Regional-Scale Porosity and Permeability Variations in Upper Devonian Leduc Buildups: Implications for Reservoir Development and Prediction in Carbonates

Abstract

Upper Devonian carbonate rocks of the Rimbey-Meadowbrook reef trend in the subsurface of central Alberta are characterized by a wide range of porosity and permeability values. Dolostones show the highest values of absolute and average horizontal and vertical permeability. For all rock types, horizontal permeability averages tens of times to several hundred times the vertical permeability. If considered irrespective of burial depth, limestones and dolomitic limestones are more porous than dolostones. There is an overall decrease of porosity and permeability in Leduc Formation carbonates with increasing burial depth and thermal maturity along the reef trend. At relatively shallow burial depths ( 2000 m), dolostones are significantly more porous and permeable than limestones. Dolostones appear to retain their porosity and permeability much better than limestones during burial, indicating that dolostones are more resistant to porosity- and permeability-reducing processes, particularly pressure solution. Thus, reservoir quality in dolostone reservoirs decreases less severely with depth. In addition, porosity enhancement in dolostones by leaching in the burial environment during and after dolomitization is important. Although these significant relationships arenot new concepts, they appear to be underappreciated. The better porosity and permeability of dolostones relative to limestones at greater depths is of economic significance for exploration in Upper Devonian carbonates of the deep Alberta basin (and probably other carbonate areas), where reservoirs are mostly found in dolostones and where limestones have a low reservoir potential and pose a considerable exploration risk. The best reservoir potential in the Leduc Formation of the deep Alberta basin occurs in dolomitized buildups that were connected to a regional conduit system.