Silurian Niagaran reefs, a majority of which contain hydrocarbons, have been found in the northern part of the Michigan basin. These are pinnacle reefs similar to those in southeastern Michigan and Ontario and are present within a narrow belt along the northern flank

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of the basin. Larger platform reefs may be present throughout a belt north of, and parallel with, the pinnacles. The productive trend, presently 15 mi wide and 150 mi long, was found as a result of renewed drilling activity in this exploration frontier. The high incidence of locating reefs is due mainly to detailed seismic evaluation. Significantly larger reserves could be contained in the platform reefs, but where found to date in the northwestern part of the current play, they are composed of nonporous dolomite.

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