Abstract

Carbonate-evaporite cycles in the Mississippian (Osagean) Midale Beds of southeastern Saskatchewan were deposited over a shallow homoclinal ramp along the northern margin of the Williston Basin. Fourteen lithofacies, including nine limestone facies (C1 to C9), two dolostone facies (D1 and D2), and three anhydrite facies (A1 to A3), were identified from cores (46 wells) and geophysical well logs (750 wells) in the Steelman-Bienfait area. These lithofacies represent a range of subtidal to supratidal settings and are interpreted to be related to four transgressive-regressive parasequences (S1 to S4, in ascending order). Paleosols and microkarst features (T1) characterize parasequence boundaries and provide evidence for subaerial exposure.

Oolitic grainstones and packstones were deposited as shore-parallel, shoals and barrier islands in the S2 and S3 parasequences along the western margin of the Steelman and Bienfait oilfields. Paleotopographic hinge-zones were an important control on the distribution of shoals, lagoons, algal marsh sediments, salinas and algal/ostracod mounds. Abrupt thickness changes and lateral, carbonate-evaporite facies transitions were observed across the hinge-zones.

Facies distributions in the Midale Beds were related to sea level fluctuations, paleotopography, and the availability of accommodation space. Minor sea level fluctuations in this shallow ramp setting resulted in major lateral shifting of facies, significant migration of the paleocoastline, and periodic exposure of the upper to middle ramp environments, producing stratigraphic traps as exploration targets, in addition to the traditional sub-crop “plays” in the study area.

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