Regional-scale hydrogeology of the Upper Devonian-Lower Cretaceous sedimentary succession, south-central Alberta Basin, Canada

The flow of formation waters in the Upper Devonian-Lower Cretaceous sedimentary succession in the south-central part of the Alberta basin is controlled mainly by (1) outcrops of Devonian and Mississippian strata at high elevation in the south in Montana, and at low elevation at the Peace River in the north; (2) subcrops of Devonian to Jurassic strata at the sub-Cretaceous unconformity; and (3) deposition of Cretaceous sediments on pre-Cretaceous relief exposed for a long period of time. Weathering of Upper Devonian strata during this long period of subaerial exposure and the concurrent paleokarsting of the Grosmont Formation led to high permeability in these aquifers. As a result, the Grosmont aquifer and the Upper Devonian aquifer system that subcrop at the sub-Cretaceous unconformity form a drainage path in a northward regional-scale flow system in the southern and central parts of the Alberta basin. This long-range flow system is fed by meteoric recharge in the south, by updip flow of connate waters from deep Paleozoic aquifers that subcrop along the western flank of the main system, and by downdip meteoric recharge through Cretaceous strata along the eastern basin edge. A plume of relatively high salinity is formed in the Lower Mannville aquifer in the area where highly saline Devonian waters discharge at the sub-Cretaceous unconformity and mix with fresh water of meteoric origin. Hydrocarbons generated in Upper Devonian to Lower Cretaceous strata of the deep foreland basin migrated northeastward updip, driven by buoyancy and supported by a concurrent hydrodynamic drive. The great majority of the generated hydrocarbons reached the sub-Cretaceous unconformity, where they were trapped in complex stratigraphic traps in the Lower Mannville Formation. Downward flow of meteoric water along the eastern flank of the basin hydrodynamically enhanced the trapping and led to hydrocarbon biodegradation in place into heavy oils and oil sands in the Cold Lake and Athabasca areas.