OCCURRENCE AND ORIGIN OF METHANE IN SHALLOW GROUNDWATER IN BEREA AND ROGERSVILLE PLAY AREAS, EASTERN KENTUCKY
In 2016, we sampled 51 water wells to provide a first assessment of dissolved methane in groundwater overlying the Berea and Rogersville plays. Our data showed that methane was detected in 78 percent of the sampled wells (40 of 51 wells) and about half of the wells (26 of 51 wells) had methane concentrations above 1 mg/L. The highest methane concentration was 78 mg/L. Higher methane concentrations were found associated with sodium-rich water and reducing redox conditions. No correlation was found between methane concentrations and distances between sampled wells and nearest vertical oil and gas wells. The 26 wells with methane concentrations greater than 1 mg/L were also analyzed for stable carbon and hydrogen isotopic composition of methane (δ13C-CH4 and δ2H- CH4). The isotopic composition of δ13C-CH4 (-84.0‰ to -58.3‰) and δ2H-CH4 (-246.5‰ to -146.0‰), along with a dominance of dry gas in most wells (21 of 26 wells), indicated that dissolved methane was primarily formed with microbial origin through CO2 reduction pathway. However, two wells with a biogenic signature based on isotope data also showed a possible thermogenic influence, as indicated from the occurrence of ethane and propane.