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Data release for Organic geochemistry and petrology of Devonian shale in eastern Ohio: implications for petroleum systems assessment (2018)

December 17, 2021

Recent production of light sweet oil from shallow (~2,000 ft) horizontal wells in the Upper Devonian Berea Sandstone of eastern Kentucky and historical oil production from conventional wells in the Berea of adjoining southern Ohio has prompted re-evaluation of Devonian petroleum systems in the central Appalachian Basin. Herein, we examined Upper Devonian Ohio Shale (lower Huron Member) and Middle Devonian Marcellus Shale organic-rich source rocks from eastern Ohio and nearby areas using organic petrography and geochemical analyses of solvent extracts. The data indicate the organic matter in the Ohio and Marcellus Shales was primarily derived from marine algae and its degradation products including bacterial biomass. Absence of odd-over-even n-alkane distributions in gas chromatograms and low gammacerane index values in Devonian source rocks are similar to properties reported for Devonian-reservoired oils in eastern Ohio, suggesting a strong oil-source rock correlation. However, petrographic and geochemical parameters presented here were unable to discriminate specific shale source rocks (e.g., Ohio Shale vs. Marcellus Shale) for the Devonian oils. Lower Paleozoic oils from eastern Ohio, in contrast, are characterized by the presence of odd-over-even n-alkane distributions and higher gammacerane values which clearly discriminate them from Devonian shale-derived oils. Measurements of solid bitumen reflectance (BRo) at the thermal maturity range of the samples (immature to peak oil conditions) tend to underestimate 'true' thermal maturity because solid bitumen has lower reflectance than co-occurring vitrinite. Because solid bitumen dominates the organic matter in Devonian shale and vitrinite is sparse, the value of reflectance as a thermal proxy is questionable and its use may lead to reports of 'vitrinite'reflectance suppression' in early mature to oil window mature areas. For example, thermal maturity estimates from equilibrium(?) biomarker isomerization ratios may suggest some of the Devonian source rock samples are at middle to peak oil window conditions e.g., approximate vitrinite reflectance values of 0.8-0.9%, whereas solid bitumen reflectance is approximately 0.52-0.54% in the same samples. If correct, this observation may require that the predicted onset of oil generation from Devonian shale source rocks in eastern Ohio is moved farther westward. As a consequence, only local to short-distance (30-50 mi) migration would be necessary for emplacement of Devonian-sourced oils into Devonian reservoirs of eastern Ohio, rather than long-distance migration (>50 mi) from 'deep' in the Appalachian basin', as suggested by previous workers, potentially impacting exploration and future assessments of undiscovered petroleum resources in the Berea Sandstone. However, biomarker isomerization ratios do not show consistent relationships to other thermal maturity parameters (BRo, Tmax), thereby preventing development of robust empirical calibrations for these thermal proxies in the Devonian of eastern Ohio.