Marked facies changes occur in the Mission Canyon and lower part of the Charles formation (Mississippian) in southeastern Saskatchewan. Northeastward from the basinal area, four successive environments of deposition with characteristic rock types and fossils may be mapped as follows.
Basin: dark brown-gray argillaceous limestone containing scattered white crinoid columnals, black bituminous shale partings, and brownish gray chert.
Open marine shelf: cream-white fossiliferous-fragmental and chalky limestones containing crinoids, bryozoans, brachiopods, and zaphrentid corals.
Barrier bank: cream-white precipitated limestones including pisolitic, oölitic, pseudo-oölitic, and lithographic types. Fossils include abundant algae and few scattered gastropods.
Lagoon: cream-white chalky argillaceous limestone containing ostracods, earthy to sucrosic secondary dolomite and anhydrite.
Evaporitic (primary?) dolomites occur in the upper Charles as a result of basin-wide evaporitic conditions. These typically dense (cryptocrystalline) and non-porous dolomites are closely associated with anhydrite.
Factors which control porosity in the limestones include sorting of fossil debris or precipitated calcareous grains, submarine erosion and re-deposition of partially lithified calcareous muds, solution, dolomitization (?), fractures, compaction, and infilling of pores by calcite, anhydrite, or chert.
Stratigraphic trap oil accumulations occur where permeable units of the Mission Canyon and Charles are truncated and overlain by pre-middle Jurassic redbeds. Factors affecting oil accumulation include the presence of the following.
Permeable reservoir beds at or near the unconformity.
Anhydrite filling pores where the permeable reservoir beds are truncated by the pre-Jurassic erosion.
Anhydrite beds conformably overlying and underlying the permeable units and thus form a roof and floor for the reservoir beds.
Topography of the Mississippian erosion surface.
Mississippian strata in the oil-producing area of southeastern Saskatchewan are characterized by the following.
Relatively steep dips (40–60 feet per mile) as compared with dips of 10-20 feet per mile in much of southwestern Saskatchewan.
Relatively rapid truncation (15–40 feet per mile) as compared with an average of approximately 5 feet per mile in southwestern Saskatchewan.
Relatively saline waters (50,000–150,000 parts per million of chloride ion content) as compared with 100–8,000 parts per million in southwestern Saskatchewan.
In exploration for new trends or extensions of known producing areas, important consideration should be given to the distribution of permeable rock facies within individual stratigraphic units. In addition, it is possible to predict the location of topographically high trends as the various lithologic types are differentially resistant to erosion. For example, the precipitated barrier bank limestones are relatively resistant to weathering and form topographic ridges in the Nottingham field. The lower limit of oil accumulation within reservoir beds may be determined by the location of a spill point, in turn controlled by the downdip limit of lagoonal anhydrite floors.