Geologic Assessment of Undiscovered Oil and Gas Resources of the Mowry Composite Total Petroleum System, Greater Green River Basin, Colorado, Utah, and Wyoming

Mark A. Kirschbaum and Laura N. R. Roberts
U.S. Geological Survey, Denver, CO

The Mowry Composite Total Petroleum System (TPS) encompasses the entire Greater Green River Basin, which includes parts of southwestern Wyoming, northwestern Colorado, and northeastern Utah. It is a composite system because hydrocarbons were generated from multiple source rocks, but mainly from the Mowry Shale. Initial petroleum generation and eastward migration from the foredeep basin, now preserved in the Idaho-Wyoming thrust belt, took place from about 100 to 80 million years ago (Ma). Later petroleum generation took place during the Late Cretaceous through Miocene (80-5 Ma) from source rocks buried during Laramide basin development. Hydrocarbons migrated into fluvial, tidal, deltaic, and shoreface sandstone reservoirs of the Bear River, Frontier, Cloverly, and Dakota Formations, and the Muddy Sandstone, and were trapped in structural, stratigraphic, and basin-centered accumulations. Seals include thick, continuous marine shale, diagenetic seals, and presumably capillary-pressure seals.

Two assessment units (AU) were defined for the Mowry Composite TPS. Mean resource estimates for the Mowry Conventional Oil and Gas AU totaled 6.6 million barrels of oil (MMBO), 1.4 trillion cubic feet of gas (TCFG), and 5.5 million barrels of natural gas liquids (MMBNGL). Mean resources estimates for the Mowry Continuous Gas AU were 8.5 TCFG and 171 MMNGL. Total gas resources for both assessment units having potential for production over the next 30 years were estimated at a mean of about 8.8 TCFG with a range from 6.8 to 10.9 TCFG.