Hydrothermal Dolomitization of Fluid Reservoirs in the Michigan Basin, U.S.A

David A. Barnes¹, Thomas M. Parris², and G. Michael Grammer^1
1Geosciences - MGRRE, Western Michigan Ubiversity, Kalamazoo, MI
²Kentucky Geological Survey, University of Kentucky, Lexington, KY

Mechanisms for dolomitization of primary carbonate facies imply flow properties, spatial distribution, and internal geometry of the resulting geo-body. Petrologic data from diagenetic carbonates in the St. Peter Sandstone, Trenton/Black River, Burnt Bluff, Niagaran, Bass Islands, and Dundee in the Michigan basin suggest that fracture-related, hydrothermal dolomitization was important in the origin of these reservoirs. The T/Br Gp is a famous example of a fracture-related hydrothermal dolostone reservoir. In all units, saddle dolomite is a replacive intergranular cement, fracture/vug fill, and/or primary carbonate matrix replacement. Other related diagenetic phases include pyrite; bitumen; quartz; fracture filling, calcite; anhydrite; and fluorite. Primary fluid inclusion Th/Tm from carbonate minerals indicate reservoir-forming episodes of diagenesis as a result of precursor carbonate mineral interaction with high salinity, hydrothermal fluids. In all of these Ordovician to Devonian units, hydrothermal carbonate minerals have oxygen isotopic composition from -5 to -12 δ18O and minimum formation temperatures of 80o-170oC. These data indicate diagenetic carbonate mineral formation from high salinity, δ18O enriched (+5 to +12 δ18O; PDB) fluids during at least one episode of fracture related, hydrothermal mineralization. Fluids resulting in hydrothermal alteration in the Michigan basin can be characterized by 1) high pressure gradients, 2) elevated temperature, 3) high salinity, and 4) δ18O enriched oxygen isotopic composition. A speculative model for hydrothermal dolomitization in the Michigan basin includes reactivated basement faults (coincident with Appalachian orogenic events), vertical migration of basinal brines (related to thick evaporite-rich units), and serpentinization of basement peridotite (associated with the deeply buried, Mid-Continent Rift System).

AAPG Search and Discover Article #90078©2008 AAPG Annual Convention, San Antonio, Texas