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The pinnacle reefs of the Michigan Basin form small, isolated hydrocarbon reservoirs encased in impermeable evaporites and mudstones, and account for most of Michigan’s hydrocarbon reserves. The temporal relations between the reef sequence and the evaporites are still in dispute, but in the currently favored model, deposition of reefs and evaporites follow each other closely in a cyclic manner but are not synchronous. Pinnacle development occurred in four stages and included periods of subaerial exposure, which enhanced reef porosity and permeability through leaching and dolomitization. Subsequent evaporite precipitation filled much of this porosity; many reefs are completely salt plugged and impermeable. Sea-level history and depositional environments of Salina evaporites are disputed, but a model of shallow-water evaporite deposition in the basin is favored.

Regional trends have been recognized across the pinnacle-reef belt, and these predict increased salt plugging of the reefs basinward, increased dolomitization and preserved secondary porosity toward the basin margin, and in the northern trend, zones of production that pass from gas to oil and finally to water toward the basin margin. The producing reefs have porosities that range from 3 to 37 percent (average 6 percent) and average permeabilities of 11 to 12 mD (ranges to more than 1 D).

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