OCCURRENCE AND ORIGIN OF METHANE IN SHALLOW GROUNDWATER IN BEREA AND ROGERSVILLE PLAY AREAS, EASTERN KENTUCKY

In eastern Kentucky, horizontal drilling and hydraulic fracturing are being used to develop and evaluate unconventional reservoirs in the Devonian Berea Sandstone and the Cambrian Rogersville Shale in recent years. With more than 100 wells drilled, the Berea play is being developed at a small scale with shallow fracturing depths of less than 600 m. The Rogersville play is in its early stages, but has the potential to become a major shale gas play with large-scale hydraulic fracturing at depths of 2,000 m to more than 4,000 m. Development of these unconventional reservoirs has prompted concerns about potential impacts to groundwater resources in the region.

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.