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 produce predominately from the Midale carbonate, which is divided into three zones. The lower zone represents a restricted, possibly 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. Early syntaxially cemented crinoid banks form local reservoirs. Field-wide leached intercrystalline porosity and microfractures are the economically most significant porosity types. Based on the knowledge of local depositional environments, diagenesis, structural contours, and isopach maps, it is possible to high grade reservoir predictability.

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