Overthrust Belt Geology
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
In 2023, the first wells since 2014 were spudded in the Overthrust Belt. These new gas storage and disposal wells indicate that the area will continue to contribute to Wyoming’s natural gas future.
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, 2024, Wyoming Oil and Gas Conservation Commission website, accessed March 2024, at http://pipeline.wyo.gov/legacywogcce.cfm.