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Mississippian reservoirs in northern Montana have produced oil in an amount almost equal to the combined total from all other producing zones. Six important fields, four in the Sweetgrass arch area and two in the Williston Basin, currently yield a total of over 350,000 barrels of oil monthly. Without exception, the producing zones are carbonate reservoirs which contain either intergranular, vuggy or fracture porosity. A discussion of each field is presented to illustrate that stratigraphic variations are equally, if not more important, than structural closure in governing accumulation. The stratigraphy of the Mississippian is treated briefly to show regional correlation of the producing zones.
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