Anomalous pressures and fluid migration within the Alberta Basin, Canada.

摘要

The primary goal of this project is to advance our understanding of how ancient and modern groundwater flow systems function within the Alberta Basin. Basin hydrodynamics played a dominant role in the regional transport of hydrocarbons and heat in western Canada; however, the fluid impelling mechanisms remain a point of some controversy. The location of oil reserves within distal portions of the basin and geothermal gradient patterns suggest that regional, topography-driven groundwater flow dominates; however, observed hydraulic gradients indicate areas of groundwater flow that oppose topography-driven flow due to subnormal fluid pressures.; Quantitative modeling was used to analyze different hydrogeologic aspects of the Alberta Basin which are presented in three separate studies. The first study quantifies the petroleum generation and migration history, and constrains the permeability structure of the basin using thermal data and primary oil migration directions inferred from oil-source rock correlations. The second study examines different mechanisms controlling long-range oil migration (buoyancy, basin hydrodynamics, permeability heterogeneity) within the Viking Formation, one of the important reservoirs. Geostatistical models of heterogeneity were developed using an extensive database of measured core permeabilities. Lastly, the third study focuses on mechanical unloading versus basin hydrodynamic controls on anomalous fluid pressure generation within the basin at the present time.; The Alberta Basin is characterized by a shallow, ‘normally’ pressured flow system which overlies a lower permeability, anomalously pressured system. Oil was sourced downward into the Viking Formation through a formerly overpressured section that extended up to 100 kilometers east of the Canadian Cordillera. Overpressures were generated by mechanical sediment loading and minor pressuring due to petroleum generation. In the Late Paleocene or Early Eocene, basin uplift reduced the buoyancy drive, while meteoric recharge in uplifted regions strengthened gravity-driven groundwater flow. Modeling demonstrates that a combination of buoyancy and basin hydrodynamics was required to impel oil over several hundreds of kilometers across the basin. Permeability alteration precludes our ability to accurately delineate oil migration pathways. The modern flow system contains subnormally pressured aquifers due to a combination of limited vertical recharge through shale aquitards, erosional unloading since the Early Eocene, and glacial unloading.