NUMERICAL STUDIES OF BRINE MIGRATION IN THE ALBERTA BASIN, CANADA

The Alberta Basin has been the subject of many basin-scale hydrogeologic studies. There is, however, a strong disparity between hydrogeologic and geochemical estimates of the residence time of brines in this basin. This project goes beyond previous hydrogeologic modeling studies of fluid and heat flow by adding variable-density solute transport and using salinity and Br/Cl ratios as geochemical constraints. 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. Whereas previous studies suggested that only topography driven flow flushed out brines from this basin, our results indicate that compaction driven flow during the foreland basin stage also contributed to the flushing of brines. Preliminary simulations suggest that a regional fluid drain develops from west to east along permeable layers below thick, over-pressured shales of the Colorado Group of rocks during the foreland basin stage (100 Ma to 60 Ma) which generates a pulse of brines that moves out of the basin at this time. Buoyancy driven flow is found to exist throughout the 100 Ma years of simulation, especially around the evaporite layers. Following uplift of the basin and formation of the Rockies, fresh water infiltrating the basin enhances dissolution of evaporites and a considerable amount of brine is generated and lost. Initial simulations using established permeability data for the rock types in the basin show that, contrary to field observations, almost all the evaporite layers dissolve by the end of the period of simulation, suggesting that previous studies may have over-estimated solute transport rates.