Abstract Reservoirs in the Upper Ordovician Trenton Limestone and the Black River Group in Indiana, Michigan, and Ohio form a major oil province, important since 1884. This province (America’s first giant oilfield) has yielded more than 600 MMBO. This includes the giant Albion-Scipio Trend (120 MMBO), discovered in 1956. Although production from the Trenton and Black River carbonates is high, little has been known about the nature and distribution of their reservoirs. Recent research indicates that they do not contain any depositional (primary) porosity. Reservoirs exist only where dolomitization or fracturing or a combination has occurred. Trenton and Black River dolomitization does not conform to facies-related models, but must be related either to fluid movement along fractures associated with tectonic features or to burial dolomitization. A comparison of fields with dolomitization along linear fault (or fracture) zones and those showing regional burial dolomitization indicates the relative importance of faults and fractures. Oil recovery from fault/fracture-dolomite reservoirs is 2500 to 12, 000 bbl/acre, and that from burial-dolomite reservoirs is only 540 to 1000 bbl/acre.