ORIGIN OF BRINES IN THE WESTERN CANADIAN SEDIMENTARY BASIN AS INFERRED FROM BROMIDE AND CHLORIDE RATIOS

Ions of bromine and chlorine are recognized as reasonably conservative tracers in natural waters. This quality has led to the use of bromide (Br-) concentrations and chloride (Cl-)/Br- mass ratios to infer the geochemical provenance of different groundwaters. For brines, Carpenter (1978) defined a “seawater evaporation trajectory” (the S-E-T) using cross plots of Cl- and Br- concentrations. On the S-E-T, the ratio of Cl- to Br- remains constant as water evaporates and becomes increasingly enriched, until the point of halite precipitation, at which point the Cl-/Br- ratio decreases. The S-E-T can be used to infer the origin of a brine, whether due to evaporative enrichment, halite dissolution or mixing with other waters.

A recent compilation of Br- and Cl- concentrations for the Western Canadian Sedimentary Basin (WSCB) allows the interpretation of brine origins in the basin. At the bottom of the sedimentary section, oilfield brines in Middle Devonian formations have high TDS concentrations (greater than 100 g/L) and plot directly on the S-E-T. Published data from evaporite mines in Saskatchewan have higher TDS (greater than 200 g/L) and overlap with the oilfield brines. The Cl-/Br- mass ratios of these brines indicate evaporation past the point of halite precipitation.

Cl-/Br- mass ratios in brines from the top of the WCSB geological section, in clastic Cretaceous formations, indicate that these waters have undergone evaporative concentration of seawater. Mannville Group (and its equivalent) brines typically have TDS concentrations of 50-90 g/L and plot directly on the S-E-T. Some of the Upper Cretaceous brines from units above the Colorado Group contain as little as 5 g/L TDS and plot close to the S-E-T, but waters from this section are often Br-enriched, suggesting an organic source of Br- in the shallow WCSB. Brines from Upper Devonian sediments indicate mixing with the underlying Middle Devonian, while Mississippian and Mesozoic sediments are often Cl- enriched, suggesting mixing with halite dissolution-derived brines.

Overall, Cl-/Br- mass ratios suggest that many of the brines in the WCSB have evidence of a syndepositional salt source that has mixed with other waters. This mixing has been especially active within the Devonian and the post-Devonian sections, with less mixing between these units.