Disequilibrium fluid pressures and groundwater flow in the western Canada sedimentary basin

A numerical model has been used to study the origin of low pressure in southern Alberta and its effects on patterns of regional groundwater flow over the past 5 million years. The model accounts for changes in basin topography, conduction and advection of heat, cooling of pore fluid, and rebound of pore volume during erosion. Results show that the ~3 MPa of underpressuring observed in this region could have formed because the pore volume of sediments expanded slightly as Pliocene-Pleistocene erosion removed some of the confining load. Calculations provide an estimate for the upper limit for the permeability of Cretaceous shales. In the simulations, erosion generates potential gradients that drive groundwater along deep aquifers toward regions of lowest pressure in adjacent aquitards. Groundwater, however, moves too slowly to transport a significant amount of heat. The observed west-to-east increase in the geothermal gradient across the study area most likely occurs because sediments that were deeply buried in the west are more compacted and hence more thermally conductive than those in the east. -from Authors