Evaluating strontium isotopes as a tracer of fluids in subsurface reservoirs and possible brine contamination in shallow aquifers related to oil-gas production

Mohammad Marza and Jennifer McIntosh

Hydrology and Atmospheric Sciences
The University of Arizona

Previous studies using limited data have suggested that Sr isotopes can provide a unique fingerprint of basinal brines from specific geologic formations at depth in the earth’s crust and leakage of brines into shallow aquifers. In basins, where hydrocarbon production can come from multiple geologic formations with similar lithologies (e.g., organic-rich shales), it is unclear if Sr isotope signatures are indeed a unique tracer of formation water sources.  This study systematically evaluates the utility of Sr isotopes as a tracer of natural or anthropogenic contamination in shallow aquifers related to oil/gas production.  Strontium isotopes, together with other solute and isotopic chemistry, were investigated for two major oil/gas producing regions in the United States: Williston, and Appalachian basins.  Initial results suggest that multiple oil/gas bearing formations with depth can have overlapping Sr isotope ratios. For example, in the Appalachian Basin, the Marcellus and Utica shales have overlapping 87Sr/86Sr values. Likewise, in Williston Basin, the Bakken Shale has overlapping Sr isotope ratios with other hydrocarbon-bearing formations (e.g., Winnipegosis and Dawson). Alternative tracers and multiple tracer approaches will be investigated to better constrain sources of salinity in oil/gas producing regions.

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