The age of oil- and gas-bearing rocks in Michigan ranges from Mississippian to Ordovician, and one gas occurrence is doubtfully in the Pennsylvanian. Gas is also locally present in the glacial drift where it has presumably accumulated from immediately underlying Paleozoic rocks. Oil is known to occur in eleven distinct horizons of which nine have proved productive, and gas occurs in nine separate horizons of which five are being actively exploited.

Deep wells have disclosed the general features of the stratigraphy of the state. The results of drilling indicate that in the deepest parts of the Michigan basin the oldest known sediments likely to contain oil and gas would be found at depths not exceeding 10,000 feet.

Examination of samples has shown the presence of several types of porosity, most of which are in some manner associated with dolomite. It is suggested that adequate knowledge of the manner in which dolomite has been formed would probably provide some valuable clues to the origin of petroleum.