Hydrothermal Dolomite: Occurrence and Mechanisms
, Michigan Basin, USA
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