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Case histories of textural and reservoir analyses of two Mississippian carbonate cycles of the Western Canada basin are presented to illustrate the relationships of grain, matrix, and cement variants of carbonate rocks to porosity and permeability determinations.
Large stratigraphic oil pools have been discovered, at or near the Paleozoic subcrop of the Mississippian "Midale" carbonate cycle, in southeastern Saskatchewan. Apart from scattered, vuggy, algal-encrusted strand line deposits, most of the carbonates of the "Midale" producing zone consist of skeletal and oolitic limestones which have a finely comminuted, commonly dolomitized, limestone matrix with intergranular and chalky porosity. Effective reservoir porosity is controlled by the relative distribution and grain size of this matrix.
Major hydrocarbon (oil and gas) reserves have been found in the Mississippian "Elkton" carbonate cycle, both in the Foothills Belt and along the subcrop, in southwestern Alberta. Effective reservoir material of this cycle was found to consist mainly of the dolomitized equivalent of an originally coarse skeletal limestone, with a variable amount of generally porous, finely comminuted (granular) skeletal matrix. Primary porosity was very important in the control of dolomitization which probably began with the replacement of this matrix by euhedral rhombohedrons and finally affected the coarse skeletal material (now generally indicated by leached fossil cast outlines). These porous dolomites grade laterally in a predictable way into tight, relatively non-dolomitized, well sorted, coarse keletal limestones with original high interfragmental porosity now completely infilled with clear crystalline calcite. This lithification by cementation took place early in the history of carbonate sedimentation of this area and before secondary dolomitization processes took effect.
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