Big horn basin

From SEG Wiki
Jump to navigation Jump to search

This page is currently being authored by a student at the University of Oklahoma. This page will be complete by May 6, 2020.


The Bighorn Basin is a large sedimentary and structural basin located in the states of Wyoming and Montana in the western US. The boundaries of the basin are defined by Laramide (Late Cretaceous through Eocene) faulted and folded uplifts (Beartooth, Absaroka, Owl Creek, and Bighorn Mountains), and the northernmost boundary is constrained by the Nye-Bowler lineament. The Phosphoria Formation is the dominant source of oil in the basin although some Cretaceous oil sources have also been recognized. [1] Most of the petroleum that has been produced from the Bighorn Basin is from Paleozoic reservoirs of the Permian Phosphoria, the Pennsylvanian Tensleep Sandstone, and the Mississippian Madison Limestone formations. [2] The earliest commercial hydrocarbon production from Cretaceous reservoirs in the basin was established at Garland field (1906) and Greybull field (1907). [3] Since then, cumulative production from Cretaceous and Tertiary reservoirs is about 94 million barrels of oil (MMBO) and 830 billion cubic feet of gas (BCFG), primarily from sandstone reservoirs associated with anticlinal traps around the margins of the basin. [4] During the Permian, the Phosphoria Basin occupied what is now southwestern Montana, eastern Idaho, northern Utah, and western Wyoming.


Lower Cretaceous Rocks

During the lower cretaceous era, three rock formations formed in the Big Horn Basin. The cloverly formation was the first of the three to form. The basal Cretaceous rocks in the Bighorn Basin are represented by the Cloverly Formation consisting of 210 to 385ft of interbedded sandstone, variegated shale, claystone, and minor amounts of conglomerate. [5] The Thermopolis Shale formed next and consists of 125 to 230 ft of marine shales and siltstones and represents continued deposition during sea-level rise in Albian (Early Cretaceous) time. The last to form in the lower cretaceous time was the muddy sandstone. The Muddy Sandstone is composed of very fine to medium-grained sandstone interbedded with minor amounts of shale, siltstone, carbonaceous shale, and coal of latest Albian (Early Cretaceous) age. The formation was deposited in fluvial, marginal marine, and estuarine environments and ranges in thickness from 7 to 125 ft. [6]

Upper Cretaceous Rocks

The Mowry shale consists of two sections, an upper and a lower section. The lower part is about 160 to 400 ft of soft fissile clay-rich shale similar to the Thermopolis Shale. The upper part is about 240 to 400 ft of hard brittle siliceous shale. The upper part of the Mowry Shale was deposited in marine environments ranging from prodelta deposits along the western margin of the seaway to fine-grained deposits that accumulated in an oxygen-starved offshore marine basin. [7] The combined thickness for the lower and upper parts of the Mowry range from about 400 ft in the southeastern part of the basin to more than 800 ft near the Beartooth uplift. The next to form was the frontier formation. The Frontier Formation consists of sandstone, siltstone, shale, and bentonite that accumulated in a marine or marginal marine setting. In the western and northwestern parts of the basin, the Frontier also includes some nonmarine strata including minor amounts of carbonaceous shale and coal. [8] The Cody Shale in the Bighorn Basin consists of marine shale, sandstone, and siltstone deposited during a major marine transgressive-regressive cycle that extended from Coniacian to Campanian (Late Cretaceous) time. The Cody ranges in thickness from about 1,700 ft in the northern part of the basin to nearly 3,800 ft in the southeastern part of the basin. [9] The Mesaverde Formation consists of basal regressive marginal marine sandstone, overlain by interbedded nonmarine sandstone, siltstone, shale, carbonaceous shale, and coal deposited in coastal plain and marginal marine environments as the western shoreline of the Cretaceous sea retreated eastward across the Bighorn Basin. The Mesaverde is Campanian (Late Cretaceous) in age and can be subdivided into a lower member, a middle or main part, and the Teapot Sandstone Member.[10]

Paleocene Rocks

The Fort Union Formation, referred to as the Polecat Bench Formation, is comprised of sandstone, siltstone, conglomeratic sandstone, conglomerate, carbonaceous shale, and coal . The conglomerates accumulated mainly as alluvial fans along the northwestern margin of the basin, and as a braidplain system that flowed northeast into the basin from surrounding highlands in the southwestern part of the basin. The sandstones are very fine to coarse-grained and represent a variety of fluvial deposits, whereas the finer grained and carbonaceous deposits represent lacustrine and swamp deposits. [11] The Fort Union is generally less than 1,000 ft thick around the margins of the basin, thinning locally to about 300 ft in the north, and thickening to more than 7,500 feet in the structurally deepest central part of the basin.

Depositional Environment

During much of Cretaceous time, the part of Wyoming and Montana that is now the Bighorn Basin was located near the western edge of the Rocky Mountain foreland basin, an elongate north-south structural depression that developed to the east of the tectonically active Western Cordilleran highlands. Throughout much of its history, the foreland basin was flooded by a broad epicontinental sea, referred to as the Western Interior Seaway (WIS) that developed in response to foreland basin subsidence and eustatic sea-level rise. [12] At its maximum extent, the seaway extended a distance of more than 3,000 mi from the Arctic Ocean to the Gulf of Mexico. [13]

Fluctuations in relative sea level and variations in sediment supply along the western shoreline of the seaway during much of Cretaceous time resulted in a complex pattern of intertonguing marine, marginal marine, and nonmarine deposits. Marine deposition ended near the close of the Cretaceous Period (early Maastrichtian), as the foreland basin gradually filled and the western shoreline of the seaway retreated eastward. The uppermost Cretaceous strata also record the onset of the Laramide orogeny, a period of crustal instability and compressional tectonics that commenced in early Maastrichtian time and fragmented the Rocky Mountain foreland basin into numerous smaller structural basins that were flanked by rising basement-cored uplifts. [14] Basins such as the Bighorn Basin subsided rapidly and became deposition centers for thick accumulations of clastic debris eroded from the surrounding uplifts during latest Cretaceous and Paleocene time. During Eocene time, the basin continued to be filled with volcaniclastic debris that originated in the Yellowstone-Absaroka volcanic area to the west. [15]

Tectonic History

Present-day structure of the Bighorn Basin developed primarily during the Laramide orogeny, a period of crustal instability that began during Late Cretaceous time and ended in early Eocene time. Many of the structures are the result of compressional deformation characterized by Precambrian basement-involved thrust faults (thick-skinned), wrench faults, and strongly folded and faulted anticlines and synclines. The northeast, east, and south margins of the basin are formed by basement-cored uplifts referred to as the Pryor, Bighorn, and Owl Creek Mountains, respectively. These uplifts are flanked by highly folded and faulted sedimentary rocks that range from Cambrian to Paleocene in age, whereas the central part of the basin is covered by nearly flat-lying lower Eocene and undifferentiated Tertiary and Quaternary rocks that mask the structure of the older rocks in the central part of the basin. [16] Major thrust faults in the Bighorn Basin include the Beartooth fault, the Line Creek fault, and the Oregon Basin fault on the west side of the basin, the Elk Basin fault in the north-central part, and the Rio thrust fault on the east.

The Beartooth fault is the major basin-bounding fault in the northwestern part of the basin. The fault plane dips to the west-southwest and displacement is generally to the east and northeast, thrusting the Beartooth Mountains over Paleozoic and Mesozoic sedimentary rocks with approximately 7.5 mi of overhang along the northwest margin of the basin. [17]

The Line Creek fault, located in the western part of the basin, is an east-directed, west-dipping thrust fault that originates in Precambrian rocks and involves strata as young as the Paleocene Fort Union Formation. The fault starts near the Montana-Wyoming state line, and extends south to southeast approximately 40 mi. [18]

The Oregon Basin fault, the major thrust fault along the west side of the basin, trends north-south to northwestsoutheast for about 80 mi and separates the relatively undeformed central deep trough of the basin to the east from the highly deformed basin margin on the west.

East of and parallel to the northern segment of the Oregon Basin fault is the Elk Basin fault, a west-dipping, east-directed thrust fault rooted in Precambrian crystalline rocks and terminating in Upper Cretaceous rocks in the subsurface.[19] The fault trends north-south about 25 mi but may extend southward several miles, merging with the Oregon Basin fault in the deep part of the basin trough.

The most important thrust fault on the east side of the Bighorn Basin is the Rio thrust. It trends about 35–40 mi northwest-southeast, dips east-northeast, is westsouthwest-vergent, and separates the relatively undeformed central deep part of the Bighorn Basin on the west from the highly deformed basin margin on the east. The fault originates in Precambrian basement rocks, with displacements of around 3,000 to 4,000 ft, and dies out in Cretaceous rocks in the subsurface.

The Tensleep fault, located in the southern part of the basin, is a major high-angle fault that extends from the Precambrian core of the Bighorn Mountains due west about 30 mi where it is obscured by Upper Cretaceous sedimentary rocks.

Around the margins of the Bighorn Basin are numerous anticlines, many of which are located on the hanging walls of the Rio, Oregon Basin and Line Creek thrust faults. These folds involve sedimentary rocks ranging from Paleozoic through Paleocene in age, are typically asymmetric, commonly thrust fault-bounded along their steep flanks, and generally trend northwest-southeast. Many important oil and gas fields are associated with these features—for example, Oregon Basin, Little Buffalo Basin, and Grass Creek fields. [20]

Tectonic map of big horn basin.png

Tectonic map of Bighorn Basin, Wyoming and Montana, showing major structural features (1) Beartooth thrust fault, (2) Line Creek thrust fault, (3) Oregon Basin thrust fault, (4) Elk Basin thrust fault, (5) Rio thrust fault, (6) Tensleep fault, western extension dashed where inferred, (7) Oregon Basin anticline, (8) Grass Creek anticline, and (9) Five Mile trend. Courtesy of the united states geological survey


Unconventionals History

The Lower Cretaceous Mowry Shale is a major source rock in the northern Rocky Mountain region The source rock contains primarily Type II organic matter with an admixture of Type III kerogen towards the west. Thermally mature Mowry Shales are closely associated with petroleum accumulations in both Lower and Upper Cretaceous reservoirs. The Mowry Shale has produced modest amounts of oil from older vertical wells in the Powder River basin. With technology improvements (horizontal drilling and multistage hydraulic fracture stimulation) over the past decade, the Mowry is now a prospective target for hydrocarbon production. Horizontal drilling is currently targeting the Mowry in the deeper parts of the Powder River Basin. The Mowry Shale overlies the Shell Creek Shale or Muddy/Newcastle sandstones and is overlain by the Frontier Formation or Belle Fourche Shale. The Mowry petroleum system consists of the Mowry source rock and the following Cretaceous reservoirs: Lakota, Fall River, Muddy, Newcastle, Mowry, Frontier, and Turner. [21]

Future Unconvetionals

Unconventional gas is a potentially large resource, although it is technically challenging to develop. Three types of unconventional gas have potential for future development within the Big Horne Basin. The first potential unconventional location is located in tight sands such as sandstone or carbonate with low permeability, which prevents the gas from naturally flowing to a borehole. The next potential unconventional type is coal-bed natural gas. Which is formed in coal deposits and adsorbed by coal particles. The last unconventional type is fractured shale gas that is formed in fine-grained shale rock with low permeability in which gas has been adsorbed by clay particles or is held within minute pores and microfractures. In addition, there is some potential for unconventional oil reservoirs. Specifically, fractured shale oil reservoirs, formed in fine-grained shale rock with low permeability could be present in the Big Horne Basin.

Risks and Uncertainties

There is a new potential for Muddy oil unconventionals in the Big Horne. This potential for new Muddy oil discoveries to be made on the west side of the basin in narrow, overpressured channel systems, could lead to profitable wells. These unconventionals come with a lot of risk but the biggest risk will be to locate channel systems containing porous and permeable sandstone. There is oil and gas still down in the formations in the ground, but the chances of ending up in an area with very low permeability is high.


  1. Lillis, Paul & Selby, David. (2013). Evaluation of the rhenium-osmium geochronometer in the Phosphoria petroleum system, Bighorn Basin of Wyoming and Montana, USA. Geochimica et Cosmochimica Acta. 118. 312-330. 10.1016/j.gca.2013.04.021.
  2. Stone, D.S., 1967. Theory of Paleozoic oil and gas accumulation in Big Horn basin, Wyoming. American Association of Petroleum Geologists Bulletin 51, 2056–2114.
  3. Dolton, G.L., and Fox, J.E., 1996, Powder River Basin Province (033), in Gautier, D.L., Dolton, G.L., Takahashi, K.I., and Varnes, K.L., eds., 1995 National Assessment of Oil and Gas Resources—Results, methodology, and supporting data: U.S. Geological Survey Digital Data Series DDS-30, one CD-ROM, Release 2.
  4. Finn, Thomas M., et al. “Cretaceous–Tertiary Composite Total Petroleum System (503402), Bighorn Basin, Wyoming and Montana.” United States Geological Survey, 2010 pg 1.
  5. Keefer, W.R., Finn, T.M., Johnson, R.C., and Keighin, C.W., 1998, Regional stratigraphy and correlation of Cretaceous and Paleocene rocks, Bighorn Basin, Wyoming and Montana, in Keefer, W.R., and Goolsby, J.E., eds., Cretaceous and lower Tertiary rocks of the Bighorn Basin, Wyoming and Montana: Wyoming Geological Association 49th Guidebook, p. 1–30
  6. Finn, T.M., 2007, Subsurface stratigraphic cross sections of Cretaceous and Lower Tertiary rocks in the Wind River Basin, central Wyoming, Petroleum Systems and Geologic Assessment of Oil and Gas Resources in the Wind River Basin Province, Wyoming: U.S. Geological Survey Digital Data Series DDS–69–J, Chapter 9, 28 p., CD–ROM
  7. Davis, H.R., Byers, C.W., and Pratt, L.M., 1989, Depositional mechanisms and organic matter in Mowry Shale (Cretaceous), Wyoming: American Association of Petroleum Geologists Bulletin, v. 73, no. 9, p. 1103–1116
  8. Siemers, C.T., 1975, Paleoenvironmental analysis of the upper Cretaceous Frontier Formation, northwestern Bighorn Basin, in Exum, F.A., and George, G.R., eds., Geology and mineral resources of the Bighorn Basin: Wyoming Geological Association 27th Annual Field Conference Guidebook, p. 85–100.
  9. Merewether, E.A., 1996, Stratigraphy and tectonic implications of upper Cretaceous rocks in the Powder River Basin, northeastern Wyoming and southeastern Montana: U.S. Geological Survey Bulletin 1917-T, p. T13
  10. Severn, W.P., 1961, General stratigraphy of the Mesaverde Group, Bighorn Basin, Wyoming, in Wiloth, G.J., Hale, L.A., Randall, A.C., and Garrison, L., eds., Symposium on late Cretaceous rocks Wyoming and adjacent areas: Wyoming Geological Association 16th Annual Field Conference Guidebook, p. 195–199.
  11. Bown T.M., 1975, Paleocene and Lower Eocene rocks in the Sand Creek-No Water area, Washakie County, Wyoming, in Exum, F.A., and George, G.R., eds., Geology and mineral resources of the Bighorn Basin, Wyoming: Wyoming Geological Association 27th Annual Field Conference Guidebook, p. 55–61.
  12. Steidtmann, J.R., 1993, The Cretaceous foreland basin and its sedimentary record, in Snoke, A.W., Steidtmann, J.R., and Roberts, S.M., eds., Geology of Wyoming: Geological Survey of Wyoming Memoir No. 5
  13. Kauffman, E. G., 1977, Geological and biological overview; Western Interior Cretaceous basin: The Mountain Geologist, v. 14
  14. Dickinson, W.R., Klute, M.A., Hayes, M.J., Janecke, S.U., Lundin, E.R., McKittrick, M.A., and Olivares, M.D., 1988, Paleogeographic and paleotectonic setting of Laramide sedimentary basins in the central Rocky Mountain region: Geological Society of America Bulletin, v. 100, p. 1023
  15. Finn, Thomas M., et al. “Cretaceous–Tertiary Composite Total Petroleum System (503402), Bighorn Basin, Wyoming and Montana.” United States Geological Survey, 2010 pg 7.
  16. Gries, R.R., 1981, Oil and gas prospecting beneath the Precambrian of foreland thrust plates in the Rocky Mountains: The Mountain Geologist, v. 18, no. 1, p. 1–18. Gries, R.R., 1983a, North-south compression of Rocky Mountain foreland structures, in Lowell, J.D., and Gries, R.R., eds., Rocky Mountain foreland basins and uplifts: Rocky Mountain Association of Geologists, p. 9–32.
  17. Bonini, W.E., and Kinard, R.E., 1983, Gravity anomalies along the Beartooth front, Montana—Evidence for a lowangle thrust, in Boberg, W.W., ed., Geology of the Bighorn Basin: Wyoming Geological Association Guidebook, p. 89–94.
  18. Blackstone, D.L., Jr., 1986a, Structural geology—Northwest margin, Bighorn basin—Park County, Wyoming and Carbon County, Montana, in Garrison, P.B., ed., Geology of the Beartooth uplift and adjacent basins: Yellowstone Bighorn Research Association-Montana Geological Society 50th Anniversary Guidebook, p. 125–135.
  19. Stone, D.S., 1983, Seismic profile—South Elk basin, in Bally, A.W., ed., Seismic expression of structural styles: American Association of Petroleum Geologists Studies in Geology Series no. 15, v. 3. p. 3.2.2-20–3.2.2-24.
  20. Johnson, R.C., and Finn, T.M., 1998a, Structure contour map on top of the Upper Cretaceous Mesaverde Formation, Bighorn Basin, Wyoming and Montana, in Keefer, W.R., and Goolsby, J.E., eds., Cretaceous and Lower Tertiary rocks of the Bighorn Basin, Wyoming and Montana: Wyoming Geological Association 49th Guidebook, p. 197–198.
  21. Sonnenberg, Steve. “The Niobrara Formation in the Southern Powder River Basin, Wyoming: An Emerging Giant Continuous Petroleum Accumulation.” Proceedings of the 6th Unconventional Resources Technology Conference, 2018, doi:10.15530/urtec-2018-2901558.

Further Reading

External Links