National and Global Petroleum Assessment

Assessment of Undiscovered Continuous Oil and Gas 

Resources in the Heath Formation, Central Montana and 

Western North Dakota, 2016

U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2017–3032

June 2017

Using a geology-based assessment methodology, the U.S. Geological Survey estimated undiscovered, technically recoverable mean 

resources of 884 million barrels of oil and 106 billion cubic feet of gas in the North-Central Montana and Williston Basin Provinces 

of central Montana and western North Dakota.

Figure 1. Location of the North-Central Montana and Williston 

Basin Provinces with continuous assessment unit (AU) boundaries. 

Cedar Creek Structure modified from Maughan (1966) and Gaswirth 

and others (2013). Big Snowy Trough outline modified from Derkey 

and others (1985).

MONTANA

WYOMING

IDAHO

NORTH DAKOTA SOUTH DAKOTA

–113° –112° –111° –110° –109° –108° –107° –106° –105° –104°

49°

48°

47°

46°

45°

44°

CANADA

MONTANA

IDAHO

WYOMING

Map area

Source: U.S. Department of the Interior National Park Service

EXPLANATION

Big Snowy Trough boundary 

Province boundary and identifier

North-Central Montana Heath Continuous Oil AU

Williston Heath Continuous Oil AU

WILLISTON

BASIN

0 50 100 KILOMETERS

0 50 100 MILES

Butte

Bozeman

HELENA

Cedar

Creek

Anticline

NORTH-CENTRAL

MONTANA BASIN

WILLISTON

BASIN

Introduction

The U.S. Geological Survey (USGS) completed a geologybased assessment of the continuous (unconventional) oil and gas 

resources in the Big Snowy Trough area of central Montana and 

western North Dakota (fig. 1). The Big Snowy Trough was an 

east-west depression during Meramec time, and the north and south 

boundaries of the trough were later enhanced by faulting and erosion (Maughan and Roberts, 1967). The Heath Formation of the Big 

Snowy Group is confined to the Big Snowy Trough. This assessment is based on the geologic concepts and criteria of a total petroleum system (TPS) and includes information on the hydrocarbon 

source rocks, reservoir rocks, and hydrocarbon traps. In this study, 

we defined the Heath TPS and two assessment units (AUs) that contain similar structural, stratigraphic, and petroleum-charge histories. 

The assessment input data for each AU are shown in table 1.

Total Petroleum System

The Heath TPS is limited to the Heath Formation, which 

consists of a heterogeneous mix of lithologies that includes black 

shales, limestone, sandstone, anhydrite, and coal. The primary 

petroleum source rock is the Cox Ranch shale member in the 

middle of the Heath Formation. The Heath TPS is self-sourced 

with total organic carbon ranging from 0.5 to 26 weight percent 

and averaging 9 weight percent (McClave, 2012). Additional minor 

sources of petroleum may include the less organic-rich shale units 

in the upper and lower Heath Formation. Evidence suggests that 

some petroleum has migrated from the Heath Formation, and much 

of it has been produced from the overlying Tyler Formation; however, the primary resource potential is within the Heath Formation.

Assessment Units

Two AUs are identified in this assessment: the North-Central 

Montana Heath Continuous Oil AU and the Williston Heath 

Continuous Oil AU. The North-Central Montana Heath Continuous 

Oil AU lies within the Big Snowy Trough of central and eastern 

Montana and covers a maximum area of 5,255,000 acres (fig. 1). 

The potential resource AUs include the areas where the Heath 

Formation is thermally mature and not near the surface. Oil has 

been produced from the Heath Formation since 1919 when it was 

discovered in the Devil’s Basin oil field on the Central Montana 

Uplift (IHS MarkitTM, 2016). The Heath Formation is the source 

of more than 95 million barrels of oil (MMBO) produced from the 

overlying Tyler Formation (Bottjer, 2014). 

The Williston Heath Continuous Oil AU is along the eastern 

side of the Cedar Creek anticlinal structure in the southwestern 

portion of the Williston Basin (fig. 1). The Heath Formation is 

present only in a limited area (Maughan, 1966; Clement, 1986) and 

is thickest along the Cedar Creek anticline (greater than 300 feet 

according to Clement, 1986). The Heath Formation eroded during the Early Pennsylvanian on the west side of the Cedar Creek 

structure and only extends a short distance east from the structure 

(Maughan, 1966). 
Undiscovered Resources Summary

Using the USGS continuous resource methodology for assessing petroleum resources (Charpentier and Cook, 2011), we quantitatively 

assessed oil and gas resources in two continuous (unconventional) AUs in the North-Central Montana and Williston Basin Provinces (table 2). 

The overall total mean resources in the Heath Formation AUs are 884 million barrels of oil (MMBO) with an F95–F5 range from 154 MMBO 

to 1,965 MMBO and 106 billion cubic feet of gas (BCFG) with an F95–F5 range from 17 BCFG to 246 BCFG.

Table 1. Key assessment input data for two continuous assessment units in the Heath Formation, Montana and North Dakota.

[AU, assessment unit; %, percent; EUR, estimated ultimate recovery per well; MMBO, million barrels of oil. EUR, well drainage area, and success ratios are partly from 

U.S. shale-oil analogs. The average EUR input is the minimum, median, maximum, and calculated mean. Shading indicates not applicable]

Assessment input data

North-Central Montana Heath Continuous Oil AU Williston Heath Continuous Oil AU

Minimum Mode Maximum Calculated 

mean

Minimum Mode Maximum Calculated 

mean

Potential production area of AU (acres) 1,200 1,437,000 5,255,000 2,231,067 600 1,309,000 2,600,000 1,303,200

Average drainage area of wells (acres) 60 120 180 120 60 120 180 120

Success ratios (%) 40 70 80 63.3 10 40 90 46.7

Average EUR (MMBO, oil) 0.02 0.05 0.12 0.053 0.02 0.05 0.12 0.053

AU probability 1.0 0.9

Table 2. Assessment results for two continuous assessment units in the Heath Formation, Montana and North Dakota.

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

risked estimates. For gas accumulations, all liquids are included in the natural gas liquids category. 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. Shading indicates not applicable]

Total petroleum system and

assessment units (AUs)

AU

probability

Accumulation

type

Total undiscovered resources

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

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

Heath Total Petroleum System

North-Central Montana Heath Continuous 

Oil AU 1.0 Oil 154 563 1,371 637 17 66 172 76 0 0 1 0

Williston Heath Continuous Oil AU 0.9 Oil 0 216 594 247 0 25 74 30 0 0 0 0

Total undiscovered continuous resources 154 779 1,965 884 17 91 246 106 0 0 1 0

References Cited

Bottjer, Richard, 2014, Stratigraphy and oil resource potential of the 

Mississippian Heath Formation, Central Montana, USA [abs.], in

American Association of Petroleum Geologists (AAPG), Rocky 

Mountain Section Meeting, Salt Lake City, Utah, September 22–24, 

2013, Proceedings: American Association of Petroleum Geologists, 

Search and Discovery Article No. 50914.

Charpentier, R.R., and Cook, T.A., 2011, USGS methodology for assessing continuous petroleum resources: U.S. Geological Survey Open–

File Report 2011–1167, 73 p., accessed April 4, 2017, at http://pubs.

usgs.gov/of/2011/1167/.

Clement, J.H., 1986, Cedar Creek—A significant paleotectonic feature 

of the Williston Basin, in Peterson, J.A., ed., Paleotectonics and sedimentation in the Rocky Mountain Region, United States: American 

Association of Petroleum Geologists Memoir 41, p. 213–240.

Derkey, P.D., Abercrombie, F.N., Vuke, S.M., and Daniel, J.A., 1985, 

Geology and oil shale resources of the Heath Formation, Fergus 

County, Montana: Montana Bureau of Mines and Geology Memoir 

No. 57, 100 p.

Gaswirth, S.B., Marra, K.R., Cook, T.A, Charpentier, R.R., Gautier, D.L., 

Higley, D.K., Klett, T.R., Lewan, M.D., Lillis, P.G., Schenk, C.J., 

 Tennyson, M.E., and Whidden, K.J., 2013, Assessment of 

undiscovered oil resources in the Bakken and Three Forks Formations, Williston Basin Province, Montana, North Dakota, and South 

Dakota, 2013: U.S. Geological Survey Fact Sheet 2013–3013, 4 p., 

accessed April 4, 2017, at https://pubs.usgs.gov/fs/2013/3013/.

IHS MarkitTM, 2016, U.S. well history and production database: 

Englewood, Colo., IHS MarkitTM, accessed June 2016 at http://

www.ihsenergy.com. [Available from IHS MarkitTM, 15 Inverness 

Way East, Englewood, CO 80112.] 

Maughan, E.K., 1966, Pennsylvanian and Permian paleogeography, 

tectonics and stratigraphy in Montana and the Dakotas, in Smith, J.R., 

and Weichman, B.E., Symposium on recently developed geologic 

principles and sedimentation of the Permo-Pennsylvanian of the 

Rocky Mountains, 20th Annual Conference, Casper, Wyo., August 

31–September 2, 1966: Wyoming Geological Association, p. 95–108.

Maughan, E.K., and Roberts, A.E., 1967, Big Snowy and Amsden 

Groups and the Mississippian-Pennsylvanian boundary in Montana: 

United States Geological Survey Professional Paper 554–B, 27 p.

McClave, G.A., 2012, Stratigraphy and source rock analyses of the 

Heath Formation in Fergus, Garfield, Golden Valley, Musselshell, 

Petroleum, and Rosebud counties, central Montana: Colorado 

School of Mines, M.S. thesis, 312 p.

Heath Formation Assessment Team

Ronald M. Drake II, Christopher J. Schenk, Timothy R. Klett, Phuong A. Le, Heidi M. Leathers-Miller, Michael E. Brownfield, 

Thomas M. Finn, Stephanie B. Gaswirth, Kristen R. Marra, and Marilyn E. Tennyson

For More Information

Assessment results are available at the USGS Energy Resources Science Center website at https://energy.usgs.gov.

ISSN 2327-6916 (print)

ISSN 2327-6932 (online)

Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government. https://doi.org/10.3133/fs20173032