Synthesis and Model of Formation-Water Flow, Alberta Basin, Canada

ABSTRACT Based on a large amount of publicly available data, several studies have previously examined the flow of formation waters in different parts of the Alberta basin, offering various interpretations as to the causes of the observed pressure regime and flow pattern; however, there has been no synthesis of these diverse studies on a basin-wide basis. Accordingly, these studies are critically reviewed in this paper and synthesized in a new basin-scale model of the flow of formation waters in the Alberta basin. The proposed regional-scale model has significant implications for understanding hydrocarbon migration pathways, ore genesis, the geothermal regime, and deep waste disposal in the Alberta basin. Several flow systems, each one driven by a different mechanism, are identified, together with the main processes leading to the nonhydrostatic pressures observed in the basin. Two megahydrostratigraphic successions and associated flow systems are recognized. The first succession corresponds to the pre-Cretaceous passive-margin stage of basin development, and consists of thick, carbonate-dominated aquifer systems separated by shaly aquitards and evaporitic aquicludes. A northeastward basin-scale flow system is driven by basin topography, with recharge in Montana and discharge in northeastern Alberta. Southwest-to-northeast regional-scale flow adjacent to the fold and thrust belt is probably the result of past tectonic processes. The salinity of formation water in pre-Cretaceous aquifers is high, and generally increases both northward and with depth. This variability is the result of increased water-rock reactions (mineral solubility) with increased temperature, and of incomplete flushing by meteoric water. As a result of salinity variations, flow-retarding buoyancy effects can be important. The second megahydrostratigraphic succession corresponds to the post-Jurassic foreland stage of basin evolution, and consists of thick, shaly aquitard systems and relatively thin sandstone aquifers. In the southwestern part of the basin, flow in isolated aquifers is driven southwestward by erosional rebound in the thick intervening shales, downdip toward the fold and thrust belt. The salinity of formation waters in the post-Jurassic aquifers is low. Mixing of waters and interference between the major flow systems takes place along the pre-Cretaceous unconformity where successively older pre-Cretaceous aquifers subcrop from west to east. Flow in shallow local systems at the top of the sedimentary succession is driven by local topography.