Geology of the W eyburn Field, Saskatchewan
The reservoirs of Mississippian Midale beds and Frobisher beds found in the Weyburn field lie at the northwestern termination of a broad productive belt extending 120 miles along the northeastern rim of the Williston basin. These carbonates, and interbedded evaporites, originated during cycles of marine transgression and regression, and were deposited as rock types characteristic of shoal, lagoon, and basinward shelf environments. The Frobisher-Alida Beds, which form a secondary reservoir to the main Midale Beds of producing zone are composed of oolit1c, fragmentalbioclastic, vugular limestone capped by an impervious dolomitic mudstone. Production is controlled by localized structural features resulting from depositional build-up. Excellent reservoir conditions are accompanied by an active water drive. The Frobisher Evaporite, consisting of massive primary anhydrite with dolomitic veinlets, forms the floor seal for the Midale beds oil accumulation. A significant facies transition into an oolitic, bioclastic, vugular limestone characterizes this unit. Beyond the depositional margin of the evaporitic facies, marked increases in porosity and permeability of the entire reservoir are noted. Carbonate units overlying the areal extent of the Frobisher Evaporite exhibit a greater degree of metasomatism, dolomitization, and more abundant secondary anhydrite. • Porous carbonates comprising the producing zone of the Midale beds are divided into an oolitic, fragmental- bioclastic, vugular limestone, and an overlying granular marly limestone unit, capped, by the Midale Evaporite. Oil is accumulated in a stratigraphic trap bounded above and below by primary anhydrite and truncated by the post-Mississippian erosional surface. Mississippian strata dip basinward from this erosional surface in a broad homocline exhibiting only minor irregularities. Rapid lateral and vertical porosity and permeability variations are common. Localized, separated, porous lenticles contain interstitial water in areas where the Midale beds are underlain by the Frobisher Evaporite. The major portion of the field, however, is capable of water-free production.