Assessing Distribution and Origin of Methane in Shallow Groundwater in Horizontal Oil and Gas Play Areas, Eastern Kentucky

Rapid implementation of horizontal drilling and hydraulic fracturing technology to produce oil and gas from tight rock formations across the country has increased public concern about possible impact on the environment, especially on shallow drinking-water aquifers. In eastern Kentucky, horizontal drilling and hydraulic fracturing have been used to develop the Upper Devonian Berea Sandstone in recent years. Although production in the Berea Sandstone is at a relatively small scale, the Rogersville Shale, a deeper, thicker, and more spatially extensive organic-rich shale, is projected to become a major shale play in eastern Kentucky. This has necessitated a better understanding of groundwater quality, especially the occurrence of dissolved methane, in aquifers overlying the Berea and Rogersville plays to help address the public's environmental concerns and protect groundwater resources. To assess baseline groundwater chemistry and evaluate distribution and origin of methane detected in the groundwater, 51 water wells in Greenup, Carter, Boyd, Lawrence, Johnson, and Elliott Counties were sampled and analyzed for major cations and anions, metals, and dissolved light hydrocarbon gases including methane. Twenty-six wells were identified as having methane concentrations greater than 1 mg/L and were further analyzed for carbon and hydrogen isotopes. The results indicate that methane is a relatively common constituent in shallow groundwater in eastern Kentucky. Correlation of methane distribution with water chemistry data shows that elevated methane concentrations were more common in sodium bicarbonate type water and in low-nitrate, low-sulfate redox conditions. Carbon and hydrogen isotope analysis suggests that the methane detected in groundwater is derived primarily from bacterial sources from the CO2 reduction pathway.