Diagenesis of the Mississippian Alida Beds, Williston Basin, Southeastern Saskatchewan

Cornelius M. Rott and Hairuo Qing
University of Regina, Regina, SK

The Mississippian Alida Beds are a suite of carbonate rocks present in the subsurface of southern Saskatchewan. They are host to significant hydrocarbon reserves and, together with the Mississippian Midale and Frobisher Beds, represent the most prolific oil bearing strata in the northern Williston Basin. In southeastern Saskatchewan the Alida Beds dip to the southwest towards the center of the Williston Basin and are truncated by a regionally extensive unconformity. Deposition likely occurred in a carbonate shelf setting in shallow marine waters. Crinoidal and coated grain pack- and grainstones make up the bulk of the sequence, interlayered with partially dolomitized mudstones and peloidal pack/grainstones.

Mississippian reservoirs characteristically exhibit a high degree of vertical and lateral heterogeneity, which results from rapid lateral facies changes as well as a complex history of diagenetic processes. A succession of 18 different diagenetic events has been identified based on petrographic and fluid inclusion work. The most important processes controlling reservoir quality appear to be early calcite cementation, early dolomitization that probably occurred during seepage-reflux of evaporative brines, at least two stages of anhydritization, and late-stage sulphate dissolution. The petrographic and fluid inclusion data also show that formation of the reservoir-sealing caprock involved further dolomitization followed by partial anhydritization and occurred before deposition of the overlying Lower Watrous siliciclastics.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005