Abstract

We sampled gases from 22 wells and 5 cores in western Pennsylvania and eastern Ohio to evaluate the possible use of stable isotope geochemistry for interpreting the origins of natural gases in the Plateau province of the central Appalachians and for correlating these gases with their probable sources. The isotope data suggest that several of these gases have multiple sources or were altered by geological or biogeochemical processes. Gases produced from Upper Cambrian Rose Run sandstones in Portage County, Ohio, are condensate-associated hydrocarbons. These gases can be correlated to local Ordovician source rocks. Gases produced from the Upper Cambrian Beekmantown Dolomite in Coshocton County, Ohio, also are thermogenic, but the maturity of these hydrocarbons is greater than that observed in the local Ordovician source rocks. The Beekmantown hydrocarbons probably migrated from deeper in the Appalachian basin along the Knox unconformity. Lower Silurian Medina Group gases in northwestern Pennsylvania are late mature and probably originated in Ordovician source rocks. Medina Group gases might be mixtures of thermogenic hydrocarbons, or their geochemistry may indicate reservoir leakage due to gas diffusion through cap rock. Gases produced from structural traps in the Devonian Ridgeley Sandstone and Huntersville Chert of western Pennsylvania are isotopically diverse and reflect both the entrapment of mature associated gases and probable second-order fractionation effects. Autogenic gases produced from Devonian black shales are postmature along the Allegheny Front and early mature along the northwest basin flank. The early-mature gas is autochthonous and thermogenic. Most Upper Devonian gases produced across the Plateau are oil-associated gases that were emplaced in reservoirs prior to maximum burial of the Paleozoic section during the Alleghanian orogeny. Some Upper Devonian gases, however, could be residues of diffusive gas leakage or possible mixtures of thermogenic gases that migrated into or from reservoirs located along regional fractures. Lower Silurian Tuscarora Formation gases produced from fractured sandstones near the Allegheny structural front in central Pennsylvania are postmature hydrocarbons that correlate with local source rocks. Gases produced from fractured Ordovician Bald Eagle Formation rocks near the Allegheny Front, however, are enigmatic. These gases might be postmature hydrocarbons of possible hydrothermal or geothermal origin, or their chemistry could be the result of postgenetic fractionation due to diffusion or oxidation. Gases produced from bituminous Pennsylvanian coals of the Appalachian Plateau have varied methane delta 13 C and methane delta D contents and may be mixtures of thermogenic gas and microbial gas

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