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Tectonic-hydrothermal brecciation associated with calcite precipitation and permeability destruction in Mississippian carbonate reservoirs, Montana and Wyoming

¹ Comparative Sedimentology Laboratory, University of Miami, 4600 Rickenbacker Causeway, Miami, Florida 33149-1098; dkatz{at}rsmas.miami.edu
² Comparative Sedimentology Laboratory, University of Miami, 4600 Rickenbacker Causeway, Miami, Florida 33149-1098; geberli{at}rsmas.miami.edu
³ Comparative Sedimentology Laboratory, University of Miami, 4600 Rickenbacker Causeway, Miami, Florida, 33149-1098; pswart{at}rsmas.miami.edu
⁴ New York State Museum, Room 3124 CEC, Albany, New York 12230; lsmith{at}mail.nysed.gov

David A. Katz received his B.S. degree from Hamilton College (1999) and his M.S. degree from the Colorado School of Mines (2002) and is currently a Ph.D. candidate at the University of Miami, where he conducts research at the Comparative Sedimentology Laboratory. His research investigates the earliest diagenesis and geochemistry of modern carbonates, dolomitization, and integration of geochemistry with sequence stratigraphy in ancient carbonates.

Gregor P. Eberli is a professor and chair of the Division of Marine Geology and Geophysics at the University of Miami and is the director of the Comparative Sedimentology Laboratory. He received his Ph.D. from the Swiss Institute of Technology (Eidgenössische Technische Hochschule) in Zurich, Switzerland. His field research focuses on sedimentology and sequence stratigraphy of carbonates. In the laboratory, he explores the influence of pore structure on petrophysical properties of carbonates. He was Distinguished Lecturer for AAPG (1996–1997), the Joint Oceanographic Institutions/U.S. Science Advisory Committee (1997–1998), and the European Association of Geoscientists and Engineers (2005).

Peter received his Ph.D. from the University of London in 1980 for work on modern coral reefs. After 3 years at the University of Cambridge, he started a project on dolomite geochemistry at the University of Miami, where he is now a professor of marine geology and geophysics in the Rosenstiel School of Marine and Atmospheric Sciences. His professional interests include carbonate geochemistry and diagenesis, hydrology, and paleoclimatology. He is also a coleader of the Comparative Sedimentology Laboratory.

Langhorne "Taury" Smith heads the Reservoir Characterization Group at the New York State Museum. He holds a B.S. degree from Temple University and a Ph.D. from Virginia Polytechnic Institute and State University and did postdoctoral work at the University of Miami. He also worked for Chevron as a development geologist, and his current research interests are focused on carbonate reservoir characterization and hydrothermal alteration of carbonate reservoirs.

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 crustal shortening during the Laramide orogeny. Brecciation is accompanied by dedolomitization, late-stage calcite precipitation, and porosity occlusion, especially in outcrop dolomites. The tectonic-hydrothermal late-stage calcite reduces permeability in outcrops and, potentially, high-quality subsurface reservoir rocks of the subsurface Madison Formation, Bighorn Basin. The reduction of permeability and porosity is increased along the margins of the Bighorn Basin but not predictable at outcrop scale. The destruction of porosity and permeability by hydrothermal activity in the Madison Formation is unique in comparison to studies that document enhanced porosity and permeability and invoke hydrothermal dolomitization models.

Hydrothermal breccias from the Owl Creek thrust sheet are classified into four categories based on fracture density, calcite volume, and clast orientation. Shattered breccias dominate the leading edge of the tip of the Owl Creek thrust sheet in the eastern Owl Creek Mountains, where tectonic deformation is greatest, whereas fracture, mosaic, and chaotic breccias occur throughout the Bighorn Basin. The breccias are healed by calcite cements with ¹⁸O values ranging between –26.5 and –15.1 Peedee belemnite (PDB), indicating that the cements were derived from isotopically depleted fluids with elevated temperatures. In the chaotic and mosaic breccia types, large rotated and angular clasts of the host rock float in the matrix of coarse and nonzoned late-stage calcite. This appearance, combined with similar ¹⁸O values across even large calcite veins, indicates that the calcite precipitated rapidly after brecciation. Values for ¹³C (5–12 PDB) from the frontal part of the Owl Creek thrust sheet indicate equilibrium between methane and CO₂-bearing fluids at about 180°C. Fluid inclusions from the eastern basin margin show that these cements are in equilibrium with fluids having minimum temperatures between 120 and 140°C and formed from relatively low-salinity fluids, less than 5 wt.% NaCl. Strontium isotope ratios of these hydrothermal fluids are more radiogenic than proposed values for Mississippian seawater, suggesting that the fluids mixed with felsic-rich basement before migrating vertically into the Madison Formation.

We envisage that the tectonic-hydrothermal late-stage calcite-cemented breccias and fractures originated from undersaturated meteoric groundwaters that migrated into the burial environment while dissolving and incorporating Ca²⁺ and and radiogenic Sr from the dissolution of the surrounding carbonates and the felsic basement, respectively. In the burial environment, these fluids were heated and mixed with hypersaline brines from deeply buried parts of the basement. Expulsion of these fluids along basement-rooted thrust faults into the overlying strata, including the Madison Formation, occurred most likely during shortening episodes of the Laramide orogeny by earthquake-induced rupturing of the host rock. The fluids were injected forcefully and in an explosive manner into the Madison Formation, causing brecciation and fracturing of the host rock, whereas the subsequent and sudden decrease in the partial pressure of CO₂ caused the rapid precipitation of calcite cements.

The explosive nature of hydrothermal fluid migration ultimately produces heterogeneities in reservoir-quality carbonates. In general, flow units in the Madison Formation are related to sequence boundaries, which create vertical subdivisions in the porous dolomite. The late-stage calcite cement surrounds hydrothermal breccia clasts and invades the dolomite, reducing porosity and permeability of the reservoir-quality rock. As a consequence, horizontal flow barriers and compartments are established that are locally unpredictable in their location and extent and regionally predictable along the margins of the Bighorn Basin.

This article has been cited by other articles:

				L. B. Smith Jr. and G. R. Davies Structurally controlled hydrothermal alteration of carbonate reservoirs: Introduction AAPG Bulletin, November 1, 2006; 90(11): 1635 - 1640. [Full Text] [PDF]