Oil and Gas Still in the Niobrara of Southwestern Wyoming: The USGS Estimate

The U.S. Geological Survey has estimated how much oil and gas could still be recovered, with today's technology, from the Niobrara Formation in the eastern Southwestern Wyoming Province — east of the Rock Springs uplift, in the Sand Wash, Washakie and Great Divide basins of southwestern Wyoming and northwestern Colorado. It is a continuous (unconventional) resource: oil and gas held throughout the rock rather than pooled in traps.

The Niobrara oil and gas assessment units in the Southwestern Wyoming Province, Wyoming and Colorado. USGS

A sea across the continent

In the Late Cretaceous (late Turonian to early Campanian), the Western Interior Seaway ran from today's Gulf through the middle of North America to Arctic waters in the Canadian Rockies. The Niobrara was laid down in it, and records one full rise and fall of sea level. Much later, Laramide mountain-building (Late Cretaceous to Eocene) raised the structures that shape the province today.

Here, near the seaway's western shore, the Niobrara differs from the same formation in the Powder River and Denver basins. It is mostly limy, but sediment eroded from the Sevier highlands to the west diluted it: the rock is limier to the east and becomes thicker, muddier and more siliceous westward, until near the Rock Springs uplift it passes into the Baxter Shale. Its most limy unit, the Fort Hays Limestone Member, marks the sea at its highest.

A system that is its own source

The Niobrara is a self-sourced petroleum system: the same rock generated the oil and gas and holds them.

• Marine organic matter, which yields oil, is preserved mainly in the limy grains; land-derived organic matter, which yields gas, in the silt and clay.
• The richest rock is the marls — up to 4% organic carbon — informally the Buck Peak, Tow Creek and Wolf Mountain benches; they are both source and main reservoir, and each shows up on well logs as a rise in gamma ray and resistivity.
• Interbedded mudstones and the overlying Mancos Shale seal them.
• Natural fractures around Laramide folds, reopened by later stretching, improve production.

Two assessment units

Almost all of both units is untested, so per-well recoveries were borrowed from analogs: oil wells in the Sand Wash and Powder River basins, vertical gas wells in the Denver–Julesburg Basin. The system's eastern edge follows Niobrara outcrops; its western edge was drawn where source-rock analyses show oil-prone marine matter giving way to gas-prone matter from the land.

The estimate

F95 is the amount with a 95 percent chance of being there; F5, a 5 percent chance.

Sources

Written from Assessment of Continuous Oil and Gas Resources in the Upper Cretaceous Niobrara Formation in the Southwestern Wyoming Province, U.S. Geological Survey Fact Sheet; a work of the United States government in the public domain. It draws on Kauffman (1977), Barlow and Kauffman (1985), Longman and others (1998), Vincelette and Foster (1992), Finn and Johnson (2005), Dellenbach (2016), Drake and Hawkins (2021), Dreier and Warden (2021), Olson and others (2022) and Timm (2025).