Residence Times of Brines in the Alberta Basin, Canada

Br/Cl and O/D ratios have often been used to explain the origin and evolution of formation waters in sedimentary basins. A strong disparity, however, exists between geochemical and hydrogeologic estimates of the residence times of brines in the Alberta Basin, Canada. Geochemical evidence suggests that these brines have been preserved at depths for hundreds of millions of years with little to no dissolution, whereas hydrogeologic models suggest that these brines should have been flushed out of the basin in less than 2 million years. This project goes beyond previous hydrogeologic models of fluid and heat flow by adding variable-density solute transport and uses the 2D finite element FORTRAN code, COMPACT, that also accounts for sediment compaction in tectonically evolving basins. COMPACT has been modified to add effects of erosion and sediment decompaction, and dissolution of evaporites. Brine migration and groundwater age are simulated over the last 100 million years, along a 700 Km, east-west cross-section of the basin, using salinity, halite dissolution, Br/Cl and O/D ratios as geochemical constraints.

Our results suggest that contrary to geochemical interpretations, halite dissolution has significantly contributed to the salinity distribution in the basin. Formation waters in this basin are not simple end members of a seawater evaporation model, but bear signatures of original residual brines formed by evaporation of seawater, halite dissolution, freshwater infiltration and mixtures of some or all of the above in various proportions. Low permeabilities are required to retain originally evaporated brines in the deeper parts of the basin. Most of the brine loss occurs during erosional unloading (55 Ma to present) but some brines are also lost during the deposition of the foreland basin (100 Ma to 60 Ma) when a west-east, regional fluid drain develops along permeable layers below thick, over-pressured shales of the Colorado Group of rocks.