sandstone. Fracture-gradients in the sandstones most 

likely control the maximum amount of overpressuring 

and the high reservoir pressures probably cause some of 

the natural fractures. 

Meissner (1978b) and Law and others (1980) proposed that hydrocarbons can be generated in lowpermeability sequences at a rate more rapid than pressure 

loss until the fracture gradient of the rocks is exceeded, 

at which time the gas (or oil) migrates out of the reservoirs and thereby reduces the pressure. The pressure is 

reduced to the closure stress, the fractures close, and 

pressure build-up begins again. 

Analyses of wells in the Pinedale area and the 

MWX site confirm the concepts of overpressuring caused 

by active gas generation developed by Law and others 

(1980) for similar tight gas reservoirs in the Pacific Creek 

area (fig. 1). Using these concepts, the depth to the top 

of the overpressured zone in tight gas reservoir sequences 

can be predicted, but the degree of overpressuring is less 

predictable. The information from these two areas also 

provides some measure of the rate of pressure decay (gas 

loss and gas shrinkage due to cooling) in tight rock sequences. In the Pinedale area, the originally, actively 

overpressured rocks have been cooling during the last 2 

to 4 m.y. The rate of heat flow also may have decreased 

slightly because the top of the 200 op (93 °C) active gas 

generation zone is now about 2,400 ft (730 m) below the 

0.74-percent R0 level at the top of the zone of inactive 

overpressuring. At the MWX site, erosion and cooling 

started about 9-10 m.y. ago, and the pressure has returned to normal or below normal through 1,600 ft 

(488 m) of strata interpreted to have been originally overpressured. The inactive zone at MWX is now about 

1,400 ft (430 m) thick and the top of the zone or active 

gas generation is about 3,000 ft (915 m) below the interpreted top of the zone of active overpressuring at the time 

of maximum heating. 

This research enables engineers and scientists to: 

(1) locate the main interval of gas saturation (almost all 

reservoirs in tight-gas overpressured sequences are gas 

saturated), (2) predict the approximate depth at which 

overpressuring will occur and thereby predict the depth 

at which intermediate casing should be set, (3) anticipate 

the maximum pressure that may be encountered during 

C14 Geology of Tight Gas Reservoirs, Wyoming and Colorado 

drilling (maximum pressure is about equal to the fracture gradient of the sandstone reservoirs), and (4) provide a paleopressure model for paleohydrologic and other 

studies. Determination of high-pressure areas is important in petroleum exploration and resource assessment 

because the higher pressure provides more reservoir 

energy to produce hydrocarbons and because more gas 

can be stored in given gas-saturated pore volume at higher 

pressures. 

At the MWX site, vitrinite reflectance levels of 

coaly organic matter appear to be slightly suppressed in 

the deep, highly overpressured parts of the wells. 

Although data are presently inconclusive, prolonged abnormally high gas pressure may have caused this apparent 

suppression of thermal maturation. 

REFERENCES CITED 

Bostick, N.H., and Freeman, V.L., 1984, Tests of vitrinite 

reflectance and paleotemperature models at the Multi well 

Experiment site, Piceance Creek basin, Colorado, in 

Spencer, C.W., and Keighin, C.W., eds., Geologic studies 

in support of the U.S. Department of Energy Multiwell 

Experiment, Garfield County, Colorado: U.S. Geological 

Survey Open-File Report 84-757, p. 110-120. 

Branagan, P., Cotner, G., and Lee, S.J., 1984, Interference 

testing of naturally fractured Cozzette Sandstone; a case 

study at the DOE MWX site: Society of Petroleum 

Engineers Unconventional Gas Recovery Symposium, 

Pittsburgh, Pa., 1984, Proceedings, p. 359-366. 

Branagan, P.T., Cipolla, C.L., Lee, S.J., and Wilmer, R.H., 

1985, Comprehensive well testing and modeling of preand post-fracture well performance of the MWX lenticular 

tight gas sands: Society of Petroleum Engineers/U.S. 

Department of Energy Joint Symposium on Low 

Permeability Reservoirs, Denver, Colo., 1985, SPE/DOE 

Proceedings 13867, p. 191-202. 

Burst, J.F., 1969, Diagenesis of Gulf Coast clayey sediments 

and its possible relation to petroleum migration: American 

Association of Petroleum Geologists Bulletin, v. 53, no. 

1, p. 73-93. 

Cecil, B.C., Stanton, R.W., and Robbins, E.I., 1977, Geologic 

factors controlling coalification and hydrocarbon maturation [abs.]: American Association of Petroleum Geologists 

Bulletin, v. 61, no. 5, p. 775. 

Chaney, P.E., 1949, Abnormal pressures, lost circulation; Gulf 

Coast's top drilling problem: Oil and Gas Journal, v. 47, 

no. 51, p. 210-215. 

Fertl, W .H., 1976, Abnormal formation pressure environments, 

in Fertl, F.H., ed., Abnormal formation pressures; implications to exploration, drilling, and producing of oil 

and gas resources; Developments in petroleum science 2: 

New York, Elsevier, p. 1-48. 

Harkins, K.L., and Baugher, J.W., 1969, Geological significance of abnormal formation pressures: Journal of Petroleum Technology, v. 21, p. 961-966. 
Hedberg, H.D., 1980, Methane generation and petroleum migration, in Roberts, W.H., III, and Cordell, R.J., eds., Problems of petroleum migration: American Association of 

Petroleum Geologists Studies in Geology 10, p. 179-206. 

Hutton, A. C., and Cook, A. C., 1980, Influence of alginite on 

the reflectance of vitrinite from Joadja, NSW, and some 

other coals and oil shales containing alginite: Fuel, v. 59, 

p. 711-714. 

Law, B.E., 1984a, Relationships of source-rock, thermal maturity, and overpressuring to gas generation and occurrence 

in low-permeability Upper Cretaceous and lower Tertiary 

rocks, Greater Green River basin, Wyoming, Colorado, 

and Utah, in Woodward, J., Meissner, F.F. and Clayton, 

J.L., eds., Hydrocarbon source rocks of the greater 

Rocky Mountain region: Rocky Mountain Association of Geologists Symposium, Denver, Colo., 1984, 

p. 469-490. 

___ ed., 1984b, Geological characteristics of low-permeability 

Upper Cretaceous and Lower Tertiary rocks in the 

Pinedale anticline area, Sublette County, Wyoming: U.S. 

Geological Survey Open-File Report 84-753, 107 p. 

___ 1984c, Structure and stratigraphy of the Pinedale Anticline, Wyoming, in Law, B.E., ed., Geological characteristics of low-permeability Upper Cretaceous and lower 

Tertiary rocks in the Pinedale anticline area, Sublette 

County, Wyoming: U.S. Geological Survey Open-File 

Report 84-753, p. 6-15. 

Law, B.E., and Dickinson, W.W., 1985, Conceptual model for 

origin of abnormally pressured gas accumulations in lowpermeability reservoirs: American Association of Petroleum Geologists Bulletin, v. 69, no. 8, p. 1295-1304. 

Law, B.E., Spencer, C.W., and Bostick, N.H., 1980, Evaluation of organic matter, subsurface temperature and 

pressure in low-permeability Upper Cretaceous and lower 

Tertiary sandstones in Pacific Creek area, Sublette and 

Sweetwater Counties, Wyoming: The Mountain Geologist, 

v. 17, no. 2, p. 23-35. 

Meissner, F.F., 1976, Abnormal electric resistivity and fluid 

pressure in Bakken Formation, Williston basin, and its 

relation to petroleum generation, migration and accumulation [abs.]: American Association of Petroleum Geologists 

Bulletin, v. 60, no. 8, p. 1403-1404. 

___ 1978a, Patterns of source-rock maturity in nonmarine 

source-rocks of some typical Western Interior basins: 

Rocky Mountain Association of Geologists Continuing 

Education Lecture Series, Chapter IX, p. 1-37. 

___ 1978b, Petroleum geology of the Bakken Formation, 

Williston basin, North Dakota and Montana: Montana 

Geological Society Annual Conference, 24th, 1978, 

p. 207-227. 

Naeser, N.D., 1986, Neogene thermal history of the northern 

Green River basin, Wyoming; evidence from fission-track 

dating, in Gautier, D.L., ed., Roles of organic matter in 

sediment diagenesis: Society of Economic Paleontologists 

and Mineralogists Special Publication 38, p. 65-72. 

Northrop, D.A., Sattler, A.R., Mann, R.L., and Frohne, K.H., 

1984, Current status of the Multiwell Experiment: Society of Petroleum Engineers Unconventional Gas Recovery 

Symposium, Pittsburgh, Pa., 1984, SPE/DOE/GRI 

Proceedings 12868, p. 351-358. 

Nuccio, V.F., and Johnson, R.C., 1983, Preliminary thermal 

maturity map of the Cameo-Fairfield or equivalent coal 

zone through the Piceance Creek basin, Colorado: U.S. 

Geological Survey Miscellaneous Field Studies Map 

MF-1575, scale 1:253,440, 2 sheets. 

___ 1984a, Thermal maturation and burial history of the Upper Cretaceous Mesaverde Group, including the Multiwell 

Experiment (MWX), Piceance Creek basin, Colorado, in 

Spencer, C. W., and Keighin, C. W., eds., Geologic studies 

in support of the U.S. Department of Energy Multiwell 

Experiment, Garfield County, Colorado: U.S. Geological 

Survey Open-File Report 84-757, p. 102-109. 

___ 1984b, Retardation of vitrinite reflectance in Green River 

oil shales, Piceance Creek basin, northwestern Colorado 

[abs.]: American Association of Petroleum Geologists 

Bulletin, v. 68, no. 4, p. 513. 

Pitman, J.K., and Sprunt, E.S., 1984, Origin and occurrence of 

fracture-filling cements in the Upper Cretaceous Mesaverde 

Formation at MWX, Piceance Creek basin, Colorado, in 

Spencer, CW. and Keighin, CW., eds., Geologic studies 

in support of the U.S. Department of Energy Multiwell 

Experiment, Garfield County, Colorado: U.S. Geological 

Survey Open-File Report 84-757, p. 87-101. 

Powers, M.C., 1967, Fluid-release mechanisms in compacting 

marine mudrocks and their importance in oil exploration: 

American Association of Petroleum Geologists Bulletin, 

v. 51, no. 7, p. 1240-1254. 

Prensky, S.E., and Dickinson, WW., 1986, Computer-generated 

well-log data plots assist in regional subsurface evaluation, 

northern Green River basin: Geobyte, v. 1, no. 2, p. 52-58. 

Rathbun, F.C., 1968, Abnormal pressures and conductivity 

anomaly northern Green River basin, Wyoming: Society 

of Petroleum Engineers Annual Fall Meeting, 43rd, 

Houston, Tex., 1968, SPE Paper 2205, p. 1-8. 

Rathbun, F.C., and Dickey, P., 1969, Abnormal pressures and 

conductivity anomaly, northern Green River basin, Wyoming: The Log Analyst, v. 10, no. 4, p. 3-8. 

Spencer, CW., 1983, Overpressured reservoirs in Rocky Mountain region [abs.]: American Association of Petroleum 

Geologists Bulletin, v. 67, no. 8, p. 1356-1357. 

___ 1984a, Overpressured tight gas reservoirs in the Pinedale 

anticline area, Sublette, County, Wyoming, in Law, B.E., 

ed., Geological characteristics of low-permeability Upper 

Cretaceous and lower Tertiary rocks in the Pinedale anticline area, Sublette County, Wyoming: U.S. Geological 

Survey Open-File Report 84-753, p. 51-59. 

___ 1984b, Overview of U.S. Department of Energy Multiwell Experiment, Piceance Creek basin, Colorado, in 

Spencer, CW. and Keighin, CW., eds., Geologic studies 

in support of the U.S. Department of Energy Multiwell 

Experiment, Garfield County, Colorado: U.S. Geological 

Survey Open-File Report 84-757, p. 1-13. 

___ 1987, Hydrocarbon generation as a mechanism for overpressuring in the Rocky Mountain region: American 

Association of Petroleum Geologists Bulletin, v. 71, no. 

4, p. 368-388. 

Spencer, CW., and Keighin, CW., eds., 1984, Geologic studies 

in support of the U.S. Department of Energy Multiwell 

Experiment, Garfield County, Colorado: U.S. Geological 

Survey Open-File Report 84-757, 134 p. 

Comparison of Overpressuring, Pinedale Anticline and Multiwell Site C15 
Walker, A.L., McCulloch, T.H., Petersen, N.F., and Stewart, 

R.J., 1983, Discrepancies between anomalously low 

reflectance of vitrinite and other maturation indicators 

from an upper Miocene oil source rock, Los Angeles 

basin, California [abs.]: American Association of Petroleum Geologists Bulletin, v. 67, no. 3, p. 565. 

Warpinski, N.R., Branagan, P.T., Sattler, A.R., Lorenz, J.C., 

Northrop, D.A., Mann, R.L., and Frohne, K-H., 1985, 

Fracturing and testing case study of paludal, tight, 

C16 Geology of Tight Gas Reservoirs, Wyoming and Colorado 

lenticular gas sands: Society of Petroleum Engineers/ 

U.S. Department of Energy Symposium on Low 

Permeability Reservoirs, Denver, Colo., 1985, Proceedings, p. 267-278. 

Warpinski, N.R., Branagan, Paul, and Wilmer, R., 1984, In 

situ stress measurements at DOE's Multi-Well Experiment, in Frohne, K-H., ed., Western gas sands subprogram review, technical proceedings: U.S. Department 

of Energy, DOE/METC 84-3, p. 181-189. 
Chapter D 

Mineral Composition, Petrography, and 

Diagenetic Modifications of Lower Tertiary and 

Upper Cretaceous Sandstones and Shales, 

Northern Green River Basin, Wyoming 

By RICHARD M. POLLASTRO 

Prepared in cooperation with the U.S. Department of Energy 

U.S. GEOLOGICAL SURVEY BULLETIN 1886 

GEOLOGY OF TIGHT GAS RESERVOIRS IN THE PINEDALE ANTICLINE AREA, WYOMING, 

AND AT THE MULTIWELL EXPERIMENT SITE, COLORADO 
CONTENTS 

Abstract Dl 

Introduction Dl 

Location and stratigraphy of wells D2 

Methodology D3 

Results D4 

Tertiary rocks D6 

Early Tertiary-Late Cretaceous interval D7 

Wagon Wheel No. 1 well D7 

New Fork wells D22 

Upper Cretaceous rocks D22 

General mineralogy and diagenetic overview D22 

Lance Formation D24 

Ericson Sandstone D27 

Rock Springs Formation D28 

Specific diagenetic phases and events D32 

Laumontite and associated framboidal pyrite and chlorite D32 

Interstratified chlorite/ smectite ( corrensite) in sandstone D34 

Alteration of feldspars to chert-chalcedony D34 

Summary D35 

References cited D38 

FIGURES 

1. Map showing structural features of the northern Green River basin and 

location of wells used in study D2 

2-6. Charts showing: 

2. Generalized stratigraphy in northern Green River basin D3 

3. General and unit- or depth-specific sequences of paragenesis for 

sandstones from El Paso Natural Gas Company Wagon Wheel No. 

1 and American Hunter New Fork Nos. 1, 2, and 4 wells D5 

4. Paragenetic sequence for laumontite-bearing sandstones of Fort 

Union Formation in Wagon Wheel No. 1 well D5 

5. Paragenetic sequence for quartzarenites of Ericson Sandstone in 

Wagon Wheel No. 1 well D6 

6. General paragenetic sequence for Upper Cretaceous sandstones in 

WASP well D6 

7. Thin section photomicrographs of sandstones of Fort Union Formation 

in Wagon Wheel No. 1 well D16 

8. Scanning electron micrographs of clays in sandstones of Fort Union 

Formation from Wagon Wheel No. 1 well D17 

9-11. Thin section photomicrographs of: 

9. Lower Tertiary sandstones from Wagon Wheel No. 1 well D18 

10. Lower Tertiary sandstones from Wagon Wheel No. 1 well D19 

11. Lower Tertiary sandstones, Wagon Wheel No. 1 and New Fork No. 

1 wells D20 
12. Scanning electron micrographs of authigenic clay minerals in sandstones D21 

13. Thin section photomicrographs of sandstones D23 

14-15. Thin section photomicrographs of sandstones: 

14. From the Rock Springs and Lance Formations, New Fork No. 2 

and WASP wells D25 

15. Lance Formation and Ericson Sandstone, Wagon Wheel No. 1 and 

WASP wells D26 

16. Graphs showing clay-mineral relationships between sandstones and 

shales from the Lance Formation, Wagon Wheel No. 1 well, as 

determined by X-ray powder diffraction D27 

17-22. Thin section photomicrographs of: 

TABLES 

17. Quartzarenites from the Ericson Sandstone, Wagon Wheel No. 1 

well D29 

18. Sandstones from the Ericson Sandstone and Rock Springs Formation, Wagon Wheel No. 1 and WASP wells D30 

19. Sandstones from the Rock Springs Formation, Wagon Wheel No. 1 

well D31 

20. Sandstones from the Rock Springs Formation, WASP and Wagon 

Wheel No. 1 wells D33 

21. Relic feldspar grains in sandstones D36 

22. Feldspar grains being replaced by silica D37 

1. Location, and stratigraphic units cored from wells used in this study, 

northern Green River basin D3 

2-4. Mineralogical data for samples from: 

2. El Paso Natural Gas Company Wagon Wheel No. 1 well D8 

3. Inexco A-1 WASP well D12 

4. American Hunter New Fork Nos. 1, 2, and 4 wells D14 

Mineral Composition, Petrography, and 

Diagenetic Modifications of Lower Tertiary and 

Upper Cretaceous Sandstones and Shales, 

Northern Green River Basin, Wyoming 

By Richard M. Pollastro 

Abstract 

Mineralogic and petrologic studies of lower Tertiary and 

Upper Cretaceous rocks of the northern Green River basin 

reveal several depositional and diagenetic relations. Although 

lower Tertiary sandstones are typically arkosic, average feldspar 

content decreases progressively with depth and most Upper 

Cretaceous sandstones contain little feldspar. Some original 

feldspar in both lower Tertiary and Upper Cretaceous sandstones has been dissolved or altered to calcite, silica, or 

kaolinite. Below depths of 16,000 ft (4,900 m), however, detrital 

feldspars are unaltered and feldspar content increases to as 

much as 15 weight percent of the rock. 

Lower Tertiary and Upper Cretaceous shales from the 

study area typically are enriched in mixed-layer illite/smectite 

(1/S) and discrete illite; chlorite and kaolinite are much less 

abundant. Shales from the Ericson Sandstone, however, are 

enriched in kaolinite indicating a major change in detrital 

source material. Clay-mineral assemblages in sandstones above 

a depth of 16,000 ft (4,900 m) are typically kaolinite and chlorite 

rich. These clay minerals in sandstones are mostly coarser, 

authigenic, pore-lining or pore-filling cements. The absence of 

kaolinite below 16,000 ft suggests that the stability of kaolinite 

may be controlled by maximum burial conditions. 

The conversion of smectite to illite is a major diagenetic 

reaction in both sandstones and shales. As precursor smectitebearing clays (defined here as 1/S) become more illitic with 

depth, illitic and sericitic overgrowths become more abundant 

on shale clasts, feldspar grains, and earlier 1/S clay cements. 

Products of the smectite-t<;>-illite reaction also contribute to the 

formation of other authigenic carbonate, clay-mineral, and silica 

phases. 

Early stages of diagenesis in these sandstones commonly 

involve compaction and precipitation of pyrite and subsequent 

quartz overgrowths and (or) 1/S cements. These earliest stages 

are commonly followed by cementation and replacement by 

iron-free and iron-bearing calcite cements; iron-bearing calcite 

commonly replaces iron-free calcite. Dolomite cements also 

increase in abundance in deeper samples. Dissolution of calcite 

cements produces much of the secondary intergranular porosity, and the dissolution of feldspars, chert, and shale clasts commonly produces significant intragranular porosity. In the case 

of quartz-rich, quartz-cemented sandstones, relatively high intragranular (moldic) porosity is attained at depths greater than 

13,000 ft (3,900 m), mostly from the dissolution of chert grains. 

INTRODUCTION 

The allogenic and authigenic mineral assemblages 

of sandstone and shale can help provide a physical, 

chemical, and petrologic history of both depositional and 

postdepositional (diagenetic) environments. Quantitative 

or semiquantitative analysis of these assemblages reveals 

important mineralogical differences, lithological relationships, and the intensity of a particular diagenetic stage. 

During the past two decades, the diagenesis of sandstone 

and shale has received considerable attention. Of particular interest are studies that relate the effects of 

diagenesis on both reservoir quality and the source-rock 

potential of organic-rich shale by evaluating organicmatter type, amount, and maturation (Scholle and 

Schluger, 1979; Longstaffe, 1981; McDonald and 

Surdam, 1984; Gautier, 1986). Combined studies of inorganic and organic diagenesis may also provide insight 

into the timing of generation and migration of 

hydrocarbons. 

Studies of clay minerals are important to the understanding of inorganic diagenesis (Burst, 1969; Perry and 

Hower, 1970; Weaver and Beck, 1971; Hower and 

others, 1976; Weaver, 1979; Hower, 1981). For example, 

the conversion of smectite to illite (illitization of smectite) 

through a mixed-layer or interstratified illite/smectite 

(1/S) series (as defined by X-ray powder diffraction profiles) is considered the major clay-mineral reaction that 

occurs as sedimentary rocks are progressively buried. The 

maximum extent of this reaction can provide information on the chemical or thermal intensity of diagenesis, 

as well as on the chemical constituents that may be related 

to stages of dissolution and precipitation of various 

Mineral Composition, Petrography, and Diagenetic Modifications, Northern Green River Basin 01 
minerals phases in sandstone, shale, and bentonite beds 

(Hower and others, 1976; Boles and Franks, 1979; Hoffman and Hower, 1979; Millikan and others, 1981; Altaner 

and others, 1984; Nadeau and others, 1984; Pollastro, 1985; 

Pollastro and Barker, 1986). 

Recent stratigraphic, geochemical, mineralogic, and 

petrologic studies on the tight-gas sandstones, shales, and 

coals of the Green River basin of Wyoming (Law, 1979, 

1984a, b; Dickinson, 1984, 1985, this volume; Keighin, 

1984; Pollastro, 1985; Law and others, 1986; Naeser, 1986; 

Pollastro and Barker, 1986; Schuster, 1986; Law and Johnson, this volume), particularly in the area of the Pinedale 

anticline, provide much of the framework for large-scale 

basin analysis and petroleum geology. None of these 

studies, however, relate the clay mineralogy and bulk-rock 

mineralogy and petrology to regional diagenetic stages. 

In this study, basic mineralogic relationships and 

spatial, textural, and morphologic observations are used 

to characterize the depositional and burial history of Upper Cretaceous and lower Thrtiary sandstones and shales 

from the northern Green River basin. The interpretations 

of this study are limited to data obtained from five wells. 

Most of the data are from the El Paso Natural Gas 

..__ 

CJ) 

:::::> 

ex: 

I 

..__ 

ex: 

w 

> 0 

Nsw Fork No. 4e 

Wagon Wbssl No. t• 

Nsw Fork No. 2 

Nsw Fork No. 1 

10 

Company Wagon Wheel No. 1 well (Law, 1984a; Law and 

Johnson, this volume, fig. 1) and are used as a mineralogic 

and stratigraphic reference set. This report presents a comprehensive mineralogic analysis of both sandstones and 

shales and describes and interprets the stages of diagenesis 

in tight-gas sandstone reservoirs in the area. The distribution and processes that cause dissolution porosity in these 

rocks will be discussed in a later report. 

Acknowledgments.-I thank Christopher J. Schenk 

and C. William Keighin for their critical reviews of the 

manuscript and helpful suggestions for improvement, 

the U.S. Department of Energy and B.E. Law for allowing the opportunity to work on the low-permeability sandstones of the Green River basin, and B.E. Law for his 

encouragement and support throughout the study. 

LOCATION AND STRATIGRAPHY OF WELLS 

Location information for the wells used in this study 

is shown in table 1, and well locations are shown in figure 

1. As previously mentioned, most of the data in the study 

are from the El Paso Natural Gas Company Wagon Wheel 

WYOMING 

20 30 MILES 

I I I 

I I 

10 20 30 KILOMETERS 

Figure 1. Structural features of the northern Green River basin and location of wells used in study. Location information for wells and stratigraphic units cored shown in table 1. 

02 Geology of Tight Gas Reservoirs, Wyoming and Colorado 
Table 1. Locations of wells and stratigraphic units cored from wells used in this study, northern Green River basin 

[Location of wells shown in figure 1] 

Well Location Stratigraphic units cored 

El Paso Natural Gas Wagon Wheel No.1 

Inexco A-1 WASP (Belco Petroleum 3-28 Merna) 

American Hunter New Fork No. 1 

American Hunter New Fork No. 2 

American Hunter New Fork No. 4 

Sec. 5, T. 30 N., R. 108 W. 

Sec. 28, T. 36 N., R. 112 W. 

Sec. 25, T. 30 N., R. 108 W. 

Sec. 2, T. 30 N., R. 108 W. 

Sec. 35, T. 31 N., R. 109 W. 

Rock Springs, Ericson, Lance, 

unnamed Tertiary unit, Fort Union 

Rock Springs, Ericson, Lance 

Lance, unnamed Tertiary unit 

Lance, unnamed Tertiary unit 

Lance, unnamed Tertiary unit 

w 

z 

w Fort Union Formation 

u 

0 

w 

....J 

<( Unnamed unit c.. -

Lance Formation 

- - en Ericson Sandstone 

0 

:::> -- w 

u a. 

~ :::J 

0 w t5 Rock Springs 0:: 

u Q) Formation "'0 0:: (i; w 

c.. > co c.. en :::> Q) 

~ 

Hilliard Shale 

Figure 2. Generalized stratigraphy in the northern Green 

River basin. Modified from Law 

and others (1986). 

No. 1 well. This well, on the crest of the Pinedale anticline, was originally planned for the nuclear stimulation of tight gas-bearing sandstones. Twelve sections of 

core, totaling about 900ft (275m), were taken at selected 

intervals at depths ranging from 5,000 to 18,000 ft 

(1,525-5,485 m) (Law and others, 1986), and for the present study approximately 180 sandstone and shale samples 

were analyzed. A total of 60 core samples from three 

American Hunter wells (designated New Fork Nos. 1, 2, 

and 4) drilled in the area of the Pinedale anticline were 

also studied. Approximately 90 sandstone and shale 

samples were analyzed from the Inexco A-1 WASP 

(Wyoming Atomic Stimulation Project) well. TheW ASP 

well, also planned for nuclear stimulation, is about 60 mi 

(97 km) northwest of the Wagon Wheel well. This well 

was later drilled deeper and renamed the Belco Petroleum 

3-28 Merna well. 

The stratigraphy and structural evolution of the 

northern Green River basin has been described by Martin 

and Shaughnessy (1969), Shaughnessy and Butcher (1973, 

1974), Law (1979, 1984a), Schuster and Steidtmann 

(1983), Schuster (1986), and Law and Johnson (this 

volume). A simplified stratigraphic section is shown in 

figure 2. 

In the deepest part of the Greater Green River 

basin, sedimentary rocks ranging from Cambrian through 

Tertiary in age reach a maximum thickness of about 

32,000 ft (9,750 m). In the wells studied, cores were taken 

from lower Tertiary and Upper Cretaceous, interbedded, 

discontinuous, low-permeability sandstone and shale between depths of 5,000 to 18,000 ft (1,500-5,500 m). Most 

of these rocks were deposited in nonmarine environments. 

The only exception is the lower 1,600 ft (490 m) of the 

Wagon Wheel well drilled in the marginal-marine lower 

part of the Rock Springs Formation. Upper Cretaceous 

rocks include the Rock Springs Formation, Ericson 

Sandstone, and Lance Formation. Tertiary rocks in 

ascending order include an unnamed unit and the Fort 

Union Formation. (See Law and Johnson, this volume, 

for stratigraphic nomenclature.) All of the above mentioned units were studied from the Wagon Wheel well; 

the WASP core was taken from the lowermost 1,200 ft 

(365m) of the Lance Formation and from parts of the 

Ericson Sandstone and Rock Springs Formation. The 

three New Fork wells were cored from the lower Tertiary 

unnamed unit and from the upper part of the Lance Formation. Cores recovered from the New Fork Nos. 2 and 

4 wells have been interpreted to contain the CretaceousTertiary (K-T) unconformity (B.E. Law, oral communication, 1986). 

METHODOLOGY 

X-ray powder diffractometry (XRD) and acidsoluble carbonate analysis were performed on half of a 

1-in.-diameter (2.5 em) cored plug sampled from the 

original core of each well; the adjacent half of each plug 

was used for thin section preparation. 

Mineral Composition, Petrography, and Diagenetic Modifications, Northern Green River Basin 03 
Samples were washed and scrubbed to remove surficial contaminants, dried, and then ground to <35 mesh. 

Each sample was split by using a Jones splitter into two 

portions, one for whole-rock XRD analysis and one for 

carbonate dissolution and clay-mineral analysis. 

Carbonate was dissolved in IN HCl and the residue 

filtered and washed with distilled water immediately after 

effervescence stopped in order to minimize solution of 

noncarbonate minerals (Pollastro, 1977). The insoluble 

residue was dried overnight at 65 OC. The weight percent 

of the residue was then determined. A small part of the 

residue was spot checked for undissolved carbonate with 

6N HCI. 

Qualitative and semiquantitative estimates of the 

minerals in whole rock were made through XRD analysis 

of randomly oriented powders that were ground to 

<44 p.m (<325 mesh) and packed into the back of aluminum specimen holders. A fine texture was imparted onto 

the surface to be irradiated in an attempt to maximize 

random orientation of the grains (Schultz, 1978a). Semiquantitative estimates of the amount (in weight percent) 

of total phyllosilicates and individual minerals were 

calculated by comparison with several prepared mixtures 

of the same minerals having similar XRD characteristics 

and by the procedures outlined by Schultz (1964) and 

Hoffman (1976) with some modification. In order to 

check the carbonate results, the semiquantitative relative 

weight-percent values calculated for total carbonates by 

XRD were compared with those determined by chemical 

dissolution. 

Oriented clay aggregates of the <2- and <0.25-p.m 

(equivalent spherical diameter) fractions were prepared 

by using a modified filter-membrane-peel technique 

(Pollastro, 1982) similar to that described by Drever 

(1973). Semiquantitative XRD analysis of the clay-size 

fractions was made by using the method of Schultz (1964) 

with some modification (Pollastro, 1985). 

Although I am aware that the interpretation of 

XRD patterns for I/S clay does not conform with the concept of interparticle diffraction (Nadeau and others, 

1984), the currently accepted terminology for these interstratified clay minerals (as described by Reynolds, 

1980) will be used for convenience. Composition and 

ordering of the I/S clay was determined on oriented, 

ethylene glycol-saturated specimens of both the <2- and 

<0.25-p.m fractions by using the methods of Reynolds and 

Hower (1970) and Schultz (1978b ). Ordering types were 

defined by using the "Reichweite" (R) notation (as 

described by Reynolds, 1980). Most diffractograms of 

naturally occurring I/S clay can be categorized into one 

of three ordering types (or any mixture of these types) 

of interstratification (Reynolds and Hower, 1970): (1) 

random I/S (R = 0), (2) allevardite or short-range-ordered 

I/S (R = 1 ,2), or (3) Kalkberg or long-range-ordered liS 

(R?3). 

04 Geology of Tight Gas Reservoirs, Wyoming and Colorado 

Thin section and scanning electron microscopy 

(SEM) analyses of sandstones were used as reconnaissance 

methods for qualitative and descriptive petrographic 

analysis and to identify general diagenetic and petrophysical relationships. 1\vo matched thin sections were prepared from each sandstone plug. One was impregnated 

with blue epoxy to aid in recognizing porosity and stained 

with potassium ferrocyanide and alizarin red for the identification of carbonate phases following the procedure 

described by Dickson (1966). The other section was impregnated with clear epoxy and stained with potassium 

cobaltinitrite to distinguish between potassium and plagioclase feldspar. SEM samples of freshly fractured rock 

chips were air dusted, mounted on aluminum stubs, and 

sputter coated with ::::: 100 A of gold/palladium. 

RESULTS 

Although the mineralogic composition of the sandstones and shales in this study is relatively simple, their 

diagenetic history is complex. Numerous diagenetic events 

have occurred since the deposition of many of these 

rocks, and their original compositions have been partly 

or extensively modified. Diagenetic events or stages identified in the sandstones of this study are not necessarily 

restricted to a particular lithologic unit. Original characteristics, such as feldspar content or the presence of 

ductile lithic grains and rock fragments, produce specific 

chemical or physical constraints that create an environment more or less susceptible to change during deep 

burial. Specific sedimentologic factors may also control 

or limit the degree and spatial extent of diagenesis. 

Moreover, it is important to note that the degree or extent of diagenesis may vary from one sandstone to 

another and over relatively short vertical distances. 

Pollastro (1985) and Pollastro and Barker (1986) 

have outlined the diagenesis, nature, and significance of 

the smectite-to-illite reaction for both sandstone and shale 

in the Wagon Wheel well, and I will only discuss the 

smectite-to-illite reaction as it relates to petrographic textures or other diagenetic events. The diagenetic events and 

modifications identified in this study are summarized in 

figures 3-6. Similar diagenetic events have occurred in 

rocks from both the area of the Pinedale anticline and 

the WASP well. In general, however, equivalent rocks 

recovered from wells in the Pinedale area are at a more 

advanced stage of diagenesis-that is, thermally and compositionally more mature-than those from the area of 

the WASP well because those in the Pinedale section have 

been buried much deeper. These differences in thermal 

maturation and diagenetic extent are shown in mean 

random vitrinite reflectance values (Law, 1984), claymineral data, and interpretations of paragenesis from 

petrographic analysis. 
UNIT(S) STAGE OR EVENT DIAGENESIS 

Early .. Late 

Framboidal pyrite 

UJ 

Syntaxial quartz overgrowths - u Dissolution of chert, feldspar, 

z and shale clasts UJ 

:J Cementation and replacement d 

UJ ALL By iron-free calcite (/) 

__J By iron-bearing calcite 

<( 

a: Conversion of smectite to illite UJ 

z lllitic and sericitic overgrowths UJ 

(!:> Dissolution of calcite cements 

Cementation and replacement 

by kaolinite 

FORT UNION Illite/smectite cement 

FORMATION Iron chlorite cement 

Replacement of feldspars 

u CRETACEOUS- by chert-chalcedony 

u::: TERTIARY Chert-overgrowth cement u 

UJ INTERVAL Iron chlorite cement a.. 

(/) Chlorite/smectite cement :I: 

1-

a.. LANCE FORMATION Alternating quartz-illite/smectite w (2,720-3,370 m) c cement 

a: Illite/smectite cement 0 ROCK SPRINGS 

!::: FORMATION Cementation and replaGement 

z (4,540- 4,560 m) by ankerite or dolomite 

=> 

ROCK SPRINGS Chert-overgrowth cement 

FORMATION Ferroan dolomite overgrowths 

(4,890 5,240m) Fracture-filling quartz and calcite 

Figure 3. General and unit- or depth-specific sequences of paragenesis for sandstones from the El 

Paso Natural Gas Company Wagon Wheel No. 1 and American Hunter New Fork Nos. 1, 2, and 4 

wells. 

STAGE OR EVENT DIAGENESIS 

Early , Late 

Precipitation of framboidal pyrite 

Quartz-overgrowth cementation ·-· •• •• 

Cementation and replacement 

by laumontite • • 

Conversion of smectite to illite ••• •• 

Precipitation of discrete 

chlorite/pyrite dissolution •• • 

Dissolution of laumontite ••• • 

Dissolution of feldspars, quartz, 

chert, and shale clasts ••• • 

Figure 4. Parage netic sequence for laumontite-bearing sandstones of the Fort Union 

Formation in the Wagon Wheel No. 1 well. 

Mineral Composition, Petrography, and Diagenetic Modifications, Northern Green River Basin 05 
STAGE OR EVENT DIAGENESIS 

Early .. 

r Late 

Syntaxial quartz-overgrowth cement • •• •• 

Replacement of feldspars 

by chert or chalcedony •• •• 

Illite/smectite clay cement 

Cementation and replacement ·-··-· 

by dolomite •• •• 

Conversion of smectite to illite •• • 

lllitic and sericitic overgrowths • • 

Dissolution of chert 

and rock fragments ••• • 

Cementation and replacement 

by kaolinite • • 

Dickite precipitation in fractures ·-· 

Figure 5. Paragenetic sequence for quartzarenites of the Ericson Sandstone in the 

Wagon Wheel No. 1 well. 

STAGE OR EVENT DIAGENESIS 

Early Late 

Precipitation of framboidal pyrite 

Syntaxial quartz-overgrowth cement ·-· 

Illite/smectite clay cement ·-··-· 

Conversion of smectite to illite ·-··-· • •• 

Ferroan dolomite overgrowths 

on detrital dolomite • • 

Dissolution of chert, feldspar, 

and shale clasts • 

Cementation and replacement ·~· 

by iron-free calcite 

lllitic and sericitic overgrowths ·-· • • 

Cementation and replacement 

by iron-rich calcite 

Dissolution of iron-rich calcite ·-· 

Precipitation of fracture-filling ·-· 

quartz, calcite, and dickite ·-· 

Figure 6. General paragenetic sequence for Upper Cretaceous sandstones in the WASP 

well. 

Tertiary Rocks 

Mineralogical data from XRD for each of the wells 

are shown in tables 2-4; paragenetic sequences for the 

Tertiary rocks are shown in figure 3. Tertiary rocks were 

cored from the Wagon Wheel and New Fork Nos. 1, 2, 

and 4 wells; however, only the lowermost Tertiary rocks 

are represented in the New Fork cores. In the Wagon 

06 Geology of Tight Gas Reservoirs, Wyoming and Colorado 

Wheel well, Tertiary sandstones are primarily feldspathic, 

many are conglomeratic, and most are classified as lithic 

arkoses. Sandstones of the Fort Union Formation (table 

2) sampled from the Wagon Wheel well average about 

50 weight percent feldspar and contain little or no carbonate; quartz content is about 25-30 weight percent. 

Biotite grains are abundant and glauconite is common. 

As a result of early compaction, poor sorting, and high 
clay matrix content (fig. 7A), most of the sandstones from 

the Fort Union have little macro porosity. 

The dominant clay minerals are randomly interstratified I/S (R = 0) and discrete chlorite; little discrete 

illite or kaolinite was observed. Most of the discrete claysize chlorite is iron rich, as interpreted by XRD characteristics (Pollastro, 1984). The chlorite has weak to absent 

odd-number basal reflections (001, 003, 005, and so forth) 

and very strong even-number basal reflections. The interbedded shales consist mostly of expandable liS clay 

and contain less feldspar and clay-size chlorite than 

adjacent sandstones (table 2). 

Diagenesis in sandstones of the Fort Union Formation involves early compaction of feldspathic sands having clay-rich matrices. Few remnants of calcite, either as 

cement or a replacement phase, were observed in thin section, and little or no calcite was detected by XRD (table 

2). The little calcite that remains selectively and partially 

replaces feldspar or infills dissolution voids (fig. 7 B). 

Dissolution of the clayey matrix produced intergranular 

porosity; some intragranular porosity was also created 

from the dissolution of shale clasts and framework 

feldspars (fig. 7B). Early cements also include smectiterich liS coatings on framework grains. liS clay cement 

also drapes or bridges secondary pores (fig. SA). 

Feldspar grains commonly are altered and appear 

sericitized (fig. 7A). Closer examination of these feldspars 

using the SEM shows that they are being altered to a 

smectitic clay (figs. 8B, C). As discussed later, these clayaltered feldspar grains are more susceptible to dissolution and (or) sericitization (smectite to illite) with deeper 

burial because the early formed, smectitic clay crystals 

create nucleation sites for dissolution or illite growth. 

About 10ft (3m) of the core from the Fort Union 

Formation in the Wagon Wheel well contains sandstones 

cemented by laumontite. The paragenetic sequence for 

these sandstones is summarized in figure 4. Although 

cementation by laumontite is interpreted to have occurred 

relatively early in the burial history of these sandstones, 

laumontite formation postdates compaction of ductile 

glauconite and brittle biotite grains and precipitation of 

syntaxial quartz overgrowth cement. Laumontite partly 

to almost completely replaces feldspars and etches, corrodes, and embays framework quartz grains, particularly 

in areas where cementation is extensive (fig. 7C). Later 

dissolution of laumontite cement creates extensive intergranular and intragranular secondary porosity (fig. 

7D). Iron-rich chlorite replaces laumonite and lines 

secondary pores. The authigenic chlorite commonly coexists with framboidal pyrite (fig. 8D); however, chlorite 

probably formed much later. 

Lower Tertiary unnamed sandstones from 7,040 to 

7,140 ft (2,150-2,180 m) in the Wagon Wheel well contain shale clasts and grains of chert, biotite, and glauconite. Many of the sandstones from this interval are 

cemented by calcite (fig. 9A). In contrast, sandstones containing little or no calcite have intergranular spaces filled 

with fine, clay-rich matrix (fig. 9B). Detrital carbonate 

grains are occasionally found in these sandstones; rarely, sandstones contain isolated rhombs of dolomite. Total 

feldspar content in this group of sandstones and shales 

is about 20-25 weight percent; potassium feldspar typically is more abundant than plagioclase, and quartz content is about 30-45 weight percent (table 2). A_.lthough 

the addition of carbonate to the bulk-rock analysis appears to dilute all other phases in these sandstones, there 

is a real decrease in the relative weight percent of feldspar 

and an increase in quartz content, as compared to sandstones above from the Fort Union Formation in the 

Wagon Wheel well. Dissolution and (or) replacement of 

feldspars by calcite, smectitic clay, or other phases 

account for some of the decrease in feldspar abundance 

(fig. 9B, C). 

Early Tertiary-Late Cretaceous Interval 

Law ( 1979) and Law and Nichols (1982) reported 

that the Cretaceous-Tertiary (K-T) unconformity is at 

a depth of about 7,520 ft (2,302 m) in the Wagon Wheel 

well. In the American Hunter New Fork wells, this unconformity is at depths of from 8,300 to 8, 700 ft 

(2,530-2,650 m) (B.E. Law, oral communication, 1986). 

The bulk mineralogy of Tertiary sandstones from the New 

Fork wells is similar to that in the Wagon Wheel well. 

In general, the most significant difference in mineralogy 

from Tertiary rocks to Upper Cretaceous rocks is an increase in total feldspar content. This change in feldspar 

content occurs near, or coincident with, the K-T unconformity and can often be recognized on gamma-ray logs 

(Prensky, 1984; this volume); it is best shown in the XRD 

data from the Wagon Wheel well (table 2). The significance of this abrupt change in feldspar content is not 

well understood. The sudden increase in feldspar content 

may, however, reflect unroofing of Phanerozoic rocks 

of the Wind River Mountains and incipient deposition 

of arkosic debris from granitic crystalline Precambrian 

rocks in the core of the Wind River Mountains (Schuster 

and Steidtmann, 1983; Prensky, 1984, this volume; 

Schuster, 1986). 

Wagon Wheel No. 1 Well 

In the lowermost Tertiary sandstones in the Wagon 

wheel well, at depths from 7,340 to 7,400 ft (2,240-

2,260 m), total feldspar content is less than 10 weight 

percent of the bulk rock, detrital dolomite is common, 

and kaolinite is abundant as an intergranular cement and 

replacement phase. The total quartz content of these 

sandstones, a significant portion of which is from chert 

Mineral Composition, Petrography, and Diagenetic Modifications, Northern Green River Basin 07 
Table 2. Mineralogical data for samples from El Paso Natural Gas Company Wagon Wheel No. 1 well 

[Analyzed using X-ray powder diffraction. Sample designation indicates depth from surface to sample; suffix refers to lithology: s, sandstone; 

shl, shale. All stratigraphic units are formations except Ericson Sandstone, and unnamed Tertiary unit. Data in relative weight percent. Sol Carb, 

soluble carbonate; Cal, calcite; Dol, dolomite; Carb, other carbonate minerals; Plag, plagioclase; K-spar, potassium feldspar, I/S, illitesmectite; Kaol, kaolinite; Chi, chlorite; C/S, chlorite-smectite; L, laumontite; A, ankerite; S, siderite; P, pyrite. tr, trace; leaders (--),none 

detected; nd, not determined] 

Sample Stratigraphic Sol Whole rock <2-1!!!! cia~ fraction 

designation unit Carb Clay guartz Cal Dol Carb Plag K-s~ar Other Illite vs Kaol Chi C/S 

500l.Os Fort Union 20 26 30 24 4 73 6 17 

5020.5s Fort Union 17 30 28 18 7-L 9 2 89 

5028.2shl Fort Union 55 20 13 12 11 74 15 

5058.9s Fort Union 18 31 31 20 3 58 39 

5072.5shl Fort Union 33 25 21 18 3-P 6 77 17 

7046.3s Tertiary 21 15 30 21 17 17 3 80 7 10 

7048.5shl Tertiary 55 25 7 13 3-P 9 54 37 

7054.6s Tertiary 35 31 13 21 12 61 27 

7057.2s Tertiary tr 23 41 2 14 20 3 64 31 

7061.4s Tertiary 14 25 30 14 10 21 

7066.7s Tertiary 4 25 35 4 19 17 3 25 11 14 47 

7083.3shl Tertiary tr 43 33 2 7 15 18 46 13 23 

7093.0s Tertiary 18 21 36 18 10 15 8 28 14 21 29 

7095.0shl Tertiary tr 50 25 2 9 14 33 34 33 

7100.7s Tertiary 33 40 8 19 5 23 72 

7102.9s Tertiary 12 34 30 9 3 9 15 5 18 5 18 27 

7106.5s Tertiary 43 19 19 43 6 13 18 50 19 13 

7108.2s Tertiary 38 40 3 19 8 34 34 24 

7110.5s Tertiary 35 38 7 20 2 20 11 30 37 

7114.2s Tertiary 10 18 37 10 16 19 2 20 11 30 37 

7116.1shl Tertiary 48 30 8 14 13 30 15 16 26 

7120.4s Tertiary 28 44 13 15 9 42 28 21 

7125.0s Tertiary 5 25 43 6 12 14 8 40 11 14 27 

7129.6s Tertiary 18 18 45 18 6 13 6 48 13 14 19 

7130.2shl Tertiary 8 54 24 8 5 11 27 53 4 9 7 

7134.1shl Tertiary 3 70 16 1 2 3 9 -15 56 13 16 

7137.2shl Tertiary 50 28 6 14 14 37 25 24 

7340.6s Tertiary 8 17 65 8 4 6 10 12 27 25 26 

7343.6s Tertiary 10 18 55 10 7 10 6 11 38 40 5 

7350.0s Tertiary 7 17 41 4 3 15 20 11 27 19 10 33 

7355.0s Tertiary 6 25 64 4 2 3 2 9 38 10 15 28 

7358.8s Tertiary 6 17 73 5 1 2 2 12 22 35 31 

7366.3s Tertiary 20 15 50 20 7 8 11 10 24 19 36 

7375.8s Tertiary 8 15 73 7 1 2 2 10 18 8 31 33 

7378.5s Tertiary 6 15 72 5 1 3 4 11 12 77 

7382.8s Tertiary 14 26 55 9 5 2 3 21 59 9 11 

7384.0shl Tertiary 12 58 28 7 5 2 33 51 9 7 

7388.0shl Tertiary 20 57 21 15 5 2 29 62 3 6 

7392.2shl Tertiary 15 44 39 11 4 2 22 64 6 8 

7395.0shl Tertiary 18 56 24 13 5 2 30 59 4 7 

8030.1s Lance 33 63 1 1 2 19 37 23 21 

8037.0shl Lance 16 52 31 10 6 1 23 61 9 7 

8045.2s Lance 30 68 1 1 19 22 20 40 

8050.8s Lance 24 72 1 3 23 41 23 13 

8059.1s Lance 2 27 70 3 19 29 26 26 

08 Geology of Tight Gas Reservoirs, Wyoming and Colorado 
Table 2. Continued 

Sample Stratigraphic Sol Whole rock <2-l!m cia~ fraction 

designation unit Carb Clay Quartz Cal Dol Carb Plag K-s~ar Other Illite 1/S Kaol Chi CIS 

8063.0s Lance 31 67 2 43 30 14 13 

8066.5s Lance 14 36 47 9 5 2 1 23 56 11 10 

8070.2s Lance 10 40 47 6 4 1 2 28 54 8 8 

8073.4shl Lance 11 64 23 7 4 1 1 27 61 7 5 tr 

8076.9s Lance 19 18 62 17 2 1 13 51 24 12 tr 

8084.6s Lance 10 14 73 11 2 22 29 27 22 tr 

8088.2s Lance 9 15 74 9 1 22 34 29 15 

8094.5s Lance 9 20 69 9 2 33 24 22 21 tr 

8100.ls Lance 10 18 70 10 2 15 17 19 28 21 

8107.3s Lance 10 17 72 10 1 15 16 35 12 22 

8113.2s Lance 10 20 69 10 1 18 38 15 13 

8117.0shl Lance 3 56 38 3 2 28 50 16 6 

8124.2s Lance 20 15 63 19 1 1 1 15 21 32 16 16 

8128.1s Lance 19 14 65 19 2 19 21 48 18 tr 

8131.6s Lance 7 18 73 7 2 12 22 24 17 25 

8134.3s Lance 12 25 60 12 2 1 16 47 16 21 

8135.8shl Lance 12 44 42 10 2 1 1 17 64 13 6 

8136.5shl Lance 7 61 32 4 2 1 16 66 12 6 

8920.5shl Lance 6 49 38 3 3 5 2 46 47 7 

8925.8s Lance 4 25 66 3 1 4 1 25 23 52 

8931.5s Lance 10 30 56 6 4 4 28 56 4 12 

8934.7s Lance 22 16 59 21 2 2 23 45 25 7 

8944.ls Lance 12 12 73 10 2 2 1 25 42 19 14 

8954.3s Lance 9 15 75 7 2 1 26 46 14 14 

8962.ls Lance 12 14 71 9 3 3 22 40 16 22 

8971.6s Lance 8 25 65 6 2 2 12 61 12 12 

8980.8s Lance 8 16 74 3 5 2 29 56 8 11 

8984.4shl Lance 6 45 47 4 2 2 2 29 36 22 29 

8988.8s Lance 15 11 72 12 3 2 20 33 10 37 

9001.3shl Lance 5 68 26 2 2 1 1 41 52 3 4 

10142.6s Lance 7 27 60 6 4 3 33 28 14 25 

10145.3shl Lance 10 35 45 1 9 10 57 37 3 3 

10153.4s Lance 11 14 75 8 3 2 42 36 24 

10163.8s Lance 8 17 73 6 2 3 36 33 31 

10171.2s Lance 6 18 73 4 2 3 29 28 6 37 

10173.5shl Lance 6 52 38 2 4 3 52 34 6 8 

10190.5s Lance 6 14 78 3 3 2 26 24 50 

10193.8s Lance 70 28 2 60 35 5 

10204.0shl Lance 45 52 3 60 33 7 

10212.2s Lance 9 32 57 2 6 3 42 27 16 15 

10217.6s Lance 13 10 74 11 3 2 35 33 32 

10235.0s Lance 6 15 77 4 2 2 26 26 32 

10245.0s Lance 8 24 65 4 4 3 44 29 27 

10252.3shl Lance 9 50 38 2 7 3 63 30 1 6 

10971.1 s Lance 16 25 57 10 6 2 35 35 15 15 

10977.7s Lance 17 13 65 15 2 2 3 36 35 7 22 

10984.6s Lance 6 18 74 4 2 2 32 31 37 

10993.9s Lance 2 10 85 2 1 2 23 21 8 48 

10995.3shl Lance 2 67 28 2 3 54 39 3 4 

11003.8s Lance 11 22 65 7 4 2 22 24 8 46 

Mineral Composition, Petrography, and Diagenetic Modifications, Northern Green River Basin 09 
Table 2. Continued 

Sample Stratigraphic Sol Whole rock <2-IJ:m cla;t fraction 

designation unit Carb Clay Quartz Cal Dol Carb Plag K-s~ar Other Illite 1/S Kaol Chi CIS 

11007.8s Lance 12 27 58 8 5 2 38 34 10 18 

11017.3s Lance 12 35 50 6 6 3 37 37 25 12 

11018.2shl Lance 8 53 40 1 3 3 55 33 2 10 

11024.8shl Lance 16 52 28 2 14 2 53 37 4 6 

11027.8s Lance 21 23 54 14 7 2 33 26 17 24 

11032.6shl Lance 58 40 2 50 41 2 7 

11035.0s Lance 10 24 60 4 6 4 27 23 19 31 

11044.0s Lance 9 15 75 3 5 2 27 14 13 46 

11046.8shl Lance 9 72 20 4 4 71 21 3 5 

11051.3shl Lance 7 65 25 7 3 57 33 5 5 

11056.5s Lance 9 10 79 6 3 2 45 23 12 20 

11066.3shl Lance 2 85 12 3 56 35 4 5 

11068.1shl Lance 18 46 35 10 8 1 50 31 5 14 

13097 .3shl Ericson 66 34 19 32 35 14 

13100.9s Ericson 11 89 32 8 21 48 

13104.1s Ericson 7 93 56 6 15 23 

13108.9s Ericson 7 93 59 18 10 13 

13114.5shl Ericson 42 56 1 1 26 16 50 8 

13119.0s Ericson 10 90 22 21 30 27 

13124.2s Ericson 8 92 26 10 38 26 

13129.0s Ericson 11 89 30 10 30 30 

13136.0s Ericson 7 93 53 28 13 6 

13141.4s Ericson 10 90 29 33 14 14 

13144.5s Ericson 12 88 22 14 41 23 

13150+-s Ericson 8 92 66 11 11 12 

13162.0s Ericson 25 75 tr 61 10 11 18 

13167.0s Ericson 20 80 39 27 25 9 

13170.0s Ericson 30 70 tr 43 24 21 11 

13176.0s Ericson 25 73 2 30 21 32 17 

13179.0s Ericson 25 74 1 32 16 36 16 

13184.0s Ericson 25 75 tr 40 18 27 15 

13190.0s Ericson nd 21 70 8 1 23 26 32 19 

13197.0s Ericson nd 23 71 6 tr 30 26 28 16 

13198.0shl Ericson 53 47 42 33 20 5 

14897 .8shl Rock Springs 28 46 24 11 17 2 64 29 4 3 

14902.3shl Rock Springs nd 67 27 3 1 2 1 66 23 6 5 

14906.2shl Rock Springs 24 50 26 6 17 1 73 18 5 4 

14914.1shl Rock Springs 66 31 2 1 70 20 2 8 

14918.4shl Rock Springs nd 54 27 2 20 2 54 33 4 9 

14920.9s Rock Springs 47 17 34 7 18 22-A 1 40 22 4 34 

14923.8shl Rock Springs 27 48 23 9 18 2 55 34 8 8 

14927.0s Rock Springs 28 25 46 9 19 1 40 25 32 3 

14930.3shl Rock Springs 66 32 2 51 32 10 7 

14932.5shl Rock Springs nd 65 26 6 2 1 60 33 3 4 

14939.5shl Rock Springs 64 35 1 48 37 11 4 

14942.5shl Rock Springs 24 54 22 4 19-A 1 55 37 6 2 

14946.1s Rock Springs 24 34 41 4 20 1 55 39 14 2 

14951.3s Rock Springs 19 25 56 5 9 5-S tr 79 17 2 2 

16062.1shl Rock Springs 25 25 45 8 17 4 1 47 31 22 

16065.2s Rock Springs 26 22 48 12 14 3 1 52 14 34 

DlO Geology of Tight Gas Reservoirs, Wyoming and Colorado 
Table 2. Continued 

Sample Stratigraphic Sol Whole rock <2-Lil!! cia~ fraction 

designation unit Carb Clay Quartz Cal Dol Carb Plag K-s~ar Other Illite vs Kaol Chi C/S 

16068.5shl Rock Springs 17 54 24 2 15 4 1 68 25 7 

16072.5s Rock Springs 17 23 51 7 10 7 2 56 16 29 

16076.5s Rock Springs 16 24 52 5 12 5 2 43 16 43 

16078.8shl Rock Springs 16 40 39 2 14 4 1 76 16 8 

16083.6s Rock Springs 13 21 61 2 11 4 1 45 14 41 

16087.0s Rock Springs 16 20 59 1 15 5 44 17 39 

16094.8s Rock Springs 15 27 50 5 10 4 4 52 21 27 

16098.0s Rock Springs 14 31 49 5 9 5 1 58 21 21 

16101.9shl Rock Springs 19 36 37 4 15 5 3 60 27 13 

16104.5s Rock Springs 26 26 40 10 16 4 4 56 15 29 

16106.8s Rock Springs 26 25 43 12 15 3 2 56 27 17 

16110.0shl Rock Springs 18 53 25 4 14 4 63 25 12 

16112.1s Rock Springs 23 20 51 10 13 4 2 48 17 35 

16115.3shl Rock Springs nd 54 3 3 15 4 1 66 33 11 

16116.6shl Rock Springs 20 39 35 4 16 4 2 71 13 16 

16117.8s Rock Springs 23 25 46 3 20 4 2 70 10 20 

16118.0s Rock Springs 26 28 40 10 16 4 2 68 12 20 

16168.8s Rock Springs 27 15 48 13 15 8 1 74 15 11 

17172.3shl Rock Springs 21 44 27 5 17 6 1 76 16 8 

17173.3s Rock Springs 28 27 37 14 15 7 79 9 12 

17176.1s Rock Springs 21 20 47 8 14 8 3 70 24 6 

17178.5s Rock Springs 20 17 52 6 14 9 2 62 20 11 

17183.2s Rock Springs 15 21 53 2 14 8 2 64 16 20 

17187.0shl Rock Springs 18 60 22 14 4 74 17 9 

17959.0s Rock Springs 32 21 32 20 12 15 67 21 12 

17960.0shl Rock Springs 21 45 26 10 12 7 74 12 14 

17965.0s Rock S:Qrings 28 29 34 12 16 6 3 72 17 11 11 

Mineral Composition, Petrography, and Diagenetic Modifications, Northern Green River Basin 011 
Table 3. Mineralogical data for samples from lnexco A-1 WASP well 

[Analyzed using X-ray powder diffraction. Sample designation indicates depth from surface to sample; suffix refers to lithology: s, sandstone; 

shl, shale. All stratigraphic units are formations except Hilliard Shale and unnamed Tertiary uniL Data in relative weight percent. Sol Carb, 

soluble carbonate; Cal, calcite; Dol, dolomite; Carb, other carbonate minerals; Plag, plagioclase; K-spar, potassium feldspar; I, illite; I/S, 

illite-smectite; Kaol, kaolinite; Chi, chlorite; A, ankerite. tr, trace; leaders ( --), none detected; nd, not determined] 

Sample Stratigraphic Sol Whole rock <2-!:!m cia)!: fraction Percent illite 

designation unit Carb Cia~ Quartz Cal Dol Carb Plag K-s~ar Illite 1/S Kaol Chi in 1/S 

10446.4shl Lance 7 48 44 3 2 3 25 41 24 25 81 

10462.5s Lance 14 24 60 10 3 3 8 27 36 27 75 

10509.2shl Lance 14 53 31 6 6 4 33 55 7 5 80 

10510.0s Lance 23 27 52 12 5 4 12 49 25 12 75 

10530.5s Lance tr 32 63 1 4 1 21 32 24 21 82 

10541.3s Lance 9 17 72 6 2 3 17 32 33 18 80 

10576.5shl Lance 19 60 23 10 6 1 nd nd nd nd nd 

11339.6shl Lance 72 25 1 2 31 60 5 4 80 

11356.4s Lance 1 34 61 1 3 1 20 49 20 11 79 

11388.2s Lance 41 57 tr 2 23 50 17 10 82 

11445.4s Lance 8 23 68 6 2 1 20 36 32 12 80 

11488.lshl Lance 12 55 30 6 4 2 2 1 31 55 7 7 80 

11504.5s Lance 21 72 5 1 1 31 37 5 27 75 

11546.0s Lance 32 18 52 23 6 1 tr 28 56 10 6 80 

11629.0shl Lance 74 25 1 46 47 3 4 80 

11625.7s Lance 10 33 55 6 3 3 15 39 30 18 80 

11656.0shl Lance 63 36 1 36 57 3 4 85 

11678.0s Ericson 10 70 20 7 1 1 14 43 29 14 80 

11736.1s Ericson 6 30 64 2 2 2 26 33 27 14 82 

11759.0s Ericson 14 16 70 11 2 1 22 31 29 18 80 

11969.2s Ericson 8 15 78 5 1 1 33 44 4 19 82 

11989.4s Ericson 6 24 70 4 2 tr 20 38 14 28 80 

12042.0shl Rock Springs 23 76 tr 38 51 6 5 80 

12055.3s Rock Springs 57 42 tr tr tr 32 40 16 12 80 

12128.0s Rock Springs 5 22 74 3 1 29 50 8 13 78 

12147.1s Rock Springs 5 35 60 3 2 27 45 16 12 78 

12153.9s Rock Springs 4 25 72 1 2 tr tr 17 30 12 41 78 

12220.0s Rock Springs 8 20 73 7 tr 18 30 27 25 78 

12274.8shl Rock Springs 53 47 26 64 6 4 80 

12296.2s Rock Springs 6 18 76 3 2 19 18 52 18 78 

12336.ls Rock Springs 2 24 75 1 tr 21 23 16 40 78 

12374.0shl Rock Springs 68 30 1 1 28 56 9 7 80 

12396.4s Rock Springs 6 21 73 4 2 tr 16 22 21 41 82 

12432.5shl Rock Springs 10 64 28 2 3 3 tr tr 31 46 13 10 80 

12451.9s Rock Springs 45 55 tr tr 24 42 22 12 78 

12494.9s Rock Springs 17 82 tr 1 29 15 24 32 82 

12531.2s Rock Springs 29 69 tr tr tr 1 19 25 25 31 82 

12542.0s Rock Springs 1 28 71 1 tr 40 38 7 15 80 

12552.0s Rock Springs 34 66 tr 1 21 27 35 17 78 

13002.3s Rock Springs 3 43 54 2 1 tr 23 44 20 13 80 

13003.4shl Rock Springs 4 43 54 2 tr 25 51 14 10 82 

13100.ls Rock Springs 6 28 67 3 2 tr 26 42 18 14 80 

13128.1shl Rock Springs 55 43 1 1 40 30 22 8 82 

13127.4s Rock Springs 18 82 29 36 16 19 82 

13181.2s Rock Springs 14 86 nd nd nd nd nd 

012 Geology of Tight Gas Reservoirs, Wyoming and Colorado 
Table 3. Continued 

Sample Stratigraphic Sol Whole rock <2-IJ.m cia~ fraction Percent illite 

designation unit Carb Clay Quartz Cal Dol Carb Plag K-sEar Illite 1/S Kaol Chi in 1/S 

13199.0shl Rock Springs 5 63 32 2 1 1 1 41 47 4 8 82 

13224.8s Rock Springs 30 70 tr 43 47 20 10 85 

13251.6shl Rock Springs 32 66 1 1 35 22 34 9 82 

13264.0s Rock Springs 26 74 tr tr tr tr 13 9 6 72 82 

13306.5s Rock Springs 26 74 tr 17 5 78 nd 

13311.6shl Rock Springs 45 54 tr tr 1 tr 25 56 12 7 87 

13364.6s Rock Springs 3 34 64 2 tr 35 23 24 18 85 

13366.1shl Rock Springs 18 46 37 1 15 1 35 44 12 9 80 

13391.9s Rock Springs 22 25 55 6 12 2 tr tr 36 14 37 13 87 

13396.0s Rock Springs 18 18 67 7 5 3 tr tr 35 13 35 17 80 

13415.8shl Rock Springs 9 62 28 2 4 3 1 tr 47 41 5 7 80 

13473.2s Rock Springs 23 29 50 6 15 tr tr 32 27 32 9 82 

13475.5shl Rock Springs 46 54 tr 27 47 15 11 nd 

13506.2s Rock Springs 31 68 tr 52 29 8 11 85 

13533.4s Rock Springs 11 24 65 8 3 tr tr 51 27 22 85 

13542.3shl Rock Springs 23 49 34 6 10 1 tr 53 27 20 82 

13579.5s Rock Springs 7 25 68 2 5 tr tr 29 44 3 24 85 

13670.3shl Rock Springs 21 60 26 3 7 4 tr 56 32 12 85 

13684.3s Rock Springs 10 22 68 6 4 tr 34 39 12 15 80 

13731.5s Rock Springs 10 26 64 4 5 tr tr 37 30 33 78 

13737 .9shl Rock Springs 7 33 61 2 3 tr 30 47 5 18 80 

13811.1s Rock Springs 1 43 57 tr tr tr 46 37 9 8 80 

13866.0s Rock Springs 23 27 53 9 8 3 tr 31 45 18 6 82 

13881.6s Rock Springs 15 18 70 8 6 tr tr 29 43 12 16 80 

13884.2shl Rock Springs 16 60 24 4 11 46 42 4 8 82 

13924.0shl Rock Springs 61 37 tr 1 1 48 39 5 8 80 

13941.8s Rock Springs 67 33 tr tr 20 51 22 7 82 

13952.9shl Rock Springs 19 61 21 5 7 6-A tr tr 46 42 4 8 85 

13969.0s Rock Springs 12 21 70 7 2 tr 40 37 5 18 78 

13987 .Os Rock Springs 18 20 62 11 5 2 tr 30 39 6 25 80 

14020.6shl Rock Springs 9 61 29 9 1 46 44 4 6 78 

14026.7s Rock Springs 23 29 50 6 13 1 1 32 46 16 6 80 

14043.2s Rock Springs 21 25 53 15 5 5 tr 37 51 12 75 

14091.5s Rock Springs 18 20 62 7 9 2 31 30 6 33 78 

141 04.5shl Rock Springs 7 53 40 6 tr 39 40 7 4 82 

14110.4s Rock Springs 2 33 65 1 1 33 32 35 82 

14121.6s Rock Springs 5 34 62 tr 3 1 tr tr 35 40 11 14 82 

14131.6s Rock Springs 9 25 68 5 2 tr 26 56 18 80 

14159.2s Rock Springs 31 20 50 13 17 tr tr 33 36 25 6 82 

14198.0s Rock Springs 13 21 68 6 5 tr tr tr 36 51 10 3 80 

14241.9s Rock Springs 20 29 53 7 9 2 39 44 14 3 75 

14286.0shl Rock Springs 22 48 32 4 15 1 47 40 6 7 80 

14327.2s Rock Springs 28 38 36 13 13 tr tr 39 45 10 6 80 

14341.6shl Rock Springs 52 46 tr tr 1 1 36 56 4 4 nd 

14352.2s Rock Springs 18 32 52 7 9 34 44 17 5 78 

14358. 7s Rock SI!ings 25 35 42 8 15 1 40 32 22 6 80 

Mineral Composition, Petrography, and Diagenetic Modifications, Northern Green River Basin 013 
Table 4. Mineralogical data for samples from American Hunter New Fork Nos. 1, 2, and 4 wells 

[Analyzed using X-ray powder diffraction. Sample designation indicates well number and depth from surface to sample; suffix refers to lithology: 

s, sandstone; shl, shale. All stratigraphic units are formations except for unnamed Tertiary unit. Data in relative weight percent. Sol Carb, 

soluble carbonate; Cal, calcite; Dol, dolomite; Carb, other carbonate minerals; Plag, plagioclase; K-spar, potassium feldspar, 1/S, illitesmectite; Kaol, kaolinite; Chl, chlorite; CIS, chlorite-smectite; P, pyrite. tr, trace; leaders (--),none detected] 

Sample Stratigraphic Sol Whole rock <2-IJ,m cia~ fraction 

designation unit Carb Clay Quartz Cal Dol Carb Plag K-sear Other Illite 1/S Kaol Chi CIS 

l-7951.4s Tertiary 20 13 63 12 6 3 3 23 21 39 17 

1-7953.2s Tertiary 15 11 65 11 7 3 3 7 32 30 30 

1-7961.2s Tertiary 12 39 49 6 4 tr 2 26 60 9 5 

1-7965.1s Tertiary 18 19 61 16 2 1 1 12 45 29 14 

1-7969.8s Tertiary 15 18 47 8 6 11 18 10 28 45 17 

1-7975.2s Tertiary 8 13 67 6 5 3 6 20 30 28 22 

1-8150.0s Tertiary 12 21 60 7 5 2 5 8 32 48 12 

l-8152.5s Tertiary 14 18 41 9 8 8 16 13 11 65 12 

l-8460.7s Lance 8 16 74 8 2 tr 11 27 45 17 

1-8486.6s Lance 11 18 72 6 4 tr 17 29 38 16 

2-8451.3s Tertiary 13 18 30 20 5 11 16 12 38 43 7 

2-8439.5s Tertiary 7 14 40 8 1 22 15 14 32 32 22 

2-8461.4s Tertiary 2 16 47 3 1 18 15 13 39 19 29 

2-8706.3s Tertiary 14 17 65 3 11 1 3 35 36 17 12 

2-8710.0s Tertiary 32 10 45 22 10 7 6 15 9 75 

2-8713.9s Lance 11 19 68 8 2 3 14 41 27 19 

2-8715.5s Lance 13 24 60 8 5 2 1 16 31 20 32 

2-8752.3s Lance 26 31 38 7 17 1 6 35 42 11 12 

2-9204.5s Lance 20 12 67 18 2 1 12 27 44 17 

2-9217 .5s Lance 16 25 59 9 6 1 13 53 20 14 

2-9238.3s Lance 20 12 67 19 2 tr 9 24 48 19 

4-8260.0s Tertiary 21 14 63 21 2 7 30 27 10 26 

4-8263.8s Tertiary 13 24 38 14 1 9 14 3 34 23 3 28 

4-8269.0s Tertiary 8 17 55 12 12 4 3 5 62 11 19 

4-827l.Os Lance 6 31 61 7 2 4 96 

4-827l.ls Lance 12 26 54 12 8 100 

4-8274.0shl Lance 8 53 36 7 3 1 24 65 7 4 

4-8274.1shl Lance 11 59 28 7 3 1 2 16 72 8 4 

4-8280.0shl Lance 10 65 25 7 2 tr tr 25 63 7 5 

4-828l.Oshl Lance tr 67 31 1 tr 1 35 51 8 6 

4-8287 .Os Lance 2 28 68 3 1 13 22 18 46 

4-8288.3shl Lance 49 47 2 2 26 42 21 10 

4-8290.0shl Lance 1 65 31 2 1 1 26 54 12 7 

4-8292.0shl Lance 65 35 44 43 10 4 

4-8510.0s Lance 10 16 73 11 tr 20 22 44 13 

4-8516.0s Lance 16 27 51 16 4 2 15 19 45 22 

4-8517 .Os Lance 19 26 55 16 2 1 18 17 65 

4-8520.0shl Lance 12 57 35 4 3 1 35 48 11 6 

4-8526.0shl Lance tr 79 20 tr tr 1 35 45 11 9 

4-8534.0shl Lance 57 40 tr 31 42 17 10 

4-8535.0shl Lance 72 25 2-P 37 50 9 4 

4-10272.2s Lance 8 24 66 6 2 2 15 46 26 14 

4-10280.0shl Lance 6 58 35 1 3 2 tr-P 40 43 11 6 

4-10282.2s Lance 18 17 64 15 2 2 22 37 28 13 

4-10287.5s Lance 6 27 65 4 2 2 20 28 21 32 

014 Geology of Tight Gas Reservoirs, Wyoming and Colorado 
Table 4. Continued 

Sample Stratigraphic Sol Whole rock <2-IJ:m cia~ fraction 

designation unit Carb Clay Quartz Cal Dol 

4-1029l.Os Lance 7 17 75 6 1 

4-10302.0shl Lance 13 72 20 6 2 

4-10310.3s Lance 10 28 59 7 3 

4-10315.8s Lance 7 23 67 6 1 

4-10316.0shl Lance 59 39 

4-10329.0s Lance 5 38 56 2 2 

4-10523.3s Lance 7 26 64 7 2 

4-10524.3s Lance 9 14 76 8 1 

4-10524.4s Lance 8 25 70 6 1 

4-10543.0s Lance 11 28 59 9 2 

4-10556.4s Lance 6 17 74 6 1 

4-10565.4s Lance 9 23 67 4 2 

4-10567.0s Lance 3 29 67 3 tr 

grains, is 55-75 weight percent. The shales from this interval in the Wagon Wheel well are more illitic than those 

higher in the section (table 2). liS and discrete illite comprise about 80-90 relative weight percent of the clay 

minerals in the <2-J.tm fraction. 

Many of the samples from the unnamed Tertiary 

unit and the uppermost part of the Lance Formation exhibit extensive diagenetic modification. Similar types and 

degrees of diagenesis can be correlated with equivalent 

units and depths in the New Fork wells of the Pinedale 

area. The particular intensity of diagenesis in this interval is defined by extensive physical and chemical modifications in sandstone samples from core recovered 

at depths beginning about 7,340 ft (2,240 m) in the 

Wagon Wheel well. Lower Tertiary unnamed sandstones 

cored from the interval above, at 7,040-7,140 ft 

(2,150-2,180 m) exhibit much less diagenetic change than 

sandstones below 7,340 ft. 

Thin section and SEM analysis of core samples 

7,343.6 and 8,084.6 from the Wagon Wheel well core 

(table 2) illustrate the maximum extent of diagenesis 

across the K-T interval in the Pinedale area. These 

diagenetic events are diagrammed in figure 3. Early compaction produced deformation fabrics, including 

squashing of ductile pelitic grains and rock fragments and 

the fracturing, crushing, and interpenetration of brittle 

framework grains (fig. 9D). Well-developed syntaxial 

overgrowths on quartz framework grains formed prior 

to other diagenetic stages. Chert overgrowth cement was 

observed on a few grains (fig. lOA). Schuster (1986) 

reported that chert cement is ubiquitous in lower Tertiary 

sandstones of the Pinedale anticline, but in this study 

chert cement was rare and limited in extent when present. 

Kaolinite cement replacing calcite was found to be ubiquitous, however, and, because of its microcrystalline 

texture and low birefringence in thin section, it can easily 

Carb Plag s~ar Other Illite 1/S Kaol Chi C/S 

1 19 47 17 17 

tr 48 39 6 7 

3 20 52 11 17 

3 tr 15 55 10 20 

2 48 42 5 4 

2 tr 38 40 13 8 

1 tr 25 34 40 

1 tr 22 35 13 30 

1 24 34 42 

2 22 61 9 7 

2 32 37 16 15 

2 19 53 5 23 

1 27 31 42 

be mistaken for chert or other forms of microcrystalline 

quartz (figs. lOB, C), particularly if SEM analysis of the 

samples is not performed. 

Chloritic clay cements commonly form rims around 

framework quartz and most syntaxial quartz overgrowths 

(fig. lOD). The precipitation of discrete chlorite cement 

is interpreted as the next stage in the paragenetic sequence. Although much of this discrete chlorite cement 

coexists with mixed-layer chlorite/smectite (C/S), the 

CIS is interpreted to have formed later, as discussed 

later. 

Precipitation of discrete chlorite clay was followed 

by cementation of sandstone by iron-free calcite. Calcite 

also partly replaced feldspars, chert, and shale fragments (fig. 9D). Iron-free calcite was later dissolved and 

(or) replaced by iron-bearing calcite cement; the iron content of these cements, as revealed by staining, increases 

with depth into sandstones of the Lance Formation 

(fig. llA). 

The later dissolution of calcite created extensive 

intergranular and intragranular secondary porosity. Feldspars, chert, and shale and carbonate fragments were also 

extensively dissolved (fig. liB). Replacement of much of 

the intergranular calcite cement by kaolinite resulted in 

a complex network of microporosity (fig. liB). Precipitation of CIS (mainly corrensite) was contemporaneous 

with formation and dissolution of iron-bearing calcite. 

CIS is a cement in many of the sandstones in the interval between 7,060 and 8,130 ft (2,150-2,480 m) (Pollastro 

and Barker, 1986). Although the C/S coexists with late 

authigenic quartz overgrowths, it postdates most 

authigenic quartz because the clay commonly drapes 

quartz crystals or etches quartz overgrowths (fig. 12A). 

In some cases, however, quartz crystal growth probably 

was inhibited by the presence of CIS clay; therefore, 

multiple stages of quartz precipitation were also likely. 

Mineral Composition, Petrography, and Diagenetic Modifications, Northern Green River Basin 015 
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Figure 7. Thin section photomicrographs of sandstones of the Fort Union Formation in the Wagon Wheel No. 1 well. Scale bars 0.2 mm. A, Poorly sorted, 

lithic arkose having high clay matrix (M) and clay-altered feldspars (Cif) (crossed nicols). F, feldspar. 8, lntergranular and intragranular dissolution porosity 

(P) in lithic arkose showing remnants of calcite (C). F, feldspar. C, Laumontite (L) cement replacing feldspars (F) in sandstone. 0 , Extensive dissolution porosity 

(P) after removal of laumontite. F, feldspar; B, biotite. 
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Figure 8. Scanning electron micrographs of clays in sandstones of the Fort Union Formation from the Wagon Wheel No. 1 well. A, Smectite grain-coating cement. 8, Feldspar (F) grain altered by smectitic clay (S). Q quartz. C, Higher magnification of B. 0, Framboidal pyrite 

(P) in laumontite (L) cement showing chlorite (CH) replacing laumonite. 
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Figure 9. Thin section photomicrographs of lower Tertiary sandstones from the Wagon Wheel No. 1 well. Scale bars 0.2 

-

mm. A, Calcite (C) extensively cemented 

sandstone having essentially little or no porosity. Q, quartz; B, biotite; F, feldspar. 8, Poorly sorted sandstone having high clay matrix (M) and clay-altered 

feldspars (Cif) (crossed nicols). F, feldspar; Q, quartz. C, Replacement of feldspars (F) by iron-free calcite (C) and ferroan (Fe) calcite (arrows). Note intergranular 

dissolution porosity (P). 0, Deformation fabrics (df) and dissolution porosity (P) in sandstone cemented by iron-free calcite (C). Note quartz overgrowth (OG) 

cement and dissolution and replacement of feldspars (F) by calcite. 
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Figure 10. Thin section photomicrographs of lower Tertiary sandstones from the Wagon Wheel No. 1 well. Scale bars 0.1 mm. A, Calcite (C) cement after 

overgrowths (OG) on quartz (Q) and chert (CT) (crossed nicols). 8, Extensive kaolinite (K) cement showing resemblance to chert (crossed nicols). C, Extent 

and similarity of microcrystalline chert (CT) and kaolinite (K) (crossed nicols). 0, Grain-coating chlorite (CH) cement. 
0 

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Figure 11. Thin section photomicrographs 

-

of lower Tertiary sandstones. Scale bars 0.2 mm. A, Ferroan calcite (Cfe) cement. Wagon Wheel No. 1 well. 

B, High secondary porosity (blue) resulting from dissolution of calcite (C) cement and dissolution of feldspars (F) and other grains mostly replaced or 

infilled by kaolinite (K). Wagon Wheel No. 1 well, 7,366 ft. C, Coarse-grained sandstone containing abundant detrital limestone (LS) and dolomite (D) 

grains partly cemented by calcite (C). Q, quartz. New Fork No. 1 well, 7,970 ft. D. Rhombic detrital dolomite (D) grains in sandstone exhibiting dissolution 

porosity (P) and remnants of calcite (C) cement. Note quartz overgrowths (OG). 
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Figure 12. Scanning electron micrographs of authigenic clay minerals in sandstones. A, Chlorite/smectite (CS) and terminated quartz (Q) 

crystals in close association with kaolinite (K). Wagon Wheel No. 1 well. 8, Coarse, vermicular kaolinite as pore-filling cement. Wagon 

Wheel No. 1 well. C, kaolinite (K) replacing earlier-formed calcite (C) cement. Ericson Sandstone, WASP well, 11,760 ft. 0, Chlorite/smectite 

(CS) honeycombs on earlier formed platelets of chlorite (CH). Wagon Wheel No. 1 well. 
The extent of diagenesis in sandstones from the 

K-T interval is variable: the high intensity of diagenesis 

was certainly selective to particular sandstones. Many 

sandstones within the same interval show few diagenetic 

events and are still well cemented with iron-free or ironbearing calcite. The minor secondary porosity that exists in sandstones well cemented by calcite is primarily 

intragranular and formed by the partial dissolution of 

feldspars, chert, and sedimentary rock fragments. These 

local variations in the intensity of diagenesis may be controlled by sedimentologic factors. 

New Fork Wells 

The feldspar content of Tertiary rocks in the New 

Fork wells varies from 2 to 40 weight percent and total 

clay content averages 15-20 weight percent. Carbonate 

minerals, mostly calcite, were observed in all Tertiary 

sandstone samples; XRD analysis indicates that dolomite 

contents are as high as 10 weight percent. Although no 

Tertiary shales were analyzed from the New Fork cores, 

shaley siltstones having clay contents of more than 30 

weight percent (as calculated by XRD) (New Fork sample 1-7961.2, table 4) and clayey matrices observed in thin 

section show a marked increase in discrete illite and liS. 

The direct relationship between increases in discrete illite and liS and an increase in detrital clay in sandstone 

suggests that shales from this interval have a similar claymineral assemblage. This illite-liS assemblage is similar 

to that for equivalent rocks from the Wagon Wheel well. 

Detrital limestone and dolomite grains and other 

sedimentary rock fragments are abundant in Tertiary 

sandstones from the New Fork Nos. 1 and 2 wells, particularly in the coarser sandstones (fig. llC). Although 

most of the detrital dolomite consists of well-rounded 

polycrystalline grains, individual rhombs of detrital dolomite commonly are dispersed throughout the finer 

grained sandstones (fig. liD). Detrital carbonate grains 

were likely derived from Paleozoic carbonate rocks. 

Rhombs of authigenic dolomite also occur as overgrowths 

on detrital dolomite grains or as replacements in chert 

fragments (fig. 13A). Similar petrographic characteristics 

for dolomite from the Wagon Wheel well were reported 

by Dickinson (1984, 1985). 

The style and intensity of diagenesis in the lower 

Tertiary sandstones and sandstones from the Lance Formation in the three New Fork wells, a depth range of 

7,950 to 10,570 ft (2,420-3,220 m), are similar in many 

aspects to those for the K-T interval described in 

the Wagon Wheel well between depths of 7,240 and 

11,070 ft (2,240-3,370 m) and are summarized in figure 

3. Most sandstones of the New Fork cores from this interval contain significant secondary porosity. They are 

characterized by well-formed, early quartz (fig. 13B) 

and rare minor chert overgrowth cementation. Silica 

022 Geology of Tight Gas Reservoirs, Wyoming and Colorado 

cementation was usually followed by the precipitation of 

liS clay around overgrowths and other framework grains. 

CIS also formed in some of the sandstones from this interval (table 4). 

Calcite and kaolinite are major cements in Tertiary 

sandstones from the New Fork wells (fig. 13B). Framework feldspar grains have been extensively dissolved or 

replaced, and sometimes infilled by calcite and (or) altered 

to kaolinite (fig. 13C). The dissolution of calcite cement 

and silicate framework grains (primarily feldspars and 

chert) are the common causes of intergranular and intragranular secondary porosity, respectively. The extent 

of dissolution is variable, however, and thus the amount 

of intragranular secondary porosity is unpredictable 

among sandstones from any one interval. 

Upper Cretaceous Rocks 

General Mineralogy and Diagenetic Overview 

In the Wagon Wheel well, total feldspar content 

generally decreases to less than 5 weight percent of the 

bulk rock in Upper Cretaceous sandstones and shales at 

depths less than 16,000 ft (4,900 m), as compared to the 

20-25 weight percent in Tertiary rocks immediately above 

the Cretaceous-Tertiary unconformity (tables 2-4). Typically, 2-3 weight percent plagioclase is present, and 

potassium feldspar is absent or present only in trace 

amounts; however, rocks deeper than 16,000 ft (4,900 m) 

in the Wagon Wheel well contain as much as 15 weight 

percent total feldspar in some samples (table 2). 

The diagenesis of Upper Cretaceous rocks from the 

Wagon Wheel well varies in some aspects from unit to 

unit, but many diagenetic events are common to most 

units; these are shown in the upper part of figure 3. Additional or unique diagenetic events are shown in the 

lower part of figure 3. Early diagenesis of Upper Cretaceous sandstones from the Wagon Wheel well typically 

involved compaction, framboidal pyrite formation, and 

the precipitation of quartz overgrowth and smectitic clay 

cements. These earliest stages were followed by partial 

dissolution of framework grains and cementation by 

calcite. Iron-bearing calcite commonly replaced earlier 

iron-free calcite, and dissolution of both calcite cements 

and feldspar, chert, and shale grains typically followed. 

The replacement of calcite cement and (or) feldspar, 

chert, and lithic grains (shale clasts) by kaolinite or the 

precipitation of kaolinite cement in secondary pores was 

almost ubiquitous during later stages of burial. 

In the WASP well, the paragenetic sequence and 

extent of diagenesis in sandstones from the Lance Formation and Ericson Sandstone and most sandstones from 

the Rock Springs Formation are similar through the depth 

interval cored (10,450-14,360 ft; 3,180-4,380 m) and are 
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Figure 13. Thin section photomicrographs of sandstones. Scale bars 0.1 mm. A, dolomite (D) rhombs replacing chert 

--

(CT) in sandstone cemented by calcite 

(C) (crossed nicols). 8, Quartz-overgrowth cement (OG) and porosity (P) from dissolution of feldspar (F) and later calcite (C) cement prior to replacement 

by kaolinite (K). C, Relic outline of rectangular feldspar (arrows) replaced by calcite (C) and kaolinite (K). Unnamed Tertiary unit, New Fork No. 4 well, 8,260 

ft. 0 , Alternating quartz-overgrowth (OG) and clay (CI) cements (crossed nicols, accessory plate). Q, quartz. Rock Springs Formation, WASP well, 12,514 ft. 
summarized in figure 6. Most of the diagenetic stages are 

similar to those in the generic sequence described above 

for the Wagon Wheel well (fig. 3). Diagenesis of Upper 

Cretaceous sandstones of the WASP well generally began 

with the formation of quartz overgrowths; the development of these overgrowths was sometimes preceded or 

interrupted by the precipitation of smectitic grain-coating 

clay (fig. 13D). Many of the sandstones are characterized by ferroan dolomite overgrowths on detrital dolomite 

grains. The ferroan dolomite overgrowth cements are 

easily recognized after carbonate staining. These overgrowths commonly are extensive, and can fill a significant volume of primary pore space (fig. 14A). Formation 

of these overgrowths of ferroan dolomite postdated formation of quartz overgrowths but predated cementation 

by calcite (fig. 14B). These diagenetic events were almost 

always followed by calcite cementation. Wherever ironfree calcite cement formed, it commonly was replaced by 

iron-bearing calcite cement. Fractures formed late in the 

burial history, probably during uplift and erosion 

(Pollastro and Barker, 1986), and were filled by quartz, 

then dickite, and finally calcite (fig. 14C). Detailed 

descriptions for each of the Upper Cretaceous units from 

these wells follow. 

Lance Formation 

The Lance Formation is represented in core samples 

from all five wells. Sandstones typically consist of 60-70 

weight percent quartz, as calculated by XRD. Chert and 

chalcedony are abundant and are framework grains or 

microcrystalline replacements of earlier phases. Volcanic 

quartz grains, including some having beta habit, were 

observed in some sandstones (fig. 14D). Clay content 

varies from about 10 to 35 weight percent of the bulk 

rock; however, the very clay-rich sandstones commonly 

contain shale clasts or laminae. 

Diagenesis within the Lance Formation between the 

cored intervals from about 8,920 ft to 11,070 ft 

(2,720-3,370 m) in the Wagon Wheel well is summarized 

in figure 3. Sandstones from this interval contain at least 

two alternating generations of quartz and liS clay cement. After formation of the second generations of 

quartz and liS cement, calcite cement was precipitated 

and chert and shale clasts were replaced by calcite. The 

later dissolution of calcite cement, chert, and sedimentary rock fragments created some moderate secondary 

porosity. If feldspar was an original component of these 

rocks, most has been altered because little or no feldspar 

was detected in the analysis. Replacement of many partly to completely dissolved rock fragments by liS and 

discrete illite suggests cannibalization (usually by solution) of earlier, more expandable clay and reprecipitation as a more illitic assemblage (Pollastro, 1985). 

Authigenic pore-lining and pore-filling chlorite was also 

024 Geology of Tight Gas Reservoirs, Wyoming and Colorado 

precipitated after calcite dissolution and is closely 

associated with the progressive growth of illitic cements. 

Calcite and dolomite are common constituents to 

the sandstones of the Lance Formation in samples from 

all of the wells studied (tables 2-4). Much of the carbonate 

material is detrital limestone fragments and polycrystalline dolomite grains (fig. 15A); rhombic dolomite grains 

are also common, particularly in some of the finer, clayrich siltstones. When observed in thin section, most sandstones and siltstones containing more abundant carbonate 

(> 10 weight percent of bulk rock as calculated by XRD) 

are partly or extensively cemented by iron-free or ironbearing calcite (fig. 15B). Iron-bearing calcite cements 

commonly replace earlier iron-free calcite cement. In 

contrast, however, iron-free calcite fills fractures that 

formed very late in the burial history (Dickinson, 1985; 

Pollastro and Barker, 1986). 

Kaolinite is a common cement in many of the sandstones, and, as in the Tertiary rocks, it replaces feldspar, 

chert grains, rock fragments, and calcite cements. 

Kaolinite cement commonly is extensive (fig. 15C). It 

typically occurs as coarse crystals in books and vermicules 

or as individual pseudohexagonal plates (fig. 12B). 

Because of its coarse nature in many of the sandstones, 

the weight percent of kaolinite relative to other clay 

minerals in the rock may be grossly underestimated 

because the XRD data only calculates the amount of 

kaolinite in the <2-JLm fraction. Other authigenic clay 

mineral phases in sandstones of the Lance Formation are 

liS, CIS, and discrete illite. In many cases, these clay 

phases are overgrowths on a precursor clay substrate 

(Pollastro, 1985). 

Shale samples from the Lance Formation are 

dominated by I/S and discrete illite clay; little kaolinite 

and (or) discrete chlorite were observed in the shales. This 

relationship is shown in the XRD data for the Lance Formation in all wells (tables 2-4). Chlorite and kaolinite, 

however, are abundant in sandstones, particularly cleaner 

sandstones; that is, those containing less than 20 weight 

percent phyllosilicate minerals. This relationship is 

demonstrated by a crossplot of XRD data for an interval of the Lance Formation. The data show an almost 

inverse relationship between the amount of <2-JLm 

discrete chlorite and discrete illite (fig. 16A). A comparison of the lithology of these samples and the XRD 

data shows that the most chlorite rich sandstones are the 

cleanest and that those sandstones containing the most 

discrete illite are true shales. Intermediate compositions 

include shale-laminated sandstones and lithic sandstones 

containing an abundance of shale clasts. 

Crossplots of total phyllosilicate content for the 

bulk rock versus relative weight percent discrete illite (fig. 

16B) or discrete chlorite (fig. 16C) also demonstrate welldefined clay-mineral populations for both sandstone and 

shale. This relationship can be explained by considering 
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Figure 14. Thin section photomicrographs of sandstones. Scale bars 0.2 mm. A, Detrital dolomite (D) with ferroan 

-

(Fe) dolomite overgrowths (stained blue). 

Rock Springs Formation, WASP well, 13,553 ft. 8, Genesis of cements. Earliest quartz overgrowth (OG) cement on quartz (Q) grains, followed by ferroan dolomite 

(Fe) overgrowths on detrital dolomite (D), then intergranular calcite (C) cement. Rock Springs Formation, WASP well. C, Cements filling fracture. Order of 

genesis: quartz (Q), dickite (D), and calcite (C) (crossed nicols). Rock Springs Formation, WASP well, 13,612 ft. D, Volcanic quartz (V) grains having beta 

habit, secondary porosity (P), and calcite (C) cement. lance Formation, New Fork No. 2 well. 
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Figure 15. Thin section photomicrographs of sandstones. Scale bars 0.2 mm. A, Detrital limestone (LS) and dolomite (D) in ferroan (Fe) calcite-cemented 

sandstone of Lance Formation. Wagon Wheel No. 1 well, 10,153 ft. 8, Low-porosity sandstone showing quartz overgrowths (OG) on quartz (Q) prior to calcite 

(C) cement. Lance Formation, WASP well, 11,546 ft. C, Quartz overgrowths (OG) and replacement of calcite (C) cement (arrows) and grains by kaolinite (K). 

Q, quartz. Lance Formation, Wagon Wheel No. 1 well, 8,124 ft. 0, Quartzarenite comprised mainly of quartz (Q) and chert (CT) grains with clay (CI) and 

quartz-overgrowth (OG) cements. Ericson Sandstone, Wagon Wheel No. 1 well, 13,124 ft. 
the origin for many of the clay minerals in sandstone ver- A 

sus those in shale. Detritus of the shales from the Lance 

Formation was derived from sources containing free silica 

(quartz, chert, and so forth), mica (mostly discrete illite), 

smectite, and (or) liS; some detrital carbonate (typically 

5-10 weight percent) is also common. Few chlorite or kaolinite clay minerals were common to the source area and, 

therefore, deposited in the shales. In sandstones, however, 

kaolinite is pervasive as an authigenic cement or replacement phase, as discussed earlier, and was observed throughout the entire Lance Formation at all localities in this study. 

Discrete iron-rich chlorite is also an authigenic clay common to these low-permeability sandstones. The weight ratio 

of chlorite and kaolinite to the total clay-mineral population in these sandstones is probably underestimated from 

the XRD data because many of the individual clay particles, as measured from thin section and SEM analysis, 

is greater than 2J.tm in diameter (fig. 12B). Therefore, these 

relationships between sandstone and shale clay mineralogy 

are probably more significant than the data plotted in figure 

16 because the analysis only accounts for relative weight 

percent as determined from the <2-J.tm clay mineralogy. 

Ericson Sandstone 

The Ericson Sandstone was sampled from core of 

the Wagon Wheel and WASP wells. In the northern Green 

River basin, the Ericson is subdivided into an upper unit, 

referred to as the Canyon Creek zone, and a lower unit, 

the Trail zone (Law, 1984b; Law and Johnson, this volume). 

The nonmarine sandstones of the Ericson are more continuous than the more common, lenticular Upper Cretaceous low-permeability sandstones in this area. 

Compositionally, the sandstones of the Ericson are more 

arenitic than the more lithic sandstones of the Lance Formation and other Mesaverde units in this part of the basin 

(Gautier and Pollastro, 1982; Edman and Surdam, 1985). 

There is some textural evidence suggesting that feldspars 

were present in many of the Upper Cretaceous sandstones 

sampled from the Wagon Wheel and WASP wells. This 

evidence will be discussed below. 

In the Wagon Wheel well, the upper zone of the Ericson contains quartzarenites that average about 90 weight 

percent quartz, as determined by XRD (table 2). These 

sandstones are composed primarily of detrital quartz, chert 

fragments, and occasional shale clasts (fig. 15D). Parts of 

the lower zone in the Wagon Wheel well (sample 13132 and 

deeper, table 2), however, contain sandstones having 20-30 

weight percent total clay, as calculated by XRD; this clay 

is predominantly mica and kaolinite minerals. 

Figure 16. Clay-m ineral relationships between sandstones and 

shales from the Lance Formation, Wagon Wheel No. 1 well, as 

determined by X-ray powder diffraction. A, Relative weight 

6 60 

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10 30 50 

PERCENT CHLORITE IN <2J.Lm CLAY FRACTION 

25 45 65 

PERCENT ILLITE IN <2J.Lm CLAY FRACTION 

. . .. . . . :: .. 

10 30 50 

PERCENT CHLORITE IN <2J.Lm CLAY FRACTION 

percent illite versus chlorite. 8, Total clay in whole rock 

versus relative weight percent illite in clay fraction. C, Total 

clay in whole rock versus relative weight percent chlorite 

in clay fraction. 

Mineral Composition, Petrography, and Diagenetic Modifications, Northern Green River Basin 027 
The diagenetic stages interpreted for quartzarenites 

of the Ericson Sandstone are shown in figure 5 for the 

Pinedale area and figure 6 for the WASP samples. 

Multiple stages of syntaxial quartz overgrowths interrupted by stages of clay cement around framework quartz 

grains are typical of sandstones of the Ericson (fig. 15D). 

Quartz cementation filled most primary intergranular 

porosity in many sandstones of the Ericson, and subsequent dissolution of chert grains and shale clasts produced 

intragranular pores (fig. 17A). The only porosity in 

quartzarenite sandstones is intragranular secondary 

porosity that formed by dissolution of chert and rock 

fragments (figs. 17A, B). Because of the dissolution of 

less stable rock fragments and chert grains (both which 

are microcrystalline composites and therefore contain 

numerous reactive surfaces relative to framework quartz 

grains and quartz overgrowth cement), sandstones of the 

Ericson have higher porosity than sandstones in adjacent 

formations. Therefore, the quartz-rich sandstones of the 

Ericson have potential for better porosities than other 

reservoir rocks at depths greater than 10,000 ft (3,050 m). 

Highly illitic clay coats many of the quartz grains. 

Grains having a sericitic appearance or coarse illitic 

overgrowths account for a major part of the micaceous 

material. Many of the sericitic grains are shale clasts that 

probably contained a precursor smectitic clay. During 

deep burial diagenesis, these grains were converted to a 

more illitic composition with lath-shaped overgrowths 

(Pollastro, 1985). Shale clasts also may have served as 

nuclei for further crystal growth of intergranular illitic 

or sericitic cements (fig. 17C). 

Authigenic kaolinite is common in sandstones of 

the Ericson. Chert grains are commonly altered to 

booklets and vermicules of kaolinite (fig. 17D). Coarsecrystalline dickite, probably of hydrothermal origin, fills 

late-formed fractures in some sandstones of the Ericson 

at a depth of about 13,200 ft (4,020 m) in the Wagon 

Wheel well. 

Sandstones of the Ericson in the WASP core are 

generally coarser and contain more lithic fragments than 

those from the Wagon Wheel well. These characteristics 

are consistent with a closer proximity to a source area 

to the northwest (B.E. Law, oral commun., 1987). Very 

quartz rich sandstones are distinctively absent in samples 

from the WASP core, as compared to samples from the 

Wagon Wheel core. Detrital dolomite occurs in sandstones of the Ericson from the WASP well; it is less common than in those from the Wagon Wheel well and occurs 

only in sandstones from the lower zone. Dolomite rhombs 

commonly replace chert or are overgrowths on polycrystalline detrital dolomite grains, and sparry or rhombic 

dolomite partly to completely replaces quartz and chert 

grains (fig. 18A). Calcite is rare or absent in Ericson 

samples from the Wagon Wheel well. In the WASP core, 

however, all sandstones of the Ericson contain calcite 

028 Geology of Tight Gas Reservoirs, Wyoming and Colorado 

cement; this calcite is commonly partly dissolved or 

sometimes replaced by kaolinite (figs. 12C, 18B). 

Shales of the Ericson from the Wagon Wheel well 

are enriched in kaolinite relative to all other shales in this 

study (table 2). No shale samples from the Ericson were 

analyzed in the WASP core. 

Rock Springs Formation 

Mesaverde Group rocks referred to as "Undivided" 

by Law (1984b) are designated as the Rock Springs 

Formation in this report (Law and Johnson, this volume) 

and are represented in samples from the Wagon Wheel 

and WASP wells. Seventy-three samples of the Rock 

Springs Formation were analyzed from the WASP core 

from 12,050 to 14,350 ft (3,670-4,370 m), and thirtyseven samples were analyzed from the Wagon Wheel 

well from about 14,000 to 17,965 ft (4,270-5,470 m) 

(table 2). 

Sandstones from the Rock Springs Formation in 

the Wagon Wheel well typically are finer grained than 

those from the WASP well and contain 45-50 weight percent quartz, 25 weight percent total clay, and 20-25 

weight percent carbonate minerals. Sandstones of the 

Rock Springs Formation commonly are rich in sedimentary rock fragments, particularly carbonate grains and 

shale clasts; chert is also a major component. Early quartz 

overgrowth cementation is common to lithic sandstones 

of the Rock Springs Formation. A few quartzarenite 

sandstones, petrographically similar to the quartz-rich 

sandstones of the upper part of the Ericson from the 

Wagon Wheel well, are present in the WASP core. These 

sandstones are extensively cemented by silica and contain mostly secondary moldic porosity from the dissolution of chert and shale clasts (fig. 18C). 

Rock Springs sandstones from the Wagon Wheel 

well are rich in detrital dolomite (fig. 18D); dolomite content averages about 15 weight percent, as determined by 

XRD. Limestone rock fragments are also common. 

Ankerite or siderite may also coexist with dolomite. In 

core samples from the WASP well, dolomite increases 

from about 2 to 15 weight percent downsection. Detrital 

dolomite is both rhombic and polycrystalline and commonly has micritic or sparry overgrowths of ferroan 

dolomite; micritic dolomite or ankerite cement sometimes 

replaces or encompasses framework grains (fig. 19A). 

Iron-free and iron-bearing calcite cements are usually 

present, and the more iron bearing varieties commonly 

replace iron-free calcite (fig. 19B). 

Sandstones from the Rock Springs Formation in 

the Wagon Wheel well contain distinctly more feldspar 

than do other Upper Cretaceous sandstones higher in the 

section. Below a depth of about 16,000 ft (4,900 m), 

plagioclase and potassium feldspar, combined, may comprise as much as 10 weight percent of the rock (table 2). 
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Figure 17. Thin section photomicrographs of quartzarenites from the Ericson Sandstone, Wagon Wheel No. 1 well. Scale bars 0.2 mm. A, 8, Porosity (P) 

from dissolution of chert (CT) and shale fragments (RF) in quartz (Q) sandstone. 13,109 ft. C, Sericitic (S) overgrowths from sericitized grain in quartz (Q) 

sandstone. 13,124 ft. 0, Chert grain or shale clast altered to kaolinite (K). 
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Figure 18. Thin section photomicrographs 

-

of sandstones. Scale bars 0.1 mm. A, Extensive replacement of chert and 

--

cementation by dolomite (D) (crossed 

nicols). Q quartz. Lower part of Ericson Sandstone, Wagon Wheel No. 1 well, 13,197 ft. B, Dissolution porosity (P) and cementation by calcite (C) and kaolinite 

(K). Q, quartz. Ericson Sandstone, WASP well. 11,757 ft. C, Moldic porosity (P) in quartzarenite. Rock Springs Formation, WASP well, 13,181 ft. 0, Quartz 

overgrowth (OG) and ferroan (Fe) calcite cements in sandstone containing abundant detrital rhombic dolomite (D) grains. Note extensive dissolution porosity 

(P). Q, quartz. Rock Springs Formation, Wagon Wheel No. 1 well, 16,084 ft. 
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Figure 19. Thin section photomicrographs of sandstones, Rock Springs Formation, Wagon Wheel No. 1 well. Scale 

-

bars 0.1 mm. A, Dolomite (D) and calcite 

(C) cements replacing framework feldspar (F) and quartz (Q) (crossed nicols). 16,087 ft. 8, Ferroan (Fe) calcite cement after iron-free calcite (C). Note porosity 

(P) infilled by blue epoxy. 16,087 ft. C, Dissolution of feldspar (F) and replacement or infilling by calcite (C). 16,095 ft. 0, Abundance of potassium feldspar 

(KF) (stained yellow). 16,076 ft. 
These feldspars may be partly or completely replaced by 

calcite; in some samples, calcite cement fills earlier 

dissolution pores in the grains (fig. 19C). Both potassium 

feldspar and plagioclase grains are commonly unaltered 

(fig. 19D). Other common mineral grains are biotite and 

glauconite. 

The common clay minerals in sandstones of the 

Rock Springs Formation are illite, illitic liS (table 3), and 

iron-rich chlorite. Kaolinite was found in almost all sandstones from the WASP core and to a depth of about 

14,950 ft (4,560 m) in the Wagon Wheel well. Illitic clay 

cement was observed between stages of quartz overgrowth 

cementation in many of the sandstones. Clay cementation may have occurred relatively early in the burial 

history and, therefore, the original clay-mineral composition may have been modified during deep burial diagenesis (Pollastro, 1985). Illitic or sericitic clay overgrowths 

also occur as elongate or lathlike crystals on shale clasts 

and as coatings on framework grains. This micaceous clay 

sometimes grows outward crystallizing from the framework grain nucleus to form a late intergranular cement 

(fig. 20A). 

Diagenesis of sandstones from the Rock Springs 

Formation in the Wagon Wheel well can be separated into 

two different sequences by depth and is shown in figure 

3. Sandstones from the upper sequence, from 14,890 to 

14,950 ft (4,340-4,560 m), exhibit less quartz overgrowth 

cementation than those from from 16,060 to 17,970 ft 

(4,890-5,480 m). Authigenic kaolinite is common to sandstones in the upper cored interval and is absent in sandstones below 16,000 ft (4,900 m). Ankerite and siderite 

cements are common in the upper sequence; dolomite is 

more common in sandstones of the lower sequence (table 

2). Sandstones below 16,000 ft (4,900 m) contain unusually well preserved feldspars, more extensively developed 

syntaxial quartz overgrowths, and rare occurrences of 

chert cement. Late diagenetic features in the lower sequence include precipitation of pore-filling chlorite and 

infilling of fractures by quartz and iron-free calcite. 

Porosity in sandstones from the Rock Springs Formation is essentially secondary and produced through the 

dissolution of intergranular cements, framework chert, 

and shale clasts (fig. 20B). The dissolution of feldspars, 

where present, also contributes to the development of 

some moldic porosity. 

Illite and liS are the major clay minerals in shales 

of the Rock Springs Formation. Chlorite and kaolinite 

each comprise less than 10 relative weight percent of the 

<2-J.Lm clay minerals of the shales from the Wagon Wheel 

well. Kaolinite and chlorite are more abundant in shales 

from the WASP well, where they commonly exceed 20 

relative weight percent of the <2-J.Lm clay minerals. The 

relative increase in abundance of kaolinite and chlorite 

in WASP shales is consistent with a closer proximity to 

provenance, assuming kaolinite and chlorite in these 

032 Geology of Tight Gas Reservoirs, Wyoming and Colorado 

shales are primarily detrital components. In the Wagon 

Wheel well, kaolinite is not present in either shale or sandstone samples below 16,060 ft (4,900 m), and the relative 

amount of clay-size chlorite markedly increases below 

that depth (table 2). This relationship may indicate that 

kaolinite is not stable at high temperatures produced by 

deep burial, as first suggested by Hower and others 

(1976). 

Two sandstone and one shale sample of the Rock 

Springs Formation were analyzed from the Wagon Wheel 

well at depths from 17,960 to 17,970 ft (5,477-5,480 m). 

The sandstones are extensively cemented by carbonate 

minerals; total carbonate averages about 30 weight percent of the bulk rock, as calculated by XRD (table 2). 

Dolomite is commonly more abundant than calcite; 

rhombs of detrital dolomite are dispersed throughout the 

sections, and micritic to sparry dolomite overgrowths 

form intergranular cements or sometimes replace feldspars, quartz, chert, or shale fragments (fig. 20C). Intergranular iron-free and iron-bearing calcite cements are 

present; iron-bearing calcite cement always replaces ironfree calcite. It also replaces dolomite rhombs, chert, and 

other rock fragments. 

Relative to most other Upper Cretaceous sandstones in the Wagon Wheel well, distinctly more feldspar 

is in the Rock Springs Formation. Many of the feldspars 

are well preserved. Plagioclase is in amounts as high as 

15 weight percent of the bulk rock, as calculated by XRD 

(table 2). Stained thin sections reveal the presence of 

unaltered potassium feldspar in moderate quantities 

(fig. 20D). Because of the relative abundance of feldspars 

at this depth, the possibility of albitization or other authigenic feldspar formation was investigated for these sandstones. No evidence for this process was found and it is 

interpreted that the feldspars in the Rock Springs Formation are original detrital components that have been 

preserved throughout its burial history. 

Specific Diagenetic Phases and Events 

Laumontite and Associated Framboidal Pyrite and Chlorite 

Laumontite-bearing sandstones are in the Fort 

Union Formation in the Wagon Wheel well, at a depth 

of about 5,000 ft (1 ,520 m). Burial reconstruction for the 

area of the well (Pollastro and Barker, 1986) suggests that 

approximately 5,600 ft (1,700 m) of section has been 

eroded. The total maximum burial estimated for 

laumontite-bearing sandstones of the Fort Union Formation in the Wagon Wheel well therefore is about 

10,600 ft (3,230 m), a depth that corresponds to a 

calculated maximum burial temperature for these rocks 

of 90 °C (Pollastro and Barker, 1986, fig. 9). Many recent studies report the formation of laumontite very 
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Figure 20. Thin section photomicrographs of sandstones. Scale bars 0.1 mm. A, lntergranular sericite (S) overgrowths nucleating from sericitized grains 

(crossed nicols). Rock Springs Formation, WASP well, 13,255 ft. 8, Detrital dolomite (D) rhombs showing ferroan (Fe) calcite cement after iron-free calcite 

(C). Note dissolution porosity (P). F, feldspar. Rock Springs Formation, Wagon Wheel No. 1 well, 16,084 ft. C, Original feldspars (F) being replaced by 

dolomite (D) in sandstone (crossed nicols). Rock Springs Formation, Wagon Wheel No. 1 well, 17,959 ft. 0, Yellow-stained potassium feldspars (KF) in 

deep sandstone. Rock Springs Formation, Wagon Wheel No. 1 well, 17,965 ft. 
late in the burial history and in the deep subsurface 

(Crossy and others, 1984; Helmold and van de Kamp, 

1984) or in environments having temperatures that approach or mark the onset of metamorphism (Ghent, 

1979). 

McCulloh and others ( 1981) studied the Sespe Hot 

Springs, Calif., and reported laumontite formation between temperatures of 43 and 89 °C. They concluded that 

laumontite can crystallize at any depth between the surface and 22,000 ft (6, 700 m) and that its formation is 

dependent upon wherever the appropriate combination 

of temperature, fluid pressure, and fluid composition are 

met and mineralogically immature, permeable, and 

porous rocks have reacted sufficiently with interstitial 

water having exceptionally low pC02. They also stated 

that the derivation of laumonite from any precursor 

zeolite is not required and that the concept of depth zonation of zeolitic minerals in burial diagenesis, as suggested 

by Miyashiro and Shido (1970) and others, is not 

necessarily valid with respect to this mineral. It is interpreted here that laumontite in the sandstones of the Fort 

Union Formation from the Wagon Wheel well likely 

formed early in the burial history of the rocks, when 

primary intergranular porosity was high and temperatures 

were much less than 90 OC, probably by processes similar 

to those of the Sespe Hot Springs. 

Framboidal pyrite and authigenic chlorite are also 

associated with laumontite cement (fig. 8D). The formation of framboidal pyrite in sedimentary rocks is reported 

to occur shortly after burial of the sediment by sulfatereducing bacteria. (See review by Berner, 1984.) Dispersed, intergranular, framboidal pyrite associated with 

laumontite in the study area may have precipitated (1) 

in primary intergranular pores shortly after deposition 

of the sediment and then later was enclosed within the 

laumontite cement or (2) in close association with the 

laumontite from warm hydrothermal fluids. The former 

interpretation is more likely because in thin sections of 

several other sandstones, texturally similar framboidal 

pyrite was observed enclosed by early calcite cement. The 

pyrite probably acted as a source of iron for the later formation of chlorite clay; in thin section concentrations of 

chlorite are found within reddish-brown stained areas 

where ·pyrite framboids either are concentrated or have 

been partly or completely dissolved. 

lnterstratified Chlorite/Smectite (corrensite) in Sandstones 

Interstratified chlorite/smectite (CIS), mainly corrensite, is in sandstones between a sampled depth interval from 7,080 to 8,130 ft (2,150-2,480 m) in the Wagon 

Wheel well. Corrensite in these rocks is thought to have 

formed, at least in part, by reacting from a precursor 

chlorite cement. This interpretation of a chlorite precursor is evidenced texturally by a honeycomb morphology, 

D34 Geology of Tight Gas Reservoirs, Wyoming and Colorado 

characteristic of smectite, superimposed on or mixed with 

an earlier "house of cards" morphology that is characteristic of discrete chlorite (fig. 12D). This interpretation 

is also supported by the occurrence of early discrete 

chlorite cements in sandstones of the Fort Union Formation above this interval and the Lance Formation below 

this interval. Pollastro (1985) reported similar relationships for the evolution of liS clay in the Wagon Wheel 

well. Iron for the formation of CIS may have been 

available during the precipitation and (or) dissolution of 

iron-bearing calcite; the dissolution of ferromagnesian 

minerals and pyrite are other possible sources of iron. 

The smectite-to-illite reaction may have supplied 

iron, magnesium, and silica for the formation of discrete 

chlorite, CIS, and quartz because the conversion from 

random liS to short-range-ordered 1/S occurred, to a 

large extent, in this depth interval (Pollastro, 1985; 

Pollastro and Barker, 1986). Authigenic 1/S is also found 

lining pores and coating grains, but liS cement is not as 

pervasive as CIS. Aluminous-rich pore fluids certainly 

were present because the CIS is always in immediate proximity to, or coexists with, authigenic kaolinite (fig. SA). 

Corrensite in the Wagon Wheel core probably formed at 

temperatures greater than 90 °C, coincident with the 

change from randomly interstratified liS (R = 0) to shortrange-ordered 1/S (R= 1) (Pollastro and Barker, 1986). 

Temperatures of formation for both the formation of 

CIS and short-range-ordered liS (R = 1) are in good 

agreement with those documented from other studies 

(Velde, 1972; Hoffman and Hower, 1979; Weaver, 1979). 

Alteration of Feldspars to Chert-Chalcedony 

Some textural evidence suggests that feldspars were 

present in sandstones of the Ericson Sandstone and the 

Lance and Rock Springs Formations in the northern 

Green River basin. Feldspars probably were replaced by 

microcrystalline silica, as evidenced by rectangular grains 

of chert or chalcedony that resemble relic feldspars (figs. 

21A, B). Feldspars may have altered directly to chert or 

chalcedony or, in part, through an intermediate stage 

after calcite. Recently, Morad and Aldahan (1987) 

described early diagenetic direct replacement of feldspars 

by fine-crystalline quartz in Proterozoic sandstones of the 

Visingso Group in Norway and Sweden. Earlier studies 

by Wallace (1976), Odom and others (1979), and Heald 

and Larese (1973) also report the diagenetic replacement 

of feldspar by quartz. \Valker (1962) reported on the 

reversible nature of chert-calcite replacement in sedimentary rocks and suggested that feldspars may first be 

replaced by calcite and then the calcite replaced by chert. 

Textural and petrographic evidence in this study 

that supports the interpretation of replacement of 

feldspars by quartz or at least suggests that feldspars were 

once present in any of these sandstones includes: (1) the 
limited variety of framework grains (mainly quartz and 

chert) and the absence of feldspar in many Upper 

Cretaceous fluvial sandstones from these wells; (2) the 

rectangular outline of many chert grains (many have 

straight, parallel, opposing sides, a habit characteristic 

of feldspar) in Upper Cretaceous sandstones from the 

wells (figs. 21A, B); (3) the orientation of microcrystalline 

quartz in chert and chalcedony grains along what may 

be relic feldspar twin lamellae (fig. 21C, D); and (4) 

textural relationships that suggest direct replacement of 

feldspar by microcrystalline quartz or chalcedony (figs. 

22A, B). 

SUMMARY 

Lower Tertiary sandstones of the northern Green 

River basin are rich in feldspars and sedimentary rock 

fragments and can be generally classified as lithic arkoses. 

The average feldspar content of these sandstones is about 

50 weight percent (as calculated by XRD) in the Fort 

Union Formation and progressively decreases to less than 

10 weight percent in lowermost Tertiary sandstones near 

the Cretaceous-Tertiary unconformity. Although this progressive reduction in feldspar content in the bulk rock 

is generally due to a relative decrease in detrital feldspar 

and increase in silica framework grains, present mineralogical compositions can be partly explained by diagenetic modifications. Feldspar dissolution and an increase 

in authigenic phases, mostly carbonate and clay-mineral 

cements and replacement phases, produces a relative 

decrease in total feldspar content. 

Typical clay-mineral suites of lower Tertiary sandstones consist of expandable interstratified illite/smectite, kaolinite, and iron-rich chlorite; interbedded shales 

are dominated by expandable interstratified illite/smectite and discrete illite clay. The conversion of smectite to 

illite in interstratified illite/smectite plays a major role, 

however, in the progressive burial of these clay minerals. 

The change from lower Tertiary to Upper Cretaceous rocks in this area is consistent with a decrease in 

feldspar content. In Tertiary sandstones, plagioclase and 

potassium feldspar grains, together with quartz, form the 

major framework. In Upper Cretaceous sandstones, 

quartz, chert, and detrital carbonate minerals are the major framework grains. Samples of sandstone and shale 

taken from core recovered above 15,000 ft (4,570 m) in 

the Wagon Wheel well contain less than 5 weight percent 

feldspar and typically average 2-3 weight percent. 

Although this reduction in feldspar content is again mostly due to a change in source material, dissolution and 

replacement of feldspars by carbonate minerals, silica, 

and kaolinite contribute to their near disappearance. In 

samples taken below present-day depths of 16,000 ft 

(4,900 m) and to about 18,000 ft (5,500 m) in the Wagon 

Wheel well, detrital feldspar may comprise as much 

as 15 weight percent of the sandstone. The excellent 

preservation of feldspars at these depths is not well understood, but sedimentologic factors may control diagenetic 

extent. 

Upper Cretaceous shales are dominated by interstratified illite/ smectite and discrete illite clay. Shales 

from the Ericson Sandstone, however, are enriched in 

kaolinite and chlorite relative to most other Upper 

Cretaceous shales. Chlorite and kaolinite are typically 

authigenic and are most abundant in sandstones. 

The typical early stages of diagenesis involve compaction, framboidal pyrite formation, and precipitation of 

smectitic clay and quartz overgrowth cements. These 

earliest stages are followed by partial dissolution of 

framework grains and cementation by calcite. Iron-bearing 

calcites replaced earlier iron-free calcites. Dissolution of 

calcite cements and feldspar, chert and shale grains, 

coupled with the replacement or precipitation of kaolinite, 

typically follow in the sequence. The illitization of smectite from earlier expandable-clay cements or in shale clasts . 

is a dynamic diagenetic process with progressive burial. The 

development of illitic and sericitic overgrowths by precipitation or solution and recrystallization during deep burial 

is also a prevalent diagenetic process in most of these sandstones. Finally, quartz, dickite, and calcite fill fractures 

formed late in the burial history of these rocks. 

Specific diagenetic events are limited to particular 

units or intervals. These include the precipitation of the 

zeolite laumontite and the clay mineral corrensite and the 

replacement of feldspars by chert or chalcedony. Laumontite in sandstones of the Fort Union Formation from the 

Wagon Wheel well is interpreted to have formed early at 

temperatures below 90 OC, whereas corrensite is interpreted 

to have formed much later in the burial history at 

temperatures in excess of 90 OC. Early replacement of 

feldspars by chert or chalcedony is suggested by textural 

evidence and in some cases may have resulted in changes 

in sandstone composition. 

Ferroan calcite cements become progressively more 

abundant with increased burial depth in these rocks. The 

paragenetic relationship of iron-rich calcite cements 

evolving from iron-free calcite cement in burial sequences 

has been reported in sandstone cores from the Wagon 

Wheel well by Dickinson (1984; 1985) and in sandstones 

from other basins (Boles, 1978; Land and Dutton, 1978; 

Pitman and others, 1982; Burley and Kantorowicz, 1986). 

Remnants of iron-free calcites commonly are enclosed by 

iron-bearing cements. The dissolution and replacement of 

calcite is consistent in sandstones from all wells and intervals and is evidenced by remnants or relics of iron-free 

calcite and (or) iron-bearing calcite cements in sandstones 

containing secondary intergranular pores or replaced by 

kaolinite cement, by dissolution of detrital limestone 

grains, and by complete or partial dissolution of calcite 

Mineral Composition, Petrography, and Diagenetic Modifications, Northern Green River Basin 035 
0 

w 

"" 

C1 

1'1> 

0 

0 

ClQ 

-< 

s. 

-t 

ao· 

~ 

C1 

~ 

;:11:1 

1'1> 

Ill 

1'1> 

~ 0 

:::;· 

~ 

~ -< 0 

3 

:;· 

ClQ 

Ql 

::;, 

Q. 

n 

0 

0 

;:: 

Q. 

0 

- -

Figure 21. Thin section photomicrographs 

--

of relic feldspar grains in sandstones. Scale bars 0.1 mm. A, 8, Rectangular 

--

chert-chalcedony (CT) grain resembling 

relic feldspar in sandstone partly cemented by calcite (C). C, 0, Chert-chalcedony (CT) grains with appearance of replacement after feldspar along twin lamellae 

(small arrows) in sandstone cemented by calcite (C). Note alteration boundary (large arrow) in C (crossed nicols). 
Figure 22. Thin section photomicrographs 

-

of feldspar grains being replaced 

by silica. Scale bars 0.1 mm (crossed nicols). Note alteration boundary 

(arrows) in both photos. A, Feldspar (F) replaced by chert-chalcedony (CT) 

and calcite (C). Lance Formation, New Fork No. 2 well, 9,238 ft. 8, Partial 

replacement of feldspar (F) by chert-chalcedony (CT) and calcite (C). Ericson Sandstone, WASP well, 11 ,759 ft. 

cement in polycrystalline detrital dolomite grains. Ferroan 

dolomite cements are also common and usually occur as 

overgrowths on grains of detrital dolomite. 

Porosity in these sandstones is mainly secondary 

and produced by dissolution processes. The dissolution 

of intergranular calcite cement is probably the most important process in producing secondary porosity in most 

of these sandstones. Intragranular or moldic porosity may 

also be extensive but is usually of secondary importance 

in lithic sandstones. In quartzarenites or very quartz rich 

(>80 weight percent quartz) sandstones, however, porosity is mainly moldic. The original intergranular porosity 

in these quartz-rich sandstones usually is completely 

destroyed by extensive early quartz cementation. At 

depth, any porosity in these sandstones is usually intragranular (moldic) and was created by the dissolution of 

mainly chert grains. For this reason, quartz-rich sandstones containing abundant chert have a high porosity 

potential; permeabilities in quartzarenites, however, may 

be much lower than those in lithic sandstones having 

Mineral Composition, Petrography, and Diagenetic Modifications, Northern Green River Basin 037 
mainly intergranular secondary porosity and fewer 

permeability barriers. 

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Chapter E 

Thermal History and Provenance of 

Rocks in the Wagon Wheel No. 1 Well, 

Pinedale Anticline, Northern Green River 

Basin-Evidence from Fission-Track Dating 

By NANCY D. NAESER 

Prepared in cooperation with the U.S. Department of Energy 

U.S. GEOLOGICAL SURVEY BULLETIN 1886 

GEOLOGY OF TIGHT GAS RESERVOIRS IN THE PINEDALE ANTICLINE AREA, WYOMING, 

AND AT THE MULTIWELL EXPERIMENT SITE, COLORADO 
CONTENTS 

Abstract El 

Introduction El 

The fission-track method E2 

Annealing of fission tracks E2 

Application to sedimentary basins EJ 

Thermal history EJ 

Provenance studies ES 

Methods E6 

Laboratory procedures E6 

Present-day temperatures in the Wagon Wheel No. 1 well E8 

Fission-track data E8 

Apatite ages E8 

Zircon ages E9 

Thermal history E9 

Provenance of the sediments ElO 

Conclusions Ell 

References cited Ell 

FIGURES 

1. Map showing structural features of the Green River basin E2 

2-6. Diagrams showing: 

2. Temperatures required to anneal fission tracks in apatite E4 

3. Expected decrease in apparent apatite age with increasing temperature 

and depth at time of maximum burial heating ES 

4. Expected distribution of apparent apatite ages in sedimentary rocks 

after uplift and (or) cooling ES 

5. Depth and present-day temperature of samples for fission-track studies 

from Wagon Wheel No. 1 well E6 

6. Fission-track ages and corrected present-day temperatures for detrital 

apatite and zircon from Wagon Wheel No. 1 well E7 

7. Histograms of fission-track ages of zircon grains in six samples from the 

Wagon Wheel No. 1 well E9 

TABLES 

1. Depth, stratigraphic position, and present-day temperature of samples from 

Wagon Wheel No. 1 well, northern Green River basin, Wyoming E7 

2. Apatite and zircon fission-track ages, Wagon Wheel No. 1 well, northern 

Green River basin, Wyoming E8 
Thermal History and Provenance of 

Rocks in the Wagon Wheel No. 1 Well, 

Pinedale Anticline, Northern Green River 

Basin-Evidence From Fission-Track Dating 

By Nancy D. Naeser 

Abstract 

Fission-track ages of detrital apatite from Upper 

Cretaceous and lower Tertiary sedimentary rocks in the El Paso 

Natural Gas Wagon Wheel No. 1 well in the northern Green 

River basin of Wyoming indicate that the rocks have undergone 

significant cooling. Apatite data suggest that the latest phase 

of cooling began about 4-2 Ma and involved a relatively rapid 

temperature decrease of at least 20 oc. The timing of this cooling is consistent with evidence for a period of widespread uplift 

and erosion in the Green River basin beginning in the Pliocene. 

Zircon fission-track data suggest that most of the Upper 

Cretaceous and lowermost Paleocene rocks in the well had a 

similar provenance. A significant change in provenance occurs 

within Paleocene rocks and probably coincided with exposure 

and erosion of the crystalline core of the Wind River Mountains to the northeast. 

INTRODUCTION 

In the present study, the thermal history and provenance of rocks in the northern Green River basin of 

Wyoming are interpreted by using fission-track ages of 

detrital apatite and zircon separated from core from the 

El Paso Natural Gas Wagon Wheel No. 1 well (fig. 1). 

The well was drilled to a depth of 19,000 ft (5,791 m) 

into the Upper Cretaceous Hilliard Shale and is the 

deepest well on the Pinedale anticline. The well penetrated 

about 11,500 ft (3,505 m) of Upper Cretaceous rocks 

from the Lance Formation to the Hilliard Shale. All but 

the lowermost 1,600 ft (488 m), including all of the rocks 

sampled for this study, were deposited in nonmarine environments in an eastward-prograding wedge of siliciclastic rocks derived primarily from western source 

terranes. By latest Cretaceous-early Tertiary time, deposition in the Pinedale area was more complex in that structural uplift on the basin margins provided multiple 

source terranes for the dominantly fluvial sediments. 

Since Eocene time, there has been no net deposition in 

the northern Green River basin (Rice and Gautier, 1983; 

Shuster and Steidtmann, 1983; Law, 1984a, b; Pollastro 

and Barker, 1984a, 1986; Dickinson and Law, 1985; Law 

and others, 1986; Dickinson, this volume; Law and 

Johnson, this volume; Pollastro, this volume). 

The Pinedale anticline trends northwest; it is about 

35 mi (56 km) long and 6 mi (10 km) wide and has structural relief of about 2,000 ft (610 m). Its structural evolution is most likely related to movement on the Wind River 

thrust fault, which is parallel with and to the northeast 

of the anticline (fig. 1). Growth of the anticline may have 

begun as early as 90 Ma (Shuster, 1986), but the main 

period of structural deformation probably occurred during Laramide deformation (Law and Johnson, this 

volume). 

Several lines of evidence, which will be reviewed 

later, suggest that rocks in the Wagon Wheel well have 

cooled below their maximum paleotemperature. The 

fission-track study was undertaken to help clarify the 

cooling history, to provide input to paleopressure studies 

(Law, 1984c; Spencer, this volume), and to study the 

provenance of the rocks encountered in the well. 

Acknowledgments.-This research was done in 

cooperation with the U.S. Department of Energy 

Morgantown Energy Technology Center and with B.E. 

Law, U.S. Geological Survey. It was supported in part 

by the U.S. Geological Survey Evolution of Sedimentary 

Basins Program. 

I thank R.A. Ullrich of the El Paso Natural Gas 

Company and B.E. Law for providing samples from the 

Wagon Wheel well; L.R. Mahrt and J.R. Shannon, U.S. 

Geological Survey, for performing mineral separations; 

and T .H. McCulloh, Mobil Exploration and Producing 

Services, for providing corrected present-day temperatures of the samples. 

Thermal History and Provenance of Rocks, Pinedale Anticline E1 
GREEN RIVER ' 

EXPLANATION 

~Anticlinal axis 

ww Wagon Wheel no. 1 well 

50 MILES 

50 KILOMETERS 

GREAT DIVIDE BASIN 

BASIN ROCK 

\ 

SPRINGS 

WAMSUTTER 

ARCH 

T WASHAKIE 

BASIN 

SAND WASH BASIN 

Figure 1. Structural features of the Green River basin, northern Wyoming. Location of El Paso Natural 

Gas Company Wagon Wheel No. 1 well (WW) also shown. Modified from Law (1984a). 

THE FISSION-TRACK METHOD 

A fission track is the zone of intense damage that 

forms when a fission fragment travels through a solid. 

Several naturally occurring isotopes fission spontaneously, but only 238U has a fission half-life (9.9x 1015 yr) 

sufficiently short to produce significant numbers of spontaneous tracks during a time period of geologic interest. 

Trace amounts of uranium are in a number of common minerals, and, because 238U fissions spontaneously 

at a constant rate, fission tracks can be used to date these 

minerals. The age of a mineral is calculated by determining the number of spontaneous tracks intersecting a 

polished surface of the mineral and the amount of 

E2 Geology of Tight Gas Reservoirs, Wyoming and Colorado 

uranium that produced those tracks (Naeser, 1976, 1979a; 

Naeser and others, 1989). The two minerals most commonly dated by the fission-track method are apatite and 

zircon. 

Annealing of Fission Tracks 

If a mineral containing spontaneous fission tracks 

is heated to a sufficiently high temperature, the ions 

displaced along the damage zone of the track move back 

to normal crystallographic positions in the mineral and 

the zone is partly to totally repaired. The repair of the 

damage zone results in the progressive shortening and 
ultimate disappearance of the fission track. The reduction in the number of spontaneous tracks produces an 

anomalously young fission-track age, as compared, for 

example, to fission-track ages determined for unannealed 

minerals or to biotite potassium-argon (K-Ar) ages for 

the same sample. 

The two most important factors controlling the 

temperature at which annealing of fission tracks occurs 

are the mineral involved-different minerals anneal at different temperatures-and the duration of heating-the 

longer a mineral is heated, the lower the temperature 

required to totally anneal its tracks. Temperature-time 

relationships that are probably most applicable to 

geologic problems have been obtained by determining the 

temperatures at which minerals are totally annealed (that 

is, yield a zero age) in drill holes in areas where the approximate duration of heating of the rocks is known. The 

most complete drill-hole data available are for apatite, 

and studies by Naeser (1981) indicate that total annealing of fission tracks in apatite occurs at temperatures that 

range from about 105 °C for long-term heating over a 

period of about 108 years duration to 150 °C for heating 

over a period of 105 years (fig. 2). 

Drill-hole data predict slightly longer heating times 

for any given temperature than do data extrapolated from 

laboratory annealing experiments (fig. 2). Laboratory 

data do not take into account the number of new tracks 

that form during long-term geologic annealing (Sanford, 

1981). In addition, laboratory data are based on essentially square-pulse heating and thus may underestimate 

heating time for most geologic annealing. On the other 

hand, heating times are commonly difficult to estimate 

for drill-hole samples, and the times used in drill-hole 

calibrations may be too long (Harrison, 1985); if so, the 

actual "effective" heating times (Hood and others, 1975) 

applicable to most long-term geologic annealing probably 

are intermediate between the times indicated by the drillhole data and those indicated by the extrapolated 

laboratory data (fig. 2). 

The temperatures at which fission tracks in zircon 

are totally annealed are not as well known as those for 

apatite. Limited data show that temperatures are higher 

than for apatite and are probably in the range of 160-240 

°C for heating over periods of geologic time (> 1 million 

years) (Harrison and others, 1979; Hurford, 1985; cf. 

Zeitler, 1985). 

APPLICATION TO SEDIMENTARY BASINS 

Fission tracks in apatite and zircon have been used 

in a variety of studies in basin analysis (Naeser and others, 

1989). The widespread occurrence of apatite and zircon 

in the detrital suites of sedimentary rocks and the close 

correspondence between apatite and zircon annealing 

temperatures and the temperatures at which oil is generated (Hood and others, 1975; Waples, 1980; Naeser and 

others, 1989) make fission-track analysis an attractive 

method for studying the overall thermal history and more 

localized temperature anomalies in sedimentary basins. 

Thermal history information is most often provided by 

apatite because most of the rocks sampled in sedimentary basins have not been subjected to temperatures sufficient to anneal zircon. The presence of unannealed 

zircons sets some limits, however, on maximum paleotemperatures, and the ages of the individual unannealed 

zircon grains can be used to determine the provenance 

of sedimentary rocks and to date marker horizons such 

as volcanic ashes within sedimentary sequences. 

Thern•al History 

Naeser (1979b) first suggested that fission-track annealing could be used to study the thermal history of a 

sedimentary basin and described the expected trend of 

fission-track ages with depth in sedimentary rocks. This 

trend typically is somewhat more complicated than that 

for basement rocks because the grains that comprise 

sedimentary rocks can be of widely differing ages. The 

grains may be equal in age to the stratigraphic age of the 

rock, as in the case of grains derived from contemporaneous volcanism, but much more commonly the 

grains will be older. 

Figures 3 and 4 illustrate relationships between 

depth and age for sedimentary sequences that are at their 

maximum burial temperature or have cooled below maximum paleotemperature. In a sedimentary sequence at 

maximum temperature (fig. 3), fission-track ages for 

apatites from shallow rocks at relatively low temperatures 

will not have been affected by annealing during burial 

of the sediments (zone of no annealing), and the ages of 

individual apatite grains in these rocks will reflect the 

age(s) of the source rocks for the detrital grains. The composite apatite age (see later discussion) calculated for each 

individual sample in this zone may remain relatively constant with depth (as depicted in fig. 3) but more commonly will vary somewhat depending on the age(s) of the 

detrital grains counted in a given sample. In rock subjected to progressively higher temperatures as a result of 

deeper burial, apatite will undergo partial annealing and 

yield progressively younger ages (zone of partial annealing). Within this zone, the apparent apatite ages will 

become younger than the stratigraphic age of the rock 

and will finally decrease to zero at the depth where the 

temperature for total annealing is attained for a sufficient time (zone of total annealing). 

Drill-hole annealing data and extrapolated 

laboratory data (fig. 2) suggest that, for heating durations on the order of 105 to 108 years, the zone of 

Thermal History and Provenance of Rocks, Pinedale Anticline E3 
300 250 

TEMPERATURE 

200 180 160 140 120 100 oc 

700 600 500 400 

108 

106 

350 300 250 a Eielson AFB, Alaska I 

a 

..,. Fenton Hill, N. Mex.: 

..J.. 

a b 

200 

Coso Hot Springs, Calif. fi 

104 

Cf) 

0:: 

<3: 

UJ 

r 102 

z 

UJ-

:?! 

i= 

10° 

1 o-2 

1 o-4 

1.5 1.7 1.9 2.1 2.3 2.5 2.7 

1 ,000/'K 

Figure 2. Temperatures required to anneal fission tracks 

in apatite for short (laboratory experiments) to very long 

(geologic) heating periods. Temperatures for geologic annealing (dashed vertical bars) (Naeser, 1981) were determined by dating apatite from drill holes in three areas 

where approximate duration of heating is known. Total 

annealing temperatures predicted by laboratory experiments were determined by using fading rates 

established by Naeser and Faul (1969) (open circles), 

Reimer (1972) (solid circles), Mark and others (1973) 

partial annealing for apatite spans a temperature interval of only 30-35 °C. It must be emphasized that the actual temperatures covered by this zone depend, as does 

E4 Geology of Tight Gas Reservoirs, Wyoming and Colorado 

(open squares), and Zimmermann and Gaines (1978) 

(solid squares). Extrapolation of laboratory data (solid 

lines) based on rate constant for fission-track annealing 

determined from least-squares fit of laboratory data by 

Zimmermann and Gaines (1978). Solid vertical bar 

represents 95-percent confidence interval for leastsquares fit. a, b, and con solid lines and dashed vertical 

bars identify data for total annealing (a), 50 percent annealing (b), and total retention (c) of fission tracks. 

Modified from Sanford (1981, fig. 2). 

the temperature for total annealing, on the duration of 

heating. At a temperature of 105 OC, for example, apatite 

heated for 108 years will be totally annealed, but apatite 
I 

I 

I UJ 

a: ZONE OF NO ANNEALING 

::J I ~ (detrital ages) I a: I UJ 

c... 

~ I UJ 

I- I 0 

z ) <( 

/ I 

I- / c... / UJ / 0 / (.9 / z / (/) / <( 

LU / a: / 

u / z / 

) } ZONE OF TOTAL ANNEALING 

0 DECREASING APPARENT 

APATITE AGE 

Figure 3. Expected decrease in apparent apatite age 

with increasing temperature and depth in sedimentary sequence at time of maximum burial heating. 

Temperature at which apatite age decreases to zero 

depends on length of time rock has been heated near 

its present-day temperature (see fig. 2). Drill-hole and 

extrapolated laboratory data (fig. 2) indicate annealing occurs over 30-35 oc temperature interval. 

Modified from Naeser (1979b). 

heated for only 1 OS years will show little or no age reduction as a result of annealing. 

When a sedimentary sequence cools below the maximum paleotemperature, in response to either uplift and 

erosion or a decrease in the geothermal gradient, apatite 

from the zone of total annealing will once again begin 

to accumulate tracks and to record a fission-track age (fig. 

4). The recorded apatite age, the slope of the temperatureage plot, and the thickness of the zone of cooling ages 

give information on the time, rate, and amount of cooling, respectively. 

Zircon fission-track ages should generate curves 

similar to those shown in figures 3 and 4, but annealing 

will occur at higher temperatures than in apatite. 

Research on apatites from the Otway Basin, 

Australia, and other areas shows that the reduction in 

mean track length and change in track length distribution that produce the observed age decrease with progressive annealing can in themselves provide important 

UJ 

a: 

::J 

~ a: 

UJ 

c... 

~ 

UJ 

I0 

z 

<( 

I 

t 

LU 

0 

(.9 

z 

CJ) 

<( 

LU 

a: 

u 

z 

j 

I 

ZONE OF NO ANNEALING I 

(detrital ages) I 

I 

/ 

/ 

/ 

./ 

I FORMER ZONE OF I TOTAL ANNEALING= 

I ZONE OF UPLIFT 

I AND (or) 

I COOLING AGES 

I 

J 

/ 

/ 

/ v/ 

0 

ZONE OF TOTAL ANNEALING 

~---DECREASING APPARENT 

APATITE AGE 

Figure 4. Expected distribution of apparent apatite ages in 

sedimentary rocks after uplift and (or) cooling. Modified from 

Naeser (1979b). 

information about thermal history (Duddy and Gleadow, 

1982; Gleadow and others, 1983; Gleadow, Duddy, 

Green, and Hegarty, 1986; Gleadow, Duddy, Green, and 

Lovering, 1986; Green and others, 1989). If a reasonably 

complete age profile cannot be obtained through a 

sedimentary sequence or if the ages alone are ambiguous, 

length measurements can often provide valuable insight 

into the thermal history. Track-length measurements may 

not be possible or practical in young or low-uranium 

apatite having low spontaneous track density, and this 

problem is compounded in rocks having a low yield of 

apatite. Samples from the Wagon Wheel well exemplify 

rocks in which apatite track-length measurements are not 

possible or practical. 

Provenance Studies 

In sedimentary rocks in which zircon has been unaffected by postdepositional annealing of fission tracks, the 

ages of the individual zircon grains provide much 

Thermal History and Provenance of Rocks, Pinedale Anticline E5 
information about the parent rocks of the sediments. 

Within a detrital grain suite, for example, it may be possible to correlate individual age populations to probable 

parent rocks in order to delineate sediment transport patterns in a basin (Zeitler and others, 1982, 1986; Hurford 

and others, 1984; Johnson, 1984; Baldwin and Harrison, 

1985; Yim and others, 1985; Baldwin and others, 1986). 

Changes in the zircon suite through time can be used to 

reconstruct changes in the sedimentation pattern, to 

define different source areas, and to reconstruct the uplift 

and erosion history of the parent rocks (Zeitler and 

others, 1982, 1986; Cerveny, 1986; Cerveny and others, 

1988). 

METHODS 

Laboratory Procedures 

Samples for fission-track analysis were taken from 

12 sandstone core samples recovered from the Wagon 

Wheel No. 1 well between depths of about 5,038 and 

16,090 ft (1 ,536-4,904 m), from rocks of the lower Tertiary Fort Union Formation to the Upper Cretaceous 

Rock Springs Formation (figs. 5, 6, table 1). 

The amount of core available for processing varied 

from 64 to 195 g. Apatite and (or) zircon were separated 

from the samples by using heavy liquid and magnetic 

separation techniques (Naeser, 1986, fig. 5). Grains 

coarser than about 74 p.m (200 mesh; very fine sand size) 

can be dated by using the fission-track method. Samples 

from the Wagon Wheel well were sieved to 7 4 to 180 p.m 

or 74 to 850 p.m ( + 200 to -80, or + 200 to -20 mesh) 

size fractions before they were treated with heavy liquids. 

This entire fraction was used when possible, but in several 

samples only the 7 4 to 125 p.m ( + 200 to -115 mesh) size 

fraction yielded a sufficiently pure separation (greater 

than 80 percent apatite or zircon) for dating. In most 

samples, the fraction lighter than methylene iodide required further treatment with a bromoform-methylene 

iodide mix (sp gr = 3.037) and with handpicking in order 

to improve the segregation of apatite from contaminating 

grains. In most samples from the Wagon Wheel well, zircon is more abundant than apatite. The apatite yield 

varied from less than 50 to about 200 grains, many of 

which were not suitable for fission-track analysis. 

Both apatite and zircon were dated by using the external detector method; a low-uranium-content muscovite 

was used as the detector, and samples were counted at 

X 1500 magnification using a X 100 oil immersion lens 

(Naeser, 1976, 1979a; Naeser and others, 1989). Thermal 

neutron fluence was determined from a calibrated 

muscovite detector covering a glass dosimeter1 placed at 

the top and bottom of each irradiation tube. The fluences 

were calibrated against the copper value determined by 

E6 Geology of Tight Gas Reservoirs, Wyoming and Colorado 

50 

(/) 2 0: 

UJ 

1-

UJ 

~ 

0 

.....J 

Q 3 

z 

::r::' 

b: 

UJ 

0 4 

TEMPERATURE, IN OF 

100 150 200 250 

• 1 

3. 5 

• 

.g 

e NN 1 

13. 

116 

24 

17,26 

20 40 60 80 1 00 1 20 

TEMPERATURE, IN oc 

4 

6 

0 

0 

0 

8 X 

1-

UJ 

10 w 

LL 

z 

::r::' 12 1-

a.. 

w 

0 

14 

Figure 5. Depth and present-day temperature (corrected) 

of samples for fission-track studies from Wagon Wheel No. 

1 well. Sample information in table 1; sample numbers 

without alphabetic prefix, WWH. 

the National Bureau of Standards (Carpenter and Reimer, 

1974), and the fluence for each sample was calculated by 

interpolation between the values determined for the standards. This method of fluence determination, if used in 

conjunction with a fission-decay constant of 7.03 X 10-11 

yr-1 (Roberts and others, 1968) and with laboratory procedures followed in the U.S. Geological Survey fissiontrack laboratory, consistently yields fission-track ages that 

are concordant with K-Ar ages of coexisting phases in 

rapidly cooled (volcanic and hypabyssal) rocks (Naeser 

and others, 1977). 

The apatite and zircon fission-track ages for each 

of the Wagon Wheel samples (table 2, fig. 6) were 

calculated by using the sums of the spontaneous and induced tracks counted in the individual grains in the sample to calculate the values for spontaneous track density 

(ps) and induced track density (pi), respectively, in the 

fission-track age equation (Price and Walker, 1963; 

Naeser, 1967). The uncertainty in the age was calculated 

by combining the Poisson errors on the spontaneous and 

induced track counts and on the track counts in the detector covering the dosimeter (Lindsey and others, 1975; 

McGee and others, 1985). These methods are commonly 

1

Zircon, National Bureau of Standards (NBS) glass standard 

SRM 962, 37.38±0.08 ppm uranium; apatite, NBS glass standard SRM 

963, 0.823±0.002 ppm uranium, or USGS glass standard calibrated 

against SRM 963. 
oc OF 

>-

50 Fort Union and younger formations ~ 

? ~ 

--~ 0:: w 

60 ---------------- fUnnamed Tertiary sandstone beds 

? t-----1 3 182 Ma 

w 

.s 5 -~---------------~-

.9 • 9 

DECREASING 

~APATITE AGE 

~ 

0:: 

:::J 

~ 80 

~ffi a... 

~ 

w 

fFigure 6. Fission-track ages and corrected present-day 

temperatures for detrital apatite (squares) and zircon 

(triangles) from sandstones from Wagon Wheel No. 1 well, 

as compared to theoretical pattern of ages expected in rocks 

that have undergone cooling (fig. 4). Sample WWH-17 is 

from upper part of Ericson Sandstone, sample WWH-26 

from lower part. Dashed horizontal bar in each stratigraphic 

20 

H 

Lance Formation 

17 Ericson 

Sandstone ~==--~ ===== 

40 60 80 100 

FISSION-TRACK AGE, IN Ma 

(f) 

:::J 

0 

w 

u 

~ w 

0:: 

u 

unit indicates approximate time of deposition of unit as 

determined from K-Ar ages of Cretaceous ammonite zones 

(Obradovich and Cobban, 1975; recalculated for new K-Ar 

constants); correlations shown in Weimer (1961 ), Law 

(1984b), and Law and Johnson (this volume); time scale of 

Palmer (1983). Sample information in table 1; sample 

numbers without alphabetic prefix, WWH. 

Table 1. Depth, stratigraphic position, and present-day temperature of samples from Wagon Wheel No. 1 well, 

northern Green River basin, Wyoming 

[Location of well shown in fig.l] 

Sample 

Depth 

(feet} 

Depth 

(meters} Stratigraphic unit 1 

Present-day 

temQerature (

0 C} 

Log Corrected2 

WWH-1 

WWH-3 

WWH-5 

WWH-9 

NN1 

WWH-13 

5,038 

7,061 

7,340 

8,087 

8,960 

1,536 

2,152 

2,237 

2,465 

2,731 

3,098 

Fort Union Formation 44 50 

Unnamed Tertiary sandstone beds 

Unnamed Tertiary sandstone beds 

Lance Formation 

59 66 

61 68 

67 74 

Lance Formation 73 81 

10,165 Lance Formation 81 90 

WWH-16 11,027 3,361 Lance Formation 84 96 

WWH-24 11,057 3,370 Lance Formation 84 96 

WWH-17 13,100 3,993 Ericson Sandstone (upper part) 101 109 

WWH-26 13,197 4,022 Ericson Sandstone (lower part) 102 110 

WWH-21 14,950 4,557 Rock Springs Formation 114 121 

NN2 16,090 4,904 Rock Springs Formation 122 128 

1From Law (1984b) and B.E. Law (oral commun., 1984, 1987). Fort Union Formation indicates interval described as "Fort Union 

and younger formations" in Law (1984b). 

2see text for description of correction technique. 

used to calculate age and uncertainty in fission-track 

dating, but they may not be strictly applicable to 

sedimentary rock samples in which the detrital grains 

were most likely derived from more than one age 

population. The "age" calculated from the ages of 

individual detrital grains in a sedimentary rock is in 

fact a composite age and has no real meaning in a 

stratigraphic sense. 

Despite the wide range in individual grain ages 

in most of the samples (Naeser, 1986, table 2), the 

Thermal History and Provenance of Rocks, Pinedale Anticline E7 
Table 2. Apatite and zircon fission-track ages, Wagon Wheel No. 1 well, northern Green River basin, Wyoming 

[Location of well shown in figure 1; sample locality data shown in table 1. ps, spontaneous track density; pi, induced track density (two times 

measured density); t, tracks; n, neutrons; cj>, neutron fluence (see text for method of determination); number in parentheses is number of tracks 

counted] 

Number of Ps Pi <!> Age1 

Sam~le grains counted (xl o6t/cm2) (xl o6t/cm2) (xl 0 15n/cm2) (xl o6 years) 

A atite 

WWH-5 8 0.299 (94) 5.73 (902) 6.72 (2,354) 20.9±4.6 

WWH-9 10 0.213 (91) 2.31 (494) 6.61 (2,354) 36.3±8.4 

NN1 8 0.321 (77) 5.45 (654) 10.4 (3,723) 36.5±8.9 

WWH-13 9 0.093 (28) 2.53 (380) 6.51 (2,354) 14.3±5.6 

WWH-16 6 0.047 (5) 2.23 (119) 6.40 (2,354) 8.0±7.3 

WWH-24 6 0.036 (8) 2.05 (230) 3.67 (2,726) 3.8±2.7 

WWH-26 11 0.023 (8) 2.03 (354) 3.61 (2,726) 2.4±1.7 

WWH-21 9 0.025 (5) 3.50 (357) 6.30 (2,354) 2.6±2.3 

NN2 6 0 {0} 4.25 {251} 10.3 {3,723} 0 

Zircon 

WWH-3 5 22.33 (394) 7.43 (65) 1.03 (4,424) 181.9±49.0 

WWH-5 9 12.48 (1,578) 6.53 (413) 0.889 (6,340) 100.9±11.4 

WWH-9 9 10.28 (1,078) 6.60 (346) 1.01 (4,424) 94.4±12.0 

WWH-13 9 11.04 (1,472) 6.92 (461) 1.01 (4,424) 95.8±10.6 

WWH-17 9 9.69 (931) 7.91 (380) 1.01 (4,424) 73.6±9.2 

WWH-21 4 13.55 {477} 10.12 {142} 1.01 {4,424} 100.7±19.5 1 Age is ±2 standard deviations. Age is calculated from fission-track age equation (Price and Walker, 1963; Naeser, 1967) using sums of 

the spontaneous and the induced track counts obtained for all grains counted in sample and the following values: 'A-o=1.55x10-10yr-1; 

Ap=7.03x1o-17yr-1 (Roberts and others, 1968); a=580xlo-24cm2; l=7.252x1o-3. Standard deviation is calculated by combining Poisson 

errors on spontaneous and induced counts and counts in detector covering dosimeter (Lindsey and others, 1975; McGee and others, 1985). 

composite age calculated for the first six grains counted 

in any given sample gives a reasonable estimate of the 

composite "age" for that sample and does not change 

significantly with the inclusion of additional individual 

grain ages. 

Present-Day Temperatures in the 

Wagon Wheel No. 1 Well 

Two sets of temperatures are given in table 1: (1) 

temperatures interpolated from log data and (2) corrected 

temperatures that have been adjusted from the log data 

by T.H. McCulloh (written commun., 1982) using the 

procedures outlined in Bostick and others (1978, p. 74-75) 

and McCulloh and Beyer (1979). 

The uncertainty associated with individual corrected 

temperatures is about ± 3 OC from the surface to depths 

of 10,000 ft (3,048 m) and about ± 5 °C from depths of 

10,000 to 19,000 ft (3,048-5,791 m) (T.H. McCulloh, oral 

commun., 1982). Corrected temperatures are 6-12 °C 

higher than temperatures interpolated from the drill-hole 

logs and closely match temperatures that would be obtained if correction procedures of the American Association of Petroleum Geologists Geothermal Survey of 

North America were used (Kehle and others, 1970; Kehle, 

1972). 

E8 Geology of Tight Gas Reservoirs, Wyoming and Colorado 

FISSION-TRACK DATA 

Apatite Ages 

Apatite ages were determined for nine samples 

from the Wagon Wheel well (fig. 6, table 2). Sample 

WWH -17, from the upper part of the Upper Cretaceous 

Ericson Sandstone, yielded insufficient apatite for an age 

determination. 

The time of deposition of Cretaceous sedimentary 

units sampled in the Wagon Wheel well is known with 

some confidence because the units can be correlated to 

ammonite zones, the ages of which have been determined 

by K-Ar dating of associated bentonite layers (Weimer, 

1961; Obradovich and Cobban, 1975; Law, 1984b; Law 

and Johnson, this volume). The unnamed Tertiary rocks 

(table 1) underlie the Paleocene Fort Union Formation 

and thus are older than the Eocene-Paleocene boundary 

(57 .8 Ma) and younger than the Tertiary-Cretaceous 

boundary (66.4 Ma) (Palmer, 1983). 

Comparison of stratigraphic and fission-track ages 

in the Wagon Wheel well indicates that all of the composite apatite ages calculated for the samples (fig. 6) and 

almost all of the individual apatite grain ages are younger 

than the stratigraphic age of the sedimentary unit from 

which they were collected. This relationship indicates that 

significant annealing of the apatite has occurred during 
burial of the sediments. If the zone of no annealing shown 

in figures 3 and 4 is preserved in the Wagon Wheel well, 

it must be above the shallowest sample analyzed 

(WWH-5; 7,340 ft, 2,237 m) because even that sample 

contains partially annealed apatite. 

Apatite fission-track ages calculated for samples 

from the Wagon Wheel well show two distinct trends (fig. 

6). In the upper part of the sampled interval, the apatite 

ages generally decrease with increasing temperature and 

approach zero at a temperature of slightly more than 

96 °C. At this point, the trend in ages changes abruptly 

and the ages remain almost constant or decrease slightly, from about 4 to 2 Ma, over a temperature interval 

of at least 20 °C, down through sample WWH-21. 

Apatite from the deepest sample in the drill hole (NN2) 

is totally annealed; that is, it contained no spontaneous 

fission tracks and yielded a zero age. 

Zircon Ages 

Zircon ages were determined for six samples from 

the Wagon Wheel well (figs. 6, 7, table 2). Sample 

WWH-16 yielded insufficient zircon for an age determination, and sample WWH -1 could not be dated 

because the zircon is metamict. 

The composite zircon fission-track age calculated 

for each of the Wagon Wheel samples and all but a few 

of the individual zircon grain ages are equal to or older 

than the depositional age of the unit from which they were 

sampled (figs. 6, 7). This relationship suggests that no 

significant annealing of the zircon has occurred in any 

of the samples since deposition. 

Lack of significant zircon annealing is also suggested by the fact that the composite zircon ages calculated for most of the samples are indistinguishable from 

one another (table 2, fig. 7). Some variation exists in the 

range of the individual zircon grain ages in the samples 

(fig. 7), but, for most samples, no obvious correlation 

can be made between the calculated sample age or the 

range in individual grain ages and the present-day 

temperature (or depth) of the sample. Samples WWH-17, 

-3, and -1 appear to be exceptions. Sample WWH-17, 

from the upper part of the Ericson Sandstone, yielded 

a significantly younger composite zircon age than did 

many of the samples, and it apparently contains a lower 

percentage of old (older than about 110 Ma) zircons (fig. 

7). As noted below, this may result, at least in part, from 

the provenance of this sandstone. Samples WWH-1 and 

WWH-3, from the upper part of the Tertiary section 

sampled in the Wagon Wheel well, contain detrital zircon 

suites that appear to be older than those in the underlying rocks. All of the zircons in WWH-1 are metamict 

and are probably of considerable age. In sample 

WWH-3, no zircon grains younger than 100 Ma were 

WWH-3 r 81.9±48.9 ~a 

0

1

[ 

L-------------~~--~--~v ~-4 0 0 A. v 

ADO ~8 ~~ 

UJUJ 

2031 ~WWH-5 ~ --i ~··.. l' 00.9± 11.4 M' ~ ~ n ~·~ 

~--~Cl-·~~~0~. ~0~--------\ ~5~ 

WWH-9 H 194.4± 1 2.0 Ma 

~f~ __ ~(J_·_··~o~·~D~------~~ Jl___~~ (:) 

z WWH-13 

::5o: 0 

u.. 

i ~t n 0 n 1

95.8± 10.6 Ma 

n 

WWH-17 1

73.6±9.2 Ma I--f 

~[ 0 D D 0 0 

WWH-21 

~t 7 1100.7±19.5 Ma 

* ~--~ 

0 0 \ 

40 60 80 100 120 140 

~ 

160 

AGE, IN Ma 

(/) 

l9Z 

L~Q 0::1-

a..<( 

(/)~ 

~0:: 

uo 

Ou.. 

0:: 

~00 600 

Figure 7. Histograms offission-track ages of individual zircon 

grains in six samples from the Wagon Wheel No. 1 well 

(Naeser, 1986). Composite age (± 2 standard deviations) 

calculated for each sample (fig. 6, table 2) indicated by arrow. 

Bar indicates approximate time of deposition of unit from which 

sample was collected (see text and fig. 6). Asterisk(*) indicates 

age is younger than stratigraphic age and does not overlap 

stratigraphic age at two standard deviations; all other ages are 

equal to or older than stratigraphic age. 

observed and the percentage of metamict grains is significantly higher than in the underlying samples. Again, the 

provenance of these units is probably different. 

THERMAL HISTORY 

Because of the temperature-time dependency in annealing (fig. 2), the degree of annealing observed in the 

apatite from the upper part of the Wagon Wheel well (as 

deep as the almost totally annealed apatite at about 

96 °C) may have been produced by the rocks either being near their present-day (or a lower) temperature for 

a long period of time (at least 108 yr) (see fig. 2), or being at a higher temperature for a shorter time before they 

cooled to their present-day temperature. 

The rocks in the upper part of the drill hole are 

latest Cretaceous (Lance Formation) and younger in age 

(less than about 70 Ma) (Weimer, 1961; Obradovich and 

Thermal History and Provenance of Rocks, Pinedale Anticline E9 
Cobban, 1975; Law, 1984b; Law and Johnson, this 

volume) (fig. 6). According to the burial reconstruction 

of Pollastro and Barker (1986, fig. 10) for the Pinedale 

anticline, the rocks were buried rapidly to maximum 

depth at about 37 Ma, and since that time no net sediment deposition has occurred. Even if the rocks had remained at maximum depth until the present time, such 

that present-day temperature equaled maximum 

temperature, they would have been near that temperature 

for considerably less than 1 os years (see also Dickinson, 

this volume). We therefore are left with the second explanation; that is, cooling has occurred in the rocks. 

The trend of the apatite ages at temperatures greater 

than about 96 °C provides evidence for the nature of this 

cooling. These ages vary little with depth down through 

sample WWH-21 and are believed to record the timing 

of an episode of relatively rapid cooling (the zone of uplift 

and (or) cooling shown in figures 4 and 6). The apatite 

ages suggest that at least the latest phase of cooling in 

the rocks began about 4-2 Ma and involved a relatively 

rapid decrease in temperature of at least 20 °C. 

The total amount of cooling since 4-2 Ma cannot 

be interpreted with certainty because the temperature of 

the deepest sample in the drill hole (NN2) is presently sufficiently high (128 °C) that, even if the apatite had initially retained tracks following cooling, these tracks 

would probably have been subsequently totally annealed. 

However, fission-track data from shallower samples in 

the Wagon Wheel well, above the zone of cooling ages, 

help place some constraints on the amount of cooling 

since 4-2 Ma. For example, apatite in sample WWH-13, 

which is presently at a (corrected) temperature of 90 °C, 

yields a composite age of 14.3 ± 5.6 m.y., indicating that 

it was not totally annealed prior to the Pliocene cooling. 

Pollastro and Barker's (1986) burial history diagram suggests that these rocks were near their maximum temperature from the end of the Eocene until the onset of cooling 

in the Pliocene. Given that long a heating duration, it 

is unlikely that apatite in WWH -13 could have retained 

fission tracks if it had been at temperatures much higher 

than about 110-115 °C prior to cooling. Therefore, 

assuming the adjusted temperature for WWH-13 of 

90 °C is correct, it is unlikely that total cooling since 

4-2 Ma has exceeded about 20-25 °C. 

Neither the rate nor exact time of cooling can be 

determined precisely because of the large analytical uncertainty in the apatite ages in this part of the drill hole (table 

2). However, the temperature of sample WWH-24, which 

yielded an apatite age of ~4 Ma and is at or just above 

the top of the cooling zone, is sufficiently low (96 °C) 

that it is unlikely the apatite has undergone significant 

age reduction as a result of postcooling annealing. Within 

the analytical uncertainty, it therefore is unlikely that the 

period of rapid cooling began significantly earlier than 

~4 Ma. If the cooling occurred in response to the 

E10 Geology of Tight Gas Reservoirs, Wyoming and Colorado 

combined effect of uplift and erosion, however, then the 

actual uplift may have begun somewhat earlier. 

Pollastro and Barker (1984a, b, 1986) studied the 

thermal history in the Wagon Wheel well by using several 

other paleotemperature indicators. Depth-temperature 

curves determined from mean random vitrinite reflectance 

measured in thin coaly seams in the rocks and from 

illite/smectite transformations in the sandstone and shale 

layers are in good agreement and indicate that the rocks 

were subjected to temperatures 30-50 OC higher sometime 

in the past. The vitrinite reflectance data suggest that 

cooling of the rocks resulted from uplift and erosion 

rather than from a major decrease in geothermal gradient 

(Pollastro and Barker, 1984a, 1986). 

The fission-track data indicate that an event 

resulting in approximately 20-25 OC cooling began in this 

area about 4-2 Ma. The timing of the cooling as indicated 

by the fission-track data is consistent with evidence 

(reviewed in Dickinson, this volume) for widespread uplift 

and erosion in the Green River basin beginning in the late 

Pliocene. Fission-track dating of apatite from rocks in 

other parts of the basin should help clarify the extent and 

magnitude of this uplift and erosion, but initial attempts 

at such a study in the Pacific Creek area (fig. 1) were unsuccessful because the rocks yielded insufficient apatite 

for fission-track analysis. 

PROVENANCE OF THE SEDIMENTS 

In most of the rocks sampled in the Wagon Wheel 

No. 1 well, the minimal variation in the range of ages 

of individual zircons in the detrital grain suites and in 

the proportion of grains of various ages suggests that the 

dominant source of sediments in the Pinedale anticline 

remained unchanged through most of the Late Cretaceous 

and earliest Paleocene. 

Apparent exceptions are samples WWH -17, 

WWH-3, and WWH-1. Independent evidence suggests 

that the young age of zircon in WWH-17 results, at least 

in part, from the provenance of this sandstone rather than 

from postdepositional annealing. Sample WWH -17 was 

collected from the upper part of the Ericson Sandstone. 

Mineralogically and petrographically the Ericson Sandstone, particularly the upper part of the unit, is very different from most other Upper Cretaceous units in the 

northern Green River basin (Rice and Gautier, 1983; 

Edman and Surdam, 1986; Law and others, 1986; 

Pollastro, this volume). It has been suggested that these 

differences, although partly diagenetic in origin, can 

largely be explained by a major change in detrital source 

material for this unit (Rice and Gautier, 1983, p. 4-24; 

D.L. Gautier, U.S. Geological Survey, oral commun., 

1984; Pollastro, this volume). In the Wagon Wheel well, 

the upper part of the Ericson Sandstone is further 
distinguished from the other Upper Cretaceous sandstones that were sampled for fission-track analysis by the 

fact that it yielded little or no detrital apatite. 

Another possible explanation for younger zircon in 

the Ericson Sandstone is that the zircon was slightly 

annealed at some time during the burial history by localized anomalously high temperatures. However, this interpretation seems unlikely because illite/ smectite 

transformation and vitrinite reflectance data from 

sandstone and shale in the upper part of the Ericson Sandstone in the Wagon Wheel well show no evidence for 

anomalously high temperatures in this unit (Pollastro and 

Barker, 1986; R.M. Pollastro, oral commun., 1989; 

Dickinson, this volume). 

The anomalous zircon ages of WWH -1 and 

WWH-3 may indicate changes in source terrane related 

to the structural evolution of the Wind River Mountains 

to the northeast. Uplift of this range began as part of 

the major Laramide activity in the latest Cretaceous, and 

by early to middle Paleocene this uplift and erosion had 

breached the crystalline core of the Wind River Mountains and produced an influx of arkosic material into the 

Green River basin (Prensky, 1984; Law and Johnson, this 

volume). A strong anomaly in the gamma-ray log for the 

Wagon Wheel well at 7,225 ft (2,202 m), between the 

depths at which samples WWH-3 and WWH-5 were collected, has been attributed to this arkose influx (Prensky, 1984, this volume). The decrease in young zircons 

and the increase in percentage of metamict grains in 

WWH-3 relative to WWH-5 are probably a direct consequence of this change in sediment source, which 

culminated in domination of the zircon suite in WWH -1 

by metamict grains. 

CONCLUSIONS 

Vitrinite reflectance and illite/smectite transformation measurements in the Wagon Wheel No. 1 well suggest that present-day temperatures are about 30-50 °C 

cooler than the maximum p-aleotemperature (Pollastro 

and Barker, 1984a, b, 1986). Fission-track apatite ages 

of sandstones from the well support the hypothesis of 

higher paleotemperatures and suggest that the latest phase 

of cooling of the rocks began about 4 to 2 Ma, during 

the Pliocene, and has produced a relatively rapid 

temperature decrease of at least 20 °C. 

Zircon ages from the well suggest that the dominant source of sediments in the Pinedale anticline did not 

change through most of the Late Cretaceous and earliest 

Paleocene. A significant change in provenance during the 

Paleocene was probably related to unroofing of the 

crystalline core of the Wind River Mountains to the 

northeast. 

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to the study of fission-track fading, in Zartman, R.E., 

ed., Short papers of the Fourth International Conference, 

Geochronology, Cosmochronology, Isotope Geology: 

U.S. Geological Survey Open-File Report 78-701, 

p. 467-468. 

Thermal History and Provenance of Rocks, Pinedale Anticline Ell 

Chapter F 

Analysis of Vitrinite Maturation and 

Tertiary Burial History, 

Northern Green River Basin, Wyoming 

By WARREN W. DICKINSON 

Prepared in cooperation with the U.S. Department of Energy 

U.S. GEOLOGICAL SURVEY BULLETIN 1886 

GEOLOGY OF TIGHT GAS RESERVOIRS IN THE PINEDALE ANTICLINE AREA, WYOMING, 

AND AT THE MULTIWELL EXPERIMENT SITE, COLORADO 
CONTENTS 

Abstract Fl 

Introduction Fl 

Depositional and tectonic history Fl 

Geothermal gradients FS 

Vitrinite reflectance data F6 

Burial and thermal reconstructions F6 

Recent folding F8 

Elevated paleogeothermal gradient F9 

Vitrinite maturation models F9 

Summary and conclusions F11 

References cited F11 

Appendix 1. Vitrinite reflectance values for Wagon Wheel, WASP, and Pacific Creek 

wells and equations used to calculate paleotemperatures F16 

FIGURES 

1. Map showing areas of peneplained surfaces on southwestern flank of Wind 

River Mountains, northern Green River basin F2 

2. Chart showing burial history for WASP, Wagon Wheel, and Pacific Creek 

wells F2 

3. Cross sections showing general geologic structures and projected surfaces of 

Wind River peneplain F4 

4. Graphs showing vitrinite reflectance values and Lopatin reconstructions for 

top of Ericson Sandstone at WASP, Wagon Wheel, and Pacific Creek 

wells F7 

5. Cross section showing relationships between stratigraphy, structure, thermal 

maturation, and burial in northern Green River basin F8 

6. Lopatin reconstruction diagram for a local increase in geothermal gradient in 

Wagon Wheel area F9 

7. Graphs showing linear regression least-squares fits of maximum 

paleotemperatures for WASP, Wagon Wheel, and Pacific Creek wells 

calculated by using selected vitrinite maturation models FlO 
Analysis of Vitrinite Maturation and 

Tertiary Burial History, 

Northern Green River Basin, Wyoming 

By Warren W. Dickinson1 

Abstract 

Vitrinite reflectance data from three wells in the northern 

Green River basin are used in conjunction with a Lopatin thermal maturation model to evaluate Tertiary burial history. 

Geologic evidence suggests that, on a regional scale, present 

and past geothermal gradients in the Green River basin are 

similar and that heating or cooling of the rocks has occurred 

as the result of surface temperature changes, burial and erosion, or vertical movement of subsurface waters. After early 

Eocene time, little deposition occurred at the northern end of 

the basin, but on the southwestern flank of the Wind River 

mountains about 600 m of sediments was deposited. Regional 

erosion occurred during and after Pliocene time. The Lopatin 

model and the vitrinite reflectance surface intercept generally 

support the geologic evidence. 

High vitrinite reflectance values in the area of the Pinedale 

anticline suggest erosion of more than 1,200 m of basin fill, 

a significantly greater amount than stratigraphic evidence can 

account for. lsoreflectance lines, which parallel the anticlinal 

structure, suggest that these high vitrinite reflectance values 

were either physically elevated into place by recent folding or 

generated by hotter than present-day temperatures around the 

anticline. The choice between these alternatives is not clear, 

but if the paleogeothermal gradient was 2 °C/km hotter than 

the present-day gradient, results of maturation modeling support geologic data. 

Vitrinite reflectance data are used in several vitrinite 

maturation models to calculate paleotemperature gradients. 

Time-independent models show anomalously high paleogeothermal gradients, and time-dependent models yield gradients 

more similar to the present-day gradient and more in accord 

with the geologic evidence. 

INTRODUCTION 

In the northern Green River basin of Wyoming 

(fig. 1), much of the tectonic and depositional history 

since early Eocene time has been lost to erosion. In spite 

1

Research School of Earth Science, Victoria University 

P.O. Box 600, Wellington, New Zealand 

of this loss, recently acquired stratigraphic and geochemical data provide considerable insight into this history. 

An understanding of the geologic history of the basin and 

adjacent thrust belt is important to the exploration of 

hydrocarbon resources. 

Vitrinite reflectance (Rm) data for a large number 

of wells in the basin recently have been collected and 

mapped by Law (1984), Pawlewicz and others (1986), 

Merewether and others (1987), and Lickus and others 

(this volume). Vitrinite reflectance records thermal 

history, which is, in turn, a function of burial history and 

present and past geothermal gradients. Models of vitrinite 

maturation can be used to evaluate the burial history of 

the basin determined from other geologic evidence. 

The WASP (Wyoming Atomic Stimulation Project), Wagon Wheel, and Pacific Creek wells (fig. 1) were 

used in this study because they penetrate Upper Cretaceous and Tertiary rock sequences that are representative 

of the basin and because vitrinite reflectance data for the 

wells are abundant. The goals of this study were threefold: (1) to synthesize published stratigraphic and 

geochemical data into a Tertiary burial history for the 

northern Green River basin; (2) to evaluate the depth and 

duration of burial by using the Lopatin model of vitrinite 

maturation; and (3) to test time-independent and timedependent models of vitrinite maturation by comparing 

predicted paleogeothermal gradients and present-day 

thermal gradients. 

DEPOSITIONAL AND TECTONIC HISTORY 

Structurally, the deepest part of the northern Green 

River basin contains about 10,000 m of sedimentary 

rocks and is east of the Pinedale anticline (Law and 

Johnson, this volume, fig. 1). About 7,000 m of these 

rocks are Late Cretaceous, Paleocene, and Eocene in 

age and include the Hilliard Shale, Rock Springs Formation, Ericson Sandstone, and Lance, Fort Union, 

and Wasatch Formations. Most of these formations have 

been truncated by thrust faults and are not exposed 

Vitrinite Maturation and Tertiary Burial History, Northern Green River Basin Fl 
I ·. 

~ 

~ 

• 

EXPLANATION 

Wind River peneplain surfaces 

Fremont surface 

Peneplain no.4 

Peneplain no.3 

Crest of Wind River Mountains 

Location of well used for study 

Wyoming 

f77Lstudy L......J area 

Utah Colorado 

D' 

c 

" c

"TT 

-l 

20 40 KILOMETERS 

Figure 1. Areas of peneplained surfaces on the southwestern 

flank of the Wind River Mountains (Westgate and Branson, 

1913; Blackwelder, 1915; Baker, 1946), northern Green River 

basin. Peneplained surfaces on northeastern flank of mountains are not as extensive. Lines of section A-A', 8-8', C-C' 

(fig. 3) and 0-0' (fig. 5) and locations of WASP, Wagon 

Wheel and Pacific Creek wells also shown. 

along the margins of the basin; lower and middle Eocene 

rocks cover most of the surface. 

Studies by Armstrong and Oriel ( 1965) and Dorr 

and others (1977) indicate that large volumes of the Upper 

Cretaceous and Tertiary sediments accumulated during 

periods of tectonic activity and uplift around the basin. 

Profile modeling by Shuster and Steidtmann (1984) suggests that basin subsidence occurred when the lithosphere 

was depressed by the weight of the Darby and Prospect 

thrust plates on the west and the Wind River thrust plate 

on the east. Erosion of these rising thrust plates and 

subsequent deposition in the adjacent depression redistributed this weight and modified the basin into its present shape. Burial histories for the three study wells (fig. 

2) were synthesized from West (1969), Zeller and Stephens 

(1969), Dorr and others (1977}, Law (1981), R. Bottjer 

(University of Wyoming, written commun., 1982), Naeser 

(1984}, and Steidtmann and others (1986). 

F2 Geology of Tight Gas Reservoirs, Wyoming and Colorado 

WASP well 

Time (m.y.) 

Eroded by regional 

uplift 

<150m 

0 

Camp Davis Formation 

<150m 

17 

PRESENT-DAY 

SURFACE 

51 

Pass Peak Formation 

460 m 

53 

Hoback Formation 

1,250 m 

60 

Erosion 

~100m 

65 

Upper Cretaceous, 

undivided 

1,847 m 73 

Ericson ~ 

Sandstone 

102m 

76 

Sohare Formation 

and Bacon Ridge 

Sandstone 

1,071 m 

86 

Hilliard Shale 

Wagon Wheel well 

Time (m.y.) 

0 

Eroded by regional 

uplift, 4 70 m; 600 m 

of structural erosion 

and uplift possible 

Bridger, Wasatch, 

and Green River 

Formations 

305m 

PRESENT. DAY 

SURFACE 

50 

Wasatch Formation 

1,370 m 

54 

Paleocene, undivided 

926 m 

65 

Erosion 

"'='100m 

68 

Lance Formation 

1,685 m 

73 

Ericson ~ 

Sandstone 

122m 

76 

Rock Springs 

Formation 

1,295 m 86 

Hilliard Shale 

Pacific Creek well 

Time(m.y.) 

0 

Eroded by regional 

uplift, 600 m 

3 

17 

24 

47 

Bridger, Wasatch, 

and Green River 

Formations 

265m 

PRESENT· DAY 

SURFACE 

48 

Green River and 

Wasatch Formations 

1,140m 

54 

Paleocene, undivided 

1,530 m 

65 

Erosion 

"'='100m 

68 

Lance Formation 

1,155m 

73 

Ericson ~ 

Sandstone 

146m 

76 

Rock Springs 

Formation 

725 m 

86 

Baxter Shale 

Figure 2. Burial history for WASP, Wagon Wheel, and Pacific 

Creek wells. Stratigraphy modified from West (1969), Zeller and 

Stephens (1969), Dorr and others (1977), Law (1981 ), R.J. Bottjer (University of Wyoming, written commun., 1982), Naeser 

(1984), Steidtmann and others (1986). Corrected elevation for 

the top of the Ericson Sandstone of -1,176 m mean sea level 

(B.E. Law, written commun., 1986) used for the WASP well. 

Thicknesses and time spans not shown in correct proportions. 

Hatching indicates hiatus. Location of wells shown in figure 1. 

From the latest Cretaceous up to the present time, 

the area of the Green River basin has been an intermontane basin and the site of sporadic synorogenic deposition. Erosion and depositional rates are highly variable 

throughout the basin because the basin receives sediment 

from the west, north, and east at a rate in accordance 
with local tectonic activity. For example, unconformities 

on the Rock Springs uplift suggest that major erosional 

events occurred between deposition of Cretaceous and 

Paleocene rocks and Paleocene and Eocene rocks 

(Roehler, 1978). In the Pinedale area, there is evidence 

for an unconformity between Cretaceous and Paleocene 

rocks (Law and Nichols, 1982; Law and Johnson, this 

volume). For purposes of burial reconstruction, 100m 

of Cretaceous rocks is estimated to have been eroded 

before deposition of Tertiary rocks (fig. 2). 

The amount of deposition after early Eocene time 

is difficult to determine because of the current cycle of 

widespread erosion. Post-Eocene deposition occurred at 

various rates and times throughout the basin, but it was 

minimal because major tectonism ended by mid-Eocene. 

Dorr and others ( 1977) and Shuster and Steidtmann 

(1984) suggested that no deposition occurred in the northern end of the basin after early Eocene time, probably 

because of the rapid filling of the narrow northern end 

of the basin and subsequent bypassing of sediment into 

the broad southern end. It is not coincidental, therefore, 

that the age of the surficial sediments in the basin (excluding Pleistocene and Holocene deposits) generally 

becomes younger toward the southeast along the Wind 

River Mountains. 

In the southern part of the basin, post-Eocene 

deposition apparently continued at a slow rate. The crest 

of the Wind River Mountains may have been uplifted in 

Oligocene time by as much as 2,000 m, and erosion of 

this uplift could produce the observed Oligocene and Miocene tectogenic conglomerates (Steidtmann and others, 

1986; Steidtmann, 1987). Zeller and Stephens (1964) 

measured a maximum of 335 m of Miocene and possibly 

Pliocene deposits in the area of the Pacific Creek well, 

and farther to the east Denson and Pipiringos (1974) 

measured a maximum of 922 m of Oligocene, Miocene, 

and possibly Pliocene rocks. Careful scrutiny of all postEocene depositional data is necessary because burial 

depths affect the temperature of the rocks and hence 

maturation modeling. 

In the area of the WASP well, the lower Eocene 

(about 51 Ma) Pass Peak Formation covers the surface, 

and, except for Pleistocene deposits, rocks younger than 

early Eocene are absent from the northern end of the 

basin (Dorr and others, 1977). A thin veneer of Miocene 

and Pliocene deposits, such as the Miocene Camp Davis 

Formation (fig. 2), possibly extended into the WASP 

area, but most of these deposits are in areas adjacent to 

uplifts in the north and east (Dorr and others, 1977). 

West (1969) mapped surface rocks in the Wagon 

Wheel area as the western facies of the New Fork Tongue 

of the Wasatch Formation, which is early Eocene in age 

(about 50 Ma). In the Wagon Wheel area, about 305m 

of lower and middle Eocene rocks presumably was eroded 

from the surface because, directly to the east and 

stratigraphically above, West (1969) measured roughly 

105m of Wasatch and Green River Formations and about 

90 m of lower Bridger Formation, and McGrew (1959) 

measured 110 m of upper Bridger Formation. These 

thicknesses may be slightly excessive if the Pinedale anticline was structurally high during early and middle Eocene 

time. With the exception of about 30m of post-Bridger 

conglomerate (McGrew, 1959; Bottjer, 1984), evidence 

for post-middle Eocene deposition is lacking in the 

Wagon Wheel area. Bottjer correlated this conglomerate 

to the South Pass Formation, which Steidtmann and 

others (1986) and Zeller and Stephens (1969) recognized 

as Miocene in age. If we assume, however, that the 

amount of post-Eocene deposition increased from the 

WASP to Pacific Creek areas, then some deposition 

probably occurred in the area between them, in the 

Wagon Wheel area. A conservative estimate for the 

amount of this deposition is 165 m, about one-half of 

the amount measured at Pacific Creek (fig. 2). The total 

thickness of post-Eocene rocks that could be missing from 

the Wagon Wheel area is about 470 m. 

In the Pacific Creek area, Zeller and Stephens 

(1969) mapped the surface rocks as undivided Wasatch 

Formation and placed them about 20m stratigraphically below the base of the Laney Member of the Green 

River Formation of middle Eocene age (48 Ma). They also 

measured about 45 m of Laney Member and about 200 m 

of Bridger Formation. These thicknesses, together with 

a maximum thickness of 335 m for Miocene and possibly 

Pliocene rocks, suggest that about 600 m of sediments 

has been removed from the Pacific Creek area. 

In a number of early studies (Westgate and Branson, 1913; Blackwelder, 1915; Baker, 1946), the Wind 

River peneplain was used to help interpret the amount 

of post-Eocene deposition around the Wind River Mountains. The peneplain is a broad dissected plateau, 

30-50 km wide, parallel with almost the entire western 

and southern flanks of the mountain range. The peneplain slopes to the southwest, having gradients of from 

40 to 10 m/km away from the crest of the range, and 

to the southeast, parallel with the crest of the range. 

Present-day elevations of the peneplain range from 

3,700-3,300 m to 2,400-2,100 m (fig. 3). Although the 

peneplain is composed of as many as five, difficult to 

recognize terracelike steps, each of which represents an 

erosional subcycle (Westgate and Branson, 1913; Baker, 

1946), it is essentially the result of a single, major erosional event. 

Blackwelder (1915) interpreted the Wind River 

peneplain as a Pliocene (more recent epoch) erosional 

feature that stands in relief because of selective denudation of basin sediments during the Quaternary (glacial 

epoch). If his interpretation is correct, then a projection 

of the peneplained surface over the basin should yield an 

estimate of pre-Pliocene basin fill. Dorr and others (1977) 

Vitrinite Maturation and Tertiary Burial History, Northern Green River Basin F3 
A 

KM 

KM 

T 

B 

Pass Peak 

WIND RIVER 

THRUST 

----

WIND RIVER 

THRUST 

----

Mz+T 

Wagon Wheel well 

c 

KM 

A 

Tw 

WIND RIVER 

THRUST 

WIND RIVER 

PENEPLAIN 

p£ 

C' 

Granite 

Peak 

PacificCreekwell \ CONTINENTAL 

A I FAULT 

- \--;: p£ ~ Tm 

PROJECTED 

PENEPLAIN 

10 

I 

15 

I I 

p£ 

20 KILOMETERS I 

10 MILES 

VERTICAL EXAGGERATION X 7 

B' 

FT 

10,000 

6,000 

FT 

10,000 

6,000 

2,000 

A' 

FT 

14,000 

10,000 

6,000 

Figure 3. Cross sections showing general geologic structures and projected surfaces of the Wind 

River peneplain (dashed lines). Projected surface in section A-A' indicates that about 350 m of 

pre-Pliocene fill has been eroded from area of WASP well. Projected surfaces in sections B-8' and 

C-C' indicate that estimated amounts of basin fill are inconsistent with stratigraphic and vitrinite 

reflectance data. Section A-A' modified from Dorr and others (1977). Seismic studies (Basham and 

Martin, 1985) used to ascertain structural and stratigraphic relationships in section C-C'; true dip 

on Wind River thrust in section C-C' about 25°. pC, Precambrian rocks; Pz, Paleozoic rocks; Mz, 

Mesozoic rocks; T, Tertiary rocks; Tw, Wasatch Formation; Tm, Miocene rocks. Lines of section 

shown in figure 1. 

F4 Geology of Tight Gas Reservoirs, Wyoming and Colorado 
projected the Fremont surface (summit peneplain of 

Westgate and Branson, 1913) and showed a minimal 

amount of basin fill in the area of the WASP well at the 

northern end of the basin (cross section A-A', fig. 3). Projection of the equivalent surface (Wind River peneplains 

-4 and -3 of Westgate and Branson, 1913) over the areas 

of the Wagon Wheel and Pacific Creek wells, however, 

yields amounts of pre-Pliocene fill inconsistent with 

stratigraphic data (cross sections B-B' and C-C', fig. 3). 

These inconsistencies probably result because the peneplain 

slopes to the southeast and is near the level of the basin 

in the Pacific Creek area. 

If the peneplain is an erosional feature, then it must 

have been lowered in the southeast sometime after its formation during the Pliocene. Downdropping may have occurred along the Continental fault and a fault proposed 

by Steidtmann and others (1984) that separates the peneplain from the crest of the range (cross sections B-B' and 

C-C', fig. 3). Alternatively, the southwestern end of the 

Wind River range may have been downdropped along the 

Continental fault after Pliocene time (Love, 1954; Zeller 

and Stephens, 1969). A seismic interpretation by Basham 

and Martin (1985) shows that post-Eocene rocks, which 

include those of the peneplain, north of the Continental 

fault in the Pacific Creek area have been lowered about 

460 m relative to rocks south of the fault. Nace (1939) and 

Zeller and Stephens (1969) estimated similar displacements. 

If we raise the elevation of the peneplain by 460 m, then 

its projected surface indicates about 300m of pre-Pliocene 

fill over the Pacific Creek well. This amount of fill generally 

agrees with the amount of missing section measured by 

Zeller and Stephens (1969) but is slightly less than the 650 

m of Miocene sediments indicated in the seismic cross sections of Basham and Martin (1985). 

Strong circumstantial evidence suggests that the current cycle of erosion, which has exhumed many intermontane basins in the Rocky Mountains, began in the late 

Pliocene when the region of the Green River basin was 

uplifted by more than 1,000 m (Blackwelder, 1915; Love, 

1960; Dorr and others, 1977), and maturation modeling 

in this study generally supports this observation. Moore 

(1960) estimated that during deposition of the Hoback Formation in Paleocene time the basin was only 300-450 m 

above sea level, much lower than its present elevation of 

2,100 m. During Paleocene time, the climate was subtropical 

(15 OC) and it probably remained warm until the Pliocene, 

when uplift decreased the average surface temperature of 

the basin to its present-day level of 4 OC (Dorr and others, 

1977). 

GEOTHERMAL GRADIENTS 

An understanding of past and present geothermal 

gradients is important when constructing maturation 

models because vitrinite reflectance depends mostly on the 

heating of the rocks. A map of uncorrected borehole 

temperatures (Law and Smith, 1983) shows that the 

average geothermal gradient for the northern Green River 

basin is 25 OC/km. Geothermal gradients for the three 

study wells were calculated by using regression analysis 

of the mean annual surface temperature (4 OC for the 

basin) and the temperatures measured for drill-stem tests 

and geophysical logs. The nonmarine rocks in the basin 

are, on a large scale, lithologically homogeneous, and 

significant kinks in the geothermal gradients caused by 

differences in thermal conductivity were neither expected 

nor observed. The slightly higher gradient at the WASP 

well (28.4 °C) as compared to the Wagon Wheel 

(22.3 OC/km) and Pacific Creek (23.6 OC/km) wells may 

be related to the high geothermal gradient in the Yellowstone area about 150 km to the north. For comparison, 

uncorrected borehole temperatures in the Jackson Hole 

area to the south, halfway between the WASP well and 

Yellowstone National Park, yield an average gradient of 

36 OC/km. 

In general, data for the study wells are insufficient 

to correct disequilibrium temperatures measured during 

open-hole testing. Naeser (1984) reported corrected borehole temperatures for the Wagon Wheel well that yield 

a gradient of 23.5 OC/km, or about 1 °C/km hotter than 

the uncorrected gradient. The study wells were extensively 

tested and no abnormally high temperatures were 

reported; presumably, the use of corrected borehole 

temperatures would not significantly modify the uncorrected gradients. 

No geologic evidence exists for a regional heat 

source near the basin from about 40 to 2 Ma. The Absaroka volcanic event began in southern Montana about 

55 Ma and migrated southeast into Wyoming, where it 

ended about 44 Ma (Smedes and Prostka, 1972). 

Volcanism in the Yellowstone area, at the northeast end 

of the Cordilleran Thermotectonic Anomaly described by 

Eaton and others (1976), began about 2 Ma and has continued sporadically until the present time (Christiansen 

and Blank, 1972). Although both of these events occurred 

more than 150 km north of the north end of the basin, 

they may have had a small effect on the geothermal gradient. For example, heating associated with Yellowstone 

volcanism currently should be at its maximum, and the 

gradient at the WASP well (the closest) is slightly higher 

than gradients in other parts of the basin. Miocene and 

Pliocene volcaniclastic rocks in the Jackson Hole area 

(Love and others, 1973) and Pleistocene alkalic rocks at 

the north end of the Rock Springs uplift (Love and Christiansen, 1985) represent local heating events, yet no 

evidence for these events is found in the regional pattern 

of vitrinite maturation (Lickus and others, this volume). 

Theoretically, the vertical movement of subsurface 

waters is very effective in altering geothermal gradients. 

Hitchon (1984) successfully demonstrated a relationship 

Vitrinite Maturation and Tertiary Burial History, Northern Green River Basin F5 
between coal rank, geothermal gradient, and hydrodynamics in the Alberta basin of Canada, but Gretener 

(1981) found many claims of such a relationship to be 

tenuous. Although geothermal gradients in several areas 

of the Green River basin appear to have been modified 

by subsurface waters (B.E. Law, written commun., 1986), 

such modification is very difficult to document without 

extensive hydrodynamic data. 

Fission track annealing studies of sandstones 

(Naeser, 1984, 1986) show that a 20-30 °C cooling event 

occurred 2 to 4 Main the area of the Wagon Wheel well. 

This cooling event coincides with a period of regional 

uplift that resulted in widespread erosion and a drop of 

approximately 10 OC in surface temperature. Theoretical 

calculations show the thermal diffusivity of sandstones 

is such that a 10 °C change in the surface temperature 

can produce, in a period of 3 m.y., a similar change in 

the temperature of rocks 3 km deep (Stegna and Dovenyi, 

1983). In the same time and depth range, the geothermal 

gradient can also reequilibrate after as much as 1,000 m 

of surface erosion. Paleotemperatures modeled for the 

subsurface therefore must account for changes in surface 

temperature as well as for deposition and erosion. 

VITRINITE REFLECTANCE DATA 

Vitrinite reflectance (Rm) data used in this study 

are from Law and others (1980), Law (1984), and 

Pawlewicz and others (1986). Pyrolysis and atomic ratio 

analysis indicate the kerogen of these samples is dominantly humic or type III in composition (Law, 1984). For 

reflectance measurements, woody or coaly fragments 

from core were handpicked and supplemented with concentrates of dispersed organic matter from core and drill 

cuttings. Because of the nature of the kerogen, the quality 

of vitrinite was good, and reflectance measurements 

yielded relatively narrow histograms (M.J. Pawlewicz, 

written commun., 1984). 

Vitrinite reflectance data for the WASP, Wagon 

Wheel, and Pacific Creek wells were used for burial 

reconstruction because the wells are the most extensively 

cored wells in the northern Green River basin. Vitrinite 

reflectance gradients (fig. 4) in the wells are similar to 

those of Dow (1977) for low geothermal gradients and 

young geologic sections. The Wagon Wheel well has the 

most core over the largest depth interval and shows the 

strongest correlation between log Rm and depth (fig. 4). 

The vitrinite reflectance gradient for the Wagon Wheel 

well (fig. 4B) is almost identical to that obtained independently by Pollastro and Barker (1984). The WASP 

well also shows a good correlation between log Rm and 

depth, but the four uppermost samples are from drill cuttings and may bias the linear fit (fig. 4). The Pacific Creek 

well actually includes reflectance measurements from core 

F6 Geology of Tight Gas Reservoirs, Wyoming and Colorado 

and drill cuttings in three closely spaced wells; selective 

sampling for vitrinite was limited, and the correlation between log Rm and depth is inferior to that of the other 

two wells because most of the available samples consist 

of drill cuttings and random core chips from shale and 

siltstone (fig. 4). 

BURIAL AND THERMAL RECONSTRUCTIONS 

In this study, vitrinite reflectance data were combined with maturation models to evaluate the Tertiary 

burial history of the basin. The evaluation depends on 

the quality of the reflectance data, an understanding of 

the geothermal gradient, and the validity of the maturation model. Results from several models were compared, 

but the Lopatin model of Waples (1980) offers the easiest 

way to test depth and duration of burial against measured 

reflectance data. The amount of section known to be 

missing at each well was compared to the maximum burial 

depth determined by projecting the vitrinite reflectance 

gradient to 0.2 percent, a value most workers (notably, 

Dow, 1977) consider to be the surface intercept in uneroded terrain (fig. 4). 

The geologic history (fig. 2) was used to construct 

burial curves for the Cretaceous-Tertiary boundary, the 

top of the Ericson Sandstone, and the tops of the Blair 

and Hilliard Shales (fig. 4). The total organic maturity 

or time-temperature index {TTl) for these horizons was 

calculated according to Waples (1980). By using the TTIRm calibration of Waples (1980), the average Rm value 

for each horizon was converted to a measured TTl value 

for comparison with the calculated TTl value. Thus, 

burial history may be used to evaluate maturation models, 

or conversely, burial curves and geothermal gradient may 

be altered to match calculated and measured TTl values. 

The Ericson Sandstone is the best horizon with 

which to compare measured and calculated TTl because 

it is the most traceable time-correlative horizon in the 

basin and also the one for which there is the most Rm 

data. Wherever the match for one horizon is good, the 

model of Waples (1980) does not accurately predict the 

maturation of the other two horizons (fig. 4). These inaccurate predictions may result from errors in Rm 

measurements, errors in the TTI-Rm relationship of 

Waples, errors in modeling the rate of vitrinite maturation, or errors in estimating the value and shape of the 

paleogeothermal gradient. 

Burial curves for the WASP and Pacific Creek 

wells, as determined by stratigraphic analysis, generally 

agree with the Rm surface intercept and produce a 

reasonable match between calculated and measured TTl 

values for the Ericson Sandstone (fig. 4), except that the 

measured TTl value for the Ericson at the WASP well 

is somewhat lower than the calculated value. The 
VITRINITE REFLECTANCE, TIME, IN Ma 

531 m-\ IN PERCENT 

A 

B 

0 0.2\ 0.5 1.0 1.5 2.0 9,-0 -.86:--..----7.-3 -,----,.65---,-__ 5,..1 _ ___,--,----.,----1,7 ---,.--.,--, 

(/) 

a: 

LU 

1- 2 LU 

~ 

0 

_J 

~ 

z 

I. 4 

b: 

LU 

0 

r=0.95 Kh Ke K/T 

o} K/T=0.55 PERCENT=7 Til 0 

o\ !J!LL_ 

.._\ Ke=0.95 PERCENT 120 oe 

28.4 oC/km 

o Cuttings 

• Core 

\--1,520 m \ 

\ 

\ 

\ 

···f! ~ =60 Til 13Qoc-----------._ ~ 

~ ----------~ 

'\h~t.5 PERCENT~300 Til 

0 0.2 \ 24 

(/) 

a: 

LU 

1-

LU 

~ 

0 

_J 

~ 

~ 

:i 4 

b: 

LU 

:::::l 

22.3 oC/km 

... • Core 

526m~ 

c 

O .-0.-2\-r-----,0._5 __ 1.--.0_1.-.5_2,....0_ 90 86 73 65 

(/) 

a: 

~ 2 LU 

~ 

0 

_J 

~ 

z 

I. 4 

b: 

LU 

0 

Kb Ke K/T 

K/T=0.72 PERCENT=23 TTl 

o X 

oxx} 

oX 90 oC ---

x Core chips X 

o Cuttings 

• Core 

47 24 

23.6 oC/km 

113 

1,140 

81 

15 

i= 

1-

X 

LU 

0 

~ 

LU 

a: 

:::> 

~ a: 

LU 

0.. 

~ 

LU 

1-;-

LU 

~ 

f= 

0 

LU 

1-

<( 

_J 

:::> 

u 

_J 

<( 

u 

i= 

'::. 

X 

LU 

0 

z 

LU 

a: 

:::> 

~ a: 

LU 

0.. 

~ 

LU 

1-

uJ 

~ 

f= 

0 

LU 

~ _J 

:::> 

u 

_J 

;) 

i= 

'::. 

X 

LU 

0 

z 

LU 

a: 

:::> 

~ a: 

LU 

0.. 

~ 

LU 

1-

uJ 

~ 

f= 

0 

LU 

~ _J 

:::> 

u 

_J 

<( 

u 

Figure 4. Vitrinite reflectance values and Lopatin reconstructions for the top of Ericson Sandstone (Ke) at the WASP (A), Wagon Wheel (8), and Pacific Creek (C) wells. Burial curves for 

the tops of the Baxter (Kb) and Hilliard (Kh) Shales and for the Cretaceous-Tertiary boundary 

(KIT) (fig. 2) are parallel with that for the Ericson Sandstone. For these horizons, calculated 

TTl values (right side) may be compared to measured TTl values (converted from measured 

vitrinite reflectance data (see appendix) by using method of Waples (1980)). Constant geothermal gradients use a surface temperature of 15 °C, which has decreased to 4 oc from 3 Ma 

to the present time. Plots of vitrinite reflectance versus depth show the elevation of the extrapolated intercept with 0.2 percent reflectance. Modified from Law and others (1980) and 

Law (1984). 

Vitrinite Maturation and Tertiary Burial History, Northern Green River Basin F7 
difference in TTl values for the WASP well reflect high 

present-day geothermal gradients in the Yellowstone and 

Jackson Hole areas that may have caused a recent increase in the gradient at the WASP well. The Rm surface intercept at the WASP well predicts slightly hotter 

paleotemperatures than can be explained by burial and 

the Lopatin model, but this may result from the four 

shallow measurements from cuttings, which may bias the 

Rm gradient. 

At the Wagon Wheel well, extrapolation of the Rm 

line to 0.2 percent reflectance indicates that 600-1,000 m 

of section has been eroded in addition to the 470 m that 

is missing from the stratigraphic section (fig. 4). If we 

assume that present-day and paleogeothermal gradients 

are similar, then the Ericson must have been buried 

1, 100-1 ,500 m deeper than at present. Stratigraphic data 

indicate, however, that no more than 470 m of section 

has been eroded from the area, and two interpretations 

are possible for this discrepancy. 

Recent Folding 

Cross sections in Lickus and others (1984, this 

volume) and in figure 5 show that isoreflectance lines are 

parallel with the Pinedale structure and suggest that the 

present-day Rm values in the Wagon Wheel well may 

have been physically elevated into their position by uplift 

CJ) 

a: 

w 

1-

D 

WASP well Wagon Wheel well 

PINEDALE ANTICLINE 

on the anticline sometime after maximum maturation or 

uplift and cooling during the Pliocene. The anticline has 

about 600 m of structural relief (Law, 1984; Law and 

Johnson, this volume), and local erosion resulting from 

uplift on the anticline could account for the additional 

missing section required by the reflectance data. Presumably, both local (600 m) and regional (470 m) erosion occurred during the cooling event indicated by the 

fission-track data (Naeser, 1984). 

Some independent data support recent uplift on the 

Pinedale anticline. A listric normal fault penetrated by 

Wagon Wheel core probably was associated with uplift 

of the Pinedale anticline. Both petrographic and isotopic 

evidence (Dickinson, 1985) suggest that fractures 

associated with this fault are filled with relatively recent 

cements. In addition, fluid inclusion studies suggest that 

quartz in some of these fractures precipitated at or near 

the present-day formation temperature (Pollastro and 

Barker, 1984). In view of post-Pliocene movement on the 

Continental fault (Love, 1954; Zeller and Stepehns, 1969), 

recent folding at Pinedale would not have been an isolated 

tectonic event. 

In spite of the above evidence, some expected indications of recent folding are lacking. The Pinedale anticline is poorly defined at the surface, whereas recent 

structural features should be easily recognized. In addition, the listric normal fault associated with the anticline 

in the subsurface does not appear to penetrate Eocene 

D' 

Pacific Creek well 

12 

~ 3 0 

...J 

BASIN FILL 

~ .......... ~. ········· ................... ,.,...,.,... ....-.::-.-. ... ~·········L .............. .. ·········-···· .......... . 

ROCK SPRINGS 

UPLIFT 1-

w 

w 

l.L. 

~ 

z 2 

z 

0 

i= 1 

~ w 

...J 

w 

~ ....... ,~, 

MEAN SEA LEVEL 

10 20 

I I 1 

10 

30 

I 

I 

20 

40 

I 

VERTICAL EXAGGERATION X 14 

I 

50 KILOMETERS I 

30 MILES 

~ 

z 

0 

4 ~ > w 

...J 

w 

Figure 5. Cross section showing relationships between stratigraphy, structure, thermal maturation, and burial in northern Green 

River basin. Wind River peneplain projected into plane of cross section (fig. 3) to show its present relationship with basin fill. 

Stratigraphic data modified from Law (1981); Rm data from Pawlewicz and others (1986). Line of section shown on figure 1. 

F8 Geology of Tight Gas Reservoirs, Wyoming and Colorado 
rocks at the surface. Structural details may be obscured, 

however, by the lack of traceable beds in the arkosic facies 

of the Wasatch Formation and by the poorly cemented 

and unconsolidated nature of this formation at the 

surface. 

(fJ 

0:: 

LU 

1- 2 LU 

~ 

0 

...J 

:;;;:: 

73 65 

90 oc 

TIME, IN Ma 

51 24 

i= 

t:.. 

X 

LU 

0 

z 

LU 

0:: 

::> 

~ 

The stratigraphic age of the 600 m of missing section that may have been lost to local erosion of the anticline is unclear and is not indicated by the Lopatin 

maturation model. If post-Eocene deposition was 

minimal, as suggested by stratigraphic evidence, and an z additional600 m of Wasatch Formation was eroded, then 

~ 4 

____/,...------, 

_/r------, 0:: 

LU 

a.. 

the calculated TTl for the Ericson Sandstone would be 

202 (dashed burial curve, fig. 4), much higher than the 

measured TTl of 130. On the other hand, if 600 m of 

Miocene rocks was eroded, a supposition not supported 

by stratigraphic evidence, then the measured (130) and 

calculated (133) TTl values for the Ericson Sandstone are 

in good agreement (dotted burial curve, fig. 4). 

Elevated Paleogeothermal Gradient 

The second interpretation for the discrepancy in 

amount of missing section is that a local heating event 

elevated the vitrinite reflectance values around the Pinedale anticline. If we assume that the Pinedale anticline 

is Laramide in age (Law and Johnson, this volume), then 

the paleotemperature gradient needs to be increased from 

the Eocene to the Pliocene by only 2 °C/km over the 

present-day gradient in order for calculated and measured 

TTl values in the Ericson Sandstone to agree (fig. 6). The 

uncorrected borehole temperature gradient is probably 

not in error by this amount because the temperature map 

of Law and Smith (1983), which uses the adjacent wells, 

shows no thermal anomaly that might explain the relationship between isoreflectance lines and structure. 

Upward migration of hot fluids along faults and fractures associated with folding on the anticline could have 

locally increased the gradient, but there is no evidence 

to explain why the migration of hot fluids suddenly 

stopped or why the geothermal gradient decreased to the 

present level. Possible hydrothermal fluids, which can 

cause thermal anomalies, are presently inactive. 

The 20-30 OC of recent cooling required by the 

fission-track data (Naeser, 1984) can be more easily explained if the paleo geothermal gradient was higher than 

the present-day gradient. If an additional 600 m of sediments was eroded as the result of uplift on the anticline, 

then about 40 °C of cooling would have occurred since 

the Pliocene (fig. 4B); however, if the geothermal gradient prior to the Pliocene was higher, then the amount 

of cooling would be about 25 °C, more similar to the 

amount of cooling predicted by fission-track annealing 

(fig. 6). 

a.. 

LU 

0 

22.3 oC/km 

~----- 137 ~ 

LU 

1-

w 

~ 

i= 

1,460 

Figure 6. Lopatin reconstruction for a local increase in geothermal gradient in the Wagon Wheel area. Burial curves for the tops 

of the Ericson Sandstone (Ke) and Hilliard Shale (Kh) and for the 

Cretaceous-Tertiary boundary (KIT) same as in figure 48, except 

without 600 m of structural uplift and erosion. Geothermal gradient 

increases shortly after possible Laramide folding of Pinedale anticline, then decreases back to 22.3 °C/km at 3 Ma. Modeled using 

method of Waples (1980). 

In support of this explanation, Meyer and McGee 

(1985) observed elevated thermal gradients associated 

with structure in 15 of the 22 oil and gas fields they examined in the Rocky Mountain region. They attributed 

the elevated gradients to upward migration of fluids. On 

the Rock Springs uplift, the 0.8-percent reflectance line 

dips more steeply than structure on the Ericson Sandstone 

(fig. 5) (Pawlewicz and others, 1986), and this steep dip 

may result from the upward migration of hydrothermal 

fluids along fractures and faults (B.E. Law, written 

commun., 1986). 

VITRINITE MATURATION MODELS 

Vitrinite maturation models are used to predict the 

maximum temperature to which organic-rich sediments 

have been exposed. Although much progress has been 

made toward the refinement of these models, the relationship between the rate of maturation and temperature 

remains uncertain because independent calibration of 

maximum rock temperature is lacking and because subtle 

differences exist in the chemical and physical properties 

of the vitrinite macerals. For example, Price (1983) argued 

that vitrinite maturation follows high-order reaction 

kinetics, whereas most workers believe it follows firstorder reaction kinetics. These differences of opinion have 

led to the development of essentially two categories 

Vitrinite Maturation and Tertiary Burial History, Northern Green River Basin F9 
of models: (1) time-dependent models, which require 

time or length of heating in the maturation process, 

and (2) time-independent models, which discount the 

time factor and suggest that vitrinite is an absolute 

paleogeothermometer. 

The time-dependent model of vitrinite maturation 

developed by Waples (1980) was used in this study mainly because it is widely accepted and allows for easy adjustment of burial curves. In view of the uncertainties, 

however, a comparison of this model and several other 

vitrinite models on a common basis with the known 

geologic history of the basin seems necessary. The comparison was done at each well, similar to the method of 

Bostick and Freeman (1984), by using selected models to 

calculate maximum paleotemperatures from measured 

Rm values (fig. 7). Paleotemperature gradients predicted 

by a regression analysis of these temperatures were compared to present-day temperature gradients and estimates 

of basin fill. If present-day and paleotemperature gradients are similar and stratigraphic estimates of basin fill 

are correct, the ideal paleotemperature gradient should 

be parallel with the present-day temperature gradient and 

intercept the elevation of the estimated basin fill at about 

15 OC, the estimated surface temperature of the basin at 

the time of maximum burial. 

For each well, the time-independent models of 

Price (1983) and Barker and Pawlewicz (1985) predict 

significantly higher paleogeothermal gradients than can 

be accounted for by geologic evidence (fig. 7). In addition, these high paleogeothermal gradients do not agree 

with fission-track annealing data for the Wagon Wheel 

well. The lack of annealing in zircon in the deepest fissiontrack samples (4,557 m) indicates that temperatures in the 

surrounding rocks could not have been hotter than 

175-200 °C, the minimum temperature at which zircon 

begins to anneal (Naeser, 1984). The present-day 

temperature at this depth is 113 OC and would have been 

only 126 OC had the paleogeothermal gradient been 

2 OC/km hotter than the present-day gradient. In contrast, the temperature calculated from measured Rm data 

at this depth is 241 OC for the model of Price (1983) and 

205 OC for the model of Barker and Pawlewicz (1985). 

The time-dependent models of Hood and others 

(1975) and Middleton (1982) provide the best agreement 

between Rm data, present-day temperature gradient, and 

estimated burial at each well (fig. 7). The equations in 

the appendix show that these two models define temperature in different ways. The model of Hood and others 

(1975) predicts the maximum paleotemperature by using 

the amount of time (terr) at which burial is within 15 OC 

of the maximum temperature. In this study, teff was 

determined as a single value for each well according to 

the burial curves shown in figure 4. In the model of Middleton (1982), temperature is constant for a given length 

WASP WELL WAGON WHEEL WELL 

--------1,520 m 

PACIFIC CREEK WELL 

(/) 

a: 

L.U 

1-

L.U 

~ 

0 

.....J 

~ 

z 

I 

1-

~ 3 0 

--------531 m 

R0 =0.2 PERCENT 

100 200 15 100 200 15 

------526 m 

R0 =0.2 PERCENT 

100 200 

TEMPERATURE, IWC 

figure 7. Linear regression least-squares fits of maximum 

paleotemperatures for the WASP, Wagon Wheel, and Pacific 

Creek wells calculated by using selected vitrinite maturation 

models and vitrinite reflectance values and equations shown 

in appendix. Present-day uncorrected temperature gradients 

(heavy solid lines) calculated from drill-stem tests and 

F10 Geology of Tight Gas Reservoirs, Wyoming and Colorado 

geophysical logs; paleogeothermal gradient (dotted line) from 

Lopatin reconstruction (fig. 6). Estimated surface temperature 

of basin during maximum burial 15 °C. Vitrinite maturation 

models: H, Hood and others (1975); W, Waples, (1980); M, 

Middleton (1982); P, Price (1983); B+ P, Barker and Pawlewicz 

(1985). 
of time, the age of the rock. The two models give similar 

results, probably because the rocks were buried rapidly 

and were subjected to a long period of constant heating 

at or near maximum temperatures. In the MWX wells 

of the Piceance basin, Bostick and Freeman (1984) 

showed that predictions from numerous time-dependent 

models are similar and fit the geologic data better than 

those from time-independent models. 

Paleotemperature gradients predicted by using the 

model of Waples (1980) are slightly lower than presentday gradients. These lower gradients, and the high elevations of the intercepts at 15 OC, reflect the low TTl values 

for the Cretaceous-Tertiary horizons and the high TTl 

values for the Hilliard and Baxter horizons calculated for 

the burial reconstructions (figs. 4, 6). The three time-

. dependent models predict almost identical temperatures 

for the Ericson Sandstone in each well (fig. 7}, temperatures close to the maximum temperatures predicted by 

the burial curves (figs. 4, 6). 

It is not clear why the Lopatin model of Waples 

( 1980) predicts paleo geothermal gradients that are lower 

than present-day gradients; either geothermal gradients 

must change significantly with depth or burial curves must 

differ at each horizon. Both of these alternatives seem 

less likely than error in the model of Waples (1980). 

Variations of the Lopatin model (Koncz, 1983) that use 

different Rm-TTl relationships also predict low paleogeothermal gradients. The solution, therefore, is not a 

simple calibration of the Rm-TTl relationship for the 

Green River basin, as suggested by Issler (1984), but 

perhaps a reevaluation of the basic time-temperature relationship used in the model. 

Discrepancies between thermal gradients do not 

necessarily discredit those models that do not fit the 

observed geologic data, but they do point out the need 

for evaluating several vitrinite maturation models at any 

specific location. Until the uncertainties of vitrinite 

maturation modeling are clarified, we may find that no 

model is universally applicable and that different models 

work better in different situations. 

SUMMARY AND CONCLUSIONS 

Analysis of vitrinite reflectance data in the northern 

Green River basin agrees with geologic evidence for depth 

and duration of burial. Extrapolation of the vitrinite 

reflectance profiles to 0.2 percent in the WASP and 

Pacific Creek wells yields elevations of basin fill that are 

consistent with stratigraphic evidence: only a minor 

amount of post-Eocene sediments has been removed in 

the northern end of the basin, and about 600 m of 

sediments has been removed by post-Pliocene erosion of 

the basin near the southern end of the Wind River Mountains. The duration of maximum burial (middle Eocene 

to Pliocene}, as indicated by time-dependent models of 

vitrinite maturation, agrees well with the depositional 

history of the basin and the fission-track data of Naeser 

(1984). 

Available geologic evidence indicates that the 

amount of post-Eocene deposition (165m) in the Wagon 

Wheel area should be intermediate between that in the 

WASP (no deposition) and Pacific Creek (335 m) areas. 

Using the present-day geothermal gradient, however, this 

intermediate amount of fill cannot account for the high 

reflectance values measured in the area. These high values 

probably result either from recent folding of the Pinedale 

anticline or from a local increase in the paleotemperature 

gradient, but stratigraphic, fission-track, and maturation 

data cannot completely resolve this choice. The Lopatin 

model is more in accord, however, with stratigraphic and 

fission-track data if the paleogeothermal gradient was 

2 °C/km higher than the present-day gradient. 

Comparison of maturation models clearly indicates 

that time plays an important role in vitrinite maturation. 

Errors in the geothermal gradient are not as crucial if 

time-independent models are used because such models 

yield unrealistically high gradients at each of the three 

areas. The differences in results produced by different 

time-dependent models indicates uncertainty about the 

role of time. At theW ASP well, vitrinite maturation does 

not appear to reflect the present-day geothermal gradient, 

which may be at a maximum because of heating to the 

north, but independent data such as fission-track annealing are needed to support this conclusion. 

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Smedes, H.W., and Prostka, H.J., 1972, Stratigraphic 

framework of the Absaroka volcanic supergroup in the 

Yellowstone National Park region: U.S. Geological Survey 

Professional Paper 729-C, 33 p. 

Stegena, L., and Dovenyi, P., 1983, Procedure to correct 

measured temperatures and temperature gradients for 

geological effects: Zentralblatt fur Geologie und Palaontologie, v. 1, no. 1/2, p. 25-34. 

Steidtmann, J.R., 1987, The "Wyomide Event;" Oligocene 

deformation in the Rocky Mountain foreland of Wyoming: Geological Society of America Abstracts with Programs, v. 19, no. 7, p. 855. 

Steidtmann, J.R., Hurst, D.J., and Shuster, M.W., 1984, 

Structural evolution of part of the Wyoming thrust belt 

and foreland; a sedimentary-tectonic approach: Geological 

Society of America Abstracts with Programs, v. 16, no. 

6, p. 667. 

Steidtmann, J.R., Middleton, L.T., Bottjer, R.J., Jackson, 

K.E., McGee, L.C., Southwell, E.H., and Lieblang, S., 

1986, Geometry, distribution, and provenance of tectogenic conglomerates along the southern margin of the 

Wind River Range, Wyo., in Peterson, J.A., ed., 

Paleotectonics and sedimentation in the Rocky Mountain 

region, United States: American Association of Petroleum 

Geologists Memoir 41, p. 321-332. 

Waples, D.W., 1980, Time and temperature in petroleum formation; application of Lopatin's method to petroleum exploration: American Association of Petroleum Geologists 

Bulletin, v. 64, no. 6, p. 916-926. 

West, R.M., 1969, Geology and vertebrate paleontology of the 

northwestern Green River basin, Wyoming: Wyoming 

Geological Association Annual Field Conference, 21st, 

Casper, Wyo., 1969, Guidebook, p. 77-92. 

Westgate, L.G., and Branson, E.B., 1913, The later Cenozoic 

history of the Wind River mountains, Wyoming: Journal of Geology, v. 21, p. 142-159. 

Zeller, H.D., and Stephens, E.V., 1969, Geology of the Oregon 

Buttes area, Sweetwater, Sublette, and Fremont Counties, southwestern Wyoming: U.S. Geological Survey 

Bulletin 1256, 60 p. 

Vitrinite Maturation and Tertiary Burial History, Northern Green River Basin F13 

APPENDIX 1 
Appendix 1. Vitrinite reflectance values for Wagon Wheel, WASP, and Pacific 

Creek wells, northern Green River basin, and equations used to calculate 

paleotemperatures 

A. Vitrinite reflectance values for study wells 

[Depth in meters; vitrinite reflectance (Rm) in percent. Data from Law (1984)] 

Wagon Wheel WASP Pacific Creek 

Depth 1m Depth 1m Depth 1m 

1,532.5 0.51 1,508.8 0.46 2,025.7 0.72 

1,545.9 .46 1,815.1 .56 2,370.7 .72 

2,148.2 .68 2,118.4 .57 2,557.9 .80 

2,173.2 .74 2,421.6 .60 2,992.8 .80 

2,175.4 .70 3,493.3 .93 4,438.5 .90 

2,251.9 .72 3,558.5 .95 5,501.0 .91 

2,254.0 .76 3,644.2 .97 5,681.2 .98 

2,460.7 .78 3,708.5 .90 5,779.0 .96 

2,479.5 .77 3,727.7 .92 5,779.0 1.43 

2,738.3 .85 3,738.7 1.11 5,992.4 .91 

3,092.2 .95 3,769.2 .99 5,992.4 1.39 

3,100.7 .91 3,789.3 1.09 6,249.3 .92 

3,360.1 .96 3,817.3 1.11 6,249.3 1.51 

3,362.6 .98 3,952.3 1.05 3,876.1 1.05 

3,368.3 1.03 3,970.9 1.16 3,880.4 1.05 

3,992.0 1.22 4,042.0 1.13 4,105.0 1.20 

4,005.4 1.15 4,059.3 1.10 1,979.7 .60 

4,022.8 1.13 4,078.5 1.16 2,401.8 .64 

4,542.1 1.47 4,087.1 1.11 3,032.8 .71 

4,545.8 1.48 4,112.1 1.13 3,564.6 .91 

4,547.0 1.53 4,187.3 1.15 3,977.6 1.10 

4,548.5 1.48 4,205.6 1.21 4,709.2 1.50 

4,550.7 1.55 4,207.8 1.23 5,471.2 2.00 

4,554.3 1.49 4,231.8 1.18 5,903.4 2.28 

4,897.5 1.61 4,264.5 1.28 

4,900.6 1.61 4,273.3 1.14 

4,907.6 1.65 4,291.9 1.31 

5,234.3 1.77 4,308.7 1.22 

5,474.2 1.98 4,321.8 1.24 

5,475.7 1.85 4,334.3 1.25 

4,350.1 1.44 

4,354.4 1.44 

4,367.8 1.49 

F16 Geology of Tight Gas Reservoirs, Wyoming and Colorado 
B. Equations used to calculate paleotemperatures 

1. Uncorrected borehole temperature gradients calculated by using data from geophysical logs and drill-stem tests in the 

following wells 

Pacific Creek 

Wagon Wheel 

WASP 1-A 

2. Price (1983) 

3. Barker and Pawlewicz (1985) 

lnRm 

4.4 + 0.0236 (depth in meters) 

2.2 + 0.0223 (depth in meters) 

8.9 + 0.0285 (depth in meters) 

(302.97log R0 ) + 187.33 

0.0078Tmax(°C)- 1.2, where 

Rm is mean random vitrinite reflectance as measured in oil 

4. Hood and others (1975) 

190log R0 - 32.5log teff + 185 

Level of maturation (LOM) converted to R0 in unpublished nomogram of A. Hood (1976). teff = time (between 10 and 

100 Ma) within 15 oc ofT max 

teffWASP 

teff Wagon Wheel 

teff Pacific Creek 

5. Middleton (1982) 

a 5.5 

b 2.8X1o-6 

c 0.065 

53 Ma 

48Ma 

49Ma 

R initial vitrinite reflectance (0.2 percent) 

T m constant temperature for a given time 

t time (in millions of years) 

6. Equation 35 of Koncz (1983) describes model of Waples (1980) 

Tmax(°C) = 125.2(log R0 + 0.2645log e + 3.197)- 273, where 

e heating rate; °C/m.y.=(Ti-T 0 )/ti 

Ti present-day temperature of rocks 

T 0 initial temperature at burial (20 °C) 

ti age of rocks, in millions of years 

Vitrinite Maturation and Tertiary Burial History, Northern Green River Basin F17 

Chapter G 

Thermal Maturity Patterns in the 

Northern Green River Basin, Wyoming 

By M.R. LICKUS, M.J. PAWLEWICZ, B.E. LAW, and 

W.W. DICKINSON 

Prepared in cooperation with the U.S. Department of Energy 

U.S. GEOLOGICAL SURVEY BULLETIN 1886 

GEOLOGY OF TIGHT GAS RESERVOIRS IN THE PINEDALE ANTICLINE AREA, WYOMING, 

AND AT THE MULTIWELL EXPERIMENT SITE, COLORADO 
CONTENTS 

Abstract Gl 

Introduction Gl 

Methods Gl 

Discussion G2 

Summary G4 

References cited GS 

FIGURES 

1. Map showing depth to 0.8-percent vitrinite reflectance level in northern 

part of Green River basin of Wyoming G2 

2-3. Linear regression plots of vitrinite reflectance values: 

2. Davis Granite Wash drill hole G2 

3. From 29 wells in northern part of Green River basin G3 

4. Cross section showing structures, top of overpressured zone, and 

0.8-percent isoreflectance line in northern part of Green River basin G4 

5. Sketches showing relationships between isoreflectance lines and structural 

attitudes of sedimentary rocks G4 
Thermal Maturity Patterns in the 

Northern Green River Basin, Wyoming 

By M.R. Lickus1, M.j. Pawlewicz, B.E. Law, and W.W. Dickinson2 

Abstract 

In the northern part of the Green River basin of Wyoming, 

thermal maturity patterns are similar to structural trends. In the 

Pinedale anticline area, thermal alteration effects caused by 

relatively hot fluids migrating upward along faults and fractures 

may render invalid techniques commonly used to determine 

the temporal relationships between structural growth and the 

development of thermal maturity. 

INTRODUCTION 

In recent years, thermal maturity studies have been 

increasingly utilized in basin analysis to study hydrocarbon generation and migration and to provide insight into 

the structural and burial history of a basin. Several 

methods are used to determine the level of thermal maturity, the most common of which is vitrinite reflectance. 

In oil-prone source rocks, the thermal maturation 

window for oil generation is between 0.6 and 1.3 percent 

vitrinite reflectance. In gas-prone source rocks, vitrinite 

reflectance values for the initiation of thermogenic gas 

are poorly known, but values ranging from 0.5 to 0.8 percent have been reported (Law and others, 1979, 1980; 

Magoon and Claypool, 1983; Monnier and others, 1983; 

Law, 1984; Pitman and others, 1985). In the Pacific 

Creek area of the Green River basin, the generation of 

significantly large volumes of thermogenic gas begins at 

a vitrinite reflectance of about 0.8 percent (Law and 

others, 1979, 1980; Law and Spencer, 1981; Spencer and 

Law, 1981; Law, 1984). 

In our study we attempt to determine the vitrinite 

reflectance level in the northern part of the Green River 

basin (fig. 1) and to provide information on the relationship between the thermal and structural history 

of the Pinedale anticline area. Our report expands on 

and supersedes the thermal maturity map of the northern 

part of the Green River basin by Lickus and others 

(1984). 

1

225 Old Bay Road, Bolton, MA 01740 2

Research School of Earth Science, Victoria University 

P.O. Box 600, Wellington, New Zealand 

METHODS 

Vitrinite reflectance values for 138 samples from 

29 drill holes (fig. 1) were used to construct a thermal 

maturity map; as many as 33 samples were collected from 

each drill hole. Coal samples were preferentially collected 

in order to minimize inclusion of recycled organic matter and to expedite sample preparation. Drill cuttings of 

shale and sandstone were processed by using an acid 

maceration technique (King and others, 1963; Saxby, 

1970). Coal samples and cuttings samples were mounted 

in epoxy on petrographic slides and polished, and vitrinite 

reflectance was then measured under oil immersion. 

Vitrinite reflectance measurements were made by M.J. 

Pawlewicz, N.H. Bostick, and B.E. Law of the U.S. 

Geological Survey and by Robertson Research (U.S.) Incorporated, Houston, Tex. 

Measurement of vitrinite reflectance of core 

samples presents few analytical problems; however, cuttings samples can be contaminated as a result of uphole 

sloughing or lignite drilling and additives. These effects 

can be minimized by altering the sample scanning method 

and by carefully selecting the organic matter from which 

vitrinite measurements are taken. For core samples, the 

least mature vitrinite is measured indiscriminantly; for 

cuttings samples, however, the most common material, 

or modal population, must be measured and choices must 

often be made between several populations or ranges of 

thermal maturity. Because of these constraints, the mean 

reflectance value was used to estimate the indigenous 

vitrinite population. In cases where the mean value did 

not accurately represent the vitrinite population, such as 

when vitrinite measurements were few and wide ranging, a pick was determined by the operator based on the 

polish of the slide and the quality of the organic matter. 

In almost all samples, the mean reflectance and the 

vitrinite reflectance of the pick are the same or almost 

the same. 

The method used to determine 0.8-percent vitrinite 

reflectance for a drill hole depends on the number and 

stratigraphic distribution of samples within the hole. For 

drill holes containing more than four samples distributed 

Thermal Maturity Patterns, Northern Green River Basin Gl 
• 

10 MILES 

I 

10 KILOMETERS 

Figure 1. Depth to 0.8-percent vitrinite reflectance level (contour interval, 1,000 ft) in the northern part of the Green River 

basin of Wyoming. Thrust faults shown by heavy lines; sawteeth on upper plate. Solid circles indicate drill holes from 

which samples for this study were collected. Line of section 

A-A' (fig. 4) also shown. Modified from Pawlewicz and others 

(1986). 

over a significantly large depth interval, a linear regression 

plot of depth and the log of the vitrinite reflectance was 

constructed (Dow, 1977), and the depth to the 0.8-percent 

level in a given drill hole was calculated by using the leastsquares regression line from the linear regression plot. For 

example, figure 2 shows data from the Davis Granite Wash 

well. 

For drill holes containing fewer than 6 samples or 

for drill holes in which samples are distributed over a relatively narrow depth interval, data from all 29 drill holes 

in the northern part of the Green River basin were used to 

calculate a regional least-squares linear regression line (fig. 

3). The slope of the regional regression line was then used 

to calculate the depth to the 0.8-percent vitrinite reflectance 

level in individual drill holes. For example, in a drill hole 

containing three samples, the depth to the 0.8-percent level 

was calculated for each sample by using the slope of the 

regional regression line, and the three calculated values were 

then averaged to produce a single value for the drill hole. 

G2 Geology of Tight Gas Reservoirs, Wyoming and Colorado 

VITRINITE REFLECTANCE (Rm) ,IN PERCENT 

1-

UJ 

UJ 

L.L. 

z 

0.4 0.6 0.8 1.0 

8000 r10,000 r-

-:. 12,000 II 

t 

UJ 

0 

14,000 r16,000 r2.0 3.0 

I I 

18,000 I...--___JI_-.~..1 __ ....__ I_IL...-___,I __ ...L..- I_.L...-I___.I __ ...L..- 1__. 

-0.4 -0.3 -0.2 -0.1 0.0 0.1 0.2 0.3 0.4 0.5 

LOG10(%Rm) 

Figure 2. Linear regression plot of vitrinite reflectance (Rm) 

values, Davis Granite Wash drill hole (sec. 16, T. 29 N., R. 110 

W.). (See Prensky, this volume, fig. 1, for township-range map.) 

Linear and logarithmic scales shown. Dashed line is the leastsquares linear regression line. 

DISCUSSION 

The thermal maturity map of the northern Green 

River basin (fig. 1) is modified from Pawlewicz and others 

(1986) and shows the depth to the 0.8-percent vitrinite 

reflectance level. This level was selected because observations in the Greater Green River basin indicate that 

significantly large volumes of gas begin to be generated 

at depths corresponding to vitrinite reflectance values of 

from 0.74 to 0.86 percent (Law and others, 1979, 1980; Law 

and Spencer, 1981; Law, 1984). The top of sustained gas 

shows is commonly associated with the top of overpressuring and an uncorrected (bottom-hole) subsurface 

temperature of about 180 op (82 OC). Figure 4 shows, in 

cross section, the 0.8-percent isoreflectance line and the top 

of the overpressured zone. The depth to 180 op (82 OC) 

was mapped by Law and Smith (1983). 

In the area of the Pinedale anticline, the top of the 

overpressured zone is within the upper part of the Upper 

Cretaceous Lance Formation at a vitrinite reflectance of 

about 0. 7 4 percent. The relationships between thermal 

maturity and overpressuring in the El Paso Wagon Wheel 

No. 1 well (fig. 4) have been discussed by Law (1984), Law 

and Dickinson (1985), and Spencer (1987, this volume). On 
VITRINITE REFLECTANCE (Rm), IN PERCENT 

0.4 

3000 

0.5 0.6 0.7 0.8 0.9 1.0 20 

I 1:::. I I I I I I 

1:::. 

1:::. 1:::. 

f-

~lsi:::. I:::. I:::. 1:::. 

1:::. 

" 1:::. 1:::. 

" r- 1:::. 1:::.~1:::.1:::. !:::.If-~:::. 1:::.1:::. 

"' 1:::. 1:::. 

1:::. 

1:::. &- ~ ~ 1:::. 

1:::.~ 1:::. 1:::. f- 1:::. 

"I:::. 1:::. 1:::. 

5000 

7000 

9000 

1:::. If-is. 

1:::. 

"~ 1:::. 

1:::. 1:::. 1:::. 

1:::. 

1-

IJ.J 

IJ.J 

1.1.. 

z 

11,000 - A 

i "'-:t."'-..-.---sr"'--.: "' 1:::. 1:::. 

1,9T feet 

b.tn ti. ~"' 

I. 

t 

IJ.J 

0 

13,000 -

15,000 1-

17,000 f19,000 

-0.4 

I 

-0.3 

1:::. 

I I 

-0.2 -0.1 

l:::._flt_/::;; ... 1:::. 

~1:::. " 1:::.~1:::.1:::. ~1:::. 

0.1 Log 1o 

(%Rm) 

I 

0.0 

LOG10(%Rm) 

0.1 

~ 

1:::.1:::. " 

I 

0.2 

" " 1:::. 

1:::. " 1:::. /::;. 

I " ' 

0.3 0.4 

Figure 3. Linear regression plot of vitrinite reflectance (Rm) values from 29 wells in the northern part of the Green 

River basin. Slope of least-squares linear regression (dashed) is 19,740 ft/log10Rm. 

the LaBarge platform, the top of overpressuring is difficult to define but apparently is within the lower part 

of the Hilliard Shale at a vitrinite reflectance of less than 

0.8 percent. The isoreflectance and top of overpressuring have similar configurations but are not coincident. 

One reason for the apparent lack of coincidence is the 

uncertainty in identifying the top of overpressuring. 

Another source of error is in the method used to calculate 

the depth of 0.8-percent vitrinite reflectance. The calculation assumes that thermal maturity (vitrinite reflectance) 

increases linearly with depth, but a recent study by Law 

and Nuccio (1986) and Law and others (1989) shows that 

vitrinite reflectance profiles in low-permeability rock sequences commonly are segmented and have one or more 

kinks or bends. 

The configuration of thermal maturity contours in 

the area of the Pinedale anticline (fig. 1) is similar to the 

structural configuration (fig. 4). Along the line of section shown in figure 4, the stratigraphically highest 

position of the 0.8-percent isoreflectance line is on the 

west flank of the Pinedale anticline. High vitrinite reflectance levels in this area may result from thermal alteration of organic matter by relatively hot fluids migrating 

upward along the fault. A similar area of unusually high 

thermal maturity has been observed in the Patrick Draw 

oil and gas field on the east side of the Rock Springs uplift 

in Wyoming and may result from hot fluids migrating 

upward along faults and fractures (Law and others, 

1986). 

Numerous attempts have been made to relate the 

configuration in cross section of an isoreflectance line and 

structural attitudes and to thereby infer the relative timing between thermal maturation and structural movement 

(Stach and others, 1975; Bustin and others, 1983). If 

isoreflectance lines are parallel with bedding (fig. 5A), 

thermal maturation is interpreted to have occurred prior 

to folding. If they are horizontal and cut across bedding 

planes (fig. 5B), maturation is believed to have occurred 

Thermal Maturity Patterns, Northern Green River Basin GJ 
SW NE 

A---------------- 55 MILES ----------------•A' 

FEET 

10,000 

5,000 

SEA LEVEL 

-5,000 

-10,000 

-15,000 I 

10 MILES 

I 

10 KILOMETERS 

El Paso Natural Gas 

Wagon WbBBI No. 1 

METERS 

3,000 

2,000 

1,000 

SEA LEVEL 

-1,000 

-2,000 

-3,000 

-4,000 

-5,000 

-6,000 

Figure 4. Cross section showing structures, top of overpressured zone, and 0.8-percent isoreflectance line in the northern 

part of the Green River basin. Line of section A-A' shown in figure 1. Modified from Law and Johnson (this volume, fig. 2). 

after folding. If they cut across bedding planes at a low 

angle and have a shallower dip than the bedding planes 

(fig. 5C), maturation and folding are interpreted to have 

occurred simultaneously. 

Along the cross section shown in figure 4, isoreflectance lines cut across bedding planes but have a steeper 

dip than bedding planes. This case is shown in figure 5D 

and may result from hot fluids ascending along already 

structurally deformed zones. The Pinedale anticline and 

associated structures may have acted as a focus for the 

movement of hot fluids and locally affected thermal 

maturation. We conclude that attempts to establish the 

timing of thermal maturation with respect to structural 

movement that are based on the cross sectional relationship between structure and isoreflectance are not valid 

in the Pinedale anticline area. 

SUMMARY 

The configuration of the 0.8-percent vitrinite 

reflectance contours in the northern part of the Green 

River basin is similar to the configuration of Tertiary 

structures, but because of the possible influences of 

migrating hot fluids, interpretation of the relative timing of thermal maturation and structural events in the 

Pinedale anticline area by using cross sectional relationships may be misleading. 

G4 Geology of Tight Gas Reservoirs, Wyoming and Colorado 

A 

MATURATION BEFORE 

FOLDING 

MATURATION AT THE 

SAME TIME AS FOLDING 

B 

MATURATION AFTER 

FOLDING 

MATURATION ALTERED BY A 

THERMAL (HOT) EVENT 

Figure 5. Relationships between isoreflectance lines (dashed) 

and structural attitudes of sedimentary rocks. Modified from 

Bustin and others (1983). Bedding planes shown by solid lines. 

A, Thermal maturation prior to folding. 8, Maturation after 

folding. C, Maturation at the same time as folding. 0, Maturation altered by a thermal event. 
REFERENCES CITED 

Bustin, R.M., Cameron, A.R., Grieve, D.A., and Kalkreuth, 

W.D., 1983, Coal petrology, its principles, methods, and 

applications: Geological Association of Canada Short 

Course Notes, v. 3, p. 161-169. 

Dow, W. G., 1977, Kerogen studies and geological interpretations: Journal of Geochemical Exploration, v. 7, no. 2, 

p. 79-99. 

King, L.H., Goodspeed, F.E., and Montgomery, D.S., 1963, 

A study of sedimented organic matter and its natural 

derivatives: Canada Department of Mines and Technical 

Surveys, Mines Branch Research Report R-114, 68 p. 

Law, B.E., 1984, Relationships of source rock, thermal maturity, and overpressuring to gas generation and occurrence 

in low-permeability Upper Cretaceous and lower Tertiary 

rocks, Greater Green River basin, Wyoming, Colorado, 

and Utah, in Woodward, Jane, Meissner, F.F., and 

Clayton, J.L., eds., Hydrocarbon source rocks of the 

greater Rocky Mountain region: Rocky Mountain 

Association of Geologists, p. 469-490. 

Law, B.E., and Dickinson, W.W., 1985, Conceptual model for 

origin of abnormally pressured gas accumulation in lowpermeability reservoirs: American Association of 

Petroleum Geologists Bulletin, v. 69, p. 1295-1304. 

Law, B.E., Lickus, M.R., and Pawlewicz, M.J., 1986, Fluid 

migration pathways; evidence from thermal maturity 

mapping in southwestern Wyoming, in Carter, L.M.H., 

ed., USGS research on energy resources, 1986; programs and abstracts: U.S. Geological Survey Circular 

974, p. 35. 

Law, B.E., and Nuccio, V.F., 1986, Segmented vitrinite reflectance proflle from the Deep Seam project, Piceance Creek 

basin, Colorado; evidence of previous high pore pressure: 

American Association of Petroleum Geologists Bulletin, 

v. 70, no. 8, p. 1047. 

Law, B.E., Nuccio, V.F., and Barker, C.E., 1989, Kinky 

vitrinite of reflectance profiles; evidence of paleopore 

pressure in low-permeability, gas-bearing sequences in 

Rocky Mountain foreland basins: American Association of Petroleum Geologists Bulletin, v. 73, no. 8, 

p. 999-1010. 

Law, B.E., and Smith, C.R., 1983, Subsurface temperature map 

showing depth to 180 op in the Greater Green River basin, 

Wyoming, Colorado, and Utah: U.S. Geological Survey 

Miscellaneous Field Studies Map MF-1504, scale 

1:500,000. 

Law, B.E., and Spencer, C.W., 1981, Abnormally highpressured, low-permeability, Upper Cretaceous and 

Tertiary gas reservoirs, northern Green River basin, Wyoming [abs.]: American Association of Petroleum 

Geologists Bulletin, v. 65, no. 5, p. 948. 

Law, B.E., Spencer, C.W., and Bostick, N.H., 1979, Preliminary results of organic maturation, temperature, and 

pressure studies in the Pacific Creek area, Sublette County, Wyoming: U.S. Department of Energy Symposium on 

Enhanced Oil and Gas Recovery and Improved Drilling 

Methods, 5th, Tulsa, Okla., 1979, Proceedings, v. 33, 

Petroleum Publishing Company, p. K2/l-2/13. 

___ 1980, Evaluation of organic matter, subsurface temperature, and pressure with regard to gas generation in 

low-permeability Upper Cretaceous and lower Tertiary 

sandstones in Pacific Creek area, Sublette and Sweetwater 

Counties, Wyoming: The Mountain Geologist, v. 17, no. 

2, p. 23-35. 

Lickus, M.R., Pawlewicz, M.J., Law, B.E., and Dickinson, 

W.W., 1984, Thermal maturity map, northern Green 

River basin, Wyoming, in Law, B.E., ed., Geological 

characteristics of low-permeability Upper Cretaceous and 

lower Tertiary rocks in the Pinedale anticline area, 

Sublette County, Wyoming: U.S. Geological Survey 

Open-File Report 84-753, p 60-65. 

Magoon, L.B., and Claypool, G.E., 1983, Petroleum geochemistry of the North Slope of Alaska; time and degree of 

thermal maturity, in Bjoroey, M., and others, eds., Advances in organic geochemistry, 1981 : International 

Meeting on Organic Geochemistry, lOth, Bergen, Norway, 

1981, John Wiley, p. 28-38. 

Monnier, F., Powell, T.G., and Snowden, L.R., 1983, 

Qualitative and quantitative aspects of gas generation during maturation of sedimentary organic matter; examples 

from Canadian frontier basins, in Bjoroey, M., and 

others, eds., Advances in organic geochemistry, 1981: International Meeting on Organic Geochemistry, lOth, 

Bergen, Norway, 1981, John Wiley, p. 487-495. 

Pawlewicz, M.J., Lickus, M.R., Law, B.E., Dickinson, W.W., 

and Barclay, C.S.V., 1986, Thermal maturity map showing subsurface elevation of 0.8 percent vitrinite reflectance 

in the Greater Green River basin of Wyoming, Colorado, 

and Utah: U.S. Geological Survey Miscellaneous Field 

Studies Map MF-1890, scale 1:500,000. 

Pitman, J.K., Anders, D.E., Fouch, T.D., and Nichols, D.J., 

1985, Depositional environments, diagenesis, and the 

origin of hydrocarbons in nonmarine Upper Cretaceous 

and lower Tertiary rocks, eastern Uinta basin, Utah [abs.]: 

American Association of Petroleum Geologists Bulletin, 

v. 69, no. 5, p. 860. 

Saxby, J.D., 1970, Isolation of kerogen in sediments by chemical 

methods: Chemical Geology, v. 6, no. 3, p. 173-184. 

Spencer, C. W., 1987, Hydrocarbon generation as a mechanism 

for overpressuring in Rocky Mountain region: American 

Association of Petroleum Geologists Bulletin, v. 71, 

p. 368-388. 

Spencer, C.W., and Law, B.E., Overpressured, lowpermeability gas reservoirs in Green River, Washakie, and 

Great Divide basins, southwestern Wyo. [abs.]: American 

Association of Petroleum Geologists Bulletin, v. 65, no. 

33, p. 569. 

Stach, E., Mackowsky, M.-th., Teichmuller, M., Teichmuller, 

R., Murchison, D.G., Taylor, G.H., and Zierke, F., 1975, 

Stach's textbook of coal petrology [translated by D.G. 

Murchison]: Berlin, Gebruder Borntrager, 535 p. 

Thermal Maturity Patterns, Northern Green River Basin G5 

Chapter H 

Gamma-Ray Well-Log Anomaly in the 

Northern Green River Basin of Wyoming 

By STEPHEN E. PRENSKY 

Prepared in cooperation with the U.S. Department of Energy 

U.S. GEOLOGICAL SURVEY BULLETIN 1886 

GEOLOGY OF TIGHT GAS RESERVOIRS IN THE PINEDALE ANTICLINE AREA, WYOMING, 

AND AT THE MULTIWELL EXPERIMENT SITE, COLORADO 
CONTENTS 

Abstract Hl 

Introduction Hl 

Gamma-ray anomaly Hl 

Regional correlation H3 

Method H4 

Results and interpretation H7 

Areal distribution of the arkosic facies H7 

Discussion Hll 

Relation to the Cretaceous-Tertiary boundary H12 

Conclusions H19 

References cited H19 

FIGURES 

1. Map showing structure contours for top of Lower Cretaceous Dakota 

Sandstone, northern Green River basin H2 

2. Chart showing generalized stratigraphic section and plot of gamma-ray 

log for Rainbow Resources No. 1-34 Pacific Creek Federal well H3 

3. Cross section across northern Green River basin showing truncation of 

Upper Cretaceous and lower Tertiary rocks westward onto La Barge 

platform H4 

4. Chart showing open-hole total gamma-ray log and potassium and 

uranium and thorium values derived from natural gamma-ray spectrometry log, Wexpro No. 1 Mesa well H8 

5. North-south gamma-ray log cross section, J-J' H9 

6-13. West-east gamma-ray log cross sections: 

6. A-A' HlO 

7. B-B' Hll 

8. C-C' H12 

9. D-D' H13 

10. E-E' H14 

11. F-F' H14 

12. G-G' H15 

13. H-H' H15 

14-16. North-south gamma-ray log cross sections: 

14. K-K' H16 

15. L-L' H18 

16. M-M' H20 

TABLES 

1. List of study wells, northern Green River basin H5 

2. Wells distant from the Wind River Mountains in the Green River basin 

from which arkose or feldspar has been reported H19 
Gamma-Ray Well-Log Anomaly in the 

Northern Green River Basin of Wyoming 

By Stephen E. Prensky 

Abstract 

An abrupt increase in gamma-ray intensity in lower Tertiary nonmarine clastic rocks relative to underlying Cretaceous 

rocks in the northern Green River basin of Wyoming results 

from an abrupt increase in the volume of arkose and potassium 

feldspar in these rocks due to the unroofing and erosion of the 

granitic core of the Wind River Mountains during the Laramide 

orogeny. In order to study this anomaly, gamma-ray well-logs 

for selected wells were digitized, and plots having a greatly 

compressed depth scale were computer generated. A series 

of east-west and north-south cross sections were constructed 

to facilitate identification and correlation of the anomaly. 

Although changes in facies and a paucity of deep wells prevents 

a definitive correlation, this gamma-ray event extends 

northward into the Hoback basin, westward to the Wyoming 

thrust belt, and southward out of the study area. The anomaly 

may be useful in studying lower Tertiary fluvial drainage systems 

in the basin and, in combination with palynological data, may 

assist in reconstructing the timing of movement on the ProspectHogsback thrust. 

INTRODUCTION 

The northern Green River basin in southwestern 

Wyoming is a foreland basin (Gries, 1983; Lowell, 1983) 

between the Wyoming thrust belt to the west and the 

Wind River Mountains to the east (fig. 1 ). It developed 

as a flexural response to loading caused by concurrent 

overthrusting to the west and uplift to the east (Hagen 

and others, 1985). The thick sedimentary sequence in the 

basin consists of Upper Cretaceous and lower Tertiary 

nonmarine low-permeability sandstones and shales overlying Upper and Lower Cretaceous marine sandstones 

and shales and older rocks of mixed lithology (fig. 2). 

The Tertiary and Upper Cretaceous section is thickest 

along the Wind River thrust: it is thicker than 20,000 ft 

in the Pacific Creek area (figs. 1, 2) and thins westward 

toward the La Barge platform (figs. 1, 3). Throughout 

the basin, varying amounts of uppermost Cretaceous and 

lowermost Tertiary rocks have been removed by the 

widespread and prolonged erosion during Laramide time 

(fig. 3). In some areas on the La Barge platform, 

Tertiary rocks rest unconformably on the Upper 

Cretaceous Hilliard Shale. 

An abrupt shift in gamma-ray intensity in the lower 

Tertiary clastic rocks in the Pinedale area of the northern 

Green River basin (fig. 1) was identified in plots of 

gamma-ray well-log data (Prensky, 1984; Prensky and 

Dickinson, 1986). Subsequent to that identification, additional data were examined to determine the extent of 

the anomaly throughout the basin. 

Gamma-ray logs for more than 170 wells were put 

into digital form in order to produce computer-generated 

displays (Prensky, 1988). Ninety-five of these logs (table 

1) were selected for presentation in subsurface cross sections. They were chosen on the basis of location, total 

depth, and the availability of gamma-ray log data for the 

entire drilled interval. Gamma-ray logs for many wells 

in desirable locations, especially along the basin axis, are 

not available either because these wells were drilled prior 

to widespread availability of the open-hole gamma-ray 

logging tool in the middle 1960's or because this tool was 

considered an unnecessary expense by the well operator. 

Acknowledgments.-The following companies provided data used in this study: American Hunter, American 

Quasar, Amoco, Apache Corporation, Belco Petroleum, 

Blackhawk Resources, Carollo Hay and Associates, 

Chevron, Enron, Davis Oil, Energetics, Gas and Oil 

Management Company, Home Petroleum, IGR, Primary 

Fuels, Rainbow Resources, Wexpro, and Woods Petroleum. 

CW. Keighin and CW. Spencer provided valuable comments. This study was funded in part by the U.S. Department of Energy's Western Tight Gas Sand Program. 

GAMMA-RAY ANOMALY 

The total gamma-ray log from the Wexpro No. 1 

Mesa well (well62, table 1) obtained from a single openhole log run is shown in figure 4. At a depth of about 

8,000 ft, there is an abrupt shift in the data and a similar 

shift is present in data for other wells along the Pinedale 

anticline (wells 70, 62, 63, 58, 52, 53, table 1) (fig. 5). 

The abrupt change in the total gamma-ray log corresponds to sudden changes in apparent potassium and 

Gamma-Ray Well-Log Anomaly, Northern Green River Basin H1 
43° 

1-

_. 

w 

aJ 

1-

en 

:::> 

0: 

J: 

1-

(!) 

z 

::?! 

~ 

R. 110 w. 

NORTHERN 

GREEN RIVER 

BASIN 

R. 105 w. 

109° 

12 18 24 MILES 

Figure 1. Structure contours (interval 2,000 ft) for the top of the Lower Cretaceous Dakota Sandstone, northern Green River 

basin, Wyoming. Dashed line delineates Pinedale anticline. Location of study wells (solid circles) and lines of sections also 

shown. Additional information for wells shown in table 1 by well number. Structure from Petroleum Information Corporation 

(1983). 

thorium contents as shown in the spectral gamma-ray log 

(fig. 4). Petrographic examination of core from wells in 

the Pinedale area shows that the potassium feldspar content of sandstones abruptly changes from as much as 25 

percent in Tertiary rocks (lithic arkose) to essentially zero 

in Cretaceous rocks (Butcher, 1971; Greenfield, 1981; 

H2 Geology of Tight Gas Reservoirs, Wyoming and Colorado 

Keighin, 1984; Law and others, 1986; Pollastro, this 

volume). X-ray diffraction studies of core from the El 

Paso Natural Gas No.1 Wagon Wheel well (well 52, table 

1) confirm these results and also indicate the presence of 

potassium micas in the Tertiary rocks (Pollastro, 1985, 

this volume). 
~ <( 

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Wasatch Formation 

Fort Union Formation 

Lance Formation 

B.lair Formation and 

Rock Springs Formation 

Baxter Shale 

tw 

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Rainbow Resources4 Pacific Creek No 1-34 

Sec. 34, T. 27 N., R. 103 W. 

Total depth 25,764 ft 

80 240 320 

GAMMA-RAY LOG. API UNITS 

400 

Figure 2. Generalized stratigraphic section and plot of gamma-ray log for Rainbow Resources No. 1-34 

Pacific Creek Federal well (well 24, table 1, fig. 1 ). Intermediate casing points and log-run breaks (LRB) 

are noted for purpose of identifying data artifacts. Formation tops from Petroleum Information Corporation (1987). 

Regional Correlation 

Large-scale lithologic and stratigraphic trends can 

be examined by using compressed-scale plots of well-log 

data; that is, data plots on which both the vertical (depth) 

and horizontal scales are much smaller than in standard 

presentations of well-log data (Prensky, 1986; Prensky 

and Dickinson, 1986). In areas where strong contrasts in 

lithology exist, such as the northern Green River basin, 

stratigraphic units can be identified by their different 

Gamma-Ray Well-Log Anomaly, Northern Green River Basin H3 
D D' 

1------------------ 60 MILES (96 KILOMETERS)----------------~ 

FEET 

10,000 

Davis El Paso 

Granite Wash No. 1 Wagon Wheel No.1 METERS 

Sec. 16, T. 29 N., R. 110 W. Sec. 5, T. 30 N., R. 108 W. West 3,000 flank of 

Davis PINEDALE 

West Pinedale No.1 ANTICLINE 

8,000 

LABARGE 

PLATFORM Sec. 33, T. 30 N., R. 109 W. 

6,000 

4,000 

2,000 

SEA 

LEVEL 

-2,000 

-4,000 

-6,000 

-8,000 

-10,000 

-12,000 

-14,000 

FEET METERS 

4000 

2000 

1000 

1000 

500 

TO 16,625 

10 KILOMETERS 

8 MILES 

2,000 

1,000 

SEA 

LEVEL 

-1,000 

-2,000 

-3,000 

-4,000 

-16,000~----------------------------------------~ 

Figure 3. Cross section across northern Green River basin showing truncation of Upper Cretaceous and lower Tertiary rocks 

westward onto La Barge platform. Line of section 0-0' shown in figure 1. Modified from Law (1984). 

well-log "signatures" or patterns (fig. 2) and regional 

correlations can be made on the basis of these differing 

signatures. Particularly distinctive signatures in the 

northern Green River basin include the high API 

(American Petroleum Institute) gamma unit tail in the 

nonmarine lower Tertiary rocks, which is mostly produced by potassium feldspar, and a similar tail in the 

marine Mowry Shale (figs. 2, 4). These two patterns form 

the basis for the correlations of the gamma-ray anomaly 

and of the Second Frontier sandstone of local informal 

usage (Merewether and others, 1984) shown in the sections (figs. 5-16). The Second Frontier sandstone has a 

distinctive signature and is present in deep wells 

throughout the basin; thus it provides a reliable 

stratigraphic correlation. 

H4 Geology of Tight Gas Reservoirs, Wyoming and Colorado 

Method 

Data for the selected wells were digitized in 1-ft increments from total natural gamma-ray logs (table 1). 

Data entry was accomplished either through the manual 

digitization of copies of analog prints or directly from 

well-logging service company digital tapes. Compressedscale plots for each well were then computer generated. 

The plotted gamma-ray log data were not corrected 

for borehole size and mud weight. If the original log was 

run prior to the use of standard API gamma units, an 

approximate conversion to API units was made. For a 

few wells, only cased-hole gamma-ray logs are available 

and no corrections were applied. Depths for casing and 

log-run splices (table 1) are provided to help identify data 
Table 1. List of study wells, northern Green River basin 

[Data from well-log headers and from Petroleum Information (1987). Well locations shown by number on figure. Location given in section, 

township, and range; grid shown on figure 1. Well names may be abbreviated. Completion date in month, day, and year. Depths and elevations 

in feet. Leaders (--)indicate not applicable; OWDD indicates old well, drilled deeper] 

Kelly Data 

Map Completion Total Bushing Casing splices 

no. Well name location date de~th elevation de~th log de~th 

1 Davis Oil 10 Storm Shelter 08, 23N, 111W 02/10/78 10,500 6,423 3,856 None 

2 Texaco 1 Government-Matthews 04, 23N, 112W 08/14/85 11,750 6,748 1,485 None 

3 Amoco 10 Shute Creek 20, 23N, 112W 08/21/82 11,900 6,523 1,519 None 

4 R. Lacy 1 USA Government 05, 24N, 108W 08/13/64 8,995 6,854 326 None 

5 General Petroleum 62-A-9 Buckhorn 09, 24N, 109W 10/30/55 9,275 6,635 1,698 None 

6 Chorney Oil 1-10 Broken Hand 10, 24N, llOW 09/02/69 7,200 6,547 335 None 

7 Davis Oil 1 Paparone Federal 25, 24N, 111W 09/02/76 10,621 6,518 528 None 

8 FMC 1-11 Pomeroy Draw 11, 24N, 113W 11/16/80 11,425 6,905 1,005 None 

9 Pan American Petroleum 1 State I 16, 25N, 105W 11/23/63 8,441 6,708 172 None 

10 American Hunter 1 Enterprise 30, 25N, 107W 04/20/82 17,045 6,764 1,486 None 

11,056 11,000 

11 American Hunter 1 Faraway 17, 25N, 108W 10/26/81 16,525 7,022 1,554 None 

10,278 10,200 

12a Davis Oil 1 Sandy Unit 25, 25N, 108W 10/31178 10,825 6,811 168 None 

b Davis Oil 1 Simpson Unit (OWDD) 25, 25N, 108W 08/17179 17,034 6,816 5,002 None 

c Merged data 25, 25N, 108W 17,034 6,811 5,002 10,550 

13 Energetics 23-14 Federal 14, 25N, 111 W 03/16/79 10,480 6,885 1,038 None 

14 Natural Gas Corp. of Calif. 2-20 Federal 20, 25N, 111 W 01/07/83 9,350 6,914 920 None 

15 Natural Gas Corp. 33-4E Fontenelle Fed. 04, 25N, 112W 10/01/85 8,450 6,803 850 None 

16 Natural Gas Corp. of Calif. 34-21E Federal 21, 25N, 112W 10/21/82 9,150 6,542 4,196 4,140 

17 Davis Oil 1 Parrish Mark 12, 26N, 109W 01/27/79 15,872 6,907 5,005 None 

18 Davis Oil 1 18-Mile Canyon 17, 26N, 109W 07/13/65 8,004 6,968 278 None 

19 Energetics 32-33 Ferguson Federal 33, 26N, 110W 07/16/78 11,555 7,037 1,028 None 

20 Belco Petroleum 8-11 Stead Canyon 11, 26N, 112W 04/05/82 8,535 6,936 326 None 

21 Belco Petroleum 84-18 Green River Bend 18, 26N, 112W 04/25/83 7,610 6,635 1,209 None 

22 Mobil Oil T21-21G Hogsback 21, 26N, 113W 04/15/69 8,300 6,919 503 None 

23a Ferguson 1 Government 18, 27N, 102W 02/01/68 8,421 7,154 522 None 

b Woods Petroleum 18, 1-A Govt (OWDD) 18, 27N, 102W 06/26/69 12,492 7,157 522 None 

c Merged data 18, 27N, 102W 12,492 7,154 522 8,300 

24 Rainbow Resources Pacific Creek 1-34 34, 27N, 103W 09/10/82 25,764 7,080 1,533 None 

11,409 11,370 

16,000 

21,550 

21,670 21,650 

25 Davis Oil 1 Bright Trail 21, 27N, 108W 06/04179 18,000 6,905 4,975 None 

26 Davis Oil 1 Yellow Point Ridge 07, 27N, 109W 10/15/78 15,772 7,085 3,932 None 

27 Energetics 20-8 Federal 08, 27N, llOW 08/25/81 14,405 7,225 994 13,400 

28 Energetics 30-23 East Bird Canyon 23, 27N, 111W 06/20/79 11,750 7,243 1,010 None 

29 Energetics 10-32 Federal 32, 27N, 111 W 07/22/80 9,730 7,235 820 None 

30 Belnorth 32-5 North LaBarge Unit 05, 27N, 113W 10/04/84 7,385 7,494 313 None 

31 Exxon 1 Graphite 16, 27N, 114W 10/13/81 17,317 9,214 60 None 

4,885 4,850 

13,191 13,150 

15,800 

32 American Quasar 1 Skinner Federal 32, 28N, 101 W 11/06!75 15,040 7,581 375 None 

2,505 2,450 

13,010 12,950 

33 California Oil 1 Government 34, 28N, 106W 08/30/62 8,502 6,989 750 None 

34 Home Petroleum 1-4 Jonah Federal 04, 28N, 108W 1987 12,594 7,139 2,550 None 

35 Woods Petroleum 2 Cutlass 24, 28N, 110W 10/30/82 17,700 7,102 4,000 None 

14,000 13,980 

36 Energetics 30-21 Chorney Federal 21, 28N, 111W 07/14/83 12,150 7,038 998 None 

37 Belco Petroleum 1-2 Chapel Canyon 02, 28N, 112W 10/13/81 11,075 6,782 1,002 None 

Gamma-Ray Well-Log Anomaly, Northern Green River Basin HS 
Table 1. Continued 

Kelly Data 

Map Completion Total Bushing Casing splices 

no. Well name location date depth elevation depth log depth 

38 Energetics 13-16 Dry Piney-State 16, 28N, 112W 07/05/81 9,738 6,952 815 None 

39 Mobil Oil T63X-27P Tip Top 27, 28N, 113W 08/21/84 7,300 7,182 827 None 

40 Belnorth Petroleum 36-33 North LaBarge 33, 28N, 113W 12/22/84 7,700 8,038 822 None 

41 Husky Oil 8-2 Federal 02, 29N, 106W 10/02/73 12,944 7,208 1,943 6,380 

11,650 

42 Quadrant Oil14-16 State 16, 29N, 106W 08/12/72 10,050 7,113 499 None 

43 Davis Oil 1 Granite Wash 16, 29N, 110W 11/30/77 16,625 7,018 4,016 None 

44 Primary Fuels 24-20 Van Buren Federal 20, 29N, lllW 09/27/82 12,394 6,870 2,025 None 

45 Chandler 1 ldeen Government 35, 29N, 111W 08/19/69 6,206 6,815 231 None 

46 Belco Petroleum 4-4 BUDD 04, 29N, 113W 03/13/80 3,250 7,359 230 None 

47 Mobil Oil T72-19G Tip Top 19, 29N, 113W 07/03/79 8,610 7,517 887 7,850 

48 Mobil Oil F12-33G Tip Top 33, 29N, 113W 10/04/77 7,900 7,542 788 None 

49 American Quasar 8-24 Riley Ridge 08, 29N, 114W 09/24/80 16,000 7,760 69 None 

1,990 2,050 

9,865 9,800 

15,450 15,420 

50 Terra Resources 1-12 North Tip Top 12, 29N, 114W 02/28/82 11,590 7,709 2,220 10,300 

51 American Hunter 2 New Fork 02, 30N, 108W 07/08/81 11,986 7,282 4,425 None 

52 El Paso Natural Gas 1 Wagon Wheel 05, 30N, 108W 08/01/71 19,000 7,089 2,479 5,065 

7,121 

12,086 12,090 

15,200 

53 American Hunter 1 New Fork 25, 30N, 108W 07/10/81 10,989 7,338 4,068 None 

54 Davis Oil 1 West Pinedale 33, 30N, 109W 11/18/75 11,507 7,052 478 None 

55 Chandler 1 James Mickelson 15, 30N, 112W 10/30/69 6,288 6,954 222 None 

56 General Petroleum 63-21 Bar Cross X 21, 30N, 112W 08/20/54 8,270 7,008 306 None 

57 Mountain Fuel Supply 3 Johnson Ridge 19, 30N, 113W 07/04/72 4,837 7,562 202 None 

58 Hay-Carollo 2 Jensen 11, 31N, 109W 08/10/83 10,302 6,925 1,046 None 

59 Belco Petroleum 2-27 Mason 27, 31N, 113W 03/14/83 13,340 7,295 2,504 None 

60 Pacific Transmission 13-4 Federal 04, 31N, 114W 08/01/75 12,586 8,437 1,025 None 

10,925 10,880 

61 Natural Gas Calif. 13-5 Castle Creek 05, 31N, 114W 03/03/81 7,058 8,698 226 None 

62 Mountain Fuel Supply (Wexpro) 1 Mesa 07, 32N, 109W 07/15/81 12,050 7,524 1,033 None 

63 Wexpro 2 Mesa 16, 32N, 109W 06/22/81 12,190 7,474 1,023 None 

64 Phillips Petroleum 4 Marbleton Unit 10, 32N, 112W 02/15/60 11,565 7,380 506 11,150 

65 Ray Smith Drilling 1 Fear Ranch Unit 13, 32N, 113W 07/20/58 9,318 7,564 312 None 

66a Davis Oil 1 Aspen Ridge 22, 32N, 113W 06/15/74 7,854 7,902 378 None 

b Davis Oill Aspen Ridge (OWDD) 22, 32N, 113W 04/23/76 14,940 7,891 2,010 None 

c Merged data 22, 32N, 113W 14,940 7,902 378 None 

2,010 None 

7,550 7,800 

67 Belco Petroleum 3-8 Ote Creek 08, 32N, 114W 11/17/80 5,575 8,058 394 None 

68 S. Louisiana (Celeron) 21-28 Castle Creek 08, 32N, 114W 07/22/82 11,875 8,167 1,400 None 

10,600 10,540 

69 Mountain Fuel Supply 8 Pinedale Unit 20, 33N, 109W 01/06/64 10,500 7,448 1,009 None 

70 Blackhawk Resources 21-24 Baumgartner 24, 33N, llOW 12/12/81 11,238 7,344 800 None 

71 Phillips Petroleum 1 Daniel 21, 33N, 111W 04/04/56 16,531 7,307 529 None 

72 Union Texas Natural Gas 1 Federal-Pure 20, 33N, 113W 11/11/60 7,268 7,827 451 None 

73 Depco 7-4 Federal 04, 35N, 113W 09/13/71 5,510 7,843 336 None 

74 Davis Oil 1 Bacon Ridge 33, 35N, 114W 09/06/78 11,820 8,937 840 None 

8,256 9,290 

75 Pacific Trans Supply 24-24 Federal 24, 35N, 115W 10/22/75 14,999 9,778 1,005 None 

8,340 8,290 

H6 Geology of Tight Gas Reservoirs, Wyoming and Colorado 
Table 1. Continued 

Kelly Data 

Map Completion Total Bushing Casing splices 

no. Well name Location date depth elevation depth log depth 

76 Anadarko Production 1 Willow Lake Unit 29, 36N, 109W 11/24170 11,753 8,633 10,237 None 

77 Pacific Transmission Supply 33-13 Fed. 13, 36N, 111W 02/12/74 11,153 7,743 1,020 None 

9,943 9,910 

78a Inexco A, 1 WASP 28, 36N, 112W 11/14/70 14,363 7,812 30 None 

1,844 2,050 

10,182 10,180 

b Belco Petroleum 3-28 Merna Unit (OWDD) 28, 36N, 112W 02/24177 18,124 7,815 16,209 14,350 

6,209 16,200 

16,978 16,950 

c Merged data 28, 36N, 112W 18,124 7,812 30 None 

1,844 2,050 

10,182 10,180 

16,209 14,350 

16,209 16,200 

16,978 16,950 

79 Apache Corporation 36-1 Robbins 36, 36N, 112W 09/27/85 16,480 7,625 1,590 None 

10,545 10,515 

80a Rainbow Resources 1-7 Cliff Creek 07, 36N, 114W 10/04177 11,548 7,649 486 None 

2,905 2,950 

8,628 8,590 

b Chevron Rainbow 1-7 Federal (OWDD) 07, 36N, 114W 07/19/81 15,561 7,659 15,531 13,510 

c Merged data 07, 36N, 114W 15,561 7,649 486 None 

2,905 2,950 

8,628 8,590 

(cased-hole gamma-ray neutron log) 15,531 13,510 

81 Home Petroleum 34-23 New Forks Carney 23, 37N, 110W 12/06/84 4,750 7,649 55 2,750 

82 Home Petroleum 32-27 New Forks Black 27, 37N, llOW 12/19/85 5,050 7,655 785 None 

83 Pure Oil 1 Bondourant Unit 22, 37N, 112W 10/19/64 5,202 7,200 378 None 

84 Pure Oil 2 Bondourant Unit 29, 37N, 112W 11/06/64 4,570 7,118 326 None 

85 Pan American 1 Empire State 25, 37N, 113W 11/09/63 4,509 6,900 303 None 

86 Chevron 31-33C Cabin Creek Federal 33, 37N, 114W 07/28/86 11,305 8,041 494 490 

3,483 3,450 

7,880 

9,008 9,000 

87 Depco 1 Bondourant Unit 09, 38N, 113W 03/12/71 13,300 7,127 1,050 None 

5,077 6,650 

Wells added after preparation of initial list 

88 Gas & Oil Management GOMI 1-7 Federal 07, 30N, llOW 1987 9,600 7,033 531 None 

89 BWAB 20-24 20, 31N, 111W 02/21/86 7,300 7,153 675 None 

90 Gas Oil Management 1-28 New Tech Fed. 28, 31N, 111W 1988 9,570 6,982 1,067 None 

91 Skyline 1 Piney Creek 23, 31N, 114W 11/21/82 5,300 7,876 410 None 

92 Natural Gas Corp of Calif. 23-16A State 16, 32N, 114W 08/14/81 7,201 7,971 34 None 

93 Pure Oil 1 USA Broady Draw 31, 34N, 112W 08/21/64 5,170 7,529 396 None 

94 Gulf-U.S. 1 West Side Federal 32, 34N, 114W 11/01/72 8,487 8,254 782 None 

95 Depco 5-27 Federal 27, 35N, 113W 09/30/71 5,490 8,094 329 None 

shifts that are artifacts resulting from the change from RESULTS AND INTERPRETATION 

cased-hole to open-hole conditions or vice versa or from 

changes in logging tools or calibrations. Areal Distribution of the Arkosic Facies 

All depths have been corrected to true elevation 

relative to sea level. Despite the schematic nature of these 

sections (they lack true horizontal scale), gross structural 

and stratigraphic elements within the study area can be 

observed. 

The data are presented in figures 6-13 as transverse 

sections A through H and in figures 5 and 14-16 as 

longitudinal sections J through M. As mentioned earlier, 

these data, at least those for the Pinedale anticline, reflect 

Gamma-Ray Well-Log Anomaly, Northern Green River Basin H7 
TOTAL GAMMA RAY POTASSIUM THORIUM URANIUM 

0~------------------~--------------------~--------------------~------------------~ 

1-

LU 5 LU 

u. 

u. 

0 

(/) 

Cl 

z 

<( 

(/) 

::J 

0 

I 

1-

z 

:i 

b: 10 LU 

Cl 

15~------------------~--------------------~--------------------~------------------~ 0 200 0 

API UNITS PERCENT 0.05 0 PPM 400 PPM 5 

Figure 4. Open-hole total gamma-ray log (API gamma units), and potassium (percent) and uranium and thorium (ppm) values 

derived from natural gamma-ray spectrometry log, Wexpro No. 1 Mesa well (well 62, table 1, fig. 1 ). Horizontal line indicates 

position of gamma-ray anomaly. Note arcuate pattern of increasing and then decreasing API gamma unit intensity in total gammaray log just above anomaly indicated by solid line. 

the presence and relative amounts of postassium feldspar 

and mica in that the magnitude of gamma-ray intensity 

(as indicated by API value) is directly related to the 

percentage of these constituents. At a given locality, the 

volume of potassium minerals, which are present in 

arkosic sandstones, is related to proximity to source terrain and to stream gradient. Consequently, gamma-ray 

intensity may be an indication of relative stream gradients; these, in turn, ultimately reflect tectonic uplift or 

subsidence. The gamma-ray anomaly is most striking in 

wells on the Pinedale anticline (fig. 5), and the magnitude 

of the anomaly (or shift) diminishes westward away from 

the Wind River Mountains (figs. 6-12). Absolute gammaray intensities in granitic intervals (wells 32 and 41, table 

1) are extremely high and decrease with distance from the 

Wind River Mountains, especially across the Wind River 

thrust where the igneous granite facies changes to a 

detrital lithic arkosic facies (figs. 6, 8). The proximity and 

areal distribution of the arkosic facies relative to these 

mountains indicate that the Precambrian core of the 

H8 Geology of Tight Gas Reservoirs, Wyoming and Colorado 

Wind River Mountains was the source of the arkose 

(Shuster, 1986). 

Gamma-ray log data for the Mesa well (fig. 4) form 

an arclike pattern of high API values with the API shift 

at the lower part of the arc. In this pattern, gamma-ray 

intensity rises rapidly to a maximum value and then gradually decreases. The pattern, as well as the absolute gammaray intensity discussed above, becomes more subdued with 

distance from the presumed source terrane (figs. 6-13). The 

pattern may parallel major Laramide activity, which began 

in latest Cretaceous time (Love, 1960) and coincides with 

uplift of the Wind River Mountains. By middle to late 

Paleocene time, the crystalline core of the Wind River 

Mountains had been breached (Berg, 1961; Keefer, 1965; 

Merin and Lindholm, 1986) and erosion had begun. Deformation continued to intensify through early Eocene time 

(Love, 1960) and began to diminish by middle and late 

Eocene time (Love, 1960; Dorr and others, 1977). By Eocene time, deposition of arkose in Rocky Mountain basins was widespread (Love, 1960; Vine and Tourtelot, 1970). 
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~ 

Ill 

:r 

:::t 

I.C 

J PINEDALE ANTICLINE J' 

Well no. 871 Well no. 841 Well no. 781 Well no. 70 I Well no. 621 Well no. 631 Well no. 581 Well no. 521 Well no. 531 Well no. 331 Well no. 24 

0 200 0 200 0 200 0 300 0 200 0 250 0 250 0 200 0 200 0 250 0 200 

+1o GAMMAAPI UNITS 

fLJ.J 

+5 

t1: 0 

L.L. 

0 

(j) 

0 

z 

<( 

en 

::> 

0 I -5 fz 

z 

0 

f= 

<( 

> LJ.J u:l-10 

-15 

L 

{Cased 5 hole 

CSG 

Figure 5. North-south gamma-ray log cross section)-]', northern Green River basin. Casing points (CSG) and log-run breaks (LRB) indicated. Solid line indicates position of gamma-ray anomaly; dashed, queried line indicates position of anomaly is uncertain. Wells listed 

by number in table 1 and shown in figure 1. Line of section shown on figure 1. Datum is sea level. 
::::t: 

..... 

= 

C'l 

~ 

g 

S, 

-1 

rjQ" 

~ 

C'l 

Ill 

"' 

~ 

~ 

~ Q 

~· 

~ 3 

:;-

CJQ 

Ill 

:I 

Q. 

n 

Q 

0 

;:: 

Q. 

Q 

1-

L.U 

L.U 

lL 

lL 

0 

(/) 

0 

z 

<( 

(/) 

:::::> 

0 

I 

1-

~ 

z 

0 

i= 

<( 

> 

A LABARGE PLATFORM A' 

Well no. 3 I Well no. 1 I Well no. 7 I Well no. 6 I Well no. 5 I Well no. 4 I Well no. 121 Well no. 10 I Well no. 9 I Well no. 241 Well no. 231 Well no. 32 

0 200 0 200 0 200 0 200 0 200 0 200 0 200 0 200 0 200 0 200 0 200 0 300 

+10 I GAMMA API UNITS I 

CSG 

+5 

-5 

,, ....... , __ Second f:. 

---~ntierp, --.!:!!~f!::.n 

--. 

----""""--

~ -10 L.U 

-15 

Figure 6. West-east gamma-ray log cross section A-A', northern Green River basin. Casing points (CSG) and log-run breaks (LRB) indicated. Solid line 

indicates position of gamma-ray anomaly; dashed, queried line indicates position of anomaly is uncertain. Wells listed by number in table 1 and shown 

in figure 1. Line of section shown on figure 1. Datum is sea level. 
B LABARGE PLATFORM B' 

Well no. 8 I Well no. 141 Well no. 131 Well no. 19 I Well no. 181 Well no. 171 Well no. 251 Well no. 33 

o 200 o 200 o 200 o 200 I o 200 o 200 o 200 o 200 

GAMMAAPI UNITS +10r-----------------------------------~~--------------------------~ 

I- +5 UJ 

UJ 

L.L 

L.L 

0 

(j) 

0 

z 

<( 

(j) 0 ::::) 

0 

I 

I-

~ 

z' 

0 

i= 

<( -5 

> 

UJ 

_j 

UJ 

-10 

Figure 7. West-east gamma-ray log cross section B-B', northern Green River basin. Casing points (CSG) 

and log-run breaks (LRB) indicated. Solid line indicates position of gamma-ray anomaly; dashed, queried 

line indicates position of anomaly is uncertain. Wells listed by number in table 1 and shown in figure 

1. Line of section shown on figure 1. Datum is sea level. 

Discussion 

Arkosic sandstones have been reported in wells in 

the Pinedale, La Barge and Big Sandy areas (Berg, 1961, 

1962; Asquith, 1966; Martin and Shaughnessy, 1969; 

Shuster, 1986). Shuster (1986) discussed the petrology and 

provenance of Upper Cretaceous and lower Tertiary rocks 

(primarily from outcrop) in the northern Green River 

basin and suggested that the feldspathic nature of the 

lower Paleocene rocks (Hoback Formation, Fort Union 

Formation, Chappo Member of the Wasatch Formation, 

unnamed unit of Law (Law and Nichols, 1982)) indicates 

a granitoid source, probably the northern Wind River 

Mountains. The Gros Ventre uplift probably served as 

a secondary and more localized source of Hoback Formation granitic debris. 

Examination of lithologic logs (American Stratigraphic Company, Denver, Colo.) indicates that arkose 

has been reported in wells throughout the basin. In the 

logs of many wells, particularly those on the La Barge 

platform, arkose has not been specifically reported, but 

feldspar grains (presumably potassium feldspar) have 

been reported (table 2). Lithologic logs do not provide 

definitive evidence for the presence or absence of these 

constituents, in part because of inconsistencies in identification and reporting on the logs and in part because 

they are not available for most drilled wells. Despite these 

problems, the logs can aid interpretations. The gammaray shift is at or below the deepest reported occurrence 

of arkose or feldspar. 

Although the only analyzed core data available for 

this study are from the Pinedale area, the abovementioned reports of arkose and feldspar suggest that, 

during lower Tertiary time, westward-draining fluvial patterns reached as far west as Rs. 112 and 113 W. Paleogeographic reconstructions of the northern Green River 

basin by Shuster (1986) indicate a southerly drainage in 

the basin throughout Upper Cretaceous and lower Tertiary times, and Shuster believed that the amount of 

detritus supplied from the Wind River and Gros Ventre 

Mountains was sufficient to force drainage westward 

toward the basin margin. In fact, arkosic material may 

exist farther west than the present-day margin of the 

basin. Reports of Tertiary sedimentary rocks underlying 

the leading edge of the Eocene-to Paleocene-age 

Prospect-Hogsback thrust, along which 31 mi of crustal 

Gamma-Ray Well-Log Anomaly, Northern Green River Basin H11 
c LABARGE PLATFORM C' 

Well no. 221 Well no. 21 I Well no. 20 I Well no. 291 Well no. 281 Well no. 271 Well no. 351 Well no. 341 Well no. 421 Well no. 41 

0 200 0 200 0 200 0 200 0 200 0 200 0 200 0 200 0 300 0 500 

+1or----------------------------------G_A_M_M_A_A __ PI_U_N_I_TS __________________________________ ~ 

rt±1 +5 L.L 

L.L 

0 

(f) 

0 

z 

<! 

(f) 

:::J 

~ 0 r-

~ 

z 

0 

i= 

<! 

> 

~ -5 UJ 

-10 

G 

Figure 8. West-east gamma-ray log cross section C-C', northern Green River basin. Casing points (CSG) and log-run breaks 

(LRB) indicated. Solid line indicates position of gamma-ray anomaly; dashed, queried line indicates position of anomaly is uncertain. Wells listed by number in table 1 and shown in figure 1. Line of section shown in figure 1. Datum is sea level. 

shortening has been proposed (Dixon, 1982), suggest the 

existence of a much broader east-west fluvial plain during lowermost Tertiary time than is indicated by presentday conditions. In addition, 15 mi of crustal shortening 

may have occurred on the Wind River thrust (Hagen and 

others, 1985). 

Relation to the Cretaceous-Tertiary Boundary 

The preliminary report on this gamma-ray log 

anomaly (Prensky, 1984) suggests a possible relationship 

between the anomaly and the Cretaceous-Tertiary boundary in the basin. This boundary has been dated by using 

palynological determinations from core samples and, in 

a few cases, drill cuttings. Unpublished data (D.J. 

Nichols, U.S. Geological Survey, to B.E. Law, written 

commun., 1984) indicate that in the Pinedale area, where 

little or no erosion of lower Tertiary and uppermost 

Cretaceous rocks (El Paso No.1 Wagon Wheel and Wexpro No. 1 Mesa; wells 52 and 62, table 1) has occurred, 

the gamma-ray anomaly is time transgressive. In the 

western part of the basin, the depths of the gamma-ray 

log anomaly and the Cretaceous-Tertiary boundary converge to within a short distance of about 100ft (General 

Petroleum No. 62-A-9 Buckhorn and No. 63-21 Bar 

H12 Geology of Tight Gas Reservoirs, Wyoming and Colorado 

Cross X; wells 5 and 56, table 1) as a result of erosional 

truncation of the lower Tertiary and uppermost Cretaceous rocks. 

A facies change occurs in lower Tertiary rocks from 

the northwestern margin of the basin (Hoback Formation) to the eastern margin of the basin (Wasatch and Fort 

Union Formations) (fig. 12). The part of the Hoback Formation in the Hoback Basin (northernmost Green River 

basin) is middle Paleocene in age (Dorr, 1952, 1958; 

Guennel and others, 1973; Dorr and others, 1977) and 

is composed of nonarkosic, recycled Mesozoic rocks derived from the Hoback Range (Spearing, 1969). The 

arkosic facies in the northern Green River Basin is generally assigned to the Eocene Wasatch Formation or its 

equivalents (Martin and Shaughnessy, 1969; West, 1969; 

Dorr and others, 1977), but arkose deposition may have 

begun earlier. In the vicinity of the Rock Springs uplift, 

the Fort Union Formation is assigned a middle Paleocene 

age based on vertebrate fossils, and Fort Union sandstones have been described as containing minor coarsegrained feldspars (Berg, 1961); an unnamed pre-Fort 

Union Tertiary arkosic sequence has been correlated 

northward from the Rock Springs area to the Merna area 

(Law, 1979; Law and Johnson, this volume). Definitive 

subsurface resolution of this facies change is hindered by 

a lack of deep wells between Ts. 34 and 38 N. and further 
~ Dol 

3 

3 

Dol 

~ '< 

$! 

~ ,... 

Q 

QQ 

> 

= Q 

3 

Dol 

~ 

z 

Q 

;::s. 

:::r 

~ 

= C') 

~ = 

"' 

~· ... 

Dol = 

fl> 

;· 

= I.N 

D LABARGE PLATFORM PINEDALE ANTICLINE D' 

Well no. 31 I Well no. 30 I Well no. 40 I Well no. 381 Well no. 371 Well no. 441 Well no. 451 Well no. 431 Well no. 541 Well no. 521 Well no. 51 

0 200 0 200 0 200 0 200 0 200 0 200 0 200 0 200 0 200 0 200 0 200 

GAMMAAPI UNITS +10~------------------------------------------------------------------------------------------------------~ 

I- +5 LlJ 

LlJ 

L.L. 

L.L. 

0 

(/) 

0 

z 

<( 

(/) 0 .:::::> 

0 

I 

I-

~ 

z0 

i= 

<( 

-5 ~ iit~~·~·· .. > . -·""!(!..~:::;.,~ .. :s. LlJ 

_J ' &co ................ '1)0' LlJ 

....... ::> ~ 

.,......_ o'Jt;&,. 

........ ....... 

t 

-10 

Figure 9. West-east gamma-ray log cross section 0-0', northern Green River basin. Casing points (CSG) and log-run breaks (LRB) indicated. 

Solid line indicates position of gamma-ray anomaly; dashed, queried line indicates position of anomaly is uncertain. Wells listed by number 

in table 1 and shown in figure 1. Line of section shown on figure 1. Datum is sea level. 
E LABARGE PLATFORM PINEDALE ANTICLINE E' 

Well no. 491 Well no. 50 I Well no. 461 Well no. 561 Well no. 551 Well no. 881 Well no. 58 

0 200 0 200 0 200 0 200 0 200 0 200 0 200 

10 ~--------------------G~A~M~M~A~A~P~I~U_N~IT_S~------------------~ 

I- CSGI tsG- w 

w +5 ··' L.L ?- . L.L -.. ~. 0 

(/) ---- 0 "" "" 

<i 

z - (/) 

:::J 

0 0 f:or I -'Jiatioi'J I-

~ 

z' 

0 

i= 

<i 

> w 

_J -5 w 

-10~--------------------------------------------------------------~ Figure 10. West-east gamma-ray log cross section E-E', northern Green River basin. Casing 

points (CSG) and log-run breaks (LRB) indicated. Solid line indicates position of gamma-ray 

anomaly; dashed, queried line indicates position of anomaly is uncertain. Wells listed by 

number in table 1 and shown in figure 1. Line of section shown on figure 1. Datum is sea level. 

F PINEDALE ANTICLINE F' 

Well no. 681 Well no. 661 Well no. 651 Well no. 641 Well no. 71 I Well no. 70 I Well no. 69 

0 200 0 200 0 200 0 200 0 200 0 300 0 200 

GAMMAAPI UNITS +10r-----------------------~--------------------------------------~ 

1--

w 

~ +5 L.L 

0 

(/) 

0 

z 

<i 

(/) 

:::J 

~ 0 I-

~ 

z 

0 

i= 

<i 

> 

~ -5 w 

----. --........... ?---

( 

...... ~"'--· 

~···,. ' 

.. ";:: 

-10~------------------------------------------------------------~ Figure 11. West-east gamma-ray log cross section F-F', northern Green River basin. Casing 

points (CSG) and log-run breaks (LRB) indicated. Solid line indicates position of gamma-ray 

anomaly; dashed, queried line indicates position of anomaly is uncertain. Wells listed by 

number in table 1 and shown in figure 1. Line of section shown on figure 1. Datum is sea level. 

H14 Geology of Tight Gas Reservoirs, Wyoming and Colorado 
fLU 

fLU 

LU 

LL 

LL 

0 

(/) 

0 

z 

<( 

(/) 

~ 

0 

I 

f-

~ 

z 

0 

f= 

<( 

> 

LU 

....J 

UJ 

G G' 

Well no. 751 Well no. 731 Well no. 781 Well no. 791 Well no. 771 Well no. 76 

0 200 0 200 0 200 0 200 0 400 0 200 

10 ~----------------~G~A~M~M~A~A~P~I~U~N~IT~S~----------------~ 

+5 

0 

-5 

-10 

j--c~G .. 

••• ••• 

LRB 

Cased 

hole 

Figure 12. West-east gamma-ray log cross section G-G', northern Green River 

basin. Casing points (CSG) and log-run breaks (LRB) indicated. Solid line indicates 

position of gamma-ray anomaly; dashed, queried line indicates position of anomaly 

is uncertain. Wells listed by number in table 1 and shown in figure 1. Line of section shown on figure 1. Datum is sea level. 

H H' 

Well no. 861 Well no. 80 I Well no. 851 Well no. 841 Well no. 831 Well no. 821 Well no. 81 

0 200 0 200 0 200 0 200 0 200 0 200 0 200 

GAMMA API UNITS +10~----------------------~~~~~~~~--------------------~ 

t1: +5 

LL 

0 

(/) 

0 

z 

<( 

(/) 

~ 

0 

I 

f-

~ 

z 

0 

f= 

<( 

> 

0 

~ -5 LU 

-10~--------------------------------------------------------------~ 

Figure 13. West-east gamma-ray log cross section H-H', northern Green River basin. Wind 

River thrust shown schematically. Casing points (CSG) and log-run breaks (LRB) indicated. 

Solid line indicates position of gamma-ray anomaly; dashed, queried line indicates position 

of anomaly is uncertain. Wells listed by number in table 1 and shown in figure 1. Line of 

section shown on figure 1. Datum is sea level. 

Gamma-Ray Well-Log Anomaly, Northern Green River Basin H15 
K 

Well no. 871 Well no. 851 Well no. 731 Well no. 951 Well no. 931 

0 200 0 200 0 200 0 200 0 200 

+10~--------------------------------------------

GJ +5 w 

lJ._ 

lJ._ 

0 

(/) 

0 

z 

<t 

(/) 

6 0 I 

f-

~ 

z 

0 

~ > -5 w 

_J 

w 

-10 

I

""· I -··· ' . . _:· . 

' 

-~::·. ~· .. ·_' . ~:·.~ ::·.:. ~- .. . _:: 

_. - -:. 

Figure 14. North-south gamma-ray log cross section K-K', northern 

of gamma-ray anomaly; dashed, queried line indicates position of 

Datum is sea level. 

H16 Geology of Tight Gas R~servoirs, Wyoming and Colorado 
K' 

I Well no. 641 Well no. 90 I Well no. 881 Well no. 431 Well no. 351 Well no. 261 Well no. 1 7 I Well no. 11 I Well no. 1 2 

0 200 0 200 0 200 0 200 0 200 0 200 0 200 0 200 0 200 

GAMMA API UNITS 

Green River basin. Casing points (CSG) and log-run breaks (LRB) indicated. Solid line indicates position 

anomaly is uncertain. Wells listed by number in table 1 and shown in figure 1. Line of section shown on figure 1. 

Gamma-Ray Well-Log Anomaly, Northern Green River Basin H17 
= = 

~ 

~ Q 

~ 

Q 

--1 

OCi" 

~ 

~ 

Ill 

Ill 

:111:1 

I'D 

~ 

::i! 

Q 

:::;· 

~ 

~ 

~ 

3 

;· 

CIQ 

Ill 

c. = 

t"') 

Q 

Q 

;: 

c. 

Q 

L L' 

Well no. 861 Well no. 80 I Well no. 741 Well no. 721 Well no. 661 Well no. 591 Well no. 561 Well no. 441 Well no. 361 Well no. 281 Well no. 191 Well no. 6 

0 200 0 200 0 200 0 200 0 200 0 200 0 200 0 200 0 200 0 200 0 200 0 200 

+1o GAMMA API UNITS 

fUJ 

LU 

LL 

LL 

0 

(f) 

0 

z 

<( 

(f) 

:::> 

0 

I 

1--

~ 

z' 

0 

i= 

<( 

> UJ 

_J 

UJ 

/CSG 

z~::: ... CSG-

+5 t- ··:· 

~ 

.. ~\t. 1- . .~ LRB rLR: __ 

~~--: ·.:: .... 1 l ~-

0 :'"f'~~\~;.~·· •. ·~ :: ·, No data~.:_. ~·· 

. ~ I LRB ·= «'"" 

tNodata ., . 

LRB_JI 

~--~~d__F<~~·~~~"-.---~----

CSGCSG-

-5 

· CSG 

-10~------------------------------------------------------------------------------------------------------J 

Figure 15. North-south gamma-ray log cross section L-L', northern Green River basin. Casing points (CSG) and log-run breaks (LRB) indicated. Solid 

line indicates position of gamma-ray anomaly; dashed, queried line indicates position of anomaly is uncertain. Wells listed by number in table 1 and 

shown in figure 1. Line of section shown on figure 1. Datum is sea level. 
Table 2. Wells distant from the Wind River Mountains in the Green River basin from which arkose or 

feldspar has been reported 

[Data from American Stratigraphic Company, Denver, Colo. Location in section, township. (north), and range (west); 

township-range grid shown in figure 1] 

Direction from 

Wind River Mountains Location Well name Constituent 

Northwest 23, 37, 113 

23, 37, 113 

Kerr McGee No. 1 Bondourant Arkose 

Pan American No. 1 Raspberry Ridge Arkose 

West 24, 35, 114 

32, 34, 114 

13, 33, 113 

02, 33, 113 

28, 31, 113 

34, 26, 113 

09, 26, 113 

Pacific Transmission Supply No. 24-24 Federal 

Gulf Oil No.1 West Side Federal 

Feldspar 

Feldspar 

Ray Smith No. 1 Fear Ranch Unit Arkose 

Texas Gulf No. 1 Horse Creek Federal Arkose 

Belco No. 1 Goat Hill Arkose 

Arkose,feldspar 

Feldspar 

Belco No.2 Rocking Chair Ranch 

Belfer No. 35 BNG 

Southwest 34, 24, 113 Amoco No. 1 Gravel Unit Arkose 

complicated by a sediment contribution from the Gros 

Ventre Mountains to the north (figs. 12, 13). Available 

data suggest that in many parts of the northern Green 

River basin the earliest preserved Tertiary rocks are probably arkosic. In these areas, the gamma-ray anomaly may 

mark the approximate location of the Cretaceous-Tertiary 

boundary. 

CONCLUSIONS 

A strong anomaly in gamma-ray intensity has been 

observed in lower Tertiary rocks of the northern Green 

River basin, in the subsurface adjacent to and parallel 

with the Wind River thrust. The anomaly can be identified in wells throughout the northern Green River basin, 

despite its reduction in amplitude with increasing distance 

from the Wind River Mountains. The anomaly results 

from detrital potassium feldspars and micas that have 

eroded from the Precambrian granitic core of the Wind 

River Mountains. The presence of arkose and feldspar 

in wells along the Wyoming thrust belt and on the La 

Barge platform implies that lower Tertiary fluvial 

drainage from the Wind River Mountains extended at 

least that far to the west. Limited palynologic data suggest that in some areas of the basin, particularly along 

its western flank, the gamma-ray anomaly is near the 

Cretaceous-Tertiary boundary. If additional data support 

this contention, the anomaly may prove useful in dating 

the timing of the Prospect-Hogsback thrusting. 

REFERENCES CITED 

Asquith, D.O., 1966, Geology of Late Cretaceous Mesaverde 

and Paleocene Fort Union oil production, Birch Creek 

Unit, Sublette County, Wyoming: American Association 

of Petroleum Geologists Bulletin, v. 50, no. 10, 

p. 2176-2184. 

Berg, R.B., 1961, Laramide tectonics of the Wind River Mountains, in Wiloth, G.J., ed., Symposium on Late Cretaceous rocks, Wyoming and adjacent areas: Wyoming 

Geological Association Field Conference, 16th, 1961, 

Guidebook, p. 70-80. 

__ 1962 Mountain flank thrusting in Rocky Mountain 

foreland, Wyoming and Colorado: American Association 

of Petroleum Geologists Bulletin, v. 46, p. 2019-2032. 

Butcher, R.H., 1971, Modal analysis of petrographic study in 

Project Wagon Wheel technical studies report, December 

31, PNE-WWl: El Paso Natural Gas Company, 

p. 203-206. 

Dixon, J.S., 1982, Regional structural synthesis, Wyoming 

salient of western overthrust belt: American Association 

of Petroleum Geologists Bulletin, v. 66, p. 1560-1580. 

Dorr, J.A., Jr., 1952, Early Cenozoic stratigraphy and vertebrate paleontology of the Hoback Basin, Wyoming: 

Geological Society of America Bulletin, v. 63, no. 1, 

p. 59-94. 

__ 1958, Early Cenozoic vertebrate paleontology, sedimentation, and orogeny in central western Wyoming: Geological Society of America Bulletin, v. 69, no. 10, 

p. 1217-44. 

Dorr, J.A., Jr., Spearing, D.R., and Steidtmann, J.R., 1977, 

Deformation and deposition between a foreland uplift and 

impinging thrust belt, Hoback basin, Wyoming: Geological Society of America Special Paper 177, 82 p. 

Greenfield, H., 1981, Resource evaluation and production 

research on tight sands in the Pinedale unit, Sublette 

County, Wyoming: Gas Research Institute, Final report 

for contract 5080-321-0328, 157 p. 

Gries, Robbie, 1983, North-south compression of Rocky Mountain foreland structures, in Lowell, J.D., and Gries, Robbie, eds., Rocky Mountain foreland basins and uplifts: 

Rocky Mountain Association of Geologists, p. 9-32. 

Guennel, G.K., Spearing, D.R., and Dorr, J.R., Jr., 1973, 

Palynology of the Hoback Basin, in Schell, E.M., ed. 

Symposium and core seminar on the geology and mineral 

Gamma-Ray Well-Log Anomaly, Northern Green River Basin H19 
M 

Well no. 861 Well no. 751 Well no.941 Well no. 671 Well no. 921 Well no. 60 I 

0 200 0 200 0 200 0 200 0 200 0 

+10~----------------------------------------------------

tUJ 

UJ 

LL 

LL 

0 

CJ) +5 0 

z 

<( 

CJ) 

::J 

0 

I 

t-

~ 

z 

0 

f= 

<( 

> UJ 

_J 

UJ 

0 

-5 

CSG 

--CSG CSG1

. CSG-

. 

- ~ - . . - . 

- --· 

.. ~-?-- --?- '·"' -" 

LRB~ 

c· ------------------~ 

Figure 16. North-south gamma-ray log cross section M-M', northern Green River 

line indicates position of anomaly is uncertain. Wells listed by number in table 1 

resources of the greater Green River basin: Wyoming 

Geological Association Field Conference, 25th, Guidebook, p. 173-185. 

Hagen, E.S., Shuster, MW., and Furlong, K.P., 1985, Tectonic 

loading and subsidence of intermontane basins; Wyoming 

foreland province: Geology, v. 13, no. 8, p. 585-588. 

Keefer, W.R., 1965, Stratigraphy and geologic history of the uppermost Cretaceous, Paleocene, and lower Eocene rocks in 

the Wind River basin, Wyoming: U.S. Geological Survey 

Professional Paper 495-A, 77 p. 

Keighin, CW., 1984, Petrographic and selected reservoir 

characteristics of some Thrtiary and Cretaceous sandstones, 

Pinedale anticline, Sublette County, Wyoming, in Law, RE., 

ed., Geological characteristics of low-permeability Upper 

Cretaceous and lower Thrtiary rocks in the Pinedale anticline 

area, Sublette County, Wyoming: U.S. Geological Survey 

Open-File Report 84-753, p. 16-21. 

Law, B.E., 1979, Section B-B'; subsurface and surface correlations of some Upper Cretaceous and Thrtiary rocks, northern Green River basin, Wyoming: U.S. Geological Survey 

Open-File Report 79-1689, 2 sheets. 

___ 1984, Relationships of source rock, thermal maturity, and 

overpressuring to gas generation and occurrence in lowpermeability Upper Cretacous and lower Tertiary rocks, 

greater Green River basin, Wyoming, Colorado and Utah, 

in Woodward, J., Meissner, F.F., and Clayton, J.L., eds., 

Hydrocarbon source rocks in the greater Rocky Mountain 

region: Rocky Mountain Association of Geologists, 

p. 468-490. 

Law, B.E. and Nichols, D.J., 1982, Subsurface stratigraphic correlations of some Upper Cretaceous and lower Tertiary 

rocks, northern Green River basin, Wyoming [abs.], in 

Steidtmann, J.R., ed., Subsurface practices in geology and 

H20 Geology of Tight Gas Reservoirs, Wyoming and Colorado 

geophysics: Laramie, University of Wyoming, Department 

of Geology and Geophysics, Earth Science Bulletin 15, 

p. 17. 

Law, B.E., Pollastro, R.M. and Keighin, CW., 1986, Geologic 

characterization of low-permeability gas reservoirs, in 

Spencer, CW., and Mast, R.F., eds., Geology of tight gas 

reservoirs: American Association of Petroleum Geologists 

Studies in Geology 24, p. 253-269. 

Love, J.D, 1960, Cenozoic sedimentation and crustal movement 

in Wyoming: American Journal of Science, v. 258-A, 

p. 204-214. 

Lowell, J.D., 1983, Foreland deformation, in Lowell, J.D., and 

Gries, R., eds., Rocky Mountain foreland basins and uplifts: Rocky Mountain Association of Geologists, p. 1-8. 

Martin, W.R and Shaughnessy, J., 1969, Project Wagon Wheel, 

in Symposium on Thrtiary rocks of Wyoming: Wyoming 

Geological Association Field Conference, 21st, Guidebook, 

p. 145-152. 

Merewether, E.A., Blackman, P.D., and Webb, J.C., 1984, The 

Mid-Cretaceous Frontier Formation near the Moxa arch, 

southwestern Wyoming: U.S. Geological Survey Professional 

Paper 1290, 29 p. 

Merin, I.S. and Lindholm, R.C., 1986, Evidence that the crystalline 

cores of uplifts adjacent to the Powder River Basin were 

breached during Paleocene time: The Mountain Geologist, 

v. 23, no. 4, p. 128-131. 

Petroleum Information Corporation, 1983, Structure contour map 

of the NWl/ 4 Green River basin, Wyoming: Denver, 1 sheet. 

___ 1987, Well history control system well completion records, 

Wyoming: Denver, microfiche. 

Pollastro, R.M., 1985, Mineralogical and morphological evidence 

for the formation of illite at the expense of illite/smectite: 

Clay and Clay Minerals, v. 33, no. 4, p. 265-274. 
LABARGE PLATFORM M' 

Well no. 91 I Well no. 571 Well no. 471 Well no. 481 Well no. 391 Well no. 21 I Well no. 151 Well no. 161 Well no. 2 I Well no. 3 

0 200 0 200 0 200 0 200 0 200 0 200 0 200 0 200 0 200 0 200 

GAMMA API UNITS 

An om 

. Forrnatio~- · .. d Frontle':_. --- - - secon ---

---

basin. Casing points (CSG) and log-run breaks (LRB) indicated. Solid line indicates position of gamma-ray anomaly; dashed, queried 

and shown in figure 1. Line of section shown on figure 1. Datum is sea level. 

Prensky, S.E., 1984, A gamma-ray anomaly associated with the 

Cretaceous-Tertiary boundary in the northern Green River 

basin, Wyoming, in Law, B.E., ed., Geological characteristics of low-permeability Upper Cretaceous and lower 

Tertiary rocks in the Pinedale anticline area, Sublette 

County, Wyoming: U.S. Geological Survey Open-File 

Report 84-753, p. 22-35. 

___ 1986, Geologic implications of large-scale trends in welllog response, northern Green River basin, Wyoming: 

Society of Professional Well Log Analysts Annual Logging Symposium, 27th, Houston, Transactions, paper 

EEE, 24 p. 

___ 1988, Catalog of gamma-ray logs for selected wells in 

the northern Green River basin, Wyoming, presented as 

compressed depth-scale X-Y plots: U.S. Geological Survey 

Open-File Report 89-82, 180 p. 

Prensky, S.E., and Dickinson, W.W., 1986, Computergenerated well-log data plots assist regional subsurface 

evaluation: Geobyte, v. 1, no. 2, p. 52-58. 

Shuster, M.W., 1986, The origin and sedimentary evolution of 

the northern Green River basin, western Wyoming: Laramie, Wyo., University of Wyoming, Ph.D. thesis, 323 p. 

Spearing, D.R., 1969, Stratigraphy and sedimentation of the 

Paleocene-Eocene Hoback Formation, western Wyoming, 

in Barlow, J.A., ed., Symposium on Tertiary rocks of 

Wyoming: Wyoming Geological Association Field Conference, 21st, Guidebook, p. 65-76. 

Vine, J.D., and Tourtelot, E.B., 1970, Prelminary geochemical 

and petrographic analysis of lower Eocene fluvial sandstones in the Rocky Mountain region, in Enyert, R.L., 

ed., Symposium on Wyoming sandstones: Wyoming Geological Association Field Conference, 22nd, Guidebook, 

p. 251-263. 

West, R.M., 1969, Geology and vertebrate paleontology of the 

northeastern Green River basin, Wyoming, in Barlow, 

J.A., Jr., ed., Symposium on Tertiary rocks of Wyoming: 

Wyoming Geological Association Field Conference, 21st, 

Guidebook, p. 77-92. 

Gamma-Ray Well-Log Anomaly, Northern Green River Basin H21 

Chapter I 

Quantitative Model for Overpressured Gas 

Resources of the Pinedale Anticline, Wyoming 

By R.R. CHARPENTIER, B.E. LAW, and S.E. PRENSKY 

Prepared in cooperation with the U .5. Department of Energy 

U.S. GEOLOGICAL SURVEY BULLETIN 1886 

GEOLOGY OF TIGHT GAS RESERVOIRS IN THE PINEDALE ANTICLINE AREA, WYOMING, 

AND AT THE MULTIWELL EXPERIMENT SITE, COLORADO 
CONTENTS 

Abstract Il 

Introduction Il 

Gas-bearing rocks Il 

General method of resource calculation I2 

Structure I3 

Calculation of gas concentrations I4 

Lithology I4 

Porosity IS 

Water saturation IS 

Pressure I6 

Temperature I7 

z factor I7 

Gas concentrations and volumes I7 

Distributions of concentrations I8 

Resource estimations I9 

Uncertainty of estimate IlO 

Conclusions IlO 

References cited IlO 

FIGURES 

1. Cross section from Wyoming thrust belt northeast to Wind River Mountains showing top of overpresssuring I2 

2. Map showing structure contours of top of Cretaceous in area of Pinedale 

anticline I3 

3-6. Diagrams showing: 

3. Amount of sandstone in the Wagon Wheel well as determined by using 

gamma-ray log values I4 

4. Regression of sandstone porosity on depth in Wagon Wheel well IS 

5. Well pressures and temperature gradients in Wagon Wheel well I6 

6. Depth of top of overpressuring versus depth to top of Cretaceous for 

nine wells in Pinedale anticline area I7 

7. Cross section showing concentration of natural gas per cubic foot of sandstone I9 

8. Graph showing concentration of natural gas per cubic foot of sandstone 

versus depth IlO 

9. Cross section showing concentration of natural gas per cubic foot of 

rock Ill 

10. Map showing concentration of natural gas in assessed part of Pinedale 

anticline 112 

TABLES 

1. Average composition of gas for several wells on the Pinedale anticline, 

northern Green River basin I8 

2. Estimate of in-place overpressured natural gas resource of the Pinedale anticline area Il2 
Quantitative Model for Overpressured Gas 

Resources of the Pinedale Anticline, Wyoming 

By R.R. Charpentier, B.E. Law, and S.E. Prensky 

Abstract 

The Pinedale anticline in the northern part ofthe Green 

River basin of Wyoming contains a large accumulation of overpressured natural gas within a low-permeability sequence of 

Upper Cretaceous rocks. The more important geologic factors 

that determine the vertical and horizontal distribution of gas 

have been incorporated into a quantitative model of the accumulation. Within the 197-mi2 (317 km2) structure, sandstones of the 1 0,000-ft-thick (3,048 m) section of overpressured 

gas-bearing reservoirs are estimated to contain 159 trillion cubic 

feet (4.50x 1012 m3) of in-place natural gas. Only a small part 

of this resource may be recoverabl~: 

INTRODUCTION 

The Pinedale anticline is a large thrust-rooted fold 

in the northern part of the Green River basin of Wyoming (Law and Johnson, this volume, fig. 1). Data 

obtained from wells drilled on the anticline demonstrate 

the occurrence of a thick (about 10,000 ft, 3,048 m) lowpermeability gas-bearing interval below a depth of about 

8,000 ft (2,438 m). 

Prior to 1978, almost all aspects of the nature of 

large unconventional low-permeability gas accumulations 

were poorly understood; subsequently, numerous studies 

have been conducted that clarify the nature of these accumulations (Meissner, 1978, 1980, 1981; Masters, 1979; 

Spencer, 1983, 1987; Law and Dickinson, 1985). In the 

Pinedale area, relevant studies include those by Law and 

others (1979, 1980), Law and Spencer (1981), Spencer and 

Law (1981), Law (1984a, b), and Spencer (1984, 1987, 

this volume). Because these investigations are quite recent, most resource evaluations of low-permeability gas 

reservoirs have not incorporated these data and, in some 

instances, have utilized irrelevant or incomplete geologic 

criteria. 

The primary purpose of our study, therefore, is to 

provide a more objective, geologically based model for 

the assessment of low-permeability gas reservoirs and, 

specifically, to evaluate the gas resources contained in 

low-permeability sandstone reservoirs in the Pinedale anticline area. 

Acknowledgments.-This work was funded by the 

U.S. Department of Energy under Interagency Agreement 

DE-A121-83MC20422. The authors thank B. Hassler, 

Celcius Energy Company, and D. Reffert, Mountain Fuel 

Supply Company, for conversations concerning exploration, drilling, and production in the Pinedale area. The 

authors also thank K.B. Krystinik, U.S. Geological 

Survey, for help with the robust regression technique and 

G.L; Dolton and C.W. Spencer, U.S. Geological Survey, 

for helpful comments on the manuscript. 

GAS-BEARING ROCKS 

The gas accumulation in the Pinedale area is a deepbasin or basin-centered type, similar to that originally 

described by Masters (1979). This type of accumulation 

commonly is very large, occupies the deeper part of a 

basin, is abnormally pressured, and is downdip from 

water-bearing rocks. The reservoirs are almost always 

low-permeability (<0.1 mD) sandstones. The 

characteristics of these accumulations are important 

because they form the essential geologic elements of the 

resource analysis. 

The gas accumulation in the Pinedale anticline is 

a small part of a much larger accumulation that occupies 

the deeper part of the Greater Green River basin of 

Wyoming, Colorado, and Utah. In the El Paso Natural 

Gas Company Wagon Wheel No. 1 well (Law and 

Spencer, this volume, fig. 2), the top of the gas-bearing 

interval is in Upper Cretaceous sandstone reservoirs at 

a depth of about 8,000 ft (2,438 m), coincident with the 

top of abnormally high pressures. The gas-bearing interval extends downward through the Upper Cretaceous 

Ericson Sandstone into the Rock Springs Formation (Law 

and Johnson, this volume). According to Law and 

Spencer (1981), Law (1984a, b), Spencer (1984, 1987, this 

volume), and Law and Dickinson (1985), the high 

pressures are caused by active or recently active gas 

generation. The low permeability of the sandstone reservoirs does not allow pressure to equilibrate to hydrostatic 

levels. 

Quantitative Model for Overpressured Gas Resources, Pinedale Anticline 11 
The seals for these low-permeability accumulations 

are not well understood. Conventional seals such as shale, 

evaporites, or some other relatively impermeable lithology 

are not significant, and the occurrence of these unconventional accumulations is not completely controlled by 

structure and stratigraphy. The relationship between 

structure, stratigraphy, and overpressuring, as shown in 

cross section (fig. 1), indicates that the top of gas-bearing 

overpressured rocks cuts across structural and stratigraphic boundaries. The more important aspects of these 

accumulations are temperature, thermal maturity, organic 

richness, and permeability, all of which have been determined to have a pronounced effect on the occurrence and 

magnitude of pressure variations in the Wagon Wheel 

well (Law, 1984a, b). 

GENERAL METHOD OF 

RESOURCE CALCULATION 

Both structural and stratigraphic boundaries were 

sw 

A 

used to delimit the volume of rock in the Pinedale anticline assessed for gas resources. The areal limits of the 

anticline are defined as the -2,000-ft (-610 m) contour 

on the top of the Cretaceous rocks and the high-angle 

reverse or thrust fault on the southwest flank of the anticline (fig. 2). The top of the assessed volume of rocks 

is defined as the top of the overpressured interval and 

the base of the assessed volume as the base of the Rock 

Springs Formation and the curving surface of the 

southwest-bounding fault. 

Calculation of gas resources in the anticline was accomplished by dividing the anticline into about 8,000 cells 

and calculating the gas concentration and volume for each 

cell. Areally, land-survey section outlines (1 mi2 each) 

were used to define cell boundaries. The column of rock 

under each of the 241 sections was divided into 25 intervals, each 500ft (152m) thick. The uppermost interval 

of each section began at an elevation 500ft (152m) below 

sea level and the lowermost ended 13,000 ft (3,962 m) 

below sea level. In order to facilitate separate estimations of gas resources for the Lance Formation, Ericson 

NE 

A' 

FEET 

15,000 

r------------------ 55 MILES------------------.. METERS 

10,000 

5,000 

SEA. 

LEVEL 

-5,000 

-10,000 

-15,000 

LABARGE 

PLATFORM 

El Paso Natural Gas 

Wagon Wheel No. 1 

4,000 

3,000 

2,000 

1,000 

SEA 

LEVEL 

-1,000 

-2,000 

-3,000 

-4,000 

-5,000 

Figure 1. Cross section from Wyoming thrust belt northeast to Wind River Mountains. High-angle reverse fault on west side 

of Pinedale anticline from Martin and Shaughnessy (1969) and Shaughnessy and Butcher (1973, 1974). Line of section A-A' 

shown on figure 1 of Law and Johnson (this volume). 

12 Geology of Tight Gas Reservoirs, Wyoming and Colorado 
L 

l 

l 

o:' 

~ & .....__ s:s: 

en= 'i ... I """' "" ~lo:' 

'""" 

l\ IT 33 N 

\I ~ 1\S: 0 ,.,.,. 

-o 

""' 

= 

\ \ c?o \ o:' 

_i \ \_ \ 1\ ~ \_ '- I"' ""' \ l\ ... !"\ ~ l 32 N. 

'\Li J 

""" 

I\_ l _\ on \. \ \ ~~ \ .... \ i\ B' 

~ ~ \ o:' 10 \ \ V\ s 

~ [',. 1\ ~0 1\ v< ~ 

'\ \ ... \'b /' 1\ o:' T 31 N .

Fauler'\~ ~ ___. ~ \ .[\_ "" l ~ !"\ ;<- ,.,.,. ' I ... -'6-,.,.,. ~-

~ ""' " o o -dQ -0 

\. o-'br- B 1\ "'' ... " 1\ 

'r-.. 

""" ~ •J .\ 

' "-..._ \ l 30 N. 

El Paso Wagon I~ "-..._ \s: Wheel No.1 I'Vr--._ I"-- ~ 

~ ~ o:' 

--.._,_!'--. 

-----r--- 1---- ~ 

l 29 N. 

Figure 2. Structure contours of the top of the Cretaceous 

in the area of the Pinedale anticline. Contour interval, 500 

ft; areal limits of assessed structure delimited by -2,000-ft 

contour. Wells: open circle, producing; solid circle, shutin; triangle, all others, drilled and abandoned. Line of section B-8' (figs. 7,9) also shown. 

Sandstone and Rock Springs Formation, some of the intervals between -5,500 and -8,000 ft (-1,676-2,438 m) 

were divided into two or three cells, one for each 

stratigraphic unit. For example, between -5,500 and 

-6,000 ft (-1,676-1,829 m), the Lance Formation and 

possibly the Ericson Sandstone can be within the interval, depending on the particular section. Thus, this interval was divided into two cells, one for the Lance from 

-5,500 to -6,000 ft and one for the Ericson. If for a particular section there is no Ericson at that depth, the volume of rock in the latter cell was set to zero. This 

procedure resulted in a total of 33 cells in each section. 

Section corners were digitized and used in combination 

with grid files of structural surfaces in order to allow computer calculation of section areas and cell volumes. 

Especially as one approaches the outer edge of the 

anticline, many of the cells fall above or below the 

limiting boundaries. These cells were given a cell volume 

of zero. 

An alternative method would be to employ a regular 

grid instead of the slightly irregular section grid. If a 

regular grid is used, calculation of areas and volumes is 

simplified; however, straight outlines for the assessment 

boundaries are required, a disadvantage that results either 

in some loss of resolution or in a large increase in the 

number of cells and the number of calculations. Use of 

the section grid allows results to be related easily to lease 

areas and other land-use categories. 

STRUCTURE 

The structural configuration of the Pinedale anticline is shown in figure 1 of Law and Johnson (this 

volume). The structural datum is the top of the Cretaceous rocks. In the Green River basin, the boundary between 

Cretaceous and Tertiary rocks is unconformable and in 

many areas its location is uncertain. In the area of the 

Pinedale anticline, it has been identified on the basis of 

palynology and subsurface correlations (Law, 1979; Law 

and Nichols, 1982), but because it is an unconformable 

surface, some uncertainty remains as to the structural 

significance of the datum. Examination of all the well 

logs in the Pinedale area indicates little evidence of 

significant erosional truncation of the underlying 

Cretaceous rocks, and we believe that the CretaceousTertiary datum in this area accurately portrays the structural configuration. 

The computer representation of anticlinal structure 

is based on a hand-contoured map of structure of the top 

of the Cretaceous (fig. 2). Because initial trials of computer contouring on picks of the top of the Cretaceous 

in 20 wells did not adequately represent the structure, 

points were digitized from the hand-contoured map and 

a regular grid calculated. Reference to a proprietary 

seismic section suggests that the structural horizons are 

reasonably parallel, and grids for other structural horizons 

were calculated by subtracting a constant from each grid 

node value of the original (top of Cretaceous) grid. 

For purposes of calculation, if depth relative to the 

top of Cretaceous was needed in an equation, the average 

elevation of the top of the Cretaceous was calculated for 

each section. The volume of the cells in a section from 

sea level down to the top of the Cretaceous was calculated 

and then divided by the area of the section. This p~ocedure 

gave the average depth below sea level of the top of the 

Cretaceous for the particular section. 

The configuration of the southwest-bounding fault 

was digitized in a manner similar to that of the top of 

the Cretaceous. A hand-contoured map of the fault surface was used, points digitized from the contour lines, and 

a grid calculated. 

Quantitative Model for Overpressured Gas Resources, Pinedale Anticline 13 
CALCULATION OF GAS CONCENTRATIONS 

Lithology 

The gas-bearing interval in the Pinedale anticline 

consists of interbedded sandstone, siltstone, and shale and 

minor amounts of carbonaceous rocks. Although all 

lithologies in the gas-bearing interval most likely are gas 

saturated, only the gas contained in sandstones was 

evaluated in this study. Based on examination of cores 

from the Wagon Wheel and other wells on the Pinedale 

anticline, the sandstones were grouped into those from 

the Lance and Rock Springs Formations and those from 

the Ericson Sandstone. The cored intervals in the Wagon 

Wheel well and their corresponding geologic units are 

shown in figure 3 of Law and Johnson (this volume). 

The Lance and Rock Springs Formations consist of 

generally very fine to fine grained, poorly to moderately 

sorted lithic sandstones. Many of the sandstones have conglomeratic bases that fine upward. In the Lance Formation, many pores are partly to completely filled with 

authigenic clays (Keighin, 1984; Pollastro, this volume). 

The Ericson Sandstone consists of quartz-rich sandstones 

that are in places conglomeratic. The sandstones commonly contain more than 90 weight percent quartz (Law and 

others, 1986; Pollastro, this volume); grain size is very fine 

to coarse, sorting is poor to good. Detailed descriptions 

of the sandstone mineralogy are in Keighin (1984), 

Pollastro and Barker (1984), and Pollastro (this volume). 

The percentage of sandstone in the gas-bearing interval is difficult to determine, but the proportion of sandstone, as determined from a visual examination of well 

logs, is 37-45 percent. In an attempt to increase precision 

of this value, we correlated core-derived lithology and the 

core-gamma log in the Wagon Wheel well. This correlation is good, in general, but a consistent source of error 

resulted from our inability to distinguish intraformational 

conglomeratic sandstones on the core-gamma log. Almost 

all the clasts in the conglomeratic beds are shale, 

mudstone, or shaly siltstone and, consequently, give a 

shale response on the core-gamma log. 

The wireline gamma-ray log was corrected for the 

influence of borehole diameter and mud density, and the 

core-gamma log was linearly depth shifted to provide the 

best fit with the wireline gamma-ray log. Core lithology, 

as determined from visual and petrographic examination, 

was used to select a core-gamma sandstone cutoff value. 

Although the two gamma-ray logs are similar in character, they are not similar in absolute values and the coregamma cutoff was transferred to the wireline gamma log 

in a proportional manner. The gamma-ray sandstone 

discriminator used corrected gamma values of 47 API 

from 8,000 to 12,099 ft (2,438-3,688 m) and 70 API from 

12,100 to 17,840 ft (3,688-5,438 m). The change in API 

values occurs at a break in log runs. 

14 Geology of Tight Gas Reservoirs, Wyoming and Colorado 

AMOUNT OF SANDSTONE, IN PERCENT 

20 40 60 80 100 

ILU 

LU 

l.L 

z 12,000 

w 

u 

<( 

l.L 

c: 

:::> 

(j) 

s 

0 

_J 

LU 14,000 a::::J 

I 

t 

LU 

0 

Bottom of Rock Spnngs Formation 

c 

0 

·;:; 

co 

E 

0 

LL 

Ql 

u 

c 

co 

....J 

c 

0 

·;:; 

co 

E 

0 

LL 

(Jl 

Ol 

c 

"§. 

(f) 

~ 

u 

0 

0::: 

Figure 3. Amount of sandstone in the Wagon Wheel well 

as determined by using gamma-ray log values. 

The wireline gamma-ray log was digitized in 1-ft increments, and from 8,024 to 17,840 ft (2,446-5,438 m) 

depth in the Wagon Wheel well determinations of 

lithology were made for each increment. The average 

percentage of sandstone for each 100-ft interval was summarized (fig. 3) and entered into the appraisal program 

as a table. 

In order to calculate the percentage of sandstone 

for a particular cell, the depth of the center of the cell 

relative to the top of the Cretaceous was determined. The 

percentage of sandstone for the cell was calculated as the 

average percentage of sandstone for a 500-ft-thick (152m) 

interval in the Wagon Wheel well centered at the same 

depth below the top of the Cretaceous. The geologic 

assumption behind this procedure is that the trends in 

sandstone percents obtained by averaging for 500-ft intervals are relatively consistent in the area of the Pinedale 

structure. This assumption cannot be firmly established, 

however, because only the Wagon Wheel well penetrated 
the entire assessed interval. For cases in which the 500-ft 

interval included part of a different formation, only that 

part of the interval in the formation of interest was used. 

Porosity 

Three types of porosity measurements were made 

in the Wagon Wheel well: acoustic, density, and sidewall 

neutron. The latter two utilize pad tools that require good 

contact with the borehole wall and thus are affected by 

hole rugosity. Severe borehole washout, as indicated by 

the caliper log, is encountered throughout much of the 

Upper Cretaceous interval between 7,000 and 16,000 ft 

(2,134-4,877 m). It occurs preferentially in shaly zones 

(as defined by gamma-ray logs) between 7,000 and 12,000 

ft (2,134-3,658 m) and frequently in sandstone intervals 

(as defined by gamma-ray logs) between 12,000 and 

17,000 ft (3,658-5,182 m). Although the preferred porosity log for low-permeability gas sandstones is the density 

log, it was not used because its overall quality as measured 

by the density correction curve was highly variable. 

Porosity determinations were made by using the 

borehole-compensated sonic log. Acoustic traveltime was 

converted to porosity by using a matrix traveltime of 55.5 

microseconds/ft (182 microseconds/m). This matrixderived value correlates best with core-derived data from 

the well (Prensky and Dickinson, 1986). Data affected 

by borehole washout generally were excluded through the 

0 

8,000 

IUJ 

UJ 

u._ 

~ 

10,000 

w· 

u 

<! 

u._ 

a: 

::> 

(.f) X 

s 

0 14,000 ....J 

UJ 

c:c 

I 

I- 16,000 a.. 

UJ 

0 

18,000 

POROSITY, IN PERCENT 

y 

30 40 50 

/ahll~~'&<a",,«,o"' '1j, o o 

60 70 

~8\~0u:ooooooc')o<:>oo o oB. 

EXPLANATION 

Least-squares regression; Y=16.460-0.0005238X 

Robust regression; Y= 14.353-0.0004954X 

Figure 4. Regression of sandstone porosity on depth in 

Wagon Wheel well. For ease of viewing, independent 

variable (depth) shown on vertical axis. 

use of the gamma-ray sandstone cutoff, and the remaining high acoustic porosities may result from the presence 

of gas. The sonic log also may have underestimated the 

fracture porosity. 

Sonic-derived porosity values for the sandstone intervals were regressed on depth (fig. 4), but, because of 

borehole washout, porosity values for many intervals 

were unreasonably high. A robust regression program 

written by K.B. Krystinik and W.F. Dumler (U.S. 

Geological Survey) was used to give a more reasonable 

fit to the data. The first step of the program is a leastsquares regression on all the points. This regression is 

followed by a weighted least-squares regression that gives 

less weight to the points having large deviations from 

the regression. The procedure is repeated until an adequate fit is obtained. For this particular data set, points 

having large deviations are those having unreasonably 

high porosity values and thus those most reasonable to 

downweight. Comparison of the two regressions (fig. 4) 

indicates that the robust regression gives a more 

reasonable fit and that the original least-squares regression overestimates porosity by about 2 percent. 

Water Saturation 

The derivation of reliable water saturation values 

in low-permeability reservoirs is an unresolved problem. 

Log-derived values commonly are higher than corederived values. Calculation of water saturation from well 

logs requires a value for formation water resistivity (Rw), 

which normally is computed by using the spontaneous 

potential (SP) curve. This method is unreliable in the 

Wagon Wheel well because of the poor character of the 

SP curve. The deflection amplitude of the curve varies 

from very low to flat, and the SP reverses; this character 

is a result of adverse changes in Rw relative to the mudfiltrate resistivity (Rmr>· Rw can be obtained independently from analysis of formation water samples 

obtained during drill-stem or production testing, but 

water generally is not recovered from these lowpermeability reservoirs, and the origin of the small 

amounts of water that have been recovered is somewhat 

uncertain. 

Observations by Law and others (1979, 1980) in 

the Pacific Creek area of the Green River basin (Spencer, 

this volume, fig. 1) indicate that these gas-bearing 

low-permeability reservoirs contain little or no 

recoverable water, and wells in the Pinedale anticline area 

commonly do not produce water. These empirical relationships indicate that, in these reservoirs, water is immobile and is in very small pores and capillaries as 

irreducible water. For purposes of our calculations, we 

therefore assume that water in these reservoirs occurs 

at irreducible saturation levels. 

Quantitative Model for Overpressured Gas Resources, Pinedale Anticline 15 
During the original evaluation of data from the 

Wagon Wheel well by El Paso Natural Gas Company, 

analyses of cores from depths of 8,100 to 10,985 ft 

(2,469-3,348 m) indicate that water saturations are from 

27.6 to 54.9 percent, with most values from 40 to 54.9 

percent. These measurements agree well with results of 

mercury capillary pressure tests from similar sandstone 

reservoirs at the U.S. Department of Energy Multiwell 

Experiment site in the Piceance basin of Colorado. 

Because irreducible water saturation probably increases 

as porosity and permeability decrease with depth and 

because there are no measurements of water saturation 

deeper than 11,000 ft (3,353 m) in the Pinedale anticline, 

we chose a water saturation value of 50 percent for the 

gas-bearing interval. 

Pressure 

Overpressuring in the Pinedale area was initially 

reported by Rathbun (1968) and later discussed by 

Rathbun and Dickey (1969), Law and Spencer (1981), 

Law (1984a, b), Spencer (1984, 1987, this volume), Law 

and Dickinson (1985), and Prensky (1986). Rathbun concluded that tectonism was the cause of overpressuring, 

but subsequent investigations attributed the abnormally 

high pressures to the thermal generation of gas from 

coal and carbonaceous rocks interbedded with lowpermeability sandstone reservoirs. The latter explanation 

of overpressuring in these rocks is more compatible with 

available data than is the former. 

The pressure gradient in the Wagon Wheel well (fig. 

5) is based on mud-weight data from mud logs, well logs, 

and drill-stem tests. Because gas is the pressuring phase, 

the total gas-bearing interval was divided into three 

pressure-related intervals that are defined on the basis of 

specific geologic conditions (Law, 1984a; Spencer, this 

volume). 

The uppermost interval in the Wagon Wheel well, 

referred to as the inactive zone (Law, 1984a), extends 

from a depth of 8,035 to about 10,500 ft (2,449-3,200 m), 

at which point the pressure gradient abruptly increases. 

The inactive zone corresponds to an interval in which 

significantly large volumes of gas previously have been 

thermally generated. As a result of a cooling event that 

occurred 2-4 Ma (Naeser, 1984, 1986, this volume), thermal generation of gas has either ceased or diminished to 

such low rates that it is lost from the reservoirs faster than 

it accumulates. 

The active zone (Law, 1984a) is below the inactive 

zone and is subdivided into two intervals. Within the active zone, thermogenic gas is thought to be actively generated. The upper part of the active zone extends from a 

depth of about 10,500 to about 14,700 ft (3,200-4,481 m), 

at which point the pressure gradient again abruptly increases. The pressure gradient in the upper part of the 

16 Geology of Tight Gas Reservoirs, Wyoming and Colorado 

FT TEMPERATURE m 100 200 300 (oF) 

2 

EXPLANATION 

4 [] Mud weight from mud log 

• Mud weight from well log 

A Mud weight from drill-stem test 

6 

e Temperature data point 

Pressure gradient 2 

. \ 

8 ' TOP OF OVERPRESSURING 

0 

0 INACTIVE ZONE o. 10 3 ..... 

c. 

I 

1- 12 \ ACTIVE ZONE a. 

LU 

0 

\ 4 

14 

\ 

16 \C? ~ 5 

\~ 18 

~ 20 6 

Psi 5,000 10,000 15,000 

PRESSURE 

Figure 5. Well pressures and temperature gradients in Wagon 

Wheel well. Pressure gradients as interpreted by C.W. Spencer 

(oral commun., 1986). Temperature data from Wagon Wheel 

No. 1 and Pinedale No. 5 wells. 

active zone probably is the result of a thermal effect described by Law (1984a). The lower part of the active zone 

extends from a depth of about 14,700 ft to the base of the 

Rocks Springs Formation at 17,700 ft (4,481-5,395 m). 

The increase in pressure gradient in the lower part of the 

active zone coincides with the top of a coal-bearing 

(organic-rich) zone. The elevated pressure gradient 

through this interval has been interpreted to result from 

the increased gas generation capacity of the coal zone 

(Law, 1984a). 

The top of overpressuring in wells was determined 

primarily by using mud-weight data. If the top of overpressuring as determined by using mud weights is plotted versus the elevation of the top of the Cretaceous as 

determined by using palynological and sedimentological 

data, a strong correlation (R2=0.8732) is evident (fig. 

6) and indicates that the surface of the top of overpressuring dips in the same directions as that of the structure. 

This relationship indicates a probable genetic link between 

thermal maturity and the structural deformation. The 

slope of the regression is greater than one, however, and 

indicates that the surface of the top of overpressuring dips 

more steeply than the structural surface (Lickus and 

others, this volume). Results of this regression were used 
800 

• .....J 1,000 UJ 

> UJ 

.....J 

<( 

UJ 1,200 (/) 

s 

0 

.....J 

UJ 1,400 co 

IUJ • UJ 

LL 

z 1,600 

d 

z 

~ >- 1,800 

(/) 

(/) 

UJ 

c:: 

0... 2,000 0::: 

UJ 

> 0 

LL 

0 2,200 0... Y=64.40+ 1 .SOX 

0 

ILL 

0 2,400 I 

b: 

UJ 

0 

2,600 

• 

2, 800 ,.__......L...---1---'-------L-...L._--l.._J.._____L__L__L_____l____J 

1 ,600 1 ,400 1 ,200 1 ,000 800 600 400 

D~PTH OF TOP OF CRETACEOUS, 

IN FEET BELOW SEA LEVEL 

X 

Figure 6. Depth of top of overpressuring versus depth to top 

of Cretaceous for nine wells in Pinedale anticline area. 

in conjunction with the grid of the top of the Cretaceous 

surface to create a grid of the surface of the top of 

overpressuring. 

In order to calculate the average pressure for a cell, 

the depth below the surface was multiplied by an appropriate pressure gradient, the product of which was 

then added to atmospheric pressure of 14.73 psi (101.6 

kPa). The three zones each have specific pressure gradients. The inactive zone was assigned a pressure gradient 

of 0.55 psilft (12.45 kPa/m), the upper part of the active zone a gradient of 0.70 psilft (15.83 kPa/m), and 

the lower part of the active zone a gradient of 0. 78 psi! ft 

(17.64 kPa/m). These zones are separated from each 

other by surfaces that correspond to the inflection points 

in the pressure/depth plot for the Wagon Wheel well (fig. 

5). The inactive zone and the upper part of the active zone 

are separated by a horizontal plane at 3,311 ft (1,009 m) 

below sea level, and the upper and lower parts of the active zone are separated by a surface parallel with structure 7,180 ft (2, 188 m) below the top of the Cretaceous. 

Temperature 

Temperature is an important aspect of overpressured gas accumulations, but reliable temperature data 

are difficult to obtain. Most temperature data are from 

measurements recorded during well-log runs (bottom-hole 

temperatures); they generally are not in equilibrium with 

true formation temperatures and are therefore minimal 

temperatures. Correction of bottom-hole temperatures to 

true formation temperatures requires information often 

not available. 

The bottom-hole thermal gradient for the Pinedale 

area is 1.24 "F/100 ft (2.26 °C/100 m). T.H. McCulloh 

(Mobil Exploration and Producing Services, written 

commun., 1981) calculated a corrected thermal gradient 

based on these bottom-hole temperatures, and this gradient (converted to degrees Rankine ( ~)) was used in our 

resource calculations. The corrected subsurface temperature (~) from the surface to a depth of 10,000 ft 

(3,048 m) is defined as 

T= 1.8*[10+0.0079*Depth] +491.69 (l) 

and from 10,000 ft (3,048 m) to total depth as 

T= 1.8*[89+0.00645*(Depth-10,000)] +491.69. (2) 

Present-day subsurface temperatures in the Pinedale area are lower than paleotemperatures. Studies involving vitrinite reflectance (Law and Spencer, 1981: 

Law, 1984a; Pollastro and Barker, 1984, 1986), clay 

transformations (Pollastro and Barker, 1984, 1986), and 

apatite fission-track dating (Naeser, 1984, 1986, this 

volume) show that present-day temperatures are 36-90 

"F (20-50 °C) cooler than paleotemperatures and that a 

cooling event occurred 2-4 Ma. 

z Factor 

The derivation of the gas compressibility factor (z) 

is based on the average composition of gas recovered from 

several wells on the Pinedale anticline (table 1). Although 

there is some uncertainty as to the depths from which the 

gas was recovered, most of the gas recovered from wells 

on the anticline has been from depths of 10,000 to 11,500 

ft (3,048-3,505 m). Because of the uncertainties as to 

changes in gas composition with depth, we calculated an 

average z of 1.34 for the entire gas-bearing interval. 

Gas Concentrations and Volumes 

The gas concentration for each cell was calculated 

in a stepwise manner such that both gas quantity per unit 

volume of sandstone and gas quantity per unit volume 

Quantitative Model for Overpressured Gas Resources, Pinedale Anticline 17 
Table 1. Average composition of gas for several 

wells on the Pinedale anticline, northern Green River 

basin 

[Data from El Paso Natural Gas Company (1971)] 

Components 

Nitrogen 

Carbon dioxide 

Methane 

Ethane 

Propane 

!-butane 

N-butane 

!-pentane 

N-pentane 

Hexanes plus 

Mole percent 

0.54 

.26 

92.14 

4.71 

1.39 

.29 

.28 

.16 

.07 

.16 

of total rock could be derived. Initially, the concentration in sandstone was calculated by using the equation 

Concentration per cubic foot of sandstone 

= (0.14353-0.000004954*Depth)*0.5 

*(Reservoir pressure/14.696) 

*(519.69/Reservoir temperature)*(l/1.34). (3) 

The concentration in total rock was then calculated by 

using the equation 

Concentration per cubic foot of rock 

=Concentration per cubic foot of sandstone 

*Percent sandstone. (4) 

Both types of gas concentrations were used to study the 

distribution of gas resources within the structure. The gas 

volume for a cell was calculated by using the equation 

Volume of gas in a cell 

=Concentration per cubic foot of rock 

*Volume of rock in cell. (5) 

A gas volume was calculated for each cell, these were then 

summed for each section; the total gas volume was then 

divided by the area of the section to give the gas concentration by surface area. The gas volumes per section were 

summed to give a total volume of gas-in-place for the entire structure. The volume for each formation was also 

tallied separately. 

DISTRIBUTION OF CONCENTRATIONS 

Gas concentrations per cubic foot of sandstone in 

a cross section through the Pinedale anticline at approximately the site of the Wagon Wheel well are shown in 

figure 7. These concentrations depend on five main factors: water saturation, porosity, pressure, temperature, 

and gas compressibility. Both water saturation and gas 

compressibility are held constant and thus do not 

18 Geology of Tight Gas Reservoirs, Wyoming and Colorado 

contribute to the pattern shown in figure 7. Pressure increases with depth and thus contributes to an increased 

concentration with depth. Porosity decreases with depth 

and temperature increases with depth; both contribute 

to a decrease in concentration with depth. 

The interaction of these trends is better shown in 

figure 8, in which concentration is plotted as a function 

of depth. The concentrations used are based primarily 

on results for sec. 5, T. 30 N., R. 108 W., an area that 

includes the Wagon Wheel well. Concentrations increase 

gradually with depth and dramatically increase at 3,500 

ft (1 ,067 m) below sea level. This Increase corresponds 

to the top of the active zone and a substantial increase 

in pressure gradient. The gas concentration again 

dramatically increases at 7,500 ft (2,286 m) below sea 

level, where the pressure gradient changes at the boundary between the upper and lower parts of the active zone. 

These jumps in concentration follow the surfaces that 

define the boundaries between zones: a horizontal boundary between the active and inactive zone and a boundary 

parallel with structure between the upper and lower parts 

of the active zone (fig. 7). 

Within each zone, the gas concentration changes 

more gradually. In the inactive zone and in the uppermost part of the active zone, the increase in concentration indicates that the effect of pressure outweighs those 

of decreasing porosity and increasing temperature. Below 

about 5,500 ft (1 ,676 m) below sea level, the decrease in 

concentration indicates that the effects of porosity and 

temperature outweigh that of pressure. If a water saturation is used that increases with decreasing porosity (and 

thus increases with depth) instead of a constant saturation, gas concentrations would further decrease with 

depth. Not only will the actual slopes in figure 8 change, 

but the point at which the slope changes probably will 

be closer to the surface. 

The concentration of gas per cubic foot of total 

rock is shown in figure 9. Comparison of figures 9 and 

3 shows that the concentration of gas is more strongly 

controlled by lithologic variation than by variation in concentration per cubic foot of sand. The high concentrations in the Ericson Sandstone are especially striking. The 

second most prominent trend of high concentration is between about 7,500 and 10,000 ft (2,286-3,048 m) below 

sea level, an interval in which a group of sandstone beds 

in the Rock Springs Formation has high concentrations 

per cubic foot of sandstone because of the increase in 

pressure in the lower part of the active zone. In the Lance 

Formation, gas concentrations are generally higher in the 

upper part of the formation because of higher proportions of sandstone. 

The concentration of gas per square mile is shown 

in figure 10. Two trends are evident, and both probably 

are primarily related to thickness of assessed section. 

A minor trend is evident over most of the structure, 
sw 

8 

El Paso 

Wagon Wheel 

No. 1 NE 

~ 8' SEALEVELr-----~----.-----------.-------~--~------------~-----------r------------~ 

2.000 

f-

~ 4,000 

u... 

z 

_..I 

L.U 

> L.U 

.....J 

<t: 

L.U 

(/) 

s 

0 

.....J 

L.U 

OJ 

I 

fa.. 

L.U 

0 

6,000 

8,000 

10,000 

12,000 

Sec 13 

T30N,R.109W 

Se c 7 Sec 5 Sec 33 

T 30 N , R 1 08 W. T 30 N, R 108W. T 31 N , R 108 W 

Sec 27 

T 31 N, R. 108W 

Sec 23 

l31N , R1 08W 

en 

Ole 

c 0 ·;::: ~ 

a. co 

(f) E ~ "-

(..) 0 aLL 

0:: 

CONCENTRATION OF GAS (IN CUBIC FEET) PER 

CUBIC FOOT OF SANDSTONE 

-

10.5-11 . 13-13.8 > 14.5 

~ 11-13 13.8-14.5 

Figure 7. Cross section showing concentration of natural gas per cubic foot of sandstone. Line of section B-8' shown in 

figure 2. 

especially to the northeast where concentration decreases 

downdip. This trend is probably the result of decreases 

in assessed thickness owing to the steeper dip of the top 

of overpressuring. A much more prominent trend is evident on the southwest flank of the structure where the 

assessed section is truncated by the thrust fault (fig. 9). 

RESOURCE ESTIMA liONS 

The results of the resource estimation are shown 

in table 2. The total in-pl~ce overpressured gas resource 

for the sandstone reservoirs is 159 trillion cubic feet 

(4.50 x 1012 m3). It must be stressed that this is an estimate of gas in-place, not recoverable resources. Recoverable amounts of gas would be considerably less. 

Other estimates of resources in the Pinedale anticline have been made, but because of differences in such 

factors as area assessed, stratigraphic interval assessed, 

and in-place versus recoverable amounts estimated, the 

estimates cannot be directly compared. 

El Paso Natural Gas Company (1971) estimated an 

in-place resource of 252.3 billion cubic feet (7.1 X 1()9 m3) 

for an area of only 640 acres (2.59 km2) around the 

Quantitative Model for Overpressured Gas Resources, Pinedale Anticline 19 
Wagon Wheel well and only for depths from 7,983.5 to 

11,796.5 ft (2,433-3,596 m). In the El Paso study, the 

interval was assigned to the Fort Union Formation, but 

we now know that it is generally correlative to the sandy 

part of the Lance Formation (fig. 3) (Law, 1979, 1984c; 

Law and Nichols, 1982). 

Greenfield and others ( 1981) estimated a total recoverable resource of 2.8 trillion cubic feet (79.3 X 109 

m3) for the Pinedale field. Their estimate was for an area 

slightly smaller (90,000 acres, 364 km2) than that used 

in our study (more than 126,000 acres, 510 km2) and having a somewhat different boundary. They also only 

estimated gas resources for selected intervals within the 

Lance. 

UNCERTAINTY OF ESTIMATE 

Despite all the sources of uncertainty in the input 

variables, some comments regarding the uncertainty of 

the results can be made. Because water saturation and 

gas compressibility are treated as constants, the concentrations and amounts of gas can be easily calculated for 

different constants. For a water saturation of X, gas concentrations and volumes will change by a factor of 

[(1-X)/0.5]. For a compressibility factor (z) of Y, gas concentrations and volumes will change by a factor of 

[ 1. 34/ Y]. The effect of variation in either of these factors with depth would be more difficult to calculate. 

A number of factors are not included in our model. 

Gas volumes from porosity in silt and shale were not 

assessed, and gas dissolved in formation water or sorbed 

on organic material was excluded from the analysis. As 

mentioned, fracture porosity and formation pressures 

may have been underestimated. All in all, estimates for 

in-place gas resources may be conservative. On the other 

hand, gas from the shaly and silty intervals is less likely 

to ever be producible, and thus the estimate of 159 trillion 

cubic feet from the sandstone reservoirs is probably of 

more interest and use than an estimate for all lithologies. 

CONCLUSIONS 

The in-place resources of natural gas within the 

overpressured part of the Upper Cretaceous in the 

Pinedale anticline is estimated to be 159 trillion cubic feet 

(4.50X1012 m3), but a number of factors suggest that this 

estimate may be conservative. Only a small part of this 

gas may be recoverable using currently available recovery 

technology. In our study, the characteristics of these unconventional accumulations have been related to specific 

geologic conditions that facilitate the resource evaluation, 

and we believe the methodology used in this study is, in 

general, applicable to similar gas accumulations elsewhere. 

110 Geology of Tight Gas Reservoirs, Wyoming and Colorado 

1-

LU 

LU 

l.L 

z 

_j 

LU 

> LU 

_J 

<( 

LU 

(/) 

s 

0 

_J 

LU 

co 

I 

1-

a_ 

LU 

0 

VOLUME OF GAS, 

IN CUBIC FEET/ CUBIC FOOT SANDSTONE 

8 10 14 16 

Or---~--,----r---.---,,---.---,---, 

12 

2,000 

4,000 

6,000 

8,000 

10,000 

I 

• •" 12,000 , 

•' 

14,000 

Figure 8. Concentration of natural gas per cubic foot of sandstone versus depth. Data primarily from sec. 5, T. 30 N., R. 

108 W.; the three deepest values from sec. 23, T. 31 N., R. 

108 w. 

REFERENCES CITED 

El Paso Natural Gas Company, 1971, Project Wagon Wheel, 

technical studies report: El Paso, Tex., 248 p. 

Greenfield, H., Ahmed, U., Holland, M.T., and Schatz, J.F., 

1981, Resource evaluation and production research on 

tight sands in the Pinedale Unit, Sublette County, Wyoming: Gas Research Institute GRI-81/0049, 157 p. 

Keighin, C. W., 1984, Petrographic and selected reservoir 

characteristics of some Tertiary and Cretaceous sandstones, Pinedale anticline, Sublette County, Wyoming, in 

Law, B.E., ed., Geological characteristics of low-permeability Upper Cretaceous and lower Tertiary rocks in the 

Pinedale anticline area, Sublette, County, Wyoming: U.S. 

Geological Survey Open-File Report 84-753, p. 16-21. 

Law, B.E., 1979, Section B-B'; subsurface and surface correlations of some Upper Cretaceous and Tertiary rocks, northern Green River basin, Wyoming: U.S. Geological Survey 

Open-File Report 79-1689, 2 sheets. 
El Paso 

Wago Wheel 

SW No. 1 NE 

8 ~ 8' 

SEALEVEL.------..-----.------------.------;------,------------.-------------.-------------, 

1-

w 

w 

u... 

z 

.... I 

w 

> w 

_J 

<{ 

w 

(/) 

s 

0 

_J 

2,000 

4,000 

6,000 

~ 8,000 

I 

1-

0... 

w 

0 

10,000 

12,000 

Sec.13 Sec7 

T. 30 N, R 109 W l 30 N, R 108 W 

Sec.5 Sec.33 Sec.27 

T. 30 N, R 108 W T. 31 N, R 108 W T. 31 N, R. 108 W 

Sec.23 

T. 31 N, R 108 W 

c 

0 

·;:; 

co 

E 

0 

LL 

(J) 

Ole 

c 0 

'§. ·~ 

(J) E ~ ..... (,) 0 OLL 

a: 

CONCENTRATION OF GAS (IN CUBIC FEEn PER 

CUBIC FOOT OF ROCK 

Em 

E§ 

[] . . 

. . . 

< 1 

1-2 

2-3 4.5-6 > 7.5 

~ 3-4.5 6-7.5 

Figure 9. Cross section showing concentration of natural gas per cubic foot of rock. Line of section 8-B' shown in figure 2. 

___ 1984a, Relationships of source-rock, thermal maturity, 

and overpressuring to gas generation and occurrence in 

low-permeability Upper Cretaceous and lower Tertiary 

rocks, Greater Green River basin, Wyoming, Colorado, 

and Utah, in Woodward, J ., Meissner, F.F., and Clayton, 

J .L., eds., Hydrocarbon source rocks of the greater Rocky 

Mountain region: Rocky Mountain Association of Geologists, p. 469-490. 

____ 1984b, Source-rock evaluation of Upper Cretaceous and 

lower Tertiary rocks in the Pinedale anticline area, 

Sublette County, Wyoming, in Law, B.E., ed., Geological 

characteristics of low-permeability Upper Cretaceous and 

lower Tertiary rocks in the Pinedale anticline area, 

Sublette County, Wyoming: U.S. Geological Survey 

Open-File Report 84-753, p. 36-50. 

___ 1984c, Structure and stratigraphy of the Pinedale 

anticline, Wyoming, in Law, B.E., ed., Geological characteristics of low-permeability Upper Cretaceous and lower 

Tertiary rocks in the Pinedale anticline area, Sublette 

County, Wyoming: U.S. Geological Survey Open-File 

Report 84-753, p. 6-15. 

Law, B.E., and Dickinson, W.W., 1985, Conceptual model for 

origin of abnormally pressured gas accumulations in lowpermeability reservoirs: American Association of Petroleum Geologists Bulletin, v. 69, no. 8, p. 1295-1304. 

Law, B.E., and Nichols, D.J., 1982, Subsurface stratigraphic 

correlations of some Upper Cretaceous and lower Tertiary 

rocks, northern Green River basin, Wyoming [abs.], in 

Steidtmann, J .R., ed., Subsurface practices in geology and 

geophysics: Laramie, Wyo., University of Wyoming, 

Quantitative Model for Overpressured Gas Resources, Pinedale Anticline 111 
CONCENTRATION OF GAS 

(IN BILLIONS OF CUBIC FEET) 

PER SQUARE MILE 

- 940-960 

920-940 

900-920 

~ 880-900 

t~::::::::::~:~:~:j 500-880 

8IJ 200-500 

[::·y:::.':\=.:·A o-2oo 

Figure 10. Concentration of natural gas in assessed part of 

Pinedale anticline. 

Department of Geology and Geophysics, Earth Science 

Bulletin 15, p. 17. 

Law, B.E., Pollastro, R.M., and Keighin, C.W., 1986, Geologic 

characterization of low-permeability gas reservoirs in 

selected wells, Greater Green River Basin, Wyoming, Colorado, and Utah, in Spencer, C.W., and Mast, R.F., eds., 

Geology of tight gas reservoirs: American Association of 

Petroleum Geologists Studies in Geology 24, p. 253-270. 

Law, B.E., and Spencer, C.W., 1981, Abnormally highpressured, low-permeability, Upper Cretaceous and Tertiary gas reservoirs, northern Green River basin, Wyoming 

[abs.]: American Association of Petroleum Geologists 

Bulletin, v. 65, no. 5, p. 948. 

Law, B.E., Spencer, C.W., and Bostick, N.H., 1979, Preliminary results of organic maturation, temperature, and 

pressure studies in the Pacific Creek area, Sublette County, Wyoming: U.S. Department of Energy Symposium on 

Enhanced Oil and Gas Recovery and Improved Drilling 

Methods, 5th, Tulsa, Okla., 1979, p. K2/1-K2/13. 

112 Geology of Tight Gas Reservoirs, Wyoming and Colorado 

Table 2. Estimate of in-place overpressured natural gas 

resources of the Pinedale anticline area 

Stratigraphic unit 

Lance Formation 

Ericson Formation 

Rock Springs Formation 

Entire structure 

Amount of gas 

(trillion cubic feet) 

62 

23 

75 

159 

___ 1980, Evaluation of organic matter, subsurface temperature, and pressure with respect to gas generation in lowpermeability Upper Cretaceous and lower Tertiary sandstones in Pacific Creek area, Sublette and Sweetwater 

Counties, Wyoming: The Mountain Geologist, v. 17, no. 

2, p. 23-35. 

Martin, W.B., and Shaughnessy, J., 1969, Project Wagon 

Wheel, in Symposium on Tertiary rocks of Wyoming: 

Wyoming Geological Field Conference, 21st, Casper, 

Wyo., 1969, Guidebook, p. 145-152. 

Masters, J.A., 1979, Deep basin gas trap, western Canada: 

American Association of Petroleum Geologists Bulletin, 

v. 63, no. 2, p. 152-181. 

Meissner, F.F., 1978, Patterns of source-rock maturity in nonmarine source rocks of some typical Western Interior 

basins: Rocky Mountain Association of Geologists Continuing Education Lecture Series, chapter IX, p. 1-37. 

___ 1980, Examples of abnormal fluid pressure produced by 

hydrocarbon generation [abs.]: American Association of 

Petroleum Geologists Bulletin, v. 64, no. 5, p. 749. 

___ 1981, Abnormal pressures produced by hydrocarbon 

generation and maturation and their relation to processes 

of migration and accumulation [abs.]: American Association of Petroleum Geologists Bulletin, v. 65, no. 11, 

p. 2467. 

Naeser, N.D., 1984, Fission-track ages from the Wagon Wheel 

No. 1 well, northern Green River basin, Wyoming; evidence for recent cooling, in Law, B.E., ed., Geological 

characteristics of low-permeability Upper Cretaceous and 

lower Tertiary Rocks in the Pinedale anticline area, 

Sublette County, Wyoming: U.S. Geological Survey 

Open-File Report 84-753, p. 66-77. 

___ 1986, Neogene thermal history of the northern Green 

River basin, Wyoming; evidence from fission-track dating, 

in Gautier, D.L., ed., Roles of organic matter in sediment 

diagenesis: Society of Economic Paleontologists and 

Mineralogists Special Publication 38, p. 65-72. 

Pollastro, R.M., and Barker, C.E., 1984, Geothermometry 

from clay minerals, vitrinite reflectance, and fluid inclusions; applications to the thermal and burial history of 

rocks cored from the Wagon Wheel no. 1 well, Green 

River basin, Wyoming, in Law, B.E., ed., Geological 

characteristics of low-permeability Upper Cretaceous and 

lower Tertiary rocks in the Pinedale anticline area, 

Sublette County, Wyoming: U.S. Geological Survey 

Open-File Report 84-753, p. 78-94. 
___ 1986, Application of clay-mineral, vitrinite reflectance, 

and fluid inclusion studies to the thermal and burial 

history of the Pinedale anticline, Green River basin, 

Wyoming, in Gautier, D.L., ed., Roles of organic matter 

in sediment diagenesis: Society of Economic Paleontologists and Mineralogists Special Publication 38, p. 73-83. 

Prensky, S.E., 1986, Geologic implications of large-scale trends 

in well-log response, northern Green River basin, Wyoming: SPWLA Annual Logging Symposium, 27th, Paper 

BEE, p. 1-24. 

Prensky, S.E., and Dickinson, W.E., 1986, Computer-generated 

well-log data plots assist in regional subsurface evaluation: Geobyte, v. 1, no. 2, p. 52-58. 

Rathbun, F.C., 1968, Abnormal pressures and conductivity 

anomaly, northern Green River basin, Wyoming: Society of Petroleum Engineers, Annual Fall Meeting, 43rd, 

SPE Paper 2205, p. 1-8. 

Rathbun, F.C., and Dickey, P., 1969, Abnormal pressures and 

conductivity anomaly, northern Green River basin, Wyoming: The Log Analyst, v. 10, no. 4, p. 3-8. 

Shaughnessy, J., and Butcher, R.H., 1973, Geology of Project Wagon Wheel Nuclear Stimulation Project, in Fassett, 

J .E., ed., Cretaceous and Tertiary rocks of the southern 

Colorado Plateau: Durango, Colo., Four Corners Geological Society, p. 185-196. 

__ 1974, Geology of Wagon Wheel Nuclear Stimulation 

Project, Pinedale field, Wyoming: American Association 

of Petroleum Geologists Bulletin, v. 58, no. 11, 

p. 2250-2259. 

Spencer, C.W., 1983, Geologic aspects of tight gas reservoirs 

in the Rocky Mountain region: Society of Petroleum 

Geologists/U.S. Department of Energy Joint Symposium 

on Low-Permeability Gas Reservoirs, Denver, Colo., 

1983, Proceedings, p. 399-408. 

___ 1984, Overpressured tight gas reservoirs in the Pinedale 

anticline area, Sublette County, Wyoming, in Law, B.E., 

ed., Geological characteristics of low-permeability Upper 

Cretaceous and lower Tertiary Rocks in the Pinedale anticline area, Sublette County, Wyoming: U.S. Geological 

Survey Open-File Report 84-753, p. 51-59. 

___ 1987, Hydrocarbon generation as a mechanism for overpressuring in rocky Mountain region: American Association of Petroleum Geologists Bulletin, v. 71, no. 4, 

p. 368-388. 

Spencer, C.W., and Law, B.E., 1981, Overpressured, lowpermeability gas reservoirs in Green River, Washakie, and 

Great Divide basins, southwestern Wyo. [abs.]: American 

Association of Petroleum Geologists Bulletin, v. 65, no. 

3, p. 569. 

Quantitative Model for Overpressured Gas Resources, Pinedale Anticline 113 

Chapter J 

Petrology and Isotope Geochemistry of 

Mineralized Fractures in Cretaceous RocksEvidence for Cementation in a 

Closed Hydrologic System 

By JANET K. PITMAN and WARREN W. DICKINSON 

Prepared in cooperation with the U.S. Department of Energy 

U.S. GEOLOGICAL SURVEY BULLETIN 1886 

GEOLOGY OF TIGHT GAS RESERVOIRS IN THE PINEDALE ANTICLINE AREA, WYOMING, 

AND AT THE MULTIWELL EXPERIMENT SITE, COLORADO 
CONTENTS 

Abstract Jl 

Introduction Jl 

Origin and distribution of fractures J4 

Fracture mineralization JS 

Carbon and oxygen isotopes J8 

Carbon J8 

Oxygen JlO 

Discussion Jll 

Temperature Jll 

Pore water otso Jll 

Water/rock ratio J12 

Pore water o13C J13 

Summary and conclusions J13 

References cited J14 

FIGURES 

1. Photographs showing mode of natural fractures in sandstone and shale, 

MWX wells J2 

2. Photomicrographs illustrating paragenetic relationships of fracture-fill 

minerals in study wells J6 

3. Scatter diagram of ol3C versus otso for fracture-fill calcite J9 

4. Crossplots of isotopic composition and depth J9 

5-6. Graphs showing: 

5. Variation in oxygen isotope composition of water in equilibrium with 

calcite of fixed isotopic composition, 120-300 Of J12 

6. Calculated o180 and ol3C compositions for calcite as a function of 

water/rock ratio in closed system at 120 Of (50 °C) J13 

TABLE 

1. Isotopic compositions and estimated temperatures for fracture-fill calcite, 

Piceance basin, Colorado, and northern Green River basin, Wyoming JlO 
Petrology and Isotope Geochemistry of 

Mineralized Fractures in Cretaceous RocksEvicience for Cementation in a Closed Hydrologic System 

By Janet K. Pitman and Warren W. Dickinsonl 

Abstract 

Well-developed, partly mineralized, vertical extension 

fractures containing abundant calcite and minor quartz, dickite, 

barite, and bitumen commonly occur in overpressured gasbearing sandstones of Cretaceous age in the Green River basin 

of Wyoming and the Piceance basin of Colorado. Methane inclusions in calcite (C.E. Barker, U.S. Geological Survey, 1987, 

oral cornmun.) and the occurrences of bitumen indicate that 

fracture cementation took place during hydrocarbon generation. Periods of tectonism and sediment loading also contributed to fracture development. Stable-isotope studies of 

fracture calcite from three wells show a narrow range in carbon and oxygen compositions (average o13c, -2.98 per mil 

PDB; average o18o, 15.07 per mil SMOW) over a thick 

stratigraphic interval (5,000-18,000 ft; 1,500-5,500 m). Typically, o13C: and o18o compositions display a slight enrichment 

trend with increased burial depth and temperature and, at any 

given depth, approximate the o 13c and o 1Bo compositions of 

matrix carbonate in the the enclosing sandstones. The isotopic 

trends, together with a knowledge of the burial and pressure 

history of the rocks, indicate that fracture carbonate inherited 

its isotopic composition from matrix carbonate through a process of dissolution and reprecipitation in a closed hydrologic 

system dominated by low water/rock ratios. The primary source 

of carbon in fracture calcite was from dissolved detrital marine 

carbonate grains that are ubiquitous in adjacent rocks. Increased o 180 com positions w ith depth suggest that, at greater 

burial depths and temperatures, fracture calcite crystallized 

from o180-enriched pore waters. On the basis of estimated formation temperatures, the calculated composition of formation 

waters in isotopic equilibrium with fracture calcite varies from 

-8 to + 2 per mil SMOW, values consistent with calcite crystallization in a rock-buffered diagenetic system. 

I NTR()DUCTION 

Thermogenic gas in the Piceance basin of Colorado 

and the Green River basin of Wyoming is widespread in 

Upper Cretaceous rocks. Most of the gas-bearing rocks 

1

Research School of Earth Science, Victoria University 

P.O. Box 600, Wellington, New Zealand 

are nonmarine and consist of lenticular, low-permeability 

sandstone interbedded with siltstone, kerogenous mudstone, and discontinuous beds of coal. The economic production of gas from these rocks depends either on the 

presence of artificially induced (hydraulic) fractures or 

on a network of open, interconnected natural fractures 

to increase effective permeability. Because artificial fracture enhancement is generally not cost effective, the exploitation of natural fractures may provide a possible 

alternative for economic gas recovery. 

This paper describes mineralized fractures from 

widely spaced wells in the Green River and Piceance 

basins and characterizes the hydrologic and thermal 

regime that existed during fracture development and 

mineralization. Cores from the El Paso Natural Gas 

Company Wagon Wheel No. 1 well and the Belco 

Petroleum WASP No. 1-A well (Law, 1984) in the Green 

River basin show numerous mineralized fractures in the 

Upper Cretaceous Lance Formation, Ericson Sandstone, 

and Rock Springs Formation. Abundant open and partly mineralized fractures are also present in core from the 

Mesaverde Formation (Upper Cretaceous) in the two 

wells from the U.S. Department of Energy's Multi well 

Experiment (MWX) in the Piceance basin (Law and 

Spencer, this volume, fig. 1). 

High pore-fluid pressures, which formed during the 

maturation of hydrocarbons, together with tectonism and 

sediment loading, may have generated stresses that initiated the formation of fractures. Knowledge of the 

factors controlling the opening, orientation, and mineralization of fractures is necessary before fractures can be 

successfully exploited for gas recovery. Mineralized fractures provide evidence of the chemical and thermal conditions that govern fracture cementation. In this study, 

petrographic and stable isotope techniques were used to 

identify the occurrence and distribution of discrete 

generations of fracture-fill minerals and to postulate the 

geochemical conditions that existed during fracture 

mineralization. The degree of rock-water interaction during fracture mineralization and the relative timing of fracture cementation also were evaluated. 

Petrology and Isotope Geochemistry of Mineralized Fractures )1 
,. ... ~ '7' 

~ ·... J 

~ "~j_ •• 

.,._ v•" .... -~~~·,."•--~ - ·~ 

Figure 1 (above and facing page). Mode of natural fractures in sandstone 

and shale, MWX wells. Scale in inches and millimeters. A, Open fracture 

in sandstone perpendicular to bedding; parallel vertical fracture displays 

calcite along fracture face. B, Multiple en echeleon vertical fractures in sandstone filled with calcite cement. C, Calcite-filled fracture in sandstone becomes 

closed and nonmineralized at shale interbeds. 0, Fracture face in sandstone 

coated with tiny euhedral crystals of calcite. E, Fracture in shale displaying 

a polished slickensided surface. 

J2 Geology of Tight Gas Rest:rvoirs, Wyoming and Colorado 
- H ... II ._ - - - - --

Petrology and Isotope Geochemistry of Mineralized Fractures )3 
Results from this study suggest that the precipitation of fracture-fill carbonate was controlled by the 

dissolution and reprecipitation of matrix carbonate in a 

closed diagenetic environment characterized by low 

water/rock ratios. The fluids that precipitated fracture 

carbonate do not appear to have been in equilibrium 

with matrix carbonate. Results from this study may be 

applicable to fractured rocks that formed in similar 

depositional and diagenetic settings in other geographic 

areas. 

Acknowledgments.-This study was funded by the 

U.S. Geological Survey Evolution of Sedimentary Basins 

Program and by the U.S. Department of Energy Morgantown Energy Technology Center. Carbon and oxygen 

isotope analyses were determined by the U.S. Geological 

Survey, Branch of Isotope Geology, and by Coastal 

Sciences Laboratory through Mobil Oil Company. 

ORIGIN AND DISTRIBUTION OF FRACTURES 

With few exceptions, the occurrence and distribution of fractures are similar in Upper Cretaceous rocks 

in the study wells. Fractures may be either open (nonmineralized) or partly to completely cemented with 

calcite, dickite, quartz, or barite. In general, open nonmineralized fractures are rare in core from the Wagon 

Wheel and WASP wells and are locally abundant in the 

MWX wells (fig. 1A). It is uncertain why some fractures 

are open and others are mineralized; however, the open 

fractures may be younger than the mineralized fractures 

and simply not have had sufficient time to become filled 

with cement. Verbeek and Grout (1984a) have shown that 

the oldest fractures in Tertiary rocks in the Piceance basin 

commonly contain more discrete generations of calcite 

than do the youngest fractures. No evidence exists for 

carbonate dissolution in partly cemented fractures in 

Cretaceous sandstones, and thus it is likely that the fractures are in the initial process of cement filling rather than 

cement removal. 

In sandstone and siltstone, fractures typically are 

single or multiple, subvertical extension features that extend across several beds and usually terminate at a change 

in lithology (figs. 1B, C). The fractures commonly are 

cemented by fine-to coarse-crystalline calcite (fig. 1D). 

Permeability is locally enhanced in these otherwise lowpermeability rocks wherever fractures are open as a result 

of incomplete development of carbonate cement. Fractures in shale are rare in the WASP and Wagon Wheel 

wells but numerous and mostly nonmineralized and 

closed in the MWX wells. The low angle to bedding of 

most fractures in shale suggest that the fractures may have 

formed during compaction and dewatering of mud shortly after deposition within a few feet of the surface 

(Verbeek and Grout, 1984a). The polished and 

)4 Geology of Tight Gas Reservoirs, Wyoming and Colorado 

slickensided surfaces of some fractures (fig. 1E) suggest 

that the fractures originated in shear. 

Fractures in Cretaceous rocks may have resulted 

from stresses associated with tectonism and sediment 

loading in conjunction with high pore-fluid stresses 

generated during burial diagenesis. Secor ( 1965) has 

demonstrated that high fluid pressures resulting from progressive loading of low-permeability sediments can play 

a significant role in the initiation and propagation of fractures at depth. Many Upper Cretaceous rocks in the Piceance and Green River basins contain beds of 

low-permeability shale that enclose and, hence, effectively 

isolate more permeable sandstone lenses. During burial 

and compaction, these shale beds may have functioned 

as barriers to fluid movement between lenticular sandstones and thereby caused high pore-fluid pressures to 

develop internally (Pitman and Sprunt, 1984). 

Law and others (1980), Law (1984), Law and 

Dickinson (1985), and McPeek (1981) have attributed abnormally high pore pressures in the Green River basin 

to the thermal generation of gas from coals and 

kerogenous mudstone. Law and others (1980) found that 

generation of significant thermogenic gas begins at 

temperatures of about 190-200 Of' (88-93 OC), which correspond to a mean vitrinite reflectance (Rm) of about 0.80 

percent. Overpressuring develops when gas is produced 

from local source rocks at a rate that exceeds the rate 

at which it can escape. Stresses associated with overpressuring may create new fractures or possibly reopen 

preexisting fractures that developed during burial and 

compaction. Under high pore-fluid pressures, fracturing 

by tectonism and sediment loading requires less effective 

stress than does fracturing under normal (hydrostatic) 

pressures. 

In each well, fractures occur in rocks that are now 

overpressured or were overpressured in the past. Fractures in the MWX wells are present from a depth of about 

4,000 ft {1,219 m) to the bottom of the hole at about 

8,000 ft {2,438 m); however, no cores were shallower than 

4,000 ft {1,219 m). Although the top of overpressuring 

is at a depth of about 5,600 ft {1,707 m), an Rm of 0.80 

percent occurs at about 4,000 ft (1,219 m). Studies by 

Bostick and Freeman (1984) show that in the area of the 

MWX wells approximately 5,000 ft (1 ,520 m) of strata 

was eroded during late Tertiary development of the Colorado River drainage system. During this erosional event, 

temperatures and the rate of gas generation decreased, 

which allowed high pore pressures to dissipate to a normal pressure regime. As the overlying sediments were 

eroded, the top of the overpressured zone was lowered 

from 4,000 to 5,600 ft (1,219-1,707 m). 

Although the MWX wells are in a structurally 

undeformed area, some extension fractures in Upper 

Cretaceous rocks may have formed as a result of stresses 

that developed during periods of epeirogenic uplift and 
erosion. Studies by Verbeek and Grout (1984b) indicate 

that regional fracture orientation patterns on the Grand 

Hogback along the eastern margin of the Piceance basin 

are very similar to those in MWX core. In addition, 

Laramide tectonism caused widespread differential 

erosion of sediment that is reflected in a regional unconformity separating Cretaceous and Tertiary strata 

throughout much of the basin. Fractures in these 

Cretaceous rocks may record stresses that occurred during the early stages of Laramide tectonism or during later 

periods of tectonic activity associated with the development of the White River plateau and the Grand Hogback 

monocline, which border the eastern margin of the basin. 

In the Wagon Wheel well, situated on the Pinedale 

anticline in the Green River basin, the top of overpressuring (0. 74 percent Rm) was lowered from 8,030 to 

10,400 ft (2,450-3,170 m) because of a decrease in 

temperature (Law, 1984). Fractures were observed in all 

of the cored intervals below the top of overpressuring 

(8,030 ft, 2,450 m), but none were found in the 200 ft 

(61 m) of core above this depth. An increase in the 

number or frequency of fractures with increasing depth 

below the top of the overpressuring may be the result of 

different lithologies or of faults at depth. Many fractures 

in the Wagon Wheel well formed, at least in part, from 

stresses caused by tectonism. The highest concentration 

of fractures is between 16,000 and 18,000 ft (4,877-

5,486 m). Seismic data (Martin and Shaughnessy, 1969) 

and steeply dipping beds in core indicate that these fractures are associated with a fault and thus are of probable 

tectonic origin. 

In the WASP well, the top of overpressuring is at 

about 10,600 ft (3,200 m) (Law, 1984). Fractures were 

not observed in the 100ft (30m) of core above the top 

of overpressuring but are common in cored intervals 

(11,329-14,900 ft, 3,453-4,542 m) within the overpressured zone. Because there is no evidence for 

widespread uplift and erosion in the area of this well during the late Tertiary, the top of overpressuring is presumed to have remained relatively constant since 

maximum burial. 

FRACTURE MINERALIZATION 

Fractures in the Upper Cretaceous rocks are partly 

to completely cemented by a mineral assemblage that is 

common to all wells and consists of quartz, dickite, 

calcite, and barite. Fine- to coarse-crystalline calcite is 

most abundant, dickite occurs in moderate amounts, and 

quartz and barite are present in minor amounts. Two 

minerals commonly coexist in a single fracture, but rarely 

are all found together. In addition to these minerals, a 

bituminous residue coats many crystals in fractures in the 

Wagon Wheel and WASP wells. Textural relations 

between these discrete mineral phases are shown in figure 2. 

The distribution of fracture-fill minerals, particularly calcite and dickite, appears to be controlled by the 

mineralogic composition of the surrounding rock. 

Calcite-filled fractures are abundant in sandstone containing authigenic matrix calcite and detrital limestone 

grains but rare or absent in rocks devoid of these 

minerals. Dickite occurs locally in fractures for which 

there is evidence for pore-fill and replacement kaolinite 

in the surrounding sandstone. In Wagon Wheel core, 

fractures in the Ericson Sandstone, a quartzarenite, are 

filled only with dickite, prob~bly because there are no 

carbonate minerals in the surrounding rocks. No apparent 

correlation exists between the fracture-fill minerals quartz 

and barite and the matrix composition of the enclosing 

sandstone. 

The paragenetic sequence of minerals in fractures 

is similar among the wells. Quartz, the earliest mineral 

to precipitate, commonly occurs as euhedral crystals lining the walls of fractures that are subsequently infilled 

by either dickite or calcite (fig. 2A). The presence of 

euhedral, syntaxial quartz overgrowths attached to 

detrital grain surfaces suggests that some fractures were 

open and permeable to fluids before the onset of carbonate precipitation. In one fracture from the Wagon 

Wheel well, large euhedral quartz crystals surrounded by 

calcite suggest that quartz was the last mineral phase to 

precipitate. 

Dickite is morphologically identical to kaolinite and 

has been identified by X-ray diffraction in several fractures from the Wagon Wheel and MWX wells (R.M. 

Pollastro, 1986, oral commun.). We assume that in many 

fractures the kaolin mineral, which has not been X-rayed, 

also is dickite. Macroscopic dickite occurs as a wellcrystallized, soft, white mineral along fracture faces (fig. 

2B). In thin section, it forms discrete pseudohexagonal 

platelets that commonly surround crystals of quartz. Both 

quartz and dickite typically are well crystallized, and textural relations between the two minerals suggest that they 

Figure 2 (next two pages). Photomicrographs illustrating 

paragenetic relationships of fracture-fill minerals in the study 

wells. A, Equant crystals of quartz (Q) lining fracture walls; proximal part of fracture is filled with calcite (C). 8, Well-crystallized 

dickite filling fracture. C, Fracture containing intergrown quartz 

(Q) and dickite (D). 0, Calcite (C) replacing early-formed dickite 

(D) within fracture; dashed lines indicate fracture walls. E, 

Fracture-fill calcite composed of elongate crystals having planar 

and curved intercrystalline boundaries. F, Fracture filled with 

ferroan calcite (Fe) cutting feldspar grain (F); note occurrence 

of iron-free matrix calcite (C). G, Fracture-fill calcite cement 

(C) replaced by multiple generations of barite (B). H, Early 

calcite (C) along fracture walls and late-stage barite (Ba) filling 

central part of fracture; voids (P) are lined with bitumen (B). 

Petrology and Isotope Geochemistry of Mineralized Fractures )5 
J6 Geology of Tight Gas Reservoirs, Wyoming and Colorado 
Petrology and Isotope Geochemistry of Mineralized Fractures )7 
may have formed simultaneously during a single 

diagenetic event rather than by replacement. Moreover, 

quartz shows no evidence of corrosion by dickite (fig. 

2C). In fractures from the MWX wells, dickite clearly 

has been replaced by calcite, whereas in the WASP and 

Wagon Wheel wells calcite and dickite interpenetrate one 

another (fig. W). Although paragenetic relations between 

calcite and dickite are somewhat obscure, it seems most 

likely that crystallization of dickite postdates development 

of quartz but predates precipitation of calcite. It is possible that reaction of kaolinite in nearby sandstones with 

high-temperature pore fluids resulted in recrystallization 

of kaolinite to dickite in fractures; however, the reported 

coexistence of nacrite, dickite, and kaolinite in ore hydrothermal deposits (Hanson and others, 1981) indicates that 

no unique environmental indicators can be used to predict 

the conditions governing the precipitation of these minerals. The crystallization of dickite may be controlled by 

trace amounts of heavy metals such as mercury, lead, 

zinc, and copper (Hanson and others, 1981). 

Fine- to coarse-crystalline calcite fills most fractures 

and is composed of discrete, tabular or equant crystals 

that coalesce to form a cement. In some fractures, calcite 

crystallization apparently occurred during a single 

episode, inasmuch as individual crystals of calcite extend 

from one fracture wall to the other and their long axes 

are oriented perpendicular to the face of the fracture (fig. 

2E). The intercrystalline boundaries commonly are either 

slightly curved or planar. In other fractures, aggregates 

of calcite crystals are intergrown and exhibit different optic orientations; in these fractures, more than one episode 

of calcite precipitation may have occurred in the same 

fracture. There is no evidence in any of the wells to 

suggest that fracture calcite has undergone extensive 

dissolution and recrystallization. The sharp, planar 

boundaries of voids within discrete calcite crystals indicate 

incomplete carbonate cementation rather than a period 

of dissolution. 

Some features in calcite from the MWX wells were 

not evident in calcite from the Wagon Wheel and WASP 

wells. Potassium ferricyanide staining indicates that 

calcite in fractures from MWX locally is enriched in iron 

(fig. 2F); thus, the chemical composition of pore fluids 

that precipitated calcite probably changed through time. 

In addition, crystals of calcite may exhibit multiple sets 

of twin lamellae reflecting stress gradients possibly caused 

by an increase in overburden pressure resulting from 

either progressive burial or periods of local tectonism. 

Barite in fractures is recognized by its moderate 

relief, blocky cleavage, and low birefringence. In the 

MWX wells, barite occurs locally as patches replacing 

calcite and dickite; individual barite crystals commonly 

display a sharp contact with these minerals (fig. 2G). In 

the Wagon Wheel well, barite typically occupies the central part of a fracture (fig. 2H). Although paragenetic 

J8 Geology of Tight Gas Reservoirs, Wyoming and Colorado 

relations between barite and the other minerals are 

unclear, barite probably was the last mineral to precipitate 

in the fractures. 

Bitumen coats crystals of quartz, dickite, calcite, 

and barite in many fractures in the Wagon Wheel and 

WASP wells. Characteristically, it lines these crystals and 

was deposited parallel with the fracture walls (fig. 2H); 

local cementation probably was episodic, at least during 

the formation of the bitumen. Numerous inclusions of 

methane were also detected in calcite (C.E. Barker, 1985, 

oral commun.) and indicate that interbedded humic-rich 

source rocks were generating thermogenic gas during the 

time calcite was infilling fractures·. 

CARBON AND OXYGEN ISOTOPES 

Representative samples of calcite in fractures from 

the study wells were analyzed for their carbon and oxygen isotope compositions in order to determine the conditions governing fracture mineralization. The calcite was 

scraped and drilled from the fractures and reacted with 

anhydrous phosphoric acid to produce carbon dioxide gas 

for analysis (McCrea, 1950). The results, summarized in 

table 1, are reported in per mil relative to the PDB standard for carbon and the SMOW standard for oxygen. For 

comparison between wells, ol3C and o180 compositions 

are shown in a scatter diagram (fig. 3) and are plotted 

against depth (fig. 4). Calcite from fractures in all three 

wells shows little isotopic variation; the average carbon 

composition is -2.98 per mil, and the average oxygen 

compositions is 15.07 per mil. The isotopic variation that 

does occur in fracture calcite may reflect the wide range 

in burial depth and temperature of formation in the different wells. 

In natural systems, the carbon and oxygen in calcite 

originate from different sources and are fractionated by 

different processes. Predominantly, equilibrium processes 

fractionate oxygen, whereas kinetic processes fractionate 

carbon. Although both ol3C and o180 compositions of 

fracture calcite become slightly enriched with increasing 

burial depth (fig. 4), the factors that influence each trend 

are different. 

Carbon 

Carbon isotopic ratios in pore waters are a function of pH, C02 partial pressure, and the dominant 

source of carbon in the reservoir. In turn, the o13C composition of the pore water and, to a lesser extent, the 

temperature at the time of crystallization control the 

isotopic composition of carbon in calcite. Isotopically 

light carbon derived from organic matter (<-20 per mil) 

and isotopically heavy carbon derived from limestones 
co 

0 

c.. 

_J 

~ 

a: 

w 

c.. 

u 

"' Go 

0 

-2 

-4 

-6 

-8 

DEEP 

Wagon Wheel 

EXPLANATION 

• Wagon Wheel 

A. WASP 

o MWX 

-10~--~--~--~--~--~----~--~--~--~ 

12 15 18 21 

o1 8 Q PER MIL (SMOW) 

Figure 3. Scatter diagram of one versus o180 for fracturefill calcite. 

and dolomites (about zero per mil) as well as from various 

inorganic minerals can be distinguished by the oBc composition of calcite. The isotopic composition of sedimentary carbonate minerals commonly reflects a mixing or 

dominance of carbon from these sources. With the exception of the samples denoted by asterisks in table 1, 

most oBC compositions for fracture calcite are within the 

range reported for freshwater and marine limestones 

( + 2.0 to -6.6 per mil PDB; Keith and Weber, 1964), suggesting that the carbon in the diagenetic fluids precipitating fracture calcite originated primarily from these 

sources. 

The oBC composition of fracture calcite generally 

becomes more positive with increasing depth of burial 

(table 1, fig. 4A). This shift may be interpreted as 

resulting from a similar shift in oBC composition of the 

formation waters. In the northern Green River basin, the 

average oBc composition for inorganic carbon in 

shallow aquifers (mean depth 164 ft, 50 m) is -8.6 per 

mil (B.A. Kimball, U.S. Geological Survey, 1984, written commun.). As burial depth increases, the relative concentration of Be in formation waters moving down a 

flow path may increase if there is a progressive addition 

of Be from the dissolution of detrital carbonate grains 

and (or) if there is a decrease in the volume of nearsurface water (usually depleted in Be) penetrating the 

deeper sandstones. Calcite depleted in Be also may 

reflect sandstone cement that contains a component of 

soil C02 that formed during meteoric diagenesis 

associated with the development of an erosion surface 

(Pitman and Sprunt, 1987). Allan and Matthews ( 1982) 

0 0 (f) 0 0 0 ct:l<8J a: 0 00 

w 2 0 1- 0 

w • • 0 0 

~ 

0 • _J • • Q • .... z .... - :i 4 ........ 1-

c.. ............ :. ........ w EXPLANATION 0 

• Wagon Wheel • • .... WASP ... 0 MWX •• 

6 

-8.0 -6.0 -4.0 -2.0 

o13 C PER MIL (PDB) 

(f) 

a: 0 0 

L1J Off6Xb;) 00 

1- 2 0 L1J 

~ • • 00 

0 

_J • • 

Q 

z 

I- 4 .... .... ........ ......... 1- .... .... c.. 

w EXPLANATION 0 • • 

-• 

• • 

6 

13 15 1 7 

81 8 0 PER MIL (SMOW) 

0 

0 

5 

10 

15 

20 

0 

5 

0 

0 

0 

X 

1-

w 

w 

u.. 

z 

:i 

1-

c.. 

w 

0 

0 

0 

0 

X 

1-

10 ~ u.. 

z 

I 

15 t 

20 

L1J 

0 

Figure 4. Crossplots of isotopic composition and depth. A, 

one. B, o18o. 

have documented that as much as 4 to 6 per mil carbon 

depletion can occur in rocks near an exposure surface. 

In each well, the oBc composition of fracture 

calcite at any given depth approximates the o13C composition of calcite cement in the host sandstone (J .K. Pitman, 1984, unpub. data; Dickinson, 1985). The general 

correlation in oBc composition between fracture and 

matrix calcite strongly suggests that the dominant source 

of carbon in fracture calcite was sandstone cement, and 

the slight shift in fracture calcite to lighter values indicates 

that the carbon reservoir underwent some modification 

during burial. Emrich and others (1970) have demonstrated that the carbon-isotope equilibrium between 

Petrology and Isotope Geochemistry of Mineralized Fractures J9 
Table 1. Isotopic compositions and estimated temperatures for fracture-fill calcite in three wells, Piceance basin, 

Colorado, and northern Green River basin, Wyoming 

[Asterisk (*) indicates value more negative than average range for freshwater and marine limestones. Paleotemperatures estimated from 

Lopatin burial history models. Location of wells shown in Law and Spencer (this volume, figs. 2, 3)] 

o13c olso o13c olso 

Depth Depth Temperature (°F) PDB SMOW Depth Depth Temperature (°F) PDB SMOW 

(feet) (meters) Past Present (eermil) (eermil) (feet) (meters) Past Present (eer mil) (eer mil) 

MWX wells Piceance basin 

5,018.0 1,534.0 243 153 -6.58 13.33 6,076.0 1,857.0 266 176 -1.72 15.29 

5,475.3 1,674.0 253 163 -4.71 12.60 6,076.0 1,857.0 266 176 -1.72 15.29 

5,731.0 1,752.0 259 169 -3.09 15.08 6,076.5 1,865.0 266 176 -1.72 15.39 

5,734.0 1,753.0 259 169 -2.90 14.46 6,131.6 1,874.0 266 176 -2.29 14.77 

5,744.6 1,756.0 259 169 -2.62 14.25 6,312.0 1,929.0 270 180 -4.57 14.36 

5,780.7 1,766.0 259 169 -2.55 14.46 7,181.4 2,194.0 298 208 -1.27 17.14 

5, 792.9 1,771.0 259 169 -10.38 17.14 7,219.0 2,206.0 300 210 -2.59 15.49 

5,828.7 1,782.0 261 171 -2.36 14.67 8,114.2 2,479.0 334 244 -2.69 15.90 

5,867.0 1 '793.0 261 171 -2.18 14.87 8,124.5 2,482.0 334 244 -1.83 15.90 

5,868.0 1,793.0 261 171 -2.17 14.98 

Wagon Wheel well, northern Green River basin 

8,096.1 2,467.7 198 153 -5.50 14.53 16,120.7 4,913.6 297 252 -0.92 15.65 

8,128.0 2,477.4 198 153 -4.85 14.16 16,119.1 4,913.1 297 252 -1.67 15.06 

8,944.2 2,726.2 208 163 -5.05 15.50 17,169.0 5,233.1 309 264 -1.26 16.16 

8,959.0 2,730.7 208 163 -5.49 15.06 17,176.2 5,235.3 309 264 -1.43 15.65 

10,154.9 3,095.2 223 178 -4.04 17.44 17,178.5 5,236.0 309 264 -1.48 15.71 

10,234.9 3,119.6 225 180 -3.96 17.57 17,960.0 5,474.2 318 273 -1.37 15.27 

10,993.1 3,350.7 234 189 -3.19 17.38 17,965.9 5,476.0 318 273 -1.12 15.99 

11,000.0 3,352.8 234 189 -3.33 17.80 

WASP well, northern Green River basin 

11,333.0 3,454.3 225 225 -7.14* 12.53 

11,591.9 3,533.2 230 230 -7.34* 12.68 

13,025.9 3,970.3 252 252 -1.79 13.62 

13,433.1 4,094.4 259 259 -1.97 14.25 

13,482.0 4,109.3 259 259 -1.69 15.09 

13,590.9 4,142.5 261 261 -0.75 13.87 

calcite and dissolved bicarbonate in pore fluids is 

temperature dependent. Although temperature may have 

been an important factor governing the isotopic composition of carbon during the precipitation of fracture calcite, 

the crystallization of calcite in fractures at temperatures 

lower than those at which matrix carbonate precipitated 

is unlikely. More likely, the slight o13C depletion exhibited in fracture calcite reflects pore waters containing 

a component of isotopically light carbon associated with 

the generation of hydrocarbons rather than a significant 

temperature decrease. Inclusions of methane occurring 

locally in calcite and quartz (C.E. Barker, 1985, oral communication) are good evidence that interbedded source 

rocks attained high levels of thermal maturity (>0. 76 percent Rm) during fracture cementation. 

Oxygen 

o18o compositions for calcite from MWX and 

shallow Wagon Wheel fractures form a trend that is 

distinct from those for the WASP and deep Wagon Wheel 

J10 Geology of Tight Gas Reservoirs, Wyoming and Colorado 

13,611.9 4,148.9 261 261 -0.68 14.05 

13,986.9 4,263.2 269 269 -0.82 14.20 

14,032.2 4,277.0 269 269 -2.15 14.82 

14,043.0 4,280.3 269 269 -1.77 15.19 

14,192.9 4,326.0 270 270 -2.63 14.73 

14,198.2 4,327.6 270 270 -2.69 13.82 

fractures (fig. 4B). Except for the deep fractures in the 

Wagon Wheel well, o18Q compositions generally become 

enriched by 1 to 3 per mil as burial depth increases. Variations in o18Q compositions of fracture calcite with depth 

may reflect changes in both the o180 composition of pore 

waters and the temperature of precipitation. The ()18Q 

composition of calcite as it undergoes recrystallization 

during burial will shift toward the o180 composition of 

the pore water in response to the decreasing isotopic fractionation that occurs between calcite and water as temperature increases. 

The degree of openness of the rock-water system 

influences the control of mineral-water reactions on the 

isotope composition of pore water. In a closed system 

in which the water /rock ratio is low, mineral-water reactions can significantly modify the o180 composition of 

the pore waters. The remarkable similarity between the 

()18Q isotope compositions of calcite in fractures and 

those of matrix calcite in sandstones (J .K. Pitman, unpub. data, 1984; Dickinson, 1985) and the apparent control by the mineralogic composition of the surrounding 

rock on the fracture mineralogy strongly suggest that, 
similar to o13C compositions, local wall-rock materials 

were the dominant source of oxygen in fracture-fill calcite. 

Thus, it is probable that a large component of the oxygen 

in fracture calcite was inherited from matrix carbonate 

through a process of dissolution and reprecipitation in a 

closed, rock-dominated system. Detrital and authigenic 

clay minerals such as illite and chlorite also may have contributed some oxygen to the system during diagenesis. 

The o180 composition of fracture calcite typically 

is depleted by 1 to 4 per mil relative to that of calcite 

cements in the surrounding sandstones (J .K. Pitman, unpub. data, 1984; Dickinson, 1985). If fracture calcite 

crystallized from pore fluids that were in isotopic 

equilibrium with those that precipitated matrix calcite, 

then the difference in o18o compositions may reflect 

burial temperatures that were higher during the crystallization of fracture calcite than during the formation of 

calcite cement in adjacent sandstones. This mechanism 

of 180 depletion is consistent with the hypothesis that the 

development and cementation of fractures may have 

taken place under the influence of an overpressured 

regime at elevated temperatures. Petrographic studies 

demonstrate that mineralization of fractures followed 

emplacement and subsequent dissolution of matrix carbonate in sandstones. In the MWX well, o180-enriched 

fracture calcite (about 13 per mil; table 1) occur locally 

and has approximately the same o18o compositions as 

matrix calcite in rocks of the same age and origin in other 

areas of the basin. These similar compositions suggest 

that fracture calcite may have precipitated at temperature 

conditions and pore-water chemistries similar to those 

from which matrix carbonate dissolved. 

The o180 compositions of calcite (15.64 per mil 

average) in the deep Wagon Wheel fractures, which are 

associated with a fault system, are depleted relative to 

the depth trend of enrichment established by the shallow 

Wagon Wheel fractures (fig. 4B). These isotopically light 

o180 compositions may reflect either high water/rock 

ratios associated with groundwater recharge through open 

and interconnected fractures or possibly deep basin brines 

that migrated upward through the fracture network. 

DISCUSSION 

The o180 composition of water, o180 composition 

of carbonate, and temperature are important variables 

in a tso;16o equilibrium carbonate system. Although the 

physiochemical conditions that govern fracture cementation are poorly understood, some constraints can be 

placed on the temperature and isotopic composition of 

the pore waters that may have characterized an overpressured restricted hydrologic regime during Late Cretaceous and early Tertiary time in the Piceance and Green 

River basins. 

Temperature 

Temperatures in the Piceance and Green River 

basins during the precipitation of fracture calcite were 

estimated from modified Lopatin burial history models 

and are reported in table 1, together with corresponding 

present-day temperatures. Based on these models, fracture calcite precipitated over a wide temperature range 

if cementation occurred at temperatures equal to or 

greater than present-day temperatures. Paleotemperatures 

for Cretaceous rocks in the MWX well are estimated to 

have varied from 240 to 330 op (116-166 °C) during maximum burial 10 to 40 Ma. Bostick and Freeman (1984) 

postulated that about 5,000 ft (1 ,400 m) of erosion has 

taken place in the area of MWX during the past 10 million 

years. Assuming that the geothermal gradient has not 

changed from the Late Cretaceous to the present, we 

estimate that burial temperatures declined about 122 Of' 

(50 °C) in response to this late stage of uplift and erosion. 

In the vicinity of the Wagon Wheel well, maximum 

burial temperatures of about 200-300 op (90-160 °C) occurred between 48 and 4 Ma ago; during the last 2 to 4 

million years, the rocks cooled by 68-86 Of' (20-30 °C) 

(Naeser, 1984). This temperature range is in good agreement with temperatures determined by Pollastro and 

Barker (1984) for fluid inclusions in fracture-fill quartz 

from the deep Wagon Wheel fractures, which formed late 

in the burial history. Pollastro and Barker estimated a 

minimum temperature of 266 op (130 °C) for quartz 

precipitation, about 50 op (1 0 °C) hotter than the present burial temperature. Uplift and erosion in the area of 

the WASP well has been minimal for the last 50 million 

years (Dorr and others, 1977), and there is no evidence 

to suggest that the geothermal gradient varied significantly through time. Formation temperatures in the Cretaceous section (225-270 Of'; 110-130 OC) probably remained 

relatively constant during this period. 

Pore Water <)1Bo 

The oxygen isotope composition of pore fluids that 

may have existed during precipitation of fracture calcite 

in the Piceance and Green River Basins was determined 

from the calcite-water fractionation equation of Friedman and O'Neil (1977) using estimated paleotemperatures 

from burial history models (table 1) and measured 

isotopic ratios of fracture calcite. Figure 5 shows the 

range in oxygen isotopic composition of pore waters in 

equilibrium with calcite over a temperature span of 

120-300 op (50-150 °C). It is evident that formation 

waters in isotopic equilibrium with fracture calcite at the 

estimated temperatures of precipitation become enriched 

from about -8 to + 2 per mil as burial depth increases 

(fig. 5). Based on studies by Dickinson (1987), these 

Petrology and Isotope Geochemistry of Mineralized Fractures J 11 
waters are isotopically more similar to modeled waters 

for a closed system than to those for an open system. 

The a1So composition of near-surface meteoric 

water in the northern Green River basin is about -20 per 

mil (B.A. Kimball, 1984, unpublished data), whereas in 

the Piceance Creek basin, some 62 mi (100 km) north of 

the MWX well, the a1So composition of recharge 

groundwater is about -17 per mil (Kimball, 1984). Formation water data from the Wagon Wheel and WASP 

wells are not available, but the also composition of 

water from the paludal zone, 3,973 ft (1,211 m) below 

the Cretaceous-Tertiary boundary, in one of the MWX 

wells is -7.5 per mil (B.A. Kimball, 1986, written commun.). Based on the fractionation equation of Friedman 

and O'Neil (1977), this water is not in isotopic equilibrium 

with fracture calcite (about 14 per mil alSo) at the 

present-day temperature of 230 "F (110 °C) (fig. 5). To 

be in equilibrium with this water, the calcite would have 

had to precipitate at 122 "F (50 OC), which is unlikely 

because the estimated temperature of precipitation is 

about 352 op (160 °C), hotter than the present temperature of 230 "F (110 °C). The oxygen isotope composition of water recovered from this MWX well probably 

represents a mixing of enriched formation water and 

depleted meteoric water. 

Compared to the measured compositions of formation water from one of the MWX wells, the estimated 

compositions of formation fluids that may have existed 

during fracture cementation are significantly enriched in 

a1so. Isotopically heavy pore waters may be expected in 

a closed hydrologic system for which water/rock ratios 

are low, such as in these low-permeability, overpressured 

sandstone. Part of this enrichment may reflect elevated 

temperatures resulting from deep burial, but the large 

component of a1So in the pore waters should compensate for a negative shift in also of calcite that might 

result from increasing temperatures. It is plausible that 

some fraction of the heavy oxygen represents a component of enriched meteoric water. Taylor (1974) estimated 

that meteoric waters present in the Rocky Mountain 

region from the Eocene to the Pliocene were enriched by 

2 to 3 per mil in a1SO relative to present-day waters. 

Water/Rock Ratio 

Calculation of the mole ratio of water to rock 

(W /R) following the method of Taylor (1977) can be used 

to model conditions governing fracture mineralization. 

The W /R ratio in atom percent is the isotopic shift in 

the rock divided by the isotopic shift in the water and 

can be determined by estimating the initial isotopic composition of the water and calcite in the system prior to 

alteration by dissolution and subsequent precipitation in 

fractures. Because isotopic composition of fracture calcite 

J12 Geology of Tight Gas Reservoirs, Wyoming and Colorado 

120 

L.L.. 

0 

~ 180 

u} 

a: 

::::> 

~ a: 

L.U 

a.. 240 

~ 

L.U 

1-

+ MWX 

WATER 

-8.0 -4.0 0 

818 0 WATER PER MIL (SMOW) 

4.0 

Figure 5. Variation in oxygen isotope composition of water 

in equilibrium with calcite of fixed isotopic composition in 

temperature range 120-300 °F (50-150 °C.) (Based on fractionation equation of Friedman and O'Neil, 1977.) o1Bo compositions of calcite are read from the curves (o1Bo = 18, 15, 

12 per mil), which depict relationship between temperature 

and water in equilibrium with calcite of indicated o18o composition. Estimated paleotemperatures used to calculate o1Bo 

(SMOW) of formation waters are from table 1. Dashed lines 

show range in isotope composition for waters in open and 

closed systems; closed system model of Dickinson (1987) uses 

o18o value of 18.5 for surface water; open system water uses 

mixing factor of 0.3 for the same water. Solid circles, Wagon 

Wheel well; open circles, MWX wells; closed triangles, WASP 

well. 

not only changes with depth but also reflects the original 

composition of detrital limestone grains in sandstone, 

meteoric water compositions (a13C, -8.6; a1so, 18.5) and 

detrital limestone compositions (al3C, -0.33; a1SO, 21) 

are best estimates for prealteration or initial values. When 

these initial values are used, W /R ratios calculated 

from the measured carbon and oxygen values in fracture 

calcite are generally similar and decrease in value with 

increasing depth. This trend probably reflects the 

decreasing porosity and permeability of the associated 

sandstones. 

The isotopic compositions of calcite, calculated 

as a function of W /R ratios at constant temperature 

and superimposed on the measured compositions of 

fracture calcite, are shown in figure 6. Values for 

present-day meteoric waters (line B) are bracketed by 

heavier and lighter values (lines A and C); all calculated 

lines converge toward the isotopic composition selected 

for the initial rock. The use of isotopic values for initial 

water, such as the water at depth in the MWX well, which 

is heavy in both carbon and oxygen, cause the calculated 

lines to rotate clockwise around values for the initial rock 

and away from the measured isotopic compositions of 

fracture calcite. To compensate for this rotation, lighter 

values for the initial rock must be used to bring the 
~ 

0:: 

0 

-2 

~ -6 

u 

(") 

co 

-8 

-10 

12 15 18 21 

<)1BQ PER MIL (SMOW) 

Figure 6. Calculated o180 and o13C compositions for calcite 

as a function of the water/rock ratio (atom percent) in closed 

system at 120 oF (50 °C) superimposed on data fields for calcite 

cement (fig. 3). Water/rock ratios calculated according to Taylor 

(1977). For initial water, values foro 1Bo and o Be are, respectively, as follows: line A, 20, -4; line 8, 18.5, -8.6; line C, 10, 

-12; line 0, 7.5, -1. For initial rock, values of o1Bo and o13C 

are, respectively, as follows: lines A, 8, and C, 21, -0.33; line 

D, 13, -7. For example, a calcite on line C having o1Bo composition of 15 and o 13c composition of -8 precipitated from 

a system having water-rock ratio of 5 if the initial water had 

o180 of 10 and o13C of -12 and the initial rock had 5180 of 

21 and o13C of -0.33. At water/rock ratios below 5, the differences between open and closed systems (as defined by 

Taylor, 1977) are negligible. 

calculated values (lineD, fig. 6) close to the measured 

values; the use of these lighter values causes the W /R 

ratios to increase toward the deep samples. 

The relationship between the calculated water/rock 

ratios and the measured isotopic compositions of the 

calcite cements places constraints on the conditions governing fracture cementation. For example, it is evident 

that the isotopic composition of the initial water from 

which the cements precipitated is similar to present-day 

meteoric values, although it is possible that the initial 

water may have been somewhat lighter in carbon and 

heavier in oxygen than assumed in the model. The fracture cements could not have precipitated from deep-seated 

or isotopically heavy water unless the initial calcite was 

isotopically light in both carbon and oxygen, which is 

unlikely. In addition, carbon or oxygen isotope trends 

in calcite for all three wells correlate closely with the 

isotopic composition of meteoric water as the water/rock 

ratio decreases with increasing burial depth. These 

calculations alone, however, cannot determine whether 

the diagenetic system in which fracture cementation occurred evolved from a closed to an open state. 

Pore Water onc 

Dissolved bicarbonate in water from the paludal 

zone in one MWX well has a o13C composition of+ 1.16 

per mil. This heavy carbon most likely reflects dissolved 

marine carbonate that originated in sandstone and shale 

beds. Assuming a temperature of about 230 op (110 OC), 

which corresponds to the present-day burial depth at the 

top of the paludal zone, fracture calcite from this interval should be enriched in Be by approximately 2.5 per 

mil relative to dissolved HCOr and C02=. Fracture 

calcite in equilibrium with this water should have a ol3C 

composition of about + 4 per mil rather than the measured -2.1 per mil average. The difference of approximately -6 per mil suggests that these fluids may contain a 

component of organically derived carbon. 

SUMMARY AND CONCLUSIONS 

Overpressured gas-bearing rocks of Upper Cretaceous age in Wyoming and Colorado contain abundant, 

open and partly mineralized natural fractures that may 

serve to enhance permeability in these otherwise tight 

rocks. Identification of the factors that control the occurrence and distribution of fracture-fill cements and the 

hydrologic regime that existed during fracture mineralization is important in order to predict the presence of fractures in rocks of the same age and origin in other 

geographic areas that have undergone similar burial and 

diagenetic histories. Fractures in sandstones from three 

study wells are cemented with abundant calcite and lesser 

amounts of quartz, dickite, and barite. Most of the fractures were filled late in the burial history when the rocks 

were at greater burial depths and higher temperatures 

than at present. 

Stable isotope studies reveal systematic o13C and 

o18o patterns in fracture calcite with depth that are 

similar to those in matrix carbonate. These trends suggest that host sandstones acted as the dominant carbon 

and oxygen reservoir during fracture cementation. In all 

three wells, the ol3C compositions for fracture calcite are 

enriched with depth and in the range reported for marine 

carbonate, suggesting that the source of most of the carbon was from detrital grains of marine carbonate in the 

host sandstones. The o180 compositions for fracture 

calcite are consistently light (15 per mil average); in the 

MWX, WASP, and shallow Wagon Wheel fractures, 

oxygen is slightly enriched (1-3 per mil) with increasing 

burial depth. This trend reflects increased temperatures 

with greater depth of burial. The small difference in o18o 

Petrology and Isotope Geochemistry of Mineralized Fractures )13 
compositions between the wells argues for a broad geographic control on the geochemical conditions governing fracture cementation. 

The isotope trends, together with a knowledge of 

the burial history of the rocks, indicate that fracture 

calcite formed through a process of dissolution and 

reprecipitation of matrix carbonate. In addition, methane 

inclusions in fracture calcite are evidence for fracture 

cementation in a thermal regime dominated by the active generation of hydrocarbons and high pore-fluid 

pressures. The slight enrichment in o18Q of fracture 

calcite with increasing depth and temperature suggests 

that precipitation occurred in a closed system in which 

the oxygen of the pore waters became sufficiently enriched with increasing depth to offset the decrease in 

isotopic fractionation between calcite and water. 

Most fractures in Cretaceous rocks probably were 

opened and cemented at different times in the burial 

history rather than during a single event. Variations in 

the isotopic composition of calcite in closely spaced fractures suggest that most fractures, except those in the 

deepest part of the Wagon Wheel well, were not open 

and interconnected during mineralization. If the fractures 

had been connected at depth, the isotopic compositions 

of the calcite cement would be homogeneous between individual fractures. The isotopic differences, together with 

petrographic evidence that indicates carbonate dissolution and reprecipitation did not occur following fracture 

cementation, imply that fracture-fill calcite underwent no 

significant postdepositional isotopic exchange during 

burial. 

Formation water from the paludal zone in one 

MWX well has a o180 composition of -7.5 per mil and 

a ol3C bicarbonate composition of + 1.6 per mil. The 

calcite-water fractionation equation of Friedman and 

O'Neil (1977) indicates that the o18Q of water having this 

isotopic composition is not in equilibrium with fracture 

calcite in the same zone at its present-day temperature 

(230 'F; 110 OC). We postulate that the water sample 

represents a mixing of enriched formation water and 

depleted near-surface meteoric water. Based on paleotemperature estimates from burial history models, 

calculated formation waters in equilibrium with fracture 

calcite become enriched in o180 from -8 to + 2 per mil 

with increasing burial depth. These systematically heavier 

values, together with dissolved bicarbonate values on the 

order of + 1.6 per mil, are consistent with fracture calcite 

precipitation in a closed rock-buffered diagenetic system. 

REFERENCES CITED 

Allan, J.R., and Matthews, R.K., 1982, Isotope signatures 

associated with early meteoric diagenesis: Sedimentology, 

v. 29, no. 6, p. 797-817. 

J14 Geology of Tight Gas Reservoirs, Wyoming and Colorado 

Bostick, N.H. and Freeman, V.L., 1984, Tests of vitrinite 

reflectance and paleotemperature models at the Multi well 

Experiment site, Piceance Creek basin, Colorado, in 

Spencer, C.W., and Keighin, C.W., eds., Geologic studies 

in support of the U.S. Department of Energy Multi well 

Experiment, Garfield County, Colorado: U.S. Geological • 

Survey Open-File Report 84-757, p. 110-120. 

Dickinson, W. W., 1985, Isotope geochemistry of carbonate 

minerals in nonmarine rocks, northern Green River basin, 

Wyoming: U.S. Geological Survey Open-File Report 

85-532, 103 p. 

___ 1987, An oxygen isotope model for interpreting carbonate diagenesis in nonmarine rocks, Green River basin, 

Wyoming: Chemical Geology (Isotope Geoscience Section), v. 65, p. 103-116. 

Dorr, J.A., Spearing, D.R., and Steidman, J.R., 1977, Deformation and deposition between a foreland uplift and an 

impinging thrust belt, Hoback Basin, Wyoming: Geological Society of America Special Paper 177, 82 p. 

Emrich, K., Ehhalt, D.H., and Vogel, J.C., 1970, Carbon 

isotope fractionation during the precipitation of calcium 

carbonate: Earth and Planetary Science Letters, v. 8, 

p. 363-371. 

Friedman, 1., and O'Neil, J.R., 1977, Compilation of stable 

isotope fractionation factors of geochemical interests, in 

Fleischer, M., ed., Data of geochemistry (6th ed.): U.S. 

Geological Survey Professional Paper 440-KK, 

p. KK1-KK12. 

Hanson, R.F., Zamora, R., and Keller, W.D., 1981, Nacrite, 

dickite, and koolinite in one deposit in Nayarit, Mexico: 

Clay and Clay Minerals, v. 29, no. 6, p. 451-453. 

Keith, M.L., and Weber, J.N., 1964, Carbon and oxygen 

isotopic composition of selected limestones and fossils: 

Geochimica et Cosmochimica Acta, v. 28, no. 11, 

p. 1787-1816. 

Kimball, B.A., 1984, Ground water age determinations, Piceance Creek basin, Colorado, in Hitchon, Brian, and 

Wallick, E. I., eds., Practical applications of ground water 

geochemistry: First Canadian/ American Conference on 

Hydrogeology, Banff, Alberta, June 1984, Proceedings, 

p. 267-283. 

Law, B.E., 1984, Relationships of source-rock, thermal maturity, and overpressuring to gas generation and occurrence 

in low-permability Upper Cretaceous and lower Tertiary 

rocks, Greater Green River basin, Wyoming, Colorado, 

and Utah, in Woodward, Jane, Meissner, F.F. and 

Clayton, J., eds., Hydrocarbon source rocks of the greater 

Rocky Mountain region: Rocky Mountain Association of 

Geologists, p. 469-490. 

Law, B.E., and Dickinson, W.W., 1985, Conceptual model for 

the origin of abnormally pressured gas accumulations in 

low-permeability reservoirs: American Association of 

Petroleum Geologists Bulletin, v. 69, no. 8, p. 1295-1304. 

Law, B.E., Spencer, C.W., and Bostick, N.H., 1980, Evaluation of organic matter, subsurface temperature, and 

pressure with regard to gas generation in low-permeability 

Upper Cretaceous and lower Tertiary sandstones in Pacific 

Creek area, Sublette and Sweetwater Counties, Wyoming: 

The Mountain Geologist, v. 17, no. 2, p. 23-25. 
Martin, W.B., and Shaughnessy, J., 1969, Project Wagon 

Wheel, in Symposium on Tertiary rocks of Wyoming: 

Wyoming Geological Association Field Conference, 21st, 

1969, Guidebook, p. 145-152. 

McCrea, J.M., 1950, The isotopic chemistry of carbonates and 

a paleo-temperature scale: Journal of Chemistry and 

Physics, v. 18, p. 849-855. 

McPeek, L.A., 1981, Eastern Green River basin, a developing 

giant gas supply from deep, overpressured Upper 

Cretaceous sandstones: American Association of 

Petroleum Geologists Bulletin, v. 65, no. 6, p. 1078-1098. 

Naeser, N.D., 1984, Fission-track ages from the Wagon Wheel 

No. 1 well, northern Green River basin, Wyo.; evidence 

for recent cooling, in Law, B.E., ed., Geological 

characteristics of low-permeability Upper Cretaceous and 

lower Tertiary rocks in the Pinedale anticline area, Sublette 

County, Wyoming: U.S. Geological Survey Open-File 

Report 84-753, p. 66-77. 

Pitman, J.K., and Sprunt, E.S., 1984, The origin and occurrence 

of fracture-filling cements in the Upper Cretaceous Mesaverde Formation at MWX, Piceance Creek basin, Colorado, in Spencer, CW., and Keighin, CW., eds., Geologic 

studies in support of the U.S. Department of Energy 

Multiwell Experiment, Garfield County, Colorado: U.S. 

Geological Survey Open-File Report 84-757, p. 87-101. 

Pollastro, R.M., and Barker, C.E., 1984, Geothermometry 

from clay minerals, vitrinite reflectance, and fluid inclusions; applications to the thermal and burial history of 

rocks cored from the Wagon Wheel No. 1 well, Green 

River basin, Wyoming, in Law, B.E., ed., Geological 

characteristics of low-permeability Upper Cretaceous and 

lower Tertiary rocks in the Pinedale anticline area, 

Sublette County, Wyoming: U.S. Geological Survey 

Open-File Report 84-753, p. 78-94. 

Secor, D.T., Jr., 1965, Role of fluid pressure in jointing: 

American Journal of Science, v. 263, p. 633-646. 

Taylor, H.P., Jr., 1974, The application of oxygen and 

hydrogen isotope studies to problems of hydrothermal 

alterations and ore deposition: Economic Geology, v. 69, 

no. 6, p. 843-883. 

___ 1977, Water/rock interactions and the origin of H20 in 

granitic batholiths: Geological Society of London Journal, v. 133, pt. 6, p. 509-558. 

Verbeek, E.R., and Grout, M.A., 1984a, Fracture studies in 

Cretaceous and Paleocene strata in and around the Piceance basin, Colorado; preliminary results and their bearing on a fracture-controlled natural-gas reservoir at the 

MWX site: U.S. Geological Survey Open-File Report 

84-156, 30 p. 

___ 1984b, Prediction of subsurface fracture patterns from 

surface studies of joints; an example from the Piceance 

Creek basin, Colorado, in Spencer, C. W., and Keighin, 

C.W., eds., Geologic studies in support of the U.S. 

Department of Energy Multiwell Experiment, Garfield 

County, Colorado: U.S. Geological Survey Open-File 

Report 84-757, p. 75-86. 

Petrology and Isotope Geochemistry of Mineralized Fractures )15 

Chapter K 

Reservoir Sedimentology of Rocks of the 

Mesaverde Group, Multiwell Experiment Site 

and East-Central Piceance Basin, 

Northwest Colorado 

By JOHN C. LORENZ 

Prepared in cooperation with the U.S. Department of Energy 

U.S. GEOLOGICAL SURVEY BULLETIN 1886 

GEOLOGY OF TIGHT GAS RESERVOIRS IN THE PINEDALE ANTICLINE AREA, WYOMING, 

AND AT THE MULTIWELL EXPERIMENT SITE, COLORADO 
CONTENTS 

Abstract Kl 

Introduction Kl 

Fluvial deposits K2 

Example of fluvial reservoir K4 

Nonchannel deposits K7 

Delta-plain deposits Kll 

Example of an upper delta-plain reservoir K13 

Examples of lower delta-plain reservoirs K16 

Shoreline-marine deposits K16 

Reservoirs in the Mancos Shale K19 

Summary K22 

References cited K23 

FIGURES 

1. Diagrammatic map showing depositional environments of sandstones at 

MWX site K2 

2. Chart showing reservoir characteristics of rocks from different depositional 

environments encountered in MWX wells and geophysical logs K3 

3. Well-to-well gamma-ray log correlation of reservoir sandstones in fluvial 

part of Williams Fork Formation in MWX wells K4 

4. Cross section showing detailed lithologic description of slabbed core and 

gamma-ray logs for typical fluvial reservoir in Williams Fork Formation in 

MWX wells KS 

5. Charts showing porosity and permeability characteristics of meander-belt 

fluvial sandstone reservoir in Williams Fork Formation in MWX wells K6 

6. Schematic cross section of flood-deposit sandstone (unnamed sandstone bed 

of Williams Fork Formation) K7 

7. Core and gamma-ray logs showing lateral variation of floodplain deposits 

in fluvial zone of Williams Fork Formation in MWX wells and schematic 

diagrams showing depositional environments K8 

8. Schematic cross section of point-bar deposits in single-story lenticular sandstone in lower delta-plain deposits of Williams Fork Formation Kll 

9. Well-to-well gamma-ray log correlation of reservoir sandstones in deltaplain interval of Williams Fork Formation in MWX wells K12 

10. Cross section showing detailed lithologic description of slabbed core and 

gamma-ray logs for composite reservoir in upper delta-plain environment 

(coastal zone of Williams Fork Formation) in MWX wells K14 

11. Charts showing porosity and permeability characteristics of upper deltaplain distributary-channel sandstone reservoirs in Williams Fork Formation 

in MWX wells KlS 

12. Cross section showing detailed lithologic description of slabbed core and 

gamma-ray logs for lenticular distributary-channel sandstone and thick 

splay deposit in lower delta-plain environment of Williams Fork Formation 

in MWX wells K17 
13. Charts showing porosity and permeability characteristics of distributary 

channel sandstone reservoir and laterally variable splay sandstone reservoir 

in lower delta-plain environment of Williams Fork Formation in MWX-1 

and MWX-2 wells K18 

14. Well-to-well gamma-ray log correlation of sandstones in shoreline-marine 

interval of lies Formation of Mesaverde Group in MWX wells K20 

15. Cross section showing detailed lithologic description of slabbed core and 

gamma-ray logs for Cozzette Sandstone Member of lies Formation of 

Mesaverde Group in MWX-1 and MWX-2 wells K21 

16. Charts showing porosity and permeability characteristics of blanket-marine 

sandstones (lower and upper sandstone units of Cozzette Sandstone 

Member of lies Formation) in MWX-1 and MWX-2 wells K22 

17. Diagrams showing lithology and setting of offshore-marine deposits in 

Mancos Shale along the Book Cliffs K23 

Reservoir Sedimentology of Rocks of the 

Mesaverde Group, Multiwell Experiment Site and 

East-Central Piceance Basin, Northwest Colorado 

By john C. Lorenz1 

Abstract 

Three types of low-permeability sandstone reservoirs are 

encountered in Upper Cretaceous rocks of the Mesaverde 

Group in the U.S. Department of Energy's Multiwell Experiment (MWX) wells in the east-central Piceance basin, northwest 

Colorado. Reservoir characteristics of each type of sandstone 

are determined by the environment in which the sandstone 

was deposited. The lower Mesaverde Group consists of a series 

of blanket sandstone reservoirs deposited in regressive wavedominated delta and shoreline environments and interbedded 

with nonmarine coal and marine shale. Above the blanket sandstone reservoirs, distributary channel reservoirs were deposited 

on a delta plain as elongate lenticular sandstone bodies. These 

are associated with mudstone in the upper delta plain (coastal) 

facies and with mudstone and coal in the lower delta plain 

(paludal) facies. Irregular sandstone bodies composed of 

stacked point bars were deposited in the overlying fluvial environment. Estimates of reservoir morphology, dimensions, 

trends, and internal characteristics for each environment can 

be made on the basis of sedimentological models and by using characteristics observed both in core and in outcrops near 

the MWX site. 

INTRODUCTION 

The U.S. Department of Energy conducted 

field-laboratory studies designed to characterize lowpermeability sandstones and to assess different technologies for the production of natural gas from these 

reservoirs. The laboratory consisted of three closely spaced wells drilled into the Upper Cretaceous Mesaverde 

Group in the east-central Piceance basin of northwest 

Colorado (Law and Spencer, this volume, fig. 1). It was 

designated the Multiwell Experiment (MWX), and the 

three wells were designated MWX-1, MWX-2, and 

MWX-3. The purpose of this report is to summarize 

reservoir characteristics of the various types of sandstones 

encountered within the Mesaverde Group in these wells. 

1

Sandia National Laboratories, Albuquerque, N. Mex. 87185 

These characterizations are supported by extensive field 

studies of Mesaverde outcrops around the Piceance basin, 

most notably at Rifle Gap, approximately 12 mi (20 km) 

northeast of the MWX site. 

The MWX wells penetrated rocks deposited in four 

distinct environments in the 4,000-ft-thick (1 ,220 m) 

Mesaverde Group. Three of these environments are part 

of a progradational sequence and are, from the stratigraphic base up, shoreline marine, delta plain (including 

coastal and paludal), and fluvial. The spatial arrangement 

of these environments at any given time is depicted in 

figure 1. As progradation occurred, the deposits were 

superimposed vertically. Variations in local subsidence 

and uplift rates significantly influenced the thickness and 

percentage of sandstone in the formation in different 

parts of the basin, but the primary control on local reservoir geometry and internal complexity was depositional 

environment (fig. 2). 

The fourth type of environment has been recognized in the Ohio Creek Member, found at the top of 

the Mesaverde Group (Lorenz and Rutledge, 1987). This 

marine-influenced paralic environment records a major 

change from the overall regressive succession recorded 

in the rest of the formation to deposition associated with 

the ensuing Lewis transgression. The rocks do not contain significant amounts of gas in the MWX wells and 

will not be described in this report, although they may 

constitute a significant reservoir facies elsewhere in the 

basin (Lorenz and Rutledge, 1985a, b). 

Most of the MWX reservoir properties, including 

porosity, matrix permeability, and gross reservoir morphology, size, and internal heterogeneity, are governed 

by the sedimentary characteristics described in this paper. 

Although overall reservoir permeability is controlled by 

the system of natural fractures and in situ stresses found 

in the Mesaverde strata at the MWX site (Lorenz and 

others, 1988), the different sedimentary characteristics of 

the different reservoirs exert significant control on the 

distribution and character of the natural fractures. 

Moreover, the sedimentology or primary depositional 

Reservoir Sedimentology of Mesaverde Group, Multiwell Site K1 
Figure 1. Depositional environments of sandstones at the 

MWX site in the east-central Piceance basin of northwest Colorado. Lateral migration of the system during progradation 

resulted in stacked fluvial deposits above delta-plain and 

marine deposits. Modified from Fisher and McGowen's (1969) 

interpretation of Eocene deposits of Texas Gu If Coast. Depositional environments of Mesaverde Group deposits are believed to be analogous. 

environment of a reservoir in large part dictates the 

petrologic characteristics of grain size, sorting, and 

maturity that affect the primary and secondary 

(diagenetic) development of porosity and permeability 

(Lorenz, Sattler, and Stein, 1989). 

Acknowledgments.-This work was performed for 

the U.S. Department of energy under contract DEACOH-76DP00769. 

FLUVIAL DE POSITS 

Rocks of the fluvial interval (MWX depths 

4,400-6,000 ft, 1,340-1 ,830 m) of the Mesaverde Group 

were deposited in an environment of meandering fluvial 

systems (fig. 2). Most of these systems were relatively 

small and probably originated on the alluvial plain rather 

K2 Geoqy of Titht Gas Reservoirs, Wyoming and Colorado 

than in the distant, contemporaneous fold and thrust belt 

of central Utah. The presence at the mid-fluvial level at 

Rifle Gap of a small coal deposit and a sandstone lens 

that contains several fossil logs with the trace fossil 

Teredolithus suggests that a minor transgression may have 

occurred at this time in the east-central part of the basin, 

and that the seaward edge of this progradational episode 

was never far beyond the Rifle Gap area. 

Study of outcrops along the Grand Hogback near 

Rifle indicates that most of the fluvial sandstones were 

deposited as arcuate point bars on the inside of river 

bends during lateral migration of rivers. A sandstone 

deposited as a point bar is commonly partially eroded and 

replaced when subsequent point bars migrate over the 

same area. Each individual point-bar unit in the MWX 

reservoirs fines upward, and the composite unit also commonly fines upward as a result of lowered gradients of 

maturing rivers. 

Sedimentologic models of depositional environments suggest that each point-bar deposit is arcuate in 

plan view. Reconstructions (Lorenz and others, 1985) suggest that the Mesaverde point bars were about 700 ft 

(200m) wide and 2-13ft (0.6-4 m) thick. These pointbar units commonly are stacked on top of each other in 

random orientations and form composite meander-belt 

sandstone reservoirs that are 20-60 ft (6-13 m) thick and 

on the order of 1,000-2,000 ft (300-600 m) wide. The 

reservoirs generally are elongate parallel with paleoslope 

and have irregular edges. At the MWX site, they are 

- significantly wider than the well spacings of 120-200 ft 

(37-61 m), and therefore well-to-well correlation of individual sandstone reservoirs is excellent (fig. 3). 

Internally, these complex reservoirs are composed 

of a series of fining-upward sequences, each of which 

corresponds to the deposit of one point bar. As many 

as seven fining-upward sequences occur within one 

reservoir. In general, each cycle has a basal scour surface 

(the floor of the ancient channel) overlain by fine- to 

medium-grained, crossbedded sandstone. The crossbedded sandstone grades upward into fine to very fine grained, usually rippled sandstone. Some of the sequences 

continue upward into siltstone or even thin mudstone, 

but many of the upper channel and channel-margin 

deposits were eroded by scouring at the base of the next 

channel as it migrated across the area. 

Figure 2 (on facing page). Reservoir characteristics of rocks 

from different depositional environments encountered in the 

MWX wells. Meander belts within the fluvial zone are composed of point bars. Geophysical logs: GR, gamma ray; ILD, 

deep induction log. Thicknesses (range and average) from MWX 

downhole geophysical logs. Methods used to calculate reservoir widths described in Lorenz and others (1985). Outcrops 

widths measured along Grand Hogback, 12 mi (20 km) northeast of MWX site. N.A. indicates not available. 
Depth 

(feet) 

Cl 

_Q 

cON 

.Ex SCHEMATIC PROFILE 

-53: OF SANDSTONES .!: ~ 

(perpendicular to 

trend of paleoslope) 

Q) 

0. 

ro 

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> "'0 

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c 

ro 

(f) 

RESERVOIR DIMENSIONS 

RESERVOIR 

CHARACTERISTICS 

DEPOSITIONAL 

ENVIRONMENT 

4000~~~~~~~~~=.F~~----~~~--~~-L--~----~~------~~~~~~--~~~--. Extens1ve, crossbedded D1str1butary channel and 

5000 

6000 

7000 

8000 

:"'2~:::::::. Crossbedding, ripples 

.A 4 £ .... £ Clay rip up clasts 

Bedding plane (clay 

surfaced) 

Coal 

0 0 

0 

r--

sand bod1es, relatively estuary, w1th muddy bays 

~ co <i 

1\ 

free of mternal and marshes 

(/) 

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1 Lengths are 1ndetermmant » Widths 

0 

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'<t 

d1scontmU1t1es. Un1form 

gram s1zes w1thm 

sandstones (square 

gamma-ray profile) 

Extens1ve heterogeneous 

sand bod1es composed of 

numerous crossbedded 

subunits: often contam 

slit and clay Interbeds . 

Grain s1zes f1ne upward: 

siltstone and mudstone 

beds usually separate 

maJor sand bod1es. but 

some Interconnections 

are expected. Rare coals: 

common d1ssem1nated 

plant debns 

Restncted-w1dth. lmear. 

crossbedded sandstone 

lenses. usually 1solated 

w1th1n siltstones and 

mudstones. Grain sizes 

fme upward abruptly at 

top. Some coals: common 

d1ssem1nated carbonaceous plant debns 

Blanket sandstones (thmner and less extensive than 

those of shorel1ne-mar1ne 

Interval) Interspersed w1th 

lent1cular channel 

sandstones. Relatively 

homogeneous Extens1ve 

coal beds abundant (about 

13 percent of column). 

Carbonaceous mudstone 

and siltstone beds common 

Extens1ve (50 X 75 m1) 

blanket sandstones. 

homogeneous on a local 

scale. Coarsen1ng-upward 

gra1n s1ze w1thin each 

sand body. Interspersed 

shales near base of each 

body. overly1ng coal beds 

common Sandstones 

crossbedded and well 

sorted 

2 Sand and s1lt beds as th1n as 2 It can be seen on logs but are economically unimportant 

3 Probable w1dth var1es with estimate of borehole separat1on at depth 

4 Outcrop d1mens1ons are apparent w1dths that may mclude s1gn1f1cant port1ons of length 

5 Data msuff1cient to make val1d calculation 

6 lnf1n1te w1th respect to effective well-bore dramage 

1 Of lies Format1on 

Meander-belt nver channels Interspersed w1th 

muddy floodplains and 

swamps 

Low-smuos1ty nver distributary channels traversIng low-grad1ent coastal 

plainS. Muddy swamps 

and lakes 

Delta-front/shoreline 

Interspersed w1th delta 

distnbutary channels. 

Flank1ng coal swamps. 

marshes. and bays 

Regress1ve deltafront/shoreline to 

shallow marine. 

Interspersed with offshoremanne. brackish bays. and 

coastal marshes 

Reservoir Sedimentology of Mesaverde Group, Multiwell Site K3 
4500 

5000 

fUJ 

UJ 

u.. 

~ 

:i 

b: 

UJ 

0 

5500 

6000 

MWX-1 MWX-2 MWX-3 

Figure 3. Well-to-wel I gamma-ray log correlation of reservoir 

sandstones (pattern) in fluvial part of the Williams Fork Formation in the MWX wells. Correlation between wells is excellent as a result of large lateral extent of these reservoirs 

relative to well spacing. Solid bar indicates cored intervals; 

dashed line indicates tentative correlations. 

Although some of the individual fining-upward 

units may be attributed to lateral accretion of diagonally 

inclined point-bar beds, there is evidence for channel-base 

scour surfaces below many of the units. Moreover, the 

erratic thickness of the units reflects the irregularities of 

K4 Geology of Tight Gas Reservoirs, Wyoming and Colorado 

partial erosion rather than the more uniform changes in 

thickness indicative of superimposed lower, middle, and 

upper point-bar facies. These stacked units therefore are 

inferred to be multistoried, superimposed point-bar 

deposits. 

The vertical trend across an entire reservoir is 

shown well in gamma-ray profile and commonly fines upward. Upper-channel siltstone and mudstone are preserved near the top of the reservoir and are overlain by 

siltstone and mudstone of levee and floodplain environments. Penetration of these uppermost zones by roots of 

plants commonly has obscured or destroyed bedding. 

Example of Fluvial Reservoir 

Lateral heterogeneity of reservoirs in the fluvial 

facies is illustrated in figure 4, which shows the lithologies 

of core in the three MWX wells for a typical MWX fluvial 

reservoir. On a gross scale, this sandstone is one of the 

more uniform lenses in the fluvial zone. Bedding characteristics in the core show, however, that the reservoir is 

heterogeneous in detail, in that each well contains multiple partial point-bar sequences. The thickest point-bar 

sequence is in the basal part of MWX-2; it is about 10ft 

(3 m) thick and is inferred to be relatively complete. 

The 10-ft (3m) thickness can be used to reconstruct 

a minimum meander-belt width of 1,000 ft (300 m) 

(technique of Lorenz and others, 1985). Dipmeter and 

oriented core data from MWX-3 provide two instances 

of northerly paleoflow in two point bars, and cross beds 

in MWX-2 core (oriented by using paleomagnetic measurements) indicate northwest paleo flow. The base of the 

sandstone is about 5 ft (1.5 m) lower in MWX-1 than 

in the other two wells; the channel probably was initially 

located in the area of MWX-1, subparallel with the 

MWX-2/MWX-3 axis. Such a location restricts possible reservoir-axis orientations to a generally north-south 

alignment, although data are insufficient to be definitive. 

Partially eroded mudstone drapes and the clay plugs 

that filled abandoned channels created irregularly spaced 

lithologic discontinuities that are permeability barriers 

within the matrix rock. Natural fractures are the dominant permeability mechanism in these tight reservoirs, but 

the extent of individual fractures is strongly controlled 

by lithologic discontinuities within the reservoirs (Lorenz 

and Smock, 1986; Finley and Lorenz, 1989; Lorenz, 

Branagan and others, 1989). Similar internal complexities in fluvial reservoirs elsewhere in the basin have also 

been described (Jones, 1984; Jones and others, 1984). 

Porosity and permeability within the fluvial reservoirs follow erratic but generally decreasing upward 

trends consistent with lithologic heterogeneity caused by 

the amalgamation of multiple point-bar deposits. Measurements from core of MWX-1 and MWX-3 (fig. 5) 
;:IC 

I'D 

"' I'D 

~ 

~· Vl 

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I'D 

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0 

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5825 

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MWX-1 

mudst,carb,burr 

sltst,burr 

mud/elay,deform,burr 

sltst,burr,lam 

mudst,bk,burr 

sltst 

sltst 

MWX-2 

MWX-3 

IN 

mudst,earb,burr 

ss,vfg,mass,burr 

bern mg,xbed 

bern vfg 

sltst,mass 

sltst/ss,npples 

slrst,detorrn,roots 

mudst,bk,earb 

MWX-1 

PLAN VIEW OF MWX WELLS 

AT DEPTH OF 5,800 FT 

MWX-3 

5825 

mudst,carb 

sltst,burr,lam 

sltstlmudst,lam 

sltst/ss,r~pples, burr 

ss,mg,mass 

ss.rng,deform 

ss,mg,xbed 

ss.rng,mass,r~pups 

ss,mg,mass 

ss,mg-cg,mass 

ss,tg,xbed 

I · ·· ''"ii'· .·.··.·· ·· ..... ..-~<"/-(I ss,mg-cg,xbed 

< -~--• mudst,burr 

clay,bk,carb 

sltstlmudst,lam,burr 

Figure 4. Detailed lithologic description of slabbed core and gamma-ray logs for a typical fluvial reservoir (pattern) in the Williams Fork Formation in the MWX 

wells. Nonsymbolized area in core log indicates core is missing. Abbreviations: bcm, becomes; bk, black; burr, burrowed; carb, carbonaceous; cg, coarse grained; 

clayst, claystone; deform, soft-sediment deformation; fg, fine grained; lam, laminated; mass, massive; mg, medium grained; mudst, mudstone; sltst, siltstone; ss, 

sandstone; vfg, very fine grained; xbed, crossbedded. 
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Q.. 

Q 

MWX-1 MWX-2 MWX-3 

Gamma-ray log Porosity Dry Klinkenberg permeability Porosity 

(in percent) at 1000 psi (in microdarcys) Gamma-ray log (in percent) Gamma-ray log 

Porosity Dry Klinkenberg permeability 

(in percent) at 1000 psi (in microdarcys) 

u 5 10 0.1 1.0 10.0 100.0 0 5 10 

5800 I ............... I I I I I I I I I ""I I I I II 1111 I I I I ""I 5 10 0.1 1.0 10.0 100.0 r------~-~---~--...., I I I '""I I I I '"'I I I I ""'I 

8 I I 0 0 I I ~ I 0 I I __, I 0 0 

5825 cR oo 0 -

0 0 0 0 0 0 00 08 0 1- 00 

LU 0<6 oo 0 0 0 

LU oo 8 0 0 

u.. a& 0 0 80 0 

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<c 0 0 0 

:i 0 0 0 0 0 0 

0 0 

t 0 0 0 0 0 0 

0 0 0 0 0 

LU 5850 cf:n 0 0 00 0 -

5875 j- - r- - .. I Ill llJ. J..Ui ul 

Figure 5. Porosity and permeability characteristics of a meander-belt fluvial sandstone reservoir in the Williams Fork Formation in the MWX wells. Reservoir quality is highly variable in sandstones deposited in this type of environment. Note logarithmic scale of permeability graphs. 
"'0 

Q) 

(jj 

> 

0 

u 

100ft 

VERTICAL 

EXAGGERATION 

;:::X 1. 6 

Figure 6. Schematic cross section of a flood-deposit sandstone (unnamed sandstone bed of Williams Fork Formation) in Estes 

Gulch, about 1 mi (1 .6 km) west of Rifle Gap. Planar-horizontal bedding predominates; subunits represent multidepositional 

events. Adjacent fine-grained deposits are covered. 

show abrupt increases in porosity and permeability at the 

bases of fluvial reservoirs and erratic but generally high 

values through the main parts. The variability in these 

measurements correlates well with grain size, increasing 

with each new coarse-grained basal point-bar deposit and 

decreasing above it. Near the top of a reservoir, where it 

becomes silty, there is a gradual but marked decrease in 

porosity and permeability. 

Porosity in core from MWX-2 shows a similar pattern, except for a more gradual initial increase at the base 

of the reservoir. In this well, numerous claystone ripup 

clasts were incorporated into the basal sandstones (fig. 4) 

and may have inhibited the development of good porosity. 

Average porosity in this reservoir is about 8 percent, 

and average dry Klinkenberg permeability at 1,000 lbs/in.2 

(6.9 MPa) net confining stress is about 7-8 microdarcys 

(A.R. Sattler, Sandia National Laboratories, oral commun., 1986). 

Nonchannel Deposits 

A wide, homogeneous sandstone body of probable 

flood origin (unnamed sandstone bed of the Williams 

Fork Formation) (fig. 6) has been observed in outcrops 

of the fluvial facies at Estes Gulch, just west of Rifle Gap. 

The closest modern analogs to this deposit are inferred 

to be the fan deltas of Scott and others (1969). These 

multistory/multidepositional event deposits result from 

storm-related torrential rainfall and flooding in local 

drainage basins. The abrupt increase in the percentage of 

sandstone in the Mesaverde Group above the flood deposit 

in Estes Gulch suggests that the storm events that caused 

the flooding also may have caused avulsion of a fluvial 

system into the Estes Gulch area. 

This fan-delta environment produced primarily 

planar-horizontal bedding in the thicker central sections 

of the sandstone body. The presence of a single fossil log 

burrowed with Teredolithus trace fossils suggests that the 

shoreline was not distant. The deposit is at about the same 

stratigraphic interval as the coal and the burrowed log 

noted previously. It is also possible, however, that the log 

was swept far inland during the storm event. The absence 

of associated shoreline deposits may indicate that the 

sandstone was deposited in an inland lacustrine basin. 

Such flood deposits should be excellent reservoirs. 

They may be significantly different in size from the more 

common meander-belt sandstone deposits, but their morphologies and internal compositions are less complex. 

Deposits of this type have not been recognized in core 

from the MWX wells but probably are not rare in the 

Mesaverde Group as a whole. 

Lacustrine claystone and thinly bedded, rooted and 

rippled siltstone and mudstone levee and splay deposits 

are common between the channel sandstones. The nonsandy deposits also include rooted and burrowed swamp 

mudstones that are carbonaceous and probably provided 

an immediate, though perhaps diluted, source for reservoir gas. These mudstones are composed of mixed clays 

and silts and should also provide good reservoir seals, even 

in fractured reservoirs, because the fractures that dominate 

reservoir permeability commonly terminate at contacts 

with these adjacent mudstones in both core and outcrops. 

The fine-grained nonchannel sediments were deposited as lateral facies equivalents of the sandy meanderbelt complexes. Paleosols are rarely found in the 

floodplain deposits, and the abundance of soft-sediment 

deformation, burrowing, and preserved organic matter indicates that the Mesaverde floodplain environments were 

rarely desiccated. The limited extent of the different 

floodplain environments is suggested by the rapid lateral 

lithologic variations in the short distances between the 

three MWX wells (Lorenz, 1985a) (fig. 7). 

Figure 7 (next three pages). Lateral variation of floodplain 

deposits in the fluvial zone of the Williams Fork Formation in 

the MWX wells. Alphabetic labels refer to units discussed in 

text. Location of interval shown in figure 3. A, Fence diagram 

of detailed logs for part of MWX wells. 8, Schematic diagrams 

showing depositional environments. 

Reservoir Sedimentology of Mesaverde Group, Multiwell Site K7 
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MWX-2 

Gamma-ray 

log Core log 

~N 

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MWX-3 

Core log 

EXPlANATION 

LITHOLOGY 

Sandstone 

Siltstone 

Mudstone 

Claystone 

No core 

LITHOLOGIC 

CHARACTERISTICS _ .... 

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Carbonaceous zone 

Soft-sediment deformation 

Crossbedding 

Ripple crossbedding 

Root traces 

Burrows 

Mudstone ripup clasts 

Pyritic nodules 

Calcareous zones 

Sediment inclusions 

Laminated bedding 

Chaotic or unbedded 
/ / 

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Reservoir Sedimentology of Mesaverde Group, Multiwell Site K9 
K10 Geology of Tight Gas Reservoirs, Wyoming and Colorado 
Unit A of the floodplain deposits shown in figure 7 

is a rooted and burrowed levee splay deposit in MWX -1 

that grades distally into swampy deposits in MWX-2 and 

into probable lacustrine deposits in MWX-3. Unit B 

probably records multiple depositional events on a splay 

that later abandoned the MWX-3 area, being replaced 

by a lacustrine environment. Unit C may record overlapping environments: a crossbedded and rippled levee and 

channel-margin deposit in MWX-3 grades into thin, 

subaqueous, burrowed and rippled siltstone in MWX-1, 

whereas the equivalent deposits in MWX-2 record a 

proximal splay derived from an entirely different source. 

Unit D records extreme facies changes: carbonaceous mudstone of a paludal environment in MWX-1 is 

equivalent to the dark, organic claystone of a lacustrine 

environment in the MWX-3 area. In the area of MWX-1, 

sandstone splay deposits encroached into these environments. More laterally extensive, but still variable, multiple 

splay deposits occur in all three wells in unit E, whereas 

unit F contains lacustrine claystone in MWX-3, rippled 

splay sandstone in MWX-2, and what are interpreted to 

be the inclined siltstone foresets of a lacustrine delta in 

MWX-1. 

DELTA-PLAIN DEPOSITS 

Rocks of the coastal (6,000-6,600 ft, 1,830-2,010 m) 

and paludal (6,600-7 ,455 ft, 2,010-2,270 m) zones in 

MWX were deposited in upper and lower delta-plain environments, respectively. On a delta plain, numerous 

relatively straight to slightly sinuous distributary channels 

radiate outward from the main river channels to the shoreline across low-gradient coastal swamps and marshes. 

Several distributary channels serve one fluvial system, and 

the channels are small relative to the meander-belt channel that feeds them. The width to depth ratios of the 

channels are commonly less than those of meander-belt 

fluvial channels, and therefore channel depths in the 

coastal and paludal zones may be greater than those in 

the fluvial zone even though channel widths are less. The 

channels lack lateral erosive power, the gradient of the 

coastal plain is low, and the channels usually do not 

migrate or meander significantly. Coals form on the lower 

delta plain, and the better drained upper delta-plain 

(coastal) interval is differentiated from the lower deltaplain interval primarily by the absence of coals. 

In the subsurface, upper delta-plain deposits would 

be difficult to distinguish from the overlying fluvial 

deposits without closely spaced wells that highlight the 

narrower sand-body widths of the coastal distributary 

channels. The sedimentology of the core, however, provides some clues; delta-plain sandstones in the MWX wells 

contain fewer internal scour surfaces and more authigenic 

pyrite than do fluvial sandstones. 

The delta-plain rocks are poorly exposed in the outcrops at Rifle Gap, but several lenticular sandstones can 

be seen and the presence of coal is well documented within 

the lower strata (Collins, 1976). However, there are excellent outcrops of delta-plain rocks in Coal Canyon, 

north of Cameo on the western side of the basin. These 

outcrops confirm both the lenticular shape and limited 

widths of the channel sandstones of this environment, 

despite locally well developed lateral-accretion (point-bar) 

surfaces (fig. 8). Internally amorphous, crossbedded, lenticular sandstones are equally abundant and probably correspond to straighter reaches of the distributary channels. 

Geophysical logs from the closely spaced MWX wells also 

document the restricted width of sandstones in the deltaplain environment (fig. 9), and core shows an internal 

structure composed of crossbedding, ripple bedding, and 

soft-sediment deformation. 

Mudstone associated with sandstone reservoirs in 

the delta-plain interval in the MWX wells is highly 

Figure 8. Cross section (traced from photograph) of point-bar deposits in a 300-ft (90 m) wide, single-story lenticular sandstone in coal-bearing, lower delta-plain deposits of the Williams Fork Formation in Coal Canyon, west of Cameo on the 

southwestern edge of the Piceance basin. Heavy line indicates clay drapes; dashed line indicates approximate edge of 

sandstone unit. 

Reservoir Sedimentology of Mesaverde Group, Multiwell Site Kll 
MWX-1 MWX-2 

6200 

UJ 

z 

0 

N 

_J 

<( 

I-

(fJ 

<( 

0 

u 

6400 

6600 

IUJ 

UJ 

u_ 

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:i 

t 

UJ 6800 0 

UJ 

z 

0 

N 

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a.. 

K12 Geology of Tight Gas Reservoirs, Wyoming and Colorado 

MWX-3 

0 

en 

Cl 

u:: 

(") 

N' 

en 

Cl 

u:: 

Figure 9. Well-to-well gamma-ray log correlation of reservoir sandstones (pattern) in the 

delta-plain interval of the Williams Fork Formation in the MWX wells. Correlation between 

wells is poorer than for fluvial part of formation 

(fig. 3) because reservoir widths are smaller. 

Solid bar indicates cored interval; hachured pattern indicates coal beds. 
carbonaceous, and the amount of coalified plant debris 

increases downhole until it is locally concentrated into 

coal beds in the paludal (lower delta plain) zone. Many 

of the rooted sandstones and siltstones were deposited 

from floods that breached channel levees and inundated 

adjacent swamps as splays. Although the sediments 

originally were rippled, many of the structures have 

been destroyed by plant roots. Roots also grew into the 

mudstone, leaving it structureless or with a churned 

texture. 

Sandstone reservoirs are isolated within mudstones 

and coals because of the avulsive behavior of distributary 

channels. Internally, the reservoirs are heterogeneous. 

Clay partings, which formed during periods of slack 

water, act as incomplete vertical permeability barriers and 

affect both reservoir conductivity and fracture distribution. Abrupt changes in grain size and sedimentary structures also disrupt vertical permeability. 

The lower boundary of delta-plain sandstone reservoirs is most commonly an abrupt sandstone-mudstone 

contact at the channel-base scour surface. The upper 

boundary is marked by an abrupt decrease in grain size 

from sandstone to mudstone over a vertical distance of 

1-2 ft, a change commonly made indistinct by root 

penetration. 

The unfractured ductile mudstones and carbonaceous mudstones that are interbedded with paludal and 

coastal reservoir sandstones provide good reservoir seals. 

The mudstones are probably the source of the gas in the 

associated sandstones, but reservoir volumes are relatively 

small in this facies. 

The average delta-plain reservoir width was determined on the basis of probability and sandstone-bed correlations between two closely spaced wells, using the 

assumption that a greater percentage of the reservoirs will 

correlate between the wells if the reservoirs are wider. 

Subsurface widths of 370-520 ft (133-159 m) (fig. 2) for 

the paludal zone are similar to those measured on outcrop. Limited subsurface data from the coastal zone 

suggest that reservoir widths in this area are 120-175 ft 

(37-53 m), although outcrops suggest that reservoir 

widths in the coastal zone are, in fact, similar to those 

of the paludal zone (Lorenz, 1985b). The delta-plain 

sandstone bodies are lenticular in cross section and of 

variable orientation. 

Detailed observations of core from the delta-plain 

sandstones allow for a tentative reconstruction of orientation of specific reservoirs. Orientations were interpreted 

from spatial relationships of the three wells at depth and 

the specific subenvironments (main channel, channel 

margin, overbank) penetrated by each well. Crossbedding 

information obtained from high-resolution dipmeter 

measurements in MWX-3 and from oriented core supplemented the conclusions. 

Example of an Upper Delta-Plain Reservoir 

A typical upper delta-plain (coastal zone) reservoir 

is shown in figure 10. Two main sandstone bodies (units 

A and B) are separated by 3-4ft (1 m) of mudstone in 

MWX-1 and MWX-3 but interconnect in MWX-2, 

where unit A has scoured down into unit B. A third bed 

(unit C) is not obviously connected but is separated from 

unit B by only 3-5ft (1-2m) of mudstone in each well. 

In all three wells, unit A is a clean, crossbedded 

sandstone that is rippled near the top. Using outcrop 

thickness to width relationships (Lorenz, 1985b), its thickness in MWX-2 suggests a minimum width of 200 ft 

(61 m). Unit A thins in MWX-1 and MWX-3, and the 

channel center may lie between these two wells and 

MWX-2 and trend northwest. Limited data (crossbeds 

in oriented core and a dipmeter log in MWX-3) suggest 

west to northwest paleo flow, although this direction is 

away from the interpreted paleoshoreline. A slight angling 

of the axis of the bed toward the northwest allows all 

three wells to be encompassed within the 200-ft (61 m) 

minimum width. Unit A is interpreted to be a channel 

deposit superimposed locally on unit B. Units A and B 

are probably unrelated and in plane view separate beyond 

the MWX site. 

Unit B also is a channel sandstone, and its central 

part is penetrated by MWX-2 and MWX-3. Oriented 

crossbeds and dipmeter data indicate east to southeast 

paleo flow, the reverse of that for unit A. Overall channel trend is best estimated to be east to southeast, based 

on paleoflow vectors and the penetration of main channel deposits along the axis of MWX-2 and MWX-3. 

The thickness of unit B is about 25 ft (7 .6 m), as 

reconstructed by estimating the amount of the top of unit 

B removed by scour during superposition of unit A in 

MWX-2. This thickness yields a minimum width estimate 

of 300 ft (91 m). 

Unit C is a splay or channel-margin deposit that 

consists primarily of rippled sandstone and siltstone in 

all three wells. Dipmeter data from MWX-3 suggest 

northeast paleoflow from a source southwest of MWX-2. 

This direction may be corroborated by the slightly thicker 

deposits of unit C in MWX-2. 

The porosity and permeability of lenticular channel sandstones of the upper delta plain are typified by 

this composite reservoir (fig. 11). In general, both the 

porosity and permeability of the two main-channel units 

(A and B) decrease upward. The trends are not uniform 

but neither are they as erratic as those in the multistoried 

fluvial reservoirs discussed previously. Porosity and 

permeability decrease abruptly within the silty to muddy 

intervals near the top and base of the reservoir. 

Porosity and permeability of unit A are slighly less 

than those of unit B, even in MWX-2 where the two units 

are directly superimposed. The multistory nature of the 

Reservoir Sedimentology of Mesaverde Group, Multiwell Site K13 
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PLAN VIEW OF MWX WELLS 

AND LITHOLOGIC UNITS A, B, AND C 

AT DEPTH OF 6,300 FT 

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Figure 10. Detailed lithologic description of slabbed core and gamma-ray logs for a composite reservoir (pattern) in the upper delta-plain environment (coastal 

zone of Williams Fork Formation) in the MWX wells. The sandstone reservoir consists of two superimposed channel sandstones (units A and B) and an underlying 

thin splay deposit (unit C). Abbreviations: bk, black; carb, carbonaceous; clayst, claystone; lam, laminated; mass, massive; mudst, mudstone; sltst, siltstone; 

ss, sandstone; xbed, crossbedded. 
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Dry Klinkenberg Dry Klinkenberg Dry Klinkenberg Gamma-ray Porosity permeability at 1000 psi Gamma-ray Porosity permeability at 1000 psi Gamma-ray Porosity permeability at 1 000 psi log (in percent) (in microdarcys) log (in percent) (in microdarcys) log (in percent) (in microdarcys) 

6400 0 5 10 0.1 1.0 10.0 100.0 0 5 10 0.1 1.0 10.0 100.0 0 5 10 0.1 1.0 10.0 100.0 

I I I """I I I '""'I I ""'I I L I I II I I '''"'I I I """I I I ""'I I ........... I I II I '''""I I '"'"'I I iliiiii 

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t- 0 LLJ 

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0 0 0 0 0 0 0 0 0 0 

0 0 0 cg 0 0 0 0 0 0 

8 '6 8 0 

64751-~ 0 

08 0 0 

0 -

I I 1111111 I 11111111 

Figure 11. Porosity and permeability characteristics of upper delta-plain distributary-channel sandstone reservoirs in the Williams Fork Formation in the MWX 

wells. Reservoir quality is slightly less heterogeneous in these reservoirs than in the meander-belt reservoirs. Note logarithmic scale of permeability graphs. 
reservoir would be difficult to detect in MWX-2 without 

core or without the adjacent wells, both of which show 

the two distinct sandstones. Average porosity in units A 

and B is 6-7 percent. Average dry Klinkenberg permeability, measured at 3,000 lb/in. 2 (20. 7 MPa) net confining 

stress, is 3-5 microdarcys (A.R. Sattler, oral commun., 

1986). 

Examples of Lower Delta-Plain Reservoirs 

An example of a lenticular distributary-channel 

reservoir, similar to the upper delta-plain reservoirs, is 

shown by the lower sandstone unit (zone 3) in figure 12. 

Core from MWX-2 shows a sequence of crossbedded and 

rippled sandstones and a few mudstones. This sequence, 

as well as basal scour surfaces and clay-ripup conglomerates, suggests penetration by the well of a main channel. 

Core from this interval in MWX-3 shows a similar but 

thinner sandstone body that probably is also part of the 

main channel but closer to its edge. This interval was not 

cored in MWX-1, but the gamma-ray log indicates a sandy 

main-channel sequence. The channel probably trends eastnortheast, generally parallel with and centered on the line 

between MWX-1 and MWX-2 but encompassing 

MWX-3 at its edge. Based on its thickness of 28 ft (8.5 m), 

its minimum width should be about 350 ft (107 m). 

The upper sandstone shown in figure 12 (zone 4) 

is a different type of lower delta-plain reservoir. Core of 

this interval from MWX-3 consists almost entirely of 

rippled and carbonaceous sandstones, without the crossbedding indicative of main channels. This type of rippled 

sandstone occurs primarily in splays, the flood deposits 

adjacent to channels. The interval is split in MWX-2 by 

a coaly mudstone, and only the lower half of the interval was cored, but the core displays the thinly laminated 

and extensively burrowed deposits typical of the subaqueous distal edges of splays. The gamma-ray log from 

MWX-1 for the interval is similar in shape and thickness 

to that of MWX-3, and thus the interval is interpreted 

to be similar to the splay sandstones in MWX-3. The 

splay deposit thins rapidly in the direction of MWX-2, 

and the parent channel may be a few hundred feet to the 

north and east of the MWX wells. 

Porosity and permeability characteristics of the lenticular reservoirs of the lower delta plain (paludal zone) 

are shown in figure 13 and are similar to those of the upper delta plain (coastal zone). The primary control on 

these petrophysical parameters is the sedimentology of 

the deposits (fig. 12). Porosity and permeability are low 

in the basal and upper zones of the reservoirs, where interbedded sandstone, siltstone, and mudstone form a heterogeneous lithology. They are higher and more uniform 

in the homogeneous and coarser grained crossbedded 

sandstones of the main-channel environment. 

K16 Geology of Tight Gas Reservoirs, Wyoming and Colorado 

The reservoir characteristics of the splay deposits 

are significantly different and vary rapidly between the 

distal and mid-to proximal regions. Porosity in the distal 

regions of the typical splay (upper unit of MWX-2) (fig. 

13) is uniformly low, and permeability of the interlaminated sandstone-mudstone deposit is too low to be measured. In the more proximal splay deposits, however, 

porosity and permeability are of reservoir quality through 

most of the sandstone; porosity is about 8-10 percent and 

Klinkenberg permeability is about 7 microdarcys at 

2,000 lb/in.2 (13.7 MPa) net confining stress (A.R. 

Sattler, oral commun., 1986). Reservoir quality decreases 

at the base and top and in a middle zone, where siltstone 

and mudstone are interbedded with the reservoir sandstone. The reservoir quality of the upper half of the splay 

deposit in MWX-3 is slightly better than that of the lower 

half. The upper and lower halves of this reservoir may 

have been deposited during different splay events or even 

from different channels, and the boundary between the 

two is marked by an interval of fine-grained sediments 

and poor reservoir quality. The two splay events deposited 

slightly different kinds of sediment, resulting in a composite reservoir having locally different properties. 

SHORE Ll N E-MARI N E DE POSITS 

From the top of the Rollins Sandstone Member of 

the Iles Formation down to the Mancos Shale 

(7 ,455-8,350 ft, 2,270-2,545 m), the rocks of the 

Mesaverde Group in the MWX wells represent an irregularly cyclic series of environments, each cycle consisting (from the base up) of marine, shallow marine, 

shoreline, and paludal components (figs. 2, 14). The 

paludal zone previously described in the delta-plain section is actually the upper and considerably thickened component of the last of these cycles. The marine units are 

dark, fissile shales of the Mancos Shale, which were 

deposited offshore and below wave base in the interior 

seaway. 

Irregularly coarsening upward shallow-marine 

deposits of thin-bedded shale, siltstone, and fine-grained 

sandstone overlie the Mancos Shale. This upward transition into the shoreline sandstone reservoirs is not well 

developed in the MWX wells, possibly because the sand 

may have been provided by longshore drift rather than 

from nearby distributary channels. A large volume of 

these deposits may therefore represent the interdeltaic 

shoreface environments depicted in figure 1 rather than 

wave-dominated delta environments. Waves dispersed 

sand evenly along the shoreline, and progradation of the 

environments produced a blanket sandstone deposit. 

In some places the cycle is completed by an uppermost coal-bearing paludal deposit, similar to that described previously. The upper sandstone of the Corcoran 
<ll 

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PLAN VIEW 

PLAN VIEW 

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w 

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7100 

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7150 

MWX-1 MWX-2 MWX-3 

sltst,lam,burrows 

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ss,rippled 

coal parting 

sltstlmudst,lam, rippled 

ss,rippled 

sltstl ss,lam,nppled 

coa l 

sltstlmudst, deformed lam, burrows 

last,carb 

Figure 12. Detailed lithologic description of slabbed core and gamma-ray logs for a lenticular distributary-channel sandstone (zone 3) and a thick splay deposit (zone 

4), both from the lower delta-plain environment of the Williams Fork Formation in the MWX wells. (No core was available for MWX-1 .) Abbreviations: bk, black; 

brn, brown; carb, carbonaceous; cg, coarse grained; clayst, claystone; gy, gray; irreg, irregular; lam, laminations; mudst, mudstone; sltst, siltstone; ss, sandstone; xbed, 

crossbedded. 
~ 

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MWX-2 MWX-3 

Porosity Dry Klinkenberg permeability Porosity Dry Klinkenberg permeability 

Gamma-ray log (in percent) at 1000 psi (in microdarcys) Gamma-ray log (in percent) at 1000 psi (in microdarcys) 

0 5 10 15 0 1 1.0 10.0 100.0 0 5 10 15 0.1 1.0 10.0 100.0 

7o5o f _, I I I I I I iiiiij I I iiiiiij I I I ilillj I\ I I I I I I I I llliij I I ""I 

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z 0 0 

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00 

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0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 oo 0 00 00 

0 0 0 0 

0 0 

0 0 

71501- ~ Coal I -II- -1 1- ~ I 0 0 

0 0 

Figure 13. Porosity and permeability characteristics of a distributary-channel sandstone reservoir and a laterally variable splay sandstone reservoir in the lower 

delta-plain environment of the Williams Fork Formation in the MWX-1 and MWX-2 wells. Note logarithmic scale of permeability graphs. 
Sandstone Member (fig. 14) in the MWX wells is in fact 

a laterally discontinuous distributary channel within lower 

delta-plain deposits. In other places at MWX (above the 

lower Corcoran and the upper Cozzette Sandstone 

Members), the ensuing marine transgression deposited 

marine shale directly above the shoreline sandstone. 

Core of the Cozzette reservoir displays good 

shallow-marine and shoreline characteristics (fig. 15), including low-angle (possibly hummocky) crossbedding in 

well-sorted, fine-grained sandstone, and marine-type burrowing. Minor scour surfaces separate crossbed sets. The 

interval between the two sandstone units of the Cozzette 

Sandstone Member (fig. 15) consists of thin-bedded and 

burrowed shallow-marine shale, siltstone, and sandstone. 

In this facies at MWX, reservoirs are limited to the 

thick, sandy shoreline deposits. These deposits are laterally extensive blanket-type sandstones that, for the most 

part, change little over a scale of miles. At the scale of 

spacing of the MWX wells, they have no axial trend and 

do not intersect. At MWX, the reservoirs are 30-100 ft 

(9-30.5 m) thick and are relatively homogeneous internally. Within the cored reservoir intervals, porosity and 

permeability are almost constant (fig. 16) and much more 

uniform than in the nonmarine sandstones. This uniformity results from well-sorted, clean sandstones that were 

reworked by and deposited in a wave-dominated, highenergy shoreline environment. Reservoir quality decreases 

in the lower part of each sandstone because of the increased siltstone and shale content associated with deeper 

marine environments. The heterogeneous sandstonesiltstone-shale lithofacies between the two sandstone units 

of the Cozzette Sandstone Member displays an erratic 

distribution of porosity and permeability. 

Within the upper 20ft (7 .3m) of the upper sandstone unit of the Cozzette Sandstone Member (fig. 16), 

porosity and permeability abruptly decrease to a slightly 

lower but still uniform level. More than half of the pore 

space within the sandstone of this interval is occupied by 

pyrobitumen (Pitman and Spencer, 1984). Water saturation increases by a factor of 2 to 3 in this zone of 

''cooked-out'' oil, probably because of the retention of 

water by stronger capillary effects in the narrowed pores. 

This interval was not cored in MWX-1, but similar subtle changes in geophysical log response in MWX -1 and 

MWX-2 suggest that it is present there as well. 

The shoreline reservoirs have no obvious lateral 

boundaries, but the presence of gas in the Corcoran and 

Cozzette Sandstone Members despite nearby outcrops 

suggests the existence of internal permeability barriers. 

Features such as the large-scale, seaward-dipping bedding 

planes noted by Balsley (1982) in excellent outcrops of 

similar rocks in the Book Cliffs of Utah or the updip 

relative permeability traps as described by Masters ( 1979) 

may have trapped the gas. High unstimulated production 

rates from these reservoirs at MWX (about a million cubic 

feet per day) indicate the existence of an important 

natural fracture system (Lorenz and Finley, 1989). 

The Rollins Sandstone Member presents an enigma 

in that it apparently does not contain gas despite its proximity to both coal beds and marine shale. The apparent 

lack of gas would suggest that the Rollins, the thickest 

of the blanket sandstones, has permeability continuity 

with the outcrops that occur as close as 12 mi (19 km) 

to the MWX site. However, the salinity of formation 

waters in the Rollins Sandstone Member is nominally 

marine, and the trend of overpressuring of the Mesaverde 

sequence is not interrupted at the Rollins; therefore, the 

zone is not a free-flowing aquifer with hydrologic continuity to meteoric waters. It may be that the clay content of the Rollins causes an erroneous log response 

indicating water saturation and this zone may in fact contain gas. It was not tested. 

The interbedded mudstone and marine shale in this 

interval should provide good vertical permeability seals. 

Both the coals and the marine organic material within 

the marine shales are possible sources of gas, although 

measurements of geochemical parameters by D.D. Rice 

(U.S. Geological Survey, written commun., 1986) indicate 

that marine organic components are the principal source 

of gas in the Corcoran and Cozzette Sandstone Members 

at MWX. 

Reservoirs in the Mancos Shale 

Potential reservoir zones also may occur locally 

within the Mancos Shale, below the blanket-type 

shoreline-marine deposits. Offshore turbidite deposits 

derived from the Cretaceous shoreline during storm 

events are present elsewhere within the upper parts of the 

Mancos Shale (Balsley, 1982; Krystinik, 1983), and three 

types of offshore deposits were observed in the Mancos 

on the slopes of Mount Garfield, just east of Grand Junction on the western edge of the Piceance basin (fig. 17). 

These deposits previously have been either ignored, or 

briefly interpreted as remnants of recent landslide 

deposits (Lohman, 1965) or as remnants of a Pleistocene 

outwash terrace or pediment (Young and others, 1981). 

Unpublished microfossil data, however, indicate a Late 

Cretaceous age. 

The potential reservoir facies within the Mancos 

Shale on Mount Garfield are composed of widespread 

thin beds of rippled and burrowed sandstone layers (unit 

A, fig. 17), large slump blocks of Mancos Shale (unit 8, 

fig. 17), or less extensive lenses of millimeter- to metersized ripup clasts of Mancos Shale and larger blocks of 

sandstone in a silty mudstone matrix (unit C, fig. 17). 

These beds are slightly more resistant to erosion than the 

surrounding Mancos Shale and form a shelf that supports 

recent talus blocks of sandstone, which in turn further 

Reservoir Sedimentology of Mesaverde Group, Multiwell Site K19 
MWX-1 

7600 

7800 

fw 

w 

u. 

z 

I' 

fc.. 

w 

0 

8000 

8200 

TOTAL DEPTH 

K20 Geology of Tight Gas Re·servoirs, Wyoming and Colorado 

MWX-2 

Tongue of 

Mancos Shale 

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E 

Cll 

~ 

Cll 

c: 

B 

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c: 

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Mancos Shale 

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Mancos Shale 

MWX-3 

Figs. 15, 16 

Q; 

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c: 

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c: 

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a: 
Figure 14 (facing page). Well-to-wel! gamma-ray log correlation of sandstones (pattern) in shoreline-marine interval of the lies 

Formation of the Mesaverde Group in the MWX wells. Correlation of these widespread and uniform blanket-type sandstones 

is excellent, except for the upper sandstone unit of the Corcoran Sandstone Member, which is inferred to be a distributary channel. Solid bar indicates cored interval; hachures indicate coal beds; dashed line indicates tentative correlation . 

MWX-1 MWX-2 

7800 

7825 

7850 

burrows 

hammocky[? ) xbed 

7875 

1-

LlJ 

LlJ sltstl mud st. lam. burrows u.. 

z 7900 sltst.xbed 

sltstlmudst,lam 

:I 7900 

t 

LlJ 

0 

7925 

792 5 

mudst.burrows.bcm sltst 

7950 

7975 

8000 

Figure 15. Detailed lithologic description of slabbed core and gamma-ray logs for the Cozzette Sandstone Member 

of the lies Formation of the Mesaverde Group (pattern) in the MWX-1 and MWX-2 wells. Logs were correlated by 

using the top of the Cozzette Sandstone Member. Abbreviations: bern, becomes; bk, black; carb, carbonaceous; fg, 

fine grained; gy, gray; lam, laminated; mass, massive; mudst, mudstone; sltst, siltstone; ss, sandstone; vfg, very fine 

grained; xbed, crossbedded. 

increase the local erosional resistance. The beds are interpreted to represent submarine slump-block (unit B), 

submarine debris-flow (unit C), and distal turbidite environments (unit A) (Lorenz and Muhr, 1987). 

Turbidite deposits have the greatest potential as 

reservoirs if their frequency, porosity, and permeability 

are sufficient. The "Mancos B" productive interval of 

the Mancos Shale in the subsurface in the northwestern 

part of the basin may consist of similar deposits. Turbidite deposits have not been encountered in the MWX 

wells because little of the Mancos Shale was penetrated. 

Both the debris-flow and slump-block deposits have a 

Reservoir Sedimentology of Mesaverde Group, Multiwell Site K21 
IUJ 

UJ 

LL 

z 

::i 

Ia... 

UJ 

0 

MWX-1 

Dry Klinkenberg 

Gamma-ray Porosity permeability at 1 000 psi 

log (in percent) (in microdarcys) 

7800 .... -..,.--o ..... -5 ..... --.1o 0.1 1.0 10 .0 100.0 

7825 

7850 

7875 

7900 

7925 

7950 

7975 

(i) 

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E 

Q) 

...... ~ 0 Q) 

...... c ·c: 0 :Jt) 

..... -o Q) c ~ ro 0(/) 

_J~ 

Q) 

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N 

8 

0) 

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0 % 0 

<o 

0 

0 

0 

0 

IQ 

0 

0 00 

& 

0 

0 0 

Gamma-ray 

log 

MWX-2 

Dry Klinkenberg 

Porosity permeability at 1000 psi 

(in percent) (in microdarcys) 

5 1 0 15 0 1 0 1 10 0 1 00 0 

.... --~-~-~-~~ ""I "'I "" 

0 - 0 

0 

0 

0 0 0 

0 0 0 

0 

0 0 0 0 

oeo 0 0 - '2J&o 8 

~ 0 

0 

~0 0 

0 

e 0 

~0 0 

0 - 0 

0 

~ 0 

c9 0 0 0 

0 

-

-

-

-

.... 1 .... 1 

Figure 16. Porosity and permeability characteristics of blanket-marine sandstones (lower and upper sandstone units of 

Cozzette Sandstone Member of lies Formation) in the MWX-1 and MWX-2 wells. Reservoir quality is relatively homogeneous 

as compared to that in overlying nonmarine sandstones. Note logarithmic scale of permeability graphs. 

muddy matrix, and although in outcrop they are more 

fractured than the adjacent Mancos Shale, their reservoir 

potential may not be significantly greater than that of the 

shale. 

SUMMARY 

Three types of gas-bearing sandstone bodies were 

encountered in the MWX wells. Their reservoir morphologies are determined by the sedimentary environments 

K22 Geology of Tight Gas Reservoirs, Wyoming and Colorado 

in which they were deposited. In the fluvial environment, 

highly irregular tabular-elongate reservoirs formed by 

amalgamation of point-bar deposits of meandering rivers. 

In the delta-plain environment, distributary channels 

deposited lenticular reservoirs that radiated out across the 

coastal plain. Near the shoreline, these channels traversed 

paludal environments and are interbedded with coalswamp deposits. Splay deposits are also common in this 

environment. 

The lowermost reservoirs at MWX were deposited 

during the initial regressions of the Cretaceous Interior 
~---:> 

/// 

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(/ ~ !\teo ··• · . . • •• lt?,.c . ·> . •U,01 ;(jL, ••• • <-lf:'' G~ ·• ... ..... '~' .. .... '1: 

Garfield 

Grand 

Junction 

/ 

/ / / 

/ / / 

5 MILES 

/ 

/ 

/ 

/ 

/ / / / / / 

/ 

/ 

/ / 

Mancos Shale 

Modern talus 

Submarine 

slump blocks 

Mancos Shale 

Laminated, 

burrowed 

sandstone 

Covered 

interval 

Mancos Shale 

c 

Mancos Shale 

Modern talus 

Submarine 

debris-flow 

mudstone 

Mancos Shale 

Figure 17. Offshore-marine deposits in the Mancos Shale along the Book Cliffs, east of Grand Junction, Colo. Inferred environments of deposition: A, submarine slump block; B, submarine debris flow; C, distal turbidite. 

Seaway as shallow-marine and shoreline blanket sandstones in wave-dominated delta and coastline environments. Hydrocarbon traps in these reservoirs probably 

are internal permeability barriers associated with delta 

abandonment and reactivation. 

Other potential Cretaceous reservoir facies in the 

Piceance basin include local turbidite sandstones in the 

Mancos Shale and flood-deposited sandstones in the nonmarine parts of the Mesaverde Group. The uppermost 

unit of the Mesaverde Group, the Ohio Creek Member, 

may also be productive locally. 

REFERENCES CITED 

Balsley, J .K., 1982, Cretaceous wave-dominated delta systems; 

Book Cliffs, east-central Utah: American Association of 

Petroleum Geologists, 219 p. 

Collins, B.A., 1976, Coal deposits of the Carbondale, Grand 

Hogback, and southern Danforth Hills coal fields, eastern 

Piceance Basin, Colorado: Colorado School of Mines 

Quarterly, v. 71, no. 1, 138 p. 

Finley, S.J., and Lorenz, J.C., 1989, Characterization of natural 

fractures in Mesaverde reservoirs at the Multiwell Experiment site: Society of Petroleum Engineers Joint Rocky 

Mountain Regional Meeting and Low-Permeability Reservoir Symposium, Denver, Colo., 1989, SPE Proceedings 

19007' p. 721-728. 

Fisher, W.L., and McGowen, J.H., 1969, Depositional systems 

in the Wilcox Group (Eocene) of Texas and their relationship to occurrence of oil and gas: American Association of Petroleum Geologists Bulletin, v. 53, no. 1, 

p . 30-54. 

Jones, J.R., Jr., 1984, Reservoir characterization of Mesaverde 

(Campanian) bedload fluvial meanderbelt sandstones, 

northwestern Colorado [abs.]: American Association of 

Petroleum Geologists Bulletin, v. 68, no. 4, p. 493. 

Reservoir Sedimentology of Mesaverde Group, Multiwell Site K23 
Jones, J.R., Jr., Scott, A.J., and Lake, L.W., 1984, Reservoir 

characterization for numerical simulation of Mesaverde 

meander belt sandstone, northwestern Colorado: Society of Petroleum Engineers Annual Technical Conference 

and Exhibition, 59th, Houston, Tex., 1984, SPE Proceedings 13502, 14 p. 

Krystinik, L.F., 1983, Turbidite fans in Upper Cretaceous Pierre 

Shale, Eagle Basin, Colorado; a new reservoir facies 

[abs.]: American Association of Petroleum Geologists 

Bulletin, v. 67, no. 3, p. 498. 

Lohman, S.W., 1965, Geology and artesian water supply of the 

Grand Junction area, Colorado: U.S. Geological Survey 

Professional Paper 451, 149 p. 

Lorenz, J.C., 1985a, Small-scale lateral variation in non-channel 

fluvial deposits, Mesaverde Formation, Colorado: International Fluvial Sedimentology Conference, 3rd, Fort Collins, Colo., 1985, Abstracts Volume, p. 26. 

___ 1985b, Predictions of size and orientations of lenticular 

reservoirs in the Mesaverde Group, northwestern Colorado: Society of Petroleum Engineers/Department of 

Energy Joint Symposium on Low Permeability Reservoirs, 

Denver, Colo., 1985, Proceedings, p. 23-31. 

Lorenz, J.C., Branagan, P., Warpinski, N.R., and Sattler, 

A.R., 1989, Fracture characteristics and reservoir behavior 

of stress-sensitive fracture systems in flat-lying lenticular 

formations: Journal of Petroleum Engineering, v. 41, no. 

6, p. 615-622. 

Lorenz, J.C., and Finley, S.J., 1989, Differences in fracture 

characteristics and related production of natural gas in 

different zones of the Mesaverde Formation, northwestern Colorado: Society of Petroleum Engineers Formation 

Evaluation, v. 4, p. 11-16. 

Lorenz, J.C., Heinze, D.M., Clark, J.A. and Searls, C.A., 

1985, Determination of widths of meander-belt sandstone 

reservoirs from vertical downhole data, Mesaverde Group, 

Piceance Creek basin, Colorado: American Association 

of Petroleum Geologists Bulletin, v. 69, no. 5, p. 710-721. 

Lorenz, J.C., and Muhr, C.A., 1987, Submarine debris-flow, 

slump-block, and turbidite deposits in Mancos Shale 

(Cretaceous) of northwestern Colorado [abs.]: American 

Association of Petroleum Geologists Bulletin, v. 71, no. 

5, p. 585. 

Lorenz, J.C., and Rutledge, A.K., 1985a, Facies relationships, 

reservoir potential of Ohio Creek interval across the 

K24 Geology of Tight Gas Reservoirs, Wyoming and Colorado 

Piceance Creek basin: Oil and Gas Journal, v. 83, 

p. 91-96. 

___ 1985b, Facies relationships and reservoir potential of the 

Ohio Creek interval across the Piceance Creek basin, 

northwestern Colorado: Sandia National Laboratories 

Report SAND84-2610, 52 p. 

___ 1987, Description of the Late Cretaceous Mesaverde 

Group outcrops at Rifle Gap, Piceance Creek basin, 

northwestern Colorado: Geological Society of America 

Centennial Field Guide 2, p. 307-310. 

Lorenz, J.C., Sattler, A.R., and Stein, C.L., 1989, Differences 

in reservoir characteristics of marine and nonmarine sandstones of the Mesaverede Group, northwestern Colorado: 

Sandia National Laboratories Report SAND 88-1963. 

Lorenz, J.C., and Smock, K., 1986, 3-D Characterization of 

fractures in Mesaverde reservoirs: or why the sugar cube 

reservoir model doesn't always work [abs.]: American 

Association of Petroleum Geologists Bulletin, v. 70, no. 

5, p. 613. 

Lorenz, J.C., Warpinski, N.R., Teufel, L.W., Branagan, P.T., 

Sattler, A.R., and Northrop, D.A., 1988, In situ stresses, 

natural fractures, and other geologic controls on reservoirs: EOS, v. 69, p. 817, 825-826. 

Masters, J.A., 1979, Deep basin gas trap, western Canada: 

American Association of Petroleum Geologists Bulletin, 

v. 63, p. 152-181. 

Pitman, J.K., and Spencer, C.W., 1984, Petrology of selected 

sandstones in the MWX wells (northwest Colorado) and 

its relationship to borehole geophysical-log analysis and 

reservoir quality, in Spencer, C.W., and Keighin, C.W., 

eds., Geologic studies in support of the U.S. Department 

of Energy Multi well Experiment, Garfield County, Colorado: U.S. Geological Survey Open-File Report 84-757, 

p. 33-66. 

Scott, A.J., Hoover, R.A., and McGowen, J.H., 1969, Effects 

of Hurricane "Beulah," 1967, on Texas coastal lagoons 

and barriers: UNAM-UNESCO Symposium, 1969, 

Lagunas Costeras, Mexico, p. 221-226. 

Young, R.G., Keighin, C.W., and Campbell, J.A., 1981, 

Second day; road log from Grand Junction to Glenwood 

Canyon and return to Grand Junction, in Epis, R.C., and 

Callender, J.P., eds., Western Slope, Colorado, western 

Colorado and eastern Utah: New Mexico Geological 

Society Field Conference, 32nd, Guidebook, p. 17-28. 
Chapter L 

Thermal History of Selected Coal Beds in the 

Upper Cretaceous Mesaverde Group and 

Tertiary Wasatch Formation, 

Multiwell Experiment Site, Colorado, 

In Relation to Hydrocarbon Generation 

By VITO F. NUCCIO and RONALD C. JOHNSON 

Prepared in cooperation with the U.S. Department of Energy 

U.S. GEOLOGICAL SURVEY BULLETIN 1886 

GEOLOGY OF TIGHT GAS RESERVOIRS IN THE PINEDALE ANTICLINE AREA, WYOMING, 

AND AT THE MULTIWELL EXPERIMENT SITE, COLORADO 
CONTENTS 

Abstract Ll 

Introduction Ll 

Vitrinite reflectance profile for the MWX site L2-

Present-day temperatures at MWX L2 

Burial reconstruction of the Mesaverde Group at the MWX site L2 

Thermal histories for the six coals L4 

TTl modeling LS 

Source-rock potential and hydrocarbon-generation thresholds LS 

Results and discussion LS 

References cited L 7 

FIGURES 

1. Vitrinite reflectance profile for MWX site L2 

2-3. Temperature logs for MWX-1 well, as measured by: 

2. Los Alamos National Laboratories L3 

3. Southern Methodist University L3 

4. Diagram showing burial history for coals from Mesaverde Group and 

Wasatch Formation L4 

TABLES 

1-3. Results of TTl modeling at MWX site using: 

1. Maximum bottom-hole temperature of 244 Of' (118.7 OC) for coals 

from Mesaverde Group and Wasatch Formation L6 

2. Maximum bottom-hole temperature of 266 Of' (131 OC) for coals from 

Mesaverde Group and Wasatch Formation L6 

3. Geothermal gradient that best fits both predicted TTl values and 

measured Rm values for coals from Mesaverde Group and Wasatch 

Formation L6 
Thermal History of Selected Coal Beds in the 

Upper Cretaceous Mesaverde Group and 

Tertiary Wasatch Formation, 

Multiwell Experiment Site, Colorado, 

In Relation to Hydrocarbon Generation 

By Vito F. Nuccio and Ronald C. Johnson 

Abstract 

The burial and thermal histories of six coal beds in the 

Upper Cretaceous Mesaverde Group and Tertiary Wasatch Formation at the Multiwell Experiment (MWX) site in northwest 

Colorado were reconstructed and the timing of certain 

hydrocarbon-generation thresholds estimated by using timetemperature index (Til) modeling. The thermal history 

reconstruction assumes that paleogeothermal gradients are the 

same as the present-day geothermal gradient. Two downhole 

temperature surveys were run at the site; however, these logs 

recorded somewhat different geothermal gradients and maximum bottom-hole temperatures. These differences probably 

result from different operators and instruments, not differences 

in equilibration times, because the surveys were conducted at 

approximately the same time. Til values were calculated for 

each of the two surveys and vitrinite reflectance (Rm) values 

predicted by using Til models were compared with measured 

values for each coal. Predicted vitrinite reflectance values for 

both temperature surveys are systematically higher than measured vitrinite reflectance values for the Mesaverde coals but 

lower than the measured value for the Wasatch coal, and we 

present a model that best fits a geothermal gradient to our 

measured vitrinite reflectance values. 

Three hydrocarbon-generation thresholds were used: the 

onset of oil generation (Til 10, 0.60 percent Rm), the begining of gas generation (Til 25, 0.73 percent Rm), and the point 

at which oil becomes unstable and breaks down to methane 

gas (Til 180, 1.35 percent Rm). Each of the geothermal gradients yields different timing for the thresholds, yet all timings 

fall within a fairly narrow range. The stratigraphically shallowest 

coal achieved an Rm of 0.60 percent about 10 Ma and the 

stratigraphically deepest coal achieved an Rm of 0.60 percent 

between 51 and 49 Ma, an Rm of 0.73 percent between 49 

and 46 Ma, and an Rm of 1.35 percent between 45 and 37 Ma. 

Overestimation of Rm values for the Mesaverde Group 

coals and underestimation for the Wasatch coal predicted by 

using the Til model may result from: (1) operator and laboratory bias in measuring vitrinite reflectance, (2) errors in 

reconstruction of burial history, (3) a recent increase in the heat 

flow of the area, (4) "kinks" in the Rm curve resulting from 

heat transfer processes, or (5) problems in comparing TTl values 

with measured Rm values. 

INTRODUCTION 

Reconstructions of the burial and thermal histories 

of the area around the Multi well Experiment (MWX) site 

in northwest Colorado (Law and Johnson, this volume, 

fig. 6) have recently been made (Bostick and Freeman, 

1984; Johnson and Nuccio, 1984, 1986; Nuccio and 

Johnson, 1984), but, as yet, the petroleum generation 

history at the site has not been reconstructed. We present results of mean random vitrinite reflectance 

measurements (Rm) for six coals from the Upper Cretaceous Mesaverde Group and Tertiary Wasatch Formation at the MWX site, reconstruct their burial history, 

and use time-temperature index modeling (TTl) of the 

selected coals to reconstruct the petroleum generation 

history of Upper Cretaceous and Tertiary rocks at the 

MWX site. 

Vitrinite reflectance measures the maximum level 

of thermal maturation of a rock, and TTl modeling 

(Waples, 1980) aids in estimating the timing of thermal 

maturation and hence the timing of hydrocarbon generation in the past. Geothermal gradients calculated from 

two different temperature logs measured at the MWX site 

were used in the TTl modeling, and vitrinite reflectance 

values predicted by using these two thermal gradients are 

compared with those measured on samples from the 

MWX wells. Using present-day geothermal gradients, 

TTl reconstructions significantly overestimate vitrinite 

reflectance values for the Mesaverde coals and underestimate the value for the Wasatch coal, so a geothermal 

gradient that best fits predicted values with our measured 

values is presented for each coal. 

Thermal History of Mesaverde Group and Wasatch Formation, Multiwell Site L 1 
VITRINITE REFLECTANCE PROFILE 

FOR THE MWX SITE 

Vitrinite reflectance data for the MWX site is available from Amoco Production Company (unpublished 

data), Bostick and Freeman (1984), and our measurements. We measured vitrinite reflectance on coal samples from core from the MWX-1 and MWX-2 wells, on 

shale cuttings from the Wasatch Formation in the 

MWX-3 well, and on coal from a Wasatch surface outcrop sample. 

The coal was crushed and mounted in epoxy, then 

planed off and polished. Composite samples of shale required maceration, a technique used to extract the organic 

matter from a rock sample. The reflectance of randomly 

oriented vitrinite grains was measured under oil immersion by using reflected-light microscopy, and mean 

reflectance values were calculated from large sample 

populations (as many as 135 selected vitrinite grains) for 

several of the samples. The vitrinite reflectance profile 

(fig. 1) shows that values from all three sources agree fairly well, although our values are generally slightly lower 

than those of both Bostick and Freeman (1984) and 

Amoco (unpublished data), and those of Bostick and 

Freeman are generally lower than those of Amoco. These 

differences probably result from laboratory and operator 

bias. The Amoco data show some scatter in the interval 

between about 5,400 and 6,200 ft (1,646-1,890 m). In this 

interval, coal is scarce and the reflectance of vitrinite from 

carbonaceous shale was measured. Individual shale 

samples commonly have a greater range of vitrinite values 

than do coals, and the data show more scatter in profile. 

In this paper, we use Rm values; they range from 

0.60 percent at the surface to 1.94 percent at 7,950 ft 

(2,423 m). A change in slope in the vitrinite reflectance 

profile occurs at about 4,500 ft. This break is coincident 

with, but not necessarily a result of, the CretaceousTertiary boundary and will be discussed later. 

PRESENT-DAY TEMPERATURES AT MWX 

Two temperature logs were measured at the 

MWX-1 well about 7 months after circulation of drilling fluids ended. In the first log (fig. 2), measured by Los 

Alamos National Laboratory, a maximum bottom-hole 

temperature of 244 "F (118 °C), corresponding to a 

geothermal gradient of 2.37 "F/100 ft (43.5 OC/km), was 

measured. In the second log (fig. 3), measured by 

Southern Methodist University, a maximum bottom-hole 

temperature of 266 "F (131 °C), corresponding to a 

geothermal gradient of 2.63 "F/100 ft (48.4 OC/km), was 

recorded. By comparison, bottom-hole temperatures of 

200 "F (94.1 °C) were recorded during some of the 

original logging runs shortly after circulation ended. 

L2 Geology of Tight Gas Reservoirs, Wyoming and Colorado 

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7000 0 Shale -- e Coal ---.,--- ---·\--t X Bostick and Free· Cameo-Fairflfld 2500 man (1984) coal zoQe 

8000 ex+. + Amoco Production 

(unpublished data) Upper Cretaceous 

Mancos Shale 

9000~----L---~~----~--~~~~ 

0.4 0.6 0.8 1.0 1.5 2.0 3.0 

VITRINITE REFLECTANCE, IN PERCENT 

Figure 1. Vitrinite reflectance profile for the MWX site. 

Both temperature logs show a distinct change in 

slope (increase in thermal gradient) through the CameoFairfield coal zone of the Williams Fork Formation of 

the Mesaverde Group between depths of 7,040 and 

7,400 ft (2, 146-2,256 m) that corresponds to differences 

in thermal conductivity between carbonaceous shale and 

coal and the enclosing sandstone and shale. The logs were 

measured at about the same time, and variations resulting 

from equilibration are not a factor. Much of the variation between the two logs results from different operators 

and equipment. 

BURIAL RECONSTRUCTION OF THE 

MESAVERDE GROUP AT THE MWX SITE 

A detailed burial history of the MWX site was 

recently presented by Johnson and Nuccio (1986). The 

MWX site is in the Colorado River canyon, and much 

of the evidence for the burial history of the MWX site 

can be determined by studying exposures on the nearby 
TEMPERATURE, IN oc 

0 

0 0 

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TEMPERATURE, IN a F 

Figure 2. Temperature log for the MWX-1 well, as 

measured by Los Alamos National Laboratories (E. H. 

Horton, written commun. to N.H. Bostick, U. S. 

Geological Survey, 1982). Note maximum bottomhole temperature Of 244 Of (118.7 °C) and kink in 

gradient through Cameo-Fairfield coal zone. 

canyon walls. The burial history of the six coals in this 

study is divided into seven time intervals (fig. 4): 

74-66 Ma, 66-60 Ma, 60-50 Ma, 50-46 Ma, 46-35 Ma, 

35-10 Ma, and 10 Ma to present. 

From 7 4 to 66 Ma (late Campanian through 

Maestrichtian time), the MWX area was subsiding and 

the Upper Cretaceous Mesaverde Group was deposited. 

The rate of subsidence can be estimated by dividing the 

thickness of Mesaverde rocks by the approximate length 

of time of deposition. Between 66 and 60 Ma, a hiatus 

(represented by an unconformity) occurred at the top of 

the Mesaverde. Although erosion was significant in other 

parts of the Piceance basin, it probably was not significant near the structural trough of the basin where the 

MWX site is located, and the reconstruction presented 

in this paper does not include any erosion at MWX during this time period. Subsidence resumed during the 

period of 60 to 50 Ma, and about 5,225 ft (1 ,593 m) of 

sediment was deposited on the unconformable surface. 

The end of this time period is represented by the Long 

Point bed of the Eocene Green River Formation. From 

50 to 46 Ma, an additional2,200 ft (670 m) of lacustrine 

Green River Formation was deposited. The end of this 

interval is represented by the top of the Mahogany oilshale ledge of the Green River Formation. The next time 

interval spans the period from 46 Ma (Eocene) to the 

end of basin subsidence at about 35 Ma (Oligocene). 

TEMPERATURE, IN o C 

1000 

500 

2000 

3000 

1000 

4000 

(j) 

cc I- LlJ LU I- LU LU u.. ~ z 

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5000 I 

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2000 

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2500 u.. 

6 

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3000 L-L.--1-..L...-...1---L----1 

0 150 300 

TEMPERATURE, IN oF 

Figure 3. Temperature log for the MWX-1 well, as 

measured by Southern Methodist University (D.O. 

Blackwell, written commun. to A.R. Sattler, Los 

Alamos National Laboratories, 1982). Note maximum 

bottom-hole temperature of 266 oF (131 °C) and kink 

in gradient through Cameo-Fairfield coal zone. 

Thermal History of Mesaverde Group and Wasatch Formation, Multiwell Site L3 
TIME, IN MILLION OF YEARS 

080 70 60 50 40 30 20 10 

2000 

4000 1-

w 

w 

u.. 6000 ~ 

:i 

b: 8000 

w 

0 

10,000 

12,000 Coal 5 1.79 

PERCENT Rm 

0 

0 

1000 

2000 

3000 

4000 

(/) 

a: 

w 

1-

w 

~ 

~ 

:i 

b: 

w 

0 

Figure 4. Burial history for coals from the Mesaverde Group 

(samples 2-6, tables 1-3) and Wasatch Formation (sample 

1, tables 1-3). Rm is measured vitrinite reflectance (in 

percent). 

Approximately 1,075 ft (328 m) of upper Eocene Green 

River Formation and Uinta Formation is preserved above 

the Mahogany ledge and beneath an erosion surface that 

is capped by a ledge of 9. 7 ± 0. 5-million-year-old basalts 

(Marvin and others, 1966). During the latest Eocene and 

early Oligocene, widespread erosion occurred across 

much of the Rocky Mountain region (Epis and Chapin, 

1975; Larson and others, 1975; Tweto, 1975; Hansen, 

1984). The erosion surface beneath the basalts is thought 

to be part of this widespread complex of late Eocene-early 

Oligocene erosion surfaces (Larson and others, 1975; 

Johnson and Nuccio, 1986). If this is the case, then little 

sedimentation or erosion occurred in the MWX area between about 35 Ma, when the surface developed, and 

9.7 Ma, when the basalts were deposited. The thickness 

of upper Eocene or possibly lower Oligocene rocks eroded 

during the development of this erosion surface is not 

known. Johnson and Nuccio (1986) suggested that during this time period erosion in the Piceance basin decreased from the basin margins inward and essentially no 

rock was removed near the axis of the basin where the 

MWX site is located. Regional uplift and erosion began 

about 10 Ma and has continued until the present. Approximately 4,600 ft (1 ,402 m) of section has been removed, and our reconstruction assumes that erosion has 

occurred at a constant rate. 

THERMAL HISTORIES FOR THE SIX COALS 

Present-day geothermal gradients were used to 

reconstruct the thermal history of coals from six stratigraphic levels in the Mesaverde Group and Wasatch Formation. The coals were collected both at the surface and 

to a depth of 4,050 ft (1 ,234 m) below the top of the 

L4 Geology of Tight Gas Reservoirs, Wyoming and Colorado 

Mesasverde. The thermal histories of the coals were used 

to predict both present-day ranks of the coals and timing of hydrocarbon generation by using the model of 

Waples (1980). For our calculations, we assume that heat 

flow has been constant through time and that the low 

thermal conductivity in the Cameo-Fairfield coal zone, 

as indicated by temperature logs (figs. 2, 3), is the same 

as that in the past. Separate geothermal gradients were 

calculated for the two temperature logs measured at 

MWX-1. We assume that the thermal gradient for the 

4,600 ft (1 ,402 m) of section eroded from the MWX site 

during the last 10 million years is the same as that 

measured through the 3,900 ft (1,189 m) of Wasatch Formation that overlies the Mesaverde Group at the MWX 

site. The thermal gradient through this interval is 2.12 

"F/100 ft (38.6 °C/km) in the LANL survey (fig. 2) and 

2.22 "F/100 ft (40.6 °C/km) in the SMU survey (fig. 3). 

Only about 1,325 ft (404 m) of the eroded section consisted of Wasatch Formation; the remaining 3,275 ft 

(998 m) consisted of about 2,700 ft (823 m) of Green 

River Formation and 575 ft (175 m) of Uinta Formation. 

The lithology of the lower 2,150 ft (655 m) of Green River 

Formation and the 575 ft (175 m) of Uinta Formation 

is generally similar to that of the Wasatch Formation and 

consists of interbedded sandstone, siltstone, claystone, 

and madstone. The thermal conductivities of these intervals probably are similar to those of the underlying 

Wasatch Formation. The upper 550 ft (168 m) of the 

Green River Formatiorl, however, consists mostly of oil 

shale, the thermal conductivity of which varies widely 

with different organic matter contents (DuBow and 

others, 1977) but is significantly lower than that of the 

typical mixed sandstone, siltstone, and shale section. If 

thermal gradients in the oil shale section are significantly higher than those in the underlying section, then 

temperatures in the Mesaverde Group at the MWX site 

at maximum burial may be underestimated by 10 "F ( 6 OC) 

or more. 

The computer program used to calculate the TTl 

index of Waples could not accommodate variations in 

geothermal gradient with depth, such as those observed 

through the Cameo-Fairfield coal zone. This did not affect our calculations of TTl for coals above the CameoFairfield coal zone because the geothermal gradient is 

relatively constant from the coal zone to the surface, but 

it did slightly affect TTl calculations for coals within the 

coal zone and below. If a constant gradient is assumed 

for the entire section, then the TTl value for the period 

of maximum burial between 35 and 10 Ma is slightly 

higher than actual TTl values obtained for those intervals when the basin was subsiding or being uplifted. The 

period of maximum burial and temperature between 35 

and 10 Ma is by far the most significant number in the 

total TTl calculation for the coals, and the error induced 

by assuming a constant gradient is not significant. 
TTl MODELING 

The thermal history of the MWX site was reconstructed by using a slightly modified version (Waples, 

1980) of Lopatin's time-temperature index (TTl) 

(Lopatin, 1971). We propose that the coals reached their 

maximum level of thermal maturity at the end of maximum burial, about 10 Ma. We calculated TTl values only 

to 10 Ma because after this time, temperatures began to 

drop as a result of uplift and erosion. The TTl model 

is one method of predicting when formations such as the 

Mesaverde and Wasatch entered or passed through oil 

and gas generation limits. In the TTl model, time and 

temperature are interchangeable and given sufficient time 

a relatively low temperature can produce a high level of 

maturity. No single temperature, therefore, can be assigned to a vitrinite reflectance value. It should be noted that 

a second time-independent model postulates that time 

does not have a significant effect on thermal maturity and 

that relatively soon (geologically speaking) after maximum temperature is attained organic matter stabilizes 

and reactions cease (Neruchev and Parparova, 1972; 

Barker, 1983; Price, 1983). If this second model is correct, then indicators of thermal maturity such as vitrinite 

reflectance can be used as absolute paleogeothermometers. Nuccio and Johnson (1984) suggested that at 

the MWX site inconsistencies exist with respect to both 

models because of problems in determining paleogeothermal gradients, but Bostick and Freeman (1984) concluded 

that the time-dependent model correlates more closely 

with vitrinite reflectance data. It is beyond the scope of 

this paper to debate the two models, and we used the TTl 

model and geothermal gradients calculated from 

temperature logs to estimate the timing of important thermal events at the MWX site. 

SOURCE-ROCK POTENTIAL AND 

HYDROCARBON-GENERATION THRESHOLDS 

In general, organic matter can be divided into three 

types: type I, sapropelic or fatty; type III, humic or coaly; 

and type II, intermediate between types I and III. 

Sapropelic organic matter is hydrogen rich; it occurs principally in rocks of marine origin and generates oil with 

some gas during thermal maturation. Humic organic matter is hydrogen poor and oxygen rich; it occurs mostly 

in nonmarine rocks and generates methane gas during 

maturation. Type II organic matter can be a source for 

both oil and gas (Tissot and others, 1974). 

At the MWX site, the Mesaverde Group contains 

a basal marginal-marine section but is primarily a fluvial 

and deltaic section. The marginal-marine section, or lies 

Formation of the Mesaverde Group, intertongues with 

the marine Mancos Shale and most likely contains a 

mixture of types II and III organic matter. A ''burnedout'' oil zone identified in the Cozzette Member of the 

lies Formation at MWX (C.W. Spencer, written commun., 1982) indicates the presence of some oil-prone 

source rocks. Hydrogen/carbon ratios determined by 

Rock-Eval pyrolysis confirm the presence of type II 

organic matter in some of the coals and carbonaceous 

shales in the lies Formation. Multiple thick (as much as 

10 ft) coal beds and associated carbonaceous shales in 

the Cameo-Fairfield coal zone in the lower part of the 

fluvial Mesaverde Group or Williams Fork Formation of 

the Mesaverde Group make this part of the Mesaverde 

an excellent source for gas. 

The overlying Wasatch Formation consists mostly 

of organically lean, varicolored shales, and lenticular 

sandstones and a few thin carbonaceous shale and coal 

intervals. A thin, coaly interval in the Wasatch Formation exposed on fresh outcrops near the MWX site was 

sampled and analyzed for our study. Most of the organic 

matter in the Wasatch Formation is of type III origin, 

as determined petrographically. 

Three important hydrocarbon-generation thresholds were recognized at the MWX site: TTl 10, 0.60 percent Rm, the point at which oil begins to be generated 

(Waples, 1980); TTl 25, 0.73 percent Rm, the approximate point at which methane gas begins to be generated 

(Juntgen and Karweil, 1966); and TTl 180, 1.35 percent 

Rm, the point at which liquid hydrocarbons break down 

to gas and condensate and the upper limit of the oil window (Waples, 1980). Methane gas is stable at relatively 

high temperatures, corresponding to 4.8 percent Rm and 

TTl 120,000-130,000 (Waples, 1980), a much higher level 

of maturation than at the MWX site. 

RESULTS AND DISCUSSION 

The results of TTl modeling for the six coals are 

shown in tables 1-3. Tables 1 and 2 show the two geothermal gradients calculated from the two temperature logs 

measured at MWX-1 and corresponding TTl and Rm 

values calculated for the end of maximum burial (10 Ma), 

the temperature at maximum burial, and the timing of 

hydrocarbon-generation events. Table 3 shows the geothermal gradient that best fits measured vitrinite reflectance values for the MWX site as well as the TTl value 

at maximum burial, the temperature at maximum burial, 

and the timing of hydrocarbon-generation thresholds. 

The geothermal gradients based on measured 

temperature logs and the best-fit geothermal gradient 

yield different times for hydrocarbon generation for the 

coals. The stratigraphically highest coal in our study 

achieved a Rm value of 0.60 percent at 10 Ma. The 

stratigraphically lowest coal in our study achieved a Rm 

value of 0.60 percent between 51 and 49 Ma, 0. 73 

Thermal History of Mesaverde Group and Wasatch Formation, Multiwell Site LS 
Table 1. Results of TTl modeling using a maximum bottom-hole temperature of 244 °F (118.7 °C) for coals from the 

Mesaverde Group and Wasatch Formation at the MWX site 

[Coal 1 from Wasatch Formation, 2-6 from Mesaverde Group. Time-temperature indices (TTI) and mean vitrinite reflectance (Rm) values for end 

of period of maximum burial; temperature at maximum burial. Time in Ma; NA indicates not attained] 

Time of h)!drocarbon Beneration 

Coal Calculated Rm TTl 10 TTl 25 TTl 180 

no. Geothermal gradient TTl (eercent) Temeerature 0.60 Rm 0.73 Rm 1.35 Rm 

1 2.12 °F/100 ft 2.1 0.45 147 °F NA NA NA 

39.0 °C/km 64.4 oc 

2 2.09 °F/100 ft 63 .96 238 °F 39 30 NA 

38.2 °C/km 115.4 °C 

3 2.07 °F/100 ft 115 1.17 253 °F 42 36 NA 

37.8 °C/km 123.8 °C 

4 2.10 °F/100 ft 400 1.65 285 °F 46 43 28 

38.4 °C/km 141.7 °C 

5 2.23 °F/100 ft 1,350 2.19 318 °F 49 47 39 

40.8 °C/km 160.2 oc 

6 2.25 °F/100 ft 2,300 2.40 333 °F 50 48 43 

41.1 °C/km 168.6 °C 

Table 2. Results of TTl modeling using a maximum bottom-hole temperature of 266 °F (131 °C) for coals from 

Mesaverde Group and Wasatch Formation at the MWX site 

[Coall from Wasatch Formation, 2-6 from Mesaverde Group. Time-temperature indices (TTI) and mean vitrinite reflectance (Rm) values for end 

of period of maximum burial; temperature at maximum burial. Time in Ma; NA indicates not attained] 

Time of h)!drocarbon Beneration 

Coal Calculated Rm TTl 10 TTl 25 TTl 180 

no. Geothermal gradient TTl (eercent) Temeerature 0.60 Rm 0.73 Rm 1.35 Rm 

2.22 °F/100 ft 2.5 0.47 152 °F NA NA NA 

40.08 °C/km 67.2 °C 

2 2.19 °F/100 ft 90 1.06 247 °F 41 34 NA 

40°C/krn 120.4 oc 

3 2.21 °F/100 ft 195 1.38 268 °F 43.5 39 13 

40.4 °C/km 132.2 °C 

4 2.19 °F/100 ft 580 1.81 295 °F 47 44 32 

40 °C/km 147.3 oc 

5 2.35 °F/100 ft 2,200 2.37 332 op 50 48 43 

43.3 °C/km 168 oc 

6 2.37 °F/100 ft 4,100 2.77 347 °F 51 49 45 

43.3 °C/km 176.4 oc 

Table 3. Results of Til modeling using geothermal gradient that best fits both predicted Til values and measured Rm 

values for coals from the Mesaverde Group and Wasatch Formation at the MWX site 

[Coall from Wasatch Formation, 2-6 from Mesaverde Group. Time-temperature indices (TTI) and mean vitrinite reflectance (Rm) values for end 

of period of maximum burial; temperature at maximum burial. Time in Ma; NA indicates not attained] 

Time of h~drocarbon Beneration 

Coal Calculated Rm TTl 10 TTl 25 TTl 180 

no. Geothermal gradient TTl (eercent) Temeerature 0.60 Rm 0.73 Rm 1.35 Rm 

3.0 °F/100 ft 10 0.60 188° F 10 NA NA 

54.6 °C/km 87.3 oc 

2 1.94 °F/100 ft 38 .82 225 op 35 22.5 NA 

35.4 °C/km 108.1 oc 

3 1.92 °F/100 ft 65 0.96 240 °F 40 31 NA 

35 °C/km 116.5 oc 

4 1.95 °F/100 ft 205 1.40 270 °F 45 40 15 

35.6 °C/km 133.3 °C 

5 2.04 °F/100 ft 550 1.79 298 °F 48 46 32 

7.2 °C/km 149 °C 

6 2.02 °F/100 ft 775 1.94 302 °F 49 46 37 

36.8 °C/km 151.2 oc 

L6 Geology of Tight Gas Reservoirs, Wyoming and Colorado 
percent between 49 and 46 Ma, and 1.35 percent between 

45 and 37 Ma. 

Present-day Rm values predicted by using the geothermal gradients calculated from the two temperature logs 

are systematically higher than actual measured values for 

the five Mesaverde coals. These results are consistent with 

those of Johnson and Nuccio (1986), which show that the 

model of Waples (1980) overestimates coal ranks in all but 

the southernmost part of the Piceance basin. This overestimation may result from operator and laboratory bias 

in measuring Rm values, from a relatively recent increase 

in heat flow in the basin, or from a "kink" in the vitrinite 

reflectance profile (fig, 1). 

Operator bias is clearly a problem in measuring 

vitrinite reflectance. For example, for the coal at a presentday depth of 5,226 ft (1,593 m), we measured an Rm value 

of 0.96 percent and Amoco measured a value of 1.14 percent. Amoco's value is quite close to the value we predict 

by using the lower bottom-hole temperature of 244 Of' 

(118 °C). On the other hand, for the coal at a presentday depth of 7,950 ft (2,423 m), our measurement of 1.94 

percent is much closer to Amoco's measurement of 2.11 

percent. For this coal, the two temperature logs predict 

Rm values of 2.40 and 2.77 percent, respectively, values 

significantly greater than either of the actual measurements. Operator bias may explain some, but probably not 

all, of the difference between predicted and measured Rm 

values. 

A relatively recent increase in heat flow at the MWX 

site, as well as throughout much of the rest of the Piceance 

basin, is an intriguing possibility that can be neither proven 

nor disproven at this time. In order to significantly lower 

TTl values for the five coals analyzed from the Mesaverde 

Group, the heat flow must have begun to increase no earlier 

than late in the period of maximum burial between 35 and 

10 Ma. Furthermore, any increase in heat flow may have 

been more substantial than is suggested by a comparison 

of the temperatures shown in tables 1-3 because, as previously discussed, we minimized the calculated temperatures 

under maximum burial by assuming the lowest possible 

maximum burial depth and the lowest possible geothermal gradient through the eroded section. 

Law and Nuccio (1986) and Law and others (1989) 

have described "kinks" in vitrinite reflectance profiles 

from several wells in Rocky Mountain basins, including 

the MWX wells. Law and others (1989) suggested that 

overpressuring associated with the maturation of coal 

beds and carbonaceous shale caused a vertical flow of 

relatively hot waters upward into normally pressured 

strata that raised the overlying vitrinite reflectance values. 

This convective heat transfer process is the most likely 

cause for our inability to model a straight-line timetemperature curve, hence underprediction of the Wasatch 

coal and overprediction for the Mesaverde coals. 

We cannot assess the possibility of other unknown 

problems in applying the TTl model of Waples (1980) 

to the Mesaverde coals of the Piceance basin. There is 

a great deal of scatter in the data that Waples used to 

calibrate TTl and Rm values, and our data is not anomalous when plotted on his curve. 

REFERENCES CITED 

Barker, C.E., 1983, Influence of time on metamorphism of 

sedimentary organic matter in liquid-dominated geothermal systems, western North America: Geology, v. 11, no. 

7' p. 384-388. 

Bostick, N.H., and Freeman, V.L., 1984, Tests of vitrinite 

reflectance and paleotemperature models at the Multiwell 

Experiment Site, Piceance Creek basin, Colorado, in 

Spencer C.W., and Keighin, C.W., eds., Geologic studies 

in support of the U.S. Department of Energy Multi well 

Experiment, Garfield County, Colorado: U.S. Geological 

Survey Open-File Report 84-757, p. 110-120. 

DuBow, J., Nottenburg, R., Rajeshwar, K., and Rosenvold, R., 

1977, Electrical and thermal transport properties of Green 

River oil shale heated in nitrogen: Oil Shale Symposium, 

lOth, Golden, Colo., 1977, Proceedings, p. 60-73. 

Epis, R.C., and Chapin, C.E., 1975, Geomorphic and tectonic 

implications of the post-Laramide, late Eocene erosion surface in the southern Rocky Mountains, in Curtis, B.F., ed., 

Cenozoic history of the southern Rocky Mountains: 

Geological Society of America Memoir 144, p. 45-74. 

Hansen, W.R., 1984, Post-Laramide tectonic history of the 

eastern Uinta Mountains, Utah, Colorado, and Wyoming: 

The Mountain Geologist, v. 21, no. 1, p. 5-29. 

Johnson, R.C., and Nuccio, V.F., 1984, Late Cretaceous through 

Early Tertiary general stratigraphy and structural geology 

of the Piceance Creek basin, Colorado, in Spencer, CW., 

and Keighin, CW., eds., Geologic studies in support of 

the U.S. Department of Energy Multiwell Experiment, 

Garfield County, Colorado: U.S. Geological Survey OpenFile Report 84-757, p. 14-20. 

__ 1986, Structural and thermal history of the Piceance 

Creek basin, western Colorado, in relation to hydrocarbon occurrence in the Mesaverde Group, in Spencer, CW., 

and Mast, R.F., eds., Geology of tight gas reservoirs: 

American Association of Petroleum Geologists Studies in 

Geology 24, p. 165-206. 

Juntgen, H., and Karweil, J., 1966, Gasbildung und 

gasspeicherung in steinkohlenflozen, part I und II [Gas 

formation and storage in bituminous coal seams]: Erdol 

and Kohle, Erdgas, Petrochemie, v. 19, p. 251-258, 339-344. 

Larson, E.E., Ozima, Minoru, and Bradley, W.C., 1975, Late 

Cenozoic basic volcanism in northwestern Colorado and 

its implications concerning tectonism and the origin of the 

Colorado River system, in Curtis, B.F., ed., Cenozoic 

history of the southern Rocky Mountains: Geological 

Society of America Memoir 144, p. 155-178. 

Law, B.E., and Nuccio, V.F., 1986, Segmented vitrinite reflectance profile from the Deep Seam Project, Piceance Creek 

basin, Colorado; evidence of previous high pore pressure 

[abs.]: American Association of Petroleum Geologists 

Bulletin, v. 70, no. 8, p. 1047. 

Thermal History of Mesaverde Group and Wasatch Formation, Multiwell Site L7 
Law, B.E., Nuccio, V.F., and Barker, C.E., 1989, Kinky vitrinite 

reflectance profiles; evidence of paleopore pressure in lowpermeability gas-bearing sequences in Rocky Mountain 

foreland basins: American Association of Petroleum 

Geologists Bulletin, v. 73, no. 8, p. 999-1010. 

Lopatin, N.V., 1971, Temperature and geologic time as factors in coalification: Akademiya nauk SSSR, Izvestiya 

Seriia Geologicheskaya, no. 3, p. 95-106. 

Marvin, R.F., Mehnert, H.H., and Mountjoy, W.M., 1966, Age 

of basalt cap on Grand Mesa, in Geological Survey 

Research 1966: U.S. Geological Survey Professional Paper 

550-A, p. A81. 

Neruchev, S.G., and Parparova, G.M., 1972, 0 roli geologicheskogo vremeni v protsessakh metamorfizma ugley i 

rasseyannogo organicheskogo veshchestva porod [The role 

of geologic time in processes of metamorphism of coal 

and dispersed organic matter in rocks]: Akademie nauk 

SSSR Sibirsk. Otdeleniye Geologiya i Geofizika, no. 10, 

p. 3-10. 

Nuccio, V.F., and Johnson, R.C., 1984, Thermal maturation 

and burial history of the Upper Cretaceous Mesaverde 

Group, including the Multiwell Experiment (MWX), 

L8 Geology of Tight Gas Reservoirs, Wyoming and Colorado 

Piceance Creek basin, Colorado, in Spencer, C.W., and 

Keighin, C.W., eds., Geologic studies in support of the 

U.S. Department of Energy Multi well Experiment, Garfield County, Colorado: U.S. Geological Survey OpenFile Report 84-757, p. 102-109. 

Price, L.C., 1983, Geologic time as a parameter in organic 

metamorphism and vitrinite reflectance as an absolute 

paleogeothermometer: Journal of Petroleum Geology, 

v. 6, no. 1, p. 5-38. 

Tissot, B.P., Durand, B., Espatalie, J., and Combaz, A., 1974, 

Influence of nature and diagenesis of organic matter 

in formation of petroleum: American Association of 

Petroleum Geologists Bulletin, v. 58, no. 3, p. 499-506. 

Tweto, Ogden, 1975, Laramide (Late Cretaceous-Early Tertiary) orogeny in the southern Rocky Mountains, in 

Curtis, B.F., ed., Cenozoic history of the southern Rocky 

Mountains: Geological Society of America Memoir 144, 

p. 1-44. 

Waples, D.W., 1980, Time and temperature in petroleum formation; application of Lopatin's method to petroleum exploration: American Association of Petroleum Geologists 

Bulletin, v. 64, no. 6, p. 916-926. 
Chapter M 

Fluid Inclusion Evidence for 

Paleotemperatures within the 

Mesaverde Group, Multiwell Experiment Site, 

Piceance Basin, Colorado 

By CHARLES E. BARKER 

Prepared in cooperation with the U.S. Department of Energy 

U.S. GEOLOGICAL SURVEY BULLETIN 1886 

GEOLOGY OF TIGHT GAS RESERVOIRS IN THE PINEDALE ANTICLINE AREA, WYOMING, 

AND AT THE MULTIWELL EXPERIMENT SITE, COLORADO 
CONTENTS 

Abstract Ml 

Introduction Ml 

Results of fluid inclusion studies M2 

Calcite veins M2 

Quartz druses M2 

Thermal history reconstruction MJ 

Present temperature M3 

Peak paleotemperature MJ 

Burial history MJ 

Timing of coalification and gas generation M4 

Timing of cementation MS 

Discussion M6 

References cited M6 

Appendix 1. Fluid inclusion analysis MlO 

Appendix 2. Paleotemperature from vitrinite reflectance Mll 

FIGURES 

1-2. Graphs showing: 

1. Homogenization temperatures of aqueous fluid inclusions, 

MWX-1 M3 

2. Reconstructed and measured geothermal gradients, MWX wells M4 

3. Diagram showing burial history reconstruction, MWX wells MS 
Fluid Inclusion Evidence for 

Paleotemperatures within the Mesaverde Group, 

Multiwell Experiment Site, Piceance Basin, Colorado 

By Charles E. Barker 

Abstract 

Gas-bearing aqueous fluid inclusions occur in veins cutting the Mesaverde Group in the MWX wells. The fluid inclusions in calcite decrepitate or reequilibrate when heated, and 

no quantitative homogenization temperature data can be obtained from them. Fluid inclusions in quartz-lined veins at 

depths of 5,578 ft (1 ,700 m) and 7,844 ft (2,390 m) were 

homogenized at abOUt 285 Of (140 °C) and 315 Of (158 °C), 

respectively. Modified homogenization temperatures suggest 

that the fluid inclusions in quartz veins formed in a geothermal profile similar to the present-day near-surface temperature 

gradient Of 2.1 °f/100 ft (38 °C/km). 

Burial temperatures calculated from vitrinite reflectance 

values indicate a peak paleogeothermal gradient of 2.0 °F/100 

ft (37 °C/km) above 4,400 ft (1 ,340m), and a higher paleogeothermal gradient of 4.1 o F/1 00 ft (7 4 °C/km) from 4,400 to 

7,950 ft (1 ,340-2,420 m). The shallow paleogeothermal gradient agrees with a geothermal gradient of 2.1 °F/100 ft (38 0 C/km) measured at depths less than 6,600 ft (2,01 0 m). Below 

a depth of 6,600 ft (2,010 m), an increase in coal content causes 

a higher gradient of about 3.4 °F/100 ft (62 °C/km). Changes 

in slope of the measured temperature profile do not correspond 

to those in the rank profile or to the gradient suggested by the 

fluid inclusions. This difference suggests that the geothermal 

gradient has changed in portions of the well. 

INTRODUCTION 

The basis of thermal history reconstruction using 

fluid inclusions is that they are trapped as a mineral 

precipitates and can record the temperature and chemistry 

of the pore water present. Fluid inclusion salinity is 

measured by freezing point depression. The brine composition is inferred from the eutectic (first melting) 

temperature. The homogenization temperature of a fluid 

inclusion is related to its formation temperature 

(Roedder, 1984) if the fluid inclusion has not reequilibrated (Goldstein, 1986). These data are used to 

reconstruct the fluid temperature and composition in the 

rock when the inclusions formed. In the MWX wells, 

fractures formed in response to several episodes of 

regional deformation. These fractures are now mineralized and contain fluid inclusions. Reconstruction of the 

regional stress history shows the time when fractures of 

a given orientation probably opened. Measurements of 

orientation bracket a geologic time when the temperatures 

shown by homogenization temperature could have existed. The sophistication of the temperature bracketing 

is increased by measurements from veins of a different 

orientation or by data from other geothermometers. Particularly useful are the peak temperatures indicated by 

vitrinite reflectance and the measured well temperatures. 

This method of comparative geothermometry shows a 

time-temperature history for the MWX site. 

These comparative studies are important because 

fluid inclusion geothermometry is being discredited as a 

viable method in sedimentary rocks (Prezbindowski and 

Larese, 1987). Physical changes in fluid inclusions 

(reequilibration) during deeper burial cause averaged 

homogenization temperatures not to fit other geologic 

data. Actually, at fault is a defective approach in defining fluid inclusion origin and thermal history, leading to 

inclusions from different trapping events being reported 

as one population. Fluid inclusion geothermometry in 

sedimentary rocks can easily be misinterpreted and lead 

to erroneous conclusions. The inaccurate temperature information arises from: (1) reequilibration of fluid inclusions when they are heated to temperatures exceeding 

their trapping conditions; (2) the assumption that fluid 

inclusions of ambiguous origin are primary; (3) overemphasis of two-phase fluid inclusions and not considering 

evidence from single-phase fluid inclusions; (4) heating 

events not being recorded by the fluid inclusion population; and (5) insufficient composition data to properly 

evaluate the meaning of heating stage measurements. (See 

Roedder 1984, and Goldstein 1986, among others, for 

reviews.) In spite of these problems, careful fluid inclusion studies provide a strong tool useful in thermal 

diagenesis studies. The potential for misinterpretation of 

fluid inclusion data is mitigated in this study by using 

other paleogeothermometers and well temperature data 

to check results. 

Fluid Inclusion Evidence for Paleotemperatures, Multiwell Site Ml 
Acknowledgments.-V.F. Nuccio and R.C. 

Johnson helped with the geology of Piceance basin. J. C. 

Lorenz, Sandia National Laboratories, provided the 

quartz-lined fractures for the fluid inclusion study. C. W. 

Spencer and B.E. Law helped guide the investigation. 

V.F. Nuccio and C.W. Spencer reviewed the paper. M.J. 

Pawlewicz helped at a critical time by typing a corrected 

version of the paper. 

RESULTS OF FLUID INCLUSION STUDIES 

Fluid inclusions in the MWX core samples occur 

in calcite veins and quartz-lined fractures. Analysis of 

these vein samples proceeded by the methods outlined in 

appendix 1. These mineralized fractures have been further described by Verbeek and Grout (1984), Pitman and 

Sprunt (1984, 1986), Lorenz and Findlay (1986), and Pitman and Dickinson (this volume). 

Calcite Veins 

Aqueous inclusions are common in calcite veins cutting the Mesaverde Group. Homogenization temperatures 

for these inclusions cannot be determined because the inclusions decrepitate or reequilibrate when heated. 

Vein samples from a shallow (5,018 ft, 1,530 m) 

and a deep (8,114 ft, 2,470 m) core were opened in a 

crushing stage (Roedder, 1984). The cleavage fragments 

are immersed in glycerin to check for noncondensable gas 

and then in kerosene to check for natural gas. The shallow 

sample contains secondary planes of two-phase fluid inclusions. Bubbles that evolved from these samples when 

they were cracked on the crushing stage did not dissolve 

in glycerin but rapidly disappeared in kerosene. Cleavage 

fragments from the deeper sample behaved similarly except that bubbling was more vigorous and a greater 

volume of gas emanated from inclusions of similar size. 

The larger volume of gas emitted from the deeper sample suggests that the gas in the inclusion was under greater 

pressure. The pressure difference is consistent with formation at a greater depth in the well. Because the gas bubbles rapidly dissolved in the kerosene, the gas is 

qualitatively identified as methane (Shepard and others, 

1985). Fluorescence microscopy detected no oil in these 

inclusions. 

Quartz Druses 

An extensive search through hundreds of fractures 

in the MWX cores (performed by Sandia Laboratories, 

Los Alamos, N. Mex.) identified only a few fractures 

lined with quartz. In some cases, quartz precipitated with 

M2 Geology of Tight Gas Reservoirs, Wyoming and Colorado 

calcite. Some of the euhedral quartz crystals that line fractures contain small rhombs (calcite?) having high relief 

and birefringence. This occurrence may be a localized 

phenomena because Pitman and Dickinson (this volume), 

in a more extensive study of the fractures, reported that 

quartz precipitated before calcite. 

Only euhedral quartz crystals or quartz overgrowth 

cements are considered usable because detrital quartz 

grains contain inherited fluid inclusions. A MWX-2 core 

sample collected from 5,572 ft (1,700 m) contains a nearvertical fracture lined with euhedral quartz crystals that 

have overgrown detrital grains. A core sample at 7,844 

ft (2,390 m) contains euhedral quartz crystals that occur 

as druses growing on a curving, subhorizontal, slickensided fracture surface. Fluorescence microscopy detected 

no oil in these inclusions. 

Aqueous inclusions in quartz did homogenize and 

provide (apparently) acceptable data. Some fluid inclusions selected for analysis are associated with inclusions 

with higher vapor to liquid ratios and homogenization 

temperatures exceeding 390 "F (200 °C). These latter inclusions were not measured because many had textures 

that suggest necking has occurred. In the sample from 

7,844 ft (2,390 m), five inclusions had a homogenization 

temperature that exceeded 340-360 op (170-180 °C). 

Cycling the heating stage at increasingly higher 

temperature did not cause homogenization in these inclusions (appendix 1). Heating was discontinued before 

damaging the sample. Both secondary fracture-bound inclusions and primary inclusions related to growth zones 

are in these samples. Although the secondary and primary 

inclusions formed at different times, their homogenization temperatures appear to be closely related and these 

values were combined to calculate an average. 

The average homogenization temperature is about 

285 "F (140 OC) at 5,578 ft (1,700 m) and 315 "F (158 

°C) at 7,844 ft (2,390 m) (fig. 1). These temperatures 

suggest a apparent geothermal gradient of 1.4 "F/100 ft 

(26 °C/km). Modifications to the homogenizationtemperature population show that the fluid inclusion data 

are consistent with the measured well temperature profile (fig. 2). The average homogenization temperature at 

7,844 ft (2,390 m) is too low because I did not include 

the five inclusions having indeterminate homogenization 

temperatures (but definitely above 340-360 "F; 170-180 

OC) in the calculations. The exclusion of these inclusions 

produces an artificially low average temperature in the 

deep sample. This is an important point, because if the 

average homogenization temperature at 7,844 ft (2,390 

m) is increased by 14 op (8 °C), the apparent geothermal 

gradient will increase to 2.1 "F/100 ft (38 °C/km) or 

equal to the present temperature profile. 

The fluid inclusions in quartz at 7,844 ft may 

contain natural gas because they coexist with gas-bearing aqueous inclusions in calcite and are difficult to 
homogenize. Crushing these inclusions did not clearly 

show that they contain natural gas. The crushing stage 

caused these small crystals to violently shatter. These 

violent movements can cause gas to exsolve from the immersion fluid and obscure any gas produced from the 

crystal. There was no continued bubbling of gas from the 

sample after cracking, which is strong qualitative evidence 

of natural gas in the fluid inclusions. Fluorescence 

microscopy detected no oil in these inclusions. 

THERMAL HISTORY RECONSTRUCTION 

Present Tern perature 

The MWX-1 borehole was spudded on September 

13, 1981, and drilling operations ceased on December 18. 

Temperature logs (fig. 2) were run the following year on 

June 28 (LANL log) and September 9, 1982 (SMU log). 

The LANL log was measured 6 months after drilling 

ceased, enough time for well temperatures to reequilibrate 

(Bullard, 1947). The LANL log appears more accurate 

because the temperature profile changes correlate more 

closely with changes in lithology in the borehole (C. W. 

Spencer, oral commun., 1987). The geothermal gradient 

changes significantly at a depth of 6,600 ft (2,010 m). It 

is 2.1 'F/100 ft (38 OC/km) above 6,600 ft and 3.4 'F/100 

ft (62 OC/km) below. 

Peak Paleotemperature 

Vitrinite reflectance (Nuccio and Johnson, this 

volume) is interpreted in this study as recording peak 

burial temperature (appendix 2). This analysis suggests 

peak paleotemperature increased from 190 op (90 °C) in 

rocks now exposed at the surface to 280 op (140 °C) at 

a depth of 4,400 ft (1,340 m) (2.0 'F/100 ft; 37 °C/km 

gradient). The gradient is higher from 4,400 to 7,950 ft 

(1 ,340-2,420 m). At 7,950 ft {2,420 m) (the deepest 

vitrinite reflectance measurement), the peak paleotemperature was 430 op (220 °C) (4.1 'F/100 ft), corresponding 

to a paleogeothermal gradient of 4.1 'F/100 ft (74 

°C/km) peak gradient. 

Other qualitative evidence for the high paleogeothermal gradients includes relationships between temperature and the type of hydrocarbon present. The observed 

coexistence of pyrobitumens and gas within the MWX-1 

well (Sattler and Lorenz, 1983) could be the result of 

burial temperatures exceeding 390 op (200 °C) (Hunt, 

1979, p. 102). 

Burial History 

The burial history (fig. 3) reconstructed by Johnson 

and Nuccio (1986) is used to estimate paleodepth for the 

period 74 to 10 Ma. For the period 10 Ma to present, their 

reconstruction is modified using data from Larson and 

others (1975). Larson and others asserted that no erosion 

10 r--

-

-

0 

100 

10 -

-

-

-

0 

100 

QUARTZ 5,572 FEET ( 1.700m) 

n = 17 ,--

x-=140° F 

r----- S= 8 G 

~ 

'G 

~ 

G r-- I>-

F 'F, 

'n , G F F I F I 

IFl F G F G I''G,I 

I 

150 200 

QUARTZ 7,844 FEET (2,390m) 

n= 19 

x-=158° 

S= 11 r-- r;:--

F f., r----- ~ 

F l, 

r---' 

F 0 F G 

'G., F F I r----- F 0 

fFl G 0 F F I fF ~ 

150 ' 200 

x 

TEMPERATURE, IN oc 

Figure 1. Homogenization temperatures of aqueous fluid inclusions, MWX-1. Fracture-bound fluid inclusions (F) are those 

trapped along a healed crack. Orphan inclusions (0) are those 

that have no apparent spatial or genetic relationship to other 

inclusions or crystal defects. Fluid inclusions in growth zones 

(G) are those that formed along quartz overgrowth contacts 

with a detrital quartz grain. Diagonal dashing across data symbol indicates difficult, suspect measurements. 

occurred between 8 and 1.5 Main the Upper Colorado 

River system. This difference in the timing of erosion 

does not effect the maximum burial depth as 

reconstructed by Nuccio and Johnson, only the timing 

of erosion in the period 10 Ma to present. The timing 

of temperature decreases resulting from erosion can be 

useful in fixing the time of diagenetic events, in particular, the temperatures recorded in secondary fluid inclusions trapped along fracture planes during uplift. 

Johnson and Nuccio (1986) assumed that minimal 

erosion occurred at unconformities in the MWX well 

because the area is near the axis of the Piceance basin. 

Because of this assumption, the burial history reconstruction of Johnson and Nuccio is a minimum estimate 

of maximum burial depth. Paleotemperature estimates 

based on this burial reconstruction are consequently 

minimum values. 

Fluid Inclusion Evidence for Paleotemperatures, Multiwell Site M3 
rLlJ 

LlJ 

u. 

z 

::C 

t 

LlJ 

Cl 

0 

1,000 

2,000 

3,000 

4,000 

5,000 

6,000 

7,000 

8,000 

9,000 

10,000 

11,000 

12,000 

------ Present depth, 

bottom of 

MWX-1 

Bottom-hole depth, 

corrected to 

maximum buriaL 

MWX-1 

____ /_ 

0 50 100 150 200 250 300 350 400 

TEMPERATURE, IN oc 

Figure 2. Reconstructed and measured geothermal gradients, MWX wells. Note that 

the paleotemperatures calculated from vitrinite reflectance data (Rm temperatures), when 

restored to maximum burial depth, fit well with the higher well temperatures in the 

coaly rocks near the bottom of well. Well temperatures from Sattler and Lorenz (1983). 

Lopatin modeling from Nuccio and Johnson (this volume). 

Timing of Coalification and Gas Generation 

Cross sections constructed by Johnson and Nuccio 

(1986) show that iso-vitrinite-reflectance lines in the vicinity of the MWX site only locally follow structure. This 

relationship infers that coalification is syntectonic (Stach 

and others, 1982). The final phase of deformation in the 

M4 Geology of Tight Gas Reservoirs, Wyoming and Colorado 

Piceance basin tha~ affected the coals occurred near the 

end of the Eocene (about 35 Ma; Johnson and Nuccio, 

1986). Coalification was apparently completed during this 

time. Coals in the Mesaverde Group at the margins of 

the Piceance basin were uplifted and cooled in the late 

Eocene. The vitrinite reflectance of these uplifted coals 

is similar to the present-day rank of stratigraphically 
0 

1,000 

2,000 

3,000 

4,000 

5,000 

~ 

LU 

LU 

u. 6,000 ~ 

:I 

b: 7,000 LU 

0 

8,000 

9,000 

10,000 

11,000 

12,000 

13,000 

80 70 60 50 40 

Coal-rich portion of Mesaverde Group 

~7.950------.... 

30 20 10 0 

en 

a: 

LU 

~ 

LU 

~ 

0 

_J 

2 ::;2 

z 

3 

:I 

b: 

LU 

0 

Figure 3. Burial history reconstruction, MWX wells. Recon1~~~ti~n ~ows the minimum depth of burial because little erosion 

occurred at the unconformities older than 10 Ma. Since 10 Ma, erosion has removed a total of 4,600 ft (1,400 m) of rock. Modified 

from Johnson and Nuccio (1986) using evidence for timing of erosion from Larson and others (1975). 

equivalent coals that are still deeply buried in the central 

part of the basin (Johnson and Nuccio, 1986). The 

similarity of rank between the shallow and deeply buried 

coals suggests that thermal maturation has not increased 

since the late Eocene. Consequently, gas generation must 

have decreased after thermal maturation stabilized (at 

about 35 Ma). 

Timing of Cementation 

The timing of fracturing is important because it 

represents the maximum age of the veins. Most fractures 

at the MWX site trend almost east-west, and Verbeek 

and Grout (1984) suggested that they opened before 

formation of the Grand Hogback at about 34 Ma. Fractures were available for cementation essentially during 

the entire maximum burial phase from 35 to 9 Ma 

(fig. 3). 

By restoring the 4,600 ft (1 ,400 m) of post-maximum-burial erosion, measured well temperatures can be 

projected to estimate peak paleotemperature. The restored temperature profile is an estimate of peak 

temperature if the heat flow has not changed. The peak 

temperatures must be adjusted for the middle Tertiary 

surface temperature of 70 "F (20 OC) (Savin, 1977). 

Samples from depths of less than 6,600 ft (2,010 m) now 

exist in a geothermal gradient of 2.1 "F/100 ft 

(38 OC/km). Samples below 6,600 ft exist in a higher gradient of 3.4 "F/100 ft (62 OC/km). Restored peak 

paleotemperature at 6,600 ft is 300 "F (150 OC). At 7,950 

ft (2,420 m), it is 350 op (175 °C). 

Fluid inclusion temperature data roughly agree with 

restored peak paleotemperatures. Aqueous inclusions 

Fluid Inclusion Evidence for Paleotemperatures, Multiwell Site MS 
in quartz-lined fractures from MWX-2 have an average 

homogenization temperature of about 280 Of' (140 °C) 

at 5,578 ft (1,700 m) depth and 315 Of' (158 °C) at 7,844 

ft (2,390 m) (fig. 1). The restored peak paleotemperatures 

at these depths are 280 Of' (140 °C) and 340 Of' (170 OC), 

respectively. The agreement between restored peak paleotemperatures and fluid inclusion temperatures argues that 

the geothermal gradient has not changed since the quartzlined fractures formed, sometime during the maximum 

burial phase 35 to 9 Ma. 

Similar homogenization temperatures for secondary 

and primary fluid inclusions in the same samples of 

MWX quartz veins suggest that the microfractures 

containing the secondary inclusions likely opened and 

healed at near maximum burial. 

DISCUSSION 

Even allowing for cooling of the system as a result 

of erosion, the shape of the measured well temperature 

profile should be similar to that of the vitrinite reflectance 

profile. The difference between the two profiles suggests 

that the peak temperatures that set the vitrinite reflectance profile in the late Eocene have changed. The higher 

than present-day geothermal profile indicated by vitrinite 

reflectance apparently existed during only the early part 

of the maximum burial phase. By the time the quartzlined fractures formed, the geothermal gradient had 

already decreased to present-day levels. The fluid inclusions appear to record conditions later in the maximum 

burial phase that existed from 35 to 9 Ma. Fluid inclusion evidence also infers that the present-day geothermal 

gradient was established before erosion commenced at 

about 9 Ma and decreased burial temperature. 

REFERENCES CITED 

Barker, C.E., and Halley, R.B., 1986, Fluid inclusion, stable 

isotope, and vitrinite reflectance; evidence for the thermal history of the Bone Spring Limestone, southern 

Guadalupe Mountains, Texas, in Gautier, D.L., ed., Roles 

of organic matter in sediment diagenesis: Society of 

Economic Paleontologists and Mineralogists Special 

Publication 38, p. 189-203. 

Barker, C.E., and Pawlewicz, M.J., 1986, The correlation of 

vitrinite reflectance with maximum temperature in humic 

organic matter, in Buntebarth, G., and Stegena, L., eds., 

Paleogeothermics: New York, Springer-Verlag, p. 79-93. 

Barker, C.E., and Reynolds, T.J., 1984, Preparing doubly 

polished sections of temperature sensitive sedimentary 

rocks: Journal of Sedimentary Petrology, v. 54, no. 2, 

p. 635-636. 

Bullard, E.C., 1947, The time necessary for a borehole to attain temperature equilibrium: Royal Astronomical Society 

Geophysical Supplement, v. 5, p. 127-130. 

M6 Geology of Tight Gas Reservoirs, Wyoming and Colorado 

Christian, S.D., and Tucker, E.E., 1986, Linear least squares 

analysis with dependent and independent variable subject 

to error: American Laboratory, v. 18, p. 33-36. 

Goldstein, R.H., 1986, Reequilibration of fluid inclusions in 

low-temperature calcium-carbonate cement: Geology, v. 

14, p. 792-795. 

Han or, J .S., 1980, Dissolved methane in sedimentary basins; 

potential effect on the PVT properties of fluid inclusions: 

Economic Geology, v. 75, no. 4, p. 603-609. 

Hunt, J.M., 1979, Petroleum geochemistry and geology: San 

Francisco, Freeman, 617 p. 

Johnson, R.C., and Nuccio, V.F., 1986, Structural and thermal history of the Piceance basin, western Colorado, in 

relation to hydrocarbon occurrence in the Mesaverde 

Group, in Spencer, C.W., and Mast, R.F., eds., Geology 

of tight gas reservoirs: American Association of Petroleum 

Geologists Studies in Geology 24, p. 165-206. 

Larson, E.E., Ozima, M., and Bradley, W.C., 1975, Late 

Cenozoic basic volcanism in northwestern Colorado and 

its implications concerning tectonism and the origin of the 

Colorado River system, in Curtis, B.F., Cenozoic history 

of the southern Rocky Mountains: Geological Society of 

America Memoir 144, p. 155-178. 

Lorenz, J.C., and Findlay, S.J., 1986, Details of fractures in 

core from the MWX marine zones: Sandia National 

Laboratories Distribution Memorandum, September 23, 

1986, 6 p. 

Love, K., 1987, Closing the trapdoors of regression analysis: 

Geobyte, v. 2, p. 18-21. 

Pitman, J.K., and Sprunt, E.S., 1984, Origin and occurrence 

of fracture-filling cements in the' Upper Cretaceous 

Mesaverde Formation at MWX, Piceance Creek basin, 

Colorado, in Spencer, C.W., and Keighin, C.W., Geologic 

studies in support of the U.S. Department of Energy 

Multi well Experiment, Garfield County, Colorado: U.S. 

Geological Survey Open-File Report 84-757, p. 87-101. 

___ 1986, Origin and distribution of fractures in lower Tertiary and Upper Cretaceous rocks, Piceance Basin, Colorado, and their relation to the origin of hydrocarbons, 

in Spencer, C.W., and Mast, R.F., eds., Geology of tight 

gas reservoirs: American Association of Petroleum 

Geologists Studies in Geology 24, p. 221-234. 

Pollastro, R.M., and Barker, C.E., 1986, Application of claymineral, vitrinite reflectance, and fluid inclusion studies 

to the thermal and burial history of the Pinedale Anticline, 

Green River basin, Wyoming, in Gautier, D.L., ed., 1986, 

Roles of organic matter in sediment diagenesis: Society 

of Economic Paleontologists and Mineralogists Special 

Publication 38, p. 73-83. 

Prezbindowski, D.R., and Larese, R.E., 1987, Experimental 

stretching of fluid inclusions in calcite; implication for 

diagenetic studies: Geology, v. 19, p. 333-336. 

Roedder, E., 1984, Fluid inclusions: Mineralogical Society of 

America, Reviews in Mineralogy, v. 12, 644 p. 

Sattler, A.R., and Lorenz, J.C., 1983, Topical discussion of 

organic maturation at Multi well Experiment site (MWX): 

Sandia National Laboratories Distribution Memorandum, 

March 24, 1983. 

Savin, S.M., 1977, The history of the earth's surface 

temperature during the past 100 million years: Annual 

Reviews of Earth and Planetary Sciences, v. 5, p. 319-355. 
Shepard, T.J., Rankin, A.H., and Alderton, D.H.M., 1985, 

A practical guide to fluid inclusion studies: London, 

Blackie, 239 p. 

Stach, E., Mackowsky, M.-Th, Teichmuler, M., Taylor, G., 

Chandra, D., and Teichmuler, R., 1982, Stach's textbook 

of coal petrology: Berlin, Gebruder Borntraeger, 535 p. 

Till, R., 1974, Statistical methods for the earth scientist; an introduction: London, Macmillan, 154 p. 

Verbeek, E.R., and Grout, M.A., 1984, Prediction of subsurface fracture patterns from surface studies of joints; an 

example from the Piceance Creek basin, Colorado, in 

Spencer, C.W., and Keighin, C.W., eds., Geologic studies 

in support of the U.S. Department of Energy Multi well 

Experiment, Garfield County, Colorado: U.S. Geological 

Survey Open-File Report 84-757, p. 75-86. 

Werre, R.W., Jr., Bodnar, R.J., Bethke, P.M., and Barton, 

P.B., Jr., 1979, A novel gas-flow fluid inclusion 

heating/freezing stage: Geological Society of America 

Abstracts with Programs, v. 11, no. 7, p. 539. 

Woods, T.L., Bethke, P.M., Bodnar, R.J., and Werre, R.W., 

Jr., 1981, Supplementary components and operation of 

the U.S. Geological Survey gas-flow heating/freezing 

stage: U.S. Geological Survey Open-File Report 81-954, 

21 p. 

Fluid Inclusion Evidence for Paleotemperatures, Multiwell Site M7 

APPENDIXES 1, 2 
Appendix 1. Fluid inclusion analysis 

Sample Preparation 

Quartz crystals lining the fractures are removed from the fracture surface by picking them off with a needle. These loose crystals 

are epoxy bonded on circular cover slips that fit directly into the 

heating stage. The mounting medium is a low-viscosity Epoxy 

(Epotek 301) that has an index of refraction close to that of quartz. 

This mounting method mutes the crystal boundaries and thus 

enhances observation of fluid inclusions in unpolished mounts. 

The calcite veins are large enough to make doubly polished 

sections or to chip off thin cleavage fragments for analysis. These 

samples are naturally heated to over 170 "F (75 CC), and thermal 

alteration of the fluid inclusions during polishing is unlikely. 

The techniques of Barker and Reynolds (1984) required modification to correctly polish the calcite veins in a harder sandstone 

matrix. Polishing the calcite required planing the rock surface on 

600-grit silicon carbide paper and polishing the sample for 60 

seconds or less on a 1-micron diamond lap or on lapping film. 

This procedure replaces the 6-micron diamond paste step used by 

Barker and Reynolds. 

Microscopy 

Auid-inclusion homogenization temperatures are measured in 

a gas-flow heating stage (Werre and others, 1979) rebuilt to the 

specifications of Auid Incorporated. Calibration of this heating 

stage uses the procedures described in Barker and Halley (1986). The 

accuracy of temperature determinations is ± 1 CC in heating mode 

and ±0.2 CC in freezing mode. The heating stage uses a Zeiss microscope equipped with a 100-watt quartz-halogen light source, Zeiss 

long-working-distance condenser, Lietz L-32 objective, and 12.5 

X, 16 X, or 20 X oculars. The higher power oculars provide a larger, 

but less distinct image. A Zeiss epi-illuminator fitted with blue and 

ultraviolet filters provides incident fluorescence microscopy. 

Often the fluid inclusions in the MWX samples are visible 

at high magnification with the heating stage chamber top off but 

become indistinct and immeasurable with the top on. Serial sections of specimens aid the search for larger and (or) more clearly 

observable small inclusions. These small inclusions occur in clean, 

solid inclusion-free parts of the crystal. Dark shadows in a fluid 

inclusion can be reduced by using oblique illumination produced 

by moving the inclusion off center in the field of view. Periodic 

cleaning of the microscope lenses and replacement of the stage 

windows is required to reduce interference by dust. Setting up the 

microscope for the Kohler illumination is required for each inclusion, and proper adjustment of the diaphragms on the condenser 

and on the L-32 objective is crucial in order to improve the fluid 

inclusion image. A polarizer can be inserted and rotated in the light 

path to cut out double refraction. 

Each fluid inclusion is described by its petrographic characteristics and occurrence. Each inclusion is heated at least twice to 

homogenization temperature to check the measurement. Only two 

fluid inclusions in each fracture-bound set are measured in order 

to estimate consistency of the vapor to liquid ratios in the group 

without unnecessarily biasing the results toward the ubiquitous 

inclusions occurring in a few secondary planes. All primary fluid 

inclusions are measured. 

MlO Geology of Tight Gas Reservoirs, Wyoming and Colorado 

Pressure Correction 

The homogenization temperatures do not require pressure 

correction. The fluids are gas saturated and they apparently 

homogenize at temperatures at or near the bubble point. The 

homogenization temperature of natural-gas-bearing aqueous fluid 

inclusions can be used as a minimum temperature of entrapment 

(Hanor, 1980). 

Dealing with Gas-Rich Inclusions 

In these gas-rich fluid inclusions it is difficult to clearly observe homogenization. When heated, the bubble merely shrinks 

until it is no longer visible. Subsequent cooling causes the bubble 

to slowly grow back, often in the same position within the crystal. 

This phenomenon suggests that homogenization has not occurred. Normally, the vapor phase renucleates at about 20-30 CC 

below homogenization. Renucleation is an abrupt event marked 

by the sudden appearance of a relatively large bubble. This type 

of homogenization was not commonly observed in the MWX 

fluid inclusions. In the cycling method (Woods and others, 1981), 

the peak stage temperature increases by small increments until 

during the cooling cycle the vapor bubble suddenly reappears 

rather than slowly grows back into view. For many MWX inclusions, incrementally increasing the peak temperature did not cause 

homogenization. The vapor bubble slowly grew back into view 

during cooling, even if the point of bubble disappearance was 

exceeded by more than 50 CC. Initially, this behavior was thought 

to result from reequilibration. In many cases, the reappearance 

temperature of the bubble did not change, despite ever higher peak 

temperatures, an indication that the fluid inclusion was not reequilibrating. Another indication of reequilibration is the increase 

of the diameter of the bubble after cooling to room temperature. 

The fluid inclusions in calcite did show this behavior and are not 

usable. Fluid inclusions in quartz did not reequilibrate when 

heated to less than 50 CC beyond the vapor-bubble disappearance. 

If the gas-rich inclusions are ignored, serious distortion of 

the data can occur. There is a tendency to not use difficult to 

measure two-phase or single-phase inclusions. In this study, the 

temperature at which the bubble disappears to the observer gives 

a crude and subjective minimum homogenization temperature estimate. This technique is only used if homogenization could not 

be directly observed, and the estimated temperatures are not included in the calculation of average homogenization temperature. 

The temperature at which the bubble first grows back into view 

should also be noted because if this temperature continues to creep 

upward during heating cycles, the inclusion may be reequilibrating. 

Reequilibration is confirmed if upon cooling to room temperature, 

the diameter of the bubble has measurably increased. 

Careful microscopy and repetition of measurements often 

permits determination of homogenization temperature. Somet~es 

if the inclusion is cooled slightly after the bubble apparently disappears, the bubble may move from where it is hidden. If the bubble is observed to move, the temperature is noted and heating 

resumed. Several heating cycles are required in order to ensure 

that the bubble truly has disappeared because the bubble may 

be hidden at the wall of the inclusion or it may have moved out 

of the focal plane. Often the inclusion can finally be observed 

to homogenize by making these small heating cycles. In these 

cases, the apparent disappearance temperature is within about 

20 CC of the true homogenization temperature. 
Appendix 2. Paleotemperature from 

vitrinite reflectance 

Barker and Pawlewicz (1986) used the ordinary leastsquares regression method to calibrate an empirical geothermometer based on vitrinite reflectance. However, because of 

measurement errors in both variables, this classical regression 

technique produces incorrect results. This problem was not 

detected in early tests of the empirical geothermometer. Barker 

and Halley (1986) and Pollastro and Barker (1986) used an equation that was computed from a smaller data set than was later 

published by Barker and Pawlewicz. Unfortunately, the final 

version of the equation published in Barker and Pawlewicz 

(1986) computes temperature estimates that are too hot for 

burial diagenesis. 

A revised equation uses major-axis regression (Till, 1974) 

to compute the curve. Major-axis regression is better because 

it is difficult to measure peak temperature and vitrinite reflectance without error. Major-axis regression considers both 

variables to have measurement errors and adjusts the fitted curve 

accordingly. In contrast, least-squares regression requires that 

one variable be known without error (Christian and Tucker, 

1986; Love, 1987), an unrealistic requirement in this case. These 

problems indicate that a data set generated by consistent 

methods of measurement is needed to assure more accuracy in 

calibrating this geothermometer. 

The major-axis regression equation is 

ln(Rm) = 0.00959(Tpeak) -1.42, 

where Rm is the vitrinite reflectance and Tpeak is the peak 

paleotemperature. This equation is used here to estimate peak 

paleotemperature by rounding to the nearest 10 OC (20 Of). 

Peak paleotemperature estimated from this equation is in 

better agreement with those from other geothermometers published in Barker and Halley (1986) and Pollastro and Barker 

(1986). 

Regardless of the statistical theory and its constraints, 

Barker and Pawlewicz's (1986) conclusion that the effect of time 

on vitrinite reflectance cannot be large still holds. This conclusion is based on the strong correlation of vitrinite-reflectance 

and peak-temperature data from diverse geologic systems having a wide range of heating duration. 

Fluid Inclusion Evidence for Paleotemperatures, Multiwell Site M11 

Chapter N 

Rock-Eval Analysis of Sediments and 

Ultimate Analysis of Coal, Mesaverde Group, 

Mu ltiwell Experiment Site, Piceance Basin, 

Colorado 

By CHARLES E. BARKER 

Prepared in cooperation with the U.S. Department of Energy 

U.S. GEOLOGICAL SURVEY BULLETIN 1886 

GEOLOGY OF TIGHT GAS RESERVOIRS IN THE PINEDALE ANTICLINE AREA, WYOMING, 

AND AT THE MULTIWELL EXPERIMENT SITE, COLORADO 
CONTENTS 

Abstract Nl 

Introduction Nl 

Rock-Eval pyrolysis Nl 

Results N2 

Conclusions N2 

References cited N2 

Appendix 1. Rock-Eval pyrolysis data, MWX-1, -2, and -3 N8 

Appendix 2. Coal analysis data, MWX-1 Nll 

FIGURES 

1-4. Graphs showing: 

1. Hydrogen index versus oxygen index from Rock-Eval pyrolysis, 

MWX-1, -2, and -3 N3 

2. Hydrogen and oxygen indices versus depth, MWX-1, -2, and -3 N4 

3. Van Krevelen diagram of coal ultimate analyses, MWX-1 NS 

4. Rock-Eval Tmax, vitrinite reflectance, and vitrinite reflectance 

equivalent versus depth, MWX-1, -2, and -3 NS 
Rock-Eval Analysis of Sediments and 

Ultimate Analysis of Coal, Mesaverde Group, 

Multiwell Experiment Site, Piceance Basin, Colorado 

By Charles E. Barker 

Abstract 

Rock-Eval data indicate that rocks in the Multiwell Experiment boreholes have a moderate to high total organic carbon 

content, an indication of good source-rock richness. Rock-Eval 

and coal ultimate analysis data both suggest that the organic 

matter is mature to overmature. The slope of the Rock-Eval 

Tmax profile confirms the high thermal maturity gradient in the 

Mesaverde Group indicated by vitrinite reflectance studies. 

Although the rocks have a moderate to high total organic carbon content, the generally thermally overmature organic matter and its low hydrogen content indicate little remaining 

petroleum-generating potential. 

INTRODUCTION 

As part of the multidisciplinary studies of the 

Multiwell Experiment (MWX) boreholes in the Piceance 

basin of northwest Colorado (Law and Spencer, this 

volume, fig. 1), Rock-Eval analysis of selected core 

samples was contracted by Sandia Laboratories, Los 

Alamos, N. Mex., to Core Laboratories, Dallas, Tex. The 

coal samples were analyzed by the U.S. Geological 

Survey. These data were intended to screen the samples 

for subsequent geochemical or other analysis (Sattler and 

Lorenz, 1983) and were not interpreted. This report 

presents the geochemical data (appendix 1) and an interpretation of the results. 

Acknowledgments.-J .C. Lorenz, Sandia National 

Laboratories, sampled the MWX boreholes and arranged 

for the Rock-Eval analysis. C.W. Spencer and B.E. Law 

helped guide the investigation and provided funding 

through the U.S. Department of Energy. 

ROCK-EVAL PYROLYSIS 

Rock-Eval pyrolysis and measurement of total organic carbon (TOC) qualitatively evaluate organic-matter 

geochemistry and thermal maturity. Rock-Eval pyrolysis 

must be used with caution in rocks having low TOC and 

if organic contamination is present (Peters, 1986). In clayrich rocks containing humic organic matter, with less 

than 0.5 weight percent TOC, the hydrogen index (HI) 

is likely to be too low and the temperature of maximumhydrocarbon generation {Tmax) too high. (See Peters, 

1986, for a discussion of terminology.) 

Organic drilling mud additives can increase HI. 

Because MWX-1 and MWX-2 were drilled with an oilbase mud and MWX-3 with a potassium chloride 

polymer-base mud (Spencer, 1984), such contamination 

could be a problem. However, no samples contaminated 

by particulate mud additives were detected by examination under a binocular microscope. The possibility of 

contamination was further reduced before analysis by 

pulverizing the core chips, washing them in hot hydrochloric acid for 2 hrs to remove carbonate, and then rinsing them with distilled water. 

Rock-Eval pyrolysis also is influenced by migrated 

oil or bitumen. Migrated oil in a sample produces an S1 

peak greater than 2 mg hydrocarbon (HC)/g of rock, an 

anomalously high transformation ratio and low Tmax as 

compared to adjacent samples, and a bimodal S2 peak. 

The migration factor must be considered because 

pyrobitumen has been observed at the top of the Cozzette 

Sandstone Member of the Iles Formation at 2,370 m 

{7,830 ft) in MWX-2 (C.W. Spencer, in Sattler and 

Lorenz, 1983). Oil contamination is probably not a 

generally important factor because the pyrobitumen is a 

localized occurrence in the MWX boreholes. Only 10 of 

109 samples (9 percent) have a S1 peak greater than 2, 

and except for 2 of the 10 samples all seem to have 

transformation ratios and Tmax values that agree with 

those of adjacent samples. Four other samples have 

bimodal S2 peaks, but the corresponding S1 peaks are 

all less than 0.1 mg HC/g of rock. Some samples, 

especially those from the shallow part of the borehole, 

have relatively low Tmax values. These samples also 

generally have a low TOC. The low Tmax may be related 

to weak S2 peaks resulting from low TOC values and not 

to organic contamination. 

Rock-Eval Analysis of Sediments and Ultimate Analysis of Coal, Multiwell Site Nl 
RESULTS 

Rock-Eval pyrolysis of carbonaceous mudrock, 

coal, and sandstone core samples indicates that the organic matter is mature to overmature and has moderate 

to high transformation ratios and moderate to high TOC 

(appendix 1), and low residual hydrogen and oxygen contents (figs. 1, 2). The mature to overmature organic 

matter indicated by the Rock-Eval pyrolysis data is confirmed by ultimate analysis of five coal samples (appendix 2, fig. 3). 

Tmax data from Rock-Eval pyrolysis generally are 

consistent with the other organic geochemical data in that 

they indicate moderate to high thermal maturation (fig. 

4), but several Tmax values are anomalously low, as compared to the general trend of the data, and may result 

from contamination in these samples. If these anomalously low points are ignored, the trend of the clustered Tmax 

data indicates an increase in thermal maturation as depth 

increases. Computation of vitrinite reflectance equivalents 

(VRE) from Tmax (fig. 4) confirms the general level of 

rank shown in the Van Krevelen diagram (fig. 3) and a 

somewhat higher rank than indicated by the vitrinite 

reflectance (Rm) measurements (Nuccio and Johnson, 

this volume). The differences between Rm and VRE 

values are not considered significant because the calibration between Rm and Tmax is diffuse and can vary from 

basin to basin. The VRE-depth profile confirms the 

relatively high vitrinite reflectance gradient in the Mesaverde Group (Nuccio and Johnson, this volume). 

CONCLUSIONS 

The data presented here support the following 

conclusions: 

1. Rock-Eval data indicate that the organic matter in the 

MWX boreholes has moderate to high TOC, an indication of good source-rock richness. 

N2 Geology of Tight Gas Reservoirs, Wyoming and Colorado 

2. Rock-Eval Tmax and ultimate coal analysis data suggest that the organic matter is mature to overmature. 

3. The generally low hydrogen content of the organic 

matter infers that little petroleum-generating potential 

remains. 

4. The slope of the Rock-Eval Tmax profile when converted to VRE indicates a high thermal maturity gradient 

in the Mesaverde Group. 

REFERENCES CITED 

Jones, R.W., and Edison, T.A., 1978, Microscopic observations of kerogen related to geochemical parameters with 

emphasis on thermal maturation, in Oltz, D.F., ed., Low 

temperature metamorphism of kerogen and clay minerals: 

Society of Economic Paleontologists and Mineralogists, 

Pacific Section, p. 1-12. 

Peters, K.E., 1986, Guidelines for evaluating petroleum source 

rock using programmed pyrolysis: American Association 

of Petroleum Geologists Bulletin, v. 70, no. 3, p. 318-329. 

Spencer, C.W., 1984, Overview of U.S. Department of Energy 

Multiwell Experiment, Piceance Creek basin, Colorado, 

in Spencer, C.W., and Keighin, C.W., eds., Geologic 

studies in support of the U.S. Department of Energy 

Multi well Experiment, Garfield County, Colorado: U.S. 

Geological Survey Open-File Report 84-757, p. 1-14. 

Sattler, A.R., and Lorenz, J.C., 1983, Topical discussion of 

organic maturation at Multi well Experiment site (MWX): 

Sandia National Laboratories Distribution Memorandum, 

March 24, 1983. 

Figure 1 (facing page). Hydrogen index versus oxygen index 

from Rock-Eval pyrolysis, MWX-1, -2, and -3, Piceance basin, 

Colorado. Thermal maturation paths for type I, II, and Ill sedimentary organic matter from Peters (1986). Data provided by 

J.C. Lorenz (Sandia National Laboratories, written commun., 

1984). 
400r------,-------------------------------------------------------------. 0 

350 

1+11 

• 

300 

• 

250 

0 

X 

w 

Q 

z 0 

z 

w 200 

(.!) 

0 0 

0: EXPLANATION 

Q 

>- • 0 MWX-1 :::c 

• MWX-2 

X MWX-3 

150 • ~ Mudstone 

0 D Sandstone 

0 0 

(All others coal, MWX-3 only) 

0 

100 

0 • 

50 

0 

• Ill 

• 0 

0~----------~------------~------------L_ ____________ L_ __________ _J 

0 50 100 150 200 250 

OXYGEN INDEX 

Rock-Eval Analysis of Sediments and Ultimate Analysis of Coal, Multiwell Site N3 
1,000 

2,000 

3,000 

1-

w 4,000 

w 

LL. 

z 0 

:i 0 b: 0 

w 5,000 

0 0 0 oo 

1-

z Q:) 

w 

en 

w 

a: 6,000 a.. 

0 

0 0 

7,000 

& 

• 

• 0 

8,000 

100 200 300 400 9,000 0 

OXYGEN INDEX 

0 

0 

0 

0 0 

00 

"0 

A • 0 

• 

EXPLANATION 

o MWX-1 

• MWX-2 

X MWX-3 

b. Mudstone 

D Sandstone 

(All others coal, MWX-3 only) 

0 

0 

0 

0 

• 

• 

100 200 300 400 

HYDROGEN INDEX 

Figure 2. Hydrogen and oxygen indices versus depth, MWX-1, -2, and -3, Piceance basin, Colorado. More than one data 

point per sample is indicated by a number. 

N4 Geology of Tight Gas Reservoirs, Wyoming and Colorado 
Iw 

w 

LL 

z 

:i 

b: 

w 

0 

u 

"' I 

(.) 

1.5 

I 

Highly oil prone 

Figure 3. Van Krevelen diagram of coal ultimate analyses, 

MWX-1, Piceance basin, Colorado. Chemical analysis data in 

appendix 2. Mean vitrinite reflectance (Rm, in percent) shown 

for each sample. Thermal maturation paths (solid lines) and 

iso-vitrinite reflectance contours (dashed lines) from Jones and 

Edison (1978). 

·E 1.o 

Ill 0 

<i: 

4,000 

5,000 

6,000 

7,000 

Gas prone 

o.5 <l.o ................ 

0 

'-......, (1.9% Rm) 

0.1 

Atomic 0/C 

EXPLANATION 

Sample Depth (feet) 

• 5,433 

A 6,601 

• 6,647 

0 6,827 

b. 7,950 

0 0 0 

cs 

0 aBo 0 eo 

0 

0 0 

8 &. 

-~ 0 A&. .&. 0 

~~ 

Bf:>o 

lit.~ 0 

0.2 

Estimates 

Figure 4 (below). Rock-Eval Tmax, vitrinite reflectance (Rm, 

in percent) and vitrinite reflectance equivalent (VRE) versus 

depth, MWX-1, -2, and -3. Piceance basin, Colorado. Vitrinite 

reflectance profile from Nuccio and johnson (this volume). Conversion of Tmax to VRE by using a line drawn through the 

midrange of Peters' calibration (1986, fig. 14). 

-- 460°C at 4,300 ft 

-- 465°C at 4,900 ft 

--4 70°C at 5,800 ft 

-- 485°C at 6,700 ft 

Anomalous data ~ } 

Beyond calibration 

of Peters ( 1986) &. 

400 450 

Tmax, IN°C 

EXPLANATION 

0 

·~ ec~. 

500 550 0.8 1.0 10.0 

VITRINITE REFLECTANCE 

EQUIVALENT, IN% Rm 

• 

MWX-2 More than one data point per 

MWX-1 } 

sample shown by number X MWX-3 

b. Mudstone } 

0 Sandstone MWX-3 only 

All others coal 

Rock-Eval Analysis of Sediments and Ultimate Analysis of Coal, Multiwell Site NS 

APPENDIXES 1, 2 
Appendix 1. Rock-Eval pyrolysis data, MWX wells, Piceance basin, Colorado 

[Data provided by J.C. Lorenz, Sandia National Laboratories. Analyses performed on a Delsi Rock-Eval system. See Peters (1986) for 

discussion of terminology. Total organic carbon (TOC) analyses of sample splits performed in a Leco WR-12 carbon determinator. Hydrogen 

index (HI)= (S2!fOC)X100. Oxygen index (OI) = (S3{f0C)x100. Transformation ratio= Sl!(S1+S2). TOC, S1, S2, and S3 in mg/g; Tmax in 

°C. Asterisk(*) indicates Tmax value considered by analyst to be unreliable due to low concentration of S2. Leaders (--)indicate sample did not 

produce a complete pyrogram, and this parameter could not be computed] 

TransDepth Depth formation 

(meters) (feet) TOC 51 52 53 HI 01 ratio Tmax 

MWX-1 

1,280 4,198.0 0.38 0.02 0.05 0.10 13.2 26.3 0.29 437* 

1,283 4,210.0 0.33 0.01 0.04 0.08 12.1 24.2 0.20 379* 

1,298 4,260.0 0.37 0.01 0.05 0.12 13.5 32.4 0.17 387* 

1,322 4,336.0 0.68 0.01 0.25 0.08 36.8 11.8 0.04 458 

1,330 4,363.0 0.58 0.01 0.18 0.05 31.0 8.6 0.05 460 

1,331 4,368.5 0.57 0.01 0.18 0.08 31.6 14.0 0.05 460 

1,375 4,510.0 0.15 0.02 0.01 0.15 6.7 100.0 0.67 285* 

1,405 4,610.0 0.31 0.01 0.04 0.05 12.9 16.1 0.20 465* 

1,416 4,647.0 3.26 0.64 3.70 0.15 113.5 4.6 0.15 461 

1,417 4,647.5 21.88 4.67 41.93 0.75 191.6 3.4 0.10 461 

1,419 4,654.0 0.21 0.02 0.02 0.03 9.5 14.3 0.50 465* 

1,435 4,707.5 0.40 0.02 0.14 0.06 35.0 15.0 0.13 404 

1,436 4,710.0 0.41 0.03 0.99 0.10 17.1 24.4 0.30 470 

1,475 4,840.0 0.83 0.06 0.25 0.12 30.1 14.5 0.19 468 

1,480 4,856.0 0.32 0.01 0.06 0.10 18.8 31.3 0.14 462 

1,486 4,875.0 0.33 0.02 0.07 0.05 21.2 15.2 0.22 372* 

1,489 4,884.0 0.63 0.08 0.50 0.07 79.4 11.1 0.14 460 

1,555 5,101.0 0.14 0.01 0.01 0.05 7.1 35.7 0.50 469* 

1,597 5,240.0 0.34 0.35 0.72 0.06 211.8 17.6 0.33 317 

1,605 5,266.0 0.21 0.01 0.01 0.11 4.8 52.4 0.50 350* 

1,608 5,275.1 1.08 0.34 0.47 0.76 43.5 70.4 0.42 460 

1,608 5,277.0 0.39 0.07 0.13 0.06 33.3 15.4 0.35 399 

1,641 5,386.0 0.49 0.22 0.43 0.08 87.8 16.3 0.34 312 

1,698 5,570.5 0.43 0.04 0.14 0.09 32.6 20.9 0.22 472 

1,702 5,584.2 1.08 0.34 0.47 0.76 43.5 70.4 0.42 460 

1,702 5,585.0 0.41 0.03 0.10 0.06 24.4 14.6 0.23 471 

1,702 5,585.0 0.43 0.01 0.34 79.1 

1,736 5,697.0 0.25 0.01 0.01 0.10 4.0 40.0 0.50 359* 

1,751 5,744.0 0.88 0.09 0.34 0.08 38.6 9.1 0.21 471 

1,759 5,771.0 0.50 0.06 0.24 0.09 48.0 18.0 0.20 396 

1,815 5,953.5 0.86 0.27 0.32 0.33 37.2 38.4 0.46 471 

1,821 5,974.5 0.39 0.01 0.05 0.09 12.8 23.1 0.17 389* 

1,831 6,006.0 2.36 0.92 2.05 0.25 86.9 10.6 0.31 472 

1,841 6,041.0 1.58 0.38 0.46 0.56 29.1 35.4 0.45 478 

1,847 6,059.7 4.43 1.97 10.74 0.35 242.4 7.9 0.15 468 

1,847 6,060.0 5.21 1.49 6.99 0.60 134.2 11.5 0.18 472 

1,859 6,099.0 1.01 0.22 0.10 0.45 9.9 44.6 0.69 479 

1,870 6,134.0 0.78 0.09 0.37 47.4 

1,890 6,200.0 0.33 0.03 0.40 121.2 

1,899 6,231.0 7.93 1.71 10.39 0.33 131.0 4.2 0.14 477 

1,908 6,259.0 1.02 0.17 0.19 0.46 18.6 45.1 0.47 481 

1,919 6,294.7 23.76 13.15 93.86 2.85 395.0 12.0 0.12 476 

1,919 6,294.8 14.90 3.45 20.50 1.39 137.6 9.3 0.14 478 

1,928 6,324.0 1.56 0.28 0.30 0.69 19.2 44.2 0.48 489 

1,949 6,395.0 0.91 0.68 0.74 0.34 81.3 37.4 0.48 364 

N8 Geology of Tight Gas Reservoirs, Wyoming and Colorado 
Appendix 1. Continued 

TransDepth Depth formation 

(meters) (feet) TOC 51 52 53 HI 01 ratio Tmax 

1,950 6,398.0 0.60 0.29 1.87 311.7 

1,964 6,443.0 1.35 0.86 0.90 0.35 66.7 25.9 0.49 480 

1,974 6,478.0 0.42 0.36 0.52 123.8 

2,000 6,562.0 0.60 0.59 1.31 218.3 

2,025 6,645.0 7.89 2.20 7.33 0.57 92.9 7.2 0.23 486 

2,028 6,653.0 0.58 0.18 1.51 260.3 

2,407 7,898.0 0.93 0.09 0.21 22.6 

2,408 7,899.0 0.12 0.05 0.20 166.7 

2,415 7,923.5 0.77 0.29 0.20 26.0 

2,420 7,938.0 1.42 0.15 0.19 13.4 

2,423 7,948.0 0.98 0.17 0.28 28.6 

MWX-2 

1,739 5,707.0 0.45 0.06 0.29 64.4 

1,750 5,741.0 0.71 0.20 0.23 0.23 32.4 32.4 0.47 469 

1,761 5,776.0 0.90 0.16 0.41 0.21 45.6 23.3 0.28 475 

1,775 5,825.0 0.33 0.05 0.14 42.4 

1,784 5,854.0 0.51 0.06 0.20 39.2 

1,784 5,854.1 0.54 0.12 0.19 35.2 

1,948 6,390.0 0.35 0.22 0.06 0.41 17.1 117.1 0.79 354 

1,951 6,401.0 0.97 0.16 0.35 0.25 36.1 25.8 0.31 476 

1,953 6,408.0 4.32 1.03 2.90 0.38 67.1 8.8 0.26 477 

1,979 6,492.0 0.33 1.03 0.12 0.33 36.4 100.0 0.90 361 

2,003 6,570.0 5.59 1.88 9.98 0.46 178.5 8.2 0.16 480 

2,158 7,081.0 3.69 0.99 1.54 0.44 41.7 11.9 0.39 497 

2,164 7,100.1 21.55 14.91 58.27 1.30 270.4 6.0 0.20 489 

2,179 7,148.0 13.83 2.74 20.44 2.19 147.8 15.8 0.12 495 

2,203 7,228.0 8.24 3.95 7.74 1.49 93.9 18.1 0.34 503 

2,207 7,241.0 4.20 1.24 1.46 0.44 34.8 10.5 0.46 495 

2,222 7,290.0 12.27 6.77 39.62 1.98 322.9 16.1 0.15 498 

2,229 7,313.0 1.68 0.34 0.13 0.53 7.7 31.5 0.72 515 

2,385 7,825.0 0.81 0.13 0.31 38.3 

2,387 7,830.0 0.73 0.02 0.22 20.1 

2,407 7,898.0 0.97 0.10 0.29 29.9 

2,408 7,900.0 0.58 0.20 0.38 65.5 

2,409 7,905.0 0.62 0.40 0.20 32.3 

MWX-3 

1,773 5,817.2 0.33 0.09 0.06 0.08 18.2 24.2 0.60 436* 

1,782 5,847.5 0.77 0.24 0.22 0.15 28.6 19.5 0.52 479* 

1,783 5,849.9 0.24 0.05 0.03 0.11 12.5 45.8 0.63 423* 

1,787 5,863.2 0.39 0.04 0.05 0.08 12.8 20.5 0.44 412* 

1,960 6,431.5 2.10 0.37 1.30 0.15 61.9 7.1 0.22 476 

1,975 6,480.2 1.10 0.25 0.39 0.35 35.5 31.8 0.39 486 

1,736 5,694.0 0.21 0.02 0.02 0.04 9.5 19.0 0.50 437* 

1,976 6,483.2 0.93 0.38 0.37 0.42 39.8 45.2 0.51 485 

1,978 6,488.2 0.99 0.46 0.48 0.39 48.5 39.4 0.49 484 

1,979 6,493.2 0.73 0.38 0.36 0.25 49.3 34.2 0.51 485 

1,987 6,519.2 1.38 0.44 0.64 1.30 46.4 94.2 0.41 490 

1,987 6,520.5 3.40 0.79 2.83 0.16 83.2 4.7 0.22 473 

1,990 6,528.3 2.91 0.64 2.04 0.10 70.1 3.4 0.24 476 

2,098 6,883.2 8.09 1.23 4.62 0.20 57.1 2.5 0.21 488 

2,098 6,882.4 1.85 0.20 0.70 0.18 37.8 9.7 0.22 490 

Rock-Eval Analysis of Sediments and Ultimate Analysis of Coal, Multiwell Site N9 
Appendix 1. Continued 

TransDepth Depth formation 

(meters) (feet) TOC 51 52 53 HI 01 ratio Tmax 

2,100 6,890.5 1.04 0.28 0.42 0.16 40.4 15.4 0.40 488 

2,103 6,898.7 8.56 1.40 7.03 0.23 82.1 2.7 0.17 487 

2,104 6,904.2 14.36 3.10 13.21 0.32 92.0 2.2 0.19 488 

2,105 6,906.6 11.14 1.31 6.21 0.27 55.7 2.4 0.17 491 

2,155 7,071.2 12.12 1.41 7.78 0.18 64.2 1.5 0.15 494 

2,156 7,072.3 14.16 1.54 12.88 0.60 91.0 4.2 0.11 504 

2,157 7,077.3 1.16 0.09 0.23 0.11 19.8 9.5 0.28 506 

2,158 7,079.8 1.75 0.49 0.64 0.19 36.6 10.9 0.43 499 

2,159 7,084.6 1.13 0.42 0.44 0.23 38.9 20.4 0.49 495 

2,164 7,100.3 1.78 0.48 0.77 0.29 43.3 16.3 0.38 494 

2,166 7,105.5 59.26 3.40 55.58 0.47 93.8 0.8 0.06 490 

2,167 7,108.4 68.07 2.92 50.69 0.32 74.5 0.5 0.05 489 

2,167 7,110.5 6.77 1.35 3.09 0.06 45.6 0.9 0.30 498 

N10 Geology of Tight Gas Reservoirs, Wyoming and Colorado 
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Appendix 2. Coal analysis data, MWX-1, Piceance basin, Colorado 

[Rm, mean randon vitrinite reflectance (in percent), except where noted mx (maximum). Elemental analysis: H, C,N in weight percent (d.a.f.); 0 in percent (d.a.f.) by difference; Sin weight percent (as 

received; ash in percent (dry). Proximate analysis: moisture in percent (as received); volatile matter and fixed carbon in weight percent (d.a.f.); ash in percent (dry); heating value in BTU/pound. N.R., 

not reported; N.D., not determined. Data from Sattler and Lorenz (1983)] 

Proximate anal~sis 

DeEth Elemental anal~sis Atomic Volatile Fixed Heating 

(meters) (feet) Rm H c 0 N s Ash H/C 0/C Moisture matter carbon Ash value 

1,656 5,433 1.52mx 4.90 90.05 3.22 1.33 0.5 N.R. 0.65 0.27 0.88 23.47 76.53 11.90 15,779 

1.43 

2,012 6,601 N.D. 5.23 88.35 3.20 2.07 1.15 38.05 0.705 0.0272 0.83 24.25 75.75 38.05 15,374 

2,026 6,647 N.D. 5.21 88.96 2.32 2.00 1.51 16.22 0.698 0.0195 0.72 21.68 78.32 16.22 15,590 

2,081 6,827.2 1.50 4.62 89.55 2.55 1.91 1.17 8.66 0.615 0.0214 0.74 21.13 78.87 8.68 15,779 

2,423 7,950.1 2.07 4.19 88.92 0.50 1.51 4.87 17.68 0.561 0.00418 0.72 14.39 95.61 17.68 15,298 

Chapter 0 

Azimuthal Vertical Seismic Profiles at the 

Multiwell Experiment Site, Northwest Colorado 

By MYUNG W. LEE 

Prepared in cooperation with the U.S. Department of Energy 

U.S. GEOLOGICAL SURVEY BULLETIN 1886 

GEOLOGY OF TIGHT GAS RESERVOIRS IN THE PINEDALE ANTICLINE AREA, WYOMING, 

AND AT THE MULTIWELL EXPERIMENT SITE, COLORADO 
CONTENTS 

Abstract 01 

Introduction 01 

Data acquisition and processing 02 

Discussion 02 

Interpretation 06 

Identification of coastal sandstone reservoirs 06 

Geometry of coastal sandstone bodies 011 

Conclusions 013 

References cited 014 

FIGURES 

1. Map showing configuration of seismic study and topography of area, 

MWX site 01 

2. Flow chart showing processing steps for far-offset vertical seismic 

profile data 03 

3-4. Logs showing: 

3. Stacked vertical-component data for MWX-3 well 04 

4. Processed vertical seismic profile data for MWX-3 well 05 

5. Schematic diagram of ray path for truncated body 06 

6. Diagram showing spatial distribution of sandstone bodies and trace of 

gamma-ray logs for the coastal zone of MWX-1, -2, and -3 wells 07 

7. Relative impedance log in two-way travel time for MWX-3 well 08 

8. Synthetic seismograms generated by using impedance log and various 

zero-phase bandpass filters 09 

9. Diagrams showing identification of lower coastal zone at MWX-3 from 

source location 1 010 

10-11. Logs showing laterally stacked and cumulatively summed verticalcomponent data at MWX-3 from: 

10. Source location 1 011 

11. Source location 2 012 

12. Log showing laterally stacked and cumulatively summed, horizontally 

polarized shear wave component data at MWX-3 from source location 

2 013 

13-14. Logs showing laterally stacked and cumulatively summed verticalcomponent data at MWX-3 from: 

13. Source location 3 014 

14. Source location 4 015 

15. Log showing laterally stacked vertical-component data at MWX-3 from 

source location 2 016 

16. Schematic interpretation of geometry of lower coastal sandstone bodies 

in MWX-3 well 016 

TABLE 

1. Summary of vertical seismic profile data 03 
Azimuthal Vertical Seismic Profiles at the 

Multiwell Experiment Site, Northwest Colorado 

By Myung W. Lee 

Abstract 

An azimuthal vertical seismic profile (VSP) experiment was 

conducted at the U.S. Department of Energy Multiwell Experiment site to delineate lenticular, very low permeability (tight), 

gas-bearing sandstone bodies in the Mesaverde Group in the 

southern Piceance basin of northwest Colorado. Profiles of two 

wells were created by using a triaxial, three-component 

downhole geophone and a 450-in.3 surface airgun source at 

four different locations. The azimuthal VSP data were analyzed 

and interpreted by using the laterally stacked vertical component of the VSP data, one- and three-dimensional seismic 

modeling, and a geological interpretation of the area. 

Individual sandstone bodies in the coastal interval were 

difficult to identify or delineate because of the lack of highfrequency components in the seismic data. The lower coastal 

sandstone bodies could be mapped, however, by using the 

azimuthal VSP data. One lower coastal sandstone probably 

trends northeast and has an average width of BOO ft, and a second probably trends northwest and has an average width of 

600 ft. The results of this study are similar to those from a 

sedimentological study of the coastal interval and suggest that 

azimuthal VSP surveys may be used to detect and delineate 

bodies of finite extent such as reservoirs. 

INTRODUCTION 

Three wells were drilled at the U.S. Department of 

Energy Multiwell Experiment (MWX) site about 9 mi 

southwest of Rifle, Colo., and are designated MWX-1, 

-2, and -3 (fig. 1) (Law and Spencer, this volume, fig. 

1). The purpose of the experiment was to delineate both 

the character of sandstone lenses within the Upper 

Cretaceous Mesaverde Group in western Colorado and 

the geometry of significant tight-gas reservoirs in order 

to better analyze these unconventional gas resources in 

the Piceance basin. To this end, a comprehensive threephase seismic program was initiated to determine how 

well seismic studies can map these discontinuous sandstone bodies. The program included a high-resolution 

three-dimensional survey, a vertical seismic profile, and 

a hole-to-hole survey (Searls and others, 1983). My studies 

focus on the second phase of the seismic program, vertical seismic profiling. 

Figure 1. Configuration of seismic study and topography 

of area, MWX site, northwest Colorado. Solid circle indicates location of well; solid square indicates seismic study 

source location. Contour interval, 200ft. Source location 

1 is on drill pad of MWX-2. 

Vertical seismic profile (VSP) data were collected 

during two separate field operations. In May 1982 data 

were collected from two source locations using the 

MWX-1 and -2 wells (Lee, 1984a), and in April1984 

data were collected simultaneously from four different 

source locations using the MWX-2 and -3 wells. 

The primary objective of the 1982 field operation was to collect VSP data that could be correlated 

to three-dimensional high-resolution surface seismic 

data previously collected. The VSP and threedimensional data would then be used to delineate the 

lateral extent of the tight-gas sandstone reservoirs 

(Searls and others, 1983). Analysis of the surface 

seismic data indicated that the spatial distribution of 

the sandstone bodies is difficult to map because only 

low-frequency seismic events were recorded at the surface. Analysis of the VSP data indicated, however, 

some possibilities for delineating lenticular sandstone 

bodies. 

Azimuthal Vertical Seismic Profiles, Multiwell Site 01 
After an extensive feasibility study of the mapping 

of lenticular sandstone bodies by using VSP techniques 

(Lee, 1984b), an azimuthal VSP survey (one near-offset 

profile and three far-offset profiles) was conducted in 

April 1984 to determine the lateral extent of the coastal 

sandstone reservoirs. This paper presents an interpretation of this second survey. 

The quality of the VSP data is not as good as 

desired because of extremely bad weather conditions and 

time limitations. Although the lack of high-frequency 

reflections made it impossible to identify or delineate individual sandstone bodies in the coastal zone, two lower 

coastal sandstone lenses were identified. These lenses are 

most likely the Yellow and Red zones of J.C. Lorenz 

(Sandia National Laboratories, written commun., 1985; 

Sattler and others, 1986); that is, the VSP study delineated 

the top of Lorenz's Yellow A zone and the bottom of 

his Red B zone. The orientation of these sandstone bodies 

was determined by using laterally stacked VSP data, 

geologic information (J .C. Lorenz, written commun., 

1985), and three-dimensional seismic modeling. 

Acknowledgments.-My sincere appreciation is extended to the CER Corporation, Sandia National 

Laboratories, and U.S. Department of Energy for their 

support of this investigation. Particular appreciation is 

expressed to C.A. Searls and A.L. Sattler of Sandia National Laboratories, to J .J. Miller, J. Leinbach, 

N. Zihlman, D. Smith and H. Oliver of the U.S. 

Geological Survey for their assistance in the data collection under adverse field conditions, and especially to 

Keith Westhusing of the U.S. Department of Energy, 

Morgantown, W. Va., who originally suggested this 

research and provided encouragement and support 

throughout the project. 

DATA ACQUISITION AND PROCESSING 

VSP data at MWX-2 and -3 were digitally recorded 

on a MDS-1 0 seismic recording system using a sampling 

interval of 1 millisecond and a record length of 5 seconds. 

The seismic sources were two Bolt LSS-1 T land airguns, 

each capable of delivering 1,500 tons of force into the 

ground. The seismic signal was detected in each well 

simultaneously by using a triaxial three-component walllocking geophone (one vertical and two horizontal components, oriented orthogonally), designed and provided 

by Sandia National Laboratories. 

Data from each component were amplified in two 

stages (high and low gain). The output of each stage 

modulated a constant-bandwidth (4 kHz), voltagecontrolled oscillator. The eight modulated signals were 

frequency-division multiplexed and transmitted along a 

single conductor wire line in each well to the surface instruments where they were demodulated. 

02 Geology of Tight Gas Reservoirs, Wyoming and Colorado 

Four source locations were distributed around the 

two wells (fig. 1). Because only two airguns were available, each well was profiled twice and a sampling interval of 25 ft was used. In the initial source configuration, 

one source was located at source location 1 and another 

at source location 3. Each source was energized alternately, four times per geophone level. 

This experiment was planned originally for late 

autumn or early winter of 1983 in order to take advantage of optimum field conditions; that is, cool weather 

and dry ground. The experiment was delayed, however, 

until April 1984. During this time period, the ground was 

wet from spring snowmelt and runoff and the weather 

unpredictable. In addition, local ranchers had opened up 

their irrigation ditches a week previously, and the fields 

in which the airguns were located became muddy 

quagmires. 

Because of the weight and impact force of the 

airgun trucks, deep holes were dug into the muddy ground 

and the operators were forced to move the trucks as often 

as every 20 shots to avoid becoming stuck; on two occasions, a bulldozer was needed to free the trucks. Because 

the trucks were moved as far as 200 ft from their original 

locations, timing discrepancies and waveform variability resulting from the large number and distribution of 

source locations caused serious processing and interpretation problems. Source location 1, located on the drill pad 

of MWX-2 and reinforced with gravel, was the only 

source location not moved extensively. 

A flow sheet showing the processing steps for faroffset VSP data is shown in figure 2. Details of data acquisition and processing are in Lee and Miller (1985), and 

a summary of the VSP data is shown in table 1. 

DISCUSSION 

One method proposed to delineate the lenticular 

sandstone bodies is to analyze the variation in reflection 

amplitude with respect to lateral distance from a well by 

using laterally stacked VSP data (Lee, 1984b). In this 

technique, the amplitude, arrival time, frequency content 

of reflected events, and propagation modes all are 

considered. 

One of the biggest problems during data acquisition was the extremely soft ground, particularly at source 

locations 3 and 4, which necessitated frequently moving 

the surface airgun source. The raw stacked verticalcomponent VSP data from source location 4 is shown 

in figure 3A. While this profile was being made, the 

source was moved at least 24 times. These moves show 

on the record either as abrupt changes of first arrival 

times or as substantial changes in reverberation patterns. 

The abrupt changes in arrival times may result from 

gradual compaction of the near-surface medium during 
MERGE 

DOWN+ UP 

Figure 2. Processing steps for far-offset vertical seismic profile (VSP) data. 

Table 1. Summary of vertical seismic profile (VSP) data 

[Configuration of seismic study shown in figure 1) 

Source Reflection Dominant 

location Timing error amplitude frequency 

1 Small Reliable 35 

2 Small Reliable 35 

3 Some Probable 25 

4 May be substantial Probable 25 

Airgun 

Maximum source movement within source location used* 

Small 1 

200 ft, parallel with source-to-well azimuth 1 

200 ft, perpendicular to source-to-well azimuth 2 

200ft, in random directions 2 

*Airgun 1 performed without problems during fieldwork; airgun 2 malfunctioned and was not resynchronized. 

the shooting, changes in distance between the well and 

the individual source point, or a combination of the first 

two. As mentioned in Lee and Miller (1985), no reliable 

monitor records are available to correct or compensate 

for the above-mentioned source variations, and some 

errors in timing and amplitude exist for all processed VSP 

data, particularly those from source location 4. 

Figure 3B shows the raw stacked verticalcomponent data from source location 2. Arrival times at 

this location are quite different from those at source location 4. For example, at a well-seismometer depth of 

2,000 ft, the arrival time of the trough following onset 

is 447 milliseconds at source location 4 and 331 milliseconds at source location 2. Some part of this time difference may result from low-velocity surface material at 

source location 4. (This source location was completely 

water saturated.) Most of this large time difference cannot be explained, however, unless a constant time delay 

difference existed between the two airgun firing circuits, 

and such a delay was not noticed during the fieldwork. 

In addition to the time-uncertainty problem, the 

low-frequency content of the source signal limits interpretation of the VSP data. The low-frequency content 

of VSP data from source locations 3 and 4 most probably 

Azimuthal Vertical Seismic Profiles, Multiwell Site 03 
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f=::t - -

-==-~ __ -

===='=-=-

:-:.::::- ·c--_--

== -_cc 

_.:;: 

t:= ;._ 

~ :::::1:::=~ 

~t~--:: 'lll E=- -:- E 

I~ 

:C- ;, 

<~ '-= •::; 

~(1:: - ~-. t=, 

c~c~ ·• 

f' 

" 

- =ff- ~ -

- -'--

r:: ; 

=-- --= 

=~- -,"~-

=t~ ~ 

____ ---=-· -=- "::~~ 

1.0 

TIME. IN SECONDS 

Figure 3. Stacked vertical-component data for the MWX-3 well. A, Source location 4. B, Source location 2. 

results from poor source coupling to the ground. Deconvolution provided for adequate contraction of the complicated and slowly decaying reverberatory downgoing 

signal and broadening of the amplitude spectrum, and 

the usable frequency content could be extended as high 

as 75 Hz for source location 2 (Lee, 1985). Although 

deconvolution of data at source location 4 substantially 

improved the data, the overall frequency content is less 

than for source location 2. 

The foregoing discussion indicates that good 

ground coupling of the source in the field can substantially improve the overall quality of the VSP data; unfortunately, during this investigation less-than-desirable 

source location position was dictated by the soft ground 

that resulted from rain and snow. 

The far-offset VSP data show complicated wave 

interference patterns, particularly in the upper part of the 

section. Figure 4 shows an example of the processed VSP 

04 Geology of Tight Gas Reservoirs, Wyoming and Colorado 

data at MWX-3 from a source (location 2) about 3,000 ft 

northwest of the well. Figure 4A was processed to emphasize mode conversions at the acoustic boundaries, and 

figure 4B was processed to enhance converted upgoing 

shear waves. Figure 4 also shows that the surface airgun 

source generated substantial amounts of shear (S) waves 

(note complicated wave fields). Near the TertiaryCretaceous unconformity, at a depth of about 3,900 ft, 

converted, transmitted, and reflected S-waves can be seen 

in addition to transmitted and reflected longitudial (P) 

waves. The mode conversions at the boundary affect not 

only processing but also interpretation. 

Shear waves on the VSP profiles provide both 

advantages and disadvantages for interpretation. 

The advantages should be fully utilized, and most of the 

disadvantages can be handled by careful processing. 

One advantage of analyzing converted vertically polarized 

shear (SV) waves is shown in figure 5. Figure 5A shows 
2,000 

fw 

w 

L.L 

~ 7,000 

I 

f- 2,000 Cl.. 

w 

0 

7,000 

1.0 

TIME. IN SECONDS 

Figure 4. Processed vertical seismic profile data for the MWX-3 well from source location 2. A, Data processed in order to analyze mode conversions at the acoustic boundary. 8, Data processed in order to analyze converted upgoing shear (5V) wave. 

Azimuthal Vertical Seismic Profiles, Multiwell Site 05 
a schematic ray-path diagram for a truncated body. The 

downgoing P-wave is reflected at the edge of the body 

and propagates as both a P-wave and a converted SVwave. The specular reflection ray path of the P-wave is 

recorded as the reflected event at a well-seismometer 

depth of ZP' and SV-wave at Zs. The depth of Zs is 

shallower than ZP by Snell's law. 

The kinematics of the different arrival times are 

shown schematically in figure 5B. If the edge of the sandstone body is identified at the well-recording depth ZP 

by P-wave analysis and the converted SV-wave extends 

vertically greater than the P-wave event (in the direction 

of the shallow well-phone depth}, then the edge of the 

truncated body can be interpreted with higher reliability. Identification of converted SV-waves at the acoustic 

boundary may also help reduce the incidence of selecting 

erroneous reflected events. 

Analysis of far-offset VSP data confirms the 

presence of converted SV-waves at the acoustic boundaries of interest. The extension of SV-wave events beyond 

P-wave events is not confirmed, partly because the SVwaves were attenuated higher than the P-waves and partly 

because interference patterns in the upper section are very 

complicated. 

Horizontally polarized shear (SH) waves are also 

useful in interpreting VSP data, and the reliability of interpreting SH-wave information derived from good VSP 

data shot by a surface source is documented by Lee 

(1984b). In this study, SH-wave analysis was not used, 

A 

SOURCE WELL 

Zp 

except at source location 2, because of the low signal-tonoise ratio of SH-waves in this azimuthal VSP survey. 

In order to estimate the frequency needed to delineate the lower coastal sandstone bodies, a one-dimensional 

model was studied. The lower coastal interval below 

6,420 ft in the MWX-3 well contains five sandstone 

lenses; they have a two-way time thickness of about 20 

milliseconds. 

The model study indicates that unambiguous identification of individual sandstone reservoirs can be made 

if the dominant frequency is about 200 Hz. In addition, 

the synthetic seismogram with the extracted source 

wavelet is very similar to that of the 30-Hz Ricker wavelet. 

This similarity implies that the dominant frequency of 

the observed VSP data is about 30Hz, well below that 

needed to resolve individual sandstone bodies (Lee, 1985). 

INTERPRETATION 

Identification of Coastal Sandstone Reservoirs 

Identification of the coastal sandstone lenses by 

using the seismic sections is difficult because of the lack 

of impedance contrast between intervening shales, the 

thinness of the lenses, and the lack of high-frequency content. Figure 6 shows the spatial distribution of individual 

sandstone bodies near the well sites, as determined by 

8 

.._.. TIME 

_L ________ _ 

J 

:I: 

1-

~ 

w 

c 

Figure 5. Schematic diagram of ray path for a truncated body (patterned area). A, Ray path for converted shear (5V) 

(dashed line) and longitudinal (P) (solid line) waves. ZP and Z5 are well-seismometer depths of P and SV waves, respectively. 8, Arrival time for reflected P- and 5V-waves. 

06 Geology of Tight Gas Reservoirs, Wyoming and Colorado 
1--

w 

w 

LL 

z 

:i 

h:: 

w 

0 

6.000 

6.100 

6.200 

6,300 

6.400 

6.500 

0 

MWX-1 

100 200 0 

MWX-2 MWX-3 

100 200 0 100 200 

Figure 6. Spatial distribution of sandstone bodies (stippled pattern) and trace of gamma-ray logs 

for the coastal zone of the MWX-1, -2, and -3 wells. Modified from J.C. Lorenz (written commun., 

1984) and Sattler and others (1986). 

J.C. Lorenz (written commun., 1984; Sattler and others, 

1986). 

Results of a previous VSP study by Lee (1984a) indicate that seismic responses of the upper coastal sandstones above a depth of 6,400 ft are too weak to interpret 

because of the thinness of the lenses, and data from the 

present study confirm this conclusion. My interpretation 

focuses, therefore, on the lower coastal sandstone bodies, 

the Yellow and Red zones of J.C. Lorenz (1984, written 

commun.). 

Azimuthal Vertical Seismic Profiles, Multiwell Site 07 
Figure 7 shows the relative impedance log, in two-way 

travel time, derived from the sonic and density logs at the 

MWX-3 well between depths of 6,000 and 7,000 ft. From 

this impedance log, synthetic seismograms were generated 

by using various zero-phase bandpass filters (fig. 8). 

The lowermost panel of figure 8 shows the seismic 

response obtained by using a 2/ 4-250/300-Hz wavelet 

bandpass filter. The top of the Yellow A sandstone appears as a strong peak, the top of the Red A as a small 

peak, and the bottom of Red B as a small trough. If the 

frequency content is lowered, the character of the seismic 

response changes dramatically. The second panel from the 

bottom of figure 8 shows the seismic response using 

21 4-72/100-Hz wavelet bandpass filter; most of the seismic energy observed in the high-frequency section has 

disappeared and overall amplitudes are much lower than 

those in the higher frequency version. The third panel 

from the bottom shows the same response plotted using 

a different gain, and the top panel of figure 8 shows the 

response obtained by using a 4/8-52/62-Hz wavelet bandpass filter. 

If the frequency content is lowered, the base of the 

Red B sandstone is the strongest trough in the seismogram 

and the peak that represents the top of the Yellow A sandstone in the high-frequency version is shifted to a later 

time. The overall seismic response in the frequency range 

of 4/8-52/62 Hz represents only the complicated interference peaks and troughs, and all of the lower coastal 

sandstone bodies are within 3/4 of the dominant period. 

The rather obvious question is how much do the 

sandstone lenses in the lower coastal interval contribute 

to the overall seismic character shown in the top panel 

of figure 8. In an attempt to answer this question, the 

lower part of the near-offset VSP data (source location 

1) was reprocessed very carefully. An inversion method 

that includes three depth intervals was used to extract 

upgoing waves in order to minimize spatial mixing of the 

upgoing waves during multichannel velocity filtering. 

The downgoing wave and its amplitude spectrum 

at a well-seismometer depth of 6,000 ft are shown in figure 

9A; the upgoing wave field was shifted in order to align 

coherent events between depths of 5,925 and 7,000 ft (fig. 

9B). The peak-trough combination shown in the top panel 

of figure 8 can be seen in the processed VSP data, and 

the overall seismic character near the coastal sandstone 

lenses is very similar to that derived from the impedance 

log. 

In the VSP section (fig. 9), the trough corresponding to the base of the Red B sandstone timewise appears 

to start at about 6,550 ft, the depth of the base of the 

Red B sandstone. The preceding peak appears to start at 

about 6,425 ft, the depth of the top of the Yellow A sandstone. The sharpness of the downgoing wave at 6,000 ft 

(fig. 9A) suggests that the preceding peak is not likely the 

result of the side lobe of the strong trough. 

08 Geology of Tight Gas Reservoirs, Wyoming and Colorado 

RELATIVE IMPEDANCE 

4 8 12 

0 ----------~=---------~ 

(/) 

0 

z 50 

0 

u 

w 

(/) 

::i 

__J 

~ 

z 

ul 

~ 

i= 

>- <( 

s I 

0 

~ 100 

150 

Figure 7. Relative impedance log in two-way 

travel time for the MWX-3 well between depths 

of 6,000 and 7,000 ft. 

Based on the above analyses and observations, I 

conclude that the peak amplitude within the lower coastal 

interval is caused by the presence of the Yellow A sandstone and that the following trough represents the base 

of the Red B sandstone. I am unable to estimate the contribution of individual sandstone bodies in the Yellow 

(Yellow A, B, and C, fig. 6) and Red (Red A and B, fig. 

6) zones to the overall seismic response, and I believe that 

the peak in the lower coastal interval represents the average 
4/8-52/62 Hz 

2/4-7 2/1 00 Hz 

I 

I 

I 

II 

I 

I 

I• 1: I I 

I I 

~ I 

I I ..... 10 I I 

MILLISECONDS I I 

I 

I I 

II I 

II I 

II I 

I I 

I I 

2/4-7 2/1 00 Hz 

2/4-2 50/300 Hz 

Bottom of Red B 

Top of Yellow A 

Top of Red A 

Figure 8. Synthetic seismograms generated by using impedance log shown in figure 7 and 

various zero-phase bandpass filters. 

seismic response from all Yell ow sandstone bodies and 

the trough represents the average response of all Red 

sandstone bodies. This peak-trough combination can be 

seen throughout the study. 

In an attempt to identify the seismic character of 

the lower coastal interval at other source locations, downgoing and upgoing waves between depths of 6,000 and 

6,250 ft were vertically summed to improve signal-tonoise ratios, and various bandpass filters were applied. 

The same peak-trough combination is at all source locations at the correct time, and differences in the peaktrough amplitude between source locations are possibly 

Azimuthal Vertical Seismic Profiles, Multiwell Site 09 
w 

A 

100 

60 

§ 20 

f-

::::::i 0 CL ~ '\}I ~ ~ ~ ./\,./' 

~ -20 <t: 

-60 

-100 

40 

8 

I 

50 60 70 80 90 

TIME, IN MILLISECONDS 

~ -24 ~--~----~--~----~--~ (l] 

u 

w 

0 -36 

-48 

-60 

0 25 50 75 100 125 

FREQUENCY, IN HERTZ 

.. 100 MILLISECONDS 

6.425 FT 

fw 

w 

LL 

z 

::i 

f- 6.425 CL 

w 

0 6,550 

Figure 9. Identification of the lower coastal zone at MWX-3 from source location 1. A, Downgoing wave and its amplitude spectrum at well-seismometer depth of 6,000 ft. 8, Vertically aligned 

upgoing wave field shifted to align coherent events between depths of 5,925 and 7,000 ft by using inversion method. 

caused by the spatial distribution of the lower coastal 

sandstone reservoirs around the well. 

Although the peak-trough combination is the result 

of sandstone reservoirs near the well, it would be very 

interesting and helpful to analyze other possible acoustic 

impedance distributions that might result in similar peaktrough combinations farther away from the well. A synthetic seismogram generated by replacing the lower 

coastal impedance by a constant relative impedance of 

8 (corresponding to the average relative impedance of 

shale within the lower coastal zone) fits the observed 

peak-trough combination quite well. This good fit implies 

010 Geology of Tight Gas Reservoirs, Wyoming and Colorado 

that if a sandstone near the well site pinches out progressively away from the well into shale, it would be very 

difficult to detect truncation of the sandstone, even 

though the amplitude response of shale is slightly less than 

that of sandstone. 

Different spatial distributions of sandstones and 

shales might provide some indication of the sandstone 

distribution, but the small amplitude variation may be 

quite difficult to analyze in real vertical seismic data. The 

possibility of a facies change into shale should be retained, therefore, in the interpretation of the sandstone 

bodies. 
A 

---===1== 

0 

Iw 

w 

LL 

z 

_j 

__J 

w 

s 

~ 

0 

a: Base of coastal zone LL 

w 

u 

z 

<t: 8 I-

(/) 105- 0 

! 

Figure 10. Laterally stacked and cumulatively summed vertical-component data at MWX-3 from source location 

1. A, Normal polarity. 8, Reverse polarity. 

Geometry of Coastal Sandstone Bodies 

The spatial distribution of the lower coastal sandstones was interpreted by using laterally stacked vertical 

seismic profiles (figs. 10-14), the geologic interpretation 

of J.C. Lorenz (written commun., 1984, 1985), well logs, 

one- and three-dimensional seismic modeling, and other 

available vertical seismic profiles. The following procedure was used. 

1. Identification of peak-trough combination for the 

lower coastal interval. 

2. Examination of continuity and character of peaktrough combination for laterally stacked data and other 

available vertical seismic profiles. 

Azimuthal Vertical Seismic Profiles, Multiwell Site 011 
tiJ 

w 

LL 

~ 

_j 

_.J 

w 

s 

:::2: 

0 

a: 

LL 

w 

u 

z 

(f) 

~ 

0 

A B 

t Base of coastal zone t 

Figure 11. Laterally stacked and cumulatively summed vertical-component data at MWX-3 from source location 2. A, Normal 

polarity. 8, Reverse polarity. 

3. Interpretation of the geometry of the Yell ow and Red 

sandstone bodies by using amplitude variation near their 

edges and available geologic information. 

4. Creation of three-dimensional seismic model based on 

results of step 3, and comparison of model results and 

real data for all four source locations. In the threedimensional modeling, a lenticular sandstone body is 

modeled as a rectangular body (Lee, 1985). 

5. Adjustment of model parameters as necessary to fit 

real data. 

The peak-trough combination that represents the 

seismic response of the lower coastal interval was 

identified for all source locations by using vertically 

summed traces. Confirmation of the same peak-trough 

combination in laterally stacked data should increase the 

reliability of both data processing and interpretation. 

Thus, the laterally stacked VSP data about 800 ft from 

MWX-3 in the direction of source location 2 were replaced by the vertically summed traces from source location 1 (fig. 15). This procedure is very similar to the 

conventional technique of inserting vertical seismic data 

012 Geology of Tight Gas Reservoirs, Wyoming and Colorado 

into surface seismic data in order to make accurate 

stratigraphic interpretations' (Balch and others, 1982). The 

remarkable match of the key stratigraphic horizons between the two data sets should rule out any ambiguity 

in identification of the lowei coastal interval, and the 

gross error in the processing should be small. 

The northwest edge of the base of the Red sandstone zone was located primarily by using the interference pattern and amplitude reduction of the laterally 

stacked SH-wave from source location 2 (fig. 12) and is 

about 400ft from the MWX-3 well. A similar seismic 

character can be observed in the vertical-component data 

(fig. 11). The northeast and southeast edges of the Red 

sandstone zone were located by using the amplitude 

reduction of the base of coastal reflection and are about 

600ft from the well (figs. 13, 14). The length of the sandstone body cannot be estimated directly because the VSP 

is not offset to the southwest but was estimated by adjusting model parameters (step 5 of interpretation procedure). 

The location and geometry of the Yellow zone were 

more difficult to determine, possibly because of the zone's 
1.0 

TIME, IN SECONDS 

2.0 3.0 

fw 

w 

LL 

z 

_J 

__J 

w 

s 

2 

0 

a: 

LL 

w 

u 

z 

(J) 

~ 

0 

1,200 

0 

Figure 12. Laterally stacked and cumulatively summed, horizontally polarized shear 

wave (SH) component data at MWX-3 from source location 2. 

low reflection amplitude as compared to that from the 

base of the Red sandstone body and because of interferences with the upper coastal sandstone bodies. By 

using the continuity of the peak reflection (fig. 11), the 

northwest edge of the Yellow zone was interpreted to be 

beyond the maximum lateral distance investigated by the 

azimuthal survey; that is, more than 1,200 ft from the 

well. The southeast edge of the Yellow zone was located 

by using an amplitude reduction (fig. 13) and is about 

250 ft from the well. 

By performing interpretative steps 4 and 5 and taking into consideration the above observations, I interpreted the shape and location of the lower coastal 

sandstone bodies (fig. 16). My interpretation is similar 

to that of J.C. Lorenz (written commun., 1985), which 

is based on the sedimentology of the sandstones. 

The similar amplitude variation between the threedimensionally modeled and real near-offset VSP data (fig. 

10) suggests that estimates for the average widths of the 

Yellow and Red sandstone bodies are reasonable. Comparison of real and synthetic data for source location 2 

indicates a similar amplitude character of trough reflections. For source location 3, the three-dimensional model 

does not indicate any amplitude reduction within 

600 ft of the lateral distance, whereas the actual VSP data 

do. 

In addition, numerous discrepancies were observed between the real and synthetic data. The synthetic model may be too simple to explain the observed 

data, or the observed data may be contaminated by 

noise. 

CONCLUSIONS 

Detection of the edges of the lower coastal sandstone bodies at the MWX site by using an azimuthal VSP 

survey was difficult because: 

1. The quality of VSP data acquired under adverse field 

conditions was substantially degraded. 

2. The high-frequency seismic data required to map 

individual sandstone bodies could not be attained 

even by using very modern technology because of the 

lack of impedance contrast between sandstone and 

shale and because of the small size of the sandstone 

bodies. 

Azimuthal Vertical Seismic Profiles, Multiwell Site 013 
tw 

UJ 

u._ 

z 

J 

_J 

UJ 

s 

~ 

0 

a: 

u._ 

UJ 

u 

z 

(f) 

~ 

0 

A 

1.000 

0 

B 

--

;:: 

::--:-: 

= 

::--: 

= ~ 

Base of coastal zone 

.. :~:= _--

- =-=~='-" -- -- r--- ---

-= ~ 

~ :: 

~ 

::::;: ::::: ":. 

~ 

::_ 

• 

-"" ~ ~ 

~~ 

~ 

- -~== 

-

-~· ;:::) 

-~ 

I~V~-- ----

- r--- -

-1------ 1---,.---- --

-- . ----+---- . - ·: 

Figure 13. Laterally stacked and cumulatively summed vertical-component data at MWX-3 from source location 3. A, Normal 

polarity. 8, Reverse polarity. 

Although some uncertainty in the interpretation exists, the following conclusions can be derived by using 

the results of the survey at the MWX-3 well. 

1. The lower sandstone bodies (Yellow and Red zones) 

were identified with a high level of confidence for all 

source locations. 

2. The Yellow sandstone zone may extend more than 

1 ,200 ft to the northwest of the well and have an average 

width of 600 ft. The truncation edge of the zone may be 

about 400 ft southeast of the well site. 

3. The Red sandstone zone probably is oriented northeast, has an average width of 800 ft, and may be truncated about 500 ft from the MWX-3 well site in the 

direction of source location 3. The position of the 

other edge of the Red sandstone zone could not be estimated directly by using the VSP data because there 

is no source location opposite to source location 3. 

A three-dimensional model indicates that the zone may 

extend more than 2,000 ft to the southwest of the MWX 

site. 

014 Geology of Tight Gas Reservoirs, Wyoming and Colorado 

4. The extent of the upper coastal sandstone bodies 

cannot be resolved because of their low and discontinuous amplitude responses and because they are 

smaller or thinner than the lower coastal sandstone 

bodies. 

5. The general orientation of the Yell ow and Red zones, 

as determined by the azimuthal VSP survey, is similar 

to that determined by a sedimentological study of these 

zones. 

6. Azimuthal VSP techniques may be applicable for the 

delineation of sandstone reservoirs. 

REFERENCES CITED 

Balch, A.H., Lee, M.W., Miller, J.J., and Ryder, R.T., 1982, 

The use of vertical seismic profiles in seismic investigations of the Earth: Geophysics, v. 47, no. 6, p. 906-918. 

Lee, M.W., 1984a, Vertical seismic profiles at the Multiwell Experiment site, Garfield County, Colorado: U.S. Geological 

Survey Open-File Report 84-168, 57 p. 
t:u 

Ll.J 

lL 

z 

_J 

_j 

Ll.J 

s 

2 

0 

a: 

lL 

Ll.J 

u 

z 

<( 

fUJ 

0 

A 8 

Figure 14. Laterally stacked and cumulatively summed vertical-component data at MWX---3 from source location 4. A, Reverse 

polarity. 8, Normal polarity. 

___ 1984b, Delineation of lenticular-type sand bodies by the 

vertical seismic profiling method: U.S. Geological Survey 

Open-File Report 84---265, 92 p. 

_ _ _ 1985, Interpretation of azimuthal vertical seismic profile survey at Multi-well Experimental site, Garfield County, Colorado: U.S. Geological Survey Open-File Report 

85---428, 44 p. 

Lee, M.W., and Miller, J.J., 1985, Acquisition and processing 

of azimuthal vertical seismic profiles at Multi-well Experiment site, Garfield County, Colorado: U.S. Geological 

Survey Open-File Report 85---427, 36 p. 

Sattler, A.R., Hudson, P.J., Raible, C.J., Gall, B.L., and 

Maloney, D.R., 1986, Laboratory studies for the design 

and analysis of hydraulic fractured stimulations in lenticular tight gas reservoirs: Society of Petroleum Engineers 

Unconventional Gas Technology Symposium, Louisville, 

Ky. , 1983, Proceedings, p. 437---447. 

Searls, C.A., Lee, M.W., Miller, J.J., Albright, J.N., Fried, 

Jonathan, and Applegate, J.K., 1983, A coordinated 

seismic study of the Multi-well Experiment site: Society 

of Petroleum Engineers/Department of Energy Symposium on Low Permeability Gas Reservoirs, Denver, 

Colo., 1983, Proceedings, p. 115---120. 

Azimuthal Vertical Seismic Profiles, Multiwell Site 015 
016 

f--

w 

w 

l.L 

~ 800 _j' 

_J 

w 

s 

2 

0 

a: 600 l.L 

w 

u 

z 

<( 

f-

(f) 

0 

CD Cii (j) 

CD (;) ""0 c -:;: CD .2 -~ 

:::J 0 CD 

u:: u 0.... 2 

Figure 15. Laterally stacked vertical-component data at MWX---3 from source location 

2. Laterally stacked data at a distance of BOO ft from the MWX-3 well was replaced by 

cumulatively summed data from source location 1. 

0 1.000 FEET 

Source 

location 3 

Geology of Tight Gas Reservoirs, Wyoming and Colorado 

Figure 16. Schematic interpretation of the geometry of the 

lower coastal sandstone bodies in the MWX-3 well. 

tr U.S. GOVERNMENT PRINTING OFFICE: 1989---773-614 REGION NO.8