U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2013–3013

 April 2013

National Assessment of Oil and Gas Fact Sheet

Assessment of Undiscovered Oil Resources in the Bakken and 

Three Forks Formations, Williston Basin Province, Montana, 

North Dakota, and South Dakota, 2013

Printed on recycled paper

Using a geology-based assessment 

methodology, the U.S. Geological Survey 

estimated mean undiscovered volumes of 

7.4 billion barrels of oil, 6.7 trillion cubic feet 

of associated/dissolved natural gas, and 

0.53 billion barrels of natural gas liquids in 

the Bakken and Three Forks Formations in 

the Williston Basin Province of Montana, 

North Dakota, and South Dakota.

is defined by the postulated maximum extent of petroleum fluids within the Bakken 

and Three Forks Formations. The Devonian and Mississippian Bakken Formation 

consists of four members in ascending order: (1) the Pronghorn Member (formerly 

known as “Sanish sand”) (LeFever and others, 2011), (2) lower shale member, 

(3) middle member, and (4) upper shale member. The sandstones and siltstones 

of the Pronghorn Member represent the first transgressive unit of the Bakken 

Formation and are limited in extent. The maximum thickness of the Pronghorn is 

58 feet (LeFever and others, 2011). The upper and lower shale members are the 

primary source rocks for the Bakken TPS, with present-day total organic carbon 

(TOC) values from <1 weight percent to 35 weight percent (Lillis, 2013). The shale 

members are present in parts of Montana and North Dakota and extend into the 

Canadian provinces of Saskatchewan and Manitoba, though the Canadian provinces 

were not assessed. The lower shale member reaches a maximum thickness of 56 feet 

along the east flank of the Nesson anticline (fig. 1). The middle member of the 

Figure 1. Map showing the Williston Basin Province, Bakken Total Petroleum System 

(TPS), and the Bakken Formation Assessment Units (AUs). Major structural features are 

also shown. Inset map shows location of the Bakken TPS (pink).

Introduction

The U.S. Geological Survey (USGS) 

recently completed a geology-based assessment of 

conventional and continuous oil and gas resources 

of the Devonian Three Forks Formation and 

Devonian and Mississippian Bakken Formation 

in the Williston Basin Province of North Dakota, 

Montana, and South Dakota (fig. 1). Since the 

2008 USGS assessment of the Bakken Formation 

(Pollastro and others, 2008), more than 4,000 

additional wells have been drilled, providing significant geologic data about the Bakken and Three 

Forks Formations. Furthermore, the Three Forks 

Formation was not assessed in 2008 by the USGS 

and accordingly warranted assessment based on 

significant increases in drilling and production 

in this formation. Oil is being produced from the 

Bakken and Three Forks Formations using both 

horizontal drilling technology, which exposes a 

larger amount of reservoir to the wellbore than 

vertical wells, and hydraulic fracturing, which 

stimulates movement of hydrocarbons in tight-oil 

reservoirs. Approximately 450 million barrels of oil (MMBO) have been produced from 

the Bakken and Three Forks Formations in the 

United States since the 2008 assessment of the 

Bakken Formation.

Geologic Summary of the Bakken 

and Three Forks Formations

The Bakken Total Petroleum System (TPS) 

encompasses strata from the Devonian Three 

Forks Formation, Bakken Formation, and lower 

part of the Mississippian Lodgepole Formation 

that may contain Bakken-sourced oil. The TPS 

MT

CANADA

WY SD

ND

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

49°

48°

47°

46°

45°

44°

CANADA

NORTH DAKOTA

MONTANA

SOUTH DAKOTA

WYOMING

OID FAULT

Poplar

dome

Cedar Creek

anticline

Billings

Nose

anticline

Little Knife

anticline BROCKTON-FROID FAULT

Nesson

anticline

Antelope

anticline

EXPLANATION

Elm Coulee-Billings Nose Continuous Oil AU

Williston Basin Province boundary

Bakken TPS

Middle Bakken Conventional AU

Nesson-Little Knife Continuous Oil AU

Central Basin Continuous Oil AU

Northwest Transitional Continuous Oil AU

Eastern Transitional Continuous Oil AU

50 MILES

0

0

50 KILOMETERS
Bakken Formation varies in thickness and lithology and includes 

sandstone, siltstone, dolomite, and mudstone (LeFever, 1991). 

The middle member reaches a maximum thickness of 90 feet 

near the basin center in North Dakota along the east flank of the 

Nesson anticline (LeFever, 2008). The upper shale member of 

the Bakken is the most geographically extensive member of the 

Bakken Formation, and it represents the maximum depositional 

limit for the Bakken Formation. It is thickest to the east of the 

Nesson anticline with localized thickening (as much as 58 feet) 

in Stark County in southwestern North Dakota.

The Three Forks Formation consists of interbedded grayishgreen dolomitic mudstones, pink to tan silty dolostones, and 

anhydrite. It is unconformably overlain by the Pronghorn Member 

of the Bakken Formation. The Three Forks Formation reaches a 

maximum thickness of 270 feet in the central portion of the basin 

(Bottjer and others, 2011) and is divided stratigraphically into 

lower and upper units, with variable oil saturations.

Geologic Model for Assessment

The geologic model for the assessment of the Bakken 

Formation and underlying Three Forks Formation is that oil generated in the upper and lower Bakken shale members migrated 

locally into low-permeability and variable-porosity reservoirs 

of the middle Bakken member, the Pronghorn Member of the 

Bakken Formation, and dolomitized units of the Three Forks 

Formation. Locally, oil was also retained in the low-porosity 

matrix and fractures of the upper and lower Bakken shale members. A hydrogen index (HI) value of 450 was used to define the 

boundary of thermally mature source rock in the upper Bakken 

shale member as indicated by recent USGS research.

The Pronghorn Member of the Bakken Formation, although 

geologically and stratigraphically defined as part of the Bakken 

Formation (LeFever and others, 2011), is assessed with the Three 

Forks Formation. Where present, the Pronghorn Member is in 

fluid communication with the underlying Three Forks reservoirs.

Assessment Units

Six continuous assessment units (AUs) and two conventional AUs were defined in the Bakken TPS. Five of the continuous AUs and one conventional AU are in the Bakken Formation, 

and one continuous and one conventional AU are in the Three 

Forks Formation. Key input data used to assess continuous AUs 

in the Bakken and Three Forks Formations are listed in table 1.

Bakken Formation

The Middle Bakken Conventional AU is a hypothetical AU 

defined by the extent of the middle Bakken member within the 

Williston Basin Province (fig. 1).

The Elm Coulee-Billings Nose Continuous Oil AU is 

defined on the north by the Devonian Prairie Evaporite dissolution edge, to the northwest by the Brockton-Froid fault zone, on 

the south by the five-foot isopach of the middle Bakken member, 

Table 1. Key assessment input data for continuous assessment units in the Bakken TPS.

[EUR, estimated ultimate recovery per well; MMBO, million barrels of oil; AU, assessment unit. The average EUR input is the minimum, median, maximum, 

and calculated mean]

Assessment input data Elm Coulee-Billings Nose Continuous Oil AU 50310161 Central Basin Continuous Oil AU 50310162

Minimum Mode Maximum Calculated mean Minimum Mode Maximum Calculated mean

Potential production area of AU (acres) 1,400,000 1,600,000 1,800,000 1,600,000 2,800,000 3,100,000 3,400,000 3,100,000

Average drainage area of wells (acres) 320 400 600 440 320 400 600 440

Percent of AU untested 51 69 80 67 80 87 91 86

Percent of AU in sweet spots 24 27 30 27 24 29 70 41

Input data for geologic sweet spots

Success ratios (%) 98 99 100 99 98 99 100 99

Average EUR (MMBO) 0.15 0.18 0.22 0.182 0.225 0.25 0.325 0.254

Input data for geologic non-sweet spots

Success ratios (%) 85 90 95 90 80 90 95 88

Average EUR (MMBO) 0.06 0.1 0.15 0.102 0.075 0.15 0.25 0.154

Nesson-Little Knife Continuous Oil AU 50310163 Eastern Transitional Continuous Oil AU 50310164

Minimum Mode Maximum Calculated mean Minimum Mode Maximum Calculated mean

Potential production area of AU (acres) 2,600,000 2,800,000 3,000,000 2,800,000 1,800,000 1,900,000 2,000,000 1,900,000

Average drainage area of wells (acres) 320 400 600 440 320 400 600 440

Percent of AU untested 65 76 84 75 70 79 93 81

Percent of AU in sweet spots 35 38 85 53 10 15 20 15

Input data for geologic sweet spots

Success ratios (%) 98 99 100 99 98 99 100 99

Average EUR (MMBO) 0.26 0.3 0.35 0.302 0.375 0.425 0.55 0.431

Input data for geologic non-sweet spots

Success ratios (%) 90 95 100 95 90 95 100 95

Average EUR (MMBO) 0.125 0.175 0.25 0.178 0.175 0.225 0.35 0.231

Northwest Transitional Continuous Oil AU 50310165 Three Forks Continuous Oil AU 50310166

Minimum Mode Maximum Calculated mean Minimum Mode Maximum Calculated mean

Potential production area of AU (acres) 500,000 2,000,000 3,100,000 1,866,667 5,000,000 10,000,000 25,000,000 13,333,333

Average drainage area of wells (acres) 320 400 600 440 220 400 600 407

Percent of AU untested 94 98.8 99.5 97 89 96.5 99.2 95

Percent of AU in sweet spots 10 15 45 23 10 50 90 50

Input data for geologic sweet spots

Success ratios (%) 80 90 95 88 80 90 95 88

Average EUR (MMBO) 0.075 0.15 0.25 0.154 0.18 0.22 0.275 0.222

Input data for geologic non-sweet spots

Success ratios (%) 10 40 80 43 10 40 80 43

Average EUR (MMBO) 0.005 0.05 0.15 0.055 0.01 0.08 0.2 0.085
and to the southeast at the boundary of the 

Nesson-Little Knife Continuous Oil AU. The 

AU includes production from the Elm Coulee 

field and along the Billings Nose anticline or 

“Bakken fairway,” which was used to define 

a geologic “sweet spot” based upon favorable 

reservoir properties in the Elm Coulee field 

and structural enhancement of the Billings 

Nose anticline.

The Central Basin Continuous Oil AU is 

defined largely by the 450-hydrogen index (HI) 

contour for the upper Bakken shale member. The 

450-HI contour includes the Poplar dome region, 

but this particular area was largely discounted 

for the addition of potential resource because of 

the migration of Bakken oil into overlying units 

in this structure (Lillis, 2013) and the absence of 

oil in the Bakken Formation. The southern and 

eastern boundaries of this AU are constrained 

by the Elm Coulee-Billings Nose and NessonLittle Knife Continuous Oil AUs, respectively. 

The geologic “sweet spot” is defined as an area 

with pressure gradients equal to or greater than 

0.68 pounds per square inch per foot (psi/ft), 

based on a contour map by Theloy and Sonnenberg (2013).

The Nesson-Little Knife Continuous Oil 

AU western boundary is defined from a structure contour map of the Bakken Formation and 

includes the Nesson and Little Knife anticlines. 

The eastern boundary is defined by the 450-HI 

contour, and the southern boundary is the edge 

of the Bakken Formation. The geologic “sweet 

spot” is defined as the area where the formation 

pressure is equal to or exceeds 0.68 psi/ft (Theloy 

and Sonnenberg, 2013).

The Eastern Transitional Continuous Oil 

AU is defined by the 450-HI contour on the west 

and wraps around to the 100-ohm resistivity line 

of Hester and Schmoker (1985) and the 650-HI 

contour line used by Pollastro and others (2008). 

Bakken production in northern Divide County, 

in northwestern North Dakota, was also included 

in this AU because the thermal maturity in that 

area is similar to that along the eastern margin 

of the Eastern Transitional Continuous AU, and 

it reflects the transitional nature of the thermal 

maturity from the center of the basin outward. 

The geologic “sweet spot” is defined as the area 

where the formation pressure gradient is equal to 

or exceeds 0.68 psi/ft (Theloy and Sonnenberg, 

2013) and includes Parshall field and the eastern 

portion of Sanish field.

The Northwest Transitional Continuous Oil 

AU is also defined on the basis of lower thermal 

maturities (>450 HI). The western boundary 

is defined by the 100-ohm resistivity line and 

650-HI contour, the northern boundary is placed 

along the U.S.-Canadian border, and the remainder of the AU follows the 450-HI contour used 

to define the Central Basin Continuous Oil AU.

Three Forks Formation

The Three Forks Conventional AU is a hypothetical AU defined outside of 

the continuous AUs by the maximum extent of the Three Forks Formation, which 

extends beyond the Bakken shale source intervals (fig. 2).

The Three Forks Continuous Oil AU is defined by the lateral extent of the 

upper Bakken shale member in conjunction with the southernmost extent of the 

Pronghorn Member in North Dakota (fig. 2). The northern boundary is placed 

along the U.S.-Canadian border. The geologic “sweet spot” encompasses the 

region of maximum hydrocarbon generation (as determined by HI values), maximum formation pressures, and structural enhancements related to lineaments and 

the Nesson and Little Knife anticlines.

Resource Summary

The USGS assessed technically recoverable continuous (unconventional) 

resources for six AUs defined in the Bakken and Three Forks Formations resulting in estimated means of 7,375 million barrels of oil (MMBO), 6,723 billion cubic feet of gas (BCFG), and 527 million barrels of natural gas liquids 

(MMBNGL) (table 2). This includes mean-estimated oil resources from the 

Elm Coulee-Billings Nose Continuous Oil AU (283 MMBO), the Central Basin 

Continuous Oil AU (1,122 MMBO), the Nesson-Little Knife Continuous Oil AU 

(1,149 MMBO), the Eastern Transitional Continuous Oil AU (883 MMBO), the 

Northwest Transitional Continuous Oil AU (207 MMBO), and the Three Forks 

Continuous Oil AU (3,731 MMBO).

The two conventional AUs (Middle Bakken Conventional Assessment Unit 

and Three Forks Conventional Assessment Unit) have estimated mean-recoverable 

resources of 8 MMBO and 3 BCFG (table 2).

Figure 2. Location of the Three Forks Formation Assessment Units (AUs) in the Williston 

Basin Province. Inset map shows location of the Bakken TPS (pink).

MT

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ND

50 MILES

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0

50 KILOMETERS

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

49°

48°

47°

46°

45°

44°

CANADA

NORTH DAKOTA

MONTANA

WYOMING SOUTH DAKOTA

Poplar

dome

Cedar Creek

anticline

Billings

Nose

anticline

Little Knife

anticline

Nesson

anticline

Antelope

anticline BROCKTON-FROID FAULT

EXPLANATION

Three Forks Continuous Oil AU Williston Basin Province boundary

Three Forks Conventional AU Bakken TPS
Acknowledgments

The authors thank Steve Sonnenberg and Cosima Theloy 

(Colorado School of Mines), Julie LeFever, Rich LeFever, 

Ed Murphy, and Stephan Nordeng (North Dakota Geological 

Survey), and Jay Gunderson (Montana Board of Oil and Gas) for 

all their assistance and guidance throughout this work. We also 

thank the many industry collaborators who provided helpful input.

References Cited

Bottjer, R.J., Sterling, R.H., Grau, D.A., and Dea, P.A., 2011, 

Stratigraphic relationships and reservoir quality at the Three 

Forks-Bakken unconformity, Williston Basin, North Dakota, 

in Robinson, J.W., LeFever, J.A., and Gaswirth, S.B., eds., The 

Bakken-Three Forks Petroleum System in the Williston Basin: 

Denver, Colo., Rocky Mountain Association of Geologists, 

p. 173–228.

Hester, T.C., and Schmoker, J.W., 1985, Selected physical properties of the Bakken Formation, North Dakota and Montana part 

of the Williston Basin: U.S. Geological Survey Oil and Gas 

Investigations Chart 126, 1 sheet.

LeFever, J.A., 1991, History of oil production from the Bakken 

Formation, North Dakota, in Hanson, W.B., ed., 1991 Guidebook to geology and horizontal drilling of the Bakken Formation: Billings, Montana Geological Society, p. 3–17.

LeFever, J.A., 2008, Structural contour and isopach maps of the 

Bakken Formation in North Dakota: North Dakota Geological 

Survey Geologic Investigations No. 59, 5 sheets.

LeFever, J. A., LeFever, R.D., and Nordeng, S.H., 2011, Revised 

nomenclature for the Bakken Formation (MississippianDevonian), North Dakota, in Robinson, J.W., LeFever, J.A., 

and Gaswirth, S.B., eds., The Bakken-Three Forks Petroleum 

System in the Williston Basin: Denver, Colo., Rocky Mountain 

Association of Geologists, p. 11–26.

Lillis, P.G., 2013, Review of oil families and their petroleum 

systems of the Williston Basin: The Mountain Geologist, v. 50, 

no. 1, p. 5–31.

Pollastro, R.M., Cook, T.M., Roberts, L.N.R., Schenk, C.J., 

Lewan, M.J., Anna, L.O., Gaswirth S.B., Lillis, P.G., 

Klett, T.R., and Charpentier, R.R., 2008, Assessment of 

undiscovered oil resources in the Devonian-Mississippian 

Bakken Formation, Williston Basin Province, Montana and 

North Dakota: U.S. Geological Survey Fact Sheet 2008–3021, 

2 p.

Theloy, Cosima, and Sonnenberg, S.A., 2013, New insights 

into the Bakken play: What factors control production [abs.]: 

American Association of Petroleum Geologists Annual Meeting, Pittsburgh, Pa., May 19–23, 2013.

Table 2. Bakken and Three Forks Formations, 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. For gas 

accumulations, all liquids are included as NGL (natural gas liquids). F95 represents 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. TPS, total petroleum system; AU, assessment unit. Gray shading indicates not applicable]

Total Petroleum Systems

(TPS)

and Assessment Units (AU)

Field

type

Total Undiscovered Resources

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

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

Bakken TPS

Elm Coulee-Billings Nose Continuous Oil AU 50310161 Oil 218 281 355 283 174 278 410 283 11 21 36 22

Central Basin Continuous Oil AU 50310162 Oil 892 1,113 1,379 1,122 699 1,103 1,604 1,122 46 85 139 88

Nesson-Little Knife Continuous Oil AU 50310163 Oil 907 1,139 1,423 1,149 714 1,130 1,648 1,149 47 87 144 90

Eastern Transitional Continuous Oil AU 50310164 Oil 706 876 1,082 883 275 435 629 441 18 33 55 35

Northwest Transitional Continuous Oil AU 50310165 Oil 90 197 357 207 57 134 268 145 4 10 22 11

Three Forks Continuous Oil AU 50310166 Oil 1,604 3,440 6,834 3,731 1,508 3,286 6,685 3,583 105 252 553 281

Total continuous resources 4,417 7,046 11,430 7,375 3,427 6,366 11,244 6,723 231 488 949 527

Bakken TPS

Middle Bakken Conventional AU 50310101 Oil 1 4 10 5 0 2 4 2 0 0 0 0

Gas 0 0 0 0 0 0 0 0

Three Forks Conventional AU 50310103 Oil 0 3 7 3 0 1 3 1 0 0 0 0

Gas 0 0 0 0 0 0 0 0

Total conventional resources 1 7 17 8 0 3 7 3 0 0 0 0

Total undiscovered oil and gas resources 4,418 7,053 11,447 7,383 3,427 6,369 11,251 6,726 231 488 949 527

Bakken-Three Forks Assessment Team

Stephanie B. Gaswirth (Task Leader; sgaswirth@usgs.gov), Kristen R. Marra, Troy A. Cook, Ronald R. Charpentier, 

Donald L. Gautier, Debra K. Higley, Timothy R. Klett, Michael D. Lewan, Paul G. Lillis, Christopher J. Schenk (Project 

Chief, schenk@usgs.gov), Marilyn E. Tennyson, and Katherine J. Whidden

For Additional Information

Supporting geologic studies of the Bakken and Three Forks Formations and the methodology used in the assessment 

are in progress. Please see the USGS Energy Resources Program Web site (http://energy.usgs.gov).