USING GEOCHEMICAL EVIDENCE TO CONSTRAIN HYDROGEOLOGIC MODELS OF BRINE FORMATION AND RESIDENCE TIME

Geochemical tracers provide crucial clues to the evolution of brines in sedimentary basins, but some geochemical clues remain ambiguous. Br/Cl ratios, which have been used to distinguish evaporatively-concentrated brines from dissolution-derived brines, have been particularly troublesome in this regard. Here we show that the power of otherwise ambiguous Br/Cl ratios can be boosted significantly when used as constraints for hydrogeologic models. We constructed models of fluid flow and reactive solute transport based on the Alberta Basin, Canada, to test the hypothesis that halite dissolution could have contributed to brine formation despite Br/Cl ratios that suggested otherwise. The models simulated basin evolution and brine migration from 100 Ma to the present, testing (a) a range of initial conditions that were based on geochemical evidence and (b) a range of permeability models that were taken from prior published models and speculations. Results were not particularly sensitive to initial conditions, but only two sets of permeabilities produced reasonable salinities and Br/Cl ratios: (1) permeabilities based on modern core samples and drill-stem tests and (2) permeabilities set so low that advection did not occur in the basin. These two possibilities exactly correspond to the ambiguity of the Br/Cl ratios, but because the chance that permeabilities in the basin have increased dramatically in recent times is extraordinarily small, we can rule out the second possibility. Brines in the basin were found to be formed by a combination of evaporative concentration, halite dissolution, and dilution by seawater and fresh groundwater, in combinations that vary across the basin. At this point owning a rare calibrated model of groundwater flow in a sedimentary basin, we also estimated the age of brines in the basin. Groundwater age varies spatially, with a maximum of ~200 My at great depths, significantly younger than the host rocks. Combining Br/Cl ratios with numerical models provided new information on the origin of the brines and should settle lingering questions about regional permeabilities and retention of brines in the Alberta Basin.