West Branch field, located near the center of the Michigan basin, is a low-relief, asymmetric, northwest-southeast-trending anticline. Since 1934, the “Dundee reservoir” (Middle Devonian Dundee and Rogers City limestones) has produced more than 12 million bbl (2 million m3) of oil from this field. Dundee sediments were deposited on a carbonate ramp that dipped gently to the northwest. Core studies and log correlations show that the reservoir is dominated by normal-marine, somewhat lenticular beds of skeletal-peloidal grainstones and packstones. Crinoids are very abundant, comprising nearly 100% of the grains in some intervals. The uppermost Dundee consists of micritic carbonates with a restricted faunal assemblage. The Dundee-Rogers City contact is a disconformity that appears to be a pyritized, extensively bioeroded hardground. The overlying Rogers City Limestone is a dark nodular wackestone with rare packstone interbeds.
Trace amounts of dolomite are common throughout the field. However, pervasive dolomite is common only in the top 10-15 ft (3-4.5 m) of the Dundee. This dolomite generally has high porosity (5-15%) but very low permeability (0-1.0 md). Primary interparticle porosity in limestones accounts for most of the reservoir porosity. Although previous workers have concluded that fracture-controlled dolomites are responsible for porosity and permeability in this field, we conclude that such dolomites are not important. We also conclude that at West Branch field there is no evidence for reefal growth over a pre-existing fold, as has been reported in other Dundee-Rogers City reservoirs.