Chapter 4

New Source Rock Data for the Thermopolis 

and Mowry Shales in the Wyoming Part of 

the Bighorn Basin

By Thomas M. Finn

Chapter 4 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:

Finn, T. M., 2010, New source rock data for the Thermopolis and Mowry Shales in the Wyoming part of the Bighorn 

Basin: U.S. Geological Survey Digital Data Series DDS–69–V, 16 p.

ISBN: 1–4113–2667–5
iii

Contents

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

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

Acknowledgments.........................................................................................................................................1

Depositional Setting ......................................................................................................................................1

Stratigraphy ....................................................................................................................................................6

Thermopolis Shale................................................................................................................................6

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

Mowry Shale..........................................................................................................................................6

Methods...........................................................................................................................................................7

Results .............................................................................................................................................................7

Quantity of Organic Matter .................................................................................................................7

Types of Organic Matter......................................................................................................................8

Distribution of Organic Matter in the Mowry Shale........................................................................9

Summary........................................................................................................................................................13

References....................................................................................................................................................13

Figures

1. Rocky Mountain region showing locations of Laramide basins...........................................2

2. Index map of the Bighorn Basin Province................................................................................3

3. Extent of the Western Interior Seaway during Campanian time..........................................4

4. Regional stratigraphic cross section of Cretaceous and Paleocene rocks.......................5

5. Stratigraphic chart and type log of Lower and lowermost Upper Cretaceous rocks.......6

6. Source rock sample localities....................................................................................................8

7. Tmax versus depth plot................................................................................................................9

8. Total organic carbon content and S2 .......................................................................................11

9. HI vs OI diagrams........................................................................................................................11

10. The ratio S2/S3 .............................................................................................................................12

11. Variations of total organic carbon and kerogen types for the Mowry Shale ...................12

Table

1. Results of Rock-Eval and total organic carbon analysis .....................................................10

Plate

1. Subsurface stratigraphic cross section of the Thermopolis and 

Mowry Shales and assocated rocks in the Bighorn Basin .............................................. link
Abstract

Thirty samples of well cuttings, collected from marine 

shales from the Cretaceous Thermopolis and Mowry Shales in 

the Bighorn Basin, were analyzed using Rock-Eval and total 

organic carbon analysis to determine their source rock potential. The samples were collected from wells located near the 

outcrop belt along the margins of the basin and are generally 

immature with respect to hydrocarbon generation. Results for 

the Thermopolis Shale indicate that it is composed of mainly 

Type-III gas-prone kerogen, however S2/S3 ratios indicate that 

the Thermopolis is capable of generating both oil and gas. 

Organic richness levels for the Thermopolis Shale are considered poor to fair. Results indicate that the Mowry Shale has a 

generative potential based on organic richness that is considered to be poor to very good, and hydrogen indices and S2

/S3

ratios indicate it is capable of generating both oil and gas.

Introduction

The Bighorn Basin is one of many structural and sedimentary basins that formed in the Rocky Mountain foreland 

during the Laramide orogeny (Late Cretaceous through early 

Eocene). The basin is nearly 180 mi long, 100 mi wide, and 

encompasses about 10,400 mi2

 in north-central Wyoming 

and south-central Montana (fig. 1). The basin is structurally 

bounded on the northeast by the Pryor Mountains, on the 

east by the Bighorn Mountains, and on the south by the Owl 

Creek Mountains. The north boundary as defined in this report 

includes a zone of faulting and folding referred to as the NyeBowler lineament (Wilson, 1936). The northwest and west 

margins are formed by the Beartooth Mountains and Absaroka 

Range, respectively (fig. 2).

The first commercial hydrocarbon production from 

Cretaceous reservoirs in the basin was established at Garland 

and Greybull fields in 1906 and 1907, respectively (Fox and 

Dolton, 1996) (fig. 2). Since then cumulative production from 

Cretaceous and Tertiary reservoirs is about 94 million barrels 

of oil and 830 billion cubic feet of gas (IHS Energy Group, 

2007). In addition, a potential unconventional basin-centered 

gas accumulation may be present in Cretaceous reservoirs 

(Surdam and others, 1997; Johnson and Finn, 1998; Johnson 

and others, 1999; Finn and others, Chapter 3, this CD–ROM). 

It has been suggested that various Cretaceous marine shales 

are the principal hydrocarbon source rocks for these accumulations in the Bighorn Basin (Burtner and Warner, 1984; Hagen 

and Surdam, 1984; Meissner and others, 1984). 

The purpose of this report is to present new data from 

results of Rock-Eval and total organic carbon (TOC) analysis 

for samples collected from immature Thermopolis and Mowry 

Shales in the Bighorn Basin and characterize their source 

rock potential. These new data supplement previously published reports by Schrayer and Zarrella (1963, 1966, 1968), 

Nixon (1973), Burtner and Warner (1984), Hagen and Surdam 

(1984), Hagen (1986), Davis (1986), and Davis and others 

(1989). This study was conducted to support the U.S. Geological Survey’s assessment of undiscovered oil and gas resources 

in the Bighorn Basin in 2007–08.

Acknowledgments

The author thanks Paul Lillis and Mike Pribil for numerous helpful discussions regarding organic geochemistry and 

source rocks, and for their assistance in interpreting the RockEval and TOC data presented in this report. Important geologic 

input was provided by Mark Kirschbaum and Tom Judkins. 

The manuscript benefited from reviews by Larry Anna, Paul 

Lillis, Dick Keefer, Doug Nichols, and Gayle Dumonceaux 

and their suggestions and comments are greatly appreciated. 

Depositional Setting

During much of Cretaceous time, the part of central 

Wyoming that is now the Bighorn Basin was located near the 

west edge of the Rocky Mountain foreland basin, an elongate 

north-south structural depression that developed to the east 

of the tectonically active Western Cordilleran highlands prior 

to the Laramide orogeny. Throughout much of its history the 

foreland basin was flooded by a broad epicontinental sea, 

referred to as the Western Interior Seaway (WIS) that developed in response to foreland basin subsidence and eustatic 

sea-level rise (Steidtmann, 1993). At its maximum extent, the 

WIS extended for more than 3,000 mi from the Arctic Ocean 

New Source Rock Data for the Thermopolis and Mowry 

Shales in the Wyoming Part of the Bighorn Basin

By Thomas M. Finn
2 New Source Rock Data for the Thermopolis and Mowry Shales in the Wyoming Part of the Bighorn Basin

0 50 100 MILES

MT

ID

MT

WY

ND

SD

WY

CO

NE

ID

UT

UT CO

AZ NM

CO

NM

Black Hills uplift

uplift

Casper

arch

Owl Creek uplift

Powder

River

Basin

Crazy

Mountains

Basin

Bull

Mountains

Basin

Absaroka

volcanics

San

Juan

volcanic

eld

San Juan

Basin

Raton

Basin

Denver

Uinta Basin

Piceance

Basin

Greater Green

River Basin

Wind River Basin

Basin

Hanna

Basin

Hartville uplift

Rawlins

uplift

Uinta uplift

Granite Mountains

uplift

Wind

River uplift

White

River

uplift

Gunnison uplift

Nacimiento

uplift

Rio

Grande rift

Sangre

de

Cristo uplift

Sangre

de

Cristo

uplift

Park

Range

uplift

Sierra Madre

uplift

Uncompahgre

uplift

Bridger

uplift

Beartooth

uplift

Miles City arch

San Rafael

Swell

Medicine

Bow

uplift

Shirley

Basin

Laramie

Basin

North and Middle

Park Basins

South Park

Basin

Sawatch

uplift

Front

Range

uplift

San Juan

uplift

Axial Basin uplift

Teton uplift

Basin

Thrust or

 reverse fault

Cordilleran

orogenic

belt

(Neogene)

NE

SD

WY

Bighorn

Bighorn

Reed Point

syncline

Pryor

Mtns.

Nye-Bowler lineament

112° 110° 108° 106° 104°

36°

38°

40°

42°

44°

46°

Figure 1. Rocky Mountain region extending from southern Montana to northern New Mexico 

showing locations of Laramide sedimentary and structural basins and intervening uplifts. Modified 

from Dickinson and others (1988).
Depositional Setting 3

T

50

N

T

55

N

T

9

S

T

45

N

T

40

N

R94W R90W R85W

R95W

R100W

R104W

T

50

N

T

55

N

T

57

N

T

9

S

T

5

S

R15E R20E R25E R35E

R101W R95W

R90W R85W

R35E R40E

R105W

T

40

N

T

45

N

T

5

N

R5W R1W R1E R5E

T

8

N

R4W

T

8

N

R3E

Greybull

Sand Creek

Manderson

Worland

Hidden Dome

Dobie Ceek

Five Mile

Grass Creek

Walker Dome

Little

Grass Creek

Meeteetse

Oregon

Basin

Oregon

Basin, SE

Little

Bualo

Basin

Badger

Basin

Silver

Tip

Heart

Mountain

Garland

Byron

Big Polecat

North Danker

Elk

Basin, NW

Clark

Fork, N

Elk Basin

Elk Basin, S

Golden

Dome

Dry

Creek

Montana

Wyoming

110°

45°

44°

109° 108° 107°

0 10 20 30 40

MILES

Precambrian rocks

Tertiary volcanic rocks

Oil field producing from

 Cretaceous or Tertiary

 reservoirs

Gas field producing from

 Cretaceous or Tertiary

 reservoirs

Bighorn Basin Province

Absaroka

Range

Beartooth

Mountains

Creek Mountains

Owl

Mountains

Bighorn 

Mountains

Pryor

Nye-Bowler

Rattlesnake Mountain

anticline

lineament

Big Trails fault

Cloverly Formation

Thermopolis anticline

Sheep 

Mountain

anticline

Byron

Greybull

Figure 2. Index map showing the location of the Bighorn Basin Province, major structural elements, and oil 

and gas fields that produce from Cretaceous or Tertiary reservoirs.

to the Gulf of Mexico (fig. 3) (Kauffman, 1977). Erosion of 

the Western Cordilleran highlands supplied sediment to the 

basin by eastward-flowing streams, whereas the eastern shore 

was part of the stable craton that was topographically low and 

supplied little sediment westward into the seaway (Molenaar 

and Rice, 1988). During much of Cretaceous time, sediments 

accumulated in or adjacent to the WIS as the western shoreline repeatedly advanced and retreated across the western part 

of the basin resulting in a complex pattern of intertonguing 

marine and nonmarine deposits (fig. 4). The marine deposits 

are represented by westward-thinning tongues of marine shale, 

siltstone, limestone, and marine sandstone. The nonmarine 

deposits are represented by eastward-thinning wedges of 

marginal marine and nonmarine sandstone, siltstone, shale, 

carbonaceous shale, and coal. The marine sediments were 

deposited during widespread marine transgressions creating 

highstand conditions that resulted in deepening of the seaway, 

limiting clastic input, and forming anoxic bottom conditions 

favorable for the preservation of organic matter (Meissner and 

others, 1984; Gries and others, 1992). Figure 5 is a generalized 
4 New Source Rock Data for the Thermopolis and Mowry Shales in the Wyoming Part of the Bighorn Basin

Study

area

?

INTERIOR

WESTERN

SEAWAY

WESTERN

CORDILLERA

PACIFIC

OCEAN

OCEAN

OCEAN

ATLANTIC

ARCTIC

GULF

OF MEXICO

ALASKA

GREENLAND

CANADA

CUBA

MEXICO

0

0 400 800 KILOMETERS

500 MILES

Figure 3. Extent of the Cretaceous Western Interior seaway during Campanian time. Brown areas show the 

approximate geographic distribution of land areas. Modified from Gill and Cobban (1973).
0

0

5 10 15 20 MILES

500

FEET

1,000

Lewis Shale

Meeteetse Formation

Lower member Claggett Member of Cody Shale

Teapot Sandstone Member

Mesaverde Formation (middle part)

Cody Shale

Cody Shale

Cloverly Formation Jurassic rocks, undivided

(part)

Thermopolis Shale

Frontier Formation

"chalk kick"

Muddy Sandstone

Mesaverde Formation

?

?

?

Bearpaw Shale

Lennep Sandstone

Eagle Sandstone

Parkman Sandstone

Telegraph Creek Formation

Judith River Formation

Claggett Shale

Virgelle Sandstone Member

Lance Formation

Lance Formation

Hell Creek Formation

Fort Union Formation

(part)

Fort Union Formation

(part)

North Montana Wyoming Sou

Nye-Bowler

lineament

Badger

Basin

McCulloch

Peak

Cottonwood

Creek

"chalk kick"

Emblem

Bench

Red

Point Fritz Dobie Creek Five Mile Worland

Mowry Shale

Cody Shale

Montana

Wyoming

Figure 4. Regional northwest-southeast stratigraphic cross section of Cretaceous and Paleocene (part) rocks, Bighorn Basin, Wyoming and Colors represent different lithologies and environments of deposition: red, fluvial sandstone and conglomerate; yellow, marine and marginal msandstone and siltstone; orange, fluvial and estuarine sandstone; tan, tidal sandstone, siltstone, and shale; various shades of gray are marine various shades of green are coastal plain and floodplain sandstones, shales, and coals; brown, sandstone, siltstone, shale, coal, and conglomcontinental origin; blue, undifferentiated sedimentary deposits.
6 New Source Rock Data for the Thermopolis and Mowry Shales in the Wyoming Part of the Bighorn Basin 1000 1500

Frontier

Formation

(part)

Upper

siliceous

part

Shell Creek

Shale

of

Eicher (1962)

Mowry

Shale

Upper Cretaceous (part)

?

Muddy Sandstone

Thermopolis

Shale

"rusty beds"

Greybull Sandstone

Member

Lower Cretaceous

Cloverly

Formation

Morrison Formation

(part)

Jurassic

(part)

Gamma ray resistivity

Apache Corp. 36-11 State

sec. 36, T. 47 N., R. 102 W.

Figure 5. Type log of Lower and lowermost Upper 

Cretaceous rocks in the Bighorn Basin showing the 

stratigraphic nomenclature used in this report. Floodplain 

deposits are shown in brown; marine and marginal marine 

sandstones, yellow; marine shales, various shades of gray; 

estuarine and fluvial sandstones shown in orange, tidal 

deposits are shown in tan, and undifferentiated deposits 

shown in blue. Sample interval, dark gray. Location shown 

in figure 6.

stratigraphic chart and type log of the Lower Cretaceous and 

lowermost Upper Cretaceous rocks in the Bighorn Basin.

Stratigraphy

Thermopolis Shale

The Thermopolis Shale (known as the Skull Creek 

Shale in some other Rocky Mountain basins) as used in this 

report refers to lower Thermopolis Shale of Lupton (1916), 

Mills (1956), and Haun and Barlow (1962), which overlies 

the “rusty beds” of the Cloverly Formation, and underlies 

the Muddy Sandstone (fig. 5 and plate 1). The Thermopolis 

consists of 125 to 230 ft of marine shales and siltstones and 

represents a continuation of marine deposition that began with 

the “rusty beds” during sea-level rise in Albian time (Burtner 

and Warner, 1984; Hagen and Surdam, 1984). The shales are 

dark gray to black, contain thin layers of siltstone, sandy claystone, and bentonite. The basal contact is gradational with the 

underlying “rusty beds” of the Cloverly Formation; the upper 

contact may be sharp and locally unconformable or gradational with the overlying Muddy Sandstone.

Muddy Sandstone

The Muddy Sandstone is composed of very fine to 

medium-grained sandstone interbedded with minor amounts of 

shale, siltstone, carbonaceous shale, and coal of latest Albian 

age (Paull, 1962). The formation was deposited in fluvial, 

marginal marine, and estuarine environments and ranges in 

thickness from 7 to 125 ft. The thickest accumulations are 

associated with an incised valley-fill complex that developed 

on the exposed surface of the Thermopolis Shale during sealevel lowstand (Dolson and others, 1991).

Mowry Shale

According to Keefer and others (1998), the Mowry Shale 

in the Bighorn Basin consists of two distinct units (fig. 5 and 

plate 1). The lower part consists of about 160 to 400 ft of soft, 

fissile clay-rich shale similar to the Thermopolis Shale and is 

referred to as the upper Thermopolis Shale by several authors 

including Mills (1956) and Haun and Barlow (1962), and as 

the Shell Creek Shale by Eicher (1962) (fig. 5). The upper part 

consists of about 240 to 400 ft of hard, brittle siliceous shale. 

Numerous gray to tan bentonite beds are common throughout 

both parts and range in thickness from a fraction of an inch to 

about 7 ft (Byers and Larson, 1979). The siliceous shales are 

dark brown to black, organic-rich, and contain an abundance 

of fish scales (Burtner and Warner, 1984). Locally, thin finegrained sandstones occur in the middle to upper part of the 

siliceous part of the Mowry. These units, referred to informally 

as the “Kimball” and “Octh Louie” sands have produced oil 
Results 7

from fields on the east side of the basin (Pierce, 1948; Mills, 

1956; Cardinal and others, 1989). The Mowry Shale is marine 

in origin with a combined thickness for the lower and upper 

parts ranging from about 400 ft in the southeastern part of the 

basin to more than 800 ft near the Beartooth uplift (Finn and 

others, Chapter 3, this CD–ROM). Paleontologic evidence 

and radiometric dating indicate that the upper siliceous part 

of the Mowry is largely early Cenomanian in age (Cobban 

and Kennedy, 1989; Obradovich and others, 1996), however, 

radiometric dates for bentonite beds in the basal Mowry (Shell 

Creek equivalent) indicate an Albian (Early Cretaceous) age 

(Obradovich and others, 1996).

Methods

Thirty samples from 15 wells were collected from well 

cuttings of Thermopolis and Mowry Shales stored at the 

U.S. Geological Survey (USGS) Core Research Center in 

Lakewood, Colorado. The selected wells are located near the 

outcrop belt along the shallow margins of the basin in order to 

obtain samples that were not subjected to the effects of deep 

burial and therefore are immature with respect to hydrocarbon 

maturation (figs. 6, 7). Fifteen samples are from the Thermopolis Shale and 15 from the Mowry Shale. The 15 samples from 

the Mowry were collected from the lower 200 ft of the upper 

siliceous part, with the exception of the sample at location two 

that includes some of the upper part of the Shell Creek Shale 

interval of Eicher (1962) (plate 1). The 15 samples from the 

Thermopolis were collected from the middle to lower part 

of the unit (plate 1). Sample intervals were determined by 

examining a gamma-ray log, if available, and the interval(s) 

with higher gamma-ray intensities were selected, based on 

work by numerous authors who described a close association 

of gamma-ray intensity with higher TOC content (for example, 

Schmoker, 1981; Zelt, 1985; Hester and others, 1990; Herron, 1991; Pratt and others, 1993; Dean and Arthur, 1998; 

Pasternack, 2005; and Rigoris and others, 2005). This relation, 

according to Hunt (1996), is a result of organic matter (OM) 

concentrating uranium from seawater.

The cuttings were examined under a binocular microscope and the darkest chips were selected for analysis based 

on observations by Hosterman and Whitlow (1981), Charpentier and Schmoker (1982), Hunt (1996), and Landon and 

others (2001), all of whom suggested that TOC content generally increases as color goes from gray to black and therefore 

is a rough (although not always reliable) indicator of organic 

richness. If a gamma-ray log was not available or there was 

insufficient cuttings material present from a high gamma interval then sample intervals were determined strictly by color. 

The cuttings were composited into one sample from thickness 

intervals that were generally 30 to 50 ft thick, but ranged from 

10 to 80 ft depending on how much material was available for 

a proper analysis (table 1).

The whole-rock samples were ground to a fine powder 

and splits were sent along with an internal USGS laboratory 

standard to an outside geochemical laboratory for analysis. 

Total organic carbon (TOC) content was determined using the 

Leco combustion method described by Jarvie (1991), and the 

pyrolysis analysis was done using a Rock-Eval 2 pyroanalyzer 

(Espitalie and others, 1977; Tissot and Welte, 1978; Peters, 

1986; and Hunt, 1996, contain detailed discussions of the 

pyrolysis method).

Results

Quantity of Organic Matter

According to Jarvie (1991), the quantity of organic matter 

in a formation measured as weight percent (TOC) is an indicator of the organic richness and generative potential. Rocks 

with less than 0.5 weight percent TOC have poor generative 

potential, rocks with 0.5 to 1 weight percent TOC are considered fair, rocks with 1–2 weight percent TOC are considered 

good, rocks with 2–4 weight percent TOC are considered very 

good, and rocks with greater than 4 weight percent TOC are 

considered to have excellent generative potential (Peters and 

Casa, 1994). Table 1 and figure 8A show the results of TOC 

analyses of Cretaceous marine shales in the Bighorn Basin.

The results of TOC analyses of the 15 samples collected 

from the Thermopolis Shale show values ranging from 1.04 to 

1.91 percent, with an average of 1.5 percent indicating good 

generative potential (fig. 8A). The 15 samples collected from 

the Mowry Shale have TOC contents that range from 1.11 to 

2.65 percent, with an average of 1.8 percent. Most of the samples fall in the 1 to 2 percent range indicating good generative 

potential, but a few samples fall into the 2 to 4 percent range 

indicating very good generative potential (fig. 8A).

Peters and Cassa (1994) pointed out that TOC is not 

always a good indicator of source rock potential because 

measurements may include inert carbon that has little or no 

generating potential. They (Peters and Cassa, 1994) stated 

that the S2

 measurement derived from pyrolysis analysis is a 

better indicator of generative potential of source rocks. The 

value S2

, expressed as milligrams of hydrocarbons per gram 

of rock, represents the fraction of original kerogen in a source 

rock capable of generating hydrocarbons that have not yet 

been converted to oil or gas or both (Tissot and Welte, 1978). 

According to Peters and Cassa (1994), rocks with S2 values 

less than 2.5 have poor generative potential, rocks with S2

values between 2.5 and 5 have fair generative potential, rocks 

with S2 values ranging from 5 to 10 have good generative 

potential, rocks with S2 values from 10 to 20 are considered to 

have very good generative potential, and rocks with S2 values 

greater than 20 have excellent generative potential.

S2 measurements for the Thermopolis and Mowry Shales 

are presented in table 1 and on figure 8B. The 15 samples from 

the Thermopolis Shale show a range of S2 values from 0.82 to 

3.49, indicating that it is a poor to fair source rock (fig. 8B). 

Six of the 15 samples from the Mowry Shale have S2

 values 
8 New Source Rock Data for the Thermopolis and Mowry Shales in the Wyoming Part of the Bighorn Basin

T

50

N

T

55

N

T

9

S

T

45

N

T

40

N

R94W R90W R85W

R95W

R100W

R104W

T

50

N

T

55

N

T

57

N

T

9

S

T

5

S

R15E R20E R25E R35E

R101W R95W

R90W R85W

R35E R40E

R105W

T

40

N

T

45

N

T

5

N

R5W R1W R1E R5E

T

8

N

R4W

T

8

N

R3E

1

2

3

4

5

6

7

9 8

10

11

12

13

14

15

Montana

Wyoming

110°

45°

44°

109° 108° 107°

0 10 20 30 40

MILES

Precambrian rocks

Sample location

Tertiary volcanic rocks

Bighorn Basin Province

Absaroka

Range

Beartooth

Mountains

Creek Mountains

Owl

Mountains

Bighorn 

Frontier/Mowry/Muddy/

 Thermopolis outcrop

Type log

Mountains

Pryor

Nye-Bowler

Rattlesnake Mountain

anticline

lineament

Big Trails fault

Thermopolis anticline

Sheep 

Mountain

anticline

Figure 6. Index map of the Bighorn Basin showing sample localities.

less than 2.5, indicating poor generative potential (fig. 8B). 

Of the remaining samples, seven are in the fair range, and two 

fall in the good range, indicating the Mowry is generally a fair 

source rock.

Types of Organic Matter

According to Jacobson (1991) and Peters and Cassa 

(1994), there are four types of kerogen in sedimentary rocks: 

Type-I, composed of oil-prone hydrogen-rich organic matter 

generally in lacustrine and some marine sediments; Type-II, 

also composed of oil-prone hydrogen-rich organic matter 

mainly in marine sediments, Type-III composed of terrestrial 

organic matter derived mainly from woody plant material 

that is low in hydrogen content and generates mainly gas, and 

Type-IV composed of dead or inert carbon that has little or no 

generating capacity. Even though oil is the main product of 

Type-II kerogen, it actually produces more gas than Type-III 

kerogen (Hunt, 1996). Using the results of pyrolysis analysis, 

the type of kerogen present in a source rock can be determined by the hydrogen index (HI) and the oxygen index (OI), 

defined as (S2 

/TOC) × 100, and (S3 /TOC) × 100, respectively 

(Espitalie and others, 1977; Tissot and Welte, 1978; and Hunt, 
Results 9

Mowry Shale

Thermopolis Shale

410 415 420 425 430 435 440

0

500

1,000

1,500

2,000

2,500

3,000

DEPTH, IN FEET

445 450

oil window

immature early peak

Stage of thermal maturity for oil generation

TMAX ( C) o

Figure 7. Tmax values plotted versus depth. Plot shows 

that all samples are immature or are in the early stages of 

thermal maturity with respect to oil generation. Parameters 

describing stages of thermal maturity for oil generation 

are from Peters and Casa (1994). Tmax, temperature (°C) 

corresponding to S2 peak from Rock-Eval pyrolysis.

1996). According to Hunt (1996), the type of hydrocarbons 

(oil or gas) generated from a source rock depends on the 

hydrogen content of the organic matter.

The HI and OI results from pyrolysis of the samples 

collected for the Thermopolis and Mowry Shales are shown 

on table 1 and plotted in figure 9. The plots for the 15 samples 

from the Thermopolis Shale show that most of the kerogen is 

Type-III indicating that the Thermopolis is a potential source 

rock mainly for gas (fig. 9). The plots for the 15 samples from 

the Mowry Shale show that most of the kerogen types are 

intermediate between Type-II and Type-III, indicating that the 

Mowry is a potential source for both oil and gas (fig. 9). 

Another method of determining the type of hydrocarbons 

generated from a source rock is by using the ratio of the values 

S2

 and S3 derived from pyrolysis analysis. According to Peters 

(1986), Peters and Cassa (1994), and Hunt (1996), the ratio 

S2 /S3

 is proportional to the amount of hydrogen in a source 

rock and is an indicator of the potential to generate oil and 

gas. According to Peters and Cassa (1994), rocks with an S2 /S3

ratio less than 1 are not likely to produce any oil or gas, those 

with ratios between 1 and 5 produce gas, those with ratios 

between 5 and 10 produce both oil and gas, and those with 

ratios greater than 10 produce mainly oil.

The S2/S3

 values for samples collected from the Thermopolis and Mowry Shales, are shown on table 1 and plotted 

in figure 10. The S2 /S3

 values for the Thermopolis Shale range 

from 1.35 to 7.76 indicating the Thermopolis is capable of 

generating both oil and gas (fig. 10). The S2/S3

 values for the 

Mowry Shale range from 1.5 to 11.84 indicating the Mowry is 

capable of generating both oil and gas (fig. 10).

Distribution of Organic Matter in the Mowry 

Shale

Maps were constructed for the Mowry Shale to 

show variations in TOC and kerogen type (HI) across the 

Bighorn Basin (fig. 11). Maps were not constructed for the 

Thermopolis Shale due to the narrow range of values in TOC 

and HI (figs. 8, 9 and table 1).

Basin-wide variations of the type and amount of organic 

matter present in the Mowry Shale are shown in figure 11. 

Figure 11A shows that all of the samples with TOC contents 

greater than 2 weight percent occur in the southeastern and 

northeastern parts of the Bighorn Basin. This trend is in general agreement with mapping presented by Burtner and Warner 

(1984; their fig. 15). A similar trend was noted by Finn (2007) 

for the Mowry in the Wind River Basin to the south. Regional 

studies by Schrayer and Zarrella (1963, 1966, 1968), Nixon 

(1973), Byers and Larson (1979), Davis (1986), and Davis 

and others (1989) discussed a similar trend of decreasing TOC 

content from east to west and suggested several possibilities 

for this trend, including 1) clastic dilution of organic matter 

due to higher sedimentation rates along the western shoreline 

of the WIS as sediments were eroded from the Western Cordillera, 2) greater input of marine organic matter in the central 

part of the seaway, and 3) post-depositional biodegradation of 

organic matter near the margins of the basin where less anoxic 

conditions existed. Differences in sampling methodology in 

this study from previous studies, and the small number of 

samples may also bias trends. Figure 11B shows little variation 

in kerogen types based on HI for the Mowry Shale in the Bighorn Basin indicating that the Mowry is capable of generating 

both oil and gas throughout the basin.
1 49017205470000 Teton Energy 19-1 Hayes Ranch 42N 91W 19 740 770 Mowry 1.27 0.19 4.61 4.8 0.84 5.5 

1 49017205470000 Teton Energy 19-1 Hayes Ranch 42N 91W 19 1,020 1,050 Thermopolis 1.55 0.17 2.36 2.53 0.57 4.14 

2 49017202360000 Brinkerhoff Drlg. Husky 3X-A 44N 96W 7 1,115 1,195 Mowry 1.8 0.31 2.31 2.62 0.82 2.81 2 49017202360000 Brinkerhoff Drlg. Husky 3X-A 44N 96W 7 1,435 1,495 Thermopolis 1.5 0.34 2.32 2.66 0.52 4.46 

3 49043202510000 Ashland Expl. Inc. 1-20 Chabot 43N 88W 20 630 690 Mowry 1.92 0.14 1.16 1.3 0.77 1.5 

3 49043202510000 Ashland Expl. Inc. 1-20 Chabot 43N 88W 20 940 1,000 Thermopolis 1.71 0.22 3.49 3.71 0.45 7.76 

4 49043201400000 Samedan Oil 1 Kellogg-Federal 46N 89W 13 520 560 Mowry 2.24 0.3 3.33 3.63 0.82 4.06 

4 49043201400000 Samedan Oil 1 Kellogg-Federal 46N 89W 13 940 980 Thermopolis 1.45 0.21 2.19 2.4 0.48 4.56 

5 49043050420000 Pacific Western 2 State Prescott 44N 92W 36 2,150 2,180 Mowry 2.13 0.44 4.47 4.91 0.75 5.96 

5 49043050420000 Pacific Western 2 State Prescott 44N 92W 36 2,500 2,550 Thermopolis 1.5 0.24 2.67 2.91 0.5 5.34 

6 49029213230000 Apache Corp. 36-11 State 47N 102W 36 930 990 Mowry 1.57 0.46 3.44 3.9 0.41 8.93 

6 49029213230000 Apache Corp. 36-11 State 47N 102W 36 1,490 1,550 Thermopolis 1.19 0.16 1.53 1.69 0.24 6.38 7 4900320695000 DATA Bighorn DATA 43-6 51N 91W 6 1,100 1,160 Mowry 1.73 0.02 0.08 0.1 0.02 4 

7 4900320695000 DATA Bighorn DATA 43-6 51N 91W 6 1,650 1,710 Thermopolis 1.42 0.15 2.06 2.21 0.37 5.57 8 49003305100000 Amerada 1 USA-Neal 49N 91W 28 2,140 2,220 Mowry 2.34 0.42 5.26 5.68 0.84 6.26 

8 49003305100000 Amerada 1 USA-Neal 49N 91W 28 2,640 2,670 Thermopolis 1.91 0.35 3.19 3.54 0.58 5.5 

9 49029210900000 National Oil Co. 33-30 Rawhide 49N 101W 30 2,000 2,010 Mowry 1.83 0.23 3.77 4 0.56 6.73 

9 49029210900000 National Oil Co. 33-30 Rawhide 49N 101W 30 2,390 2,410 Thermopolis 1.84 0.2 3.09 3.29 0.42 7.36 

10 49003205130000 Northern Natural Gas Fed. 362 1-32 54N 94W 32 2,240 2,260 Mowry 1.75 0.23 4.38 4.61 0.37 11.84 

10 49003205130000 Northern Natural Gas Fed. 362 1-32 54N 94W 32 2,690 2,720 Thermopolis 1.42 0.11 2.15 2.26 0.34 6.33 11 49029212000000 National Oil Co. 42-10 Federal 57N 98W 10 1,950 1,960 Mowry 1.11 0.28 1.98 2.26 0.38 5.21 

11 49029212000000 National Oil Co. 42-10 Federal 57N 98W 10 2,510 2,530 Thermopolis 1.34 0.12 1.41 1.53 0.41 2.78 

12 49003050480000 True Oil 1 Dever 57N 96W 7 1,080 1,130 Mowry 1.71 0.48 3.71 4.19 0.54 6.87 

12 49003050480000 True Oil 1 Dever 57N 96W 7 1,500 1,530 Thermopolis 1.07 0.21 1.04 1.25 0.77 1.35 

13 49003063720000 Graves-Davis Oil 3 Asay 56N 96W 23 1,790 1,820 Mowry 2.65 0.65 8.41 9.06 0.92 9.14 

13 49003063720000 Graves-Davis Oil 3 Asay 56N 96W 23 2,370 2,410 Thermopolis 1.04 0.21 0.82 1.03 0.5 1.64 

14 49029055880000 Aztec Oil and Gas Federal-H M 1 51N 102W 27 930 970 Mowry 1.56 0.17 1.68 1.85 0.51 3.29 

14 49029055880000 Aztec Oil and Gas Federal-H M 1 51N 102W 27 1,400 1,430 Thermopolis 1.53 0.14 2.29 2.43 0.54 4.24 15 49029214170000 Texaco Inc. 1 H C Sheets 54N 102W 21 1,610 1,650 Mowry 1.38 0.22 1.79 2.01 0.78 2.29 15 49029214170000 Texaco Inc. 1 H C Sheets 54N 102W 21 2,110 2,160 Thermopolis 1.62 0.21 2.52 2.73 0.54 4.67 

Table 1. Rock-Eval and total organic carbon data for the Wyoming part of the Bighorn Basin. Map number in column 1 refers to the map location shown on figure S1, milligrams of hydrocarbons per gram of rock; S2, milligrams of hydrocarbons per gram of rock; S3, milligrams of CO2 per gram of rock; Pl, production index; TOweight percent; HI, hydrogen index; OI, oxygen index; API, well number assigned by American Petroleum Institute; Tmax, temperature (°C) corresponding to S2 pea

Map no. API Operator Well name Township Range Section Top depth Bottom depth Formation TOC S1 S2 S1+S2 S3 S2/S3 
Results 11

0 5

Thermopolis

Mowry

1 2 3 4 6

WEIGHT PERCENT TOC

Poor Fair Good Very good Excellent

avg. 1.8 %

avg. 1.5 %

Thermopolis

Mowry

0 5

Poor Fair Good Very good Excellent

10 15 20 25 30

S2 (mg HC/g rock)

A

B

Figure 8. Plots showing source rock potential of the Thermopolis and Mowry Shales based on 

(A) total organic carbon content and (B) S2 values derived from Rock-Eval pyrolysis. Parameters 

describing source rock generative potential are from Peters and Casa (1994). TOC, total organic 

carbon; mg HC/g rock, milligrams of hydrocarbons per gram of rock.

0 50 100 150 200

OXYGEN INDEX

0

200

400

600

800

1000

HYDROGEN INDEX

(Thermopolis Shale)

100

300

500

700

900

I

II

III

IV

Oil

Oil and gas

Gas

None

0 50 100 150 200

OXYGEN INDEX

0

200

400

600

800

1000

HYDROGEN INDEX

(Mowry Shale)

100

300

500

700

900

I

II

III

IV

Oil

Oil Oil

Oil and gas

Gas

None

Figure 9. Plots of hydrogen index versus oxygen index showing kerogen types for samples 

(black dots) from the Thermopolis and Mowry Shales in the Bighorn Basin. Hydrocarbon type (oil 

or gas) from Peters and Casa (1994). 
12 New Source Rock Data for the Thermopolis and Mowry Shales in the Wyoming Part of the Bighorn Basin

0 5 10

Mowry

Thermopolis

None Gas Oil and gas Oil

S2/S3

Figure 10. S2/S3 ratios for Thermopolis and Mowry Shales in the Bighorn Basin. Parameters describing type of 

hydrocarbons generated are from Peters and Cassa (1994). S2

, milligrams of hydrocarbons per gram of rock; S3, 

milligrams of CO2

 per gram rock (Peters, 1986).

T

50

N

T

55

N

T

9

S

T

45

N

T

40

N

R94W R90W R85W

R95W

R100W

R104W

T

50

N

T

55

N

T

57

N

T

9

S

T

5

S

R15E R20E R25E R35E

R101W R95W

R90W

R85W

R35E R40E

R105W

T

40

N

T

45

N

T

5

N

R5W R1W R1E R5E

T

8

N

R4W

T

8

N

R3E

T

5

S

R15E R20E R25E R35E

T

55

N

T

57

N

T

9

S

R101W R95W

R35E

R90W

R104W

T

50

N

R105W

T

40

N

T

45

N

R5W

T

8

N

R4W

R94W

R95W

T

5

N

R1W R1E R5E

T

8

N

R3E

R100W

R90W R85W

R85W

T

50

N

T

55

N

T

9

S

T

45

N

T

40

N

Montana R40E

Wyoming

Montana

Wyoming

110°

45°

44°

45°

44°

109° 108° 107° 110° 109° 108° 107°

0 10 20 30 40

MILES

Precambrian rocks

Tertiary volcanic rocks

Bighorn Basin Province

Absaroka

Range

Beartooth

Mountains

Creek Mountains

Owl

Mountains

Bighorn 

Frontier/Mowry/Muddy/

 Thermopolis outcrop

1–1.49%

1.5–1.99%

2.0–2.49%

2.5–3.0%

Mowry weight % TOC

Owl

Mountains

Pryor

Nye-Bowler lineament

Big Trails fault

0 10 20 30 40

MILES

Beartooth

Mountains

Nye-Bowler

Mountains

Pryor

lineament

Bighorn 

Absaroka

Range

Mountains

Big Trails fault

Creek Owl

Mountains

Precambrian rocks

 Mowry Hydrogen index

Tertiary volcanic rocks

Bighorn Basin Province

Frontier/Mowry/Muddy/

 Thermopolis outcrop

201–300

51–200

<50

301+

A B

Figure 11. Maps of Bighorn Basin showing variations of (A) total organic carbon (TOC) content and (B) kerogen 

type based on hydrogen index for the Mowry Shale.
References 13

Summary

Results of TOC and Rock-Eval analyses of potential 

hydrocarbon-source rocks for the Thermopolis and Mowry 

Shales in the Bighorn Basin are summarized as follows:

1. The Mowry Shale has poor to good generative potential, with organic matter intermediate between kerogen Types-II and -III and the capability of generating 

both oil and gas.

2. The Thermopolis Shale has poor to fair generative 

potential, and contains mainly Type-III gas-prone 

kerogen based on hydrogen indices. However; 

S2/S3 ratios indicate that the Thermopolis is capable 

of generating both oil and gas.

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