Biogenic Gas Resource Potential of the Upper Cretaceous Pierre Shale in the Denver-Julesburg (Dj) Basin

Charles R. Nelson
Blackrock Resources, LLC, Golden, CO

The Upper Cretaceous Pierre Shale Formation is the thickest sedimentary rock interval in the Denver-Julesburg (DJ) Basin. Oil and natural gas are produced from the Terry and Hygiene Sandstone Members of the Pierre Shale Formation, and oil is also produced from several naturally fractured areas in the Pierre Shale. The natural gas in the sandstones is chemically wet, i.e., has a high, 16-25 mole percent content of C2-5 hydrocarbons, which is a distinctive, thermogenic gas-origin geochemical signature. Previous geochemical analyses of source-rock potential and C15+ hydrocarbon compositions of oils indicated that the organic matter in the surrounding Pierre Shale is too thermally immature to be the hydrocarbon source-rock. Carbon and hydrogen isotope analyses reveal, however, that the methane in the Terry and Hygiene sandstones actually has a mixed biogenic-thermogenic origin isotope signature. Geologic analog comparisons suggest that the source for the biogenic methane in the Terry and Hygiene sandstones was breakdown by anaerobic bacteria of thermally immature organic matter in the surrounding Pierre Shale or underlying Niobrara chalk beds. In the eastern Denver-Julesburg Basin, biogenic methane is produced from thermally immature, fractured Niobrara chalk beds. In the northern Great Plains, southeastern Alberta, and southwestern Saskatchewan, biogenic methane is produced from thin sandstones and siltstones encased in thick Cretaceous shale sequences. The presence of biogenic methane in these Pierre Shale sandstones indicates that others areas in the Denver-Julesburg Basin where the Pierre Shale contains thin, interbedded sandstones and siltstones or is naturally fractured may have potential for significant future biogenic gas resource discoveries.

AAPG Search and Discovery Article #90092©2009 AAPG Rocky Mountain Section, July 9-11, 2008, Denver, Colorado