ORIGIN OF DIFFERENT BRINES IN THE WILLISTON BASIN (CANADA-USA)
Geochemical and isotopic data collected from more than 2000 wells across the basin over the past 15 years provide insight(s) into the origins of the different brines found in the Williston Basin. First, the presence of numerous and extensive evaporite layers, along with areas of dissolved evaporites, initially were thought to be the source (i.e., via evaporite dissolution) of the brines in the basin. These brines are of the Na-Cl type with TDS up to 300 g/L. Second, geochemical results, particularly bromine/chlorine ratios and stable isotopes of oxygen and hydrogen indicated a second type of brine in the basin: namely mixtures of evaporatively-concentrated seawater. These brines contain increased amounts of Ca-Cl2 and are often found towards the center (often deeper) portions of each aquifer. Third, there are rare extremely-saline (TDS > 425 g/L) brines of the Ca-Cl2 type that appear to be evaporated Devonian seawater. Finally, there are mixtures of these different types through out the basin.
Origin, mixing with each other, and mixing with recharging groundwaters appears to control brine compositions. Fluid mixing events can be traced within aquifers and often the different brine types are evident along flow paths in individual aquifers. In other areas, there is a stratigraphic affinity to brine type. The complicated nature of the brine origin in this basin is only becoming understood after years of detailed sampling. Insights gained from these hydrochemical data provide an improved understanding of brines in this basin and elsewhere.