Carbon and hydrogen isotopic evidence for the origin of combustible gases in water-supply wells in north-central Pennsylvania

摘要

The origin of the combustible gases in groundwater from glacial-outwash and fractured-bedrock aquifers was investigated in northern Tioga County, Pennsylvania. Thermogenic methane (CH _4) and ethane (C _2H _6) and microbial CH _4 were found. Microbial CH _4 is from natural in situ processes in the shale bedrock and occurs chiefly in the bedrock aquifer. The δ ~(13)C values of CH _4 and C _2H _6 for the majority of thermogenic gases from water wells either matched or were between values for the samples of non-native storage-field gas from injection wells and the samples of gas from storage-field observation wells. Traces of C _2H _6 with microbial CH _4 and a range of C and H isotopic compositions of CH _4 indicate gases of different origins are mixing in sub-surface pathways; gas mixtures are present in groundwater. Pathways for gas migration and a specific source of the gases were not identified. Processes responsible for the presence of microbial gases in groundwater could be elucidated with further geochemical study.