USING BRACKISH GROUNDWATERS AS INDICATORS FOR POTENTIAL IMPACTS FROM DEEPER OIL AND GAS ACTIVITIES

A variety of mechanisms (both natural and failures of human infrastructure) have been proposed as potential causes for gas and/or brine migration from producing oil and gas fields into nearby shallow drinking water resources. We propose that geochemical and physical properties of brackish groundwater systems can be used to provide insight into the likelihood for upward migration of fluids from underlying oil and gas reservoirs into these groundwater systems through natural conduits. This study targets a brackish groundwater aquifer in the Triassic Dockum Formation of west Texas, which is located above the eastern half of the Permian Basin, the largest tight oil producing province in the United States (>2 million barrels per day). A combination of reservoir pressure and ionic and isotopic geochemical (δ²H, δ¹⁸O, and ⁸⁷Sr/⁸⁶Sr) data from brackish and produced water wells were compared. In this case, substantial isotopic differences between the brackish aquifers versus waters from the underlying oil and gas reservoirs were noted, suggesting a lack of hydraulic connection between the two zones. Moreover, reservoir pressure gradient data show under-pressuring immediately below the brackish aquifers, indicating that the vertical component of fluid flow in the Dockum is downward, and there is little potential for upward migration of fluids from deeper oil and gas reservoirs despite local, natural over-pressuring. We propose that similar methods can be used in other geologic basins to better understand the potential for long-term verticalmigration of fluids as a result of oil and gas production activities, including hydraulic fracturing.