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Abstract

(Begin page 1231)

AAPG Bulletin, V. 85, No. 7 (July 2001), P. 1231-1252.

Copyright ©2001. The American Association of Petroleum Geologists. All rights reserved.

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

Karsten Michael, Stefan Bachu

1Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, Alberta, Canada
2Alberta Geological Survey, Alberta Energy and Utilities Board, 4999-98 Avenue, Edmonton, Alberta, Canada, T6B 2X3; email: [email protected]

AUTHORS

Karsten Michael received a Diplomaed degree (M.Sc. degree equivalent) in geology from the Berlin Technical University, Germany. He recently completed his Ph.D. in the Department of Earth and Atmospheric Sciences at the University of Alberta in Edmonton, Canada. His interests are in regional-scale fluid flow in sedimentary basins and the effects of fluid and aquifer heterogeneity.

Stefan Bachu has engineering and M.Sc. degrees and a Ph.D. in hydraulics, hydrogeology, and transport processes. After postdoctoral research at Cornell University, he joined in 1983 the Alberta Geological Survey in Edmonton, Canada, where he currently is geoscience advisor and leader of the Energy Section. His areas of interest and specialization include hydrogeology and geothermics of sedimentary basins, reservoir and aquifer analysis and characterization, and, more recently, sequestration of CO2 in geological media in response to climate change. He applies his interests to the platform-margin and foreland Alberta basin.

ABSTRACT

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. Overpressures developed in this region as a result of compaction and hydrocarbon generation. Since then, the basin has undergone tectonic 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 generation 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 erosional 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 repressurize the system. The thick shaly aquitards retard the recharge from the ground surface, leading to subhydrostatic pressures in various 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 (Begin page 1232) as a result of hydrodynamic and possibly physical barriers. The distributions of gas, oil, and water and of underpressure, subhydrostatic pressure, and overpressure in various strata in the area indicate absolute and relative permeability barriers to flow that impede pressure transmission, hydrocarbon migration, and water flow.

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