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Department of Geology and Geophysics, University of Calgary, Calgary, Alberta, Canada T2N 1N4 jchopkin{at}ucalgary.ca
Dolomudstones of the Pekisko Formation in western Canada form small but important oil and gas reservoirs. The reservoirs are irregularly shaped bodies 1 km or so wide and commonly 5–8 m thick. Porosity development within the dolomudstones is a complex function of sedimentation, early facies-selective dolomitization and later telogenetic leaching of calcareous components.
The carbonate sediment precursor of the dolomudstone, interpreted from relict textures preserved in chert nodules, was a microwackestone with abundant silt-sized skeletal fragments. Dolomudstone reservoirs are comprised of dolomudstones, calcareous dolomudstones, and subordinate interbedded dolowackestones and dolograinstones. Some dolomudstone reservoirs are contained entirely within grainstones. Others are capped by tight fenestral lime mudstone that has been dolomitized locally. Dolomitization has been most intense within the centres of these reservoirs, and dolomudstones grade laterally into calcareous dolomudstones.
The association of facies indicates that microwackestones were deposited in subtidal intershoal and lagoonal environments on an inner ramp. Grainstone shoals provided a broad barrier that absorbed wave energy seaward of the lagoon. Fenestral lime mudstones accumulated in peritidal environments in restricted areas of the inner ramp, landward of the lagoon. Dolomitization is interpreted to have been early and selective to the microwackestone facies because it retained permeability or was reactive during early burial. Dolomitizing fluids were most probably derived from overlying formations and made their way downwards through spatially separated conduits. The Pekisko Formation was exposed and sculptured at several Jurassic-Early Cretaceous unconformities. During these times, sandstones and shales were deposited in solution cavities developed within the dolomudstones. Concomitant leaching of calcite increased porosity of the dolomudstone reservoirs.
