Upper Cretaceous and Tertiary Petroleum Systems and Assessment Units, Wind River Basin, Wyoming

Ronald C. Johnson¹, Thomas M. Finn², Stephen B. Roberts², Laura N.R. Roberts², Paul G. Lillis², and Phillip H. Nelson². (1) U.S. Geological Survey, U.S. Geological Survey, Box 25046 MS 939, Denver Federal Center, Denver, CO 80225, phone: 303 236-5546, fax: 303-236-0459, [email protected], (2) U.S. Geological Survey, Denver, CO

Total petroleum systems and assessment units were defined for Upper Cretaceous and Tertiary strata of the Wind River Basin. Potential source rocks include the Mowry Shale, lower shaly member of the Cody Shale, Mesaverde Formation, and Meeteetse Formation in Upper Cretaceous strata, and the lower unnamed member and Waltman Shale Member of the Fort Union Formation in the Paleocene strata.

Chemical and isotopic compositions of gases indicate complex origins and highly variable vertical migration distances. These interpretations are supported by drillstem tests indicating the presence of “gas plumes” on highly faulted and fractured anticlines including Madden and Pavillion-Muddy Ridge, where gas saturation extends thousands of feet stratigraphically higher than in surrounding rocks. Because of the likelihood that gases are co-mingled from several different sources in these gas plumes, a composite total petroleum system that includes several source intervals was established.

Coalbed methane assessment units are identified within the Mesaverde and Meeteetse Formations and the lower member of the Fort Union Formation, and are charged largely with migrated thermogenic gas and biogenic gas generated in the coals. Continuous sandstone gas assessment units are identified in the Frontier Formation, Cody Shale, Mesaverde, Meeteetse Formations, and lower member of the Fort Union Formation in the deeper areas of the basin. Conventional gas and oil assessment units cover much of the basin and include the same stratigraphic intervals as the coalbed methane assessment units in the shallower areas of the basin and are present above continuous gas assessment units in the deeper parts.