Diagenesis of the Mississippian Midale Beds, Steelman Midale Pool, Southeastern Saskatchewan

The Mississippian Midale Beds in southeastern Saskatchewan contain the most productive hydrocarbon reservoirs in the northern part of the Williston Basin. Previous works in this area focused mainly on the stratigraphic and lithologic aspects, and systemic petrographic and geochemical works are needed in order to recognize the burial history, diagenetic processes and their impacts on the quality of hydrocarbon reservoirs in this area.;Stratigraphic, petrographic, and geochemical evidence suggests that 1) dolostones in the Midale strata in the Steelman Midale Pool have experienced re-crystallization during burial. A combination of sabkha-type and reflux models can be used to interpret the dolomitization process. 2) CaSO4 rich fluids altered the host carbonates with the formation of equant anhydrite by replacement and precipitation of fibrous to bladed anhydrite in open space. Entrapment of hydrocarbon during deep burial altered equant replacement anhydrite crystals, and stained them a glassy brown color.;In the Midale Beds of the Steelman area, good original interparticle porosity in fossiliferous wackestone/packstone and peloidal/algal wackestone has been decreased by calcite cementation, physical compaction, formation of metasomatic anhydrite and anhydrite cementation. However, dissolution molds and vugs, fractures, and extensive matrix dolomitization substantially improved porosity and permeability in these reservoirs. Anhydrite cementation significantly decreased the porosity in patterned dolostone in strata above bioturbated dolomudstone to produce effective seals for the hydrocarbon reservoirs. This study confirms that bioturbated dolomudstone and dolomitic lime-mudstone in the upper part of the Midale carbonates are good targets for hydrocarbon exploration.;In this study, nine lithofacies and fourteen diagenetic features were identified based on core samples and thin section examination. Core samples were examined from ten wells in the Steelman Midale Pool. Seventy-five thin sections from selected samples were examined using a petrographic microscope. Forty thin sections with well developed dolomite crystals were examined by cathodoluminescence. Fifty-two carbonate samples were analyzed for carbon and oxygen isotopes and twenty-one anhydrite samples for sulphur isotope. Twenty-one selected dolomite samples and six anhydrite samples were analyzed for strontium isotopes, and fifteen primary, two-phase fluid inclusions in bladed replacement anhydrite sample were analyzed by microthermometry.