REGIONAL-SCALE POROSITY AND PERMEABILITY VARIATIONS IN UPPER DEVONIANLEDUC BUILDUPS - IMPLICATIONS FOR RESERVOIR DEVELOPMENT AND PREDICTION IN CARBONATES

Upper Devonian carbonate rocks of the Rimbey-Meadowbrook reef trend in the subsurface of central Alberta are characterized by a wide range o f 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 o f burial depth, limestones and dolomitic limestones are more porous th an dolostones. There is an overall decrease of porosity and permeabili ty in Leduc Formation carbonates with increasing burial depth and ther mal maturity along the reef trend. At relatively shallow burial depths (<2000 m), limestone and dolomitized buildups have comparable porosit y values and distributions. Dolomitization has resulted mainly in redi stribution of primary limestone porosity to secondary dolomite porosit y types with a concomitant minor increase in permeability. At greater 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-redu cing processes, particularly pressure solution. Thus, reservoir qualit y in dolostone reservoirs decreases less severely with depth. In addit ion, porosity enhancement in dolostones by leaching in the burial envi ronment during and after dolomitization is important. Although these s ignificant relationships are not new concepts, they appear to be under appreciated.The better porosity and permeability of dolostones relati ve to limestones at greater depths is of economic significance for exp loration in Upper Devonian carbonates of the deep Alberta basin (and p robably other carbonate areas), where reservoirs are mostly found in d olostones 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 buildu ps that were connected to a regional conduit system.