Cl/Br compositions as indicators of the origin of brines: Hydrogeologic simulations of the Alberta Basin, Canada

摘要

The origin and residence time of brines in the Alberta Basin have been debated for more than 40 years, with conflicting conclusions reported by geochemical and hydrogeologic studies. Here, numerical models were used to determine hydrogeologically feasible scenarios for the origin of brines in the Alberta Basin, using salinity and Cl/Br ratios as geochemical constraints. The models simulated variable-density fluid flow, heat transport, solute transport, and sediment compaction and decompaction in the Alberta Basin over the last 100 m.y. Simulation results suggest that pore fluids in this basin represent a mixture of four geochemical end members: seawater, freshwater, brines formed by evaporation of seawater, and, contrary to prior interpretations of Cl/Br ratios, brines derived from halite dissolution. Sensitivity studies revealed that similar distributions of salinity and Cl/Br ratios could be obtained without dissolution of halite if extremely low permeabilities were used in the model, but this scenario conflicts with field-based permeability data and prior simulations of petroleum migration in the basin. The residence time of brines in the Alberta Basin has thus likely been overestimated. The presence of evaporites introduces significant uncertainty in the use of Cl/Br ratios for interpreting the origin of brines in sedimentary basins, but salinity and Cl/Br ratios provide valuable new constraints for regional-scale models.