A CRITICAL EVALUATION OF STRONTIUM ISOTOPES AS A TRACER OF POSSIBLE BRINE CONTAMINATION IN SHALLOW AQUIFERS RELATED TO OIL/GAS PRODUCTION

Both unconventional and conventional oil/gas production have led to instances of brine contamination of near-surface environments from saline produced waters spills. Strontium isotope ratios (⁸⁷Sr/⁸⁶Sr) have been used as a sensitive tracer of brine contamination sources in streams and shallow aquifers in areas where oil/gas production are limited to only a few reservoirs and produced water sources are well-defined. Recent expansion of unconventional oil/gas production to additional tight formations within sedimentary basins has resulted in production of formation waters from multiple oil/gas reservoirs that may have similar chemical and isotopic ratios, including ⁸⁷Sr/⁸⁶Sr ratios. This study critically evaluates the utility of ⁸⁷Sr/⁸⁶Sr ratios as a tracer of brine contamination related to conventional and unconventional oil/gas production in two major hydrocarbon provinces: the Williston and Appalachian basins in North America. Multiple stacked oil/gas reservoirs within each basin have overlapping ⁸⁷Sr/⁸⁶Sr ratios of formation waters based on a non-parametric statistical test. For example, in the Appalachian Basin, produced waters from unconventional gas production in the Middle Devonian Marcellus and Upper Ordovician Utica shales have overlapping ⁸⁷Sr/⁸⁶Sr ratios. Sr-isotopes alone may not be a useful tracer of saline produced water contamination in regions with both Marcellus and Utica shale oil/gas and associated saline formation water production. Likewise, in the Williston Basin, produced waters from unconventional oil production in the Late Devonian to Early Mississippian Bakken Formation have overlapping ⁸⁷Sr/⁸⁶Sr ratios with conventional oil production from the Middle Devonian Winnipegosis and the Mississippian Madison groups. In addition, there is significant spatial variability in ⁸⁷Sr/⁸⁶Sr ratios of formation waters across the basins likely from changes in lithology. Improved spatial distribution of ⁸⁷Sr/⁸⁶Sr ratios and other isotopic ratios of produced waters are needed to constrain geographic variability in hydrocarbon producing regions. As unconventional oil/gas production expands to include additional reservoirs and in areas of existing conventional oil/gas production, multiple isotopic tracers will be required to delineate the saline produced water sources.