ABSTRACT

Based on analysis of sedimentological attributes in cores, the Winnipeg Group can be divided into a number of symmetrical small, intermediate, and large-scale base level cycles which record the vertical movement of base level relative to the surface of the Earth. Trends of increasing and decreasing reservoir quality correlate to both intermediate and large-scale base level cycles. Better reservoir attributes tend to occur at the fall to rise turnaround points in both scales of cycles. Primary reservoir quality is overprinted by diagenesis. Three dominant cementation associations occur in the Winnipeg Group and consist of: 1) silica cemented sandstones common in bench fades, 2) anhydrite and dolomite cemented sandstones present in beach and upper shoreface facies, and 3) clay cemented sandstones present in upper and middle shoreface facies. Silica cemented sandstones have low intergranular porosity. Fracturing enhances permeability. Anhydrite and dolomite cemented sandstones show good to poor intergranular porosity. Porosity is best developed in zones where anhydrite and dolomite cements have been dissolved. Clay cemented sandstones display the thickest permeable reservoir intervals examined. The illite cemented sandstones are typically bioturbated shoreface facies that could have a large lateral extent. Grain coating clay cements are early formed and served to inhibit silica cementation. Pyrite is a common accessory mineral. The presence of clay coatings and pyrite lower formation resistivity. Proper treatment of reservoirs to prevent formation damage in reservoirs containing authigenic clays is critical.