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Abstract

In the Alberta Basin, carbonates of the 120-km-long Upper Devonian Bashaw reef complex and the nearby 320-km-long Upper Devonian Rimbey-Meadowbrook reef trend, together with the underlying Cooking Lake platform, act as regional subsurface conduits for oil, gas and water. Evidence of regional fluid migration in these carbonates includes: (1) huge deposits of hydrocarbons that originate from a single source rock that have migrated up-dip for hundreds of km, (2) basin-scale studies of water movement indicating that these carbonates funnel fluids from nearby formations through the deeper parts of the basin towards its edge and (3) extensive pervasive dolomitization of the carbonate “eservoir rocks by a mechanism attributed to long-distance fluid migration.

Analysis of potentiometric surfaces, pressure-depth plots and formation-fluid chemistries reveal that regional fluid flow occurs through two main aquifers. One is the Cooking Lake-Leduc aquifer that underlies and comprises the Rimbey-Meadowbrook trend, where fluids migrate laterally northward. The other is the Bashaw reef complex, where fluids move vertically upward across the Ireton aquitard and into the overlying Nisku aquifer. Regionally, fluids in the Nisku aquifer move up-dip to the northeast, except over the Bashaw area, where they are met from below by the ascending fluids from the Leduc aquifer.

The critical control on petroleum trapping in both the Leduc and Nisku formations appears to be the thickness of the intervening Ireton aquitard. Geological mapping has identified 28 wells in the Bashaw area where the Ireton aquitard is very thin or absent, creating breaches that allow for cross-formational movement of water and hydrocarbons. Trapping in both the Leduc and Nisku formations can be correlated with the thickness of the Ireton aquitard. In the southern part of the Rimbey-Meadowbrook reef trend, trapping conditions are slightly different with many Leduc reefs filled to capacity. However, some hydrocarbons must have migrated across the Ireton aquitard, because there has been scattered production from the Nisku Formation.

The results of this study demonstrate the role regional fluid flow and cross-formational flow play in hydrocarbon migration and entrapment in the subsurface. Considering the findings from the Bashaw area, it may be possible to apply the concept of vertical migration through thin shales to explore for other traps above Leduc reefs.

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