Burial, Thermal, and Petroleum Generation 

History of the Upper Cretaceous 

Steele Member of the Cody Shale 

(Shannon Sandstone Bed Horizon), 

Powder River Basin, Wyoming

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Chapter A

Burial, Thermal, and Petroleum Generation 

History of the Upper Cretaceous 

Steele Member of the Cody Shale 

(Shannon Sandstone Bed Horizon), 

Powder River Basin, Wyoming

By VITO F. NUCCIO

A multidisciplinary approach to research studies of 

sedimentary rocks and their constituents and the evolution of 

sedimentary basins, both ancient and modern

U.S. GEOLOGICAL SURVEY BULLETIN 1917

EVOLUTION OF SEDIMENTARY BASINS POWDER RIVER BASIN
DEPARTMENT OF THE INTERIOR 

MANUEL LUJAN, JR., Secretary

U.S. GEOLOGICAL SURVEY 

Dallas L. Peck, Director

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

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

UNITED STATES GOVERNMENT PRINTING OFFICE: 1990

For sale by the

Books and Open-File Reports Section

U.S. Geological Survey

Federal Center

Box 25425

Denver, CO 80225

Library of Congress Cataloging-in-Publication Data

Nuccio, Vito F.

Burial, thermal, and petroleum generation history of the Upper Cretaceous 

Steele Member of the Cody Shale (Shannon Sandstone Member Horizon), 

Powder River Basin, Wyoming / by Vito F. Nuccio.

p. cm. (Evolution of sedimentary basins Powder River Basin ; 

ch. A) (U.S. Geological Survey bulletin ; 1917-A)

"A multidisciplinary approach to research studies of sedimentary rocks and 

their constituents and the evolution of sedimentary basins, both ancient and 

modern."

Includes bibliographical references.

Supt. of Docs, no.: I 19.3:1917A

1. Geology, Stratigraphic Cretaceous. 2. Geology Wyoming Steele 

Shale. 3. Petroleum Geology Wyoming. 4. Steele Shale (Wyo.) 

I. Title. II. Series. III. Series: U.S. Geological Survey bulletin ; 1917-A. 

QE75.B9 no. 1917-A 

[QE688]

557.3s dc20 90-3116 

[551.7'7'097873] CIP
CONTENTS

Abstract Al 

Introduction Al 

Regional geology A3

Upper Cretaceous strata A3

Paleocene strata A4

Eocene strata A4

Oligocene strata AS

Miocene strata AS 

Methods AS 

Results A10

Source rock evaluation A10

Organic matter quantity A10 

Organic matter quality A10

Vitrinite reflectance A10

Rock-Eval pyrolysis All

Time-temperature and kinetic models All

Northwestern part of the study area All 

Southwestern part of the study area A12 

Discussion A13 

Summary A15 

References cited A16

FIGURES

1. Map showing geology, location of present-day trough line, and sample 

localities, Powder River Basin, Wyoming A2

2. Chart showing stratigraphy of rocks in the western Powder River 

Basin A3

3. Map showing area of vitrinite reflectance values greater than 0.60 

percent, Powder River Basin A8

4. Modified van Krevelen diagram showing type of kerogen in samples of

Steele member A9 

5-10. Diagrams showing:

5. Time-temperature model for northwestern part of study 

area All

6. Kinetic model, type II kerogen, for northwestern part of study 

area A12

7. Kinetic model, type III kerogen, for northwestern part of study 

area A12

8. Time-temperature model for southwestern part of study 

area A13

9. Kinetic model, type II kerogen, for southwestern part of study 

area A14

10. Kinetic model, type III kerogen, for southwestern part of study 

area A14

Contents III
TABLES

1. Data used to reconstruct burial history of the Shannon Sandstone Bed of the 

Steele Member of the Cody Shale, Powder River Basin, Wyoming A4

2. Mean random vitrinite reflectance data for Cretaceous rocks, Powder River 

Basin, Wyoming A6

3. Rock-Eval pyrolysis data for selected samples of the Steele Member of the 

Cody Shale, Powder River Basin, Wyoming A9

CONVERSION FACTORS FOR SOME SI METRIC AND U.S. UNITS OF MEASURE

____To convert from_________ To______________Multiply by_____ 

Feet (ft) Meters (m) 0.3048 

Miles (mi) Kilometers (km) 1.609 

Pounds (Ib) Kilograms (kg) 0.4536 

Degrees Fahrenheit (°F) Degrees Celsius (°C) Temp °C = (temp °F-32)/1.8

IV Contents
EVOLUTION OF SEDIMENTARY BASINS-POWDER RIVER BASIN

Burial, Thermal, and Petroleum Generation 

History of the Upper Cretaceous Steele Member of the 

Cody Shale (Shannon Sandstone Bed Horizon), 

Powder River Basin, Wyoming

By Vito F. Nuccio

Abstract

The burial, thermal, and petroleum generation history of 

the Upper Cretaceous Steele Member of the Cody Shale at 

approximately the Shannon Sandstone Bed horizon has 

been reconstructed for the northwestern and southwestern 

parts of the Powder River Basin of Wyoming by using vitrinite 

reflectance and Rock-Eval pyrolysis data and timetemperature and kinetic modeling. Organic geochemical and 

petrographic data indicate that the Steele Member contains 

enough kerogen of the proper types (II and III) to be 

considered a potential source rock for both oil and gas in the 

western part of the Powder River Basin.

In the northwestern part of the basin, the Shannon 

Sandstone Bed was buried to approximately 12,000 ft (3,660 

m) and attained a paleotemperature of 230 °F (110 °C) at 

maximum burial 10 Ma. From 10 Ma to present, erosion of 

2,700 ft (824 m) of section has caused a lowering of 

temperatures. Vitrinite reflectance and Rock-Eval pyrolysis 

data indicate that the Steele Member is immature to mature, 

and thus initiation of oil generation is constrained to at or near 

maximum burial 10 Ma. Time-temperature and kinetic 

modeling (assuming type II kerogen) puts the beginning of 

oil generation between 35 and 25 Ma, after the Laramide 

orogeny and before maximum burial. Kinetic modeling (type 

III kerogen) indicates that some hydrocarbon generation 

began as early as 36 Ma.

In the southwestern part of the basin, the Shannon 

Sandstone Bed was buried to approximately 13,000 ft (3,965 

m) and reached a temperature of 248 °F (120 °C) at 

maximum burial. Erosion of 2,070 ft (631 m) of section has 

resulted in a lowering of temperatures. Vitrinite reflectance 

and Rock-Eval pyrolysis data again show that the Steele

Manuscript approved for publication November 2, 1989.

Member is immature to mature and that oil generation began 

at or near maximum burial 10 Ma. Time-temperature and 

kinetic modeling of type II kerogen, on the other hand, 

constrain oil generation to between 47 and 38 Ma both 

during and after the Laramide orogeny and before maximum 

burial. Kinetic modeling using type II kerogen shows that the 

Steele Member attained a transformation ratio or production 

index of 0.40 at 10 Ma and that thermal cracking of oil to gas 

possibly occurred. Results of this modeling, along with the 

fact that the Steele Member has generated some thermogenic gas (based on kinetic modeling of type III kerogen), 

suggest that Shannon Sandstone reservoirs could contain 

some volumes of gas.

Each of the four independent maturity indicators (vitrinite reflectance, Rock-Eval pyrolysis, time-temperature 

modeling, and kinetic modeling) yields a different time for the 

beginning of oil generation. Vitrinite reflectance and RockEval pyrolysis data indicate the most recent time for oil 

generation. Time-temperature and kinetic modeling indicate 

a higher level of maturity and an earlier time for petroleum 

generation. These different results indicate the importance of 

using as many techniques as possible when assessing the 

thermal maturity and timing of petroleum generation for a 

potential source rock.

INTRODUCTION

The lower Campanian Shannon Sandstone Bed of 

the lower and middle Campanian Steele Member of the 

Cody Shale is a prolific oil and gas reservoir in parts of 

the Powder River Basin, Wyoming (fig. 1), and probably 

contains oil and gas from several different source rocks. 

Nixon (1973), in a study of the Upper Cretaceous Mowry 

Shale, and Merewether and Claypool (1980), in a study 

of Lower and Upper Cretaceous source rocks, concluded

Petroleum Generation History of Steele Member of Cody Shale, Wyoming A1
107 C 106° 105°

, 'SHERIDAN

.L..L.U

Kmv

EXPLANATION 

Wasatch Formation

Fort Union Formation 

Lance Formation

Lance Formation, Fox Hills 

Sandstone and Lewis Shale, 

undivided

Fox Hills Sandstone and 

Lewis Shale

Mesaverde Formation

Pre-Mesaverde rocks 

Contact 

Fault 

""" T "** Present-day troughlinc

' Sample locality; referred to 

by number in table 2

1 I T 

10 20 30 KILOMETERS

Figure 1. Geology of the Powder River Basin of Wyoming. Location of present-day trough line also shown. Sample localities 

(solid circles) are described by number in table 2.

that these strata are important source rocks for oil in the Niobrara Members of the Cody Shale and stated that

Powder River Basin. Momper and Williams (1984) these source rocks expelled most of the discovered oil in

presented geochemical data for the Upper Cretaceous the Powder River Basin. Burial, thermal, and geoMowry Shale and the Upper Cretaceous Sage Breaks and chemical data presented in this study suggest that the

A2 Evolution of Sedimentary Basins Powder River Basin
Steele Member of the Cody Shale is also a potential 

petroleum source rock over large areas of the western 

part of the Powder River Basin. It is likely that oil and gas 

generated from the Steele is contained in Shannon 

reservoirs.

The purposes of this study are to (1) present burial 

histories representative of the northwestern and 

southwestern parts of the Powder River Basin (south of 

lat 45° N.), (2) show the maximum level of thermal 

maturity for the Steele Member and its Shannon 

Sandstone Bed, and (3) show the source-rock potential 

and timing of petroleum generation for the Steele. It is 

hoped that data presented in this study will also lead to a 

better understanding of the burial and temperature 

history of the Shannon Sandstone Bed, an understanding 

crucial for diagenetic studies, fluid-flow modeling, and 

reservoir-rock characterization.

REGIONAL GEOLOGY

Only the geologic history of the Steele Member of 

the Cody and overlying stratigraphic units (fig. 2) is 

discussed because this study concerns the burial history 

of the Steele (Shannon Sandstone Bed horizon).

Upper Cretaceous Strata

During the Cretaceous, the Powder River Basin 

was part of a larger subsiding trough (fig. 1) that 

occupied much of the central United States and central 

Canada. This trough formed on the east side of the rising 

Sevier orogenic belt and was filled by an epicontinental 

sea during much of the Cretaceous. Upper Cretaceous 

strata in the Powder River Basin are characterized by 

deposition during a series of westward transgressions and 

eastward regressions of the epeiric sea. Upper 

Cretaceous rocks thicken from about 4,000 ft (1,220 m) 

at the northern margin of the basin to about 10,000 ft 

(3,050 m) at the southern margin (O.K. Higley and B.L. 

Crysdale, U.S. Geological Survey, written commun., 

1988) and contain coarser grained detritus in the western 

part of the basin.

The Steele Member of the Cody Shale of eastern 

Wyoming is a thick marine sequence deposited in the 

seaway during middle Late Cretaceous time (early and 

middle Campanian). These dark-gray marine shales are 

considered to be fair to good petroleum source rocks 

(Merewether and Claypool, 1980; Momper and 

Williams, 1984). A succession of marine sandstones, 

possibly nearshore deposits (in ascending order, the 

"Fishtooth" sandstone and the Shannon and Sussex 

Sandstone Beds of the Steele), were also deposited in the 

seaway. These shallow-water marine sandstones have 

been the object of oil and gas exploration in many parts 

of the Powder River Basin.

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Eocene

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Age (Ma)

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35-

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Stratigraphic Unit

"O^XJ/" ^LJ^ ^vL>" XJX"^

Arikaree 

Formation KcannrarD

White River Group

d>a>^axoxD^D^

Wasatch 

Formation

Tongue River Mbr

hort Union Lebo Shale 

Formation Member

Tullock Member

Lance 

Formation

Fox Hills 

Sandstone

Lewis 

Shale

Mesaverde 

Formation

Shannon Ss Bed I

Steele Member (part) of Cody Shale

Figure 2. Stratigraphy of Cretaceous and Tertiary rocks in 

the western part of the Powder River Basin, Wyoming. 

Nomenclature and ages of units correspond to those in 

table 1.

In middle Campanian to early Maastrichtian time, 

coincident with the eastward withdrawal of the 

Cretaceous sea, deposition in the study area changed 

from marine to marginal marine and nonmarine. The 

Mesaverde Formation, deposited in fluvial, coastal-plain, 

and shallow-water marine environments, is 

representative of this time period (fig. 2). The overlying 

marine Lewis Shale represents another westward 

transgression of the seaway during early to middle Maastrichtian time. In middle Maastrichtian time, in response 

to the beginning of the Laramide orogeny, there was a 

final eastward withdrawal of the sea from the Powder 

River Basin. During this regressive stage, marginalmarine sediments of the Fox Hills Sandstone and fluvial 

sediments of the Lance Formation were deposited. The

Petroleum Generation History of Steele Member of Cody Shale, Wyoming A3
Table 1 . Data used to reconstruct burial history of the Shannon Sandstone Bed of the Steele Member of the Cody Shale,

Powder River Basin, Wyoming

[Mean annual surface temperature of 50 °F (10 °Q is assumed for the entire burial history]

Age

(Ma) Formation or event

Thickness 

(feet) (meters)

Northwestern Powder River Basin

0-10

10-18

18-31

31-35

35-38

38-55

55-66

66-81

0-10

10-18

18-31

31-35

35-38

38-55

55-66

66-81

Uplift and erosion

Arikaree Formation

Hiatus

White River Formation

Hiatus

Wasatch Formation

Fort Union Formation

Steele Member (part) and overlying Cretaceous formations

Uplift and erosion

Arikaree Formation

Hiatus

White River Formation

Hiatus

Wasatch Formation

Fort Union Formation

Southwestern Powder River Basin

Steele Member (part) and overlying Cretaceous formations

-2,735

1,000

0

800

0

2,000

5,100

3,125

-2,068

1,000

0

1,000

0

1,000

4,400

5,500

-834

305

0

244

0

610

1,556

953

-631

305

0

305

0

305

1,342

1,678

Fox Hills Sandstone is dominantly sandstone and sandy 

shale, whereas the Lance Formation comprises shale, 

lenticular sandstone, and, in the lower half, several thin 

coal beds (Seeland, 1985).

Upper Cretaceous strata (Shannon Sandstone Bed 

of the Steele Member through the Lance Formation) in 

the northwestern part of the study area are about 3,125 ft 

(953 m) thick and in the southwestern part about 5,500 ft 

(1,678 m) thick, based on the selection of formation tops 

from geophysical borehole logs (table 1, fig. 2).

Paleocene Strata

The contact between the Lance Formation and the 

nonmarine Paleocene Fort Union Formation is 

gradational and in most areas difficult to determine. The 

boundary between the Cretaceous and Tertiary may be at 

the contact between these two formations or within the 

Lance Formation (Bohor and others, 1987; Fox and 

others, in press). Laramide uplifts were not prominent 

features in eastern Wyoming until late Paleocene time 

(Seeland, 1985), when the first coarse clastic orogenic 

sediments of Tertiary age were deposited in the Powder 

River Basin. The Fort Union Formation is divided into 

three members, in ascending order, the Tullock, the 

Lebo Shale, and the Tongue River Members.

Isopach maps of the Tullock Member (Curry, 

1971) indicate that there was subsidence, represented by 

moderate thickening, in the southern and southwestern 

parts of the basin; however, no major subsidence along

the present structural axis of the basin is indicated. The 

Tullock Member is interpreted as a fluvial and paludal 

unit consisting of thin sandstone and mudstone and 

discontinuous, mostly thin coal beds (Seeland, 1985; 

Ayers, 1986).

The Lebo Shale Member may represent 

subsidence along the basin axis. A lake could have 

formed where the mudstone, shale, discontinuous 

sandstone, and sparce carbonate rocks of the Lebo are 

found (Ayers, 1986).

The upper Paleocene Tongue River Member 

shows the first evidence of significant Laramide activity. 

Uplift along the western margin of the basin is 

represented by coarse sandstone and conglomerate 

(Weaver and Flores, 1985).

For this study, thicknesses are reported for the 

entire Fort Union Formation and not for individual 

members. In the northwestern part of the study area, the 

Fort Union Formation is about 5,100 ft (1,556 m) thick, 

as determined from geophysical borehole logs. In the 

southwestern part, it is about 4,400 ft (1,342 m) thick 

(table 1, fig. 2).

Eocene Strata

By early Eocene time, the basins of Wyoming were 

well defined, and boulder conglomerates of the Kingsbury and Moncrief Members of the Paleocene and 

Eocene Wasatch Formation had been deposited in 

alluvial fans along the east side of the Bighorn Mountains 

in the Powder River Basin (Seeland, 1985). The

A4 Evolution of Sedimentary Basins Powder River Basin
composition of the Wasatch Formation has been 

described as arkosic sandstone, lenticular conglomerate, 

siltstone, carbonaceous shale, many coal beds, and some 

variegated mudstone (Seeland, 1985). The Wasatch 

Formation ranges from 2,000 ft (610 m) thick in the 

northwestern part of the study area to 1,000 ft (305 m) 

thick in the southwestern part (Curry, 1971; Seeland, 

1985; N.M Denson, U.S. Geological Survey, oral commun., 1989) (table 1, fig. 2). It should be noted that 

Tschudy (1976) documented the lower 300 ft (100 m) of 

the Wasatch Formation in the Powder River Basin as 

being Paleocene in age.

Oligocene Strata

Laramide deformation abruptly ceased before 

Oligocene time, during which volcaniclastic deposits of 

the White River Group buried the Precambrian cores of 

mountains and filled the basin (Curry, 1971). Seeland 

(1985) stated that the Oligocene White River Group 

overlies a late Eocene land surface that formed on many 

different stratigraphic units throughout the northern 

Great Plains. The White River Group is dominantly a 

fine-grained fluvial unit derived in large part from air-fall 

volcanic ash, but it also contains pre-Oligocene sands and 

gravels derived from Laramide uplifts (Seeland, 1985).

Determining the thickness of the Oligocene White 

River Group in the Powder River basin is difficult 

because only a few remaining occurrences of these strata 

remain. McKenna and Love (1972) diagrammatically 

reconstructed the thickness of Eocene, Oligocene, and 

Miocene rocks across the Powder River Basin by 

correlating units from the Bighorn Mountains on the 

west margin of the basin with those from North Pumpkin 

Buttes in the middle of the basin and from the Black Hills 

on the east side of the basin. In the northern part of the 

study area, the White River Group is about 800 ft (244 

m) thick (N.M Denson, oral commun., 1989). In the 

southern part, the group is about 1,000 ft (305 m) thick 

(McKenna and Love, 1972; N.M Denson, oral commun., 

1989) (table 1, fig. 2).

Miocene Strata

Reconstructing the thickness and paleogeography 

of the Miocene Arikaree Formation in the study area is 

difficult due to the lack of preservation of these strata. 

The closest remaining occurrences of the Arikaree that 

can be studied in detail are in the northwestern part of 

Nebraska.

In northwestern Nebraska, fluvial erosion in the 

late Oligocene produced an unconformity that separates 

the Arikaree Group from older Cenozoic units (Swinehart and others, 1985). The estimated stratigraphic 

position of this unconformity in the study area is shown in 

figure 2. The Arikaree Group was deposited on this 

unconformable erosion surface and is made up of gray 

and brownish-gray, volcaniclastic silty sandstone and 

local occurrences of coarser grained sandstone at the 

base (Swinehart and others, 1985).

Using published data for the Bighorn Mountains 

(McKenna and Love, 1972) and northwestern Nebraska 

(Swinehart and others, 1985) and unpublished work by 

N.M. Denson (oral commun., 1989), the original 

thickness of the Arikaree Formation in the Powder River 

Basin is estimated as 1,000 ft (305 m) for both the 

northern and southern parts of the study area.

Beginning approximately 10 Ma, regional uplift 

and erosion removed most Oligocene and Miocene 

strata, leaving only Eocene and older rocks exposed in 

the Powder River Basin.

METHODS

Mean random vitrinite reflectance (Rm) analyses 

(Stach and others, 1982) for samples of the Steele 

Member from 15 wells throughout the central and 

western parts of the study area were used to determine 

levels of thermal maturity (fig. 1, table 2). Samples 1-8 

are from core of Steele collected immediately below the 

Shannon Sandstone Bed. Samples 9-15 were collected by 

Amoco Oil Company, and the vitrinite reflectance data 

(unpublished) represent the thermal maturity of the 

Steele at approximately the Shannon horizon. Figure 3, a 

vitrinite reflectance map of the Steele Member at the 

Shannon Sandstone Bed horizon in the central and 

western parts of the basin in Wyoming, illustrates where 

the Steele is sufficiently thermally mature to have 

generated petroleum.

Rock-Eval pyrolysis analysis, a rapid hydrocarbon 

source rock evaluation technique and maturation 

indicator, was used for samples 2-8 (table 3). Details of 

the analytical technique are given in Espitalie and others 

(1977).

The burial histories of the Shannon and younger 

rocks in the two parts of the study area (northwestern 

and southwestern parts of the basin) were constructed by 

using stratigraphic thicknesses (table 1) obtained mainly 

from Curry (1971), Seeland (1985), Swinehart and others 

(1985), Ayers (1986), N.M. Denson (oral commun., 

1989), F.W. Pierce (U.S. Geological Survey, oral commun., 1989), and D.A. Seeland (U.S. Geological Survey, 

oral commun., 1989).

Time-temperature index (TTI) modeling was 

performed to define levels of thermal maturity and the 

timing of petroleum generation for the Steele Member in 

the two parts of the study area. Kinetic modelingPetroleum Generation History of Steele Member of Cody Shale, Wyoming A5
Table 2. Mean random vitrinite reflectance data for Cretaceous rocks, Powder River Basin, Wyoming

[Sample numbers and locations keyed to index map (fig. 1) and vitrinite reflectance (Rm) map (fig. 3). Location is in section, township (north), and

range (west)]

Sample 

number

1

2

3

4

5

6

7

8

9

10

11

12

13

USGS Core 

Library 

number

A729

B626

A771

A817

A915

C651

C652

C221

Location

1, 48 N., 78 W.

36, 46 N., 77 W.

16,45N.,76W.

22, 44 N., 77 W.

4,43N.,74W.

7, 42 N., 75 W.

13, 41 N., 76 W.

10, 38 N., 78 W.

9.54N..77W.

16, 51 N., 81 W.

2, 47 N., 78 W.

13, 48 N. 72 W.

25, 45 N., 72 W.

Formation

Steele Member

Steele Member

Steele Member

Steele Member

Steele Member

Steele Member

Steele Member

Steele Member

Steele Member

Steele Member

Steele Member

Steele Member

Steele Member

Steele Member

Steele Member

Steele Member

Steele Member

Steele Member

Steele Member

Steele Member

Steele Member

Steele Member

Niobrara Member

Niobrara Member

Carlile Shale

Mowry Shale

Mowry Shale

Mowry Shale

Mowry Shale

Muddy Sandstone

Skull Creek Shale

Skull Creek Shale

Muddy Sandstone

Muddy Sandstone

Steele Member

Steele Member

Steele Member

Steele Member

Steele Member

(feet)

8,606

8,610

9,377

9,385

9,389

9,447

9,886

9,892

9,947

9,327

9,344

9,352

9,374

10,197

10,228

10,242

10,807

10,828

10,832

258

352

359

7,065

7,200

7,275

7,545

8,505

8,550

8,630

8,675

8,770

8,845

8,890

13,842

13,900

9,290

9,095

8,820

8,040

8,130

Depth 

(meters)

2,625

2,626

2,860

2,862

2,864

2,881

3,015

3,017

3,034

2,845

2,850

2,852

2,859

3,110

3,120

3,124

3,296

3,303

3304

79

107

110

2,155

2,194

2,217

2,300

2,592

2,606

2,630

2,644

2,673

2,696

2,710

4,219

4,237

2,833

2,774

2,690

2,452

2,480

Rm 

(percent)

0.61

.59

.58

.56

.56

.55

.58

.57

.57

.55

.54

.54

.59

.64

.62

.62

.66

.65

.66

.39

.40

.39

.49

.50

.47

.50

.56

.58

.55

.50

.51

.65

.66

1.30

1.45

.50

.54

.51

.49

.47

estimating the temperatures and chemical reactions of 

organic matter during burial and thermal metamorphism was also used to determine the level of thermal 

maturity and the timing of petroleum generation 

maturation for the Steele Member. The computer 

program BasinMod (Platte River Associates, 1989) was 

used for the TTI modeling, and the program MATOIL 

(Institut francais du Petrole, 1987) was used for the 

kinetic modeling.

Variable paleogeothermal gradients were assumed 

for the TTI and kinetic models. A paleogeothermal 

gradient of 1.60 °F/100 ft (29.2 °C/km), consistent with 

that of stable continental platforms and foreland basins

(Tissot and Welte, 1984), was assumed for part of the 

Late Cretaceous (80-66 Ma) in the Powder River Basin. 

From the beginning of the Tertiary (66 Ma) through the 

Eocene (38 Ma), a paleogeothermal gradient of 1.40 

°F/100 ft (25.4 °C/km) was used. This lower gradient 

represents a cooling of geothermal gradients in the 

western part of the basin resulting from circulation of 

relatively cool meteoric water into the basin along faults 

associated with the Bighorn mountain front during the 

Laramide orogeny (Momper and Williams, 1984; J.L. 

Clayton and J.D. King, U.S. Geological Survey, written 

commun., 1988). From 38 Ma to present, a paleogeothermal gradient of 1.60 °F/100 ft (29.2 °C/km) was

A6 Evolution of Sedimentary Basins Powder River Basin
Table 2. Continued

USGS Core 

Sample Library 

number number Location Formation

14 30, 44 N., 80 W. Steele Member

Steele Member

Steele Member

Steele Member

Steele Member

Steele Member

Steele Member

Steele Member

Steele Member

Niobrara Member

Niobrara Member

Niobrara Member

Niobrara Member

Sage Breaks Member

Sage Breaks Member

Sage Breaks Member

Frontier Formation

Frontier Formation

Frontier Formation

Frontier Formation

Frontier Formation

Frontier Formation

Frontier Formation

Mowry Shale

Mowry Shale

Mowry Shale

Mowry Shale

Muddy Sandstone

Skull Creek Shale

Skull Creek Shale

Fuson Shale

15 26, 38 N., 75 W. Steele Member.

Niobrara Member

Niobrara Member

Niobrara Member

Sage Breaks Member

Sage Breaks Member.

Frontier Formation

Frontier Formation

Frontier Formation

Frontier Formation

Mowry Shale

Mowry Shale

Skull Creek Shale

(feet)

9,870

10,235

10,655

10,880

11,045

11,180

11,315

11,585

11,750

11,900

12,005

12,075

12,155

12,290

12,410

12,510

12,600

12,720

12,865

12,955

13,040

13,135

13,230

13,305

13,380

13,450

13,520

13,585

13,630

13,690

13,760

7,875

8,050

8,150

8,200

8,360

8,455

8,850

9,050

9,265

9,370

9,455

9,550

9,680

Depth 

(meters)

3,010

3,122

3,250

3,318

3,369

3,410

3,451

3,533

3,584

3,627

3,659

3,680

3,705

3,746

3,783

3,813

3,840

3,877

3,921

3,949

3,975

4,004

4,033

4,055

4,078

4,100

4121

4,141

4,154

4,173

4,194

2,402

2,454

2,484

2,499

2,548

2,577

2,697

2,758

2,824

2,856

2,882

2,911

2,950

Rm 

(percent)

.54

.55

.58

.53

.58

.55

.53

.56

.56

.52

.57

.57

.65

.72

.68

.66

.69

.65

.66

.68

.69

.71

.68

.70

.82

.85

1.02

1.00

1.07

1.10

1.18

.61

.61

.62

.63

.66

.72

.66

.69

.71

.69

.73

.71

.74

assumed; this is the present-day, measured geothermal 

gradient in the western part of the Powder River Basin 

(Geothermal Survey of North America Subcommittee, 

1976).

Unlike vitrinite reflectance and Rock-Eval pyrolysis, which are direct measurements, TTI modeling is a 

method of calculating thermal maturity. In TTI 

modeling, both time and temperature are considered 

important to the maturation process. The TTI models, as 

outlined by Waples (1980, 1985), were used to estimate 

the time during which the Steele Member was in the oil

window (the time when a source rock has reached the 

proper thermal requirements to generate petroleum). 

Rock-Eval and organic petrographic data from this study 

and from studies by Merewether and Claypool (1980) 

and Momper and Williams (1984) suggest that organic 

matter of the Steele Member contains a mixture of types 

II and III kerogen, capable of generating both oil and 

gas. For type II source rocks, a TTI of 10 corresponding 

to a Rm of 0.60 percent is assumed to be the point at 

which oil will begin to be generated, and a TTI of 180 

corresponding to a Rm of 1.35 percent is assumed to be

Petroleum Generation History of Steele Member of Cody Shale, Wyoming A7
45°

107 C 106° 105°

Tf

Tw

Tf

Km

EXPLANATION 

Wasatch Formation 

Fort Union Formation 

Lance Formation

Lance Formation, Fox Hills 

Sandstone and Lewis Shale, 

undivided

Fox Hills Sandstone and 

Lewis Shale

Mesaverde Formation 

Pre-Mesaverde rocks

 Contact

 Fault

 -"y"-- Present-day troughline

£ 1 Sample locality; referred to 

by number in table 2

WYOMING

I

10 20 30 KILOMETERS

Figure 3. Area of vitrinite reflectance (Rm) values greater than 0.60 percent (the beginning of oil generation), Powder River 

Basin, Wyoming. Sample localities (solid circles) described by number in table 2; Rm values given in table 2.

approximately the point at which liquid hydrocarbons percent is assummed to be the onset of oil generation,

begin to be thermally destroyed and break down to wet and a TTI of 180 corresponding to a Rm of 1.35 percent

gas and condensate (Waples, 1980, 1985). For type III is assumed to be the oil deadline and onset of thermally

source rocks, a TTI of 15 corresponding to a Rm of 0.65 generated gas (Waples, 1980, 1985). The onset of

A8 Evolution of Sedimentary Basins Powder River Basin
Table 3. Rock-Eval pyrolysis data for selected samples of the Steele Member of the Cody Shale, Powder River Basin,

Wyoming.

[Sample numbers keyed to index map (fig. 1); USGS Core Library numbers follow in parentheses]

Sample 

number

2 (B626)

3 (A771)

4(A817)

5 (A915)

6 (C651)

7 (C652)

8 (C221)

Thickness 

(feet) (meters)

9,377

9,385

9,389

9446

9,886

9,947

9,352

9,374

10,197

10,228

10,828

258

2,860

2,862

1,864

2,881

3,015

3,034

2,852

2,859

3,110

3,120

3,303

79

TOC

0.84

.80

.71

.77

.83

.67

.89

.82

.89

1.15

.79

1.67

Tmax

438

437

438

436

439

441

431

438

441

441

444

403

HI

138

91

125

122

115

98

125

100

114

135

120

240

01

23

18

21

36

22

37

85

30

21

34

25

51

PI

0.12

.13

.19

.21

.12

.21

.22

.15

.21

.43

.10

.59

S1

0.16

.11

.20

.25

.24

.09

.31

.14

.27

1.16

.11

5.70

S2

1.16

.73

.89

.94

.96

.66

1.12

.82

1.02

1.56

.95

4.01

S3

0.20

.15

.15

.28

.19

.25

.76

.25

.19

.40

.20

.86

Abbreviations and units of measurements: 

TOC Total organic carbon (in weight percent)

Tmax Temperature at which maximum yield of hydrocarbons occurs during pyrolysis of organic matter (in °C) 

HI Hydrogen index (S2/TOC) 

OI Oxygen index (S3/TOC) 

PI Production index (S1/S1+S2)

51 Integral of first peak (existing hydrocarbons volatized at 250 °C for 5 minutes (in milligrams per gram)

52 Integral of second peak (hydrocarbons produced by pyrolysis of solid organic matter (kerogen between 250 and 550 °C)(in milligrams per 

gram)

53 Integral of third peak (CO2 produced by pyrolysis of kerogen between 250 and 390 °C)(in milligrams per gram)

significant gas generation from type III source rocks is 

assumed to begin at a TTI of 25 corresponding to a Rm 

of 0.75 percent (Juntgen and Karweil, 1966).

Kinetic modeling was also used to estimate the 

time when the Steele Member entered the oil window. 

Kinetic modeling is the best method for predicting the 

timing of hydrocarbon generation and the amount of 

hydrocarbon generated because it is based on the kinetic 

reactions of organic matter (in this case, types II and III) 

during the thermal maturation process. For types I, II, 

and III kerogen, the kinetic models of petroleum 

generation are described as two-step processes: kerogen 

to oil and oil to gas. All reaction steps are treated as 

kinetically first order; that is, the rate is proportional to 

the amount of reactant. The Arrhenius equation is used 

to describe the temperature dependence of the rate 

constant. The model that results for a particular kerogen 

relates amounts of oil and gas generation to time and 

temperature. For a complete explanation of kinetic 

modeling, see discussions in Tissot and Espitalie (1975), 

Ungerer (1983), Yukler and Kokesh (1984), Sweeney 

and others (1987), and Tissot and others (1987).

For this study, I used kinetics for types II and III 

organic matter as defined by workers at the Institut 

franc.ais du Petrole (1987) and a total transformation 

ratio (amount of petroleum actually formed by kerogen 

compared to amount the kerogen is capable of 

producing) of 0.1 percent for the beginning of significant 

oil generation. Figures 5-10 show burial and thermal

1,000

800

X

LJJ

Q 600

g 400

200

II

50 100 

OXYGEN INDEX

150

Figure 4. Modified van Krevelen diagram for samples 

of Steele Member showing type of kerogen and location 

along thermal evolutionary path. Data are given in 

table 3.

Petroleum Generation History of Steele Member of Cody Shale, Wyoming ,A9
histories for the northwestern and southwestern parts of 

the study area and the temperatures and predicted oil 

windows as determined by TTI and kinetic modeling.

RESULTS

Source Rock Evaluation

Organic Matter Quantity

Total organic carbon content is a useful parameter 

for evaluating the quantity of organic material in a source 

rock. Depending on the type of organic matter, finegrained rocks having a total organic carbon content of 

1-2 percent are considered good source rocks (Tissot 

and Welte, 1984), and the minimum amount of organic 

carbon required for generation of commercial 

hydrocarbon deposits is believed to be 0.50 weight 

percent (Hunt, 1979).

The total organic carbon content of samples of 

carbonaceous Steele shale collected immediately below 

the Shannon Sandstone Bed is from 0.67 to 1.67 percent 

(samples 2-8, table 3); the average content is 0.90 

percent. Except for samples 6 and 8, total organic carbon 

contents are very consistent across the study area. If a 

total organic carbon content of 0.50 weight percent is 

assumed to be the minimum for petroleum generation, 

then the carbonaceous shales analyzed for this study are 

potential petroleum source rocks in terms of quantity of 

organic matter.

Organic Matter Quality

The type of organic matter in a source rock is 

determined by the organisms present during deposition, 

the state of preservation (dictated by physical conditions 

such as burial and thermal diagenesis), and biological 

degradation. Organic matter type determines the quality 

and kind of hydrocarbons that will be generated, and it 

can be determined by using several methods. For this 

study, Rock-Eval pyrolysis of whole-rock samples (Espitalie and others, 1977) and visual kerogen analysis 

(organic petrography) were used to determine organic 

matter type.

In general, there are four types of organic matter 

or kerogen (Tissot and Welte, 1984; Waples, 1985). Type

I kerogen is of lacustrine origin; it is derived from algal 

material, has a high hydrogen index and low oxygen 

index, and produces mainly oil during catagenesis. Type

II kerogen is generally of marine origin; it is derived from 

a mixture of phytoplankton, zooplankton, and other 

micro-organisms, has intermediate hydrogen and oxygen 

indices, and can generate oil and gas. Type III kerogen is

of terrestrial origin; it is derived mainly from woody plant 

remains, has a low hydrogen index and high oxygen index, 

and generates mainly gas during its burial and thermal 

history. Type IV kerogen is comprised of inert organic 

material (oxidized and biologically altered organic 

material, charcoal, and recycled organic matter); it has 

low hydrogen and oxygen indices and virtually no 

hydrocarbon-generating potential.

The Steele Member of the Cody Shale is a marine 

shale and thus should contain type II organic matter; 

however, visual kerogen analysis indicates a significant 

quantity of types III and IV kerogen, suggesting 

proximity to a terrestrial source. Figure 4 shows a 

modified van Krevelen plot of the hydrogen and oxygen 

indices, as determined by Rock-Eval pyrolysis, for 

samples 2-8. (van Krevelen diagrams are used to 

illustrate the type of kerogen and its relative position on 

the maturation evolutionary path (van Krevelen, 1961; 

Tissot and Welte, 1984).) The samples plot near the type 

III category and show a fairly high level of thermal 

maturity; that is, they have low hydrogen and oxygen 

indices. They plot near the convergence of types I, II, and 

III, a location that usually indicates a high level of 

maturation; in this case, however, the samples plot there 

because of their relatively low total organic carbon 

content. It is likely that the kerogen in the Steele had 

higher hydrogen indices in the past. These data, in 

conjunction with the total organic carbon content data 

(table 3), indicate that the samples could be considered 

potential source rocks for oil and gas. Because the Steele 

is a marine shale, it probably contains a mixture of both 

types II and III kerogen, and kinetic models for both 

types of kerogen will be presented and compared.

Vitrinite Reflectance

The level of thermal maturity required for oil 

generation generally is believed to be 0.60 percent Rm 

for type II source rocks and 0.65 percent Rm for type III 

source rocks (Waples, 1985). For type II source rocks, 

the conversion of oil to gas occurs at a vitrinite 

reflectance of 1.35 percent. Type III source rocks, 

however, begin to generate significant amounts of gas at 

a vitrinite reflectance of 0.73 percent. Mean random 

vitrinite reflectance data from this study indicate that the 

Steele Member at the horizon of the Shannon Sandstone 

Bed is either immature or marginally mature with 

respect to oil generation and immature for gas 

generation (table 2). Figure 3 shows the area in which 

vitrinite reflectance values are greater or equal to 0.60 

percent favorable maturation levels for oil generation 

from types II and III source rocks.

The vitrinite reflectance data for the Steele are 

remarkably consistent and do not seem to increase with 

increasing depth. For instance, sample 1 has a vitrinite

A10 Evolution of Sedimentary Basins Powder River Basin
reflectance value of 0.61 percent at a depth of 8,606 ft 

(2,625 m), whereas sample 14, to the southwest, has a 

vitrinite reflectance value of 0.56 percent at 11,750 ft 

(3584 m). Even vitrinite reflectance values from 

stratigraphically lower formations are relatively low 

considering their depth of burial. Samples from well 15 of 

the Mowry Shale, 2,000 ft deeper than the Steele 

Member, have vitrinite reflectance values of only about 

0.73 percent, or marginally mature with respect to oil and 

gas generation. A sample of the Fuson Shale (Lower 

Cretaceous) at a depth of almost 14,000 ft from well 

number 14 has a vitrinite reflectance value of only 1.18 

percent. There are two possible explanations for these 

low values. (1) Circulation of relatively colder meteoric 

water along the mountain-front faults of the Big Horn 

Mountains resulted in a westward cooling of geothermal 

gradients. (2) Although the Steele Member was more 

deeply buried in the southwestern part of the basin 

during the Late Cretaceous than in the northwestern 

part, burial was rapid enough that the vitrinite did not 

equilibrate to reflect the difference in burial depths.

Rock-Eval Pyrolysis

Rock-Eval pyrolysis data indicate that the samples 

are marginally mature to mature (Tmax 431-444 °C and 

production index 0.10-0.43). Sample 8 yielded a very low 

Tmax value of 403 °C and a relatively high production 

index of 0.59. These extreme values are the result of oil 

staining of the sample and should not be considered 

representative of the entire Steele Member. If Rock-Eval 

data record maximum thermal maturity of the kerogen, 

which occurred at maximum burial and temperature, 

then the timing of petroleum generation is constrained to 

about 10 Ma.

Time-Temperature and Kinetic Models

Northwestern Part of the Study Area

Figure 5 shows the TTI model for the northwestern 

part of the study area and illustrates the burial and 

thermal history for the various stratigraphic units. The

- 1,000

80

- 2,000

DC 

LU

UJ

Q

- 3,000

4,000

70 60

TIME, IN Ma

Figure 5. Time-temperature model for northwestern part of study area. Shaded area represents oil window 

(10-180 TTI); isotherms show temperatures with depth and through time.

Petroleum Generation History of Steele Member of Cody Shale, Wyoming A11
model was constructed using the burial data in table 1. 

The TTI model indicates that oil generation from the 

Steele Member began approximately 35 Ma and that the 

Steele is still in the oil window today. Thermal cracking of 

oil to gas and condensate has not occurred. The Shannon 

Sandstone Bed was buried to approximately 12,000 ft 

(3,660 m) in the northwestern part of the study area at 

maximum burial 10 Ma. Isotherms show that 

temperatures for that horizon were as high as about 230 

°F (110 °C) before cooling occurred in response to uplift 

and erosion of about 2,700 ft (824 m) of overburden.

Figures 6 and 7 show kinetic models for the Steele 

Member (Shannon Sandstone Bed horizon) in the 

northwestern part of the study area. Because the Steele 

has relatively low total organic carbon contents, it was not 

possible to define kinetic parameters for the shale. 

Therefore, hydrocarbon kinetic reaction parameters for 

types II and III kerogen from the Institut fran9ais du 

P6trole (IFF) (1987) were used to calculate the level of 

thermal maturity, timing of oil generation, and 

milligrams of hydrocarbons produced per gram of 

organic carbon (mg HC/g TOC) for the Steele Member.

Results of kinetic modeling for type II kerogen (fig. 

6) indicate that the Steele Member in the northwestern 

part of the study area began to generate minor amounts 

of hydrocarbons as early as 70 Ma (Cretaceous). A 

transformation ratio of 0.10 was assumed for the 

beginning of significant oil generation (Tissot and Welte, 

1984). Using this value, significant hydrocarbon 

generation, in amounts sufficient for migration and 

accumulation in the Shannon Sandstone Bed, probably 

did not occur until about 25 Ma (Oligocene) at about 212 

°F (100 °C) and at a depth of about 11,550 ft (3,523 m). 

At the present time, after uplift and erosion and at a 

depth of 9,250 ft (2,821 m), the Steele has a total

IUU90-

80-

70-

50-

40- 

30-

20-

10-

0 - 

8

Beginning of oil window

I I I ~~l 7 [ 1 1

0 70 60 50 40 30 20 10 C 

TIME, IN Ma

- i,uuu 

- 900

- 80° go

- 700 5-5 

o°

- 600 gra 

QE 

- 500 >JZ

- 400 o£ 

Hb 

- 300 §2

- 200 |g 

- 100

0

)

50 40 30 

TIME, IN Ma

Figure 6. Kinetic model, assuming type II kerogen, for 

northwestern part of study area. Shaded area represents oil 

generation; heavy line indicates total transformation ratio 

through time.

Figure 7. Kinetic model, assuming type III kerogen, for 

northwestern part of study area. Shaded area represents oil 

generation; heavy line indicates total transformation ratio 

through time.

transformation ratio of about 0.20; hence, it is still in the 

oil window. The type II kerogen kinetic model indicates 

that the Steele has reached the thermal requirements to 

have generated about 100 mg HC/g TOC (as oil, shaded 

area in fig. 6) and a relatively small quantity of gas.

Results of kinetic modeling for type III kerogen 

(fig. 7) suggest that minor amounts of hydrocarbons 

(probably both oil and gas) were being generated at 40 

Ma (Eocene) at a temperature of about 203 °F (95 °C) 

and a depth of 10,700 ft (3,264 m). Using type III 

kerogen, the total transformation ratio for the Steele 

Member is less than 0.10 and the Steele has not reached 

the thermal requirements for significant oil or gas 

generation.

Southwestern Part of the Study Area

Figure 8 shows the TTI model for the southwestern 

part of the study area and illustrates the burial and 

thermal history of the various stratigraphic units. The 

data used to construct this model are presented in table 

1. The TTI model shows that oil generation began at 

about 47 Ma (Eocene), about 10 million years earlier 

than in the northwestern part of the study area. The 

Steele Member is still in the oil window today and has not 

reached the thermal requirements for cracking of oil to 

wet gas and condensate. The Shannon Sandstone Bed 

was buried to almost 13,000 ft (3,965 m) at 10 Ma, 875 ft 

(267 m) more than in the northwestern part of the study 

area, and reached a temperature of more than 248 °F 

(120 °C) (10 °C higher than in the northwestern part of 

the study area). About 2,070 ft (631 m) of overburden 

has been eroded in the southwestern part of the study 

area.

A12 Evolution of Sedimentary Basins Powder River Basin
1,000

13,000

GC 

LU

- 2,000 2

Q_

UJ

Q

3,000

50 40 30 

TIME, IN Ma

Figure 8. Time-temperature model for southwestern part of study area. Shaded area represents oil 

window (10-180 TTI); isotherms show temperatures with depth and through time.

Figure 9 illustrates the results of kinetic modeling 

of type II kerogen for the Steele Member in the 

southwestern part of the study area. The Steele began to 

generate some hydrocarbons about 70 Ma (Cretaceous). 

Significant amounts of hydrocarbons were being 

generated by 38 Ma (Eocene-Oligocene) at a 

temperature of 212 °F (100 °C) and a depth of 11,600 ft 

(3,538 m). The Steele has reached the thermal 

requirements to have generated as much as 210 mg HC/g 

TOC (as oil) in the southwestern part of the study area, 

twice the amount generated in the northwestern part of 

the study area. Today the Steele has a total 

transformation ratio of about 0.40; hence it is still in the 

oil window.

Figure 10 shows the kinetic model for type III 

kerogen in the southwestern part of the study area. This 

model indicates that the Steele Member began to 

generate some minor amounts of oil and gas about 44 Ma 

(Eocene) at a temperature of about 212 °F (100 °C) and 

a depth of 11,600 ft (3,538 m). Significant amounts of oil,

sufficient for migration and accumulation, were probably 

never generated. Assuming type III kerogen parameters, 

only 10 mg HC/g TOC (oil and gas) has been generated. 

It should be noted that the kinetic models for type 

III kerogen are two-stage conversions: kerogen to oil and 

oil to gas. Gas generation from type III kerogen, 

however, is more likely a kerogen to gas conversion 

without the oil-cracking stage. Thus, the type III kinetic 

models probably underestimate the amount of gas that 

has been generated by type III kerogen in the Steele 

Member.

DISCUSSION

In the southwestern part of the study area, the 

depth of burial for the Steele Member of the Cody Shale 

at the Shannon Sandstone Bed horizon is only 875 ft (267 

m) greater than in the northwestern part and the 

maximum temperature only about 10 °C higher, yet the 

hydrocarbon generation history is vastly different. The

Petroleum Generation History of Steele Member of Cody Shale, Wyoming A13
o

£

/IATION

QC

o

(A

z

QC

90 -

80-

70-

50-

40-

30-

20-

0 - 

8<0 70 60 50 40 30

TIME, IN Ma

X

|

X

20

/

10

_ « 

c

- i,uuu 

- 900

- 800

- 700

- 600 

- 500

- 400

- 300

- 200 

- 100

{- 0

)

20

oo

oc.ra

O 1 

If

ss

1^

<s

Figure 9. Kinetic model, assuming type II kerogen, for 

southwestern part of study area. Shaded area represents oil 

generation; heavy line indicates total transformation ratio 

through time.

IUU

90-

80-

70-

50-

40 -

30 -

20 -

10 -

0 -

Beginning of oil window

_ - 

I I I I «|^H& y - J- |-

- i,uuu 

- 900

- 800 gg

- 700 J£5?

- 600 0* 

Q E

- 500 >z

- 400 o£

- 300 ig

- 200 |g

- 100

0

80 70 60 50 40 30 

TIME. IN Ma

20 10

Figure 10. Kinetic model, assuming type III kerogen, for 

southwestern part of study area. Shaded area represents oil 

generation; heavy line indicates total transformation ratio 

through time.

four independent maturity indicators (vitrinite 

reflectance, Rock-Eval pyrolysis, time-temperature 

modeling, and kinetic modeling) yield different timing of 

hydrocarbon generation for the two areas. The vitrinite 

reflectance data suggest that the two areas are not very 

different and that the Steele Member is immature to 

marginally mature with respect to the oil window over 

much of the western part of the basin (fig. 3). If the burial 

and thermal models are representative, maximum 

temperatures for the northwestern and southwestern 

parts of the study area were 230 °F (110 °C) and more 

than 248 °F (120 °C), respectively. These temperatures 

are well in excess of those needed for significant oil

generation. Most petroleum generation occurs during 

catagenesis, commencing at about 122 °F (50 °C) and 

ending at the threshold of dry gas generation at about 392 

°F (200 °C) (Hunt, 1979; Ammosov, 1981). Recently, 

Quigley and Mackenzie (1988) suggested that most oil is 

formed between 212 and 302 °F (100-150 °C). Even this 

higher estimate for the temperature of significant oil 

generation is within the limits suggested by the burial and 

thermal models in this study. Vitrinite reflectance 

records the maximum temperature achieved by a rock. 

Maximum temperatures were achieved approximately 10 

Ma at maximum burial; thus, based on the vitrinite 

reflectance data, oil generation for the Steele Member 

probably did not begin much earlier.

Rock-Eval pyrolysis data indicate that the Steele 

Member is marginally mature to mature and constrain 

the timing of oil generation to about 10 Ma.

Time-temperature index (TTI) and kinetic 

modeling for the Steele yield an earlier timing for oil 

generation than do vitrinite reflectance and Rock-Eval 

pyrolysis data. For the northwestern part of the study 

area, estimates from TTI modeling place the beginning 

of the oil window at approximately 35 Ma (Oligocene), 

and the conservative estimate from the type II kerogen 

kinetic modeling (transformation ratio of 0.10) puts the 

beginning of significant oil generation at around 25 Ma 

(Oligocene). Therefore, the beginning of oil generation 

could be constrained to between 35 and 25 Ma after the 

end of the Laramide orogeny and before maximum 

burial. Oil could have migrated updip, along structure, 

from the deeper parts of the basin or could have 

accumulated in stratigraphic traps. If type III kerogen is 

assumed, gas generation may have begun approximately 

40 Ma (Eocene) and could be found in Shannon 

Sandstone Bed reservoirs.

For the southwestern part of the study area, 

estimates from TTI and kinetic modeling again yield a 

much earlier timing for oil generation from the Steele 

than do the vitrinite reflectance data. The modeling also 

suggests an earlier timing for oil generation than does 

modeling for the northwestern part of the study area. 

Estimates from TTI modeling place the beginning of oil 

generation for the Steele at 47 Ma (Eocene), whereas the 

conservative estimate from kinetic modeling puts the 

beginning of the oil window at 38 Ma (EoceneOligocene). Thus, as indicated by the two models, oil 

generation began during and after the Laramide orogeny. This timing has important implications because oil 

generated in the structurally deeper parts of the basin 

could have migrated updip and been deposited in less 

mature and less deeply buried reservoirs. Alternatively, 

structural movement associated with the Laramide orogeny may have adversely affected potential structural and 

stratigraphic traps, and oil generated before the end of 

the structural deformation associated with the Laramide

A14 Evolution of Sedimentary Basins Powder River Basin
orogeny could have been lost. Results of the TTI 

modeling suggest that the Steele Member is still in the oil 

window; however, even if the Steele is still in the oil 

window, oil generation will not occur if present-day 

temperatures are too low. Results of the type III kerogen 

modeling indicate that generation of hydrocarbons began 

about 44 Ma, timing that suggests Shannon Sandstone 

Bed reservoirs in the southwestern part of the study area 

are likely to contain some volumes of methane gas.

To a large extent, rate and depth of burial during 

the Late Cretaceous controlled the thermal maturity and 

hence the timing of the oil window for the Steele Member 

in the study area. In the southwestern part of the study 

area, the Upper Cretaceous section is 2,375 ft (724 m) 

thicker than in the northwestern part. The Tertiary 

section, however, is 1,500 ft (458 m) thicker in the 

northwestern part. Because the difference in the overall 

burial history is only 875 ft (267 m) (that is, thicker in the 

southwestern part of the basin), the Steele Member in 

the southwestern part of the study area was more deeply 

buried earlier and thus for a longer period of time than in 

the northwestern part. Petroleum generation for the 

Steele in the southwestern part of the study area, 

therefore, began earlier and produced a greater volume.

In a study of petroleum geochemistry in the 

Powder River Basin, Momper and Williams (1984) 

concluded that oil expulsion from the Mowry Shale 

began in the late Paleocene or early Eocene in the deep, 

southwestern part of the basin. This timing of oil 

generation for the Mowry Shale agrees well with the 

timing scenario for oil generation of the Steele Member 

determined in the present study because the Mowry 

Shale is as much as several thousand feet deeper than the 

Steele; therefore, oil generation should have begun 

earlier.

Kinetic modeling is probably the best method for 

estimating the petroleum generation history of a source 

rock because it takes into account the kinetic properties 

of the kerogen during catagenesis. Unfortunately, I was 

not able to use the kinetic properties of the kerogen in 

the Steele Member for this study, but the kerogen kinetic 

parameters of IFF are a good approximation of the 

source rock generation history in the basin. The Steele 

Member is probably a mixture of types II, III, and IV 

kerogen, and the kinetic models should be used with this 

in mind.

Production records from Shannon Sandstone Bed 

reservoirs show that large volumes of oil and gas have 

been produced throughout the basin (Wyoming 

Geological Association, 1981). Further work must be 

done in order to correlate hydrocarbons in the Shannon 

reservoirs with the various source rocks in the basin.

Geochemical studies will help show to what extent the 

Steele Member has generated and contributed to the oil 

and gas in the Shannon reservoirs.

SUMMARY

In the northwestern part of the study area, the 

Steele Member of the Cody Shale at the Shannon 

Sandstone Bed horizon was buried to about 12,000 ft 

(3,660 m) and reached a maximum temperature of about 

230 °F (110 °C) at 10 Ma at maximum burial. From 10 

Ma to present, cooling has occurred in response to 

erosion of about 2,700 ft (824 m) of section. In the 

southwestern part of the study area, the Shannon 

Sandstone Bed was buried to almost 13,000 ft (3,965 m) 

and reached maximum temperatures of about 248 °F 

(120 °C) during maximum burial 10 Ma. About 2,070 ft 

(631 m) of overburden has been eroded since 10 Ma in 

the southwestern part of the study area.

Rock-Eval pyrolysis data for selected samples of 

Steele indicate that these rocks contain enough organic 

matter of the proper type to be potential source rocks for 

both oil and gas in the western part of the Powder River 

Basin. On a modified van Krevelen diagram the samples 

plot closer to the type III kerogen category, and organic 

petrography results show an abundance of types III and 

IV kerogen. Because the Steele is a marine shale, yet 

contains significant amounts of type III kerogen, results 

of kinetic modeling should be compared and 

extrapolated in order to determine the petroleum 

generation history of the Steele Member.

Vitrinite reflectance data for samples of the Steele 

are very consistent over the entire western part of the 

Wyoming part of the Powder River Basin and do not vary 

substantially with depth. Vitrinite reflectance data 

indicate that the Steele Member is either immature or 

just entering the oil window. Assuming that vitrinite 

reflectance records maximum maturity, vitrinite 

reflectance values were set at maximum burial and 

temperature 10 Ma; hence, oil generation began about at 

that time.

Rock-Eval pyrolysis data indicate that the Steele 

Member is marginally mature to mature and constrain 

the timing of oil generation to about 10 Ma.

Results of time-temperature and kinetic modeling 

constrain the timing of oil generation to between 35 and 

25 Ma in the northwestern part of the study area and 

between 47 and 38 Ma in the southwestern part. Kinetic 

modeling of type III kerogen shows that some gas 

generation from the Steele may have begun about 40 Ma 

in the northwestern part of the study area and about 44 

Ma in the southwestern part. The Shannon reservoirs 

therefore could contain some quantities of gas.

Petroleum Generation History of Steele Member of Cody Shale, Wyoming A15
The four methods used to determine thermal 

maturity and timing of oil generation in this study yield 

quite different sets of results. There are innate problems 

with all of the techniques; hence, the "true" maturity and 

timing of petroleum generation is probably somewhere 

between the two extremes. These differences stress the 

importance of using as many techniques or methods as 

possible when assessing the thermal maturity and 

constraining timing of petroleum generation of a 

potential source rock.

REFERENCES CITED

Ammosov, LI., 1981, Petrographic features of solid organic 

materials as indicators of paleotemperatures and oil 

potential: International Geology Review, v. 23, 

p. 406-416.

Ayers, W.B., Jr., 1986, Lacustrine and fluvial-deltaic 

depositional systems, Fort Union Formation (Paleocene), Powder River Basin, Wyoming and Montana: 

American Association of Petroleum Geologists Bulletin, 

v. 70, p. 1651-1673.

Bohor, B.F., Triplehorn, D.M., Nichols, D J., and Millard, H.T., 

Jr., 1987, Dinosaurs, spherules, and the "magic" 

layer A new K-T boundary clay site in Wyoming: 

Geology, v. 15, p. 896-899.

Curry, W.H., III, 1971, Laramide structural history of the 

Powder River Basin, Wyoming, in Renfro, A.R., 

Madison, L.V., and Jarre, G.A., eds., Wyoming tectonics 

and their economic significance: Wyoming Geological 

Association, Twenty-third Field Conference Guidebook, 

p. 49-60.

Espitalie, J., Laporte, J.L., Madec, M., Marquis, F., Leplat, P., 

Paulet, J., and Boutefeu, A., 1977, Rapid method for 

source rock characterization and for evaluating their 

petroleum potential and their degree of evolution: Institut francais du Petrole, v. 32, p. 23-42.

Fox, J.E., Dolton, G.L., and Clayton, J.L., in press, Powder 

River Basin, in Decade in North American geologyEconomic geology of the United States, oil and gas 

section, regional synthesis of selected provinces: 

Geological Society of America.

Geothermal Survey of North America Subcommittee of the 

American Association of Geologists' Research 

Committee, 1976, Geothermal gradient map of North 

America: Scale 1:5,000,000, two sheets.

Hunt, J.M., 1979, Petroleum geochemistry and geology: San 

Francisco, W.H. Freeman, 617 p.

Institut fran9ais du Petrole, 1987, IFF kinetics, in MATOIL 

program version 1.2 manual: Bureau d'Etudes Industrielles et de Cooperation de 1'Institut frar^ais du Petrole, 

appendix C.

Juntgen, H., and Karweil, J., 1966, Gasbildung und gasspeicherung in steinkohlenflozen, I und II [Formation 

and storage of gas in bituminous coal seams, parts I and 

II]: Erdol and Kohle, Erdgas, Petrochemie, v. 19, p. 

251-258 and 339-344

McKenna, M.C., and Love, J.D., 1972, High-level strata 

containing Early Miocene mammals on the Bighorn

Mountains, Wyoming: American Museum Novitates, no. 

2490, p. 1-31.

Merewether. E.A., and Claypool, G.E., 1980, Organic 

composition of some Upper Cretaceous shale, Powder 

River Basin, Wyoming: American Association of 

Petroleum Geologists Bulletin, v. 64, p. 488-500.

Momper, J.A., and Williams, J.A., 1984, Geochemical 

exploration in the Powder River Basin, in Demasion, G., 

and Morris, R.J., eds., Petroleum geochemistry and basin 

evaluation: American Association of Petroleum 

Geologists Memoir 35, p. 181-191.

Nixon, R.P., 1973, Oil source beds in Cretaceous Mowry Shale 

of northwestern interior United States: American 

Association of Petroleum Geologists Bulletin, v. 57, 

p. 136-161.

Quigley, T.M., and Mackenzie, A.S., 1988, The temperature of 

oil and gas formation in the sub-surface: Nature, v. 333, 

p. 549-552.

Seeland, D.A., 1985, Oligocene paleogeography of the 

Northern Great Plains and adjacent mountains, in Flores, R.M., and Kaplan, S.S., eds., Cenozoic paleogeography of west-central United States: Rocky 

Mountain Section, Society of Economic Paleontologists 

and Mineralogists, p. 187-205.

Stach, E., Mackowsky, M. Th., Teichmuller, M., Taylor, G.H., 

Chandra, D., and Teichmuller, R., 1982, Stach's textbook 

of coal petrology (3rd ed.): Berlin-Stuttgart, Borntraeger, 

535 p.

Sweeney, J J., Burnham, A.K., and Braun, R.L., 1987, A model 

of hydrocarbon generation from type I kerogen Application to Uinta Basin, Utah: American Association of 

Petroleum Geologists Bulletin, v. 71, p. 967-985.

Swinehart, J.B., Souders, V.L., DeGraw, H.M., and Diffendal, 

R.F., Jr., 1985, Cenozoic paleogeography of western 

Nebraska, in Flores, R.M., and Kaplan, S.S., eds., Cenozoic paleogeography of west-central United States: 

Rocky Mountain Section, Society of Economic 

Paleontologists and Mineralogists, p. 209-229.

Tissot, B.P., and Espitali£, J., 1975, L'evolution thermique de la 

matiere organique des sediments; applications d'une 

simulation mathimatique; potential petrolier des basin 

sedimentaires et reconstitution de 1'histoire thermique 

des sediments [Thermal evolution of organic material in 

sediments; applications of a mathematical simulation]: 

Revue de 1'Institut francais du Petrole, v, 30, p. 743-777.

Tissot, B.P., Pelet, R., and Ungerer, P., 1987, Thermal history 

of sedimentary basins, maturation indices, and kinetics of 

oil and gas generation: American Association of 

Petroleum Geologists Bulletin, v. 71, p. 1445-1466.

Tissot, B.P., and Welte, D.H., 1984, Petroleum formation and 

occurrence (2nd ed.): Berlin-Heidelberg-New YorkTokyo, Springer-Verlag, 699 p.

Tschudy, R.H., 1976, Pollen changes near the Fort UnionWasatch boundary, Powder River Basin, in Laudon, 

R.B., ed., Geology and energy resources of the Powder 

River Basin: Wyoming Geological Survey Annual Field 

Conference, 28th, Casper, 1976, Guidebook, p. 73-81.

Ungerer, P., 1983, Models of petroleum formation How to 

take into account geology and chemical kinetics, in

A16 Evolution of Sedimentary Basins Powder River Basin
Durand, B., ed., International symposium on thermal 

phenomena in sedimentary basins: Paris, Editions Technip, p. 235-246.

van Krevelen, D.W., 1961, Coal: Amsterdam, Elsevier, 514 p.

Waples, D.W., 1980, Time and temperature in petroleum 

formation; application of Lopatin's method to petroleum 

exploration: American Association of Petroleum 

Geologists Bulletin, v. 64, p. 916-926.

____ 1985, Geochemistry in petroleum exploration: 

Boston, International Human Resources Development 

Corporation, 232 p.

Weaver, J.N., and Flores, R.M., 1985, Stratigraphic framework 

of the upper Fort Union Formation, Ta Hills, western 

Powder River Basin, Wyoming: United States Geological 

Survey Miscellaneous Field Studies Map MF-1779, 1 

sheet.

Wyoming Geological Association, 1981, Oil and gas fields, 

Powder River Basin: Symposium, 1981, v. 1 and 2, 472 p.

Yukler, M. Arif, and Kokesh, Fritz, 1984, A review of models 

used in petroleum resource estimation and organic geochemistry: Advances in Petroleum Geochemistry, v. 1, 

p. 69-112.

Petroleum Generation History of Steele Member of Cody Shale, Wyoming A17

U.S. GOVERNMENT PRINTING OFFICE: 1990 573-049/26,003 REGION NO. 8



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