U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Map 3285

Maps Showing Thermal Maturity of Upper Cretaceous 

Marine Shales in the Bighorn Basin, Wyoming and Montana
Cover photograph: Upper part of Cody Shale near Greybull, Wyoming, east flank of the Bighorn Basin. Photograph by R.C. Johnson, 1994.
Maps Showing Thermal Maturity of Upper 

Cretaceous Marine Shales in the Bighorn 

Basin, Wyoming and Montana

By Thomas M. Finn and Mark J. Pawlewicz

Scientific Investigations Map 3285

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

SALLY JEWELL, Secretary

U.S. Geological Survey

Suzette M. Kimball, Acting Director

U.S. Geological Survey, Reston, Virginia: 2014

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Suggested citation:

Finn, T.M., and Pawlewicz, M.J., 2014, Maps showing thermal maturity of Upper Cretaceous marine shales in the 

Bighorn Basin, Wyoming and Montana: U.S. Geological Survey Scientific Investigations Map 3285, 14 p., 

3 sheets, scale 1:500,000, http://dx.doi.org/10.3133/sim3285.

ISSN 2329-132X (online)
iii

Contents

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

Stratigraphy ....................................................................................................................................................1

Mowry Shale..........................................................................................................................................1

Frontier Formation.................................................................................................................................6

Cody Shale .............................................................................................................................................6

Thermal Maturity Maps ................................................................................................................................8

Acknowledgments.......................................................................................................................................10

References....................................................................................................................................................10

Figures

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

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

3. Correlation chart showing the stratigraphic relation of lowermost Upper

Cretaceous rocks in the Bighorn Basin ....................................................................................4

4. Regional stratigraphic cross section of Cretaceous rocks in the Bighorn Basin .............5

5. Type log ..........................................................................................................................................7

6. Log(Ro) versus depth plot for the Amoco Production 1 Chapman Bench well..................8

7. West–east structural cross section of the central part of the Bighorn Basin...................9

Sheets

1. Map showing variations in vitrinite reflectance at the top of the Cody Shale,

Bighorn Basin, Wyoming and Montana............................................................................... link

2. Map showing variations in vitrinite reflectance at the top of the lower shaly

member of the Cody Shale, Bighorn Basin, Wyoming and Montana.............................. link

3. Map showing variations in vitrinite reflectance at the top of the Mowry Shale,

Bighorn Basin, Wyoming and Montana............................................................................... link

Introduction

The Bighorn Basin is one of many structural and 

sedimentary basins that formed in the Rocky Mountain 

foreland during the Laramide orogeny, a period of crustal 

instability and compressional tectonics that began in latest 

Cretaceous time and ended in the 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 bounded on the northeast by the Pryor Mountains, 

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

Owl Creek Mountains (fig. 2). The north boundary includes 

a zone of faulting and folding referred to as the Nye-Bowler 

lineament (Wilson, 1936). The northwest and west margins 

are formed by the Beartooth Mountains and Absaroka Range, 

respectively (figs. 1 and 2).

Important conventional oil and gas resources have been 

discovered and produced from reservoirs ranging in age from 

Cambrian through Tertiary (Fox and Dolton, 1989, 1996a, 

b; De Bruin, 1993). In addition, a potential unconventional 

basin-centered gas accumulation may be present in Cretaceous 

reservoirs in the deeper parts of the basin (Ryder, 1987; 

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

and others, 1999; Finn and others, 2010). It has been 

suggested by numerous authors, including Burtner and Warner 

(1984), Hagen and Surdam (1984), Meissner and others 

(1984), Ryder (1987), and Fox and Dolton (1989, 1996a, b), 

that various Cretaceous marine shales are the principal source 

rock for these accumulations. Numerous studies of various 

Upper Cretaceous marine shales in the Rocky Mountain region 

by several authors, including Schrayer and Zarrella (1963, 

1966, 1968), Nixon (1973), Clayton and Swetland (1977), 

Merewether and Claypool (1980), Burtner and Warner (1984), 

Hagen and Surdam (1984), Momper and Williams (1984), 

Davis (1986), Davis and others (1989), Nuccio (1990a, b), 

Longman and others (1998), Landon and others (2001), 

Roberts and others (2004, 2007, 2008), and Finn (2010a) have 

led to the general conclusion that these rocks have generated 

or are capable of generating oil and (or) gas. 

In recent years, advances in horizontal drilling and 

multistage fracture stimulation have resulted in increased 

exploration and completion of wells in Cretaceous marine 

shales in other Rocky Mountain Laramide basins that were 

previously thought of only as hydrocarbon source rocks. 

Important parameters controlling hydrocarbon production 

from these shale reservoirs include: reservoir thickness, 

amount and type of organic matter, and thermal maturity 

(Milici, 1993; Curtis, 2002; Passey and others, 2010). The 

purpose of this report is to present maps and a cross section 

showing levels of thermal maturity, based on vitrinite 

reflectance (Ro

), for selected Upper Cretaceous marine shales 

in the Bighorn Basin.

Stratigraphy

Figure 3 is a correlation diagram showing the stratigraphic nomenclature for Upper Cretaceous marine shales 

and associated rocks in the Bighorn Basin, and the relationship to various localities in the Powder River Basin to the 

east. The Bighorn Basin nomenclature was compiled from 

Keefer (1972), Gill and Burkholder (1979), Merewether and 

Cobban (1986), Cobban and Kennedy (1989), Obradovich 

(1993), Merewether (1996), Obradovich and others (1996), 

Merewether and others (1997), Keefer and others (1998), 

and Kirschbaum and others (2009); the Powder River Basin 

nomenclature was modified from Merewether (1996). The 

stratigraphic relationships and nomenclature for the Bighorn 

Basin are also shown on the regional cross section in figure 4.

Mowry Shale

The Mowry Shale was defined by Darton (1904) for 

exposures in the northwestern part of the Powder River 

Basin. The top of the Mowry was revised by Rubey (1931) 

to include the Clay Spur Bentonite Bed. In the Bighorn Basin 

no definitive correlation can be made to the Clay Spur, but 

based on comparisons with well logs in the northern Wind 

River Basin and the Casper arch area, the top of the Mowry is 

placed at a distinctive high-gamma geophysical log response 

(Finn, 2010b; Johnson, 2010). According to Keefer and others 

(1998), the Mowry Shale in the Bighorn Basin consists of two 

Maps Showing Thermal Maturity of Upper Cretaceous 

Marine Shales in the Bighorn Basin, Wyoming and 

Montana

By Thomas M. Finn, and Mark J. Pawlewicz
2 Maps Showing Thermal Maturity of Upper Cretaceous Marine Shales in the Bighorn Basin, Wyoming and Montana

Sangre de Cristo uplift

Thrust or reverse fault

EXPLANATION

0 50

0 50 100 KILOMETERS

100 MILES

112° 110° 108° 106° 104°

46°

44°

40°

36°

38°

42°

Hartville uplift

Cordilleran

orogenic

belt

Denver

Bull

Mountains

Basin Crazy

Mountains

Basin Bridger

uplift

Powder

River

Basin

Black Hills uplift

Miles City arch

Teton

uplift

Casper arch

Owl Creek uplift

Shirley

Basin uplift

Rawlins

uplift

Sierra Madre uplift

Hanna

Basin 

Laramie

Basin

Greater Green

River Basin

Wind

River uplift

Basin

North and Middle

Park Basins

Medicine

Bow

uplift

Park

Range

uplift

Uinta Basin

White

River

uplift

Sawatch uplift

South Park

Basin

Front

Range

Uplift

UINTA UPLIFT

San Rafael

Swell

Uncompahgre

uplift

Gunnison

uplift

San Juan

uplift

San

Wind River Basin

Juan

volcanic

field

Raton

Basin

San Juan

Basin

Nacimiento uplift

Beartooth uplift

Piceance

Basin

Reed Point

syncline

lineament Nye-Bowler

Bighorn

Basin

uplift

Bighorn

Zuni uplift

rift

(Neogene)

Grande

Rio

Pryor Mountains

Absaroka volcanic field

Granite Mountains

Axial Basin uplift

CO

WY NE

ID WY

ID

MT

UT

ND

MT

WY

SD

SD

NE

CO

NM

UT

AZ

CO

NM

Figure 1.

Figure 1. Map of Rocky Mountain region extending from southern Montana to northern 

New Mexico showing locations of Laramide sedimentary and structural basins (in brown) and 

intervening uplifts. Modified from Dickinson and others (1988).
Stratigraphy 3

0 2 10 0 30 40 MILES

0 10 20 30 40 KILOMETERS

EXPLANATION

Tertiary volcanic rock

Precambrian rock

Mesaverde Formation outcrop

Bighorn Basin Province area

Bighorn Basin Province boundary

Wrench fault—Arrows show relative motion

Anticline

High-angle fault

Syncline

Line of section (see fig. 7)

Thrust fault—Sawteeth on upper plate

Type log

R105W

T

40

N

R95W

R94W R90W R85W

T

5

N

T

8

N

T

8

N

R100W

T

45

N

R4W

R3E

R104W

R35E R40E

R30E

R25E

R95W

R90W R85W

R101W

R15E R20E R25E

T

5

S

T

9

S

T

57

N

T

55

N

T

50

N

T

9 MONTANA S

WYOMING

T

5

S

T

55

N

T

50

N

T

45

N

T

40

N

R5W R1W R1E R5E

-5000

0

0

0

5000

0

0

5000

-5000

0

-10000

0

-5000

-5000

Basin Oregon

fault

axis

Basin 

Big Trails fault

lineament

Mountains

Pryor

Nye-Bowler

Range

Mountains

Bighorn 

Beartooth

Mountains

Absaroka

Clark's Fork sub-basin

Owl

Creek

Mountains

Type

log 44°

45°

110° 109° 108° 107°

MONTANA

WYOMING

Bighorn

Basin

Province

Figure 2.

Figure 2. Index map of the Bighorn Basin in Wyoming and Montana showing major structural and physiographic features. Structure 

contours are drawn on top of the Mesaverde Formation. Bighorn Basin Province boundary from Finn and others (2010). Outcrops from 

Green and Drouillard (1994), and Raines and Johnson (1995). Contour interval = 5,000 feet.
85 Colinoceras tarrantense Inoceramus involutus Claggett Mbr. "chalk kick" Collignoniceras woollgari

Santonian

Baculites obtusus

Mesaverde

Formation

(part)

Mowry Shale

Upper

Clioscaphites choteauensisPrionocyclus quadratus

Belle Fourche

Member

?

Belle Fourche

Member

(equivalent)

98.5

LK

(part)

upper sandy

member

Neogastroplites haasiScaphites depressusMammites nodosoides

"chalk kick"

Mowry Shale

upper sandy

member

Prionocyclus percarinatus

Northern Bighorn Basin

Baculites gregoryensis

lower shaly

member

95

Frontier Formation

Sage Breaks Mem

Turonian

Claggett Mbr.

Neogastroplites muelleri

Lower

Prionocyclus hyatti

Western and southwestern

Bighorn Basin

Baculites reduncus

Virgelle Sandstone Mbr.

Series

Muddy Sandstone

Pseudaspidoceras flexuosumNeogastroplites maclerni

Eagle Sandstone

Inoceramus erectus

Muddy Sandstone

lower shaly

member

Mid L L

Belle Fourche

Member

(equivalent)

Clioscaphites saxitonianus

Parkman Sandstone

U

unnamed member

Sussex and

Shannon Ss. Be

Didymoceras nebrascense

lower shaly

member

Nigericeras scotti

CRETACEOUS (part)

Mowry Shale

Lower

86.3

Mesaverde

Formation

(part)

Prionocyclus wyomingensisBaculites sp. (smooth)

lower member

Clioscaphites vermiformis

Muddy Sandstone

Baculites scotti

Cody Shale

Niobrara Memb

Telegraph Creek Formation

Cody Shale

Middle

lower member

Scaphites whitfieldi

Campanian (part)

Montana

Upper

Mesaverde

Formation

(part) 80

Dunveganoceras problematicum

100

93.3

unnamed

lower part

Cody Shale

Judith River

Formation

(part)

Baculites perplexus (late form)

Steele Member

Desmoscaphites erdmanni

"chalk kick"

Middle

90

Frontier Formation

Coniacian

Baculites mclearni

"chalk kick"

Upper

Muddy Sandstone

Scaphites hippocrepis I

83.5

System

Baculites sp. (smooth)

unnamed

upper par

Belle Fourche

Member

(equivalent)

Belle Fourche

Member

(equivalent)

Vascoceras birchbyi

Muddy Sandstone

Wall Creek Memb

upper sandy

member

Mowry Shale

Frontier Formation

Claggett Shale

Fossil zone

Frontier Formation

unnamed member

Cody Shale

Cenomanian

Scaphites hippocrepis III

unnamed member

Plesiacanthoceras wyomingenseBaculites sp. (weak flank ribs)

Lower

upper sandy

member

Prionocyclus macombiBaculites perplexus (early form)

unnamed member

Stage

Desmoscaphites bassleri

Emigrant Gap

Member

lower shaly

member

Acanthoceras bellense

Mesaverde

Formation

(part)

UPPER CRETACEOUS (part)

Mowry Shale

Upper

Ma Western Powder River Basin

Inoceramus deformisBurroceras clydense

Albian

(part) Middle 88.7 Acanthoceras granerosense 

Scaphites hippocrepis II

Frontier Formation

Cody Shale

Baculites asperiformisBaculites gilberti

Eastern and southeastern

Bighorn Basin

Middle

Figure 3.

Figure 3. Correlation chart showing the stratigraphic relation of lowermost Upper Cretaceous rocks in the Bighorn Basin. Radiometric ages and fossObradovich (1993), and Merewether and others (1997). LK, Lower Cretaceous.
Figure 4. Regional northwest-southeast stratigraphic cross section of Cretaceous rocks in the Bighorn Basin. Modified from Finn (2010b).

MONTANA

WYOMING

Mowry Shale

Co

Telegraph Creek Formation

Eagle Sandstone

ParkmanSandstone Mesaverde Formation

Judith River Formation (part)

Cody Shale

Mesaverde Formation (middle part)

Badger Basin Red Point

Frontier Formation

NORTHWEST

Lewis Shale

?

DobieCreek

Claggett Shale

Upper Cretaceous

?

Lower

Cretaceous

Jurassic

?

Claggett Member of Cody Shale

lower shaly member

Five

Mile

Muddy Sandstone

Frontier Formation

Thermopolis Shale

McCulloch

Peak

Meeteetse Formation (part)

Virgelle SandstoneMember

Cloverly Formation

Emblem

Bench Fritz

upper sandy member

Teapot Sandstone Member

?

Co

lower member

"chalk kick"

Nye-Bowler lineament Worland

Jurassic rocks undivided

(part)

1,000

500

0

10 MILES 20

0 2010 KILOMETERS

FEET

BighornBasinProvince

LOCATION OF CROSS SECTION

MONTANA

WYOMING

Stratigraphic markUnconformity

EXPLANATION

Fluvial sandstone and conglomerateFluvial and estuarine sandstoneInterbedded sandstone and shale, marineMarine and marginal marine Marine shales sandstone and siltstone

Tidal sandstone, siltstone, and shale

Floodplain and lac shale, and congloUndifferentiated dCoastal plain and f shales, and coals

Figure 4.
6 Maps Showing Thermal Maturity of Upper Cretaceous Marine Shales in the Bighorn Basin, Wyoming and Montana

distinct units. The lower unit consists of about 150 to 400 ft 

of soft fissile clay-rich shale similar to the older 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). The 

upper unit consists of about 240 to 400 ft of hard brittle siliceous shale. Numerous gray to tan bentonite beds are common 

throughout the Mowry, ranging in thickness from less than 

one 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, 

there are thin fine-grained sandstones in the middle to upper 

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

as the “Kimball” and “Octh Louie” sands, have produced 

oil from fields on the east side of the Bighorn Basin (Pierce, 

1948; Mills, 1956; Cardinal and others, 1989). Davis (1987) 

also described two sandstone bodies in the Mowry Shale along 

the east and northeast sides of the basin. These sandstones 

are fine- to medium-grained, as much as 15 ft thick, and form 

elongate bodies that he believed (Davis, 1987) were deposited during a fall in sea level. 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 Mountains (Finn and 

others, 2010). Paleontologic evidence and radiometric dating 

indicate 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 Shale of Eicher, 1962) 

indicate an Albian (Early Cretaceous) age (Obradovich and 

others, 1996). 

Frontier Formation

The Frontier Formation is early Cenomanian to early 

Coniacian in age and contains at least one major unconformity 

in the upper part (Merewether and others, 1975; Kirschbaum 

and others, 2009). The contact with the underlying Mowry 

Shale is placed at a distinctive gamma response on geophysical logs that is believed to correspond to the Clay Spur Bentonite Bed that is considered the top of the Mowry Shale in 

the Powder River Basin. The upper contact is typically placed 

at the top of the uppermost sandstone below the Cody Shale. 

The Frontier interval, as defined between the top of the Mowry 

and uppermost sandstone bed, ranges in thickness from less 

than 200 ft in the northern part of the basin to nearly 700 ft 

in the southeastern part of the basin. The Frontier Formation 

consists of marine and minor marginal-marine shale, siltstone, 

and sandstone that are generally interpreted to be deltaic 

or shoreface in origin (Hunter, 1952; Goodell, 1962; Van 

Houten, 1962; Merewether and others, 1975; Siemers, 1975; 

Merewether and others, 1998; Kirschbaum and others, 2009). 

The sandstones are continuous for tens of miles, but pinch out 

into marine shale or are truncated along their upper surfaces 

by marine-flooding surfaces (Kirschbaum and others, 2009). 

Minor amounts of carbonaceous shale and coal are present in 

the northwestern part of the basin (Merewether and others, 

1975; Siemers, 1975). 

Cody Shale

The Cody Shale in the Bighorn Basin consists of marine 

shale, sandstone, and siltstone deposited during a major 

marine transgressive-regressive cycle that extended from 

Coniacian to Campanian time (Kauffman, 1977). The lower 

and upper parts of the Cody Shale interfinger extensively with 

the underlying Frontier and overlying Mesaverde Formations (fig. 4). The Cody ranges in thickness from about 1,700 

ft in the northern part of the basin to nearly 3,800 ft in the 

southeastern part (Finn and others, 2010). This southeastward 

thickening is due to the eastward stratigraphic rise and intertonguing of the contact between the Cody and the overlying 

Mesaverde Formation (fig. 4). In the Bighorn Basin, three 

members are recognized: (1) the informally named lower shaly 

member, (2) the informally named upper sandy member, and 

(3) the Claggett Member (Keefer, 1972; Keefer and others, 

1998).

The lower shaly member is about 700-ft thick in the 

northern part of the basin and reaches a maximum thickness 

of about 1,200 ft in the southern part (Finn, in press). It is 

composed of gray to black shale, calcareous shale, bentonite, 

and minor amounts of sandstone and siltstone. The contact 

between the lower and upper members is poorly defined and 

indistinct, and in this report is drawn at a distinctive well log 

marker that can be traced in the subsurface throughout the 

basin (fig. 5). A persistent zone referred to as the “chalk kick” 

marker by Keefer (1972) is recognized in geophysical logs in 

the lower part of the lower shaly member (fig. 5). This zone 

can be traced throughout the basin and separates calcareous 

shales above, from noncalcareous shales below. Based on 

regional correlations across the Casper arch and Wind River 

Basin, the calcareous interval extending from the “chalk 

kick” marker to the base of the overlying sandy member is 

equivalent to the lower shaly member of the Cody Shale in 

the western part of the Wind River Basin, and the basal part of 

the Niobrara Formation in the Powder River Basin to the east. 

This interval reaches a maximum thickness of about 1,000 ft 

in the southwestern part of the basin and thins to about 500 ft 

in the northeastern part of the basin (Finn, in press). The interval extending downward from the “chalk kick” marker to the 

top of the uppermost sandstone bed of the Frontier Formation 

ranges in thickness from about 100 to 400 ft, and is equivalent 

to the uppermost part of the Carlile Shale of the Powder River 

Basin (Finn, in press).

The upper sandy member of the Cody Shale is about 

2,700 ft thick in the southern part of the Bighorn Basin and 

thins to around 1,000 ft in the northern part. It consists of 

interbedded light to medium gray, or buff sandstones, and gray 

shales with the amount of sandstone increasing in the upper 

500–1,200 ft (Johnson and others, 1998). In the subsurface, 

the sandstones in the upper part of the Cody can be traced 

over many miles and appear to pinch out into marine shale 
Stratigraphy 7

7,000

7,500

8,000

8,500

9,000

9,500

10,000

Snyder Oil 1-26 Worland-Fee

sec. 26, T. 47 N., R. 93 W.

GR Res.

Mowry

Shale

Thermopolis Shale

upper

siliceous

part

Clay Spur Bentonite Bed

"chalk kick

marker"

lower shaly member

Cody Shale (part)

Frontier

Formation

Cloverly

Formation

Carlile

Shale

equivalent

Jurassic

(part)

Muddy Sandstone

Basal Niobrara Formation

equivalent

Morrison Formation

(part)

lower part

upper sandy

member

(part)

Lower

Cretaceous Upper Cretaceous (part)

Pryor Member

"rusty beds"

Greybull Ss. Mbr.

Figure 5.

Figure 5. Type log of Lower Cretaceous and lowermost 

Upper Cretaceous rocks in the southeastern part of 

the Bighorn Basin. Sandstones and conglomerates of 

predominantly fluvial origin are shown in red; marine 

and marginal marine sandstones and siltstones, yellow; 

marine shales, various shades of gray; estuarine and fluvial 

sandstones shown in orange; floodplain and lacustrine 

sandstone and shale, brown; tidal sandstone, siltstone, 

and shale, light brown; interbedded marine sandstone and 

shale, tan; undifferentiated rocks, blue. GR, gamma ray; 

Res, resistivity. Location shown in figure 2.
8 Maps Showing Thermal Maturity of Upper Cretaceous Marine Shales in the Bighorn Basin, Wyoming and Montana

in all directions (Finn, 2010b; Johnson, 2010). The uppermost several hundred feet of the sandy member of the Cody 

is commonly referred to as the Telegraph Creek Formation 

in the Montana part of the basin (figs. 3 and 4). The Cody is 

poorly exposed; however, limited outcrop studies by Johnson 

and others (1998) of the uppermost Cody Shale indicate that 

the sandstones are very fine- to medium-grained, generally 

laterally persistent, and exhibit a variety of bedding features 

including hummocky cross-bedding, indicative of a nearshore 

marine origin.

According to Keefer and others (1998), the Claggett 

Member (known as the Claggett Shale in Montana) is a 

westward-thinning tongue of marine shale and fine-grained 

sandstone that is separated from the main part of the Cody 

Shale by the lower member of the Mesaverde Formation or 

its equivalent in Montana, the Eagle Sandstone (fig. 4). The 

Claggett Member extends across the eastern and central parts 

of the Bighorn Basin, and thickens eastward from zero to 

nearly 500 ft thick at its eastern limit, where it merges with the 

main part of the Cody Shale (fig. 4) (Finn and others, 2010).

Thermal Maturity Maps

Maps showing thermal maturity levels based on vitrinite 

reflectance (Ro) for Upper Cretaceous marine shales are shown 

on sheets 1–3. The maps were constructed using Ro data 

published by Hagen and Surdam (1984), Hagen (1986), Yin 

(1997), Nuccio and Finn (1998), Finn and Pawlewicz (2007), 

Roberts and others (2008), and Pawlewicz and Finn (2012) 

from samples collected from oil and gas well cuttings, and 

outcrops. For wells reporting Ro measurements over a range of 

depths, these data were plotted on a log(Ro) versus depth graph 

for each well and a visual best-fit line was drawn through the 

data, for example see figure 6. Data points appearing to be 

anomalously high or low with respect to the main trend were 

disregarded. In some cases, data from closely associated wells 

were combined into a composite log(Ro) versus depth plot. For 

a single well or outcrop location where few Ro measurements 

were available over a short depth range or stratigraphic 

interval, the available data were averaged and the calculated 

Ro

 were plotted for that location.

15,000

Percent Ro

Amoco Production 1 Chapman Bench

sec. 3, T. 55 N., R. 102 W. 

0

10

5,000

Depth, in feet

0.1 1.0

10,000

Kmr

Kcu

Kf

Kme

Kcl

Kl

Kmv

Mowry Shale

Lower shaly member

of Cody Shale

 Outcrop point from sec. 24, T. 55 N., R. 103W.

 Top of Cody Shale

Figure 6. Log(Ro) versus depth plot for the Amoco 

Production 1 Chapman Bench well. Kl, Lance Formation; 

Kme, Meeteetse Formation; Kmv, Mesaverde Formation; 

Kcu, upper sandy member of Cody Shale; Kcl, lower 

shaly member of Cody Shale; Kf, Frontier Formation; 

Kmr, Mowry Shale.
Oregon Basin

thrust fault

1 Adams 1 Otto

(projected)

?

33-20 Rio Unit

Oregon Basin

WEST

Modifie

Stone 

Five Mile trend

1 Emblem Bench

Modified from

Blackstone (1986)

?

shaly member

Loch Katrine Unit 1

sandy member

Michaels Ranch 20-1

Greybull 

-5,000

5,000

FEET

-25,000-10,000

SEA LEVEL 

-15,000-20,000

0.6

2.0

0.8

1.35

1.5

1.1

Bighorn Basin

Province

EXPLANATION

Lower Eocene—Willwood FormationPaleocene—Fort Union FormationUpper Cretaceous—Lance FormationUpper Cretaceous—Meeteetse Formation

and Lewis Shale

Upper Cretaceous—Mesaverde Formation

Kmv

Upper Cretaceous—Frontier Formation and Mowry Shale

Upper Cretaceous—Cody Shale

Kf

Kmr

Lower Cretaceous—Muddy Sandstone and Thermopolis Shale

Kmt

Kc

0 5 10 MILES

5 10 KILOMETERS

VERTICAL EXAGGERATION ×4 

0

Kcv Lower Cretaceous—Cloverly Formation

Paleozoic sedimentary rock—undivided

Jurassic to Triassic sedimentary rocks— undividedPrecambrian crystalline rocks

J–



p

Thrust fault—Arrows show relative motion 2.0 Isoreflectance line, in percent Contact—Dashed where inferred LOCATION OF CRO

Kc 

J–

Kmr

p

Kc

Tw

Kf

Tfu

Kl

Kmv

Kml



Kcv



J–

Kmt



J–

Kf

Kmv

Kmr

Kc

Tw

Tfu

Kl

Kml

Figure 7.

Figure 7. West–east structural cross section of the central part of the Bighorn Basin, Wyoming and Montana, showing relation between present-daof thermal maturity based on vitrinite reflectance. Line of section shown in figure 2, and sheets 1–3. The Oregon Basin part of the section is modified fthe Greybull monocline part of the section is modified from Stone (2004).
10 Maps Showing Thermal Maturity of Upper Cretaceous Marine Shales in the Bighorn Basin, Wyoming and Montana

Three maps were constructed to show the thermal 

maturity of Upper Cretaceous marine shales in the Bighorn 

Basin: (1) top of the Cody Shale, (2) top of the lower shaly 

member of the Cody Shale (basal Niobrara Formation 

equivalent), and (3) the top of the Mowry Shale (sheets 1–3). 

• Due to the intertonguing nature of the upper contact of 

the Cody Shale and the overlying Mesaverde Formation it was necessary to change the contoured horizon across the map from west to east (figs. 3, 4, and 

sheet 1). In the western and southwestern part of the 

basin the contours are drawn at the base of the main 

part of the Mesaverde Formation. In the central and 

northern part of the basin the contours are drawn at the 

base of the lower member of the Mesaverde Formation 

or its equivalent, the Eagle Sandstone in Montana. In 

the eastern and southeastern parts of the basin, where 

the lower member is absent, the contours are drawn 

at the base of the Mesaverde Formation (figs. 3 and 4, 

and sheet 1).

• The Ro map for the top of the lower shaly member of 

the Cody Shale is contoured at the contact between the 

shaly member and the overlying sandy member (fig. 5, 

and sheet 2). 

• The Ro map for the Mowry Shale is drawn on a distinctive high gamma marker on geophysical logs that 

corresponds to the Clay Spur Bentonite Bed (fig. 5, and 

sheet 3).

All three mapped horizons show a general increase 

in thermal maturity from the margins of the basin into the 

deeper parts (sheets 1–3). In each case, the Ro ranges from 

less than 0.6 to around 0.8 percent along the shallow margins 

of the basin, increasing to greater than 1.1 percent in the 

deeper parts along the main basin trough east of the Oregon 

Basin fault, in the Clark’s Fork sub-basin, and a small area 

near the eastern end of the Beartooth Mountains uplift (sheets 

1–3). In these structurally deeper parts of the basin, both 

the lower Cody Shale and Mowry Shale Ro values exceed 

1.35 percent. In the deepest part of the basin along the main 

synclinal trough, adjacent to and east of the Oregon Basin 

fault they both exceed 2.0 percent Ro

 (sheets 2–3). These 

thermal maturity trends generally reflect the structural 

configuration of the basin and indicate that large areas of the 

basin are thermally mature (Ro > ~0.6 percent) to overmature 

(Ro > ~1.35 percent) with respect to oil generation based on 

parameters defined by Peters and Cassa (1994). A structural 

cross section extending west–east across the central part of 

the basin illustrates the relation between the present-day 

structural configuration of the basin and thermal maturity 

(fig. 7). The isoreflectance lines are sub-parallel, nearly 

horizontal and cut across present-day structures indicating 

that maturation developed mainly after basin formation and 

deformation during the Laramide orogeny and subsequent 

accumulation of basin fill (Bustin and others, 1983). 

Acknowledgments

The manuscript and sheets benefited from reviews 

by Ron Johnson, Laura Biewick, Dave Ferderer, and Tom 

Judkins, and their suggestions and comments are greatly 

appreciated.

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