Since 2011, the Devonian Berea Sandstone in northeastern Kentucky has produced oil where thermal maturity studies indicate that likely source rocks, namely, the Devonian Ohio Shale and Mississippian Sunbury Shale, are thermally immature. Downdip, where source rocks are mature for oil, the Berea Sandstone and Ohio Shale primarily produce gas. To investigate this thermal maturity discordancy, the molecular and isotopic composition of gases from the Berea Sandstone (oil associated) and Ohio Shale (nonassociated) were analyzed to understand the gas generation and migration history.
Collected along a northwest-southeast transect in eastern Kentucky, samples range from 1079 to 4602 ft, respectively. All are wet gases with a thermogenic origin (δ13C-CH4 = −53.5‰ to −46.1‰). This is mostly consistent with a reevaluation of thermal maturity in a companion study that expands the area mature for oil. Gas migration is required, however, in updip parts of the Berea play where vitrinite reflectance (VRo) values are less than 0.6%. Southeast regional dip exerts a first-order influence on thermal maturity as gases become drier and isotopically heavier downdip. Correlation of δ13C values for heavier hydrocarbon gases in a natural gas plot with VRo contour spacing demonstrates the influence of nearby source rock thermal maturity on gas composition. Downdip, migration of oil and the attendant increase in permeability relative to gas may account for the dominance of gas production in the Ohio Shale. Migration along with basin uplift after the Alleghany orogeny may have contributed to development of a gas phase that exsolved from oil.