U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2008–3021

April 2008

National Assessment of Oil and Gas Fact Sheet

Assessment of Undiscovered Oil Resources in the Devonian-Mississippian 

Bakken Formation, Williston Basin Province, Montana and North Dakota, 2008

Printed on recycled paper

Introduction

The U.S. Geological Survey (USGS) completed an assessment of the undiscovered oil and associated gas resources of 

the Upper Devonian–Lower Mississippian Bakken Formation 

in the U.S. portion of the Williston Basin of Montana and North 

Dakota and within the Williston Basin Province (fig. 1). The 

assessment is based on geologic elements of a total petroleum 

system (TPS) that include (1) source-rock distribution, thickness, organic richness, maturation, petroleum generation, and 

migration; (2) reservoir-rock type (conventional or continuous), 

distribution, and quality; and (3) character of traps and time 

of formation with respect to petroleum generation and migration. Detailed framework studies in stratigraphy and structural 

geology and the modeling of petroleum geochemistry, combined 

with historical exploration and production analyses, were used 

to aid in the estimation of the undiscovered, technically recoverable oil and associated gas resources of the Bakken Formation 

in the United States. Using this framework, the USGS defined a 

Bakken-Lodgepole TPS (fig. 1) and seven assessment units (AU) 

within the TPS. For the Bakken Formation, the undiscovered oil 

and associated gas resources within six of these assessment units 

were quantitatively estimated (fig. 2, table 1). A conventional 

AU within the Lodgepole Formation was not assessed.

Bakken Formation and Bakken-Lodgepole Total 

Petroleum System

The Upper Devonian–Lower Mississippian Bakken Formation is a thin but widespread unit within the central and deeper 

portions of the Williston Basin in Montana, North Dakota, and the 

Canadian Provinces of Saskatchewan and Manitoba. The formation consists of three members: (1) lower shale member, (2) middle 

sandstone member, and (3) upper shale member. Each succeeding member is of greater geographic extent than the underlying 

member. Both the upper and lower shale members are organic-rich 

marine shale of fairly consistent lithology; they are the petroleum 

source rocks and part of the continuous reservoir for hydrocarbons 

produced from the Bakken Formation. The middle sandstone 

member varies in thickness, lithology, and petrophysical properties, 

and local development of matrix porosity enhances oil production 

in both continuous and conventional Bakken reservoirs. Within 

Using a geology-based assessment methodology, the U.S. 

Geological Survey estimated mean undiscovered volumes 

of 3.65 billion barrels of oil, 1.85 trillion cubic feet of 

associated/dissolved natural gas, and 148 million barrels of 

natural gas liquids in the Bakken Formation of the Williston 

Basin Province, Montana and North Dakota.

the Bakken-Lodgepole TPS, the upper and lower shale members 

of the Bakken Formation are also the source for oil produced from 

reservoirs of the Mississippian Lodgepole Formation.

Geologic Model and Assessment Units

The geologic model used to define AUs and to assess the 

Bakken Formation resources generally involves thermal maturity 

of the Bakken shale source rocks, petrophysical character of the 

middle sandstone member, and structural complexity of the basin. 

Most important to the Bakken-Lodgepole TPS and the continuous AUs within it are (1) the geographic extent of the Bakken 

Formation oil generation window (fig. 2), (2) the occurrence and 

distribution of vertical and horizontal fractures, and (3) the matrix 

porosity within the middle sandstone member. The area of the 

oil generation window for the Bakken continuous reservoir was 

determined by contouring both hydrogen index and well-log

resistivity values of the upper shale member, which is youngest 

and of greatest areal extent.

The area of the oil generation window for the Bakken

Formation was divided into five continuous AUs: (1) Elm

Coulee–Billings Nose AU, (2) Central Basin–Poplar Dome AU,

(3) Nesson–Little Knife Structural AU, (4) Eastern Expulsion 

Threshold AU, and (5) Northwest Expulsion Threshold AU. A sixth 

hypothetical conventional AU, a Middle Sandstone Member AU, 

was defined external to the area of oil generation.

Fargo

Minot

Pierre Gillette

Bismarck

Sheridan

Jamestown Dickinson

Williston

Rapid City

Grand Forks

Glendive

Miles 

City

Sidney

0 50Miles

MT

CANADA

WY

SD

ND

108° 107° 106° 105° 104° 103° 102° 101° 100° 99° 98° 97°

49°

48°

47°

46°

45°

44°

CANADA

UNITED STATES

NORTH DAKOTA

MONTANA

SOUTH DAKOTA

WYOMING

Bakken-Lodgepole TPS

Williston Basin 

Province

POPLAR

DOME

CEDAR CREEK

 CENTRAL 

MONTANA

 UPLIFT

ANTICLINE

BILLINGS

ANTICLINE

LITTLE KNIFE

ANTICLINE

NESSON

ANTICLINE

Figure 1. Map showing Williston Basin Province boundary (in red), 

Bakken-Lodgepole Total Petroleum System (TPS) (in blue), and major 

structural features in Montana, North Dakota, and South Dakota.
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

Bakken-Lodgepole TPS

Elm Coulee–Billings Nose AU Oil 374 410 450 410 118 198 332 208 8 16 29 17

Central Basin–Poplar Dome AU Oil 394 482 589 485 134 233 403 246 10 18 35 20

Nesson–Little Knife Structural AU Oil 818 908 1,007 909 260 438 738 461 19 34 64 37

Eastern Expulsion Threshold AU Oil 864 971 1,091 973 278 469 791 493 20 37 68 39

Northwest Expulsion Threshold AU Oil 613 851 1,182 868 224 411 754 440 16 32 64 35

Total Continuous

 Resources 3,645 1,848 148

Middle Sandstone Member AU Oil 1 4 8 4 1 1 3 2 0 0 0 0

Total Conventional

 Resources 4 2 0

Total Undiscovered 

 Oil Resources 3,649 1,850 148

Continuous 

Oil Resources

Conventional 

Oil Resources

Table 1. Bakken Formation, Williston Basin Province assessment results.

[MMBO, million barrels of oil. BCFG, billion cubic feet of gas. MMBNGL, million barrels of natural gas liquids. Results shown are fully risked estimates. 

F95 represents a 95 percent chance of at least the amount tabulated; other fractiles are defined similarly. TPS, total petroleum system; AU, assessment unit]

Resource Summary

The USGS assessed undiscovered oil and associated gas resources in five continuous (unconventional) 

AUs and one conventional AU for the Bakken Formation (fig. 2; table 1). For continuous oil resources, the 

USGS estimated a total mean resource of 3.65 billion 

barrels of oil, which combines means of 410 million 

barrels in the Elm Coulee–Billings Nose AU, 485 

million barrels in the Central Basin–Poplar Dome AU, 

909 million barrels in the Nesson–Little Knife Structural AU, 973 million barrels in the Eastern Expulsion 

Threshold AU, and 868 million barrels in the Northwest Expulsion Threshold AU. A mean resource of 

4 million barrels was estimated for the conventional 

Middle Sandstone Member AU.

The assessment of the Bakken Formation indicates that most of the undiscovered oil resides within a 

continuous composite reservoir that is distributed across 

the entire area of the oil generation window (fig. 2) and 

includes all members of the Bakken Formation. At the 

time of this assessment, only a limited number of wells 

have produced from the Bakken continuous reservoir 

in the Central Basin–Poplar Dome AU, the Eastern 

Expulsion Threshold AU, and the Northwest Expulsion 

Threshold AU. Therefore, there is significant geologic 

uncertainty in these estimates, which is reflected in the 

range of estimates for oil (table 1).

Bakken Formation, Williston Basin Province 

Assessment Team

Richard M. Pollastro (Bakken Formation Task Leader; 

pollastro@usgs.gov), Troy A. Cook, Laura N. R. Roberts, 

Christopher J. Schenk, Michael D. Lewan, Lawrence O. Anna, 

Stephanie B. Gaswirth, Paul G. Lillis, Timothy R. Klett, and 

Ronald R. Charpentier. For Further Information

Supporting geologic studies of the total petroleum system and assessment units and the methodology used in the Bakken 

Formation, Williston Basin Province assessment are in progress. Assessment results are available at the USGS Central Energy 

Team website, http://energy.cr.usgs.gov/oilgas/noga.

Figure 2. Map showing boundary of Bakken-Lodgepole Total Petroleum System 

(TPS) (in blue), five continuous assessment units (AU) (in green), and one conventional AU (in orange) defined for the assessment of undiscovered oil resources in 

the Upper Devonian–Lower Mississippian Bakken Formation in the U.S. portion of 

the TPS. The outermost green line defines the area of oil generation for the upper 

shale member of the formation.

Minot

Bismarck

Williston

Glendive

Miles 

City

Sidney

NORTH DAKOTA

MONTANA

Northwest Expulsion 

Threshold AU

Eastern Expulsion 

Threshold AU

Nesson-Little Knife 

Structural AU

Central BasinPoplar Dome AU

Elm CouleeBillings 

Nose AU

Middle Sandstone Member AU

Northwest Expulsion 

Threshold AU

Eastern Expulsion 

Threshold AU

Nesson-Little Knife 

Structural AU

Central BasinPoplar Dome AU

Elm CouleeBillings 

Nose AU

Middle Sandstone Member AU

Middle Sandstone Member AU

0 50Miles

Dickinson