Geochemistry of Cherokee Group oils of southeastern Kansas and northeastern Oklahoma

Abstract

We made organic carbon determinations, Rock-Eval pyrolysis analysis, and vitrinite reflectance measurements on 72 samples of organic-matter-rich (>1.0% total organic carbon) Middle Pennsylvanian (Desmoinesian) Cherokee Group and Marmaton Group shales and coals and on 13 samples of Upper Devonian-Kinderhookian Chattanooga Shale from southwestern Missouri, southeastern Kansas, and northeastern Oklahoma. These analyses show that the Cherokee Group and Marmaton Group offshore shales and coals and the Chattanooga Shale are thermally mature with respect to petroleum generation and still have good potential to generate oil and/or natural gas. In contrast, the organic matter in the Cherokee Group and Marmaton Group nearshore shales is hydrogen deficient and has little or no generating potential. Comparisons of saturated hydrocarbon, terpane and sterane distributions, pristane/phytane ratios, and carbon isotope compositions of saturated and aromatic hydrocarbon fractions from oils and rock extracts show that oils occurring in sandstone reservoirs of the Cherokee Group are similar to extracts from the Chattanooga Shale and are dissimilar to extracts of Cherokee Group and Marmaton Group offshore shales and coals. We conclude from these correlations that the Chattanooga Shale contains the source rocks for the Cherokee Group oils. These results imply that any porosity trends in Mississippian rocks above the Chattanooga Shale and/or in Paleozoic rocks immediately below it have the potential to be charged with the same oil found in Cherokee Group reservoirs. The excellent hydrocarbon-generating potential shown for the Middle Pennsylvanian offshore shales and coals suggests that these rocks, where thermally mature, may be sources for other midcontinent oils and/or natural gas.