Basin-Scale Fluid Flow, Hydrochemistry, and Petroleum Entrapment in Devonian Reef Complexes, West-Central Alberta, Canada

Benjamin J. Rostron, Jozsef Toth, H-G. Machel

The 120 km long Bashaw reef complex and the nearby 320 km long Rimbey-Meadowbrook reef trend act as pipelines in the Western Canada Sedimentary Basin by transporting oil, gas, and water through the subsurface. Evidence of regional fluid migration in these reef trends includes: (1) Huge deposits of hydrocarbons that originate from a single source rock that have been subsequently distributed up the reef trends for hundreds of kilometers. (2) Basin-scale studies of water movement indicating that these reefs funnel fluids from nearby formations through the deeper parts of the basin towards its edge. (3) Extensive pervasive dolomitization of the reservoir rocks by a mechanism attributed to long-distance fluid migration. Despite the known effects of regional fluid movement, the e have been few detailed studies of the flow systems, fluid chemistries, and the relationship between fluid flow and petroleum entrapment in these reef trends. To better understand migration and entrapment in these reef trends, a petroleum hydrogeological study on an 18,000 km² area of west-central Alberta was completed.

Analysis of potentiometric surfaces, pressure-depth plots, and formation-fluid chemistries reveal that regional fluid flow occurs through two main aquifers. In the Leduc-Cooking Lake aquifer, saline brines move in distinctly different directions in each reef trend. In the Rimbey-Meadowbrook trend, fluids migrate laterally northward. In the Bashaw reef complex, fluids move vertically upward across the Ireton aquitard, into the Nisku Formation. Regionally, fluids in the Nisku aquifer move up-dip to the northeast, except over the Bashaw area where they are met from below by ascending waters from the Leduc aquifer.

Given the ascending flow system in the Bashaw reef complex, the critical control on trapping in both the Leduc and Nisku Formations appears to be the thickness of the intervening Ireton shale. Geologic mapping has identified 28 wells in the Bashaw area where the Ireton is absent, creating direct breaches in the shale and allowing for vertical movement of water and hydrocarbons. Trapping in both the Leduc and Nisku Formations can be correlated with the thickness of the Ireton shale. In the Rimbey-Meadowbrook reef trend, trapping conditions are slightly different with Leduc reefs filled to capacity. However, some hydrocarbons must have migrated across the Ireton shale because there has been scattered production from the Nisku Formation.

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

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995