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.