Diagenesis and Reservoir Characterization of Ordovician Yeomen Carbonate Rocks, Southeastern Saskatchewan, Canada

Qing, Hairuo, University of Regina, Regina, SK

Dolostones occur as main reservoirs in the Ordovician Yeomen carbonate in Midale area, southeastern Saskatchewan. Three types of dolomite were identified in the Yeomen Formation: 1) dolomitized burrow mottling that is widespread in the entire formation; 2) dolomitized host carbonate matrix preferentially in the upper 50 meters of the formation; and 3) minor saddle dolomite cement in vugs and fractures. The reservoirs display a great degree of heterogeneity related to the burrow texture and dolomitization. The hydrocarbons are produced primarily from dolostone reservoirs, indicating that dolomitization has played a major role in forming reservoir quality porosity. The preferential dolomitization of burrow mottling and its widespread occurrence suggest that burrow mottling was likely dolomitized contemporaneously in a normal marine environment by seawater. The dolomitization could be related to the activities of the burrowing organisms, which created a better permeability in the burrowing network. This would have facilitated the fluid flows in the burrow networks and responsible for the preferential dolomitization of burrow mottling. The dolomite in the host matrix is interpreted to have occurred at very shallow burial (ten’s of meters) by evap­oritic seawater during precipitation of Lake Alma anhydrite because the vertical and lateral distribution of matrix dolomite is closely related to the Lake Alma anhydrite. The trace amount of saddle dolomite cement is probably related to cannibalization of earlier replace­ment dolomite through chemical compaction during the deep burial as indicated by their low oxygen isotopic values and high homogenization temperatures.