Since 1929 many important gas fields have been discovered in the central part of the Southern Peninsula of Michigan. These fields are on three main northwest-striking anticlines and produce from rocks of Mississippian age. The major problems are the stratigraphy of the gas sands and the relation of gas production in the Michigan formation to oil production in the Dundee, where the two (on the same general structure) are not directly superposed.

The term "stray sand" was first applied to the gas-producing stratum in the Clare field because the sand was believed to be of Michigan age rather than Marshall, as had been previously supposed. A diastem or unconformity between the Michigan and Marshall formations was postulated on evidence found in the eastern part of the state, even though no petrological distinction was recognized between the "stray sand" and the upper strata of the Marshall. It was suggested that the "stray" might be reworked Marshall sand deposited on flanks of islands in the Michigan seas.

After detailed examination of all the available information that could be assembled, it is concluded that the gas sands in the various fields are not all of the same stratigraphic horizon. The stray sands in the eastern part of the state appear to be a facies and close time equivalent of the upper part of the Marshall farther west. The break which is postulated in the eastern part of the state does not seem to extend to the fields in the west-central part, hence some of the gas sands are apparently of Marshall age.

Available evidence shows that the most important factor in limiting gas occurrence is the regional structure, although variations in sand thickness and depositional structures are locally important. Pre-Marshall structure apparently controls Dundee oil production locally, for the "Marshall to Dundee interval" is thinnest over the oil-producing areas.

Regional correlations suggest that the lower Michigan and upper Marshall formations are a part of a deltaic or littoral deposit, and that the lower part of the Michigan formation in the eastern part of the area studied is an off-shore phase of the upper part of the Marshall farther west.

Dark-colored shales in the Michigan or Coldwater formation may have been source beds for the gas in the central Michigan area. The lack of commercial gas in Mississippian rocks in the southeastern part of the area is explained by a paucity of bituminous matter in the dark shales due, possibly, to near-shore deposition.

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