Good producers, large lost circulation zones, blowouts, and high proration have focused the attention of the petroleum industry on southern Michigan. The Trenton-Black River formation play in this area and the basal Salina-Niagaran plays in eastern and western Michigan have rejuvenated a declining oil industry in Michigan.

Structurally, Michigan can be defined as a part of an intracratonic basin, flanked on the east and southeast by the Cincinnati arch, on the south and southwest by the Kankakee arch, on the west by the Wisconsin arch and peninsula and on the north and northeast by the Canadian shield.

Previous to the present activity, all the oil production of any consequence had been associated with sediments of Devonian age. However, the present developments have been mainly in Silurian and Ordovician sediments. This activity has been more or less confined to the flanks of the structural basin where the older formations are at a relatively shallow depth.

In eastern Michigan, St. Clair County, the Silurian production is from reef-type build-ups which occurred during late Niagaran deposition. Production has also been obtained from the overlying basal carbonates of the Salina group.

In Western Michigan, Allegan, and Ottawa counties, the producing units are the basal carbonates of the Salina group, namely, in descending order, the A-2 dolomite and the A-1 dolomite. Unlike eastern Michigan, this production is associated with regional structures which appear to have been affected by the lowermost evaporite unit of the Salina group, the A-1 salt.

The present Trenton-Black River development of Middle Ordovician carbonates is located in south-central Michigan on the southern flank of the Michigan basin. Production has been from a secondary dolomite confined to a fracture zone. Structurally, the fracture zone is directly associated with a shallow narrow depression which plunges in a north-northwesterly direction about 50 feet per mile. Approximately 25 linear miles has been partially proven productive with efforts being made to extend the trend and also to prove the inside acreage.

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