ABSTRACT

Based on sedimentological analysis of cores, the Winnipeg Group is divided into a number of symmetrical small, intermediate and large-scale base level cycles that record the vertical movement of base level relative to the surface of the Earth. Variations in 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: 1) silica cemented sandstones common in beach facies, 2) anhydrite and dolomite cemented sandstones present in beach and upper shoreface facies and 3) illite 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 developed in zones from where anhydrite or dolomite cements have been dissolved. Clay cemented sandstones are the thickest of the permeable reservoir beds examined. The illite cemented sandstones are typically bioturbated shoreface facies that potentially have a large lateral extent. Grain coating clay cements formed early and inhibited silica cementation. Pyrite is a common accessory. Clay coatings and pyrite lower formation resistivity. Proper treatment of reservoirs is critical to preventing formation damage in reservoirs with authigenic clays.