LONG-TERM SOLUTE TRANSPORT IN DEEP SEDIMENTARY BASINS: EFFECTS OF SHALE COMPRESSIBILITY

There is a strong disparity between hydrogeologic and geochemical estimates of the residence time of brines in the Alberta Basin, Canada, which translates to significant uncertainty in transport rates for such processes as petroleum migration, carbon sequestration, sedimentary diagenesis, and ore formation. Geochemical evidence suggests that these brines have been preserved at depths for hundreds of millions of years, whereas hydrogeologic models suggest that these brines should have been flushed out of the basin in less than 2 million years. Hydrogeologists have attempted to reconcile this difference by presenting a lower permeability scenario, but no previous study included the extensive evaporites that underlie a major portion of the basin. This project goes beyond previous hydrogeologic modeling studies of fluid and heat flow by adding variable-density solute transport and using salinity, halite dissolution and groundwater age as geochemical constraints to test the impact of both permeability and shale compressibility on brine migration in the basin. Here, we simulate brine migration over the last 100 my, along a 700 Km east-west cross-section, using COMPACT, a 2D finite element FORTRAN code that simulates variable-density fluid flow, heat transport, solute transport, and sediment compaction in tectonically evolving basins. COMPACT has been modified to add effects of erosion and sediment decompaction, and dissolution of evaporites.

Our results show that even hydrogeologic simulations using low permeability scenarios overpredict evaporite dissolution. Although the salinity distribution is sensitive to the permeability structure of the basin, it is also sensitive to the fluid pressure distribution and the compressibility of shale formations in the basin. Most of the brine loss occurs during erosional unloading (55 Ma to present) with associated decompaction and fluid pressure release, but some brines are also lost during the deposition of the foreland basin (100 Ma to 60 Ma) when a regional fluid drain develops from west to east along permeable layers below thick, over-pressured shales of the Colorado Group of rocks. Maintaining the same permeability structure, and changing only the compressibility of shales, however produces different rates of solute transport.