Powder River Basin Geology
The Powder River Basin area encompasses the Powder River structural basin and Powder River energy basin. The structural basin is an asymmetric trough in southeastern Montana and northeastern Wyoming that trends north-south for approximately 401 km (250 mi) and is 161 km (100 mi) wide. It is bounded to the south by the Casper Arch, Laramie Mountains, and Hartville Uplift; to the west by the Bighorn Mountains; to the north by the Miles City arch in Montana; and to the east by the Black Hills. The Powder River energy basin is loosely defined by the Cretaceous–Tertiary boundary observed in outcrops.
Formations in the Powder River Basin have near-vertical to overturned dips along the western margin and gentle sub-horizontal (basinward) dips along the eastern margin. Laramide structural deformation began in the Powder River Basin region during deposition of the Upper Cretaceous (Maastrichtian) Lewis Shale and ended during deposition of the Eocene Wasatch Formation (Curry, 1971). Together, Sevier subsidence and Laramide deformation resulted in structural relief greater than 7,620 m (25,000 ft; Blackstone, 1981). Nearly 2,438 m (8,000 ft) of syn-Laramide sedimentary rocks are preserved within the Powder River Basin (Curry, 1971).
Conventional hydrocarbon fields within the Powder River Basin generally occur as stratigraphic traps or in basin-bounding anticlinal structures in Paleozoic and lower Mesozoic strata. The Tensleep Sandstone and Minnelusa Formation are the major Paleozoic oil producers. According to Dolton and others (1990), the Tensleep primarily produces from structural (anticlinal) traps and the Minnelusa produces from both structural and stratigraphic traps.
The hydrocarbon source rocks that likely charged the Tensleep and Minnelusa reservoirs were Permian shales in western Wyoming. Dolton and others (1990) argue that the hydrocarbons were generated by Jurassic time, migrated east, and were trapped until the Laramide orogeny.
Subsequent uplift allowed some hydrocarbons to escape, while some remained in the Tensleep and Minnelusa reservoirs. In the eastern parts of the basin, lower Minnelusa reservoirs may have been locally sourced from interbedded black shales (Clayton and Ryder, 1984).
Formations deposited during the Cretaceous represent the other major hydrocarbon reservoirs in the Powder River Basin. Although operators did produce from conventional wells in Cretaceous reservoirs such as the Muddy Sandstone (western equivalent of the Newcastle Sandstone), Frontier Formation, and the Carlile Shale, historically less-productive Cretaceous-age unconventional reservoirs are now the main focus of exploration and production in the Powder River Basin. These unconventional reservoirs include the Lakota Formation, Fall River (Dakota) Sandstone, Mowry Shale, Wall Creek Sandstone Member of the Frontier Formation, the Turner Sandstone Member of the Carlile Shale, Niobrara Formation, Shannon and Sussex sandstone members of the Cody Shale, Teapot and Parkman sandstone members of the Mesaverde Formation, and the Teckla Sandstone Member of the Lewis Shale.
The source rock for most of the Upper Cretaceous hydrocarbon reservoirs is the Mowry Shale, with significant contributions from the Niobrara Formation and Carlile Shale (Momper and Williams, 1984; Dolton and others, 1990). Hydrocarbons were generated in the deeper western part of the basin and migrated up-dip toward the east into the Cretaceous reservoirs. Estimates suggest nearly 12 billion barrels of oil were generated in the Mowry Shale (Momper and Williams, 1984).
Production
Development of the Powder River Basin (WSGS oil and gas map) as a hydrocarbon-producing basin occurred more slowly than in the other Laramide basins. The first producing oil well in the basin was drilled in 1889 north of Salt Creek field, which is still the most cumulatively productive oil field in Wyoming. More than 725 named fields and numerous wildcat wells have since been developed in the basin.
Oil production in the Powder River Basin has fluctuated through several boom and bust cycles. In 2010, oil production from unconventional reservoirs in the basin started increasing dramatically, and in 2019 reached levels not seen since the late 1980s (WOGCC, 2024). Beginning in 2014, more than half of all oil produced in Wyoming each year has come from the Powder River Basin, and its contribution has increased each year. In each of 2022 and 2023, the basin accounted for more than 68 percent of the state’s total oil production (WOGCC, 2024).
Gas occurrences in the Powder River Basin were historically rare and were usually gas caps associated with oil reservoirs. However, coalbed natural gas (CBNG) development in the late 1990s and 2000s changed the Powder River Basin into a significant natural gas-producing region. At its peak in 2009, the Powder River Basin produced more than 584 billion cubic feet of natural gas (WOGCC, 2024). Except for a slight uptick in 2018 and 2019 associated with prolific oil wells, natural gas production has been declining or flattening in the Powder River Basin since 2009. This trend is largely due to low gas prices, depleted CBNG reservoirs, and competition from large unconventional gas plays.
Future Development
Thick sequences of stacked, unconventional reservoirs will continue to be the focus of development within the Powder River Basin. The Frontier, Niobrara, Parkman, and Turner sandstones, along with the Mowry Shale, were the basin’s five most productive reservoirs in 2023 (WOGCC, 2024). Large-scale oil and gas developments such as the Converse County oil and gas project are currently targeting these and other Upper Cretaceous units (WSGS oil and gas map).
Technology will also play a vital role in developing the basin’s previously-uneconomic tight sands and shales. Horizontal well laterals are now being drilled up to 9,000 feet in length in the Powder River Basin, increasing the intersected reservoir surface area and potentially boosting yields. Advances in hydraulic fracturing, such as intense multi-stage “cloud fracking”, will continue to improve primary recovery. In addition, the University of Wyoming, along with industry partners, is establishing a laboratory to identify the well fracturing, design, and completion technologies that will enhance production from the Powder River Basin’s more-challenging Mowry and Belle Fourche shale reservoirs.
References
Blackstone, D.L., Jr., 1981, Compression as an agent in deformation of the east-central flank of the Bighorn Mountains, Sheridan and Johnson counties, Wyoming: Contributions to Geology, v.19, no. 2, p. 105–122.
Clayton, J.L., and Ryder, R.T., 1984, Organic geochemistry of black shales and oils in the Minnelusa Formation (Permian and Pennsylvanian), Powder River Basin, Wyoming, in Woodward, J., Meissner, F.F., and Clayton, JL., eds., Hydrocarbon source rocks of the greater Rocky Mountain region: Denver, Colo., Rocky Mountain Association of Geologists, p. 231–244.
Curry, W.H., Ill, 1971, Laramide structural history of the Powder River Basin, Wyoming: Casper, Wy., Wyoming Geological Association, 23rd annual field conference, Guidebook, p. 49–60.
Dolton, G.L., Fox, J.E., and Clayton, J.L., 1990, Petroleum geology of the Powder River Basin, Wyoming and Montana: U.S. Geological Survey Open-File Report 88-450-1), 64 p.
Hughes, L., 1983, Case histories of an electromagnetic method for petroleum exploration: Prepared by Zonge Engineering and Research Organization, Inc., Tucson, Ariz., 332 p.
Momper, J.A., and Williams, J.A., 1984, Geochemical exploration in the Powder River Basin, in Demaison, G., and Murris, R.J., eds., Petroleum geochemistry and basin evaluation: Tulsa, Okla., American Association of Petroleum Geologists Memoir 35, p. 181–191.
WOGCC, 2024, Wyoming Oil and Gas Conservation Commission website, accessed March 2024, at http://pipeline.wyo.gov/legacywogcce.cfm.