A detailed petrologic investigation of a Mississippian carbonate-evaporite rock unit interrelates depositional environment, sequence of diagenesis and porosity-permeability in each of 8 rock types distinguished. Structural considerations are introduced and hydrocarbon potential assessed.

Deposited on the northern slopes of the Williston basin, the strata of the Ratcliffe Beds in this area include 3 basin-fringing evaporites, the Midale, the Oungre, and the upper Ratcliffe evaporites. Successively younger evaporites overlap toward the southwest and in much of the western part of the area the Ratcliffe Beds are exclusively carbonates.

Most of the original sediments of the microcrystalline limestones, saccharoidal dolomites, and the sparry, oolitic, pelletoid limestone were deposited in an open marine, shallow-water environment. The bedded anhydrites, dolomitic anhydrites, and anhydritic dolomites were formed in supratidal evaporitic pans, maritime lakes, or lagoons. The microcrystalline dolomites and associated nodular anhydrite reflect a supratidal environment.

The microcrystalline dolomites and associated anhydrite nodules are both early diagenetic in origin, having formed under sabkhalike conditions. The saccharoidal dolomites are late diagenetic; formed after the initial lithification of the sediments by refluxing magnesium-rich, hypersaline waters, derived from supratidal zones. The accompanying anhydrite masses probably had a similar origin.

Intercrystalline porosity combined with small interconnected vugs in the dolomites make this the best reservoir rock in the area studied. Solution vugs in the limestone also may provide effective porosity for hydrocarbons. The oil accumulation is due to structural-stratigraphic trapping.

The collapse of the Ratcliffe Beds into salt-leached areas has affected the structural pattern and influenced hydrocarbon accumulation. Possibilities of further oil discoveries in this area are good.

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