Geochemical and stable isotope comparison of New Albany Shale Group source rocks and characterization of shale-gas potential along the western margin of the Illinois Basin

Unconventional shale gas resources have been extracted from the Middle to Upper Devonian age black shales throughout the United States. In the Illinois Basin organic-rich black shales occur in the Devonian and Lowermost Mississippian age units which are called the New Albany Shale Group. As part of a USGS-STATEMAP project, two holes were drilled in central Calhoun and southwestern Greene counties in western Illinois on the margin of the Illinois Basin. Shale samples were collected from drill core (1-2' intervals) in the Mississippian age Hannibal Shale, and the Devonian age Grassy Creek and Saverton formations. This study characterized the samples by measuring bulk total organic carbon (TOC), inorganic carbon, stable isotopes (organic delta (super 13) C), gamma, single-point resistivity, and major and trace elements. Geochemical and geophysical data of the Nutwood member of the Hannibal shale yielded comparable results to the local Devonian shale units, including a C3 terrestrial plant source for organic carbon, elevated sedimentation rates, and potential for dry-gas maturation. The data suggest that economic shale-gas may be present in the Devonian age shales as well as the Mississippian age Nutwood Member of the Hannibal Shale in thermally mature portions of the Illinois Basin.