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The Sweetgrass arch is a positive structural feature extending from central Montana into southern Alberta. Rock units ranging in age from Precambrian Belt to Late Cretaceous Montanan are exposed along the 350 mi axis. The 3 major features are the South arch, culminating in the Beltian exposures on the south end; the Kevin-Sunburst dome and Sweetgrass volcanic uplifts in the center; and a broad, northward-plunging nose in southern Alberta.
Although structural traps would be expected to be the dominant controlling factors in hydrocarbon accumulation on so large a positive feature, the fact is that stratigraphic traps predominate on this arch. Structural closure on the large Kevin-Sunburst dome does not by
itself cause the entrapment of oil and gas there; rather, irregular porosity development in the Mississippian carbonate rocks and lensing and pinchout of Cretaceous sandstones is primarily responsible for those accumulations.
The largest oil and gas reserves on the arch are found in channel sandstones of the Lower Cretaceous basal Mannville Group, extending from Cutbank field, Montana, northward through the Taber, Hayes, and Bantry fields of Alberta. The middle Mannville glauconitic Moulton zone produces from a series of north-trending sandbars in the Darling area of Montana, in the Taber area, and in scattered trends in the Jenner, Countess, and Hussar fields of Alberta.
Large gas reserves are found in thin blanket sandstones of Late Cretaceous age, principally in the Medicine Hat and Second White Specks zones. A thick deposit of sandy shale in the Milk River Formation probably contains several trillion cubic feet of gas, but the low productivity of early wells kept these reserves off the market until 1970. A small drilling boom is now under way to develop and exploit what will be possibly the largest single gas field in areal extent in Canada.
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