Chapter 1

Executive Summary—Assessment of 

Undiscovered Oil and Gas Resources of 

the Bighorn Basin Province, Wyoming 

and Montana, 2008

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

Chapter 1 of 

Petroleum Systems and Geologic Assessment of Oil and Gas in 

the Bighorn Basin Province, Wyoming and Montana

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

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

U.S. Department of the Interior

U.S. Geological Survey

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U.S. Department of the Interior

KEN SALAZAR, Secretary

U.S. Geological Survey

Marcia K. McNutt, Director

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

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Suggested citation:

U.S. Geological Survey Bighorn Basin Province Assessment Team, 2010, Executive Summary—Assessment of 

undiscovered oil and gas resources of the Bighorn Basin Province, Wyoming and Montana, 2008: U.S. Geological 

Survey Digital Data Series DDS–69–V, 4 p.

ISBN 1-4113-2667-5 
iii

Contents

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

Total Petroleum Systems ..............................................................................................................................1

Resource Summary .......................................................................................................................................1

Bighorn Basin Province Assessment Team ..............................................................................................4

References Cited............................................................................................................................................4

Figure

1. Bighorn Basin Province in north-central Wyoming and southern Montana......................2

Table

1. Bighorn Basin Province assessment results...........................................................................3
Using a geology-based assessment methodology, the U.S. 

Geological Survey estimated a mean of 989 billion cubic feet 

of undiscovered natural gas, a mean of 72 million barrels of 

undiscovered oil, and a mean of 13 million barrels of undiscovered natural gas liquids in the Bighorn Basin Province of 

Wyoming and Montana.

Introduction

The U.S. Geological Survey (USGS) recently completed 

an assessment of the undiscovered oil and gas potential of the 

Bighorn Basin Province (fig. 1), which encompasses about 

6.7 million acres in north-central Wyoming and southern 

Montana. The assessment is based on the geologic elements of 

each total petroleum system defined in the province, including 

petroleum source rocks (source-rock maturation, petroleum 

generation, and migration), reservoir rocks (sequence stratigraphy and petrophysical properties), and traps (trap formation and timing). Using this geologic framework, the USGS 

defined two total petroleum systems: (1) Phosphoria, and (2) 

Cretaceous–Tertiary Composite. Within these two systems, 

eight assessment units (AU) were defined, and undiscovered 

oil and gas resources were quantitatively estimated within 

each AU (table 1).

Total Petroleum Systems

Two petroleum systems were defined in the Bighorn 

Basin based on geochemical analyses of oil types. Two oil 

families were defined: one with low API gravity and low to 

moderate sulfur interpreted to be from a Permian Phosphoria 

Formation source, and the other showing higher API gravities and low to absent sulfur content interpreted to be from 

several stratigraphic units in the Cretaceous and Tertiary. The 

geochemical data indicate that there has been little mixing of 

the two oils in the basin, as the two oil families are effectively 

sealed by thick mudstone units in the Triassic Chugwater 

Formation and a thick anhydrite bed in the Jurassic Gypsum Springs Formation. The Phosphoria-derived oils largely 

Executive Summary—Assessment of Undiscovered 

Oil and Gas Resources of the Bighorn Basin Province, 

Wyoming and Montana, 2008

By USGS Bighorn Basin Province Assessment Team

migrated into the Bighorn Basin from the west and have partly 

cracked to gas with increasing thermal maturity. The Cretaceous and Tertiary oils were generated in the basin largely 

from the Mowry Shale, and little of the oil is interpreted to 

have cracked to gas. The Cretaceous and Tertiary petroleum 

source rocks are combined into one composite petroleum 

system because there are insufficient data to differentiate gas 

types from the various potential source rocks.

Most of the conventional petroleum traps in the Bighorn 

Basin are anticlinal traps and few of these structures remain 

untested by drilling. New conventional resource potential is 

interpreted to be from stratigraphic traps mainly in carbonate 

mounds in the Permian Park City Formation and in sandstone 

stratigraphic pinch-out traps in the Upper Cretaceous Frontier 

Formation and Cody Shale. Six continuous type accumulations—three basin-centered gas, one oil, and two coalbed 

gas—are interpreted to be present in the basin. These AUs, 

as defined by their limits, have little or no drillstem tests and 

little production data, so geologic analogs from similar AUs in 

the Wind River Basin and Southwestern Wyoming Provinces 

were used to infer production potentials. The potential for 

undiscovered gas is relatively low in the Bighorn Basin compared to these analog basins because of having: (1) overall less 

coal-bearing source rocks compared to the analog basins, (2) 

lower thermal maturity, (3) less volume of sandstone (reservoirs), and (4) fewer fractured structures in the central part of 

the basin. The Mowry Formation fractured oil accumulation in 

the Bighorn Basin is hypothetical, and the Mowry Formation 

in the Powder River Basin was used as a geologic and production analog.

Resource Summary

The USGS assessed both undiscovered conventional oil 

and gas and undiscovered continuous (unconventional) oil 

and gas in the Bighorn Basin Province (USGS Bighorn Basin 

Province Assessment Team, 2008), resulting in estimated 

means of 989 billion cubic feet of gas (BCFG), 72 million 

barrels of oil (MMBO), and 13 million barrels of total natural 
2 Executive Summary—Assessment of Undiscovered Oil and Gas Resources of the Bighorn Basin Province

Figure 1. Bighorn Basin Province in north-central Wyoming and southern Montana.

MONTANA

WYOMING

Billings

Sheridan

Cody

110° 109° 108° 107°

45°

44°

43°

0 20 40 Miles

MT

ID

WY

WIND

 RIVER

 MOUNTAINS

BIGHORN

MOUNTAINS

PRYOR

MOUNTAINS BEAR TOOTH

MOUNTAINS

ABSAROKA

RANGE
Resource Summary 3

Table 1. Bighorn Basin Province assessment results.

[MMBO, million barrels of oil; BCFG, billion cubic feet of gas; MMBNGL, million barrels of natural gas liquids; TPS, Total Petroleum System; 

AU, Assessment Unit; CBG, coalbed gas. Results shown are fully risked estimates. For gas fields, all liquids are included under the NGL (natural gas 

liquids) category. F95 denotes a 95-percent chance of at least the amount tabulated. Other fractiles are defined similarly. Fractiles are additive under 

the assumption of perfect positive correlation. Gray shading indicates not applicable.]

Total Petroleum System 

and Assessment Unit

Field 

Type

Total Undiscovered Resources

Oil (MMBO) Gas (BCFG) NGL (MMBNGL)

F95 F50 F5 Mean F95 F50 F5 Mean F95 F50 F5 Mean

Cretaceous–Tertiary Composite TPS

Muddy–Frontier Sandstone and 

Mowry Fractured Shale Gas AU Gas 119 298 743 348 0 0 1 0

Mowry Fractured Shale Oil AU Oil 2 4 11 5 1 2 6 2 0 0 0 0

Cody Sandstone Gas AU Gas 14 33 80 38 0 0 0 0

Mesaverde Sandstone Gas AU Gas 13 28 63 32 0 0 0 0

Mesaverde–Meeteetse Formation Coalbed Gas AU CBG 38 87 196 98 0 0 1 0

Fort Union Formation Coalbed 

Gas AU CBG 14 29 59 32 0 0 0 0

Total Continuous Resources 2 4 11 5 199 477 1,147 550 0 0 2 0

Phosphoria TPS

Paleozoic–Mesozoic 

Oil and Gas AU

Oil 13 49 110 54 12 48 117 54 0 2 5 2

Gas 43 149 332 164 1 5 13 6

Cretaceous–Tertiary Composite TPS

Cretaceous–Tertiary 

Oil and Gas AU

Oil 4 12 24 13 9 29 66 32 1 2 4 2

Gas 53 176 370 189 1 3 6 3

Total Conventional Resources 17 70261 134 67 117 402 885 439 3 12 28 13

Total Undiscovered 

Oil and Gas Resources 19 65 145 72 316 879 2,032 989 3 12 30 13

Continuous Resources Conventional Resources
4 Executive Summary—Assessment of Undiscovered Oil and Gas Resources of the Bighorn Basin Province

gas liquids (MMBNGL) for the two total petroleum systems 

(TPSs) (table 1). More than half of the undiscovered gas 

resource, about 550 BCFG, is interpreted as continuous 

and is contained within the Cretaceous–Tertiary Composite 

TPS. The continuous gas is contained within five AUs of the 

Cretaceous–Tertiary Composite TPS. Mean estimates are as 

follows: Muddy–Frontier Sandstone and Mowry Fractured 

Shale Continuous Gas AU (348 BCFG), Cody Sandstone 

Continuous Gas AU (38 BCFG); Mesaverde Sandstone 

Continuous Gas AU (32 BCFG); Mesaverde–Meeteetse 

Coalbed Gas AU (98 BCFG); and Fort Union Coalbed Gas 

AU (32 BCFG). The remainder of the undiscovered gas (439 

BCFG) is associated/dissolved gas in oil accumulations or 

is in conventional nonassociated gas accumulations in the 

Paleozoic–Mesozoic Oil and Gas AU, and Cretaceous–Tertiary 

Conventional Oil and Gas AU (table 1).

Bighorn Basin Province Assessment 

Team

Mark A. Kirschbaum (Task Leader), Steven M. Condon, 

Thomas M. Finn, Ronald C. Johnson, Paul G. Lillis, Philip H. 

Nelson, Laura N.R. Roberts, Stephen B. Roberts, Ronald R. 

Charpentier, Troy Cook, Timothy R. Klett, Richard M. Pollastro, and Christopher J. Schenk.

References Cited

USGS Bighorn Basin Province Assessment Team, 2008, 

Assessment of undiscovered oil and gas resources of the 

Bighorn Basin Province, Wyoming and Montana: U.S. 

Geological Survey Fact Sheet FS 2008-3050, 2 p.

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