The Middle Devonian Winnipegosis Formation of Saskatchewan is divisible into upper and lower members on the basis of a regionally developed argillaceous interval which forms the uppermost part of the lower member.

The lower member consists of a regionally dolomitized marine carbonate of relatively constant thickness and lithology, attaining a maximum observed thickness of 54 ft. The upper member is a varied marine carbonate sequence, with three major facies developments. In the southwest lies a wedge of lithologically relatively consistent carbonates, reaching a maximum thickness of about 130 ft. along a northwest-trending axis extending through Weyburn and Elbow. In the north and east, bioclastic-pelletoidal carbonate banks with true reef intervals (up to 345 ft. thick), and finely laminated, interbank carbonates (up to 68 ft. thick) occur.

The lower member was laid down in a broad epicontinental sea. The relatively shallow, open marine conditions culminated on two occasions in basin-wide, reducing, lagoonal conditions, evidenced by the medial and upper very bituminous argillaceous intervals containing impoverished faunas. The upper member appears to have been deposited in a shallow shelving sea which deepened toward the northeast. Using a regionally developed Amphipora zone as a datum, three pre-Amphipora tectonic-sedimentational regions are discernible: in the southwest, the Elbow-Weyburn basin, subsiding relatively rapidly to accommodate thick, shallow-water carbonates; in the north and east, the comparatively stable Saskatoon shelf, with thin laminated carbonates and basal carbonates of the bank sediments; and the mo e rapidly subsiding Meadow Lake-Sayese basin complex, with similar sediments, except that bank sedimentation was further advanced. In post-Amphipora time, subsidence continued in the north, accelerated in the shelf area to accommodate thick bank accumulations, whereas, in the southwestern basin, carbonate deposition was almost complete.

Although no commercial quantities of oil or gas have been found in Saskatchewan, production of oil from the Winnipegosis in the Outlook and Redstone fields of northeastern Montana is encouraging. The occurrence of oil and gas "shows" in some drill-stem tests and of oil staining in the Winnipegosis Formation in Saskatchewan offers further encouragement, especially in the Elbow-Weyburn basin, which is a northwestern extension of the oil-producing facies of northeastern Montana. The presence in the southwestern basin of permeability traps resulting from differential dolomitization and recrystallization, and in the north and east, of important thickening in short distances, associated with variably porous bank carbonates, together with the widespread development of an excellent top seal i the form of the Prairie Evaporite salt or carbonate-anhydrite sequences, provide excellent conditions for the entrapment of hydrocarbons.

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