Groundwater Ages and Mixing in Western Energy Basins: Implications for Monitoring

Identifying the effects of energy development on groundwater quality can be difficult because interactions between hydrocarbon reservoirs and aquifers can be complex and involve both natural and human processes. Groundwater age and mixing data can strengthen the design of monitoring programs and interpretation of monitoring data by providing better understanding of the groundwater flow systems. Age, chemical, and isotopic tracers were used to examine age and mixing characteristics of shallow groundwater in the Piceance Basin, Colorado and Williston Basin, Montana and North Dakota. In the Piceance Basin, natural gas in tight sands of the Mesaverde Formation is extracted using hydraulically-fractured vertical wells. In the Williston Basin, oil in tight clastic and carbonate rocks of the Bakken Formation is extracted using hydraulically-fractured horizontal wells. In both basins, age-tracer data (sulfur hexafluoride, tritium, helium-3, helium-4, carbon-14) revealed complex patterns in shallow groundwater ages (< 10 to > 50,000 years). Chemical and isotopic data (chloride, bromide, sulfate, methane, strontium, δ²H-H₂O, δ¹⁸O-H₂O, δ¹³C-CH₄, ³He/⁴He, ⁸⁷Sr/⁸⁶Sr) showed that shallow groundwater mixed with water from adjacent formations and with water from much deeper formations. The age data provide information on time scales at which water quality changes in aquifers might be expected to occur. This information could be used to establish appropriate distances of monitoring wells from energy development activity and the necessary duration of monitoring. The mixing data provide information on hydraulic connections between shallow and deep formations that could be used to guide more detailed monitoring efforts and also indicate areas where additional care in drilling oil and gas wells could be warranted to avoid further enhancing that connectivity.