During the 1980's, deep gas exploration in the Michigan basin targeted middle Ordovician sandstones in the Prairie Du Chien and St. Peter formations. Deeper-pool gas discoveries occur in simple closures associated with drape-fold anticlines interpreted to overlie reactivated, oblique-slip basement faults. The Prairie Du Chien and St. Peter formations consist of up to 1300 feet (400 m) of interbedded sandstones and shaly dolomites that pinch out along the southern margin of the basin. Tight, shaly tidal-flat shales within the St. Peter provide good source rocks and multiple top seals found in many fields. Clean, cross-bedded and skolithos-burrowed sandstones capping shelf sand bar or prograding shoreface sequences produce the best reservoirs.

A multi-disciplinary study of the Rose City Field, a deeper-pool field discovered in 1985, served as an analogue for development and production of Shell's other fields. The depositional environments of the sandstones were found to have significantly influenced their subsequent diagenesis and ultimately their present reservoir characteristics. The geologic model and calculated log values were combined with seismic structural interpretations to characterize the reservoir system and map the distribution of hydrocarbon-bearing pore volume (SgφH). Volumetric and material balance estimates of OGIP agree within 10%, which is considered excellent. This approach was applied to other multi-well fields to help ensure equity and improve ultimate recovery.

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