Abstract

Vitrinite reflectance (R_m) analyses indicate that samples from Upper Cretaceous and Paleocene formations collected around the margins of the Wind River Basin are immature to marginally mature with respect to oil and thermogenic gas generation, but have the potential to generate biogenic methane. R_m values for the Upper Cretaceous Frontier Formation are in the 0.40 to 0.60 percent range; the mean is 0.48 percent; Upper Cretaceous Cody Shale samples are in the 0.30 and 0.40 percent R_m range, the mean is 0.41 percent; most Upper Cretaceous Mesaverde Formation samples fall within the range of 0.40 to 0.60 percent R_m the mean R_m value is 0.45 percent; Upper Cretaceous Meeteetse Formation samples have R_m values in the 0.39 to 0.49 percent range, the mean is 0.43 percent; Upper Cretaceous Lance Formation values range from 0.39 to 0.55 percent R_m, the mean is 0.46 percent; and Paleocene Fort Union Formation R_m values, the lowest of all formations analyzed, have a mean R_m of 0.38 percent.

The majority of R_m values from each formation are within a very narrow range and are highly consistent throughout the margins of the basin, especially considering the large areal distribution of samples. This indicates similar burial and thermal histories throughout the area sampled. Additionally, the mean R_m value for each formation generally increases from younger to older, a direct result of greater depth of burial for the older formations. Strata around the periphery of the basin were never buried as deeply as those near the basin center, and these marginal strata probably achieved their measured levels of maturity between the end of Laramide deformation (end of early Eocene; about 52 Ma) and 10 Ma.