ABSTRACT

The Midale oil field in southeastern Saskatchewan lies on the northeastern flank of the Williston Basin. Oil occurs mainly in Mississippian strata that dip south-southwestward and are truncated progressively northward by a Late Mississippian-Early Jurassic erosion surface.

The reservoir is in the Midale Beds, a suite of carbonates and evaporites that was deposited during several transgressive-regressive episodes in a shallow shelf environment.

The Midale Beds consist of the Frobisher Evaporite and Midale Carbonate, with oil production predominantly from the latter. The Midale carbonate is divided into 3 zones: the lower zone represents a restricted (lagoonal?) environment in which moderate energy conditions occurred intermittently; the middle zone formed in a transgressive, moderate to high energy shoal environment; and the upper zone carbonate originated in restricted subtidal conditions. Oil reservoirs are coarsely crystalline vuggy dolomite and fractured, bioturbated calcareous dolomite of the middle and upper zones, respectively.

Diagenesis resulted in the formation of various stratigraphic traps. Leached intercrystalline porosity and micro-fractures are the economically most significant porosity types.