Rock-eval pyrolysis and vitrinite reflectance trends in the Cleveland Shale Member of the Ohio Shale, eastern Kentucky

Abstract

Within eastern Kentucky, organic petrographic and geochemical data indicate a southeastwards increase in maturation of the Cleveland Shale Member of the Ohio Shale (Devonian-Mississippian). Reflectance levels of dispersed organic material in the Cleveland Shale increase from 0.5% in the outcrop belt in central Kentucky, to slightly over 1.0% in Pike County, eastern Kentucky. A decrease in fluorescence intensity of liptinitic components, such as Tasmanites, accompanies this reflectance increase, as does a shift in fluorescence color of the alginite towards the red. In the highest rank areas, fluorescence is almost absent. Groundmass fluorescence is observed also, and follows trends similar to those shown by the alginite. Kerogen in the Cleveland Shale is primarily Type II, as indicated by petrographic observations and Rock-Eval data. Total organic carbon contents of this shale decrease from over 12% in the outcrop belt, to less than 2% in the eastern-most part of the study area. Source rock potential ranges from good to very good, with the highest potentials occurring in the outcrop belt. T max values for this unit increase from under 430°C in the outcrop belt to over 450°C in the southeast, confirming the increase in maturation trends indicated by vitrinite reflectance data. Both petrographic and geochemical maturation parameters place the Cleveland Shale of eastern Kentucky within the oil window. Comparison with maturation data for Pennsylvanian-age coals upsection suggests that maturation indices are retarded, or suppressed, in the Devonian shales. The level of maturation indicated for the Cleveland may be problematical considering that the Ohio Shale is a major gas-producer in this area.