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The Mississippian Frobisher-Alida interval is an upward-shoaling cycle that began with open-marine sedimentation and culminated with the deposition of a widespread sabkha-salina evaporite. This cycle is the most prolific oil-producing interval in the North Dakota portion of the Williston basin. Most Frobisher-Alida production in the southern Williston basin is from dolomite reservoirs.
The six major facies defined in this paper are lithologic suites that represent sediments and precipitates deposited in similar environments. Dolomite formed as a replacement of muddy sediments deposited in three facies: II, IV, and V. Facies II sediments consisted primarily of muds deposited in offshore lagoons. Facies IV sediments were composed of burrowed muds deposited in a restricted-marine environment in front of a barrier bar and island complex. Fossiliferous muds, which formed in an open-marine environment, were the dominant sediments deposited in facies V.
The original sediment textures strongly influenced the textures and distribution of replacement dolomite. Some dolomite zones display relatively uniform textural and petrophysical properties over large portions of southwest North Dakota. Porous dolomite zones with larger average crystal sizes display better producing characteristics than zones with smaller average crystal sizes. The average crystal size in Frobisher-Alida dolomites ranges from about 8 to 44 µm.
Most large Frobisher-Alida dolomite reservoirs in southwest North Dakota occur in combination structural-stratigraphic-hydrodynamic traps. Stratigraphic traps in dolomites are found where facies changes occur or where an updip decrease in the dolomitization intensity formed lower permeability rocks. A regional hydrodynamic gradient aids stratigraphic trapping in many dolomite zones that would not trap oil under hydrostatic conditions.
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