FLOW OF FORMATION WATERS, AQUIFER CHARACTERISTICS, AND THEIR RELATIONTO HYDROCARBON ACCUMULATIONS, NORTHERN ALBERTA BASIN

Based on the wealth of data generated by the oil industry, the regiona l-scale characteristics of rocks, the flow of formation waters, and th eir relation to hydrocarbon accumulations were analyzed for the northe rn 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 southwe st at the fold belt and Bovie Lake fault, to discharge in the northeas t 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 ar e due to reefs, fracturing, dolomitization, and karst processes. Very high permeability probably leads to relatively high flow rates along t he Presqu'ile barrier reef, resulting in local advective effects on th e terrestrial heat transport to the surface. On a regional scale, all of the aquifers are underpressurized due to upstream propagation throu gh high-permeability zones of low hydraulic heads at discharge elevati ons. The flow pattern is corroborated by salinity distributions, with comparatively lower salinity in each aquifer at recharge in the southw est and at discharge in the northeast caused by mixing with fresher me teoric water, and higher salinity between recharge and discharge areas . Salinity distributions show that the aquifers are not completely flu shed of the original formation waters. Dissolution of salt and anhydri te from adjacent strata leads to high salinity in the Elk Point aquife r system and Beaverhill Lake aquifer. Hydrocarbons generated in the so uthwest in Devonian and Carboniferous strata, at maximum burial depth during the Laramide orogeny, migrated undip to the northeast driven by their own buoyancy and entrained by the flow of formation waters. Unl ess stratigraphically trapped by reefs and at the edge of semi-open aq uifers, migration to the discharge areas led to loss of the volatile c omponents and biodegradation into altered bitumens. The hydrostratigra phy and direction of the flow of formation waters in the northern part of the Alberta basin indicate that hydrocarbons generated in this reg ion did not contribute to the formation of the giant Athabasca oil san d deposit located southeast of the study area.