The Coldwater field, discovered in August, 1944, is in the central part of the Lower Peninsula of Michigan, a few miles west of the center of the Michigan structural basin. The principal producing zone is the Rogers City (Devonian) dolomite. A typical Rogers City dolomite field, it has a producing closure of 55 feet covering an area of 3,160 acres. A greenish amber oil of 48° A.P.I. gravity is produced from an average depth of 3,750 feet. The reservoir is a structural trap, anticlinal in nature, the major axis striking northwest-southeast parallel with the major axis of the basin and modified by a strong cross-fold at right angles to the major axis. An important reserve in the Michigan area, the original oil in place was estimated as 43,000,000 barrels, and the expec ed ultimate recovery more than 5,000 barrels per acre.

On the same anticlinal structure, at an average depth of 1,400 feet, the Michigan series 'Stray' sandstone (Mississippian) forms a reservoir for dry gas with a productive area of 2,400 acres.

The limits of the producing area are well defined and little drilling is foreseen for the future.

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