DAVID A. BARNES, Department of Geosciences and Michigan Basin Core Research Laboratory, Western Michigan University, 1903 W. Michigan Ave, Kalamazoo, MI 49009, phone: (269) 387-8633, [email protected], G. Michael Grammer, Department of Geosciences, Western Michigan University, Kalamazoo, MI 49008, William Harrison, III, Western Michigan University, Kalamazoo, MI 49008, and Robb Gillespie, Michigan Basin Core Research Laboratory, Department of Geosciences, Western Michigan University, Kalamazoo, MI 49008.

Analysis of petroleum producing formations, including the Ordovician Trenton/Black River (T/BR) and St. Peter Sandstone (aka "PdC"), and Devonian Dundee Formation, indicates a pervasive overprint of hydrothermal dolomitization in the Michigan basin. Hydrothermal origin of volumetrically significant dolomite is supported by new analytical data including light (relative to initial sea water) oxygen isotopic composition and elevated (relative to inferred ambient burial temperatures) fluid inclusion homogenization temperatures. Mapping and log analysis in the T/BR and Dundee suggest close spatial correlation among gross hydrothermal dolomite reservoir facies (HTDRF) distribution and interpreted, wrench fault-related NW-SE and NE-SW structures.

The T/BR and Dundee are separated by thick, Silurian and Devonian age, salt-bearing formations and other units thought to be too plastic to support brittle fracture. Episodic reactivation of basement wrench faults may have episodically propagated fractures and hydrothermal fluid conduits upwards through the Paleozoic cover rock section. These fracture conduits must have annealed in shale and evaporite sections in geologically short time periods in order to create effective seals for important hydrocarbon accumulations.

Emerging models, incorporating reactivated basement faulting, downward migrating saline formation fluids, and serpentinization of basement peridotite, are intriguing mechanisms for the origin of economically significant HTDRF in the Michigan basin. These mechanisms are consistent with regional structural grain (probably resulting from repeated regional basement wrench fault reactivation); dense, saline formation fluids; and possible Proterozoic mafic and ultramafic crust genetically related to the Mid-Continent rift in the central Michigan basin. Rift-derived, magnesium-rich, high pressure hydrothermal fluids may have been transported through "regional aquifer units" including the St Peter Sandstone and/or other regional aquifers, and delivered through wrench fault related fractures to form HTDR in several carbonate formations throughout the Paleozoic succession.

AAPG Search and Discovery Article #90059©2006 AAPG Eastern Section Meeting, Buffalo, New York