Jason Kislak1, Langhorn Smith2, David Peacock3, Gregor Eberli1, Peter Swart1
(1) University of Miami, Miami, FL
(2) New York State Museum, Albany, NY
(3) State University of New York at Buffalo, Buffalo, NY
ABSTRACT: Classification, Distribution and Origin of Hydrothermal Breccias, Madison Formation, Wyoming
A variety of hydrothermal calcite-cemented breccias occur in the thrusted carbonates of the Mississippian Madison Formation at Wind River Canyon, Wyoming. Similar breccias found in nearby oil and gas fields can have a major impact on reservoir permeability and compartmentalization.
The hydrothermal breccias can be classified into four categories: fracture, mosaic, chaotic, and shattered. Fracture breccia consists of relatively large, un-rotated clasts and less than 5% cement. Mosaic breccia has fitted clasts and up to 20% cement. Chaotic breccia consists of randomly oriented clasts and up to 80% cement. Shattered breccia consists of tightly compacted, relatively small clasts and less than 10% cement. Fracture, mosaic and chaotic breccia are common throughout the canyon and typically do not obscure bedding. Shattered breccias occur near the leading edge of the thrust sheet and obliterate bedding.
The calcite cements have d18O values between -15 and -25 PDB, which suggests that the calcite precipitated from hot hydrothermal fluids. We propose a hydrothermal fracture model for the breccias. During thrusting, geo-pressured hydrothermal fluids sourced from more deeply buried portions of the thrust sheet fractured the frontal portion of the thrust. Brecciation was followed by rapid precipitation of calcite cement that may have been caused by a pressure drop or release of CO2.
The intrusive nature of the cemented hydrothermal breccias produces permeability barriers in otherwise highly porous reservoirs. A better understanding of the distribution of the various hydrothermal breccia types may lead to improved reservoir modeling, completion practices and ultimate recovery.
AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado