Abstract

The Pekisko Formation in western Canada is a third-order sequence comprised of an open-marine grainstone through restricted carbonate mudstone succession. Truncation of the Pekisko along an unconformity edge prior to the Jurassic and several periods of incision from Early Jurassic-Early Cretaceous have formed an intricately sculptured subcrop belt. In the Medicine River field, oil is trapped in discrete pools close to the unconformity edge. Reservoir facies comprise three lithologies. Medium-crystalline dolostone with intercrystalline and vuggy porosity forms the reservoir in an elongate dolostone body that has replaced grainstone. Fine-crystalline dolostone is facies selective within a lime-mudstone unit in which the common reservoir rock is bioturbated dolomudstone with intercrystalline porosity. Grainstones have secondary porosity formed though leaching of microcrystalline calcite allochems and intergranular cements. Reservoir quality is assessed from consideration of orthogonal permeability values (K max = maximum horizontal permeability, K 90 = horizontal permeability at 90 degrees to the maximum, and K v = vertical permeability) obtained from conventional whole-core analyses. Sedimentary lamination primarily affects the K v , and the presence of fine-grained lamination dictates that K v < K 90 in laminated rocks. An empirical relationship between the three orthogonal permeability values records the presence of fractures is where K max > 3K 90 and K v > K 90 . From this relationship, the fracture density in all three lithologies appears to be similar. Low and variable porosity and permeability values in grainstones indicate that the fracture system is poorly connected to the matrix. Despite leaching and fracturing at the unconformity edge, reservoir distribution closely follows patterns of facies that were susceptible to dolomitization. Future exploration potential lies in fine-crystalline dolostone bodies that may form stratigraphic traps downdip from the unconformity edge.

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