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Identifying the Sources of Methane in Shallow Groundwaters in South-Central Texas Through Noble Gas Signatures

Abstract

With rising demands for cleaner domestic energy resources, horizontal drilling and hydraulic fracturing techniques in unconventional hydrocarbon exploration have been extensively developed. However, the observation that some water wells have showed elevated concentrations of dissolved methane and other light hydrocarbons has caused public concern regarding unconventional energy extraction. Here, we present noble gas data of production shale gases from the Barnett and Strawn Formations, as well as nearby groundwater samples in south-central Texas. The Barnett Shale located in the Fort Worth Basin at an average depth of ∼2300 m is one of the most prominent shale gas plays in the U.S. This RPSEA-sponsored study explores the potential of noble gases for fingerprinting shale gas and thus, for identifying the sources of gas in aquifers overlying the Barnett and Strawn Shales, due either to natural hydrocarbon occurrences or potentially related to gas production from unconventional energy resources. Groundwater samples were collected in Parker and Hood counties in areas where high amounts of methane (>10 mg/L) were detected in shallow groundwater. Gas samples were also collected directly from groundwater wells where bubbling methane was present. Preliminary results show that He concentrations in water samples, in excess of up to three orders of magnitude higher than expected atmospheric values are directly correlated with methane concentrations. 3He/4He ratios vary from 0.030 to 0.889 times the atmospheric ratio with the lowest, more purely radiogenic contributions being associated with highest methane levels. The presence of crustally-produced 40Ar is also apparent in groundwater samples with 40Ar/36Ar ratios up to 316. Results suggest that the source of this methane could be the Strawn Formation rather than the heavily exploited Barnett Shale.