U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2008–3050

June 2008

National Assessment of Oil and Gas Fact Sheet

Assessment of Undiscovered Oil and Gas Resources of the 

Bighorn Basin Province, Wyoming and Montana, 2008

Printed on recycled paper

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 

migrated into the Bighorn Basin from the west 

and have partly cracked to gas with increasing 

thermal maturity. The Cretaceous and Tertiary 

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.

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 drill-stem tests and 

little production data, so geologic analogs from similar AUs in the Wind River Basin and 

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

MONTANA

WYOMING

Billings

Sheridan

Cody

0 20 40 MILES

MT

ID

WY

WIND

 RIVER

 MOUNTAINS

BIGHORN

MOUNTAINS

PRYOR

MOUNTAINS BEAR TOOTH

MOUNTAINS

ABSAROKA

RANGE

110° W 109° W 108° W 107° W

45° N

44° N

43° N
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, 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 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 

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

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.]

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).

For Further Information

Supporting geologic studies of total petroleum systems and 

assessment units, and reports on the methodology used in the 

Bighorn Basin Province assessment, are in progress. Assessment 

results are available at the USGS Central Energy Team website: 

http://energy.cr.usgs.gov/oilgas/noga/

Bighorn Basin Province Assessment Team

Mark A. Kirschbaum (Task Leader; mkirsch@usgs.gov), 

Stephen 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.