Peters Reef, one of several oil fields on the southeast side of Michigan basin within the Engadine Dolomite of uppermost Middle Silurian, was studied by means of well cuttings and logs. The reef is 1 3/4 mi long, 1 mi wide, about 350 feet high, and complex with core, back reef, and steeply dipping fore-reef facies. The main part was produced by organisms growing in situ. Original calcium carbonate was replaced by dolomite which altered the reef and organic structures but did not increase porosity appreciably; porosity is due to solution openings, formed during periods of erosion, and to fractures. Evaporites characterize post-reef sedimentation. Oil pool is sealed by anhydrite, and source beds are assumed to be underlying shales of the Clinton Group. Initial oil production is 2,500 b.p.d. plus.