Characterization of the Natural Gas Systems of the Wind River Basin, Wyoming

Paul G. Lillis, U.S. Geological Survey, Box 25046, MS 977, Denver Federal Center, Denver, CO 80225, phone: 303-236-9382, fax: 303 236-3202, [email protected] and Ronald C. Johnson, U.S. Geological Survey, Box 25046, MS 939, Denver Federal Center, Denver, CO 80225.

In a re-evaluation of gas systems from the Wind River Basin, new molecular and isotopic composition data combined with data from the literature was used to characterize and identify at least four gas types. Type-A gas is found in Tensleep Sandstone and the Phosphoria, Amsden, Embar, and Sundance Formations in the Beaver Creek, Riverton Dome, Steamboat Butte, Pine Mountain, Big Sand Draw fields, and is most likely derived from the Phosphoria Formation. The gas is typically wet, has high H₂S, He, N₂, and/or CO₂, and is usually found in fields that also contain Phosphoria-sourced oil. Type-B gas has charged Lakota, Dakota, and Muddy Sandstones, and Morrison and Frontier Formations in the Beaver Creek, Riverton Dome, Big Sand Draw, Pilot Butte, Steamboat Butte, Clark Ranch, Muskrat, Sheldon and Wallace Creek fields, and is most likely derived from the Mowry Shale and possibly the Muddy. Type-C gas is found in Upper Cretaceous and Tertiary reservoirs in numerous fields in the northern part of the basin. Type-C gas includes a wide range in gas composition reflecting the variation in organic facies and maturity level of Upper Cretaceous source rocks. Type-D gas is microbial gas found in shallow coal beds of the Mesaverde Formation and is produced from the Wind River Formation at Waltman field. In some fields, gas quality has been adversely impacted by elevated non-hydrocarbon gas content (CO₂ at Madden, N₂ at Pavillion and East Riverton Dome, H₂S at several fields) and by biodegradation (Pavillion and Riverton Dome).