Greater Green River Basin Geology
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 20th 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, 2024). The giant Pinedale and Jonah gas fields, (Wyoming’s 1st and 3rd 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, 2024).
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, 2024, Wyoming Oil and Gas Conservation Commission website, accessed March 2024, at http://pipeline.wyo.gov/legacywogcce.cfm.