HYDROTHERMAL MINERALIZATION IN THE MICHIGAN BASIN, USA

In carbonate systems, dolostone often forms the best fluid (petroleum, water and liquid waste, CO₂ storage, etc.) reservoirs. Many mechanisms for dolomitization of primary carbonates facies in the Michigan basin have been proposed with implicit flow properties, spatial distribution, and internal geometry. Previous investigations and new petrologic data from the St. Peter Sandstone (AKA “PDC Sand”), Trenton/Black River (T/Br), Burnt Bluff, Niagaran, Bass Islands, and Dundee units in the Michigan basin suggest that fracture-related, hydrothermal alteration was an important mechanism in the origin of these proven and prospective fluid reservoirs. Petroleum reservoirs in the Ordovician T/Br Group are classic, fracture-related hydrothermal dolostone. Saddle dolomite occurs in all of the units studied as replacive intergranular cement, fracture/vug fill, and/or primary carbonate matrix replacement. Other common, later diagenetic phases include pyrite; bitumen; quartz; fracture filling, sparry calcite; anhydrite; and rare fluorite. Preliminary characterization of two phase fluid inclusion homogenization and melting temperatures from carbonate minerals in these units indicates that important, reservoir-forming episodes of diagenesis occurred as a result of precursor carbonate mineral interaction with high salinity, hydrothermal fluids. In all units, ranging in age from Middle Ordovician to Middle Devonian, diagenetic carbonates have oxygen isotope compositions ranging from -5 to -12 d¹⁸O (PDB) and minimum temperatures of formation from 80^o-170^oC. Petrologic data in each of these important fluid reservoir units indicates that diagenetic carbonate minerals were formed from high salinity, d¹⁸O enriched (+5-+12 d¹⁸O) hydrothermal fluids during at least one episode of fracture related mineralization. Emerging models, incorporating reactivated basement faulting (coincident with major Appalachian orogenic events), downward migrating high salinity basinal fluids (related to thick evaporite-rich successions), and serpentinization of basement peridotite (associated with the deeply buried, Mid-Continent Rift), are intriguing mechanisms for the origin of hydrothermal alteration in important fluid reservoir units in the Michigan basin.