Fluids and pressure distributions in the foreland-basin succession in the west-central part of the Alberta basin, Canada: Evidence for permeability barriers and hydrocarbon generation and migration

Maximum burial was attained in the Alberta basin at the peak of the Laramid e orogeny, when most foreland-basin strata in the west-central part of the basin entered the oil and gas windows. Overpressures developed in this regi on as a result of compaction and hydrocarbon generation. Since then, the ba sin has undergone tectonic relaxation, uplift, and erosion, and the rate of hydrocarbon generation has decreased. Overpressures are still maintained i n strata of the Cretaceous Mannville and Colorado groups in areas adjacent to the deformation front as a result of continuing gas generation at rates higher than gas escape. In other regions in these strata, hydrocarbon loss is not fully compensated by hydrocarbon generation. Significant underpressu res in hydrocarbon- and water-saturated regions in the west-central part of the Alberta basin are the combined result of this effect and of Tertiary t o Holocene erosional and postglacial rebound in thick shales. The flow is i nward in places, mostly toward the underpressured gas-saturated regions and the sinks created by erosional and postglacial rebound. Because of low rec harge rates in a low-permeability environment, water is not capable of imbi bing these regions at rates that would repressurize the system. The thick s haly aquitards retard the recharge from the ground surface, leading to subh ydrostatic pressures in various aquifers. Only the post-Colorado succession , where topography drives the groundwater flow, seems to have adjusted to t he new ground surface. The entire foreland-basin succession up to the post- Colorado aquifers is cut from recharge from the fold and thrust belt as a r esult of hydrodynamic and possibly physical barriers. The distributions of gas, oil, and water and of underpressure, subhydrostatic pressure, and over pressure in various strata in the area indicate absolute and relative perme ability barriers to flow that impede pressure transmission, hydrocarbon mig ration, and water flow.