Dolomitization and Neomorphism of Mississippian (Visean) Upper Debolt Formation, Blueberry Field, Northeastern British Columbia: Geologic, Petrologic,  and Chemical Evidence

Abstract

Petrographic, chemical, and isotopic studies of the Mississippian (Visean) upper Debolt Formation in the Blueberry field, British Columbia, Canada, reveal that dolomitization was the result of several diagenetic events and that neomorphic alteration of these dolomites significantly modified their original chemical signatures. These studies also demonstrate how tectonics play an important role in controlling and modifying reservoir dolomites in the area. Petrographic investigations have documented two early dolomite phases, (1) early matrix dolomite and (2) pervasive dolomite, and two later generations, (3) coarse cement and (4) pseudomorphic replacement of crinoids. Early matrix dolomite occurs as small (average 25 µm) subhedral to euhedral crystals that replace the matrix of carbonate mudstones, wackestones, and packstones. Petrographic evidence suggests that early matrix dolomite had a relatively early, precompaction origin, possibly from marine fluids. However, geochemical evidence indicates that later fluids have altered their original geochemical signatures. ©Copyright 1997. The American Association of Petroleum Geologists. All rights reserved.1Manuscript received February 1, 1996; revised manuscript received August 16, 1996; final acceptance January 19, 1997. 2Department of Earth Sciences, University of Windsor, Windsor, Ontario, Canada N9B 3P4. Present address: Chauvco Resources, Ltd., 2900, 255-5th Avenue S.W., Calgary, Alberta T2P 3G6 Canada. 3Department of Earth Sciences, University of Windsor, Windsor, Ontario, Canada N9B 3P4. This research was supported by individual operating grants from Canada's Natural Sciences and Engineering Research Council (NSERC) awarded to I. S. Al-Aasm and additional funding from PanCanadian Petroleum Resources Limited, and Suncor Inc. S. Durocher acknowledges support from an Ontario Graduate Scholarship. S. Brame (Talisman Energy), L. Williams (Phillips Petroleum Resources, Ltd.), G. Burrowes and I. Mcllreath (PanCanadian Petroleum Ltd.), and J. Packard (Rhomb Carbonate Consulting Ltd.) are thanked for the valuable information, insights, and ideas for tackling the complexities of the Blueberry field. Thanks are also due to J. Veizer (University of Bochum and University of Ottawa) for strontium isotope analysis. An earlier version of this paper benefited from critical reviews by M. Malone (Texas A&M University), C. Viau (Shell Canada, Ltd), S. Schutter, J. Dravis (Dravis Interest Inc.), and an anonymous reviewer.