Identifying the Sources of Methane in Shallow Groundwaters in Parker and Hood Counties, 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. In this contribution, 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 DOE-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 Shale, due either to natural hydrocarbon occurrences or potentially related to gas production from unconventional energy resources.

A total of 35 groundwater samples were collected in Parker and Hood counties in areas where high amounts of methane (>10 mg/L) were detected in shallow groundwater. Two 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 ratio values vary from 0.030 to 0.889 times the atmospheric ratio with the lowest, more pure radiogenic contributions being associated with highest methane levels. The presence of crustally-produced radiogenic ⁴⁰Ar is also apparent in groundwater samples with ⁴⁰Ar/³⁶Ar ratios up to 316. A combined analysis of ⁴⁰Ar/³⁶Ar ratios from groundwater wells bubbling gas and that of shale gas suggests that the source of this methane is not the heavily exploited Barnett Shale, but rather, the Strawn Formation.