Doug N. Harvey¹, Don M. Kent², Hairuo Qing³
(1) University of Regina, S4S 0A2 Regina, SK
(2) University of Regina, Regina, SK
(3) University of Regina

Abstract: Characterization of Mississippian Midale and Ratcliffe Reservoir Dolostones of Southeast Saskatchewan

Petrography, diagenesis, and reservoir characteristics of dolomite reservoirs were studied from Mississippian carbonates of the northern Williston Basin in southeastern Saskatchewan. These dolomites are hosted in the Midale and Ratcliffe Beds and were sampled from the Benson Midale, Glen Ewen Midale, Oungre Ratcliffe, and Hummingbird Ratcliffe fields. The Midale and Ratcliffe are part of the Madison Group which represents shallowing upward cycles deposited in an epeiric sea. It consists of deeper water carbonates at the base and shallow marine carbonates and tidal flat carbonates and evaporites in the upper portions. Dolomites in the Midale and Ratcliffe dolomites represent the dolomitization of lime muds deposited in restricted lagoonal to subtidal-intertidal environments.

Petrographically and petrophysically, two types of dolomites are recognized in these beds. Type 1 is cryptocrystalline (<4 µm) to microcrystalline (4-8 µm) dolomites associated with tidal flat sediments and evaporitic anhydrites. These dolomites generally have low porosity and permeability and occur in sections both above and below the current producing reservoirs. These dolomites are interpreted to have formed penecontemporaneously by evaporitic sea water. Type 2 is microcrystalline to microsucrosic (8-14 µm) reservoir dolomites with good intercrystalline porosity (~21%) and permeability (~12 md). The reservoir dolomites are found in all four fields and their effectiveness as reservoirs appears to depend on degree of dolomitization. Geochemical analysis will be carried out to determine the source and timing of fluids associated with the reservoir quality dolomites.

AAPG Search and Discovery Article #90914©2000 AAPG Annual Convention, New Orleans, Louisiana