Evolution of Fluid Flow Systems, Geothermal Regime, and Hydrocarbon Migration and Accumulation in the Alberta Basin, Canada

摘要

The Alberta Basin is the result of basin-margin deposition of mainly carbonates and evaporites during Late Proterozoic to Middle Jurassic time, followed by foreland-basin deposition of siliciclastics derived from the Cordillera during the accretion to the North American craton of allochthonous terranes until the Paleocene. Since then, the basin underwent uplift and significant erosion. Various mechanisms drove the flow of formation waters in the past, such as compaction, tectonic compression, topography and, probably, free convection. Today, the flow in deep Cambrian and Middle Devonian strata is driven by past tectonic compression, the flow in Upper Devonian to Jurassic strata is driven by basin-scale topography in a long range system from southern to northeastern Alberta, the flow in Cretaceous strata in southwestern Alberta is driven inward by erosional rebound in thick shales, while the flow close to the surface is driven by local topography. As a result of very low permeability of basin rocks on a regional scale, the transfer of terrestrial heat from the crystalline Precambrian basement to the surface is dominated by conduction, except for a reef barrier at the northern edge of the basin, where, because of high permeability, forced convection driven by topography is strong enough to distort the conductive geothermal regime and create an anomaly at aquifer discharge near Great Slave Lake. At the basin scale, geothermal gradients and basement heat flow increase northward, being controlled by basement structure and age, Local-scale anomalies superimposed over this trend are due to basement heterogeneity, variations in the structure and thermal conductivity of the sedimentary cover, and local effects of formation-water flow. The westernmost part of the basin is in the stage of gas generation, the middle section is in the stage of oil generation, while the eastern, shallow part did not reached the oil window. Hydrocarbons generated in the western, deeper part of the basin migrated updip northeastward, toward the shallow basin edge, where they were trapped in stratigraphic and structural traps, lost their volatile component and were biodegraded in place by fresh meteoric water, forming the huge Athabasca, Peace River and Cold Lake oil sands deposits.