EVALUATING STRONTIUM ISOTOPES AS A TRACER OF FLUIDS IN SUBSURFACE RESERVOIRS AND POSSIBLE BRINE CONTAMINATION IN SHALLOW AQUIFERS RELATED TO OIL/GAS PRODUCTION

Incidences of accidental brine contamination of near-surface environments have increased along with the exponential increase in unconventional production of oil/natural gas over the past decade. Strontium isotopes (⁸⁷Sr/⁸⁶Sr) have been used as a sensitive tracer of sources of brine contamination in surface waters or groundwater in areas where different salinity sources are relatively limited. As oil and gas production expand and produced waters may come from multiple formations with similar lithologies, it is unclear if Sr isotopes will continue to be an adequate tracer in cases of brine contamination. Similar issues may occur where unconventional oil and gas development occurs in regions where historic conventional oil and gas development has occurred. This study systematically evaluated the utility of ⁸⁷Sr/⁸⁶Sr as a tracer of natural brine migration or anthropogenic contamination in shallow aquifers related to oil/gas production. Strontium isotopes of formation waters, together with other tracers, were investigated for three major oil and gas producing regions in the United States: the Williston, Appalachian, and Permian basins. Multiple formations with depth in the individual basins have overlapping Sr isotope ratios of formation waters based on a non-parametric statistical test. For example, in the Appalachian Basin, formation waters in the Middle Devonian Marcellus and Upper Ordovician Utica shales have overlapping ⁸⁷Sr/⁸⁶Sr. Likewise, in the Williston and Permian basins, the Late Devonian to Early Mississippian Bakken Shale and thePennsylvanian to Permian Wolfcamp Shale, respectively, have overlapping ⁸⁷Sr/⁸⁶Sr values with the Middle Devonian Winnipegosis and the Middle Pennsylvanian Canyon formations, respectively. As unconventional oil and gas production expands in these basins from multiple formations in a given geographic location, it will be important to characterize produced waters isotopic signature using multiple tracers, as Sr isotopes alone may not distinguish fluid sources. In addition, there is significant spatial variability in ⁸⁷Sr/⁸⁶Sr of formation waters across the basins related mostly to changes in lithology. More spatially-distributed studies of fluid isotopic signatures are needed to constrain geographic variability in hydrocarbon producing regions.