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Katz, David Allen1, Gregor P. Eberli1, Langhorne B. Smith2, Jason Kislak1, Peter K. Swart1 
(1) University of Miami, Miami, FL 
(2) New York State Museum, Albany, NY

ABSTRACT: Hydrothermal Brecciation Associated with Calcite Precipitation and Permeability Destruction in Mississippian Carbonate Reservoirs, Montana and Wyoming

The Mississippian Madison Formation contains abundant fracture zones and breccias that are hydrothermal in origin based on their morphology, distribution and geochemical signature. The hydrothermal activity is related to thrusting during the Laramide orogeny. Brecciation is accompanied by dedolomitization, late stage calcite precipitation, and porosity occlusion in dolomites. Thus, the hydrothermal calcite locally reduces permeability in otherwise high quality reservoir rocks of the Madison Formation. 
Hydrothermal breccias from the Owl Creek thrust sheet are classified into four categories based on fracture density, calcite volume, and clast orientation. Hydrocarbon shows are present in breccia vugs and along fracture planes in Shoshone Canyon. In the Owl Creek thrust sheet, shattered breccias dominate the southern leading edge of the thrust where tectonic deformation is greatest, whereas fracture, mosaic, and chaotic breccias dominate the northern edge of the thrust sheet. The late stage calcite cements have ä18O values between -12 to –25 ‰ PDB, and the matrix values range between –2 to +2 ‰ PDB. These values show late stage calcite cements precipitated in equilibrium from waters of 80 to 120° C. 
Hydrothermal fluids and associated brecciation and cementation introduce reservoir heterogeneity by locally decreasing reservoir quality in the Madison gas fields. In muddy fine-crystalline dolomites from sequence III calcite healed breccia and fracture zones have low permeability while unaffected intervals are of excellent reservoir quality. Permeability reduction also occurs in coarse crystalline dolomites of sequences I and II, where hydrothermal fluids invade the formation away from the brecciated zones.


AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.