Chapter 5

Sonic Velocity and Other Petrophysical 

Properties of Source Rocks of Cody, 

Mowry, Shell Creek, and Thermopolis 

Shales, Bighorn Basin, Wyoming

By Philip H. Nelson

Chapter 5 of 

Petroleum Systems and Geologic Assessment of Oil and Gas in the 

Bighorn Basin Province, Wyoming and Montana

By U.S. Geological Survey Bighorn Basin Province Assessment Team

U.S. Geological Survey Digital Data Series DDS–69–V

U.S. Department of the Interior

U.S. Geological Survey

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Volume Title Page
U.S. Department of the Interior

KEN SALAZAR, Secretary

U.S. Geological Survey

Marcia K. McNutt, Director

U.S. Geological Survey, Reston, Virginia: 2010

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Suggested citation:

Nelson, P.H., 2010, Sonic velocity and other petrophysical properties of source rocks of Cody, Mowry, Shell Creek, and 

Thermopolis Shales, Bighorn Basin, Wyoming: U.S. Geological Survey Digital Data Series DDS–69–V, 39 p.

ISBN: 1–4113–2667–5
iii

Contents

Abstract...........................................................................................................................................................1

Introduction.....................................................................................................................................................1

Geologic Setting.............................................................................................................................................2

Definition of Shale Units ......................................................................................................................2

Mineralogy.............................................................................................................................................7

Total Organic Carbon............................................................................................................................7

Gas Shows and Overpressure............................................................................................................7

Modeling of Gas Generation.............................................................................................................10

Shale Units and Shale Trendlines Within An Interrupted Compaction Trend....................................10

Compaction Trends.............................................................................................................................10

Selection of Four Shale Units ...........................................................................................................16

Selection of End Points for Shale Trendlines .................................................................................16

Pressure Transition Zone at Base of Cretaceous..........................................................................16

Characteristics of Shale Trendlines.................................................................................................19

Petrophysical Properties of Four Shale Units.........................................................................................19

Discussion.....................................................................................................................................................24

Summary........................................................................................................................................................31

Acknowledgments.......................................................................................................................................33

References Cited..........................................................................................................................................33

Appendix 1.....................................................................................................................................................35

Figures

1. Bighorn Basin, showing locations of wells used in this study..............................................3

2. Stratigraphic chart of the Bighorn Basin, Wyoming and Montana......................................5

3. Pressure-depth ratio as a function of depth............................................................................8

4. Burial history plot for the base of Thermopolis Shale in the Tatman Mountain

Federal well .................................................................................................................................11

5. Vitrinite reflectance, computed from one-dimensional burial history models.................11

 6. Gamma-ray, resistivity, and sonic slowness logs in four wells ..........................................12

7. Well logs from the lower 5,000 feet of the Tatman Mountain Federal well.......................17

8. Expanded well logs from the lower 1,000 feet of the Tatman Mountain Federal well.....18

 9. Sonic slowness as a function of depth in 23 wells ...............................................................20

10. Sonic velocities in four shale units with shale trendlines ...................................................21

11. Borehole enlargement in four shale units as a function of depth......................................21

12. Resistivity for four shale units as a function of depth ..........................................................23

13. Density for the Mowry Shale unit as a function of depth ....................................................23

14. Sonic velocity plotted against resistivity, density, neutron porosity, and 

gamma-ray activity.....................................................................................................................25

15. Neutron and density values from Mowry shale unit.............................................................26

16. Box and whiskers plots of petrophysical properties from well logs..................................27

17. Parameters affecting sonic velocity........................................................................................28
iv

18. Trendline velocity minus shale unit velocity as a function of depth ..................................30

19. Evolution of pore pressure, effective stress, and sonic velocity in the 

Thermopolis shale unit, Tatman Mountain Federal well ......................................................32

Tables

1. Location and other information for 23 wells examined for petrophysical responses, 

Bighorn Basin, Wyoming.............................................................................................................4

2. Lithologic descriptions for Mowry, Shell Creek, Thermopolis Shales, and the 

Muddy Sandstone.........................................................................................................................6

3. Mineralogy of shale formations from immature outcrop samples along 

margin of basin..............................................................................................................................8

4. Weight percent of total organic carbon from samples from the Bighorn Basin, 

from various sources. ..................................................................................................................8

5. Pressure-depth ratios (PDR, in psi/ft) in wells in the Bighorn Basin, ordered 

by formation and by depth...........................................................................................................9

6. Lithologic description and depositional environment of Morrison, Cloverly, 

and Sykes Mountain Formations..............................................................................................19

7. Average values of sonic velocity as a function of borehole enlargement, 

as determined by the difference between caliper and bit size, for three 

shale units....................................................................................................................................22
Abstract

The petrophysical properties of four shales are 

documented from well-log responses in 23 wells in the 

Bighorn Basin in Wyoming. Depths of the shales examined 

range from 4,771 to 20,594 ft. The four shales are the 

Thermopolis (T) Shale, the Shell Creek (SC) Shale, the 

Mowry (M) Shale, and the lower part of the Cody (C) Shale, 

all of Cretaceous age. These four shales lie within a 4,000-

ft, moderately overpressured, gas-rich vertical interval in 

which the sonic velocity of most rocks is less than that of 

an interpolated trendline that reflects a normal increase of 

velocity with depth. Sonic velocity, resistivity, neutron, 

caliper, and gamma-ray values were determined from well 

logs at discrete levels in each of the four shales in 23 wells.

Sonic velocity in all four shales increases with depth to 

a present-day depth of about 10,000 ft; below this depth sonic 

velocity remains relatively unchanged. Velocity (V), resistivity 

(R), neutron porosity (N), and hole diameter (D) from caliper 

logs in the four shales vary, such that VM > VC > VSC > VT, RM

> RC > RSC > RT, NT > NSC ≈ NC > NM, and DT > DC ≈ DSC > 

DM. These orderings can be partially understood on the basis 

of rock descriptions. The Mowry Shale is highly siliceous 

and by inference comparatively low in clay content, resulting 

in high sonic velocity, high resistivity, low neutron porosity, 

and minimal borehole enlargement. The Thermopolis Shale, 

by contrast, is a black fissile shale with very little silt; high 

clay content causes low velocity, low resistivity, high neutron 

response, and results in the greatest borehole enlargement. 

The properties of the Shell Creek and lower Cody Shales are 

intermediate to the Mowry and Thermopolis Shales.

The sonic velocities of all four shales are less than that of 

an interpolated trendline that is anchored in shales above and 

below the interval of moderate overpressure. The reduction in 

velocity varies among the four shales, such that the amount 

of offset (O) from the trendline is OT > OSC > OC > OM, that 

is, the velocity in the Mowry Shale is reduced the least and 

the velocity in the Thermopolis Shale is reduced the most. 

Velocity reductions are attributed to increases in pore pressure 

during burial, caused by the generation and retention of gas. A 

model combining concepts and constraints from burial history, 

pore pressure increase, and sonic velocity-effective stress 

relations shows how present-day reduced sonic velocities 

could have developed during burial as a consequence of 

loading and pore pressure changes.

Introduction

Shales with organic carbon content in excess of one 

weight percent, when subjected to sufficient temperatures, are 

recognized as important sources of oil and gas in sedimentary 

basins. But the petrophysical properties of shales vary considerably as a result of variations in mineralogy, stress, thermal 

maturity, and even the generation of hydrocarbons. Attempts 

to use petrophysical properties to infer a desired parameter, 

such as pore pressure or thermal maturation, must first take 

into account the properties of individual shales. Because well 

logs are generally used to obtain an estimate of recoverable 

hydrocarbons in pore space, much of the petrophysical literature deals with the properties of reservoir rocks, not source 

rocks, although this disparity is narrowing as some shales are 

now proving to be prolific reservoirs as well as source rocks. 

Petrophysical properties of shales are controlled by effective stress and rock composition; both factors are important, 

and both receive attention in this study. As effective stress 

increases, rocks compact, and the effects of compaction can be 

observed in well logs as a general increase of sonic velocity 

and density with depth. Japsen and others (2007) reviewed 

the petrophysical models that link sonic velocity, porosity, 

and depth in shales and sandstones; normal pore pressure is 

assumed. Where pore pressure has been elevated above normal hydrostatic pressure during burial history, effective stress 

is reduced and sonic velocity is less than that expected from 

a normal compaction trend. In some geologic settings, the 

resulting reductions in sonic velocity can be used to estimate 

pore pressure (Bowers, 1994). 

Shales are fine-grained rocks with silt-sized and claysized components. Mineralogically, shales consist of varying 

amounts of quartz, feldspar, carbonate minerals, clay minerals, 

and organic matter. Size mix and mineralogical composition 

Sonic Velocity and Other Petrophysical Properties 

of Source Rocks of Cody, Mowry, Shell Creek, and 

Thermopolis Shales, Bighorn Basin, Wyoming

By Philip H. Nelson
2 Sonic Velocity and Other Petrophysical Properties of Source Rocks, Bighorn Basin, Wyoming

are interdependent (an increase in clay mineral fraction 

results in an increase in clay-size material and a complementary decrease in silt-sized material) and both determine the 

petrophysical properties. For example, increases in the volume 

fraction of clay minerals, organic matter, or porosity generally produce decreases in sonic velocity and density but cause 

increases in neutron porosity. Gamma-ray activity is commonly used to infer the fraction of the clay-sized component, 

and other petrophysical techniques have evolved to infer shale 

properties from well-log data (Katahara, 2008). MacQuaker 

and Jones (2002) examined the connection between petrophysical properties and mudstone characteristics in Upper Jurassic 

strata in the North Sea, relating the neutron-density response 

to mudstone composition and grain-size trends. Such studies 

show that mudstone facies form trends on well-log crossplots, 

but shale mineralogy or grain-size mix cannot be uniquely 

resolved from a suite of conventional well-log responses.

The systematic decrease in sonic velocity associated 

with the occurrence of gas and overpressure has been noted 

by Surdam and others (1997) in several basins in the Rocky 

Mountain area, including the Bighorn Basin. The cause of this 

decrease is currently a topic of some debate in the literature. 

Surdam and others (1997) attributed it mainly to the presence 

of gas. In a study of sonic logs in strata of comparable age in 

the Greater Green River Basin, Gutierrez and others (2006) 

reported that neither pressure nor the presence of gas could 

account for the effect on velocity, and attributed the velocity decrease primarily to lithology. This paper addresses the 

question of the anomalous responses in sonic velocity and 

other petrophysical properties by (1) summarizing previous 

work on the mineralogy and organic content of shales, gas 

shows, and overpressure in the Bighorn Basin; (2) illustrating 

the suppressed sonic velocity and resistivity logs associated 

with overpressure and the presence of gas in Cretaceous rocks; 

(3) documenting the petrophysical responses—sonic velocity, electrical resistivity, density, neutron porosity, gamma-ray 

activity, and borehole enlargement—of four shales in the 

Bighorn Basin that are considered to be source rocks; and 

(4) illustrating how reduced sonic velocity can be maintained 

during burial and uplift. Primary outcomes of this work are 

(1) each of the four shales examined here differs from the 

others in its petrophysical expression and these differences 

are attributed to differences in composition, and (2) sonic 

velocity and resistivity in the four shales are less than their 

expected value based on examination of depth trends. These 

reductions in sonic velocity and resistivity are believed to be 

the result of overpressure development and the cessation of 

increased effective stress during burial and maturation. As a 

consequence, the present-day petrophysical response of an 

individual shale results from the combined effects of lithology 

and effective stress history.

Wells selected for this study are distributed across the 

southern two-thirds of the Bighorn Basin (fig. 1). Petrophysical properties were drawn from the 23 wells listed in table 1. 

All 23 wells had satisfactory sonic velocity and gammaray logs; however, in some wells, the neutron, density, and 

gamma-ray logs were not available or were of inadequate quality. The four shales studied in this paper were penetrated by 22 

of 23 wells, with one well penetrating two of the four shales. 

Geologic Setting

The following descriptions of stratigraphy, mineralogy, 

organic carbon content, and gas shows and overpressure provide a context for understanding the petrophysical responses 

of the four shales discussed in this report. The structure of the 

Bighorn Basin, which is illustrated at the stratigraphic level of 

interest to us by contours on top of the Cloverly Formation (fig. 

1), is discussed by Finn and others (Chapter 3, this CD-ROM), 

as are the depositional setting, stratigraphy, and other topics relevant to the assessment of the hydrocarbon potential of 

Cretaceous and Tertiary rocks. Most of the 23 wells used in this 

study (fig. 1) are incorporated into stratigraphic cross sections 

by Finn (Chapter 6, this CD-ROM) that display well-to-well 

stratigraphic correlations, hydrocarbon shows, and production 

throughout the basin. 

Definition of Shale Units

The sonic velocity trendline, which extends from the 

Claggett Member of the Cody Shale to the upper part of the 

Cloverly Formation, encompasses the strata of anomalous 

log character. The four shale formations under study are the 

Thermopolis, Shell Creek, Mowry, and Cody Shales. An entire 

formation is generally comprised of shale, siltstone, and sandstone, so it is not desirable to determine petrophysical properties over the entire interval of a formation. Instead, well logs 

were sampled at discrete points within each shale formation, 

which were characterized by high gamma-ray values. These 

locations are referred to as shale units in this report and are 

selected to be representative of their respective shale formations. For example, the Mowry shale unit refers to the short 

interval within the Mowry Shale from which values of sonic 

velocity and other logs were determined. The approximate 

stratigraphic position of four shale units are shown in figure 2, 

and are the:

1. lower part of the Cody Shale, above a marker referred 

 to as the “chalk kick,”

2. Mowry Shale, siliceous unit, sometimes referred to as 

 the upper part of the Mowry Shale,

3. Shell Creek Shale as designated by Eicher (1962), 

 sometimes referred to as the lower part of the Mowry 

 Shale, and

4. the upper shale of the Thermopolis Shale from Eicher 

 (1962).

The latter three of these four shale units are within shale 

formations that are summarized in table 2. Pertinent to our 
Geologic Setting 3

0

4000

6000

5000

3000

-7000

2000 1000

-8000

-9000

-6000

-1000

-10000

-2000 -3000

-11000

-4000

-5000

-12000 -13000

-14000

7000

8000

-150

00

-16000

9000

-6000

-7000

-5000

6000

3000

6000

3000

-2000

5000

4000

4000

4000

-4000

2000

-13000

6000

0

0

-8000

-5000

4000

0

-8000

1000

-6000

-13000

-6000

0

0 -4000

6000

-7000

-110

10 00

00

-8000

2000

-3000

-7000

3000

-7000

-4000

-6000

-2000

5000

2000

-1000

2000

50

00

6000

4000

-8000

4000

4000

6000

3000

-2000

6000

70

00

2000

-3000

3000

-9000

3000

1000

-11000

0

0

7000

-6000

4000

-12000

4000

2000

-2000

30

00

3000

-3000

-3000

4000

-1000

4000

4000

3000

-8000

5000

-7000

-10000

3000

0

1000

-4000

-6000

-9000

2000

4000

-2000

2000

4000

0

-4000

-10000

1000

8000

1000

-5000

3000

5000

-5000

-7000

6000

2000

4000

1000

5000

-9000

6000

-3000

-12000

1

24

25 2 3

4 5

6 26

27

7 8

9

28

10

11

12

14

17

13

15 16

29 19

20

21 22

23

18

Wyoming

Montana

T54N

R89W

T46N

R103W

Bighorn Basin Province

Wells with sonic logs

Structure contour, top of 

 Cloverly Formation

Wells with pressure data

EXPLANATION

Fault

0

0 3.5 7 14 21 28 MILES

5 10 20 30 40 KILOMETERS

44°N

108°W

Figure 1. Map of the Bighorn Basin, showing locations of wells used in this study. Wells with sonic logs are listed 

in table 1 and wells with pressure data are listed in table 5. Structural contours on top of Cloverly Formation from 

Roberts and others (2008).
4 Sonic Velocity and Other Petrophysical Properties of Source Rocks, Bighorn Basin, Wyoming

Table 1. Location and other information for 23 wells examined for petrophysical responses, Bighorn Basin, Wyoming. First column 

(No.) is used to identify wells on map in figure 1. All wells penetrated all four shales examined in this report, except the Red Point II 

Unit well, which did not penetrate the Shell Creek and Thermopolis Shales.

[TD, total depth; API, American Petroleum Institute]

No. Operator Lease Well no. Section Township Range TD (ft) API number

1 Excel Energy Corp State 56-16 16 57N 100W 9,570 49029211050000

2 Amoco Production Co. McCullough II 3-27 27 54N 100W 13,800 49029209000000

3 Kewanee Oil Co. Gulch 1 16 54N 96W 12,718 49003201820000

4 True Oil Co. Spear-Federal 33-31 31 54N 95W 13,380 49003203640000

5 Amoco Production Co. Bridger Trail Unit 2-A 2 53N 98W 17,060 49029208550000

6 Texas Pacific Oil Red Point II Unit 1 30 53N 98W 18,500 49029205320000

7 Hunt/Impel Katrine Loch 1 2 51N 100W 23,990 49029210090000

8 Forest Oil Corp. Emblem Bench 1 2 51N 98W 18,636 49029210640000

9 Eastern Minerals Michaels Ranch 17-15 17 51N 96W 16,536 49003203790001

10 Amoco Production Co. Adams 1 17 51N 94W 11,895 49003205970000

11 Gulf Oil Fritz-Federal 1-16 2B 16 50N 95W 13,496 49003205720000

12 Gulf Oil Predicament 1-31-3D 31 50N 94W 14,569 49003205790000

13 Panther Exploration Inc. Bar TL 2 20 49N 100W 7,325 49029207300000

14 Santa Fe Energy Corp. Tatman Mtn Federal 1-20 20 49N 96W 16,145 49003205740000

15 Gulf Oil Buffalo Creek 1 32 48N 99W 8,526 49029206010000

16 American Quasar Petr. Sellers Draw Unit 1 21 48N 98W 23,081 49029204890000

17 American Quasar Petr. Ridge Unit 1-21 21 48N 97W 18,286 49043203800000

18 Union Oil of California Worland Unit 47T-M28 28 48N 92W 10,569 49043202210000

19 BP America Gillies Draw Unit 1 28 47N 96W 17,333 49043200930000

20 Midwest Oil Corp. Gywnn Ranch 1 29 46N 97W 9,854 49017060990000

21 Oil Development of Texas Blue Mesa 1-30 30 46N 96W 10,530 49043202500000

22 Equity Oil Co. Federal 15-22 1 22 46N 95W 11,948 49043202390000

23 Tipperary Oil and Gas Co. Federal 1-2 2 46N 94W 13,245 49043204110000
Geologic Setting 5

TERTIARY (part)

PALEOCENE

CRETACEOUS

UPPER CRETACEOUS LOWER CRETACEOUS EOCENE (part)

“rusty beds” “rusty beds” “rusty beds”

Shell Creek Shale Shell Creek Shale Shell Creek Shale

Thermopolis Shale Thermopolis Shale Thermopolis Shale

 CloverlyFormation

 CloverlyFormation

 CloverlyFormation

Greybull Ss. Mbr. Greybull Ss. Mbr. Greybull Ss. Mbr.

Mowry Shale Mowry Shale Mowry Shale

Muddy Sandstone Muddy Sandstone Muddy Sandstone

Cody Shale Cody Shale Cody Shale

Judith River

 Formation

Teapot Ss. Mbr. Teapot Ss. Mbr.

Pryor Conglomerate

Mbr.

Pryor Conglomerate

Mbr.

Pryor Conglomerate

Mbr.

Montana Wyoming

Parkman Sandstone

Virgelle Ss. Mbr.

Eagle Sandstone

Claggett Shale Claggett Mbr.

Lower member

Mesaverde

Formation

Mesaverde Formation

Fort Union

Formation

Fort Union

Formation

Lewis Sh.

Meeteetse

Fm.

Hell Creek

Formation Lance

Formation

Morrison Formation Morrison Formation

Wasatch Formation

Willwood Formation

Lennep Sandstone

Bearpaw Shale Bearpaw Shale

Northern Bighorn Basin Northern Bighorn Basin Southern Bighorn Basin

Willwood Formation

Lewis Sh.

Frontier Formation Frontier Formation Frontier Formation Mesaverde Formation

Mesaverde

Formation

Lower member

Claggett Mbr.

Tatman Formation

Morrison Formation

Fort Union

Formation

Lance

Formation

Meeteetse Formation

Telegraph Creek Fm.

"chalk kick" "chalk kick" "chalk kick"

System Series

?

?

JURASSIC

(part)

Shale 

trendline

Four shale

units examined

in this report

Figure 2. Stratigraphic chart of the Bighorn Basin, Wyoming and Montana, modified from Finn 

(Chapter 6, this CD-ROM). Black dots and horizontal dotted lines show approximate positions of 

four shale units where petrophysical values are determined. Shale trendlines are anchored in the 

Clagget Member of the Cody Shale and the “rusty beds” of the Cloverly Formation.
6 Sonic Velocity and Other Petrophysical Properties of Source Rocks, Bighorn Basin, Wyoming

Table 2. Lithologic descriptions for Mowry, Shell Creek, Thermopolis Shales, and the Muddy Sandstone from Eicher (1962). 

Petrophysical data analyzed in this report are taken from the Mowry Shale, the Shell Creek Shale, and the upper shale of the 

Thermopolis Shale.

Rock unit nomenclature of 

Eicher (1962) Thickness Lithologic description Depositional environment

Mowry Shale (upper part of 

Mowry according to Keefer 

and others, 1998). 

200 to 300 ft Consists mainly of dark shale with 

occasional interbeds of bentonite...shales are extremely hard ... 

because they are composed of a 

very high proportion of uncombined silica. Unusually resistant.

Shell Creek Shale (lower part of 

Mowry according to Keefer 

and others, 1998).

200 to 300 ft Dark gray and black shale containing 

a few ironstone beds ...and a few 

prominent white bentonite beds...

some of the shale beds are silty

Continuous deposition in 

deepening water.

Muddy Sandstone Generally 30 to 50 ft Highly variable unit of siltstone, 

sandstone, shale, and bentonite 

beds.

A variety of shallow water 

environments.

Upper shale of Thermopolis 

Shale.

110 ft in north, 75 ft in south Fissile black shale which contains 

very little silt. Contains some 

ironstone beds and one or two 

very thin bentonite beds in upper 

20 ft.

Deep-water deposition, depleted 

faunas.

Middle silty shale of Thermopolis Shale. About 30 ft Similar to the rusty beds...gray 

and tan silty shale, thin siltstone 

and silty limestone beds. More 

resistant to erosion than the shales 

above and below.

Possibly shallower water.

Lower shale of Thermopolis 

Shale.

About 90 ft Fissile, commonly silty shale that 

generally contains a few very thin 

sandstone, ironstone, or sandy 

limestone beds.

In deeper water than the rusty 

beds.

Rusty beds of Thermopolis Shale 

(Cloverly in USGS nomenclature).

About 120 ft Interlaminated and very thinly 

interbedded dark shales and gray 

siltstones...contain much carbonaceous material...also generally 

contain some brown shale and a 

few thin beds of ironstone.

Brackish marine water with 

active currents.
Geologic Setting 7

study are the descriptions of the siliceous upper Mowry Shale 

as being unusually resistant (ridge forming), of the Shell Creek 

Shale as a dark gray and black shale, and of the upper shale of 

the Thermopolis Shale as a fissile black shale with little silt. 

In this paper, the term “Shell Creek Shale” will be used rather 

than “lower part of the Mowry Shale.”

Mineralogy

Studies of shale properties by Ferrell (1962) and Davis 

(1967, 1970) were based upon x-ray diffraction analysis of 

outcrop samples. The usefulness of these results to the present 

study is reduced by the fact that outcrops at the periphery of 

the basin have never been buried deeper than approximately 

5,000 ft, and cannot be expected to represent the mineral 

assemblages of rocks that had been buried to greater depths. 

In particular, the conversion of montmorillonite to illite is 

unlikely to be represented in outcrop samples. 

Ferrell (1962) sampled the Thermopolis Shale, Muddy 

Sandstone, and Shell Creek Shale at three sites on the east 

flank of the Bighorn Basin. Approximately 105 samples 

from these sites were analyzed with semi-quantitative x-ray 

diffraction, meaning that clay minerals could be estimated as 

a fraction of total clays but not as a fraction of whole-rock 

mineralogy. As the author’s purpose was to interpret the 

paleoenvironments on the basis of the clay mineralogy, scant 

attention was paid to the non-clay minerals except to report 

that “quartz was by far the most abundant non-clay mineral 

in the samples” and “feldspar and gypsum were present in 

lesser quantities”. X-ray diffraction results for the relative 

abundances of clays were presented graphically (fig. 4 of 

Ferrell, 1962), and are summarized in table 3. Ferrell stated 

that most of the montmorillinite appeared to be derived from 

volcanic ash. 

Davis (1967, 1970) presented the results of x-ray 

diffraction analyses in tabular and map form, based upon 

samples from 29 outcrop sites throughout Wyoming, seven 

of which were on the periphery of the Bighorn Basin. He 

noted that the dominant facies was a siliceous shale that could 

be characterized as organic-rich siliceous argillite and silty 

argillite. His study appears to exclude the Shell Creek Shale, 

although he does divide the Mowry into upper and lower units, 

without specifying the criteria for the division. The results of 

Davis (1970) provide a general description of the mineralogy 

of the Mowry Shale with a single comment on the Shell Creek 

Shale (table 3). Davis (1967, p. 25) provided a petrologic 

summary “representative of this (siliceous) facies along the 

eastern margin of the Bighorn Basin: fine groundmass, 40–45 

percent; organic material, 20–25 percent; quartz silt, 30–35 

percent; radiolarian tests, <5 percent; and feldspar(?) silt, <5 

percent.” Regarding this analysis, he notes that “much excess 

silica that characterizes this facies and produces its distinctive 

hardness must be clay-sized, because the amount of quartz silt 

observed petrographically seems typical of ordinary marine 

shales”. Davis (1967, p. 85) also stated that “siliceous Mowry 

Shale is permeated with clay-sized quartz which in part is a 

cement between detrital clay and silt particles.”

Total Organic Carbon

Finn and others (Chapter 3, this CD-ROM) examined 

the available data for total organic carbon. Most of the total 

organic carbon values for the Mowry and Thermopolis Shales 

fall between one and two weight percent (fig. 19 of Finn 

and others, Chapter 3, this CD-ROM), with averages of 1.36 

and 1.23 weight percent, respectively, as shown in table 4. 

Some samples listed as Thermopolis in table 4 are likely to 

be from the lower Mowry or Shell Creek Shale of Eicher 

(1962). Rock-Eval data indicate that the gas- or oil-generating 

potential of these two shales, overall, is generally fair to good, 

as summarized in table 4. The volume percentage of kerogen 

can be computed from weight percent of total organic carbon 

by assuming values for the densities of kerogen and total rockdry fraction (Vernik and Landis, 1996); the volume percent 

kerogen is roughly three times the weight-percent organic 

carbon. Consequently, the average kerogen content is around 

four volume percent, with typical high and low values of 

around six and three volume percent, respectively. 

Gas Shows and Overpressure

Gas shows in mud returns, gas and oil recoveries in drillstem tests, and modest overpressure have been reported in a 

number of wells in the center of the Bighorn Basin by Johnson 

and Finn (1998), who cited these findings as evidence for a 

basin-centered gas accumulation. Gas shows and production 

are also posted on stratigraphic cross sections (Finn, Chapter 

6, this CD-ROM). Gas shows are observed in many wells 

throughout the basin in the Frontier Formation as well as in 

the Muddy Sandstone, Greybull Sandstone Member of the 

Cloverly Formation, Cody Shale, and Mowry Shale. Johnson 

and Finn (1998) outlined an overpressured area of approximately five townships at the base of the Mesaverde Formation 

and an area of approximately 21 townships in the Frontier 

Formation. No deep drilling has occurred since their review, 

so no further compilation regarding the distribution of gas and 

pressure is offered here. Most indications of overpressure are 

noted in the Cody Shale, Frontier Formation, Mowry Shale, 

Muddy Sandstone, and Cloverly Formation (figs. 2, 3). These 

formations are also the locus of decreases in sonic velocity 

that are the subject of this investigation.

Indicators of present-day pressures are summarized as 

the ratio of pressure to depth in units of psi/ft (table 5, fig. 1, 

fig. 3). Fresh water creates a pressure-depth ratio of 0.433 

psi/ft, which is referred to as the hydrostatic gradient. The 

pressure-depth ratio obtained from drillstem tests is less than 

0.4 psi/ft in the Fort Union and Lance Formations, ranges from 

0.30 to 0.53 in the Meeteetse and Mesaverde Formations, and, 

except for outliers of 0.37 and 0.62, ranges from 0.42 to 
8 Sonic Velocity and Other Petrophysical Properties of Source Rocks, Bighorn Basin, Wyoming

Table 3. Mineralogy of shale formations from immature outcrop samples along margin of basin. X-ray diffraction results from Davis 

(1970) and Ferrell (1962) were semi-quantitative, so clay mineralogy was not quantified as a fraction of whole-rock composition.

Formation Mineralogy Reference

Cody Shale No analyses No reference

Mowry Shale

Quartz content exceeds 50 percent in the Bighorn Basin. Feldspar averages 

around 5 percent. Quantitative estimates of clay and zeolite minerals could 

not be obtained. In general, mixed-layer clay is the primary clay constituent 

in the Mowry Shale; montmorillonite and kaolinite are also present. Illite is a 

minor component. Zeolites are present, not surprising as the Mowry contains 

tuffaceous units.

Davis, 1970

Shell Creek Shale High in montmorillonite. Mixed layer clays are present. Kaolinite and illite 

absent or present in small amounts. Ferrell, 1962

Shell Creek Shale Montmorillonite is dominant in the Shell Creek Shale. Davis, 1970, p. 494.

Upper part of Thermopolis Shale High in montmorillonite. Kaolinite and mixed layer clays also present. Illite 

absent or present in small amounts. Ferrell, 1962

Table 4. Weight percent of total organic carbon from samples from the Bighorn Basin, compiled by Finn and others (Chapter 3, 

this CD-ROM) from various sources.

Formation Range Average Generating

potential

Gas versus 

oil

Cody Shale 0.12–5.86 ≈1.0 Fair, with excellent potential 

locally Mainly a gas-prone source rock

Mowry Shale 0.08–3.6 1.36 Fair to good Potential to generate oil and gas

Thermopolis Shale 0.37–1.91 1.23 Fair to good Predominantly gas prone, but has generated oil.

Figure 3. Pressure-depth ratio as a function of depth, 

from drillstem tests and other sources in the Bighorn 

Basin. Green symbols designate strata—Cody Shale, 

Frontier Formation, Mowry Shale and Muddy Sandstone, 

and Cloverly Formation—where low sonic velocities are 

commonly observed. Vertical red line shows hydrostatic 

pressure-depth ratio of 0.433 psi/ft. Values for this graph 

are taken from table 5.

EXPLANATION

PRESSURE–DEPTH RATIO (psi/ft)

DEPTH, IN FEET

0

5,000

10,000

15,000

20,000

Fort Union and Lance Formations .

Meeteetse Formation

Mesaverde Formation

Cody Shale

Frontier Formation

Mowry Shale and Muddy Sandstone

Cloverly Formation

Phosphoria Formation

0.2 0.3 0.4 0.5 0.6 0.7
Geologic Setting 9

Table 5. Pressure-depth ratios (PDR, in psi/ft) in wells in the Bighorn Basin, ordered by formation and by depth.Two drillstem 

tests from the Frontier Formation in CIG McCulloch Peak 1 well were characterized as “near normal” and a value of 0.44 psi/ft 

was assigned to that interval in this table.

[PDR, pressure-depth ratio; psi, pounds per square inch; DST, drillstem test; wgt, weight; ppg, pounds per gallon; avg, average; JF, data from Johnson 

and Finn (1998); TP, data from this paper based on IHS Energy (2007)]

No. Lease Formation

Average

depth

(ft)

Method PDR

(psi/ft) Source

2 McCullough Peak 3-27 Fort Union 

Fm.(?) 5,639 DST 0.38 TP

26 Stone Barn 14-25 Fort Union Fm. 9,954 DST 0.37 JF

5 Bridger Trail Unit 2-A Lance Fm. 11,231 DST 0.37 JF

26 Stone Barn 14-25 Meeteetse Fm. 13,877 DST 0.47 JF

6 Red Point Unit 11-1 Meeteetse Fm. 14,354 DST 0.53 JF

12 Predicament Mesaverde Fm. 7,850 DST 0.37 TP

12 Predicament Mesaverde Fm. 8,046 DST 0.30 TP

17 Ridge 1-21 Mesaverde Fm. 13,712 DST 0.47 JF

27 Gilmore Hill 1-10 Mesaverde Fm. 13,822 DST 0.39 JF

17 Ridge 1-21 Mesaverde Fm. 14,393 DST 0.44 JF

28 Michael 1-16-4D Cody (upper) 11,659 DST 0.43 JF

14 Tatman Mountain Federal Cody Shale 13,307 DST 0.49 TP

6 Red Point Unit 11-1 Cody (upper) 15,500 Mud wgt 12 ppg 0.62 JF

23 Federal 1-2 Frontier Fm. 10,243 DST avg 4 tests 0.46 TP

12 Predicament Frontier Fm. 11,600 DST 0.42 TP

9 Michaels Ranch 20-1 Frontier Fm. 13,614 DST 0.37 JF

25 CIG McCulloch Peak 1 Frontier Fm. 14,210 DST “near normal” 0.44 JF

24 McCulloch Peak 2-9 Frontier Fm. 14,700 DST 0.48 JF

11 Fritz Federal Mowry Shale 12,590 DST 0.49 TP

4 Spear Federal Muddy Sandstone 11,168 DST 0.51 TP

2 McCullough Peak 3-27 Muddy Sandstone 13,129 DST 0.47 TP

16 Sellers Draw Muddy Sandstone 19,595 Tubing pressure 0.48 JF

23 Federal 1-2 Cloverly Fm. 11,289 DST 0.44 TP

25 CIG McCulloch Peak 1 Cloverly Fm. 15,254 DST 0.56 JF

29 Tenneco 1-29 Federal Cloverly Fm. 16,347 DST 0.46 JF

4 Spear Federal Phosphoria Fm. 12,991 DST 0.38 TP
10 Sonic Velocity and Other Petrophysical Properties of Source Rocks, Bighorn Basin, Wyoming

0.56 psi/ft in the Cody Shale, Frontier Formation, Mowry 

Shale, Muddy Sandstone, and Cloverly Formation. In summary, normal pressure to moderate overpressure is encountered in rocks between the Cody Shale and Cloverly Formation (fig. 3). Together with other considerations, the gas shows 

and pressure data form the basis for defining the MuddyFrontier Sandstone and Mowry Fractured Shale Continuous 

Gas Assessment Unit (fig. 31 of Finn and others, Chapter 3, 

this CD-ROM). Half of the 23 wells considered in this report 

lie within this assessment unit.

Modeling of Gas Generation

The generation of oil and gas can alter the stress state and 

impact the petrophysical response in source rocks, especially 

the sonic velocity (Vernik and Landis, 1996). Generation of 

gas and oil was modeled for eight wells in the Bighorn Basin 

(Roberts and others, 2008); the Tatman Mountain Federal well 

(location 14 on fig. 1), which is intermediate in terms of burial 

depths and maximum level of thermal maturity, is used here as 

a basis for discussion of oil and gas generation in the basin. A 

summary of oil and gas generation in the Thermopolis Shale 

in the Tatman Mountain Federal well is shown in figure 4, 

superposed on the burial history. Gas generation is modeled 

as beginning at 62 Ma, when the Thermopolis Shale was 

buried to a depth of 10,000 ft, and ending at 10 Ma, as uplift 

commences. As indicated in figure 4, oil generation (from 

Type-II kerogen) commences after the commencement of gas 

generation (from Type-III kerogen) and peak oil generation 

occurs soon after peak gas generation. Rapid burial commences around 70 Ma with deposition of the Lance Formation, followed by deposition of the Fort Union and Willwood 

Formations. The model on which figure 4 is based matches the 

vitrinite reflectance data in the Tatman Mountain Federal well 

(fig. 4 of Roberts and others, 2008); measurements show that 

vitrinite reflectance (Ro) in the Thermopolis Shale is around 

1.75 percent, well past the criterion of 0.8 percent that is 

assumed for peak gas generation from Type-III kerogen. 

The Thermopolis and Cody Shales are reported to be predominantly gas-prone and the Mowry Shale to have potential 

to generate both oil and gas (Finn and others, Chapter 3, this 

CD-ROM), so the plot of Ro versus depth for Type-III organic 

matter (fig. 5) pertains to all four shale formations considered in this report. Modeling of hydrocarbon generation from 

Type-II organic matter, which pertains to the Mowry Shale, 

and to a lesser extent the Thermopolis Shale, does not account 

for gas generation directly from kerogen. Thermal maturation was sufficient to generate oil in both the Thermopolis and 

Mowry Shales, but was insufficient to crack that oil to gas in 

the Tatman Mountain Federal well (table 5 of Roberts and others, 2008). The relation between thermal maturity as indicated 

by modeled Ro and depth shows that in most wells an Ro of 

0.5 percent is reached at a burial depth of 10,000 to 12,000 ft 

and an Ro of 0.8 percent is reached at burial depths greater 

than 15,000 ft (fig. 5). The right-hand depth scale in figure 

5 accounts for approximately 5,000 ft of uplift and shows 

the equivalent present-day depths for which gas generation 

commenced and reached a maximum. The plot indicates that 

substantial amounts of gas were generated in rocks that are 

presently buried 10,000 ft in the center of the basin. 

Shale Units and Shale Trendlines 

Within An Interrupted Compaction 

Trend

Compaction Trends

As rocks are buried, porosity decreases due to the effects 

of mechanical compaction and chemical alteration. The 

decrease in porosity causes gradual decreases in the sonic 

slowness1 and, to a lesser extent, increases in the resistivity 

logs that are manifest as rightward shifts in the logs in the 

upper 10,000 feet of figures 6A–D. The compaction trends are 

interrupted by leftward excursions of resistivity and sonic logs 

in the Cody, Mowry, and Thermopolis Shales; the diminished 

values of resistivity and increased values of sonic slowness 

within the interval indicated by the orange line and shading in 

figures 6A–D are examined in detail in this paper. Note that the 

leftward excursions in sonic slowness and resistivity become 

more clearcut as depth increases (compare the deepest well 

in fig. 6, Emblem Bench (fig. 6A) with the shallowest well, 

Federal 1–2 (fig. 6D)). 

Consider the character of the resistivity and sonic logs 

in the Emblem Bench well (fig. 6A). Resistivity increases 

slightly from an average of 10 ohm-m at 1,000–2,000 ft to 

an average of 14 ohm-m at 10,100–11,100 ft at the base of 

the Fort Union Formation. Resistivity ranges from 10 to 100 

in the Lance, Meeteetse, and Mesaverde Formations, and the 

Claggett Member. Resistivity declines from 22 ohm-m at 

the top of the Cody Shale to a value of 8 ohm-m at the base 

of the Cody Shale. The lowest values of 4 and 3 ohm-m are 

reached in the Mowry and Thermopolis Shales. The sonic log 

has not been edited, so the numerous spikes to the left, with 

some values exceeding 140 μs/ft, are due to measurement 

errors sometimes referred to as “cycle skips.” Ignoring these 

sharp leftward spikes, sonic slowness shows a pronounced 

compaction gradient, decreasing at a fast rate at shallow depth 

and a slow rate at deeper depth, from greater than 90 μs/ft at 

1

Sonic logs record the time, in microseconds, required for an acoustic wave 

to travel the distance from source to receiver, in feet. Consequently, the unit 

of measure is microseconds per ft (μs/ft), variously named transit time, travel 

time, or slowness. The term slowness is used in this report. High values mean 

that the acoustic signal is traveling slowly. Well logs are scaled in slowness, 

but reflection seismologists and laboratory workers use velocity, which is the 

reciprocal of slowness. Velocity in ft/s is equal to 1,000,000/slowness (μs/ft). 

For example, a slowness of 80 μs/ft is equivalent to a velocity of 12,500 ft/s. 

In this paper, slowness is used when examining well logs, and velocity is used 

when summarizing and comparing results.
Shale Units and Shale Trendlines Within An Interrupted Compaction Trend 11

Thermopolis Shale

VITRINITE REFLECTANCE, Ro, IN PERCENT

1.0

BURIAL DEPTH, IN FEET

0

5,000

10,000

15,000

20,000

25,000

5,000

10,000

15,000

20,000

EQUIVALENT PRESENT-DAY DEPTH, IN FEET

0

0.5 0.8 2.0

Start of gas generation

Peak gas generation

End of gas generation

Figure 5. Vitrinite reflectance, 

computed from one-dimensional 

burial history models for TypeIII source rocks, as a function 

of burial depth (left scale) and 

equivalent present-day depth 

(right scale). Blue ellipse at 10,000 

ft present-day depth encloses 

computational results for the Red 

Point II, Emblem Bench, Tatman 

Mountain Federal, Sellers Draw, 

and McCulloch Peak wells, which 

are wells 6, 8, 14, 16, and 25–26 

respectively in figure 1. From table 

4 of Roberts and others (2008).

Figure 4. Burial history plot 

for the base of Thermopolis 

Shale in the Tatman Mountain 

Federal well, from Roberts 

and others (2008), with 

hydrocarbon generation, 

depositional, and tectonic 

events shown as special 

symbols. Rapid burial of the 

Thermopolis Shale occurred 

during deposition of the Lance, 

Fort Union, and Willwood 

Formations. Heat flow of 

49 mW/m2, constrained by 

vitrinite reflectance data, and 

a temperature gradient of 

1.30°F/100 ft, based on well 

data, were used to calculate 

thermal maturation. The 

Tatman Mountain Federal well 

is well 14 in figure 1.

Base of Thermopolis Shale

Tatman Mountain Well

TIME (Ma)

120 100 80 60 40 20 0

DEPTH, IN FEET

0

5,000

10,000

15,000

20,000

25,000

62 Ma Start gas generation

55 Ma Start oil generation

53 Ma Peak gas generation

51 Ma Peak oil generation

47 Ma End oil generation

10 Ma Start uplift and erosion

104 Ma 

 Start deposition

Lance Formation

Fort Union Formation

Willwood Formation

Deposition of

and end of signicant gas

generation
12 Sonic Velocity and Other Petrophysical Properties of Source Rocks, Bighorn Basin, Wyoming

2000

3000

4000

5000

6000

7000

8000

9000

10000

11000

12000

13000

14000

15000

16000

17000

18000

Fort Union

Lance

Meeteetse

Mesav erde

Claggett

Lower Mesav erde

Cody Shale

Frontier

Mowry

Muddy

Thermopolis

Clov erly

Morrison

GammaRay

0 200

(API)

Resistivity (ohm-m)

1

140 40

Sonic slowness (us/ft)

10 100

90

Emblem Bench Well

M.Depth (ft)

Shell Creek

A

Figure 6 (above, facing page). Gamma-ray, resistivity, and sonic slowness logs in (A) Emblem Bench, (B) 

Tatman Mountain Federal, (C) Predicament, and (D) Federal 1–2 wells. Locations of wells shown in figure 1; well 

designations are in table 1. Orange line represents the unperturbed sonic slowness in shale, interpolated between 

an upper point in the Claggett Shale or Mesaverde Formation and a lower point in the Cloverly Formation. Orange 

shading represents anomalous sonic slowness. Yellow squares are selected sonic slowness values in the Cody, 

Mowry, Shell Creek, and Thermopolis Shales. Stratigraphic ages of geologic formations are given in figure 2. 
Depth (ft) 3000

4000

5000

6000

7000

8000

9000

10000

11000

12000

13000

14000

15000

16000

Fort Union

Lance

Meeteetse

Mesav erde

Claggett

Lower

Mesav erde

Cody Shale

Frontier

Mowry

Muddy

Thermopolis

Clov erly

Morrison

GammaRay

0 200

(API)

Resistivity (ohm-m)

1

140 40

Sonic slowness (us/ft)

10 100

90

Tatman Mountain Federal Well

Shell Creek

B

Shale Units and Shale Trendlines Within An Interrupted Compaction Trend 13

Figure 6.—Continued
14 Sonic Velocity and Other Petrophysical Properties of Source Rocks, Bighorn Basin, Wyoming

Gamma Ray

0 200

(API)

M.Depth (ft)

3000

4000

5000

6000

7000

8000

9000

10000

11000

12000

13000

14000

140 40

1 100

Fort Union

Lance

Meeteetse

Lewis

Mesaverde

Claggett

CodyShale

Frontier

Mowry

Muddy

Thermopolis

Cloverly

Morrison

Predicament Well

Resistivity (ohm-m)

Sonic slowness (us/ft)

10

90

Shell Creek

C

Figure 6.—Continued
GammaRay

0 200

(API)

M.Depth (ft) 2000

3000

4000

5000

6000

7000

8000

9000

10000

11000

12000

13000

Resistivity (ohm-m)

1

140 40

Sonic slowness (us/ft)

Lance

Meeteetse

Lewis

Mesav erde

Claggett

Lower Mesav erde

Cody Shale

Frontier

Mowry

Muddy

Thermopolis

Clov erly

Morrison

10 100

Fort Union

90

Federal 1-2 Well

Shell Creek

D

Figure 6.—Continued

Shale Units and Shale Trendlines Within An Interrupted Compaction Trend 15
16 Sonic Velocity and Other Petrophysical Properties of Source Rocks, Bighorn Basin, Wyoming

depths less than 2,000 ft to a value of 70 μs/ft at the base of 

the Fort Union Formation. Variations in lithology at the scale 

of tens and hundreds of feet cause variations in sonic slowness 

visible as a band of finite width that is particularly visible in 

the Fort Union and Lance Formations—the left-hand edge of 

the sonic log represents high gamma-ray or “shaly” rocks and 

the right-hand edge corresponds to low gamma-ray or “sandy” 

rocks. Like the resistivity log, the sonic log breaks to the left at 

the top of the Cody Shale at a depth of 14,860 ft and reaches a 

value of 85 μs/ft (11,765 ft/s) designated by the yellow square 

at 16,850 ft at the base of the Cody Shale. High slowness 

(low velocity) values, also designated by yellow squares, are 

reached near the base of the Mowry Shale at 17,953 ft 

(85 μs/ft; 11,765 ft/s) and in the Thermopolis Shale at 

18,148 ft (98 μs/ft; 10,200 ft/s). The orange shading indicates 

the amount of increased slowness (decreased velocity) from 

an interpolated trendline that represents the expected value of 

slowness in shales in this interval. There is no unambiguous 

method to establish such a trendline; the method used in this 

paper is described below. 

Selection of Four Shale Units

The four shale units that are the subject of detailed study 

in this report are highlighted by yellow squares in figure 7. 

The value for the lower Cody Shale is taken from a shale unit 

near the base of the Cody Shale. The sonic slowness of the 

lower three units, in the Mowry, Shell Creek, and Thermopolis 

Shales, are also shown on an expanded section in figure 8. The 

resistivity log provides a clear separation of the Shell Creek 

and Mowry Shales (fig. 8). The Mowry is more siliceous than 

the Shell Creek (table 2) and consequently has a lower value 

of sonic slowness and a higher resistivity than does the Shell 

Creek. Bentonitic units in the Mowry and Shell Creek Shales 

characterized by high gamma-ray, low resistivity, and low 

sonic velocity spikes are avoided in selecting the shale units. 

The Thermopolis Shale is represented by the sonic slowness 

of the “upper shale” described by Eicher (1962) in his threepart description of the Thermopolis (fig. 8, table 2) as a fissile 

black shale. In each well, the shale unit was selected at about 

the same proportional distance from a top or base of the shale 

formation, no claim is made that a shale unit is traceable 

across the basin. From figure 8, it can be seen that variations 

in resistivity and sonic slowness within a shale formation 

(Mowry Shale, Shell Creek Shale, and upper shale of Thermopolis Shale) are substantially less than variations among 

the three formations. As will be seen, the variations of sonic 

slowness among the lower Cody, Mowry, Shell Creek, and 

the Thermopolis shale units are consistent from well to well 

and indicate that a lithologic control on sonic velocity must 

be taken into account in extracting an overpressure signature 

from the sonic logs. 

Selection of End Points for Shale Trendlines

The two end points defining the shale trendline in the 

sonic slowness log are placed in the shallowest and deepest 

“shaly” units that lie above the Cody Shale and below the 

Thermopolis Shale, and are within rock units deposited in a 

marine and coastal environment. The upper end point lies in 

a high gamma-ray unit, placed in the Claggett Shale Member 

in figure 7, or in the Mesaverde Formation or Cody Shale in 

other wells. The lower end point lies in a shale in the “rusty 

beds” of the Cloverly Formation (fig. 8). Shale in the “rusty 

beds” is characterized by high gamma-ray activity and sonic 

slowness that is similar to that in the lower part of the Cloverly and the underlying Morrison Formation—thus the shales 

of the “rusty beds” seem to provide a consistent representation 

of the normally pressured sequence below the source rocks of 

the Thermopolis, Mowry, and Cody Shales. Note that sonic 

slowness in the sandstones of the Cloverly Formation, which 

are recognized by gamma-ray values less than 50 API units, 

is less than that of the trendline (fig. 8). Sonic slowness in 

the siltstones and shales just below the Greybull Sandstone 

Member (15,960–16,000 ft in fig. 8) is greater than that of the 

extrapolated shale trendline and may represent an overpressured zone within the Cloverly Formation; this unit was not 

analyzed in this paper. 

Pressure Transition Zone at Base of Cretaceous

As previously stated, the base of the trendline, which is 

thought to correspond to the base of a pressure compartment, 

is placed within the Cloverly Formation, although in some 

wells it could easily have been placed within the Morrison 

Formation. Rock units deeper than the Morrison Formation 

are considered normally pressured, based on a variety of 

measurements from oil and water wells and outcrop (Bredehoeft and others, 1992). The transition from normal pressure 

in Paleozoic strata to overpressure (or paleo-overpressure) 

in Cretaceous strata appears to take place within a vertical 

thickness of several hundred feet in a heterogeneous sequence 

of mudstone, siltstone, and sandstone. Moberly (1962) noted 

that “the great bulk of all these formations (Morrison through 

Mowry) is fine volcanic ash from the Nevadan orogeny that 

altered under a variety of nonmarine and marine conditions to 

produce the main stratigraphic units, mudstones and shales.” 

This observation is germane to the nature of the pressure transition zone, because of (1) the predominance of mudstones, 

claystones, and fine-grained sandstones which are expected 

to have high capillary entry pressures; and (2) the prevalence 

of rocks derived in part from volcanic ash, which commonly 

alters to clay minerals that occlude pores and pore throats, 

even in shales. Descriptions taken from Moberly (1960, 1962), 

are cited in table 6 to detail the heterogeneity of the Morrison 

and Cloverly Formations that play an important role in establishing a pressure transition zone.
Shale Units and Shale Trendlines Within An Interrupted Compaction Trend 17

Depth (ft) 12000

13000

14000

15000

16000

Mesav erde

Claggett

 Lower

Mesav erde

Cody Shale

Frontier

Mowry Muddy

Thermopolis

Morrison Clov erly

GammaRay

0 200

(API)

Resistivity (ohm-m)

1

140 40

Sonic slowness (us/ft)

10 100

90

Tatman Mountain Federal Well

Caliper (inches)

10 20

Shell Creek

Figure 7. Well logs from the lower 5,000 ft of the Tatman Mountain Federal well, illustrating the selection of 

the shale trendline and four sonic slowness values, designated by yellow squares, in shale units in the Cody 

Shale, Mowry Shale, and Thermopolis Shale. Shaded area indicates departure of sonic slowness from a shale 

trendline. 
18 Sonic Velocity and Other Petrophysical Properties of Source Rocks, Bighorn Basin, Wyoming

0 GammaRay(API) 200

8 Caliper (in.) 48

Depth (ft) 15200

15300

15400

15500

15600

15700

15800

15900

16000

16100

1 Resistivity (ohm-m) 300 140 Sonic slowness (us/ft) 40

Mowry

Shale

Muddy Ss.

Thermopolis Shale

Cloverly Formation

Morrison Fm.

Cloverly Formation

Keefer and

others (1998)

Eicher (1962)

Rusty Beds

Mbr.

Greybull Ss. 

Mbr.

Rusty beds

Lower shale

Middle silty shale

Upper shale

Muddy Ss.

Shell

Creek 

Shale

Mowry 

Shale

Thermopolis Shale

Tatman Mountain Federal Well

Figure 8. Well logs from the lower 1,000 ft of the Tatman Mountain Federal well, showing lithologic units discussed 

by Keefer and others (1998) and Eicher (1962). Squares show three sonic slowness values in the Mowry and 

Thermopolis Shales, vertical orange line is the shale trendline for the sonic log. Lithology descriptions by Eicher 

(1962) are summarized in table 2.
Petrophysical Properties of Four Shale Units 19

Characteristics of Shale Trendlines

Values for the upper and lower end points of trendlines 

determined for 23 wells are shown in figure 9, where both 

slowness and velocity scales are shown. Sonic slowness at 

the upper end of the trendline (Claggett Shale and upper part 

of the Mesaverde Formation) decreases rapidly with depth to 

about 8,000 ft and then exhibits only a further slight decrease 

with increasing depth (fig. 9A). Velocity in the Claggett Shale 

and upper part of the Mesaverde Formation reaches a high 

value of 15,400 ft/s (65 μs/ft) at 7,500 ft present-day depth 

(fig 9A). Sonic slowness at the lower end of the trendline 

(Cloverly Formation) undergoes a break in slope around 

9,000 to 10,000 ft present-day depth, below this depth range 

the slowness displays only a further small decrease with 

increasing depth (fig. 9B). The highest velocity of 15,900 ft/s 

(63 μs/ft) is reached in shaly rocks of the upper Cloverly 

Formation at 17,000 ft. 

These limiting values of sonic slowness are reflected 

in the near-vertical line segments at 10,000 ft and deeper 

(fig. 10), where the horizontal scale is sonic velocity rather 

than sonic slowness as in previous plots. The slopes of the 

trendlines also show a change at around 10,000 ft (fig. 10). 

Below 10,000 ft, sonic travel time at the upper end of the 

trendline is slightly less than at the lower end, giving these 

trendlines a much steeper slope than those above 10,000 ft. 

Overall, the line segments reflect shale compaction as a function of depth in the Bighorn Basin; the line segments at depths 

less than 10,000 ft have lower velocities and smaller slopes 

than the line segments at depths greater than 10,000 ft. Compaction appears to have greatly diminished below present-day 

depths of 10,000 ft. 

Petrophysical Properties of Four Shale 

Units

The values of five petrophysical properties—sonic 

velocity, electrical resistivity, density, neutron porosity, and 

gamma-ray activity—were determined from well logs for the 

four shale units. The caliper reading was also noted as an indicator of the tendency for the borehole wall to slough. Wells 

were drilled and logged by Schlumberger and Dresser Atlas 

between 1972 and 1982 with the exception of the Gywnn 

Ranch well, which was drilled in 1965. Values and statistics 

for each shale unit are tabulated in Appendix 1. 

Shales tend to wash out when drilled, resulting in a 

borehole diameter that is greater than the bit diameter. The 

interval of the Tatman Mountain Federal well shown in 

figure 7 was drilled with a bit diameter of 8.5 in. but the 

hole diameter is greater than 10 in. throughout much of the 

interval and exceeds 14 in. in the Thermopolis Shale. The 

borehole diameter in each of the shale intervals was tabulated 

(Appendix 1) and subtracted from the bit size. Borehole 

enlargement ranges from about 1 to 9 in. in the lower Cody, 

Shell Creek, and Thermopolis shale units, is generally greatest 

in the Thermopolis shale unit, and is generally less than 2 in. 

in the Mowry shale unit (fig. 11). All wells were drilled with 

Table 6. Lithologic description and depositional environment of Morrison, Cloverly, and Sykes Mountain Formations, from Moberly 

(1960, 1962). The Sykes Mountain Formation of Moberly is included as part of the Cloverly Formation in this paper. 

[Mbr, member; Fm., formation]

Rock unit nomenclature of

Moberly, 1960

Thickness range 

and (average), 

in ft

Lithologic description Depositional environment

Sykes Mountain Fm. 100– 300

(136)

Rusty weathered ironstone, siltstone, and sandstone, 

interbedded with thin dark shales. All common 

clay minerals are present.

Tidal flats and lagoons peripheral to 

Thermopolis seaway.

Himes Mbr. of 

Cloverly Fm.

60–120 

(92)

Claystone and sandstone channeled with better sorted 

sandstone; cliff former. (Includes Greybull sandstone of other authors).

Leaching and subaerial weathering of 

volcanic debris and other sediments.

Little Sheep Mudstone 

Mbr. of Cloverly 

Fm.

75–290 

(250)

Variegated bentonitic mudstone with some chert 

pebble sandstone lenses. Bedding is generally 

absent.

Subaqueous weathering of volcanic 

debris in lakes and swamps.

Pryor Conglomerate 

Mbr. of Cloverly 

Fm.

 -- Conglomerate beds or conglomeratic sandstone --

Morrison Formation 130–280

(190)

Greenish mudstone and sandstone, red-banded in part, 

with calcareous quartz arenites and subordinate 

limestones.

Fluvial and lacustrine
20 Sonic Velocity and Other Petrophysical Properties of Source Rocks, Bighorn Basin, Wyoming

Lower end of shale trendline in the Cloverly

Formation

90 80 70 60 50

DEPTH, IN FEET

0

5,000

10,000

15,000

20,000

Upper end of shale trendline in Claggett Shale

and upper part of the Mesaverde Formation

90 80 70 60 50

0

5,000

10,000

15,000

20,000

VELOCITY (1,000 ft/s)

12 13 14 15 16 17 18 19

12 13 14 15 16 17 18 19

B

A

SONIC SLOWNESS ( s/ft)

VELOCITY (1,000 ft/s)

DEPTH, IN FEET

SONIC SLOWNESS ( s/ft)

water-based muds, except for the Gywnn Ranch well which 

was drilled with an oil emulsion, and the Katrine Loch well 

which was drilled with an oil-base mud (Appendix 1). As a 

consequence of drilling with an oil-base mud, the borehole 

diameter in the Katrine Loch well was close to bit size for the 

entire drilled interval and there was no borehole enlargement 

in the four shale units (fig. 11). Log properties from the 

shale units in the Katrine Loch well should be unperturbed 

by borehole effects. None of the four shale units show any 

consistent trend of borehole enlargement with depth (fig. 11). 

The distinction between transport and bulk properties is 

useful in considering petrophysical responses. Sonic velocity 

and resistivity are called transport properties because energy is 

propagating through the rock and the values of these properties depends upon direction of propagation. Neutron porosity, 

density, and gamma-ray activity are bulk properties and their 

determination is insensitive to the mode or direction of energy 

propagation. The two transport properties are presented first, 

followed by the three bulk properties.

Sonic velocity was determined with the boreholecompensated sonic log, which measures the travel time of the 

compressional wave between two receivers spaced 3 and 5 ft 

from the transmitter. The combined effect of borehole enlargement and formation damage could potentially cause erroneous 

velocity determinations in shales. If the formation is damaged, 

as it can be when water in the borehole causes clay minerals 

to expand, then there is a concern that the sonic log measures 

the reduced velocity of the altered shale close to the borehole 

rather than the velocity of the unaltered shale. Indeed, the 

depth of investigation of a sonic log is quite limited for the 

case where the velocity of rock next to the borehole is greater 

than that of the unperturbed formation, but it is not so limited 

for the case of reduced velocity next to the borehole (Baker, 

1984), which corresponds to the situation examined here. An 

algorithm based on ray-path theory (Appendix B of Baker, 

1984), can be used to calculate the minimum thickness of 

an altered zone required to produce an erroneous result. For 

example, if the true formation velocity is 14,000 ft/s and the 

altered zone velocity is 13,000 ft/s, then the sonic log will 

yield the correct velocity of 14,000 ft/s unless the altered zone 

is greater than 4 in. thick. It seems unlikely that the thickness 

of an altered shale layer extends more than 4 in., due to low 

permeability of shales. Two pieces of field evidence also indicate minimal errors in measuring the formation velocity. The 

Katrine Loch well was drilled with oil-base mud rather than 

the water-base mud used in most other wells, which results 

in a borehole with little enlargement (fig. 11) and no water 

in contact with shale, so that clay swelling is checked. The 

sonic velocities determined in four shale units in the Katrine 

Loch well are close to average values for all wells (Appendix 

1 and fig. 10), rather than being the highest of all values as 

would be the case if all other logs were adversely affected by 

altered shale. Further inspection found no interdependence 

between sonic velocity and borehole enlargement in the shale 

units, as indicated by the summary displayed in table 7. Thus 

both a theoretical model pertinent to formation alteration and 

Figure 9. Sonic slowness as a function of depth in 23 wells. 

A, Value selected in Claggett Shale for upper end of shale 

trendline, and B, value selected in Cloverly Formation for 

lower end of shale trendline. 
Petrophysical Properties of Four Shale Units 21

VELOCITY, IN CUBIC FEET PER SECOND

8,000 10,000 12,000 14,000 16,000

DEPTH, IN FEET

0

5,000

10,000

15,000

20,000

Lower Cody Shale

Mowry Shale

Shell Creek Shale

Thermopolis Shale

Claggett Member

Cloverly Fm

EXPLANATION

Shale

 Trendline

Katrine Loch Well

CALIPER MINUS BIT SIZE, IN INCHES

0 2 4 6 8 10

DEPTH, IN FEET

0

5,000

10,000

15,000

20,000

Katrine Loch well

EXPLANATION

Lower Cody Shale

Mowry Shale

Shell Creek Shale

Thermopolis Shale

Figure 10. Sonic velocities in four shale 

units, with shale trendlines, as a function of 

depth for 23 wells in the Bighorn Basin. Four 

encircled data are from the Katrine Loch well, 

which was drilled with oil-base mud. The shale 

trendlines represent unperturbed velocity 

trends, as depicted in figures 6 A–D.

Figure 11. Borehole enlargement in four 

shale units as a function of depth. The Katrine 

Loch well was drilled with an oil-base mud. 
22 Sonic Velocity and Other Petrophysical Properties of Source Rocks, Bighorn Basin, Wyoming

practical examples incorporating the effects of both borehole 

enlargement and formation alteration indicate that errors in 

sonic velocity determination are minimal in the four shale 

units.

Velocities in the lower Cody, Mowry, Shell Creek, and 

Thermopolis shale units from 23 wells are plotted with the 

trendlines in figure 10. The increasing velocities with depth for 

each of the shales form a trend that generally tracks the overall 

pattern of the trendlines, for which the increase in velocity 

with depth lessens at present-day depths greater than 10,000 ft. 

Separation between the shale velocities and the trendlines 

is proportional to the orange shading patterns of figure 6. 

Velocities of the Thermopolis shale unit are systematically less 

than for the other shales, and velocities of the Mowry shale 

unit are consistently greater. For depths greater than 8,790 ft 

(18 wells), the average velocities for the four shale units are: 

lower Cody, 12,528 ft/s; Mowry, 13,943 ft/s; Shell Creek, 

12,083 ft/s; and Thermopolis, 10,826 ft/s. The average velocity in the black, fissile Thermopolis shale unit is 3,117 ft/s less 

than in the siliceous Mowry shale unit.

Resistivity was determined with the deep laterolog tool 

in 11 wells and with the deep induction tool in 12 wells. 

Corrections to the deep laterolog are less than 10 percent for 

conditions encountered in these wells (Schlumberger, 1996). 

Corrections to the deep induction log can be greater than 10 

percent due to electrical signals generated in conductive mud. 

The correction depends upon standoff (separation of the tool 

from the borehole wall), but standoff is difficult to determine. 

Plots of resistivity with depth showed that resistivity values 

from the deep laterolog and the deep induction log were 

intermingled with no offset, indicating that any need for 

corrections was minor, so no corrections were applied to 

either the deep laterolog or the deep induction logs. As was 

apparent in the plot of sonic velocity with depth (fig. 10), 

resistivity in the shale units increases with depth to depths of 

around 10,000 ft (fig. 12). However, below the depth range 

10,000–12,000 ft, resistivity decreases with depth rather than 

remaining constant, as was the case for sonic velocity. 

Some of the borehole factors that affect the neutron 

response2 (Schlumberger, 1996)—mudcake thickness, borehole salinity, formation water salinity, and mud weight—have 

little impact in the wells studied here. A correction for 

borehole enlargement was made at the time of logging, as 

automatic compensation was applied to most neutron logs. But 

three other factors—tool standoff, temperature, and pressure—

do affect the neutron response although no correction has been 

applied here. A tool standoff of 1 in. from the borehole wall 

requires a correction of -2 to -3 percent, but this correction is 

difficult to apply due to lack of a measurement of standoff. For 

depths ranging from 7,000 to 20,000 ft, the correction for pressure ranges from -0.7 to -2 percent. Bottom-hole temperatures 

ranging from 140°F to 250°F require neutron porosity corrections ranging from +4 to +9 percent. Given that these three 

corrections are of opposite sign and somewhat compensating 

and given the uncertainty involved in applying the corrections, 

the neutron response values have not been corrected for use in 

this study. The tabulated neutron response values (Appendix 

1) do not show any depth dependence and therefore no depth 

plot is shown. From Appendix 1, the averages and standard 

deviations for all samples from all depths are in percent 

porosity: lower Cody 27, 4; Mowry 18, 2; Shell Creek 30, 7; 

and Thermopolis 38, 8. The Mowry shale unit has the lowest 

neutron porosity and the lowest standard deviation of the four 

shale units, and the Thermopolis shale unit, the greatest.

The density log can produce erroneously low density 

values if the borehole is not smooth. In the Mowry Shale, the 

caliper log consistently showed a smooth borehole and the 

density log had very little correction, but in the other three 

shale formations, the density correction generally exceeded 

0.1 g/cc and the caliper log indicated an enlarged rough 

borehole (figs. 7 and 11). For this reason, density values are 

shown for the Mowry shale unit but not for the lower Cody, 

Shell Creek, or Thermopolis shale units (fig. 13). The density values at depths less than 10,000 ft are less than values 

at depths greater than 10,000 ft; this increase of density with 

depth, which is similar to the velocity-depth relation for the 

Mowry in figure 10, is attributed to compaction. The deepest 

well in the study, Katrine Loch, was drilled with an oil-base 

mud instead of a water-base mud, thereby preserving a smooth 

borehole that is not much greater than bit size. Consequently, 

valid density data were obtained for all four shale units in the 

Katrine Loch well: Thermopolis, 20,594 ft, 2.65 g/cc; Shell 

Creek, 20,399 ft, 2.67 g/cc; Mowry, 20,246 ft, 2.63 g/cc; and 

lower Cody, 19,926 ft, 2.64 g/cc. Thus, at high levels of effective stress in the deep Katrine Loch well, the density values of 

the four shale units are similar at 2.65±0.02 g/cc. 

Table 7. Average values of sonic velocity as a function 

of borehole enlargement, as determined by the difference 

between caliper and bit size, for three shale units. Because 

there was little borehole enlargement in the Mowry shale unit, it 

is not included in this table.

Shale Unit Average Sonic Velocity 

in ft/s

Caliper Minus Bit 

Size < 5 inches

Caliper Minus 

Bit Size > 5 inches

Lower Cody 12,124 12,442

Shell Creek 11,818 11,374

Thermopolis 10,651 10,687

2

The neutron log responds primarily to hydrogen concentration. Most of 

the hydrogen present in rocks is in water and hydrocarbons, so a neutron log 

responds to the fluid in the pore space and the term “neutron porosity” is 

applicable for porous rocks with little clay content. However, because most 

clay minerals contain substantial amounts of hydroxyls or bound water, the 

“neutron porosity” response is much greater than actual porosity in siltstones 

and shales. The neutron porosity values shown in this paper can be considered 

to be responding primarily to clay content, with only a minor contribution 

from porosity. 
Petrophysical Properties of Four Shale Units 23

RESISTIVITY (ohm-m)

1 10 100

DEPTH, IN FEET

0

5,000

10,000

15,000

20,000

EXPLANATION

Lower Cody Shale

Mowry Shale

Shell Creek Shale

Thermopolis Shale

Katrine Loch Well

DENSITY (g/cc)

2.2 2.4 2.6 2.8

DEPTH, IN FEET

0

5,000

10,000

15,000

20,000

EXPLANATION

Mowry Shale

Figure 12. Resistivity for four shale units 

as a function of depth. Resistivity appears 

to reach maximum values at depths of 

10,000 to 12,000 ft. Four encircled points 

are from the Katrine Loch well, drilled with 

oil-base mud.

Figure 13. Density for the Mowry shale 

unit as a function of depth. A compaction 

effect is evident.
24 Sonic Velocity and Other Petrophysical Properties of Source Rocks, Bighorn Basin, Wyoming

Corrections to the gamma-ray log are needed to account 

for gamma-ray attenuation in the borehole if a heavy mud is 

used and the borehole is greatly enlarged. Mud weights used in 

the wells considered here ranged from 8.8 to 10.7 pounds per 

gallon (Appendix 1) and enlargement of borehole diameter, as 

previously discussed, ranged from 0 to 9 in. Using guidelines 

from Schlumberger (1996) it was found that corrections for 

gamma-ray attenuation were generally less than 10 percent, 

so no corrections were applied. One well (Fritz-Federal 1–16 

2B, location 11 in fig. 1) was drilled with potassium-chloride 

mud and exhibited an unusually high gamma-ray response 

in all shale units, so the gamma-ray values for this well were 

dropped from the data set. Gamma-ray responses show no 

dependence upon depth, but there is a dependence upon shale 

unit as discussed below.

To examine the relationships among petrophysical 

responses, the values of resistivity, density, neutron porosity, and gamma-ray activity are plotted against sonic velocity 

(fig. 14). To reduce the effect of compaction, five wells were 

excluded from the plots, so that no values are shown from 

depths shallower than 8,750 ft. Sonic velocity is positively 

correlated with resistivity, with both velocity and resistivity 

values lowest in the Thermopolis Shale and highest in the 

Mowry Shale (fig. 14A). Density values are available only for 

the Mowry Shale, where there is no clear relation between 

sonic velocity and density (fig. 14B). Sonic velocity is negatively correlated with neutron porosity (fig. 14C). Velocity is 

highest and neutron porosity is lowest in the Mowry Shale, 

whereas velocity is lowest and neutron porosity is highest in 

the Thermopolis Shale. Sonic velocity tends to increase with 

increasing values of gamma-ray activity (fig. 14D), a rather 

unexpected result because high gamma-ray activity is usually 

an indicator of shalier, hence mechanically weaker, rocks. Possible causes of these relations will be discussed below. 

Neutron-density plots, a common tool of petrophysical 

analysis, provide a way of evaluating the degree of shaliness 

in siliciclastic sequences (Katahara, 2008). The neutron and 

density values of the Mowry shale unit are tightly clustered on 

the neutron-density plot (fig.15). The sandstone line represents a zero clay line. To show the utility (and limitations) 

of a neutron-density plot, a modified illite point forms one 

corner of a skewed three-component triangular diagram that 

is superposed on the neutron-density coordinates (fig.15). The 

modified illite point has a neutron porosity of 34.4 percent, as 

given by Ellis and Singer (2008), and a density of 2.60 g/cc 

which is greater than their computed value of 2.52 g/cc. Three 

parallel lines show the illite fraction and the two long sides of 

the triangle indicate porosity bounds of zero and 10 percent. 

This diagram indicates that the illite content in the Mowry 

shale unit ranges from 30 to 55 percent. There is no assurance 

that this shale unit consists of only pore space, quartz, and 

illite, so the triangular diagram with a single illite clay point is 

only an idealization of the true mineralogy. According to the 

semi-quantitative analyses of Mowry Shale outcrop samples 

(table 3), mixed-layer clay is the primary clay constituent, 

with montmorillonite present and illite a minor constituent. It 

is quite possible that conversion of montmorillonite to illite 

has increased the illite content of rocks in the deep subsurface 

that are represented in figure 15. Quantitative mineralogic 

analysis on samples from deep wells is required to substantiate 

or improve the interpretation of figure 15. 

A statistical summary of the six petrophysical responses 

(fig. 16) is restricted to depths greater than 8,750 ft to eliminate data points that have not reached the maximum compaction state. The box-and-whiskers summary shows the following order for sonic velocity and resistivity responses:

Mowry > lower Cody > Shell Creek > Thermopolis

This order is reversed for the neutron porosity response. 

This systematic ordering in four of five petrophysical 

responses (ordering for the gamma-ray log differs from the 

others) is related to mineral composition, as discussed next.

Discussion

The petrophysical responses documented thus far are 

controlled by shale composition and stress history. Petrophysical responses are represented schematically by four horizontal 

bars showing the range of sonic velocity for depths greater 

than 8,750 ft in figure 17, along with the range of expected 

(trendline) velocities for depths greater than 10,000 ft (gray 

box in fig. 17). To a first approximation, the effect of rock 

composition is manifested by the variation in velocity among 

the four shale units, and the effect of stress history is manifested by the reduction of velocity below the range of expected 

velocities, that is, by the leftward displacement of the four 

horizontal bars from the gray box. The elements of composition—mineral content, porosity, fluids, and kerogen content—

and the manifestations of stress history—overpressure and 

microfracturing—are summarized in the lower part of figure 

17, which serves as a guide for the following discussion. 

The mineralogy of shales can vary considerably. Pettijohn (1975) stated that the clay mineral content of shale varies 

from 40 to 100 percent, although most shales have a large silt 

fraction comprised of detrital quartz and lesser amounts of 

feldspar. Kerogen is another common constituent. The estimation of a petrophysical quantity such as sonic velocity from 

rock composition represents a central activity of experimental 

and theoretical petrophysics (Hearst and others, 2000, chapters 

1 and 12). For example, the dependence of the sonic velocity 

of siliciclastics upon porosity and clay content has been examined by Vernik (1997); shales are presented as a high-clay end 

member of a siliciclastic sequence. An increase in the volume 

fraction of clay minerals and kerogen, both being low modulus 

(soft) materials, results in lower sonic velocity. Conversely, 

an increase in silica or carbonate, whether present as grains 

or cementing material, is expected to increase sonic velocity 

(arrows in fig. 17). Without mineralogical analyses of samples 

that correspond to the well-log data, these relations cannot be 

quantified in this report, but some observations can be made:
Discussion 25

RESISTIVITY (ohm-m)

1 10 100

SONIC VELOCITY (ft/s)

10,000

11,000

12,000

13,000

14,000

15,000

EXPLANATION

Lower Cody Shale

Mowry Shale

Shell Creek Shale

Thermopolis Shale

A

DENSITY (g/cc)

2.0 2.2 2.4 2.6 2.8 3.0

SONIC VELOCITY (ft/s)

10,000

11,000

12,000

13,000

14,000

15,000

EXPLANATION

Mowry Shale

z > 8,700 ft

B

NEUTRON POROSITY (percent)

0 10 20 30 40 50

SONIC VELOCITY (ft/s)

10,000

11,000

12,000

13,000

14,000

15,000

EXPLANATION

Lower Cody Shale

Mowry Shale

Shell Creek Shale

Thermopolis Shale

> 8,700 ft

C

GAMMA RAY (API)

0 50 100 150 200

SONIC VELOCITY (ft/s)

10,000

11,000

12,000

13,000

14,000

15,000

EXPLANATION

Lower Cody Shale

Mowry Shale

Shell Creek Shale

Thermopolis Shale

D

Figure 14. Values of sonic velocity plotted against A, resistivity; B, density; C, neutron porosity; and D, gamma-ray activity 

for four shale units in the Bighorn Basin. All values are determined by an average of a 10 to 20-ft well-log interval in 

selected shale units. Only values from depths greater than 8,750 ft are shown.
26 Sonic Velocity and Other Petrophysical Properties of Source Rocks, Bighorn Basin, Wyoming

NEUTRON POROSITY, SANDSTONE SCALE, IN PERCENT

0 10 20 30 40 50

DENSITY (g/cc)

2.0

2.2

2.4

2.6

2.8

Mowry Shale

Modied illite clay point

Sandstone line

0.5

0.75

10% sandstone porosity

0% sandstone porosity

0.0

0.25 clay fraction

Figure 15. Neutron and density values from the Mowry shale unit. 

Sandstone line gives porosity-density relation for a sandstone with grain 

density of 2.66 g/cc. Point labeled illite is selected such that sandstone 

porosity lines for 0 and 10 percent bracket the data.

1. The range of gamma-ray values (fig. 16) for all four 

shale units are within the range that is typical of shales. 

Gamma-ray activity is independent of depth and does not 

correlate with sonic velocity as do resistivity and neutron 

response (fig. 14). The level of gamma-ray activity of 

the Mowry and Shell Creek shale units are similar, and 

greater than the gamma-ray activity ranges of the lower 

Cody and Thermopolis shale units (fig. 16F). 

2. The relatively low neutron porosity and high density of 

the Mowry shale unit indicates low porosity (five percent 

or less), high silica content (grain density of 2.66 g/cc),

and some illite and kerogen. Based on total organic 

carbon analyses, kerogen content is around six volume 

percent. Diagnosis of mineralogical composition from 

the neutron-density plot (fig. 15) is limited to the Mowry 

shale unit, the only shale unit for which reliable density 

data are available. This diagram, an idealization of the 

true mineralogical composition, indicates a porosity less 

than 10 percent, and a range of illite content between 30 

to 55 percent. 

3. The Mowry shale unit is distinguished from the other 

three shale units by having the highest velocity (fig. 16B), 

the highest resistivity (fig. 16C), and the lowest neutron 

porosity (fig. 16E). Although density data could not be 

reliably obtained from shale units other than the Mowry, 

it was noted that the density of the Mowry is greater than 

that of the other three shale units. In addition, a smoother 

borehole is obtained when drilling the Mowry than in the 

other three shale units (fig. 16A). All of these findings 

are attributed to the high siliceous content of the Mowry 

Shale, which has been noted by several authors (tables 2 

and 3). High silica (quartz) content must be balanced by 

low clay content, which is compatible with low neutron 

response and high resistivity. And high quartz content, 

along with silica cementation of grains, is compatible with 

high sonic velocity. However, retention of hydrocarbons 
SONIC VELOCITY (ft/s)

9,000 10,000 11,000 12,000 13,000 14,000 15,000

Lower Cody ShaleMowry Shale

Shell Creek Shale

Thermopolis Shale

RESISTIVITY (ohm-m)

1 10 100

CALIPER MINUS BIT SIZE (in.)

-2 0 2 4 6

Lower Cody ShaleMowry ShaleShell Creek ShaleThermopolis Shale

Thermopolis Shale

8

Lower Cody ShaleMowry Shale

Shell Creek Shale

DENSITY (g/cc)

2.0 2.2 2.4 2.6 2.8 

Mowry Shale

GAMMA RAY (API units)

0 50 100 150 

Lower Cody Shale Mowry Shale Shell Creek Shale Thermopolis Shale 

NEUTRON POROSITY (percent)

0 10 20 30 40 

Lower Cody Shale 

Mowry Shale 

Shell Creek Shale 

Thermopolis Shale 

EXPLANATION

Percent of population

0 25 50 75 100

A

B

C

D

E

F

Figure 16. Box-and-whiskers plots of petrophysical properties from well logs, for depths greater than 8,750 ft. Each whisker and each represents one-fourth of population; vertical line in middle of box represents the median value. A, Borehole enlargement as determined log minus the bit size. B, Sonic velocity. C, Electrical resistivity. D, Density, which is valid only for the Mowry Shale. E, Neutron porosity. ray response. 
28 Sonic Velocity and Other Petrophysical Properties of Source Rocks, Bighorn Basin, Wyoming

VELOCITY (m/s)

2,000 3,000 4,000 5,000

Porosity

Kerogen content

Clay content

Silica content

Carbonate content

Rock

composition

VELOCITY (ft/s)

8,000 10,000 12,000 14,000 16,000

Present-day overpressure

Paleo-overpressure

Presence of gas

Microfractures

Stress

effects

Lower Cody Shale

Mowry Shale

Shell Creek Shale

Thermopolis Shale

Range of 

expected

velocities

below

10,000 feet

from shale

trendlines

Figure 17. Parameters affecting sonic velocity. Upper part of diagram: range of sonic 

velocity in four shale units and range of expected velocity in shales from all trendlines below 

10,000 ft. Lower part of diagram: nine arrows show consequence of an increase in a given 

parameter (six related to rock composition and three related to changes in stress) upon sonic 

velocity. For example, an increase in overpressure is expected to reduce sonic velocity but an 

increase in silica content is expected to increase sonic velocity.
Discussion 29

generated by a good source rock such as the Mowry is 

not compatible with high velocity, high resistivity, or low 

neutron porosity. It is possible that low porosity and low 

compressibility (high modulus) led to the early formation 

of microfractures so that hydrocarbons were expelled as 

they were generated, thereby preserving high velocity and 

high resistivity. 

4. Sonic velocity is expected to be affected by clay mineral 

orientation and location more than it is affected by clay 

mineralogy. Anisotropy in shales, the change in sonic 

velocity with direction of propagation, is caused by the 

presence of flattened grains and microcracks (Vernik and 

Landis, 1996). Anisotropy affects both sonic velocity and 

resistivity, which are transport properties, but not neutron 

and density, which are bulk properties. Laminae and fissility are two macroscopic descriptors related to anisotropy, 

and neither has been applied to the Mowry Shale, providing another possible explanation for the high velocity and 

resistivity of the Mowry. One unexplained observation, 

which is probably related to anisotropy, is the decrease 

of resistivity with depth below 10,000 ft in all four shale 

units (fig. 12).

5. The sonic velocity and resistivity of the lower Cody shale 

unit are less than that of the Mowry shale unit but greater 

than that of the Shell Creek and Thermopolis shale units 

(fig. 16). In addition, the neutron porosity of the lower 

Cody shale unit is less than that of the Shell Creek or 

Thermopolis shale units. These latter relations are indicative of lower clay content and higher carbonate content 

in the lower Cody shale unit than in the Shell Creek or 

Thermopolis shale units. The possibility of high carbonate content rather than high silica content is suggested by 

the deposition of carbonate-rich sediments in the Niobrara 

Formation, which is equivalent to the lower Cody Shale, 

in areas east of the Bighorn Basin (Longman and others, 

1998). 

6. The Thermopolis Shale is described as a black fissile 

shale (table 2), and the Thermopolis shale unit has the 

lowest values of sonic velocity and resistivity (figs. 16B

and 16C) and the highest neutron response (fig. 16E). It 

also tends to create a somewhat larger and rougher borehole than the other shale units (fig. 16A). These responses 

indicate that the Thermopolis shale unit has the highest 

clay mineral content of the four shale units.

The reductions in sonic velocity and resistivity (fig. 6) 

and the associated presence of overpressure (fig. 3) highlight an interval that extends from the Cloverly Formation 

to the base of the Mesaverde Formation (fig. 2). Generation 

of hydrocarbons, particularly gas, occurred as source rocks 

were buried to depths of about 15,000 ft (fig. 5). Constrained 

by the low permeability of fine-grained shales and siltstones, 

gas could not migrate freely, causing pressure to increase 

to the point where microfractures were induced, at which 

point gas could migrate into newly accessible rock volumes. 

This stepwise migration process of gas generation, pressure 

increase, fracture opening, and migration continued until gas 

had migrated into the more accessible units within an interval 

that extends from the Cloverly Formation upwards into the top 

of the Cody Shale. The pervasive pressure increase throughout 

this depth interval caused a drop in effective stress that in turn 

caused drops in sonic velocity and resistivity. The high pressures that were attained during gas generation at maximum 

burial have likely decreased with uplift and erosion during 

the last 10 million years (fig. 4), leaving a record of modest 

present-day pressures (fig. 3) which are less than maximum 

paleopressures. As pore pressure builds, the normal increase in 

effective stress with burial depth is arrested, and the increase 

in sonic velocity is also arrested or can even decrease if the 

rocks are unloaded. The reduced values of sonic velocity and 

resistivity are found in tight sandstone, siltstones, and shales, 

resulting in the well-log excursions shown in figure 6. Rocks 

of the Morrison and Cloverly Formations, containing abundant clay minerals of volcanic origin (table 6), form the lower 

bound of the system. The upper bound of the gas system, 

located approximately at the top of the Cody Shale, indicates 

the top of gas migration, which is determined by the amount 

of gas generated and the concomitant overpressures. 

The velocity of shales approaches a limiting value of 

around 16,000 ft/s at the base of the Cretaceous, as shown 

by the asymptotic behavior of the lower end of the trendlines 

(fig. 9B), which is established in the “rusty beds” of the 

Cloverly Formation. The limiting velocity value of each of 

the four shale units probably differs from that of the “rusty 

beds,” due to variations in the properties of each shale unit, 

as previously discussed. All four shale units exhibit a marked 

increase in velocity with depth to a present-day depth of about 

10,000 ft, followed by a much smaller rate of increase below 

that depth (fig. 10), and the density-depth plot for the Mowry 

shale unit is compatible with the velocity-depth patterns 

(fig. 13). The separation, or difference, between the velocity 

in the shale unit and the corresponding trendline velocity 

does not appear to change much with depth, as demonstrated 

by plotting the difference in the two velocities as a function 

of depth (fig. 18). None of the four shale units exhibits any 

depth dependence in the velocity difference. The compaction 

process has left an imprint on the velocity of each shale unit, 

but with an offset that differs for each shale unit, as indicated 

by the average values posted in figure 18. The true offset 

for each shale unit is unknown because the true trendline 

(compaction curve) will differ among the shale units due to 

variations in mechanical properties. It is proposed in this paper 

that the sonic velocity has been reduced by overpressure that 

developed during the course of burial. The steady increase 

in effective stress that would normally occur with increasing 

depth of burial was slowed, halted, or even reduced by an 

increase in pore pressure. As there is no deviation of data 

shallower than 10,000 ft from the average velocity difference 

in figure 18, the data suggest that the onset of internal pressure 

increase occurred at a present-day depth less than 5,000 ft 

which corresponds to a paleodepth of less than 10,000 ft. 
30 Sonic Velocity and Other Petrophysical Properties of Source Rocks, Bighorn Basin, Wyoming

TRENDLINE VELOCITY MINUS SHALE VELOCITY (ft/s)

6,000 5,000 4,000 3,000 2,000 1,000 0

DEPTH, IN FEET

0

5,000

10,000

15,000

20,000

Thermopolis Shale Lower Cody Shale

TRENDLINE VELOCITY MINUS SHALE VELOCITY (ft/s)

6,000 5,000 4,000 3,000 2,000 1,000 0

DEPTH, IN FEET

0

5,000

10,000

15,000

20,000

Shell Creek Shale Mowry Shale 

1,115 3,115

4,215 2,406

Figure 18. Trendline velocity minus shale unit velocity as a 

function of depth. Average value for each shale unit is shown as 

a vertical line. Trendline velocities and shale unit velocities are 

shown in figure 10. Trendline velocity is selected from the trendline 

in the same well and at the same depth as the shale unit velocity. 
Summary 31

Petroleum generation modeling indicates that gas generation 

commenced at a burial depth of around 10,000 ft (figs. 4 and 

5). 

Sonic velocity in the four shale units remains low to 

the present day, after uplift and possible reduction in pore 

pressure from maximum pressures reached when strata 

were more deeply buried. A plausible explanation for the 

persistence of low sonic velocity is offered in figure 19. The 

burial history curve for the Thermopolis Shale in the Tatman 

Mountain Federal well (fig. 4) is reproduced in figure 19A

with four times at 65, 53, 10, and 0 Ma representing events 

that impact the evolution of pore pressure and sonic velocity. 

These four events also appear in figures 19B, C, and D, 

which display the interdependence of depth, pore pressure, 

effective stress, sonic velocity, and geologic time. As burial 

continues and pore pressure builds after time 1, effective stress 

drops, reaching a minimum at time 3 at the point of deepest 

burial, where pore pressure reaches 0.9 of lithostatic stress 

(fig. 19B). Sonic velocity follows a loading curve, increasing 

with burial to time 1 when effective stress stops increasing 

due to the increase in pore pressure (fig. 19C). After time 1, 

velocity drops, following an unloading curve with a slope 

that differs markedly from the loading curve. (The loadingunloading scenario is based on the model of Bowers (1994), 

who noted that rocks do not follow the same stress-strain path 

for loading and unloading). At time 3 when pore pressure 

reaches a maxium and effective stress reaches a minimum, 

sonic velocity also reaches a minimum (fig. 19C). With 

uplift, pore pressure declines from 0.9 to 0.68 of lithostatic 

stress, effective stress increases correspondingly, and velocity 

increases to its present-day value at time 4 (figs. 19B and C). 

The variation of sonic velocity with depth is plotted in fig. 

19D. The overall effect of the pore pressure increase is to 

freeze sonic velocity of the Thermopolis shale unit at a value 

of around 10,500 ft/s, the observed present-day value in the 

Tatman Mountain Federal well. 

The scenario pictured in figure 19 rests on assumed 

values of time of commencement of pore pressure increases 

(time 1), assumed values of pore pressure and effective stress 

at time 3 and 4, and assumed values defining the loading and 

unloading curves (fig. 19C), although only the initial velocity 

at zero stress and the slope of the loading curve play a role in 

the outcome. Other choices for these parameters could easily 

result in a similar outcome. The main features of the model 

are the burial history for the Tatman Mountain Federal well 

(fig. 19A), the complementarity of pore pressure and effective 

stress (fig. 19B), and the concept of loading and unloading 

curves and the cessation of velocity increase (figs. 19C and 

D). The result is that a velocity observed in present-day rocks 

differs little from the velocity obtained at maximum burial. 

Others have postulated that sonic velocity remains at levels 

reached during overpressured conditions without undergoing 

the increases expected due to subsequent drops in pore pressure (Magara, 1978; Cluff and Cluff, 2004).

Summary

Petrophysical properties from well logs have been 

compiled in 23 wells for four shale units in the Cody, Mowry, 

Shell Creek, and Thermopolis Shales of the Bighorn Basin. 

No borehole corrections were applied to any of the measurements, although the need for such corrections was considered 

before deciding that either corrections were unnecessary or the 

errors incurred in applying a correction were too great to justify making a correction. Measurements were unavailable for 

some shale units in some wells due to incomplete penetration 

of all shale units, incomplete logging runs, poor log quality, 

or excessive washouts. Statistical summaries of the data are 

tabulated in Appendix 1 and are shown as summary diagrams 

in figure 16. 

The petrophysical properties of the Mowry Shale are consistent with its overall higher siliceous content relative to the 

other three shales: sonic velocity is higher, resistivity is higher, 

and neutron porosity is lower in the Mowry than in the lower 

Cody, Shell Creek or Thermopolis shale units. The mineral 

composition of the Mowry Shale appears remarkably similar 

from well to well within the Bighorn Basin, based upon the 

tight clustering of the neutron and density well-log responses 

(figs. 16D and 16E). In addition, the gamma-ray response 

of the Mowry is greater than that of the other shale units, 

although this is not an expected property of highly siliceous 

rock. Uranium, commonly found in organic matter, or illite, 

a potassium-bearing clay mineral, could be producing the 

high gamma-ray response. Illite also has high density and low 

neutron response and hence its presence is compatible with the 

density and neutron response of the Mowry Shale (fig.15). 

This is not the case however, for the two transport properties: sonic velocity varies as much in the Mowry Shale as in 

the other three shales and resistivity varies even more (figs. 

16B and C). Moreover, sonic velocity decreases as resistivity 

decreases—a correlation that is strong when considering all 

four shale units and weak when considering the Mowry shale 

unit singly (fig. 14A). This correlation between sonic velocity 

and resistivity can be attributed to the opening of microcracks, 

causing a slowing of velocity perpendicular to bedding as 

measured by the sonic velocity log, and a decrease in resistivity parallel to bedding, as measured by the induction log. 

All four shale units examined in this study show an 

increase in sonic velocity to a depth of about 10,000 feet. 

Below 10,000 ft, there is little or no further change of sonic 

velocity with depth. 

A model combining burial history, the decrease of effective stress with increasing pore pressure, and Bower’s model 

for the dependence of sonic velocity on effective stress has 

been proposed to explain the persistence of low velocity in 

shale units. As mentioned in the Introduction, interruptions 

to compaction gradients associated with gas occurrences and 

overpressure are observed in correlative strata in other basins 

in Wyoming, so the general results for shales in the Bighorn 

Basin established in this paper should be applicable elsewhere.
TIME (Ma) 120 100 80 60 40 20 0

DEPTH, IN FEET

DEPTH, IN FEET

0

5,000

10,00015,00020,00025,000

53 Ma Peak gas generation

10 Ma Start uplift and erosion

104 Ma Start deposition

1 65 Ma Start pore pressure increase

2

3

4 0 Ma Present day

A

PRESSURE (psi)

0 5,000 10,000 15,000 20,000 25,000

0

5,000

10,00015,00020,00025,000

Lithostatic stress

Hypothetical porepressure

Hydrostaticpressure

Hypotheticaleective stress

104 Ma

53 Ma

10 Ma

0 Ma

65 Ma 1

2

3

4

B

VELOCITY (ft/s)

4,000 6,000 8,000 10,000 12,000 14,000 

EFFECTIVE STRESS (psi)

0

2,0004,0006,0008,000

10,00012,000

1

4

2

3

Loading 

Unloading curve

VELOCITY (ft/s)

4,000 6,000 8,000 10,000 12,000 14,000 

DEPTH (ft)

0

5,000

10,00015,00020,00025,000

1

4

2

3

Withpres

With porepressureincrease

D

C

Figure 19. Evolution of pore pressure, effective stress, and sonic velocity at four times in the Thermopolis shale unit, Tatman MounFederal well. A, Burial history from Roberts and others (2008) with times of four events. B, Pore pressure and effective stress as funcdepth. C, Sonic velocity as a function of effective stress. D, Sonic velocity as a function of depth. 
References Cited 33

Acknowledgments

During the course of this investigation, useful discussions 

were held with Tom Finn, Mark Kirschbaum, Neil Fishman, 

Paul Lillis, and Chris Schenk of the U.S. Geological Survey, 

and Mike Batzle and Manika Prasad of the Colorado School of 

Mines. Reviews by Tom Finn, Lori Burke, and Neil Fishman 

led to improvements in the manuscript. 

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34 Sonic Velocity and Other Petrophysical Properties of Source Rocks, Bighorn Basin, Wyoming

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Appendix 1. File containing petrophysical responses in four shale units in 23 wells.

Map Lease Well Bit Size Type fluid Mud weight

Number Name Number (in.) in well (lb/gal)

15 Buffalo Creek 1 8.75 Fresh gel mud 8.9

13 Bar TL 2 7.875 Chem-Gel 9.5

20 Gywnn Ranch 1 7.875 Oil emulsion 9.4

1 State 56-16 8.75 Gel Chem 8.8

18 Worland Unit 47T-M28 7.875 Chem-Gel 9.3

21 Blue Mesa 1-30 7.875 Fresh gel mud 9.1

3 Gulch 1 8.75 Fresh gel mud 9.3

23 Federal 1-2 8.75 Gel Chem 9.2

4 Spear-Federal 33-31 8.75 Fresh gel mud 10.2

10 Adams 1 8.75 Chem-Gel 9.1

22 Federal 15-22 1 7.875 Chem Gel 9.1

12 Predicament 1-31-3D 8.75 KCl mud 8.9?

11 Fritz-Federal 1-16 2B 8.75 KCl-Dispac 9.1

2 McCullough II 3-27 8.5 KCl mud 9.6

9 Michaels Ranch 17-15 8.5 Fresh gel mud 9.2

19 Gillies Draw Unit 1 8.5 Fresh gel mud 9.4

14 Tatman Mtn Federal 1-20 8.5 Dispersed 9.04

5 Bridger Trail Unit 2-A 8.5 KCl mud 9

8 Emblem Bench 1 7.875 KCl mud 10.7

17 Ridge Unit 1-21 8.5 Fresh gel mud 9.2

6 Red Point II Unit 1 6 Fresh gel mud 10.1

16 Sellers Draw Unit 1 8.75 Fresh gel mud 9.3

7 Katrine Loch 1 8.9 Oil base mud 11.0?

11/24/2009

Well Information

Statistical 

summary for 

all 23 wells

Statistical 

summary for 

depths 

greater than 

8,750 ft 

Appendix 1

Appendix 1 35
36 Sonic Velocity and Other Petrophysical Properties of Source Rocks, Bighorn Basin, Wyoming

Depth Vtrendline Vshale unit Vdifference Neutron Density Resistivity Gamma ray Caliper Caliper-Bit

(ft) (ft/s) (ft/) (ft/s) (percent) (g/cc) (ohm-m) (API) (in.) (in.)

4771 13388 11111 2277 4.3 114.2 11.7 3.0

5203 12781 10870 1911 3.9 109.6 9.0 1.1

5968 13291 11111 2180 6.3 103.4 9.4 1.5

6418 14489 11710 2780 28.0 6.3 89.6 15.0 6.3

6861 12582 11161 1421 9.2 118.8 10.5 2.6

8792 14745 12821 1924 10.8 111.1 9.0 1.1

8849 15250 12180 3069 10.8 89.6 15.4 6.7

9779 14078 12453 1625 28.9 11.2 81.2 10.6 1.9

9792 15522 12658 2864 28.6 15.6 82.8 12.0 3.3

10114 14883 12392 2491 32.5 12.6 98.8 11.5 2.8

10321 14704 12771 1932 20.4 98.8 9.8 1.9

11184 14674 12500 2174 22.9 11.0 113.3 14.5 5.8

11744 15113 12594 2519 18.5 8.3 10.7 2.0

11848 15062 12626 2436 25.0 7.0 104.9 11.3 2.8

13176 15065 12019 3046 30.0 6.7 91.0 14.2 5.7

13572 15169 13369 1800 34.0 81.8 14.5 6.0

14451 14846 12438 2408 29.0 14.8 83.3 12.8 4.3

15280 15687 13141 2546 21.5 11.5 114.0 10.5 2.0

16851 15068 11765 3303 26.9 8.2 105.2 14.5 6.6

16871 15418 12658 2760 26.4 11.5 92.3 16.5 8.0

17467 14522 11364 3158 23.5 5.7 94.1 8.9 2.9

18428 15534 13333 2201 28.0 16.5 83.3 16.1 7.4

19296 14848 12346 2503 22.0 2.64 9.2 112.7 8.6 -0.3

10870 1421 18.5 3.9 81.2 8.9 1.1

13369 3303 34.0 20.4 118.8 16.5 8.0

12234 2406 26.6 10.1 98.8 12.2 3.9

23 23 16 22 22 22 22

727 503 4.2 4.2 12.5 2.5 2.2

Minimum 11364 18.5 5.7 81.2 -0.3

25th percentile 12357 23.2 8.3 83.3 2.0

Median 12547 26.9 11.0 94.1 3.1

75th percentile 12743 28.9 12.6 105.2 5.9

Maxiumum 13369 34.0 20.4 114.0 8.0

Minimum

Maximum

Average

Count

Standard deviation

Lower Cody Shale

Appendix 1. File containing petrophysical responses in four shale units in 23 wells.–-Continued
Appendix 1 37

Depth Vtrendline Vshale unit Vdifference Neutron Density Resistivity Gamma ray Caliper Caliper-Bit

(ft) (ft/s) (ft/) (ft/s) (percent) (g/cc) (ohm-m) (API) (in.) (in.)

5617 13461 12953 508 2.52 9.2 151.2 9.5 0.8

6057 13103 12019 1084 2.39 6.3 137.3 8.6 0.7

6871 13773 12413 1360 12.6 121.9 8.6 0.7

7175 14721 13557 1164 20.0 2.55 10.8 125.0 10.3 1.6

7871 12846 12315 530 2.51 7.8 132.7 8.7 0.8

9674 14995 13158 1837 22.7 142.0 8.5 0.6

9824 15481 14286 1195 2.60 29.6 106.4 10.2 1.5

10702 14212 13514 698 22.1 2.58 21.0 116.9 10.6 1.9

10835 15588 14535 1053 19.4 2.61 52.9 100.8 10.0 1.3

11100 14911 13866 1045 19.0 2.57 31.3 137.3 10.0 1.3

11273 14775 14205 571 40.9 135.8 9.0 1.1

12123 14834 13736 1098 16.8 2.55 24.9 140.4 9.5 0.8

12682 15287 13721 1566 14.7 2.59 26.7 10.0 1.3

12751 15113 13866 1247 19.0 2.60 16.5 118.8 9.7 1.2

14033 15118 13514 1604 13.3 108.0 12.5 4.0

14497 15305 14124 1180 19.0 2.56 117.3 10.5 2.0

15364 14896 13587 1309 16.0 2.60 18.3 118.8 9.5 1.0

16111 15796 14451 1345 15.2 2.58 20.4 145.3 9.1 0.6

17769 15120 14021 1098 16.6 2.59 17.4 130.3 8.5 0.6

17691 15541 13889 1652 18.1 2.59 18.4 130.6 10.0 1.5

18379 14570 14451 119 15.0 2.51 11.4 145.1 5.5 -0.5

19282 15664 14451 1213 17.7 2.62 28.4 132.4 9.3 0.6

20246 14898 13736 1161 17.0 2.63 7.0 135.8 9.0 0.1

12019 119 14.7 2.39 6.3 100.8 5.5 -0.5

14535 1837 22.1 2.63 52.9 151.2 12.5 4.0

13668 1115 17.7 2.56 20.4 128.6 9.5 1.1

23 23 15 19 22 22 22 22

697 406 2.1 0.06 11.5 13.6 1.3 0.8

Minimum 13158 14.7 2.51 7.0 100.8 -0.5

25th percentile 13725 16.1 2.57 17.4 117.3 0.6

Median 13877 17.3 2.59 21.0 130.6 1.2

75th percentile 14265 19.0 2.60 28.4 137.3 1.4

Maxiumum 14535 22.1 2.63 52.9 145.3 4.0

Mowry Shale

Minimum

Maximum

Average

Count

Standard deviation

Appendix 1. File containing petrophysical responses in four shale units in 23 wells.–-Continued
38 Sonic Velocity and Other Petrophysical Properties of Source Rocks, Bighorn Basin, Wyoming

Depth Vtrendline Vshale unit Vdifference Neutron Density Resistivity Gamma ray Caliper Caliper-Bit

(ft) (ft/s) (ft/) (ft/s) (percent) (g/cc) (ohm-m) (API) (in.) (in.)

5939 13489 10526 2963 2.8 134.2 11.5 2.8

6227 13169 10417 2753 2.6 137.3 9.0 1.1

7097 13900 11013 2886 4.3 101.9 9.0 1.1

7508 14826 10460 4365 3.5 121.9 13.0 4.3

8106 12909 10050 2858 4.3 111.1 14.5 6.6

9931 15070 11862 3207 5.7 123.4 8.7 0.8

10069 15540 12034 3507 6.3 112.7 12.0 3.3

10900 14241 12063 2178 31.3 5.8 120.4 13.5 4.8

11099 15605 12195 3410 38.6 7.8 100.6 13.5 4.8

11290 14916 12136 2780 37.0 8.3 117.3 12.5 3.8

11517 14794 12407 2387 8.7 140.4 9.5 1.6

12302 14865 12300 2565 26.4 6.8 151.1 10.5 1.8

12891 15327 12195 3132 23.0 7.0 9.7 0.9

13052 15130 11429 3702 37.0 4.3 120.3 14.0 5.5

14268 15133 10953 4180 3.7 97.2 14.5 6.0

14720 15338 12674 2664 97.2 16.1 7.6

15518 14905 12500 2405 20.0 7.4 98.8 11.5 3.0

16303 15821 11806 4015 30.7 4.6 130.0 10.0 1.5

17954 15130 11820 3310 34.6 3.7 122.3 12.5 4.6

17957 15581 11765 3817 34.6 4.7 118.2 15.0 6.5

Not reached

19536 15703 13263 2440 26.8 6.4 125.6 9.5 0.8

20399 14906 11905 3001 23.5 2.67 3.7 138.9 8.6 -0.3

10050 2178 20.0 2.6 97.2 8.7 0.8

13263 4365 38.6 8.7 151.1 16.1 7.6

11717 3115 30.3 5.4 120.0 11.9 3.5

22 22 12 21 21 21 21

825 613 6.2 1.8 15.4 2.3 2.2

Minimum 10953 20.0 3.7 97.2 -0.3

25th percentile 11820 25.7 4.5 109.6 1.5

Median 12063 31.0 6.1 120.4 3.3

75th percentile 12300 35.2 7.1 126.7 4.8

Maxiumum 13263 38.6 8.7 151.1 7.6

Shell Creek Shale

Minimum

Maximum

Average

Count

Standard deviation

Appendix 1. File containing petrophysical responses in four shale units in 23 wells.–-Continued
Appendix 1 39

Depth Vtrendline Vshale unit Vdifference Neutron Density Resistivity Gamma ray Caliper Caliper-Bit

(ft) (ft/s) (ft/) (ft/s) (percent) (g/cc) (ohm-m) (API) (in.) (in.)

6060 13500 10204 3295 3.0 106.4 13.0 4.3

6484 13271 10152 3118 3.5 108.0 11.0 3.1

7243 13982 9833 4149 4.6 75.7 12.0 4.1

7645 14869 10246 4623 47.0 3.2 100.3 18.0 9.3

8197 12933 10000 2933 4.6 91.0 14.0 6.1

10037 15101 10905 4196 5.4 101.9 9.5 1.6

10216 15576 10638 4938 4.8 88.0 16.0 7.3

11031 14260 10881 3379 44.4 4.9 92.8 12.0 3.3

11344 15620 10256 5364 46.1 10.3 91.3 12.5 3.8

11438 14920 10593 4327 6.7 95.7 15.0 6.3

11618 14801 11299 3502 7.0 111.1 10.0 2.1

12447 14890 11338 3552 30.2 6.2 108.6 15.0 6.3

13024 15352 11338 4014 23.5 8.3 10.5 1.8

13160 15136 11038 4099 36.0 3.9 91.0 15.0 6.5

14390 15140 9785 5355 46.0 2.8 80.2

14867 15360 10929 4431 78.7 >16

15724 14916 10526 4390 3.7 89.6 14.0 5.5

16580 15858 11468 4390 33.0 4.6 120.8 14.0 5.5

18148 15141 10256 4885 37.6 2.9 107.6 14.0 6.1

18064 15598 10101 5497 41.0 4.3 92.3 16.0 7.5

Not reached

19656 15721 11348 4373 6.3 15.0 6.3

20594 14916 10989 3927 30.0 2.65 2.6 125.0 8.6 -0.3

9785 2933 23.5 2.6 75.7 9.5 1.6

11468 5497 47.0 10.3 125.0 18.0 9.3

10642 4215 37.7 4.9 97.8 13.5 5.1

22 22 11 21 20 19 19

537 725 7.9 2.0 13.3 2.3 2.1

Minimum 9785 23.5 2.6 78.7 -0.3

25th percentile 10526 30.9 3.9 90.3 2.7

Median 10905 36.8 4.9 92.8 5.5

75th percentile 11299 43.6 6.4 108.1 6.3

Maxiumum 11468 46.1 10.3 125.0 7.5

Average

Count

Standard deviation

Minimum

Maximum

Thermopolis Shale

Appendix 1. File containing petrophysical responses in four shale units in 23 wells.–-Continued

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