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

摘要

Maximum burial was attained in the Alberta basin at the peak of the Laramide orogeny, when most foreland-basin strata in the west- central part of the basin entered the oil and gas windows. Over- pressures developed in this region as a result of compaction and hydrocarbon generation. Since then, the basin has undergone tec- tonic relaxation, uplift, and erosion, and the rate of hydrocarbon generation has decreased. Overpressures are still maintained in strata of the Cretaceous Mannville and Colorado groups in areas adjacent to the deformation front as a result of continuing gas gen- eration at rates higher than gas escape. In other regions in these strata, hydrocarbon loss is not fully compensated by hydrocarbon generation. Significant underpressures 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 to Holocene ero- sional and postglacial rebound in thick shales. The flow is inward in places, mostly toward the underpressured gas-saturated regions and the sinks created by erosional and postglacial rebound. Because of low recharge rates in a low-permeability environment, water is not capable of imbibing these regions at rates that would repres- surize the system. The thick shaly aquitards retard the recharge from the ground surface, leading to sub hydrostatic pressures in vari- ous aquifers. Only the post-Colorado succession, where topography drives the groundwater flow, seems to have adjusted to the 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 result of hydrodynamic and possibly physical barriers.The distributions of gas,oil,and watera nd of underpressure,subhydrostatic pressure,and overpressure in various strata in the area indicate absolutea nd relative permeability barriers to flow that impede pressure transmission,hydrocarbon migration,and water flow.