The discovery of the Howell gas field, Livingstone County, Michigan, with gas production from the base of the Salina formation and the top of the Niagara group, has added to the interest in the possibility of commercial production from these rocks in other areas in the Michigan basin. Gas and oil have been produced from the Cataract formation and Niagara group in fields in southwestern Ontario for many years, and showings of oil and gas were reported in Niagara rocks penetrated by wells in various areas in Michigan.

Cataract rocks of lower Silurian age and Niagara rocks of middle Silurian age occur in the subsurface in the Michigan basin, and they crop out, or underlie glacial drift, around the margin of the basin. The rocks are exposed in places in the Northern Peninsula of Michigan, eastern Wisconsin, northeastern Illinois, northern Indiana, northwestern Ohio, and southwestern Ontario, Canada.

The Cataract formation which includes the "White Medina" and "Red Medina" gas-producing sandstones in Ontario west of Niagara River, is 45-190 feet thick in the Michigan basin. The Cataract is represented by dolomite and shale in the Michigan basin, and sandstones of the eastern facies are absent.

The Niagara group of the Michigan basin is composed almost entirely of dolomite, with some chert and shale in places. Shale, which grades laterally into dolomite, occurs in the group in northern Indiana. The lower part of the Niagara in eastern Michigan and the Northern Peninsula of Michigan

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is dolomite, argillaceous dolomite, and shale. The thickness of the Niagara varies from 66 feet in Kent County in southwestern Michigan to more than 700 feet in Mackinac County in the Northern Peninsula of Michigan. The Niagara increases in thickness southward from Kent County, Michigan, to 425 feet in northwestern Indiana near the area of outcrop. The sequence of Niagara rocks in southern Michigan is thin due to non-deposition of pre-Salina erosion. Outcrop and subsurface data indicate that pre-Salina erosion occurred in some areas. Local variations in thickness are due in part to reefs which in places are numerous. Niagara rocks dip at the average rate of 45 feet per mile from the outcrop area in northern Indiana to the center of the Michigan basin in Clare and Gladwin counties, Mic igan, where the top of the group is more than 9,000 feet below sea level. The average dip per mile from the outcrop area in the Northern Peninsula to the center of the basin is approximately 70 feet per mile.

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