Photographs by R.C. Johnson, U.S. Geological Survey, Energy Team
Burial History, Thermal Maturity, and 

Oil and Gas Generation History of 

Source Rocks in the Bighorn Basin, 

Wyoming and Montana

By Laura N.R. Roberts, Thomas M. Finn, Michael D. Lewan, 

and Mark A. Kirschbaum

Scientific Investigations Report 2008–5037

U.S. Department of the Interior

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

DIRK KEMPTHORNE, Secretary

U.S. Geological Survey

Mark D. Myers, Director

U.S. Geological Survey, Reston, Virginia: 2008

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Suggested citation:

Roberts, Laura N.R., Finn, Thomas M., Lewan, Michael D., and Kirschbaum, M.A., 2008, Burial History, Thermal 

Maturity, and Oil and Gas Generation History of Source Rocks in the Bighorn Basin, Wyoming and Montana: U.S. 

Geological Survey Scientific Investigations Report 2008–5037, 28 p.

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reproduce any copyrighted materials contained within this report.
iii

Contents

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

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

Methods—Burial History..............................................................................................................................5

Age ..........................................................................................................................................................5

Thickness and Lithology ......................................................................................................................5

Stratigraphic Intervals .........................................................................................................................5

Unconformities......................................................................................................................................5

Post-Middle Eocene Deposition and Erosion ..................................................................................5

Methods—Thermal History........................................................................................................................11

Methods—Petroleum Generation History...............................................................................................11

Gas-Prone Source Rocks ..................................................................................................................11

Oil-Prone Source Rocks ....................................................................................................................11

Results—Burial History ..............................................................................................................................12

Emblem Bench, Red Point/Husky, and Sellers Draw....................................................................12

Amoco BN 1.........................................................................................................................................12

Santa Fe Tatman..................................................................................................................................19

McCulloch Peak..................................................................................................................................19

Dobie Creek..........................................................................................................................................19

Doctor Ditch.........................................................................................................................................19

Results—Maturation History.....................................................................................................................19

Results—Petroleum Generation History .................................................................................................19

Gas Generation from Source Rocks ................................................................................................24

Oil Generation from Source Rocks ..................................................................................................24

Oil Cracking to Gas .............................................................................................................................24

Summary........................................................................................................................................................24

References Cited..........................................................................................................................................25
iv

Figures

1. Base map of Bighorn Basin, Wyoming and Montana, showing eight burial history 

 locations and structure contours on top of the Cloverly Formation.....................................2

2. Generalized stratigraphic chart of Pennsylvanian (part) through middle Tertiary 

 (part) rocks in the Bighorn Basin, Wyoming and Montana ...................................................4

3. Maps and cross section showing method used to estimate post-middle Eocene 

 deposition and erosion in the Bighorn Basin, Wyoming and Montana. A, trend 

 surface using elevations of erosion surfaces described in Hagen and Surdam 

 (1984) and Roberts and others (2007) and locality of Miocene vertebrates at 

 Darton’s Bluff. B, cross section showing estimated basin fill. C, isopach map 

 of estimated post-middle Eocene erosion..............................................................................10

4. Burial history curves at eight locations in the Bighorn Basin, Wyoming and 

 Montana. A, locations where source rocks were most deeply buried. B, 

 locations where source rocks were buried to intermediate depths, C, locations 

 where source rocks were buried to shallowest depths ......................................................14

5. Timing of gas generation from Type-III source rocks for each source rock and 

 burial history location in the Bighorn Basin, Wyoming and Montana...............................22

6. Timing of oil and gas generation from Type-II source rocks for each source rock 

 and burial history location in the Bighorn Basin, Wyoming and Montana .......................23

Tables

1. Information on wells used for burial history curves, Bighorn Basin, Wyoming 

 and Montana .................................................................................................................................3

2. Data used to generate burial history curves for eight locations, Bighorn Basin, 

 Wyoming and Montana ...............................................................................................................6

3. Present-day depth, calculated maximum depth of burial, and calculated

 temperature at maximum depth of burial for base of source rock intervals 

 and for Phosphoria/Park City Formation from burial history reconstructions, 

 Bighorn Basin, Wyoming and Montana..................................................................................13

4. Timing of gas generation for Type-III source rocks at burial history locations, 

 Bighorn Basin, Wyoming and Montana..................................................................................20

5. Timing of oil generation for Type-II source rocks and timing of oil cracking 

 to gas, Bighorn Basin, Wyoming and Montana.....................................................................21
Abstract

Burial history, thermal maturity, and timing of oil and gas 

generation were modeled for seven key source-rock units at 

eight well locations throughout the Bighorn Basin in Wyoming 

and Montana. Also modeled was the timing of cracking to 

gas of Phosphoria Formation-sourced oil in the Permian Park 

City Formation reservoirs at two well locations. Within the 

basin boundary, the Phosphoria is thin and only locally rich in 

organic carbon; it is thought that the Phosphoria oil produced 

from Park City and other reservoirs migrated from the IdahoWyoming thrust belt. Other petroleum source rocks include 

the Cretaceous Thermopolis Shale, Mowry Shale, Frontier 

Formation, Cody Shale, Mesaverde and Meeteetse Formations, 

and the Tertiary (Paleocene) Fort Union Formation. 

Locations (wells) selected for burial history reconstructions include three in the deepest parts of the Bighorn Basin 

(Emblem Bench, Red Point/Husky, and Sellers Draw), three 

at intermediate depths (Amoco BN 1, Santa Fe Tatman, and 

McCulloch Peak), and two at relatively shallow locations 

(Dobie Creek and Doctor Ditch). The thermal maturity of 

source rocks is greatest in the deep central part of the basin 

and decreases to the south, east, and north toward the basin 

margins. The Thermopolis and Mowry Shales are predominantly gas-prone source rocks, containing a mix of Type-III 

and Type-II kerogens. The Frontier, Cody, Mesaverde, Meeteetse, and Fort Union Formations are gas-prone source rocks 

containing Type-III kerogen. Modeling results indicate that in 

the deepest areas, (1) the onset of petroleum generation from 

Cretaceous rocks occurred from early Paleocene through early 

Eocene time, (2) peak petroleum generation from Cretaceous 

rocks occurred during Eocene time, and (3) onset of gas generation from the Fort Union Formation occurred during early 

Eocene time and peak generation occurred from late Eocene 

to early Miocene time. Only in the deepest part of the basin 

did the oil generated from the Thermopolis and Mowry Shales 

start generating gas from secondary cracking, which occurred 

in the late Eocene to Miocene. Also, based on modeling 

results, gas generation from the cracking of Phosphoria oil 

reservoired in the Park City Formation began in the late 

Eocene in the deep part of the basin but did not anywhere 

reach peak generation.

Introduction

The Bighorn Basin is a large Laramide (Late Cretaceous through Eocene) structural and sedimentary basin that 

encompasses about 7,500 mi2 in north-central Wyoming and 

south-central Montana (fig. 1). The basin boundaries are 

defined by fault- and fold-bounded Laramide uplifts that surround it except in the northernmost part where the basin is 

bounded by the northwest-trending Nye-Bowler Lineament. 

Adjacent mountain ranges include the Beartooth Mountains to 

the northwest, the Absaroka Mountains to the west, the Owl 

Creek Mountains to the south, and the Bighorn Mountains 

to the east (fig. 1). This report summarizes the burial history, 

thermal maturity, and timing of petroleum generation at eight 

locations (table 1) for seven key source-rock intervals throughout the basin. The petroleum source-rock intervals studied are 

(1) the Cretaceous Thermopolis Shale, (2) Mowry Shale, (3) 

Frontier Formation, (4) Cody Shale, (5) Mesaverde Formation, (6) Meeteetse Formation, and (7) the Tertiary (Paleocene) 

Fort Union Formation (fig. 2). Also modeled was the timing of 

cracking to gas of Phosphoria-sourced oil in the Permian Park 

City Formation reservoirs at two well locations. The resulting 

data and interpretations support the assessment of undiscovered oil and gas resources of the Bighorn Basin undertaken by 

the U.S. Geological Survey.

Previous studies of the burial/thermal history of parts 

of the Bighorn Basin were presented in Hagen and Surdam 

(1984), Hagen (1986), Ryder (1987), and Nuccio and Finn 

(1998). All wells examined by Hagen and Surdam (1984) are 

confined to the northern part of the basin, and their interval of 

study included only Thermopolis, Mowry, Frontier, and Cody 

strata. Hagen (1986) discussed only burial/thermal history as it 

relates to oil generation from the Mowry Shale in the northern 

part of the basin. Ryder (1987) presented time-temperature 

reconstructions for two wells in a small area just north of 

the central part of the basin, and Nuccio and Finn (1998) 

presented thermal maturity data for Cretaceous and Tertiary 

source rocks across the basin.

Burial History, Thermal Maturity, and Oil and Gas 

Generation History of Source Rocks in the Bighorn Basin, 

Wyoming and Montana

By Laura N.R. Roberts, Thomas M. Finn, Michael D. Lewan, and Mark A. Kirschbaum
2 Burial History, Thermal Maturity, and Oil and Gas Generation History of Source Rocks in the Bighorn Basin

Figure 1. Base map of Bighorn Basin, Wyoming and Montana, showing eight burial history locations and structure 

contours on top of the Cloverly Formation. 

110°W 109°W 108°W



 













0

-5,000

-10,000

0

5,000

-15,000

-5,000

5,000

5,000

-10,000

-10,000

-5,000

-5,000

-5,000

-5,000

-10,000

-5,000

0

-5,000

-5,000

0

0

5,000

0

0 0

0

0

5,000

5,000

5,000

0

0

0

-10,000

-5,000

0

Red Point/Husky

Amoco BN 1

McCulloch Peak

Emblem Bench

Santa Fe Tatman

Dobie Creek

Sellers Draw 

110° 109° 108° 107°

44°

45°

Montana

Wyoming

Bighorn

Mountains

Owl Creek Mountains

Beartooth

Mountains

Nye-Bowler 

Lineament

Absaroka

Mountains

Doctor Ditch

Bighorn Basin boundary

Fault

Cloverly (Kootenai)/Thermopolis outcrop 

 in Montana (Raines and Johnson, 1995)

Middle Jurassic through Lower Cretaceous 

 outcrop in Wyoming (Green and Drouillard, 1994)

Structure contours on top of the Cloverly

Formation contour interval, 1,000 ft

 Modeled well

EXPLANATION

0 10 20 30 40 MILES
Table 1. Information on wells used for burial history curves, Bighorn Basin, Wyoming and Montana.[Map name is shortened well name for use on figures, in tables, and for discussion in text. Asterisk indicates data were composited from two wells. N, north; S, south; E, easting in feet; GL, ground level in feet; DF, derrick floor in feet; TD, total depth in feet]

Map name Operator Lease Well Section Township Range Elevation TD 

Emblem Bench Forest Oil Corp. Emblem Bench 1 2 51N 98W 5,224 (KB) 18,63Red Point/Husky* Texas Pacific Oil Red Point II Unit 1 30 53 N 98W 5,270 (KB) 18,50Red Point/Husky* Husky Oil Unit 14–25 25 53 N 99W 5,304 (KB) 15,91Sellers Draw Amercan Quasar Petro Sellers Draw Unit 1 21 48N 98W 5,874 (KB) 23,08Santa Fe Tatman Santa Fe Energy Corp. Tatman Mtn Federal 1–20 20 49N 96W 4,662 (KB) 16,14McCulloch Peak* Mich Wisc Pipe Line McCulloch Peak II 2–9 9 54N 101W 5,142 (KB) 14,96McCulloch Peak* CIG Exploration McCulloch Peak 1 10 54N 101W 4,977 (KB) 15,30Amoco BN 1 Amoco Prod Co. Amoco-BN 1 11 9S 20E 6,140 (KB) 14,84Dobie Creek Exxcel Energy Dobie Creek Unit 1–11 11 49N 94W 4,457 (KB) 10,80Doctor Ditch* Gas Prod. Entp Inc. Burlington Northern 23–58–102 23 58N 102W 4,714 (GL) 13,60Doctor Ditch* Phillips Petroleum Co. NP A–1 33 9S 21E 4678 (DF) 13,83
4 Burial History, Thermal Maturity, and Oil and Gas Generation History of Source Rocks in the Bighorn Basin PALEOCENE UPPER CRETACEOUS TERTIARY (part) LOWER

?

?

System Series Western and southwestern

Bighorn Basin

Mesaverde Formation

Cody Shale

Teapot Ss Member

Meeteetse Formation

Lance

Formation

Fort Union

Formation

Tatman Formation

Claggett Mbr.

Lower member

Mesaverde

Formation

Mesaverde Formation

Teapot Ss Member

 Eastern and southeastern

Bighorn Basin

Mesaverde

Formation

Lance

Formation

Meeteetse Formation

Cody Shale Cody Shale

Fort Union

Formation

Bearpaw Shale?

Lewis Shale

Willwood Formation Willwood Formation

Northern Bighorn Basin

Willwood 

Formation

Lance

Formation

Meeteetse Formation

Lewis Shale

Fort Union

Formation

Mesaverde Formation

Mesaverde

Formation

Lower member

Claggett Mbr.

Teapot Ss Member

? ? ?

JURASSIC/ Jurassic and Triassic rocks undifferentiated

TRIASSIC

Cloverly Formation

Thermopolis Shale

EOCENE

 (part)

Mowry Shale Mowry Shale

Muddy Sandstone Muddy 

Sandstone

Frontier Formation Frontier Formation Frontier Formation

PERMIAN

PENNSYLVANIAN

(part)

Phosphoria/Park City/ Goose Egg Formation

Tensleep Sandstone

Santa Fe Tatman

 Amoco BN 1

Doctor Ditch

Red Point/ Emblem Bench Husky

Dobie Creek 

Sellers Draw

McCulloch 

Peak

West and 

Southwest

East and 

Southeast

North

Montana

Wyoming

Figure 2. Generalized stratigraphic chart of Pennsylvanian (part) through middle Tertiary (part) rocks in 

the Bighorn Basin, Wyoming and Montana. Time spans and thicknesses not shown in correct proportions. 

Hatching indicates time periods of erosion or nondeposition; wavy line represents unconformity. Inset map 

shows burial history locations and basin subdivisions.
Methods—Burial History 5

Methods—Burial History

One-dimensional (1-D) modeling of burial history and 

thermal maturity was performed for eight well locations (fig. 

1, table 1) using PetroMod1D® (version 9.0) of Integrated 

Exploration Systems (2005) GmbH (IES), Germany. The well 

locations were chosen because they (1) were drilled to depths 

that penetrated a significant part of the geologic section of 

interest, (2) represent different geologic settings within the 

basin, and (3) have measured vitrinite reflectance and downhole temperature data to aid in calibrating maturation models. 

Age 

Ages of stratigraphic units older than the Cretaceous 

Cloverly Formation (table 2) were estimated using Love and 

others (1993) as a guide for the ages of stratigraphic units and 

ages of regional unconformities throughout the Bighorn Basin. 

Table 2 shows the age, thickness, and generalized lithologic 

data used to construct the burial history curves. The ages at 

system and series boundaries were adjusted to the 1999 Geologic Time Scale (Palmer and Geissman, 1999).

Thickness and Lithology

Thicknesses of the stratigraphic units in the subsurface 

were interpreted from geophysical well logs or were determined from tops of units recorded in the Petroleum Information/Dwights PetroROM well-history database (IHS Energy 

Group, 2004). Lithologies of the stratigraphic units were interpreted from geophysical well logs and American Stratigraphic 

Company lithology logs and from published cross sections 

(Keefer and others, 1998). The lithologies were generalized 

for modeling purposes (table 2).

Stratigraphic Intervals

The models include stratigraphic units from the base of 

the Permian Phosphoria Formation through the middle Eocene 

Tatman Formation at the surface. The Triassic and Jurassic 

sections do not contain major petroleum source rocks so were 

considered as a single unit (fig. 2). The Upper Cretaceous 

Mesaverde Formation consists of a lower part overlain by the 

Teapot Sandstone Member; in the eastern, southeastern, and 

northern parts of the basin, the Claggett Member of the Cody 

Shale is included in the lower part of the formation. The Upper 

Cretaceous Lewis Shale is present in the eastern, southeastern, 

and northern parts of the basin, where it is included with the 

Meeteetse Formation. The Bearpaw Shale, which is only in the 

northern part of the basin, also is included with the Meeteetse 

Formation in the models.

Unconformities

Figure 2 shows the major unconformities that were used 

for the burial history reconstructions. The area of the Bighorn 

Basin during the Permian through the Early Cretaceous (Lawton, 1994) existed in a relatively stable cratonic setting; therefore, the unconformities at the Permian-Triassic and JurassicCretaceous boundaries (Love and others, 1993) and within the 

Lower Cretaceous (Merewether and others, 1997) probably 

do not represent long enough periods of sedimentation and 

erosion to significantly affect either the thermal maturity of the 

reservoired oil of the underlying Phosphoria/Park City Formation or the thermal maturity of the Thermopolis Shale. At least 

one hiatus occurred during deposition of the Upper Cretaceous 

Frontier Formation (Love and others, 1993), when the area 

of the Bighorn Basin was in a distal foreland basin setting 

(Lawton, 1994) that prevented a sufficient amount of sedimentation and subsequent erosion to have taken place that would 

affect the thermal maturity of the underlying source rocks; 

therefore, a hiatus was not included in the model (and not 

shown in figure 2). We included a hiatus for the unconformity 

at the base of the Teapot Sandstone Member of the Mesaverde 

Formation that occurred from 78 to 73 Ma (Merewether, 1996; 

and Merewether and others, 1997).

The erosional event within and at the base of the Paleocene Fort Union Formation depicted in figure 2 occurred 

toward the margins of the basin. We assumed continuous 

deposition across the Cretaceous-Tertiary boundary in the 

models, based on their locations, which are a considerable 

distance from the basin margin, and based on the premise that 

unconformities at the basin margins become less significant 

toward the interior, as is evidenced by the much greater thicknesses of the stratigraphic units involved. 

Post-Middle Eocene Deposition and Erosion

No post-middle Eocene rocks are preserved in areas 

where the burial histories were reconstructed, and in the 

northern part of the basin no post-Paleocene rocks are preserved. Because of this, we made assumptions in regard to the 

amounts of post-middle Eocene and post-Paleocene sedimentation and erosion, based on a method described in Roberts 

and others (2007), to generate a trend surface of the minimum 

elevation of basin fill across the Bighorn Basin. Hagen and 

Surdam (1984, and references therein) described locations 

of subsummit erosion surfaces and of post-middle Eocene 

outcrops that they used to estimate the amount of erosion from 

the northern part of the basin for their study. We incorporated 

the locations used by Hagen and Surdam (1984) with those in 

the Absaroka Mountains (Roberts and others, 2007) to construct a first-order polynomial trend surface (fig. 3A, B) using 

EarthVision (Dynamic Graphics, Inc., 2005) that extends from 

surfaces in the Absarokas eastward to a locality of Miocene 

mammal fossils at an elevation of about 9,000 ft on top of 

Darton’s Bluff (fig. 3A) (sec. 19, T. 48 N., R. 85 W.) in the 
6 Burial History, Thermal Maturity, and Oil and Gas Generation History of Source Rocks in the Bighorn Basin

Table 2. Data used to generate burial history curves for eight locations, Bighorn Basin, Wyoming and Montana.

[Models only include stratigraphic units from the base of the Phosphoria Formation regardless of whether the well was drilled to deeper units. Mbr., Member; Fm., Formation; Ss., Sandstone; ft, feet; Ma, mega-annum; %, percent; ss, sandstone; sh, shale; ls, limestone; slst, siltstone; dolo, dolomite. Heat flow 

and thermal gradient used to calibrate model are given for each location. mW/m2, milliWatts per square meter; °F, degrees Fahrenheit]

System/Series, Unit, 

or Event

Present 

thickness 

(ft)

Age 

range 

(Ma)

Deposited,

later 

eroded (ft)

Amount of 

erosion 

(ft)

Generalized lithology

%ss %sh %slst %dolo %coal

Emblem Bench

Erosion 10 – 0 5,000

Post–middle Eocene rocks 0 52 – 10 5,000 40 60

Willwood Fm. 3,555 55 – 52 40 60

Fort Union Fm. 7,565 65.4 – 55 50 50

Lance Fm. 1,735 69.4 – 65.4 70 30

Meeteetse Fm. 835 72 – 69.4 50 50

Mesaverde Fm. (Teapot Ss. Mbr.) 100 73 – 72 100

Hiatus 0 78 – 73

Mesaverde Fm. (includes Claggett 

Sh and lower Mesaverde Fm.) 1,040 82 – 78 60 40

Cody Shale 2,130 89 – 82 20 80

Frontier Fm. 540 97.2 – 89 50 50

Mowry Shale 550 99 – 97.2 100

Muddy Sandstone 15 101 – 99 100

Hiatus 0 102 – 101

Thermopolis Shale 168 104.4 – 102 100

Cloverly Fm. 297 134 – 104.4 30 70

Hiatus 144 – 134

Jurassic rocks (part) 215 175 – 144 60 40

Heat flow 51 mW/m2 Thermal gradient 1.35°F/100 feet

Red Point/Husky

Erosion 10 – 0 5,000

Post–middle Eocene rocks 0 51 – 10 5,000 50 50

Willwood Fm. 4,825 55 – 51 40 60

Fort Union Fm. 7,125 65.4 – 55 50 50

Lance Fm. 1,815 69.4 – 65.4 70 30

Meeteetse Fm. 695 72 – 69.4 50 50

Hiatus 0 78 – 73

Mesaverde Fm. 850 83 – 78 50 50

Cody Shale 2,250 88.5 – 83 100

Frontier Fm. 470 97.2 – 88.5 60 40

*Mowry Shale 540 99 – 97.2 100

*Muddy Sandstone 35 101 – 99 100

Hiatus 0 102 – 101

*Thermopolis Shale 185 104.4 – 102 100

*Cloverly Fm. 289 134 – 104.4 50 50

Heat flow 47 mW/m2 Thermal gradient 1.25°F/100 feet

*

Note: Thickness of Mowry, Muddy, Thermopolis, and Cloverly are projected from a nearby well.
Methods—Burial History 7

Table 2. Data used to generate burial history curves for eight locations, Bighorn Basin, Wyoming and Montana.—Continued

[Models only include stratigraphic units from the base of the Phosphoria Formation regardless of whether the well was drilled to deeper units. Mbr., Member; Fm., Formation; Ss., Sandstone; ft, feet; Ma, mega-annum; %, percent; ss, sandstone; sh, shale; ls, limestone; slst, siltstone; dolo, dolomite. Heat flow 

and thermal gradient used to calibrate model are given for each location. mW/m2, milliWatts per square meter; °F, degrees Fahrenheit]

System/Series, Unit, 

or Event

Present 

thickness 

(ft)

Age range 

(Ma)

Deposited,

later 

eroded (ft)

Amount of 

erosion 

(ft)

Generalized lithology

%ss %sh %slst %dolo %coal

Sellers Draw

Erosion 10 – 0 4,900

Post–middle Eocene rocks 0 51 – 10 4,900 50 50

Tatman and Willwood Fms. 5,080 55 – 51 50 50

Fort Union Fm. 7,430 65.4 – 55 50 50

Lance Fm. 1,250 69.4 – 65.4 80 20

Meeteetse Fm. 1,255 72 – 69.4 45 45 10

Mesaverde Fm. (Teapot Ss. Mbr.) 135 73 – 72 100

Hiatus 0 78 – 73

Mesaverde Fm. 1,040 83 – 78 50 50

Cody Shale 2,440 88.5 – 83 100

Frontier Fm. 420 97.2 – 88.5 50 50

Mowry Shale 515 99 – 97.2 100

Muddy Sandstone 45 101 – 99 100

Hiatus 0 102 – 101

Thermopolis Shale 145 104.4 – 102 70 30

Cloverly Fm. 285 134 – 104.4 50 50

Triassic and Jurassic rocks 1,594 248 – 144 50 50

Hiatus 0 253 – 248

Phosphoria/Park City Fm. 230 278 – 253 60 40

Heat flow 44 mW/m2 Thermal gradient 1.18°F/100 feet

Amoco BN 1

Erosion 10 – 0 4,500

Post–Paleocene rocks 0 55 – 10 4,500 30 70

Fort Union Fm. 9,030 65.4 – 55 70 30

Lance Fm. 1,290 69.4 – 65.4 30 70

Meeteetse Fm. 980 72 – 69.4 30 70

Mesaverde Fm. (Teapot Ss. Mbr.) 50 73 – 72 100

Hiatus 0 78 – 73

Mesaverde Fm. 1,155 83 – 78 40 60

Cody Shale 1,810 88.5 – 83 20 80

Frontier Fm. 395 97.2 – 88.5 40 60

Mowry Shale 600 99 – 97.2 100

Heat flow 51 mW/m2 Thermal gradient 1.35°F/100 feet
8 Burial History, Thermal Maturity, and Oil and Gas Generation History of Source Rocks in the Bighorn Basin

Table 2. Data used to generate burial history curves for eight locations, Bighorn Basin, Wyoming and Montana.—Continued

[Models only include stratigraphic units from the base of the Phosphoria Formation regardless of whether the well was drilled to deeper units. Mbr., Member; Fm., Formation; Ss., Sandstone; ft, feet; Ma, mega-annum; %, percent; ss, sandstone; sh, shale; ls, limestone; slst, siltstone; dolo, dolomite. Heat flow 

and thermal gradient used to calibrate model are given for each location. mW/m2, milliWatts per square meter; °F, degrees Fahrenheit]

System/Series, Unit, 

or Event

Present 

thickness 

(ft)

Age range 

(Ma)

Deposited,

later 

eroded (ft)

Amount of 

erosion 

(ft)

Generalized lithology

%ss %sh %slst %dolo %coal

Santa Fe Tatman

Erosion 10 – 0 6,000

Post–middle Eocene rocks 0 52 – 10 6,000 50 50

Fort Union Fm. 5,480 65.4 – 55 50 50

Lance Fm. 1,350 69.4 – 65.4 70 30

Meeteetse Fm. 785 72 – 69.4 30 70

Mesaverde Fm. (Teapot Ss. Mbr.) 115 73 – 72 100

Hiatus 0 78 – 73

Mesaverde Fm. (includes Claggett 

Sh.and lower Mesaverde Fm.) 1,100 82 – 78 70 30

Frontier Fm. 440 97.2 – 89 70 30

Mowry Shale 520 99 – 97.2 100

Muddy Sandstone 50 101 – 99 100

Hiatus 0 102 – 101

Thermopolis Shale 160 104.4 – 102 100

Cloverly Fm. 258 134 – 104.4 70 30

Hiatus 0 144 – 134

Jurassic rocks (part) 37 150 – 144 50 50

Heat flow 49 mW/m2 Thermal gradient 1.30°F/100 feet

McCulloch Peak

Erosion 10 – 0 5,000

Post–middle Eocene rocks 0 51 – 10 5,000 50 50

Willwood Fm. 2,485 55 – 51 30 70

Fort Union Fm. 6,105 65.4 – 55 50 50

Lance Fm. 1,320 69.4 – 65.4 50 50

Mesaverde Fm. (Teapot Ss. Mbr.) 210 73 – 72 70 30

Hiatus 0 78 – 73

Mesaverde Fm. 740 83 – 78 50 50

Cody Shale 2,115 88.5 – 83 20 80

Frontier Fm. 535 97.2 – 88.5 50 50

Mowry Shale 614 99 – 97.2 100

Muddy Sandstone 46 101 – 99 100

Hiatus 0 102 – 101

Cloverly Fm. 186 134 – 104.4 30 70

Heat flow 49 mW/m2 Thermal gradient 1.29°F/100 feet
Methods—Burial History 9

Table 2. Data used to generate burial history curves for eight locations, Bighorn Basin, Wyoming and Montana.—Continued

[Models only include stratigraphic units from the base of the Phosphoria Formation regardless of whether the well was drilled to deeper units. Mbr., Member; Fm., Formation; Ss., Sandstone; ft, feet; Ma, mega-annum; %, percent; ss, sandstone; sh, shale; ls, limestone; slst, siltstone; dolo, dolomite. Heat flow 

and thermal gradient used to calibrate model are given for each location. mW/m2, milliWatts per square meter; °F, degrees Fahrenheit]

System/Series, Unit, 

or Event

Present 

thickness 

(ft)

Age range 

(Ma)

Deposited,

later 

eroded (ft)

Amount of 

erosion 

(ft)

Generalized lithology

%ss %sh %slst %dolo %coal

Dobie Creek

Erosion 10 – 0 6,500

Post–middle Eocene rocks 0 52 – 10 6,500 50 50

Willwood Fm. 2,315 55 – 52 50 50

Fort Union Fm. 2,585 65.4 – 55 50 50

Lance Fm. 865 69.4 – 65.4 70 30

Meeteetse Fm. (includes Lewis 

Sh.)

555 72 – 69.4 30 70

Mesaverde Fm. (Teapot Ss. Mbr.) 90 73 – 72 100

Hiatus 0 78 – 73

Mesaverde Fm. 530 82 – 78 70 30

Cody Shale 2,785 89 – 82 100

Frontier Fm. 445 97.2 – 89 70 30

Mowry Shale 470 99 – 97.2 100

Muddy Sandstone 60 101 – 99 100

Thermopolis Shale 105 104.4 – 102 100

Heat flow 55 mW/m2 Thermal gradient 1.49°F/100 feet

Doctor Ditch

Erosion 10 – 0 4,500

Post–Paleocene rocks 0 55 – 10 4,500 30 70

Fort Union Fm. 5,410 65.4 – 55 50 50

Lance Fm. 910 69.4 – 65.4 30 70

Meeteetse Fm. (includes

Bearpaw Sh.)

1,080 72 – 69.4 50 50

Mesaverde Fm. (Teapot Ss. Mbr.) 30 73 – 72 100

Hiatus 0 78 – 73

Mesaverde Fm. 850 83 – 78 50 50

Frontier Fm. 580 97.2 – 88.5 30 70

Mowry Shale 470 99 – 97.2 100

Hiatus 0 102 – 99

Thermopolis Shale 320 104.4 – 102 30 70

Cloverly Fm. 243 134 – 104.4 70 30

Hiatus 0 144 – 134

Triassic and Jurassic rocks 1,347 248 – 144 34 33 33

Hiatus 0 253 – 248

Phosphoria/Park City Fm. 60 278 – 253 50 50

Heat flow 51 mW/m2 Thermal gradient 1.37°F/100 feet
10 Burial History, Thermal Maturity, and Oil and Gas Generation History of Source Rocks in the Bighorn Basin

West East

12,000

11,000

10,000

9,000

8,000

East Pryor Mountain

N of 

Burgess 

Junction

Line Creek 

 Plateau

Carter 

Mountain Darton’s Bluff

Darton’s Bluff

Square Top

12,000

10,000

8,000

6,000

4,000

12,000

10,000

8,000

6,000

4,000

Basin fill

Trend surface

Absaroka

Mountains

12,000

11,000

9,000

11,500

ELEVATION, IN FEET

Distance (feet × 1,000)

0

0 25 MILES

B

A

0-500

500-1,000

1,000-1,500

1,500-2,000

2,000-2,500

2,500-3,000

3,000-3,500

3,500-4,000

4,000-4,500

4,500-5,000

5,000-5,500

>5,500

Estimated thickness (ft) of 

basin-fill strata removed by 

post-middle Eocene erosion

0 25 MILES

Amoco BN 1 

McCulloch Peak

Red Point/Husky

Emblem Bench

Sellers Draw

Santa Fe Tatman

Dobie Creek

C

Doctor Ditch

100 200 300 400 500 600 700

Figure 3. Maps and cross 

section showing method 

used to estimate post-middle 

Eocene deposition and erosion 

in the Bighorn Basin, Wyoming 

and Montana. A, trend surface 

using elevations of erosion 

surfaces described in Hagen 

and Surdam (1984) and Roberts 

and others (2007) and locality 

of Miocene vertebrates at 

Darton’s Bluff. B, cross section 

showing estimated basin fill. 

C, isopach map of estimated 

post-middle Eocene erosion.
Methods—Petroleum Generation History 11

Big Horn Mountains (McKenna and Love, 1972). The general 

eastward slope of the surface is consistent with interpretations 

of drainage patterns during the Oligocene (Lillegraven and 

Ostresh, 1988). Subtracting the grid of the modern topography 

(generated from a Digital Elevation Model) from this trend 

surface grid resulted in an isopach map of estimated eroded 

thickness of post-Paleocene to post-middle Eocene rocks 

(fig. 3C), which was then used as a guide for estimating thickness of erosion across the basin. The actual values applied 

in the modeling were determined by matching a calculated 

Ro profile at each location with the measured data by using a 

combination of input parameters as described below.

Modeling results indicate that the amount of post-early 

Eocene deposition (from about 50 to 10 Ma) that was subsequently eroded (from 10 Ma to the present) at the eight 

burial history locations is estimated to range from 4,500 ft 

in the northern part of the basin to 6,500 ft in the central part 

(fig. 3C). These erosion estimates are consistent with those 

reported by McMillan and others (2006), who used a similar 

method to estimate basin fill, but whose area of study included 

the entire Rocky Mountain orogenic plateau. In a summary of 

Cenozoic erosional events in the Bighorn Basin, Bown (1980) 

reported that between the early Miocene and the present day, 

more than 7,800 ft of rock was removed by erosion from the 

Absaroka area and more than 5,000 feet from the eastern Bighorn Basin. Nuccio and Finn (1998) assumed a uniform gradient between an area in the Absaroka Range and Darton’s Bluff 

and estimated the thickness of Tertiary rocks stripped from the 

central part of the Bighorn Basin to be about 4,900 ft. 

Methods—Thermal History

For each of the eight burial history locations, bottomhole temperatures, temperatures from drill-stem tests, vitrinite reflectance (Ro

) data, and assumed average paleosurface 

temperatures were used to calibrate thermal models. The 

temperature data were from records on log headers of the individual wells used in modeling. The bottom-hole temperatures 

were corrected according to Waples and others (2004). Ro data 

are from Nuccio and Finn (1998) and Finn and Pawlewicz 

(2007). The paleosurface temperature data from the beginning of the Late Cretaceous (100 Ma) to the present for all of 

the burial history locations were taken from those reported by 

Barker (2000) for the neighboring Wind River Basin to the 

south, except that, on the basis of annual surface temperature 

near Worland, Wyoming, in the Bighorn Basin, a present-day 

temperature of 7 ̊C (44.5 ̊F) was used. For the surface temperature history before 100 Ma, we used a constant paleosurface 

temperature of 20 ̊C (68 ̊F) from the beginning of deposition 

to 144 Ma and gradually decreased the temperature to 17 ̊C 

(63 ̊F) at 100 Ma based loosely on a module that is included 

with the PetroMod1D® software that calculates paleosurface 

temperature through time at chosen latitudes (Wrygala, 1989). 

Heat flow value, which is an input parameter for the 

PetroMod1D® program, is used for the burial history reconstructions (table 2). A present-day heat flow at the base of the 

stratigraphic column at each modeled location was determined 

within the PetroMod1D® program by calibrating thermal 

histories using the measured downhole temperatures and the 

present-day and assumed paleosurface temperatures. Heat 

flow values used at the modeled well locations range from 

44 to 55 milliWatts per square meter (mW/m2), which is in 

agreement with the range of values documented by Decker 

and others (1980) for eastern Wyoming (25 to 67 mW/m2). 

Assuming our estimated burial depths and amounts of erosion 

to be geologically correct, using a constant heat flow through 

time resulted in an acceptable match between calculated and 

measured Ro values, as determined by EASY%Ro (Sweeney 

and Burnham, 1990).

Methods—Petroleum Generation 

History

Timing of petroleum generation was determined for the 

seven source rocks at the eight burial history locations, and 

timing for the gas generation from the cracking of Phosphoria oil in Park City reservoirs was determined for two of the 

locations. The source rocks are either gas-prone or predominantly gas-prone, with the possibility of also being oil-prone 

as a result of variations in organic facies (for example, in the 

Thermopolis and Mowry Shales). 

Gas-Prone Source Rocks

The extent of gas generation in source rocks with TypeIII kerogen can be equated to Ro

 levels because it is measured 

on the macerals that generate the gas. Based on discussions 

in Roberts and others (2004, 2007), we used the values of 

0.5 percent Ro for the start of gas generation, 0.8 percent Ro for 

peak generation, and 2.0 percent Ro for the end of gas generation from Type-III kerogen. Accordingly, EASY%Ro

 was used 

to model gas generation from gas-prone source rocks at these 

values. Gas generation discussed in this report refers only to 

thermogenic gas and not to biogenic gas.

Oil-Prone Source Rocks

Thermopolis and Mowry Shales in the Bighorn Basin 

consist of some Type-II kerogen (Burtner and Warner, 1984; 

Hagen and Surdam, 1984; unpublished RockEval data). 

Although they are predominantly gas-prone, they are potential 

oil-prone source rocks. Low-sulfur oils in Cretaceous reservoirs have been attributed to these two source rocks (Paul 

Lillis, oral commun., 2007). Oil generation from Type-II 

kerogen in these source rocks was modeled at the relevant 

burial history locations with hydrous pyrolysis kinetic 
12 Burial History, Thermal Maturity, and Oil and Gas Generation History of Source Rocks in the Bighorn Basin

parameters from Lewan and Ruble (2002) as discussed by 

Roberts and others (2004). The activation energy (Ea) is 52.16 

kilocalories per mole with a frequency factor (Ao) of 5.707 × 

1026 million years. Source rock oil-generation kinetic parameters from hydrous pyrolysis experiments, combined with 

burial history and thermal maturity, determine the timing of 

the generation of expelled oil because expulsion is considered 

a consequence of generation (Momper, 1978; Lewan, 1997; 

Lewan and Ruble, 2002).

Although gas is generated during oil generation from 

oil-prone kerogen, this type of gas generation was not modeled; however, cracking of generated oil to gas was modeled. 

Hydrothermal-pyrolysis kinetic parameters used to determine 

the timing of gas generation from oil cracking are from Tsuzuki and others (1999; C15+ saturates). The activation energy 

(Ea) is 76 kilocalories per mole with a frequency factor (Ao) of 

3.419 × 1033 million years.

Phosphoria oil in the Bighorn Basin is thought to have 

migrated eastward from the Idaho-Wyoming thrust belt and 

accumulated in upper Paleozoic and lower Mesozoic reservoirs (Sheldon, 1967; Stone, 1967). Within the boundary of 

the Bighorn Basin, the Phosphoria Formation is thin and only 

locally rich in organic carbon (Claypool and others, 1978; 

Maughan, 1984; and Hiatt, 1997). For this reason we do 

not present modeling results of the timing of oil generation 

from Phosphoria source rocks but present only the timing of 

cracking of Phosphoria-sourced oil pooled in reservoirs of the 

Permian Park City Formation. 

Each set of kinetic parameters previously described 

has its own transformation ratios that express the extent of 

oil generation from Type-II kerogen and oil cracking to gas. 

The transformation ratios are the decimal fraction of reaction 

completed (that is, 0.00 = no reaction; 1.00 = completed reaction) as determined by their kinetic parameters for the various 

source rocks at the burial history locations. Oil generation 

and oil cracking to gas are defined in this study by transformation ratios between 0.01 and 0.99. Immature source rocks 

and uncracked oils have transformation ratios less than 0.01, 

and source rocks and crude oils that have completed both oil 

generation and the cracking of oil to gas have transformation ratios greater than 0.99. Peak oil generation and peak oil 

cracking to gas are defined at a transformation ratio of 0.50 

when the maximum rate of reaction is reached. Because the 

kinetics for oil generation and for oil cracking are different, 

a transformation ratio of 0.50 for oil generation occurs at a 

lower thermal maturity than a transformation ratio of 0.50 for 

oil cracking.

Results—Burial History

Table 3 lists information on the present depth, calculated 

maximum depth of burial, and calculated temperatures at 

maximum depth for the source-rock intervals and for the 

Phosphoria/Park City Formation from the burial history 

reconstructions. Based on Nuccio and Finn (1998) and 

McMillan and others (2006), the time of maximum burial for 

all stratigraphic units at the modeled locations was placed at 

10 Ma (late-middle Miocene). Figure 4 presents the burial history curves from 120 Ma (late Early Cretaceous) to the present; the burial history prior to 120 Ma is not shown because 

it is comparatively consistent from location to location across 

the basin, indicating fairly constant and relatively slow subsidence/sedimentation rates within a stable tectonic setting. The 

onset of the Laramide orogeny in the Late Cretaceous triggered the changes in sedimentation rates and created depocenters at different areas of the Bighorn Basin at different times. 

Following is a brief description of the burial history at each 

location shown in figure 1: three in the deepest parts of the 

basin (Emblem Bench, Red Point/Husky, and Sellers Draw); 

three at intermediate depths (Amoco BN 1, Santa Fe Tatman, 

and McCulloch Peak); and two at relatively shallow locations 

(Dobie Creek and Doctor Ditch).

Emblem Bench, Red Point/Husky, 

and Sellers Draw

The Emblem Bench, Red Point/Husky, and Sellers Draw 

locations are all on the gently dipping flanks of the relatively 

deep part of the Bighorn Basin trough and represent the 

greatest depth to which the source rocks were buried (fig. 1, 

fig. 4A, table 3). From 278 Ma to about 89 Ma, subsidence/

sedimentation rates appear to have been fairly constant and 

relatively slow. The rate of deposition increased substantially 

between 89 and about 52 Ma, resulting in a total of between 

about 17,000 and 18,600 ft (table 2) of section represented by 

this period of time. A brief episode of nondeposition occurred 

between 78 and 73 Ma, and, in the last 10 million years, an 

estimated 5,000 ft of rock was removed by erosion from this 

part of the basin.

Amoco BN 1

The burial history reconstruction at Amoco BN 1, located 

in a small subbasin in the northern part of the Bighorn Basin 

(fig. 1), represents an intermediate depth to which source rocks 

were buried (table 3, fig. 4B). Between 88.5 Ma and about 

83 Ma, only about 1,800 ft of Cody Shale was deposited there, 

whereas to the south and east the Cody thickens to as much 

as 2,400 ft (as at Sellers Draw). A big difference in the burial 

history at the Amoco BN 1 locality, as compared with other 

locations, is that during Paleocene Fort Union time, almost 

2,000 ft more sediment was deposited (9,000 ft) than in the 

deep part of the basin (7,000 ft) and almost 6,500 ft more than 

at the Dobie Creek location (2,500 ft) in the east-central part 

of the basin. After maximum burial from the addition of postPaleocene sediments, uplift and erosion caused the removal of 

an estimated 4,500 ft of section in the modeled area.
Results—Burial History 13

Table 3. Present-day depth, calculated maximum depth of burial, and calculated temperature at maximum depth of burial for base 

of source rock intervals and for Phosphoria/Park City Formation from burial history reconstructions, Bighorn Basin, Wyoming and 

Montana.

[ND = no data. Depth values are in feet. Temperature values are degrees Fahrenheit. Fm., Formation; Sh, Shale] 

Burial history 

location

Source rock

Phosphoria 

Fm.

Thermopolis 

Sh.

Mowry 

Shale

Frontier 

Fm.

Cody 

Shale

Mesaverde 

Fm.

Meeteetse 

Fm.

Fort Union 

Fm.

Emblem Bench

Present-day depth ND 18,233 18,050 17,500 16,960 14,830 13,690 11,120

Maximum depth ND 23,250 23,030 22,530 21,940 19,810 18,690 16,110

Temperature ND 381 377 369 361 328 313 277

Red Point Unit/Husky

Present-day depth ND 18,930 18,710 18,170 17,700 15,450 14,500 11,950

Maximum depth ND 23,730 23,530 23,010 22,520 20,270 19,310 16,780

Temperature ND 358 355 348 341 308 297 264

Sellers Draw

Present-day depth 21,864 19,755 19,565 19,050 18,630 16,190 15,015 12,510

Maximum depth 26,770 24,620 24,440 23,930 23,510 21,060 19,900 17,400

Temperature 372 346 343 336 331 297 284 254

Amoco BN 1

Present-day depth ND ND 15,310 14,710 14,315 12,505 11,300 9,030

Maximum depth ND ND 19,780 19,180 18,750 16,980 15,730 13,490

Temperature ND ND 337 327 321 294 276 243

Santa Fe Tatman

Present-day depth ND 15,850 15,640 15,120 14,680 12,300 11,085 8,950

Maximum depth ND 21,580 21,640 21,130 20,710 18,280 17,080 14,950

Temperature ND 345 342 334 328 291 275 247

McCulloch Peak

Present-day depth ND 15,114 14,904 14,290 13,755 11,640 10,690 8,590

Maximum depth ND 19,610 19,400 18,840 18,230 16,210 15,220 13,150

Temperature ND 316 313 304 296 267 254 228

Dobie Creek

Present-day depth ND 10,805 10,640 10,170 9,725 6,940 6,320 4,900

Maximum depth ND 17,300 17,120 16,660 16,220 13,420 12,780 11,390

Temperature ND 311 313 306 299 251 241 220

Doctor Ditch

Present-day depth 13,020 11,370 11,050 10,580 10,000 8,280 7,400 5,410

Maximum depth 17,530 15,860 15,530 15,070 14,490 12,760 11,890 9,880

Temperature 305 281 276 267 260 234 222 194
14 Burial History, Thermal Maturity, and Oil and Gas Generation History of Source Rocks in the Bighorn Basin

Figure 4. Burial history curves at eight locations in the Bighorn Basin, Wyoming and Montana. A, locations where source rocks were 

most deeply buried. B, locations where source rocks were buried to intermediate depths, C, locations where source rocks were buried 

to shallowest depths. Burial history model locations are shown in figure 1. Data used to construct the curves are presented in table 2. 

Ages are given in millions of years before present (Ma, mega-annum). Abbreviations: Fm., Formation; Mbr., Member; Ss., Sandstone; Sh., 

Shale. Calibration of models, using vitrinite reflectance (Ro) and down-hole temperature, is shown with each associated burial history 

curve. Red dots on the left graph are measured Ro values. Green dots on the right graph are down-hole temperatures (corrected, in 

degrees Fahrenheit); brown dots are temperatures from drill-stem tests. Colors on the burial history curves represent calculated Ro with 

burial depth and time (see color key).

0

Red Point/Husky

5,000

10,000

0

Lance Fm.

Willwood Fm.

Mesaverde Fm.

Teapot Ss. Mbr

Muddy Ss.

Thermopolis Sh.

BURIAL DEPTH (FEET)

Fort Union Fm.

0.50–0.60

0.60–0.80

0.80–1.10

1.10–1.35

1.35–2.00

%Ro range (calculated)

Emblem Bench

Age (Ma)

5,000

10,000

15,000

20,000

25,000

30,000

0

Mesaverde Fm.

Teapot Ss. Mbr

Jurassic rocks (part) Cloverly Fm.

Muddy Ss./

Thermopolis Sh

BURIAL DEPTH (FEET)

Jurassic (part)

Muddy Ss. and Thermopolis Sh.

 Clovery Fm.

A

120 100 80 60 40 20

EASY%Ro (Sweeney and Burnham, 1990)

45 200 300

CENOZOIC

CRETACEOUS PALEOGENE NEOGENE& QUATERNARY

MESOZOIC

Lance Fm.

Willwood Fm.

Frontier Fm. Mowry Sh.

Meeteetse Fm.

Mesaverde Fm.

Fort Union Fm.

Cody Sh.

Mesaverde Fm.

Frontier Fm. Mowry Sh.

Teapot Ss. 

member

Meeteetse Fm.

Lance Fm.

Fort Union Fm.

Willwood Fm.

Cody Sh.

0.10 1.00 10.00 100

5,000

10,000

15,000

CENOZOIC

CRETACEOUS PALEOGENE NEOGENE & QUATERNARY

MESOZOIC

EASY%Ro (Sweeney and Burnham, 1990) Temperature (oF)

Fort Union Fm.

Willwood Fm.

0.10 1.00 10.00 100 200 300 0

5,000

45

Thermopolis Sh.

Cloverly Fm. Frontier Fm. Mowry Sh.

 Teapot Ss. member

Lance Fm.

Cody Sh.

10,000

15,000

Frontier Fm.

Mowry Sh.

Meeteetse Fm.

Mesaverde Fm.

Cody Sh.

20,000

25,000

30,000

15,000

0

Age (Ma)

120 100 80 60 40 20 0

Temperature (oF)

EXPLANATION
Results—Burial History 15

Sellers Draw

CENOZOIC

CRETACEOUS PALEOGENE NEOGENE & QUATERNARY

MESOZOIC

Age (Ma)

100 80 60 40 20 0

5,000

10,000

15,000

20,000

25,000

30,000

0

Lance Fm.

Tatman and 

Willwood Fms.

Frontier Fm.

Mowry Sh.

Meeteetse Fm.

Mesaverde Fm. (part)

Mesaverde Fm.

Teapot Ss. Mbr

Cloverly Fm.

Phosphoria Fm.

Jurassic and 

Triassic rocks

Muddy Ss./

Thermopolis Sh

BURIAL DEPTH (FEET)

Fort Union Fm.

Cody Sh.

EASY%Ro (Sweeney and Burnham, 1990) Temperature (oF)

Mesaverde Fm. Teapot Ss. member

Meeteetse Fm.

Lance Fm.

Fort Union Fm.

0.10 1.00 10.00 100 200 300 0

5,000

10,000

15,000

20,000

45

Muddy Ss. and Thermopolis Sh. Frontier Fm. Mowry Sh.

 Clovery Fm. Phosphoria/Park City Fm. Jurassic and 

Triassic rocks

Cody Sh.

Willwood and 

Tatman Fms.

120

0.50–0.60

0.60–0.80

0.80–1.10

1.10–1.35

1.35–2.00

%Ro range (calculated)

EXPLANATION

A

Figure 4. Burial history curves at eight locations in the Bighorn Basin, Wyoming and Montana. A, locations where source rocks were 

most deeply buried. B, locations where source rocks were buried to intermediate depths, C, locations where source rocks were buried 

to shallowest depths. Burial history model locations are shown in figure 1. Data used to construct the curves are presented in table 2. 

Ages are given in millions of years before present (Ma, mega-annum). Abbreviations: Fm., Formation; Mbr., Member; Ss., Sandstone; Sh., 

Shale. Calibration of models, using vitrinite reflectance (Ro) and down-hole temperature, is shown with each associated burial history 

curve. Red dots on the left graph are measured Ro values. Green dots on the right graph are down-hole temperatures (corrected, in 

degrees Fahrenheit); brown dots are temperatures from drill-stem tests. Colors on the burial history curves represent calculated Ro with 

burial depth and time (see color key).—Continued
16 Burial History, Thermal Maturity, and Oil and Gas Generation History of Source Rocks in the Bighorn Basin

0.50–0.60

0.60–0.80

0.80–1.10

1.10–1.35

1.35–2.00

%Ro range (calculated)

5,000

10,000

15,000

20,000

25,000

30,000

0

BURIAL DEPTH (FEET)

5,000

10,000

15,000

20,000

25,000

30,000

BURIAL DEPTH (FEET)

0

Santa Fe Tatman

Age (Ma)

120 100 80 60 40 20 0

Frontier Fm.

Mesaverde Fm.

Teapot Ss. Mbr

Cloverly Fm.

Jurassic rocks (part)

Muddy Ss./

Thermopolis Sh

EASY%Ro (Sweeney and Burnham, 1990) Temperature (oF)

0.10 1.00 10.00 100 200 300 0 45

5,000

10,000

15,000

Lance Fm.

Willwood Fm.

Mowry Sh.

Meeteetse Fm.

Mesaverde Fm.

Fort Union Fm.

Cody Sh.

CENOZOIC CRETACEOUS PALEOGENE NEOGENE & QUATERNARY

MESOZOIC

Jurassic 

 Clovery Fm. (part)

Thermopolis Sh.

Muddy Ss.

Mesaverde Fm.

Frontier Fm. Mowry Sh.

Teapot Ss. 

member

Meeteetse Fm.

Lance Fm.

Fort Union Fm.

Willwood Fm.

Cody Sh.

CENOZOIC PALEOGENE

NEOGENE

CRETACEOUS & QUATERNARY

MESOZOIC

Age (Ma)

120 100 80 60 40 20 0

Lance Fm.

Frontier Fm.

Mowry Sh.

Meeteetse Fm.

Mesaverde Fm.

Mesaverde Fm.

Teapot Ss. Mbr

Fort Union Fm.

Cody Sh.

EASY%Ro (Sweeney and Burnham, 1990) Temperature (oF)

Mesaverde 

Fm.

Mowry Sh.

Frontier Fm.

 Teapot Ss.

 member

 Value in Mowry from Hagen and Surdam (1984)

Meeteetse Fm.

Lance Fm.

Fort Union Fm.

Cody Sh.

0.10 1.00 10.00 45 100 200 300 0

5,000

10,000

15,000

B Amoco BN 1

EXPLANATION

Figure 4. Burial history curves at eight locations in the Bighorn Basin, Wyoming and Montana. A, locations where source rocks were 

most deeply buried. B, locations where source rocks were buried to intermediate depths, C, locations where source rocks were buried 

to shallowest depths. Burial history model locations are shown in figure 1. Data used to construct the curves are presented in table 2. 

Ages are given in millions of years before present (Ma, mega-annum). Abbreviations: Fm., Formation; Mbr., Member; Ss., Sandstone; Sh., 

Shale. Calibration of models, using vitrinite reflectance (Ro) and down-hole temperature, is shown with each associated burial history 

curve. Red dots on the left graph are measured Ro values. Green dots on the right graph are down-hole temperatures (corrected, in 

degrees Fahrenheit); brown dots are temperatures from drill-stem tests. Colors on the burial history curves represent calculated Ro with 

burial depth and time (see color key).—Continued
Results—Burial History 17

Age (Ma)

McCulloch Peak

5,000

10,000

15,000

20,000

25,000

30,000

0

BURIAL DEPTH (FEET)

120 100 80 60 40 20 0

Frontier Fm.

Mesaverde Fm.

Teapot Ss. Mbr

Cloverly Fm.

Muddy Ss./

Thermopolis Sh.

EASY%Ro (Sweeney and Burnham, 1990) Temperature (oF)

CENOZOIC

CRETACEOUS PALEOGENE NEOGENE & QUATERNARY

MESOZOIC

Lance Fm.

Willwood Fm.

Mowry Sh.

Meeteetse Fm.

Mesaverde Fm.

Fort Union Fm.

Cody Sh.

0.10 1.00 10.00 45 100 200 300 0

5,000

10,000

15,000 Muddy Ss.

Mesaverde Fm.

Frontier Fm.

Mowry Sh.

 Clovery Fm.

 Teapot Ss. member Meeteetse Fm. Lance Fm.

Fort Union Fm.

Willwood Fm.

Cody Sh. Thermopolis Sh.

0.50–0.60

0.60–0.80

0.80–1.10

1.10–1.35

1.35–2.00

%Ro range (calculated)

EXPLANATION

B

Figure 4. Burial history curves at eight locations in the Bighorn Basin, Wyoming and Montana. A, locations where source rocks were 

most deeply buried. B, locations where source rocks were buried to intermediate depths, C, locations where source rocks were buried 

to shallowest depths. Burial history model locations are shown in figure 1. Data used to construct the curves are presented in table 2. 

Ages are given in millions of years before present (Ma, mega-annum). Abbreviations: Fm., Formation; Mbr., Member; Ss., Sandstone; Sh., 

Shale. Calibration of models, using vitrinite reflectance (Ro) and down-hole temperature, is shown with each associated burial history 

curve. Red dots on the left graph are measured Ro values. Green dots on the right graph are down-hole temperatures (corrected, in 

degrees Fahrenheit); brown dots are temperatures from drill-stem tests. Colors on the burial history curves represent calculated Ro with 

burial depth and time (see color key).—Continued
18 Burial History, Thermal Maturity, and Oil and Gas Generation History of Source Rocks in the Bighorn Basin

Age (Ma)

5,000

10,000

15,000

20,000

25,000

30,000

BURIAL DEPTH (FEET)

Doctor Ditch

Age (Ma)

5,000

10,000

15,000

20,000

25,000

30,000

BURIAL DEPTH (FEET)

C Dobie Creek

Mesaverde Fm.

Teapot Ss. Mbr

Thermopolis Sh.

Cloverly Fm.

Phosphoria Fm.

Frontier Fm.

Mowry Sh.

Mesaverde Fm.

Teapot Ss. Mbr

Muddy Ss.

Thermopolis Sh.

0

120 100 80 60 40 20 0

120 100 80 60 40 20 0

CENOZOIC CRETACEOUS PALEOGENE NEOGENE & QUATERNARY

MESOZOIC

Lance Fm.

Frontier Fm.

Triassic and 

Jurassic rocks

Mowry Sh.

Meeteetse Fm.

Mesaverde Fm.

Fort Union Fm.

Cody Sh.

CENOZOIC

PALEOGENE NEOGENE & QUATERNARY

CRETACEOUS

MESOZOIC

Lance Fm.

Willwood Fm.

Fort Union Fm.

Meeteetse Fm.

Mesaverde Fm. Cody Sh. Temperature (oF)

0.10 1.00 10.00 45 100 200 300

EASY%Ro (Sweeney and Burnham, 1990)

Muddy Ss.

Mesaverde Fm.

Frontier Fm.

Mowry Sh.

 Teapot Ss. member

Meeteetse Fm. Lance Fm.

Fort Union Fm.

Willwood Fm.

Cody Sh.

0

5,000

10,000 Thermopolis Sh.

EASY%Ro (Sweeney and Burnham, 1990) Temperature (oF)

0.10 1.00 10.00 45 100 200 300

Triassic and Jurassic rocks

Thermopolis Sh.

Phosphoria/Park City Fm.

Mesaverde Fm.

Frontier Fm.

Mowry Sh. Clovery Fm.

 Teapot Ss. member

Meeteetse Fm.

Lance Fm.

Cody Sh.

0

5,000

10,000

0.50–0.60

0.60–0.80

0.80–1.10

1.10–1.35

1.35–2.00

%Ro range (calculated)

0

Fort Union Fm.

EXPLANATION

Figure 4. Burial history curves at eight locations in the Bighorn Basin, Wyoming and Montana. A, locations where source rocks were 

most deeply buried. B, locations where source rocks were buried to intermediate depths, C, locations where source rocks were buried 

to shallowest depths. Burial history model locations are shown in figure 1. Data used to construct the curves are presented in table 2. 

Ages are given in millions of years before present (Ma, mega-annum). Abbreviations: Fm., Formation; Mbr., Member; Ss., Sandstone; Sh., 

Shale. Calibration of models, using vitrinite reflectance (Ro) and down-hole temperature, is shown with each associated burial history 

curve. Red dots on the left graph are measured Ro values. Green dots on the right graph are down-hole temperatures (corrected, in 

degrees Fahrenheit); brown dots are temperatures from drill-stem tests. Colors on the burial history curves represent calculated Ro with 

burial depth and time (see color key).—Continued
Results—Petroleum Generation History 19

Santa Fe Tatman

The Santa Fe Tatman location is situated on the gently 

west-dipping flank of the Bighorn Basin trough (fig. 1). The 

burial history here is most like that at the Emblem Bench location (fig. 4A); however, sedimentation/subsidence rates began 

to decrease in the Santa Fe Tatman area at about the end of 

deposition of the Meeteetse Formation, resulting in an interval 

of Lance and Fort Union rocks that is about 2,500 ft thinner 

than at the Emblem Bench location. An estimated 6,000 ft of 

post-middle Eocene sediments was deposited and then eroded 

at this location. Cretaceous source rocks at Santa Fe Tatman 

were buried to depths that were 2,000 to 3,000 ft shallower 

than those at locations in the deep part of the basin but were 

about 4,000 ft deeper than at the Doctor Ditch location in the 

northern part of the basin (table 3). 

McCulloch Peak

The McCulloch Peak location is on a subtle anticlinal 

structure in the west-central part of the Bighorn Basin trough 

(fig. 1). The burial history here (fig. 4B) also closely resembles that at Emblem Bench, except that the rate of sediment 

accumulation was not as rapid during deposition of the Lance 

through the Fort Union Formations, resulting in a lesser thickness of about 2,000 ft for that interval. We estimate that the 

thickness of post-middle Eocene deposition and subsequent 

erosion was about 5,000 ft at the McCulloch Peak location.

Dobie Creek

The burial history at Dobie Creek (fig. 4C), located 

on a northwest-trending anticline in the southeastern part of 

the Bighorn Basin (fig. 1), is virtually the same as that to the 

west at the Santa Fe Tatman location until about the end of 

Mesaverde deposition. At that time the sedimentation/subsidence rates began to decrease, and about one-half the thickness 

of Meeteetse through Willwood sediments were deposited 

as compared to the Santa Fe Tatman location. An additional 

6,500 ft of post-middle Eocene sediments is estimated to have 

been deposited at Dobie Creek before the final erosional event. 

Doctor Ditch

The Doctor Ditch location lies on a small anticlinal 

structure less than 10 mi updip to the southeast of the Amoco 

BN 1 location (fig. 1). This location, along with the Dobie 

Creek location, represents one of the two areas where source 

rocks were buried to the least depths (fig. 4C, table 3). The 

burial history here closely resembles that of the Amoco BN 1 

location, except that almost 4,000 ft more Paleocene sediments 

were deposited at Amoco BN 1. This difference in thickness in 

such a short distance can probably be attributed to the growth 

of the anticline (where Doctor Ditch is located) during the 

Paleocene and the consequential thinning of the Fort Union 

Formation across the structure. As in the area of the Amoco 

BN 1 location, we estimate that about 4,500 ft of post-Paleocene rocks was eroded from this area.

Results—Maturation History

Maturation history is calibrated based on matching as 

closely as possible, the measured and calculated Ro for each 

burial history location. Figure 4 shows selected ranges of Ro 

superimposed on the burial history curves. The range of 0.5 to 

0.8 %Ro represents the prescribed start-to-peak gas generation 

from humic coals. The Ro values of 1.10 and 1.35 percent were 

added for information purposes only and represent intermediate maturities between peak (0.8 %Ro

) and end (2.0 %Ro) of 

gas generation. All of the Cretaceous and Paleocene source 

rocks are considered gas-prone (mostly composed of TypeIII kerogen) or possibly oil-prone (Thermopolis and Mowry 

Shales), so Ro can be used to estimate extent of gas generation. 

Table 4 summarizes the time (Ma) that the base of the source 

rock intervals attained the %Ro values of 0.5, 0.8, and 2.0, and 

the depths of the source rock at the specific Ro maturity level.

Results—Petroleum 

Generation History

The timing and extent of petroleum generation from the 

prescribed source rocks at the eight burial history locations 

are listed in tables 4 and 5 and summarized in figures 5 and 6. 

With the exception of the Dobie Creek and Doctor Ditch locations, the burial history curves represent relatively deep parts 

of the Bighorn Basin (fig. 1). As a result, the other six curves 

represent the earliest timing and greatest extent of petroleum 

generation in the identified source-rock intervals. Updip 

from these locations, the time at which petroleum generation 

occurred would be later and the extent of petroleum generation 

would be less. 

A vertical dashed line at 10 Ma on each of figures 5 and 6 

represents the onset of regional uplift, erosion, and subsequent 

cooling at all of the burial history locations. Based on kinetic 

and burial history modeling, rates of gas generation, oil generation, and oil cracking to gas were significantly reduced after 

that time, with negligible generation during the last 10 m.y.

As expected, the most extensive petroleum generation 

occurred at the locations where source rocks were most 

deeply buried, which include the Emblem Bench, Red Point/

Husky, and Sellers Draw locations (figs. 5, 6). The Amoco 

BN 1, Santa Fe Tatman, and McCulloch Peak locations, which 

represent intermediate burial depths, had moderately extensive 

petroleum generation, and the shallowest burial depths at the 

Dobie Creek and Doctor Ditch locations resulted in the least 

extensive petroleum generation. Burial depths at the Sellers 
Table 4. Timing of gas generation for Type-III source rocks at burial history locations, Bighorn Basin, Wyoming and Montana. [Age values are in Ma. Depth values are calculated depth of burial at which vitrinite reflectance (%Ro) value was reached (in feet). Double hyphen indicates horizon did not at

Burial history location Source rock Thermopolis Shale Mowry Shale Frontier Formation Cody Shale Mesaverde Formation Meeteetse Formation 

Age Depth Age Depth Age Depth Age Depth Age Depth Age Dept

Emblem Bench 0.5%Ro - Start 63 10,240 63 10,270 62 10,340 61 10,510 58 10,740 56 10,71

0.8%Ro - Peak 55 16,120 55 16,150 54 16,360 54 16,360 51 16,000 49 14,942.0%Ro - End 26 21,770 22 21,880 12 22,330 -- -- -- 

Red Point Unit/Husky 0.5%Ro - Start 62 10,820 62 10,860 60 11,110 60 11,190 56 11,570 54 11,57

0.8%Ro - Peak 53 17,500 53 17,560 53 17,580 52 17,580 49 16,690 48 15,872.0%Ro - End -- -- -- -- -- -- 

Sellers Draw 0.5%Ro - Start 62 11,570 61 11,760 60 11,810 59 12,040 55 12,410 54 12,68

0.8%Ro - Peak 53 19,120 52 19,170 52 19,260 51 19,310 48 17,590 44 16,712.0%Ro - End -- -- -- -- -- -- 

Amoco BN 1 0.5%Ro - Start no data 63 10,110 62 10,300 61 10,240 58 10,690 57 10,62

0.8%Ro - Peak no data 55 16,090 54 15,770 54 15,390 47 14,030 39 13,522.0%Ro - End no data -- -- -- -- -- 

Santa Fe Tatman 0.5%Ro - Start 62 10,030 61 10,190 60 10,220 59 10,280 54 10,930 53 10,96

0.8%Ro - Peak 53 16,490 52 16,520 52 16,370 51 15,990 42 14,570 32 14,572.0%Ro - End -- -- -- -- -- -- 

McCulloch Peak 0.5%Ro - Start 61 10,570 60 10,520 59 10,560 58 10,650 54 10,600 53 10,43

0.8%Ro - Peak 51 15,820 51 15,600 49 15,170 48 14,740 31 14,270 15 14,742.0%Ro - End -- -- -- -- -- -- 

Dobie Creek 0.5%Ro - Start 60 8,330 59 8,330 57 8,330 56 8,260 49 8,260 47 7,96

0.8%R

o

 - Peak 47 12,590 46 12,510 44 12,370 40 12,310 -- -- 

Doctor Ditch

0.5%R

o

 - Start 60 9,880 59 9,950 58 10,020 57 10,040 53 9,320 49 8,67

0.8%R

o

 - Peak 42 13,280 40 13,180 33 13,250 23 13,460 -- -- 

2.0%R

o

 - End -- -- -- -- -- -- 
Results—Petroleum Generation History 21

Table 5 . Timing of oil generation for Type-II source rocks and timing of oil cracking to gas, Bighorn Basin, 

Wyoming and Montana.

[Start, peak, and end of oil generation and of oil cracking to gas are represented by transformation ratios of 0.01, 0.50, and 0.99, 

respectively, and are calculated at a horizon in the middle of the source-rock interval. Age values are Ma. Timing of Phosphoria oil 

cracking to gas was modeled for Park City reservoirs. Blank cell indicates source rock did not attain minimum transformation ratio. 

Values in brackets are transformation ratios at the present time (0 Ma)]

Source 

rock

Oil generation Oil cracking to gas

Start Peak End Start Peak End 

Emblem Bench

Thermopolis Shale 58 54 52 36 0[0.11]

Mowry Shale 57 53 51 32 0[0.07]

Red Point/Husky

Thermopolis Shale 56 52 50 16 0[0.02]

Mowry Shale 55 52 49 no gas

Sellers Draw

Phosphoria/Park City Formation 37 0[0.04]

Thermopolis Shale 56 51 48 no gas

Mowry Shale 55 50 47 no gas

Amoco BN 1

Mowry Shale 58 53 46 no gas

Santa Fe Tatman

Thermopolis Shale 55 51 47 no gas

Mowry Shale 54 50 44 no gas

McCulloch Peak

Thermopolis Shale 55 48 38 no gas

Mowry Shale 54 46 32 no gas

Dobie Creek

Thermopolis Shale 52 40 26 no gas

Mowry Shale 51 37 19 no gas

Doctor Ditch

Phosphoria/Park City Formation no gas

Thermopolis Shale 52 24 0[0.82] no gas

Mowry Shale 50 16 0[0.64] no gas
22 Burial History, Thermal Maturity, and Oil and Gas Generation History of Source Rocks in the Bighorn Basin

Burial depth

30

Age (Ma)

80 70 60 50 40 20 10 0

80 70 60 50 40 30 20 10 0

Dobie Creek

0.5%RO - Start of gas

0.8%RO - Peak gas

2.0%RO - End of gas

Minor gas generation 

may continue

Timing of 

gas generation

Source rock (base)

B Mowry Shale

A Thermopolis Shale

D Cody Shale

C Frontier Formation

E Mesaverde Formation

F Meeteetse Formation

G Fort Union Formation

Deep

Intermediate

Shallow

Sellers Draw

Amoco BN 1

Emblem Bench

Santa Fe Tatman

Red Point/Husky

Doctor Ditch

McCulloch Peak

Late Cretaceous Paleocene Eocene Oligocene Miocene

G F

E

D

C

B

A

F

E

D

C

B

A

G

F

E

D

C

B

A

G

F

E

D

C

B

A

G

F

E

D

C

B

G

F

E

D

C

B

A

G

F

E

D

C

B

A

G

F

E

D

C

B

A

G

Santa Fe Tatman

 Amoco BN 1

 Doctor Ditch

Emblem Bench

Red Point/Husky

Dobie Creek 

Sellers Draw 

McCulloch Peak

Figure 5. Timing of gas generation from Type-III source rocks for each source rock and burial history location in the Bighorn 

Basin, Wyoming and Montana. Vertical dashed line at 10 Ma represents beginning of regional uplift, erosion, and subsequent 

cooling. If gas generation has not ended by present day based on the models, the rate of gas generation was significantly 

reduced with negligible generation during the last 10 million years.
Results—Petroleum Generation History 23

Late Cretaceous Paleocene Eocene Oligocene Miocene

70 60 50 40 30 20 10

AGE (Ma)

80 0

80 70 60 50 40 30 20 10 0

McCulloch Peak

1

2

1

2

Red Point/Husky

TR 2%

1

2

Emblem Bench

TR 6%

TR 9%

Santa Fe Tatman

1

2

Dobie Creek

1

2

Amoco BN 1

2

Doctor Ditch

2

2

1

1

0 no gas generation from cracking of Phosphoria oil

1

2

Sellers Draw

0 TR 6%

Santa Fe 

Tatman

 Amoco BN 1

 Doctor Ditch

Emblem Bench

Red Point/Husky

Dobie Creek 

Sellers Draw 

McCulloch Peak

Mowry Shale

Thermopolis Shale 

Start of oil

Peak oil

End of oil

Start of gas

Minor generation 

may continue

Source rocks (middle)

Minor generation 

may continue

Timing of 

oil generation

Timing of 

oil cracking to gas

Cracking to gas of Phosphoriasourced oil in Park City reservoirs 

0

Burial depth

Deep

Intermediate

Shallow

Figure 6. Timing of oil and gas generation from Type-II source rocks for each source rock and burial history location in 

the Bighorn Basin, Wyoming and Montana. Vertical dashed line at 10 Ma represents beginning of regional uplift, erosion, 

and subsequent cooling. If oil generation or oil cracking to gas has not ended by present day based on the models, the rate 

of these reactions was significantly reduced with negligible generation during the last 10 million years. TR, transformation 

ratio.
24 Burial History, Thermal Maturity, and Oil and Gas Generation History of Source Rocks in the Bighorn Basin

Draw location in the deep part of the Bighorn Basin are 

slightly greater than at Emblem Bench and Red Point/Husky, 

but because of a lower thermal gradient (table 2), the timing of 

petroleum generation at Sellers Draw was slightly later and the 

extent of petroleum generation was less (figs. 5, 6). 

Gas Generation from Source Rocks

The onset of gas generation from all the gas-prone source 

rocks buried the deepest (16,000 to >24,000 ft), as well as 

those buried to intermediate depths (from about 13,500 to 

20,000 ft) extended over a narrow range of only 12 m.y. — 

starting at 63 Ma for the Thermopolis Shale to 49 Ma for the 

Fort Union Formation. At the two locations where source 

rocks were buried to the shallowest depths (from about 10,000 

to 17,000 ft), the timing of onset of gas generation from Cretaceous source rocks is similar to that of the other locations; 

however, Fort Union source rocks did not start gas generation 

until much later, 38 to 33 Ma. The most complete gas generation in the Bighorn Basin occurred in the gas-prone source 

rocks in the deepest basin settings — at the Emblem Bench, 

Red Point/Husky, and Sellers Draw locations (fig. 5), where 

gas generation from the Cretaceous source rocks peaked from 

55 to 44 Ma. Only at these deep basin locations did the source 

rocks of the Fort Union Formation reach peak generation. The 

results of timing of gas generation are generally consistent 

with those obtained by Nuccio and Finn (1998) for the Sellers 

Draw location. 

At locations where gas-prone source rocks were buried 

to intermediate depths, gas generation from the Thermopolis stratigraphically upward through the Cody source rocks 

reached a peak at about the same time as source rocks at locations in the deep part of the basin; however, peak generation 

of gas from Mesaverde source rocks, occurred later (middle 

to late Eocene) than in the deep part of the basin. Thermal 

maturity of the Fort Union source rocks at these intermediate 

depths was not high enough to reach peak gas generation.

As expected at locations where gas-prone source rocks 

were buried to the shallowest depths, fewer source rocks 

reached the maturity level necessary to attain peak gas generation. Also, the time span from onset to peak gas generation is 

greater for those source rocks that ultimately did generate gas. 

Results of modeling indicate a similar timing for the start of 

gas generation from the shallower buried Thermopolis through 

the Cody source rocks as at locations where these source rocks 

were buried to intermediate depths. Peak generation, however, 

did not occur in Mesaverde and Meeteetse source rocks. 

Oil Generation from Source Rocks

Timing of the start, peak, and end of oil generation 

(transformation ratios of 0.01, 0.50, and 0.99, respectively) 

for Type-II source rocks was calculated at a horizon in the 

middle of the source-rock intervals. The results are given in 

table 5 and shown in figure 6. The beginning of oil generation 

from the two oil-prone source rocks considered in this study 

extended over a narrow range of only 8 m.y. — starting 

at 58 Ma for the Thermopolis and 50 Ma for the Mowry 

—regardless of depth of burial or location. 

Oil generation peaked between early and middle Eocene 

(54 – 46 Ma), at all locations for the Thermopolis and Mowry, 

except at the shallowest locations (Dobie Creek and Doctor 

Ditch) where peak oil generation for the Mowry occurred later 

(37 – 16 Ma). Similarly, oil generation ended for these source 

rocks at all locations except Dobie Creek by the early Miocene 

(19 Ma).

Oil Cracking to Gas

Timing of the start, peak, and end of oil cracking to gas 

(transformation ratios of 0.01, 0.50, and 0.99, respectively) is 

given in table 5 and shown in figure 6. These transformation 

ratios pertain to generated oils that are retained in their host 

source rock or in immediately adjacent reservoirs. In the case 

of generated Phosphoria oils that migrated from outside the 

basin, the transformation ratios pertain to oil that may have 

migrated long distances into the Park City Formation and 

immediately adjacent reservoirs.

It is apparent from the modeling that the gas generated 

from the cracking of oil does not contribute significantly to 

the overall gas potential of the Bighorn Basin. Figure 6 shows 

that the cracking of oil to gas took place only in the locations 

where source rocks were buried the deepest. For Phosphoriasourced oils, timing of the cracking could only be calculated 

for the Sellers Draw and Doctor Ditch locations because the 

other wells did not penetrate the Park City Formation. The 

cracking of Phosphoria oil at Sellers Draw began in the early 

Oligocene, but none took place at the Doctor Ditch location 

(table 5). It is certain, however, that Phosphoria oil did crack 

to gas at Emblem Bench and Red Point/Husky because oil 

from the younger, less deeply buried Cretaceous source rocks 

cracked to gas. Gas from cracking of Cretaceous oil started 

from the late Eocene to middle Miocene. It is important to 

note that the tranformation ratios at the present are all less than 

0.10, indicating that cracking only just started before uplift, 

erosion, and cooling essentially shut down petroleum generation.

Summary

Burial history, thermal maturity, and timing of petroleum 

generation were modeled for seven petroleum source-rock 

intervals at eight locations throughout the Bighorn Basin, as 

was the timing for the generation of gas from the cracking 

of Phosphoria oil in Park City Formation reservoirs at two 

locations. The results indicate that in areas where source rocks 

were buried the deepest (16,000 to >24,000 ft), gas generation from Type-III source rocks started in the early Paleocene 

(about 63 Ma for Cretaceous source rocks) to early Eocene 
References Cited 25

(52 Ma for Fort Union Formation source rocks), reached peak 

by the middle Eocene (44 Ma for Cretaceous source rocks), 

and ended only at the Emblem Bench location by the late Miocene (12 Ma for Frontier Formation source rocks). The results 

are similar for source rocks buried at intermediate depths 

(from about 13,500 to 20,000 ft), except that Mesaverde and 

Meeteetse Formations did not reach peak generation until later 

in the Eocene to early Miocene (47 to 15 Ma) and Fort Union 

source rocks did not reach peak generation. Timing of gas generation from source rocks buried to shallowest depths (from 

about 10,000 to 17,000 ft), ranges from onset of gas from the 

middle Paleocene (60 Ma for the Thermopolis Shale) to early 

Oligocene (33 Ma for the Fort Union) and peak generation 

was reached only in the Thermopolis, Mowry Shale, Frontier Formation, and Cody Shale sources from middle Eocene 

(47 Ma) to early Miocene (23 Ma).

Modeling results indicate that in areas where oil-prone 

source rocks of the Thermopolis and Mowry Shales were buried the deepest, oil generation started and ended in a relatively 

narrow age range from late Paleocene to middle Eocene (58 

to 47 Ma). Oil generation started and ended for these source 

rocks buried at intermediate depths, except the age range is 

greater, from late Paleocene to late Eocene (58 to 35 Ma). 

Timing of the onset of oil generation from source rocks buried 

to shallowest depths occurred in early Eocene (52 Ma), and 

ended only at Dobie Creek by early Miocene (19 Ma). 

The gas generated from the cracking of oil does not contribute significantly to the overall gas potential of the Bighorn 

Basin, according to the models. Only in the locations where 

source rocks were buried the deepest did the the cracking of 

oil to gas take place. For Phosphoria-sourced oil at Sellers 

Draw, cracking began in the late Eocene (37 Ma) and probably 

occurred at about the same time in the other two deep locations. Gas from cracking of oil sourced from the Thermopolis 

and Mowry Shales started in the late Eocene to middle Miocene (37 to 16 Ma).

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28 

Publishing support provided by:

Denver Publishing Service Center, Denver, Colorado

Manuscript approved for publication, March 2008

Edited by Mary A. Kidd

Layout by Sharon Powers
Roberts and others—Burial History, Maturity, and Oil and Gas Generation History of Source Rocks in Bighorn Basin, Wyoming and Montana—SIR 2008–5037

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