Alkali earth ratios differentiate conventional and unconventional hydrocarbon brine contamination

The large increase in unconventional shale gas extraction has raised concerns about potential water contamination from leaks and spills. Shale gas produced water is challenging to detect in areas impacted by legacy contamination, particularly from conventional sources. Previous studies have proposed combinations of Br, SO₄, Ba, Cl, and other more specialized stable isotope systems to delineate shale gas produced water from 1) non-impacted waters and 2) other sources of water contamination. In general, the efforts that rely on relatively simple chemistry do not allow differentiation between conventional and unconventional brine chemistry. We examined variations in Ca/Mg and Ca/Sr ratios that seem to arise from variation in temperature with depth, to differentiate among conventional brines, unconventional brines, and non-impacted waters. This approach was applied to four sedimentary basins in the USGS produced water database: Williston, Michigan, Appalachian, and the Green River basin. In addition, the utility of the system was demonstrated with field samples taken during periods of known unconventional brine releases to surface waters. The Ca/Mg and Ca/Sr ratios allow distinction among these three water types in all basins, suggesting a relatively simple and direct way to evaluate water chemistries in landscapes dominated by unconventional shale gas extraction.