In southeastern Saskatchewan the Mississippian strata dip south-southwest and are progressively truncated northward against the post-Mississippian erosion surface. Impermeable “red beds” of the Watrous Formation unconformably overlie the Mississippian carbonate sequence.

The Benson field is a structural and stratigraphic trap in which oil has accumulated in dolomite layers of the Midale Beds beneath the erosion surface. The dolomite rocks which are microsucrosic and commonly burrowed have good to excellent intercrystalline and moldic porosity. Dolomite layers are present at several stratigraphic positions within the lagoonal carbonate rocks of the Midale Beds and range from 0.1 to 3.2 m in thickness. Either limestones (wackestones and micritic packstones), highly calcareous dolomites (microsucrosic), or anhydritic dolomites (cryptocrystalline) provide the caprocks and the bottom seals for the porous layers. Laterally, the dolomites grade into highly calcareous dolomites and/or wackestones. The porous layers result from selective dolomitization of carbonate sediments.

Close to the erosional edge, porosity in the dolomites has been reduced or obliterated by secondary anhydrite.

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