Undiscovered Oil and Gas in the Mowry Shale System of Southwestern Wyoming

The U.S. Geological Survey (USGS) assessed undiscovered, technically recoverable conventional and continuous (unconventional) oil and gas in the Mowry Composite Total Petroleum System, of Early to Late Cretaceous age (Albian to Coniacian), in the Southwestern Wyoming Province. The system covers the entire province: most of southwestern Wyoming plus small parts of northwestern Colorado and northeastern Utah.

Three conventional and two continuous assessment units in the Mowry Composite Total Petroleum System. U.S. Geological Survey.

The USGS last assessed the system in 2005, when it defined one conventional and one continuous assessment unit that overlapped. This time it defined seven, based on how the rocks vary across the area, their thickness, how they were deposited, natural fractures, and the thermal maturity of the source rock:

• Conventional: Mowry Eastern Conventional Oil and Gas, Mowry Rock Springs Uplift Conventional Gas, and Mowry Moxa Arch Conventional Oil and Gas
• Continuous: Mowry Moxa Arch Continuous Gas, Mowry Deep Continuous Gas, Mowry Shale Oil, and Mowry Shale Gas

The rocks

From oldest to youngest, the system comprises the Lower Cretaceous Cloverly Formation and Dakota Sandstone, Thermopolis Shale and Muddy Sandstone Member, and Shell Creek Shale, and the Upper Cretaceous Mowry Shale and Frontier Formation.

Type log of the Federal-Culver 1 well, showing the system's rock layers. U.S. Geological Survey.

The Mowry Shale is a marine, organic-rich, silica-rich shale laid down early in the Greenhorn transgressive cycle, with up to 5% organic carbon by weight. It is the system's main source of oil and gas, though the Permian Phosphoria Formation may also contribute. Westward, toward the old source of sediment, it changes from oxygen-poor, finely layered mudstone into oxygenated, burrowed mudstone and sandstone. The main reservoirs are river, estuary and nearshore marine sandstones of the Dakota Sandstone, Muddy Sandstone Member and equivalent rocks, and the Frontier Formation. Sandstone reservoirs occur throughout, and most gas production is along the Moxa Arch and Rock Springs Uplift.

The assessment units

Conventional units. Uplifts bound the Eastern and Rock Springs Uplift units, and their deep edge is set where the top of the Mowry Shale is 13,000 feet deep (measured depth); below that the rocks are overpressured with very tight sandstones. Their sandstones have normal pressure, average porosity above 10%, permeability above 0.1 millidarcy, and clear oil-water or gas-water contacts. Traps are mainly structural, such as anticlines and faults, and abundant faults and natural fractures are critical to well performance. The Moxa Arch conventional unit is bounded by uplift to the south, by the 3,000-foot thickness contour of the overlying Hilliard Shale to the east and west, and by the northernmost oil-gas-water contact along the arch. Its wells are among the deepest in the province yet have some of the lowest gas-oil ratios, which researchers attribute to cold groundwater recharging along faults beside the Uinta Mountains Uplift.

Continuous gas units. The Moxa Arch continuous gas unit lies north of the conventional one, bounded by the Wyoming Thrust Belt and the 13,000-foot cutoff. Its gas is a merged "regional" accumulation with no distinct gas-water contacts, in overpressured reservoirs with porosity below 10% and permeability below 0.1 millidarcy, and production benefits from natural fractures along the arch crest. The Deep continuous gas unit covers the area between the 13,000-foot cutoff and 19,000 feet; deeper areas, in the Great Divide, Washakie, Sand Wash and northern Green River Basins, were excluded as too deep to drill horizontally with current technology.

The two continuous shale assessment units. U.S. Geological Survey.

Shale units. The Mowry Shale Oil and Shale Gas units lie only on the east side of the Rock Springs Uplift. Their western edge is the change to oxygenated, burrowed mudstone, where the shale formed in shallower water closer to the sediment source and has less petroleum potential; data from the neighboring Wind River Basin also show organic content falling from east to west. The Shale Gas unit covers the area where thermal maturity exceeds 1.3% vitrinite reflectance, excluding areas deeper than 19,000 feet. Because no horizontal Mowry Shale wells had produced in the province, production data from the Powder River, Wind River and Bighorn Basins were used as analogs.

The estimates

The conventional units hold a mean of 20 million barrels of oil and 112 billion cubic feet of gas; the continuous units, 453 million barrels of oil and 27,193 billion cubic feet of gas. By assessment unit, mean estimates were:

F95 means a 95% chance of at least that amount; results are fully risked.

Sources

• U.S. Geological Survey, "Assessment of conventional and continuous oil and gas resources in the Mowry Composite Total Petroleum System in the Southwestern Wyoming Province," Fact Sheet 2025–3023, by the Mowry Shale Assessment Team (Jane S. Hearon and others). https://pubs.usgs.gov/publication/fs20253023/full
• Assessment input data: Hearon (2025); USGS Energy Resources Program: https://www.usgs.gov/programs/energy-resources-program
• The fact sheet defines both the Mowry Shale Oil and Mowry Shale Gas units by the area above the 1.3% vitrinite reflectance contour "for gas"; the threshold is given here only for the gas unit.