Diagenesis of Mississippian Alida Beds, southeastern Saskatchewan, Canada

The Mississippian Alida Beds in the subsurface of the northern Williston Basin, southeastern Saskatchewan, preserve dominantly shallow-marine carbonate sediments truncated by the regional Mississippian unconformity. The Alida Beds have undergone extensive diagenetic alteration, which has significantly influenced reservoir quality. The diagenetic history of the Alida Beds, particularly the formation of dolomite, anhydrite, and the Mississippian caprock beneath the Mississippian unconformity, is examined in this thesis.; The diagenetic history of the Alida Beds is a complex series of at least 30 diagenetic events that are interpreted to have occurred in four main phases: near-surface, initial shallow to intermediate burial, uplift and exposure related to the Mississippian unconformity, and reburial during the Mesozoic and Cenozoic. Dolomite is the most common diagenetic product in the Alida Beds and formed in multiple stages. Early matrix replacement dolomite (type 1 dolomite) is the most common and is interpreted to have formed from slightly evaporated penecontemporaneous seawater and was partially recrystallized during burial diagenesis. Minor pore-lining dolomite cement (type 2 dolomite) formed during initial burial by mechanical compaction. Cryptocrystalline dolomite cement and replacement dolomite (type 3 dolomite) is associated with the Mississippian unconformity and precipitated from seawater-derived brines.; Diagenetic anhydrite is common throughout the study area. Three stages of sulphate mineral formation are identified: early postdepositional sulphate (group 1 anhydrite), pervasive reservoir-plugging sulphate (group 2 anhydrite) derived by remobilization of exposed primary evaporite beds, and unconformity-related sulphate (group 3 anhydrite) precipitated from hypersaline brines. The sulphate minerals are interpreted to have precipitated as gypsum and later transformed into anhydrite during reburial of the Alida Beds. Brown, metasomatic anhydrite formed by entrapment of hydrocarbons within anhydrite after complete growth of the anhydrite crystals.; The Mississippian caprock is a regional diagenetic alteration zone in the study area and forms the seal for many oil pools of southern Saskatchewan. In this thesis, the caprock is interpreted to have formed during the Early Triassic when the tilted and truncated Mississippian strata were partially covered by shallow, restricted marine water. Downward flux of evaporated seawater led to overdolomitization of the Alida host rocks and precipitation of abundant gypsum that was later converted to anhydrite.