Bighorn Basin Geology

Cross Sections

Geologic Map

Type Log

The Bighorn Basin is an elongate, northwest-trending structural basin in north-central Wyoming. It is approximately 193 km (120 mi) long and up to 145 km (90 mi) wide. Along the axis of the basin, the total thickness of Paleozoic, Mesozoic, and Cenozoic rocks exceeds 7,620 m (25,000 ft). The basin is bounded on the north and east by the Pryor and Bighorn mountains, and on the south and west by the Owl Creek, Absaroka, and Beartooth mountains.

The present structural configuration of the Bighorn Basin resulted from the Late Cretaceous through Early Eocene Laramide orogeny (Blackstone, 1963), during which the peripheral mountain uplifts experienced their major growth. The folding and faulting that formed oil-producing anticlines in the Bighorn Basin occurred during pulses of compressional stress, mainly oriented northeast-southwest.

The source of the oil and gas found in the basin’s Paleozoic reservoirs is the dark, phosphatic fine-grained, marine facies of the Phosphoria Formation (Stone, 1967). Primary hydrocarbon migration began immediately after deposition of Triassic sediments and was completed by Early Jurassic time.

Both structural and stratigraphic traps occur in Paleozoic and Cretaceous source-rock/reservoir systems in the Bighorn Basin. Structural plays include basin margin subthrusts, basin margin anticlines, deep basin structures, and sub-Absaroka-volcanics (Fox and Dolton, 1996). Later Laramide folding may have been superimposed on or near the primary structural traps. Although most of the basin’s production comes from anticlinal or other structural traps, Lawson and Smith (1966) suggest that many of the structurally-controlled traps are influenced by stratigraphic effects, including intraformational variations in permeability and, as in the Bonzanza and Nowood fields, incised channels in the Tensleep surface that were filled with impervious Goose Egg sediments.

Pure stratigraphic traps are also productive within the Bighorn Basin, including Phosphoria Formation pinchouts (up-dip facies change), Tensleep Sandstone paleogeography (dune fields versus interdune regions), and irregular truncation of thick Tensleep Sandstone beds prior to deposition of the Phosphoria/Goose Egg Formation (Stone, 1967; Fox and Dolton, 1996). The largest of these is the Cottonwood Creek field in the southeast corner of the basin, a trap resulting from an eastward, updip facies change from Phosphoria carbonate to the impermeable red shale and anhydrite facies of the Goose Egg Formation. Oil and gas in some of these stratigraphic traps were later released by fracturing and faulting associated with Laramide folding. During the Laramide orogeny, these hydrocarbons moved into older Paleozoic reservoir rocks and older structures where they were trapped in pools. The occurrence of a common oil-water contact is attributed to fractures joining the reservoirs. The oil-water contact is also often tilted as a result of hydrodynamic flow (Stone, 1967).

Mesozoic formations produce a much lower percentage of the Bighorn Basin’s oil and gas, with most production coming from the Upper Cretaceous Frontier Formation. Source rocks in the Mesozoic include the Cody, Frontier, Mowry, and Thermopolis black shale units (Stone, 1967).

Production

The Bighorn Basin is primarily an oil-producing basin (WSGS oil and gas map). Oil was first discovered in the basin in 1904 as a spring on the Bonanza anticline. Oil has since been produced from more than 125 fields in the basin and from more than 30 reservoirs and/or co-mingled reservoirs ranging in age from Cambrian to Paleocene. Seven of these fields are in the top 10 cumulative oil-producing fields in Wyoming (WOGCC, 2023).

Future Development

Despite the decreasing production levels, most fields in the Bighorn Basin still contain a significant quantity of recoverable oil. In response, operators are utilizing secondary and tertiary recovery techniques to revitalize old fields and to activate fields that were not economically feasible in years past. Future oil production in the Bighorn Basin will rely on the success of these recovery techniques.

Unconventional reservoir plays could also improve oil and gas production in the Bighorn Basin. Fox and Dolton’s (1996) resource assessment identified basin center/deep gas and coalbed natural gas as significant potential future plays within the basin. Many of the same Cretaceous formations currently being exploited as unconventional reservoirs in other Wyoming basins also exist in the Bighorn Basin. Future exploration for similar unconventional plays, horizontal drilling, and hydraulic fracturing could again make the Bighorn Basin a major player in state oil and gas production.

►References

Blackstone, D.L., Jr., 1963, Development of geologic structures in central Rocky Mountains, in Childs, O.E., and Beebe, B.W., eds., Backbone of the Americas—Tectonic history from pole to pole: Tulsa, Okla., American Association of Petroleum Geologists Memoir, p. 160–179.

Fox, J.E., and Dolton, G.L., 1996, Petroleum geology of the Bighorn Basin, north-central Wyoming and south-central Montana, in Bowen, C.E., Kirkwood, S.C., and Miller, T.S., eds., Resources of the Bighorn Basin: Casper, Wyo. , Wyoming Geological Association, 47th annual field conference, Guidebook, p. 19–39.

Lawson, D.E., and Smith, J.R., 1966, Pennsylvanian and Permian influence on Tensleep oil accumulation, Bighorn Basin, Wyoming: American Association of Petroleum Geologists Bulletin, v. 50, no. 10, p. 2,197–2,220.

Stone, D.S., 1967, Theory of Paleozoic oil and gas accumulation in Bighorn Basin, Wyoming: American Association of Petroleum Geologists Bulletin, v. 51, no. 10, p. 2,056–2,114.

WOGCC, 2023, Wyoming Oil and Gas Conservation Commission website, accessed April 2023, at http://pipeline.wyo.gov/legacywogcce.cfm.

Denver Basin Geology

Cross Section (from the Colorado Geological Survey)

Geologic Map

Type Log

The Denver Basin is an asymmetrical Laramide-age basin that covers more than 180,000 square km (70,000 square mi) in parts of Colorado, Wyoming, South Dakota, Kansas, and Nebraska. The bulk of the basin is in Colorado. The basin is often termed the Denver-Julesburg Basin or the Denver-Julesburg-Wattenburg Basin. The Wyoming portion of the Denver Basin is in the southeastern corner of the state, and is bounded on the west by the Laramie Range and on the north by the Hartville Uplift.

The Denver Basin has typical foreland basin-style geometry with a north- south-trending basin axis. The strata on the western side of the basin dip steeply toward the east, while the strata in the eastern Denver Basin gently slope to the west. The basin is more than 3,962 m (13,000 ft) deep, as defined by the 1.6-billion-year-old Precambrian basement. The bulk of the strata preserved in the Denver Basin were deposited during the Cretaceous in the Sevier foreland basin, and later separated from similar strata in other Wyoming basins by the Laramide orogeny. Surface outcrops in the Denver Basin are generally Tertiary in age.

The Silo field, discovered in 1981 (Sonnenberg, 2011), is the most productive oil and (associated) natural gas field in the basin. Production in the Silo field is primarily from the self-sourced Cretaceous Niobrara Formation, which is predominantly fractured chalk (reservoir) encased in tight shales and mudstones (seal). This unconventional reservoir is conducive to horizontal drilling and hydraulic fracturing, which significantly enhance production.

In addition to the Niobrara Formation, other possible source rocks in the Denver Basin include the Cretaceous Belle Fourche (Graneros), Mowry, and Carlile shales, as well as the Greenhorn Formation. Oil generation in these formations began during the Laramide orogeny and continued through the Miocene (Higley and Cox, 2007).

Production

Oil and gas was first discovered in the Denver Basin in 1901, and it now includes approximately 1,500 hydrocarbon fields spanning several states (Higley and Cox, 2007). The Wyoming portion of the Denver Basin has 31 named oil and gas fields, 12 of which are currently producing oil or gas (WSGS oil and gas map). Production from these 12 fields and numerous wildcat wells steadily increased by more than an order of magnitude since 2009, thanks in part to some of Wyoming’s most productive oil wells. Denver Basin wells each consistently average more than 1,200 barrels of oil per month (WOGCC, 2023). Since 2017, the Denver Basin has also accounted for at least 10% of the state’s total oil production (WOGCC, 2023).

Future Development

Although production efforts in the Denver Basin have historically focused on the Niobrara Formation, operators are beginning to explore other unconventional plays in the basin. Horizontal drilling and hydraulic fracturing have increased recent production from the tight sand formations of the Upper Cretaceous Muddy "J" Sandstone and the Codell Sandstone Member of the Carlile Shale. As drilling techniques and reservoir characterization in the Denver Basin are refined and improved, there is the potential for increased production from unconventional reservoirs.

►References

Higley, D.K., and Cox, D.O., 2007, Oil and gas exploration and development along the Front Range in the Denver Basin of Colorado, Nebraska, and Wyoming, in Higley, DK., comp., Petroleum systems and assessment of undiscovered oil and gas in the Denver Basin Province, Colorado, Kansas, Nebraska, South Dakota, and Wyoming—USGS Province 39: U.S. Geological Survey Digital Data Series DDS-69-P, chap. 2, 41 p.

Sonnenberg, S.A., 2011, Silo field summary, in Estes-Jackson, J.E., and Anderson, D.S., eds., Revisiting and revitalizing the Niobrara in the central Rockies: Denver, Colo., Rocky Mountain Association of Geologists, p. 494–497.

WOGCC, 2023, Wyoming Oil and Gas Conservation Commission website, accessed April 2023, at http://pipeline.wyo.gov/legacywogcce.cfm.

Greater Green River Basin Geology

Cross Sections

Geologic Map

Type Log—Moxa Arch

Type Log—Rock Springs Uplift

The Greater Green River Basin encompasses the southwest portion of Wyoming and extends south into northeastern Utah and northwestern Colorado. The footprint of the basin covers 54,269 square km (20,953 square mi) in Wyoming. The Greater Green River Basin is bounded on the west by the Sevier overthrust belt, on the north by the Wind River Range, to the east by the Rawlins Uplift and Sierra Madre Mountain Range, and to the south by the Uinta Mountains.

The Greater Green River Basin is an amalgamation of several sub-basins, including the Green River, Great Divide, Washakie, and Sand Wash basins. These sub-basins were formed during the Late Cretaceous to Early Eocene Laramide orogeny with the uplift of the Moxa arch, Rock Springs Uplift, Cherokee Ridge arch, and Wamsutter arch. An intermittent record of sedimentation from the Cambrian through present is preserved in the basin, with total compacted sediment fill that can be greater than 9,144 m (30,000 ft) thick.

Many oil and gas fields in the Greater Green River Basin occur in anticlinal traps, which are secondary folds on the larger-scale Laramide uplifts. The giant Jonah gas field is another type of structural trap where natural gas is trapped in the Lance Formation sandstones within a fault-bounded wedge (Cluff and Cluff, 2004). With the exception of the Cretaceous formations, stratigraphic traps are rare in the basin. An example of a Cretaceous stratigraphic trap is found in Patrick Draw field, where an up-dip pinch-out of the Almond Formation traps a significant accumulation of oil (Weimer, 1965; Weimer, 1966).

Primary oil production is from the Upper Cretaceous Frontier Formation and Mesaverde Group, followed by the Lower Cretaceous Cloverly Formation. (The Cloverly Formation is often called the "Dakota Sandstone" by the hydrocarbon industry, which is an informal name borrowed from neighboring states. Its official name in Wyoming is the Cloverly Formation.) Carbon dioxide and helium production on the LaBarge platform is from the Mississippian Madison Limestone.

Hydrocarbons are also commonly produced from the Pennsylvanian and Permian Tensleep/Weber sandstones. Nearly all formations are known to contain hydrocarbons at one location or another within the Greater Green River Basin.

Hydrocarbon source rocks vary by location within the section and within the basin. The U.S. Geological Survey (USGS Southwestern Wyoming Province Assessment Team, 2005) determined nine regional total petroleum systems in the southwestern Wyoming province (the bulk of which includes the Greater Green River Basin of Wyoming). These nine systems imply source rocks in the Phosphoria Formation, Mowry/Aspen Shale, Hilliard/Baxter Shale, Niobrara Formation, Mesaverde Group, Lewis Shale, Lance and Fort Union formations, and the Wasatch and Green River formations. Excluding the Phosphoria (Permian), Wasatch and Green River formations (Eocene), and Fort Union Formation (Paleocene), all other source rocks are Cretaceous—primarily Upper Cretaceous.

The source rock facies within the Phosphoria Formation are contained within the Meade Peak and Retort members. The Phosphoria was deposited in a sediment-starved, restricted basin on the western edge of the Wyoming shelf (Piper and Link, 2002). Within this complex, the Meade Peak and Retort members were formed in areas favorable for upwelling, high organic productivity, and preservation of organic matter (e.g., Piper and Link, 2002). Total organic content values are as high as 30 weight percent in this organic-rich source rock. High amounts of sulfur suggest original oil composition within the Phosphoria was Type-IIS kerogen, with oil generation beginning during the Late Cretaceous (Johnson, 2005).

The Cretaceous source rocks were deposited during seaway transgression and regression within a foreland basin that was subsiding due to the advancing Sevier orogeny. These source rocks are all marine shales, some of which were deposited under anoxic conditions that preserved an unusual amount of carbonaceous matter. Of the Cretaceous shales, the Mowry/Aspen Shale has the highest total organic content (Burtner and Warner, 1984) and is primarily responsible for charging the Dakota Sandstone and Frontier Formation reservoirs throughout the Rocky Mountain region (Warner, 1982; Burtner and Warner, 1984), with additional gas locally sourced from the Frontier coals.

Eocene Wasatch and Green River source rocks are lacustrine organic-rich shales and marginal marine and terrestrial coal and carbonaceous mudstones (Roberts, 2005). Lacustrine source rocks contain Type-I and mixed Type-I and Type-Ill kerogen, while the coal and carbonaceous units contain Type-Ill kerogen (Grabowski and Bohacs, 1996; Carroll and Bohacs, 2001). These source rocks are responsible for significant oil shale deposits in the Green River Formation and biogenic gas accumulations (i.e., coalbed natural gas) in both the Wasatch and Green River formations.

Production

The Greater Green River Basin is a mature hydrocarbon province that has been under production since the early 20^th century. There are 295 named fields in the basin, the majority of which have primarily produced natural gas, with some associated oil (WSGS oil and gas map). The basin is home to an accumulation of CO₂ greater than 100 trillion cubic feet on the crest of the Moxa arch, as well as the nation's primary helium reserve. Thirteen of Wyoming’s top 100 highest-producing oil fields and 63 of the state’s top 100 highest-producing gas fields are in the Greater Green River Basin (WOGCC, 2023). The giant Pinedale and Jonah gas fields, (Wyoming’s 1^st and 3^rd most productive gas fields, respectively) and successful CO₂-EOR projects in the Lost Soldier, Wertz, and the Patrick Draw (Monell unit) fields have helped maintain and even increase Greater Green River Basin oil and gas production.

Future Development

In general, drilling has decreased in the Greater Green River Basin over the past few years and production has followed this trend. Production continues to decline from the basin’s 2009 high of 15.9 million barrels of oil and 1.43 trillion cubic feet of gas (WOGCC, 2023).

However, horizontal drilling and hydraulic fracturing in the Washakie Basin, the far eastern flank of the basin, and the giant Jonah and Pinedale fields have resulted in productive natural gas wells. These horizontal wells are targeting the Fort Union and Lance formations and the Mesaverde Group. Continued use of these drilling and production technologies may further increase production from what were historically thought of as tight and unproductive sandstone or shale reservoirs, but are now rather typical unconventional reservoirs. Horizontal projects, along with continued infill drilling and possible development in the Normally Pressured Lance federal project, may help offset the declining production in the Greater Green River Basin. However, as always, production remains susceptible to market factors such as price, supply, and demand.

►References

Burtner, R.L., and Warner, M.A., 1984, Hydrocarbon generation in Lower Cretaceous Mowry and Skull Creek shales of the northern Rocky Mountain areas, in Woodward, J., Meissner, F.F., and Clayton, J.L., eds., Symposium on hydrocarbon source rocks of the greater Rocky Mountain Region: Denver, Co., Rocky Mountain Association of Geologists, p. 449–467.

Carroll, A.R., and Bohacs, K.M., 2001, Lake-type controls on petroleum source rock potential in nonmarine basins: American Association of Petroleum Geologists Bulletin, v. 85, no. 6, p. 1,033–1,053.

Cluff, R.M., and Cluff, S.G., 2004, The origin of Jonah field, northern Green River Basin, Wyoming, in Robinson, J.W., and Shanley, K.W., eds., Jonah field—Case study of a giant tight gas fluvial reservoir: American Association of Petroleum Geologists Studies in Geology 52 and Rocky Mountain Association of Geologists Guidebook, chap. 8, p. 127–145.

Grabowski, G.J., Jr., and Bohacs, K.M., 1996, Controls on compositions and distributions of lacustrine organic-rich rocks of the Green River Formation, Wyoming [abs.]: San Diego, Calif., American Association of Petroleum Geologists and Society of Economic Paleontologists and Mineralogists Annual Meeting, v. 5, p. 55.

Johnson, R.C., 2005, Geologic assessment of undiscovered oil and gas resources in the Phosphoria Total Petroleum System, Southwestern Wyoming Province, Wyoming, Colorado, and Utah, in U.S. Geological Survey Southwestern Wyoming Province Assessment Team, Petroleum systems and geologic assessment of oil and gas in the Southwestern Wyoming Province, Wyoming, Colorado, and Utah: U.S. Geological Survey Digital Data Series DDS-69-D, chap. 4, 46 p.

Law, B.E., 1988, Geologic framework and hydrocarbon plays in the Southwestern Wyoming Basins Province: U.S. Geological Survey Open-File Report 88-450-F, 29 p.

Piper, D.Z., and Link, P.K., 2002, An upwelling model for the Phosphoria sea—A Permian, ocean margin sea in the northwest United States: American Association of Petroleum Geologists Bulletin, v. 86, no. 7, p. 1,217–1,235.

Roberts, S.B., 2005, Geologic assessment of undiscovered petroleum resources in the Wasatch—Green River Composite Total Petroleum System, Southwestern Wyoming Province, Wyoming, Colorado, and Utah, in U.S. Geological Survey Southwestern Wyoming Province Assessment Team, petroleum systems and geologic assessment of oil and gas in the Southwestern Wyoming Province, Wyoming, Colorado, and Utah: U.S. Geological Survey Digital Data Series DDS-69D, chap. 12, 22 p.

USGS Southwestern Wyoming Province Assessment Team, 2005, Petroleum systems and geologic assessment of oil and gas in the Southwestern Wyoming Province, Wyoming, Colorado, and Utah: U.S. Geological Survey Digital Data Series DDS-69-D.

Warner, M.A., 1982, Source and time of generation of hydrocarbons in the fossil basin, western Wyoming thrust belt, in Powers, R.B., ed., Geologic studies of the Cordilleran thrust belt: Rocky Mountain Association of Geologists, p. 805–815.

Weimer, R.J., 1965, Stratigraphy and petroleum occurrences, Almond and Lewis formations (Upper Cretaceous), Wamsutter arch, Wyoming: Casper, Wyo., Wyoming Geological Association, 19th annual field conference, Guidebook, p. 65–81.

Weimer, R.J., 1966, Time-stratigraphic analysis and petroleum accumulations, Patrick Draw field, Sweetwater County, Wyoming: American Association of Petroleum Geologists Bulletin, v. 50, no. 10, p. 2,150–2,175.

WOGCC, 2023, Wyoming Oil and Gas Conservation Commission website, accessed April 2023, at http://pipeline.wyo.gov/legacywogcce.cfm.

Hanna Basin Geology

Cross Section

Geologic Map

Type Log

The Hanna Basin is a small yet anomalously deep (9,144 m, 30,000 ft) intermontaine Laramide-style basin, approximately 64 km (40 mi) east-west and 40 km (25 mi) north-south. The basin is bounded to the north by the Shirley and Seminoe mountains, to the east by Simpson Ridge, to the south by the Medicine Bow Mountains and Park Range, and to the west by the Rawlins Uplift.

The structural development of the Hanna Basin occurred in multiple stages. The Hanna Basin was first isolated from the Greater Green River Basin by the uplift of the Shirley and Granite mountains during the early Paleocene, followed by middle-Paleocene growth of the Sweetwater Uplift. The Medicine Bow Mountains and Rawlins Uplift occurred during the late Paleocene. The Cambrian- through Jurassic-age sedimentary strata that accumulated before the structural development of the Hanna Basin are less than 762 m (2,500 ft) thick.

During the Laramide orogeny, the Hanna Basin was isolated from the surrounding basins and became a closed drainage. This structural configuration resulted in a thick succession of Upper Cretaceous to Lower Paleogene fluvial and lacustrine sedimentary deposits. These fluvial and lacustrine strata account for the bulk of the strata in the basin center, and can be up to 5,791 m (19,000 ft) thick.

Production

Conventional oil and gas exploration occurred in the Hanna Basin throughout the 20th century. There are currently 25 named fields in the Hanna Basin, 13 of which are abandoned or shut-in (WSGS oil and gas map). The basin’s most productive oil field, Big Medicine Bow field, has produced from the Frontier Formation, Muddy Sandstone, Cloverly (Dakota) Formation, Morrison Formation, Sundance Formation, and Tensleep Sandstone (WOGCC, 2023). The most productive gas field, Separation Flats field, produced from the Muddy Sandstone but is no longer active. The Hanna Basin has not been extensively explored for undiscovered petroleum accumulations, and there are potential conventional and unconventional undiscovered accumulations (Dyman and Condon, 2007).

Coal mining has been active in the Hanna Basin since 1868 (Flores and others, 1999). Mines operated at the town site of Carbon until 1900, when mining operations moved to the town of Hanna after the railroad was rerouted. Most of the coal extraction in the Hanna as well as Carbon basins (which is separated from the Hanna Basin by the northeast- southwest-trending Saddleback Hills anticline) has been from the Hanna coal field (Pierce, 1996). Coal is primarily mined from the Upper Cretaceous and Paleocene Ferris Formation, as well as the Paleocene Hanna Formation.

Dyman and Condon (2005) define the Hanna-Mesaverde coalbed gas total petroleum system in the Hanna Basin, as including portions of the Mesaverde (Almond), Medicine Bow, Ferris, and Hanna formations. Two coalbed natural gas (CBNG) pilot projects in the basin, established by 2005, have produced very little gas from this petroleum system. The Seminoe Road CBNG pilot project contained 16 wells that produced 1,400 cubic feet of gas per day; the Hanna Draw CBNG pilot project had nine wells that averaged less than 1,000 cubic feet of gas per day (Dyman and Condon, 2005).

Future Development

Because of the anomalous structure of the Hanna Basin relative to other Laramide basins (that is, it is a small but very deep basin), exploration targets are limited to the flanks of the basin— the basin center is considered too deep for most exploration. There has been very little exploration in the basin over the past few decades, mostly limited to CBNG projects. No drilling projects on federal lands have been proposed in the Hanna Basin.

►References

Dyman, T.S., and Condon, S.M., 2007, 2005 geologic assessment of undiscovered oil and gas resources, Hanna, Laramie, and Shirley basins Province, Wyoming and Colorado, in U.S. Geological Survey Hanna, Laramie, and Shirley Basins Province Assessment Team, Petroleum systems and geologic assessment of undiscovered oil and gas, Hanna, Laramie, and Shirley basins Province, Wyoming and Colorado: U.S. Geological Survey Digital Data Series DDS-69-K, chap. 2, 62 p.

Flores, R.M., Cavaroc, V.V., Jr., and Bader, L.R., 1999, Ferris and Hanna coal in the Hanna and Carbon basins, Wyoming—A synthesis: U.S. Geological Survey Professional Paper 1625-A, chap. HS, 49 p.

Pierce, B.S., 1996, Quality and petrographic characteristics of Paleocene coals from the Hanna Basin, Wyoming: Jackson, Wyo., 12th Annual Meeting of the Society of Organic Petrology, Organic Geochemistry, v. 24, no. 2, p. 181–187.

WOGCC, 2023, Wyoming Oil and Gas Conservation Commission website, accessed April 2023, at http://pipeline.wyo.gov/legacywogcce.cfm.

Laramie Basin Geology

1:100,000-scale bedrock geologic maps

• Geologic Map of the Laramie Quadrangle
• Preliminary Geologic Map of the Rock River 30’ x 60’ Quadrangle

The Laramie Basin, in southeast Wyoming, is a complexly downfolded Laramide basin. It trends north-south and is approximately 80 km (50 mi) long by 50 km (31 mi) wide. The Laramie Basin is bounded by the Medicine Bow Mountains on the west, Hanna Basin on the northwest, Shirley Mountains on the north, and Laramie Mountains on the east.

Overthrust Belt Geology

Cross Sections

The Overthrust Belt is a zone of highly deformed rock layers stretching from northern Alaska to Mexico. The portion of the Overthrust Belt in Wyoming that has been the target of oil and gas exploration efforts is more than 160 km (100 mi) wide and 320 km (200 mi) long. It is bounded to the north by Jackson Hole, Wyoming, to the east by the Darby and Prospect faults, and to the south by the Uinta Uplift.

The Overthrust Belt is not part of the Laramide Basin system, but was instead created by the Cretaceous-age Sevier orogeny approximately 150 million to 55 million years ago. The Sevier orogeny was a shortening event that resulted in "thin-skinned" thrust faults that do not involve the Precambrian basement rocks.

Often termed the Thrust Belt or Sevier Belt, the Overthrust Belt contains a series of anticlinal traps that can store hydrocarbons. The complexity of the Overthrust Belt's geology, including highly folded and faulted strata, has contributed, and continues to contribute, to the difficulty of exploring for oil and gas in this area.

The Jurassic Nugget Sandstone and the Mississippian Madison Limestone have been the most prolific oil and gas producing formations in Wyoming's Overthrust Belt. Other, mostly gas-producing, formations in the Overthrust Belt include the Ordovician Big Horn Dolomite; Pennsylvanian Amsden Formation; Permian Phosphoria Formation and Weber Sandstone; Triassic Thaynes Limestone; Jurassic Twin Creek Limestone; Cretaceous Baxter, Mesaverde, Muddy, and Bear River formations; and Eocene Almy Formation.

The main source rock in the Overthrust Belt is presumed to be the Cretaceous Mowry Shale. The Permian Phosphoria Formation and other Cretaceous organic-rich formations, such as the Bear River and Frontier formations, may also be minor sources of oil and gas in the region (USGS Wyoming Thrust Belt Province Assessment Team, 2003).

Production

Exploration began in the late 1800s and early 1900s in the Overthrust Belt region, primarily in shallow fields associated with oil seeps. These small fields were unsuccessful. Despite the discovery of the large La Barge and Dry Piney fields in the mid-1900s in the transition zone between the Greater Green River Basin and the Overthrust Belt, intensive exploratory efforts did not commence until the discovery of several fields during the mid-1970s (Ver Ploeg, 1979).

Twenty-four fields within the Overthrust Belt have been reported as having produced oil or natural gas (WSGS oil and gas map). In the 1980s, these fields accounted for more than half of the state’s total gas production. While this percent has declined, the Overthrust Belt has continued to contribute between 15 and 19 percent of Wyoming’s natural gas throughout the last decade. Most of the area’s oil and gas production is from the Whitney Canyon-Carter Creek, Painter Reservoir, Painter Reservoir East, and Ryckman Creek fields, with the Whitney Canyon-Carter Creek and Painter Reservoir East fields being the state’s 7th and 8th most productive gas fields, respectively (WOGCC, 2023).

Future Development

No productive wells have been drilled in the area since 2012, and at this point, there appears to be no new Overthrust Belt oil or gas projects on the horizon.

►References

USGS Wyoming Thrust Belt Province Assessment Team, 2003, Assessment of undiscovered oil and gas resources of the Wyoming Thrust Belt Province: U.S. Geological Survey World Energy Assessment Project Fact Sheet.

Ver Ploeg, A.J., 1979, The Overthrust Belt—An overview of an important new oil and gas province: Geological Survey of Wyoming [Wyoming State Geological Survey] Public Information Circular 11, 15 p.

WOGCC, 2023, Wyoming Oil and Gas Conservation Commission website, accessed April 2023, at http://pipeline.wyo.gov/legacywogcce.cfm.

Powder River Basin Geology

Cross Section

Geologic Map

Type Log

Upper Cretaceous Strata

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 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, 2023). Beginning in 2014, more than half of all oil produced in Wyoming each year has come from the Powder River Basin, with a contribution nearing 69 percent in 2022 (WOGCC, 2023).

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, 2022). 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, Turner, and Shannon sandstones were the basin’s five most productive reservoirs in 2022 (WOGCC, 2023). 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, 2023, Wyoming Oil and Gas Conservation Commission website, accessed April 2023, at http://pipeline.wyo.gov/legacywogcce.cfm.

Wind River Basin Geology

Cross Sections

Geologic Map

Type Log

The Wind River Basin, in central Wyoming, is an east-west elongate structural basin of typical Laramide style, 115 km (71 mi) wide by 300 km (186 mi) long. The primary basin axis trends northwest-southeast, and is asymmetrically located near the northern basin margin. The basin is bounded by the Wind River Range on the west, the Owl Creek Mountains on the north, the Casper Arch to the east, and the Granite Mountains to the south.

Significant deformation occurred in the Wind River Basin during the Laramide orogeny that resulted in a basin center greater than 7,620 m (25,000 ft) deep, beds dipping 10–20 degrees toward the basin center on the south and western margins, and near-vertical to overturned strata on the north and eastern margins (Keefer, 1969). A thick succession of undeformed post-Laramide basin-fill strata was deposited unconformably on the pre-Laramide (pre-Eocene) rocks.

In the Wind River Basin, hydrocarbon traps typically consist of structural features such as domes, anticlines, or faulted anticlines (Keefer, 1969) situated on the basin margins in rocks deposited previous to or coincident with Laramide-age faulting. Other types of structural traps include anticlines or domes near the basin axis, traps beneath the basin-bounding thrust faults, or sub-thrust plays (Fox and Dolton, 1996).

Some purely stratigraphic traps are also found in the Wind River Basin. The most frequent stratigraphic traps are lateral up-dip (to the east) facies changes in the Phosphoria, Park City, and Goose Egg formations. Others include sandstone pinch-outs in the reservoirs of the Frontier Formation, Mesaverde Group, and Muddy Sandstone, as well as vertical and lateral cementation variations in Tensleep Sandstone reservoirs. In some cases, structural traps are enhanced by the effects of stratigraphic-trapping. For example, oil and natural gas in Morrison Formation sands are held by a combination structural-stratigraphic trap on the nose of a domal structure in the Poison Spider West field (Gouger, 1989).

The Tensleep Sandstone and Phosphoria and Park City formations comprise the primary reservoir rocks in the basin. The bulk of the hydrocarbons in these formations were sourced from the black shales of the Mead Peak and Retort members of the Phosphoria Formation in western Wyoming and eastern Idaho (Sheldon, 1967; Stone, 1967; Kirschbaum and others, 2007). Migration began soon after generation, and may have been associated with Sevier orogenesis. Hydrocarbons moved up-dip, likely via the porous and permeable Tensleep Sandstone, into the area that is now the Wind River Basin and were trapped by the overlying impermeable Goose Egg Formation (Stone, 1967; Kirschbaum and others, 2007). Laramide faulting and folding was responsible for the subsequent rearrangement of the hydrocarbons into their present day structural and stratigraphic traps. Phosphoria-sourced hydrocarbons are commonly high in sulfur, exhibit high American Petroleum Institute (API) gravities, and are classified as Type-IIS (Kirschbaum and others, 2007).

Production

Oil production in the Wind River Basin and Wyoming began with the completion of the Mike Murphy #1 well in 1884, just five years after America's first commercial oil well was drilled. The Mike Murphy #1 well was completed in the Chugwater Group to a depth of 91 m (300 feet; Mullen, 1989; De Bruin, 2012), and was the discovery well for the Dallas (or Dallas Dome) field in the southwestern Wind River Basin. The basin also contains the first logged well in Wyoming (Atlantic Richfield Company Muskrat 2C, September 1936) and the deepest completed well in the Rocky Mountain region (Burlington Resources Big Horn 1-5, Madden field), which was completed between 23,758 and 23,902 ft in the Madison Limestone.

Of the 118 named fields in the Wind River Basin, 63 primarily produce oil and 55 primarily produce natural gas from reservoirs ranging in age from Mississippian to Eocene (WSGS oil and gas map). Oil production in the Wind River Basin declined from 1978 through 1994, but has since remained relatively steady at 3–5 million barrels per year (WOGCC, 2023). This consistent production is due to the use of efficient secondary and tertiary production techniques such as the CO₂-EOR project at Beaver Creek field.

Future Development

Although it is possible that small undiscovered petroleum accumulations exist in the Wind River Basin (Fox and Dolton, 1996), it is more likely that any significant future increases in oil production will be the result of improved recovery methods.

After fifteen years of declining gas production, natural gas production in the Wind River Basin began to increase in 2020. This upward trend is expected to continue as Aethon Energy and Burlington Resources develop the 4,250 wells in northeastern Fremont County and northwestern Natrona County approved as part of their Moneta Divide project.

►References

De Bruin, R.H., 2012, Oil and gas map of Wyoming: Wyoming State Geological Survey Map Series MS-55, scale 1:500,000.

Fox, J.E., and Dolton, G.L., 1996, Wind River Basin Province (035), in Gautier, D.L., Dolton, G.L., Takashashi, K.I., and Varnes, K.L., eds., 1995 National assessment of United States oil and gas resources—Results, methodology, and supporting data: U.S. Geological Survey Digital Data Series DDS-30, release 2, 21 p.

Gouger, B.l., 1989, Poison spider west, in Cardinal, D.F., Miller, T., Stewart, W.W., and Trotter, J.F., eds., Wyoming oil and gas fields symposium Bighorn and Wind River basins: Casper, Wyo., Wyoming Geological Association, p. 380–383.

Johnson, R.C., Finn, T.M., Kirschbaum, M.A., Roberts, S.B., Roberts, L.N.R., Cook, T., and Taylor, D.J., 2007, The Cretaceous-Lower Tertiary Composite Total Petroleum System, Wind River Basin, Wyoming, in U.S. Geological Survey Wind River Basin Province Assessment Team, Petroleum systems and geologic assessment of oil and gas in the Wind River Basin Province, Wyoming: U.S. Geological Survey Digital Data Series DDS-69-J, chap. 4, 96 p.

Keefer, W.R., 1969, Geology of petroleum in Wind River Basin, central Wyoming: American Association of Petroleum Geologists Bulletin, v. 53, no. 9, p. 1,839–1,865.

Kirschbaum, M.A., Lillis, P.G., and Roberts, L.N.R., 2007, Geologic assessment of undiscovered oil and gas resources in the Phosphoria Total Petroleum System of the Wind River Basin Province, in U.S. Geological Survey Wind River Basin Province Assessment Team, Petroleum systems and geologic assessment of oil and gas in the Wind River Basin Province, Wyoming: U.S. Geological Survey Digital Data Series DDS-69-J, chap. 3, 27 p.

Mullen, C., 1989, Dallas, in Cardinal, D.F., Miller, T., Stewart, W.W., and Trotter, J.F., eds., Wyoming oil and gas fields symposium Bighorn and Wind River basins: Casper, Wyo., Wyoming Geological Association, p. 114–116.

Sheldon, R.P., 1967, Long-distance migration of oil in Wyoming: Mountain Geologist, v. 4, no. 2, p.53–65.

Stone, D.S., 1967, Theory of Paleozoic oil and gas accumulation in Bighorn Basin, Wyoming: American Association of Petroleum Geologists Bulletin, v. 51, no. 10, p. 2,056–2,114.

WOGCC, 2023, Wyoming Oil and Gas Conservation Commission website, accessed April 2023, at http://pipeline.wyo.gov/legacywogcce.cfm.