Subsurface Study of Lower Ordovician Prairie du Chien Group and Underlying Cambrian Formations and Their Relation to Pre-Glenwood Unconformity in Southern Peninsula of Michigan

Charles T. Wheeler, Jr.

Prior to discovery of commercial gas in Lower Ordovician Prairie du Chien sandstones at Falmouth field, Missaukee County, in April 1981, only 119 wells had reached underlying Cambrian rocks in Michigan's Southern Peninsula. The resulting deep basin drilling activity to January 1, 1987, has resulted in 17 Prairie du Chien gas discoveries in the north-central part of the Michigan basin and an additional 46 Cambrian penetrations. This study also utilized 235 Prairie du Chien partial penetrations.

It is now possible to correlate with relative certainty the standard Cambrian formations of the subsurface of southern Michigan to wells within and on the other margins of the basin. These, in ascending order, are Mount Simon, Eau Claire, Dresbach, Franconia, and Trempealeau. Because of the previous lack of control, these units were commonly misidentified in northern wells in previous studies. The present study largely corroborates previous recent work.

Four ascending formations of the Prairie du Chien Group were also correlated throughout the basin: Umlor, Foster, Bruggers, and Goodwell (newly proposed). Two type wells demonstrate the character of the Goodwell Formation. The Bruggers Sandstone has been further divided into four informal zones, based on extending correlations of thin, silty dolomite marker beds on gamma-ray logs from the Goodwell and Woodville fields area, Newaygo County. Gas is produced from these Bruggers zones.

Twelve detailed stratigraphic cross sections (using 183 wells) and a pre-Glenwood paleogeologic map demonstrate marked basin-wide pre-Middle Ordovician erosional truncation.

AAPG Search and Discovery Article #91041©1987 AAPG Eastern Section Meeting, Columbus, Ohio, October 7-10, 1987.