Sedimentology and hydrocarbon potential of the Devonian Three Forks and Mississippian Bakken Formations, Sinclair Area, southeast Saskatchewan-southwest Manitoba
Karasinski, Daniel R.
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The Mississippian Bakken Formation is a prolific oil producer throughout the Williston Basin. In southwest Manitoba and areas of southeast Saskatchewan, the Middle Bakken Member unconformably overlies the Devonian Three Forks Formation. In the Sinclair Area of Manitoba (Twp. 6-8, Rge. 29-30 W1), both these units host commingled production. An integrated method combines sedimentology, stratigraphy, and reservoir engineering to the study of the Thee Forks Formation and Middle Bakken Member of the Sinclair Area. This approach permits interpretation of depositional and diagenetic history of both units, reservoir characterization, and ultimately the development of an exploration model for the Sinclair Area. Complexly interbedded dolomite and claystone of the Three Folks Formation reflect deposition along a prograding, temperate, carbonate tidal flat, which graded basinward towards an unrimmed, carbonate platform. Three Forks peritidal and platform facies form a series of shallowing-upward successions typical of shallow-platform carbonates. Sporadically throughout the Three Forks section are subaqueous, debris-flow conglomerates, representing local disturbances and margin instability during latest Devonian time. Following Late Devonian regression and erosion, Middle Bakken dolomites and sandstones were deposited along a transgressive, non-barred shoreline. Both the Three Forks and Middle Bakken units have undergone complex diagenetic alteration concurrently via similar fluids. Diagenetic processes which have modified Three Forks and Middle Bakken facies include: dolomitization; phosphate, pyrite halite, hematite, silicate, and anhydrite precipitation; as well as solution- and fracture-porosity development. Based on the stratigraphic distribution of oil saturation and permeability, the commingled Three Forks-Middle Bakken reservoir is determined to be the interval spanning the uppermost Three Forks Formation and lowermost Middle Bakken Member. Three Forks reservoir rocks include inner-shelf to mixed-flat facies, whereas Middle Bakken reservoir rocks include transgressive-lag, lower-shoreface, and upper-inner-shelf facies. Three Forks reservoir quality was diagenetically enhanced by early dolomitization and secondary porosity development; however, late-stage dolomite, pyrite and anhydrite cementation reduced reservoir quality of both Three Forks and Middle Bakken facies. Oil accumulation is structurally controlled, where oil saturation is greatest within local structural highs, and reservoir quality is largely depositionally controlled. Utilizing structural and depositional trends, an exploration model is developed for the Sinclair Area.