Flow of Formation Waters, Aquifer Characteristics, and Their Relation to Hydrocarbon Accumulations, Northern Alberta Basin

Abstract

Based on the wealth of data generated by the oil industry, the regional-scale characteristics of rocks, the flow of formation waters, and their relation to hydrocarbon accumulations were analyzed for the northern Alberta basin. The flow of formation waters in several aquifers and aquifer systems separated by intervening aquitards is at steady state and is driven by the present-day topography both on a regional and a local scale. The flow is generally from a recharge area in the southwest at the fold belt and Bovie Lake fault, to discharge in the northeast at the Great Slave Lake, the lowest point in the basin. The flow in Devonian aquifers is in open systems from recharge to discharge areas, whereas the flow in Carboniferous and Cretaceous aquifers is in semi-open systems, discharging into adjacent aquitards. Very high porosity and permeability in places in the Devonian Elk Point aquifer system are due to reefs, fracturing, dolomitization, and karst processes. Very high permeability probably leads to relatively high flow rates along the Presqu'ile barrier reef, resulting in local advective effects on the terrestrial heat transport to the surface. On a regional scale, all of the aquifers are underpressurized due to upstream propagation through high-permeability zones of low hydraulic heads at discharge elevations. The flow pattern is corroborated by salinity distributions, with comparatively lower salinity in each aquifer at recharge in the southwest and at discharge in the northeast caused by mixing with fresher meteoric water, and higher salinity between recharge and discharge areas. Salinity distributions show that the aquifers are not completely flushed of the original formation waters. Dissolution of salt and anhydrite from adjacent strata leads to high salinity in the Elk Point aquifer system and Beaverhill Lake aquifer. Hydrocarbons generated in the southwest in Devonian and Carboniferous strata, at maximum burial depth during the Laramide orogeny, migrated updip to the northeast driven by their own buoyancy and entrained by the flow of formation waters. Unless stratigraphically trapped by reefs and at the edge of semi-open aquifers, migration to the discharge areas led to loss of the volatile components and biodegradation into altered bitumens. The hydrostratigraphy and direction of the flow of formation waters in the northern part of the Alberta basin indicate that hydrocarbons generated in this region did not contribute to the formation of the giant Athabasca oil sand deposit located southeast of the study area.