ALBERT

Description: The term “beef” describes bedding-parallel calcite veins found commonly in the organic-rich matrix of unconventional resource plays. Although some authors have interpreted beef to be an early diagenetic feature, these calcite veins are commonly attributed to precipitation at high temperatures and localized overpressure during hydrocarbon generation. The temperature at which the beef formed is thus crucial to ascertain the process of beef genesis. We use the novel methodology of clumped isotope analysis to constrain both the temperature at which beef forms and the isotopic composition of fluids present during formation.For this study, we use beef from basinal sections of the Vaca Muerta Formation in the Neuquén Basin, where veins are commonly up to approximately 10 cm (∼4 in.) thick and are laterally continuous over 1 km (0.6 mi). The calcite veins occur in isolation or in association with concretions and ash layers. Sequence stratigraphic boundaries have little influence on distribution, and only a low correlation between beef and total organic content or beef and ash layers exists. The internal crystal structure of beef varies largely, suggesting both syntaxial and antitaxial growth forms. The δ18O values of beef range from approximately −12‰ to −9‰, and the δ13C values vary between approximately −1‰ and 1‰. The surrounding mudstone and concretion fracture fills (calcite) show little difference isotopically when compared to the beef itself. The δ18O values of nearby concretions range from approximately −3.5‰ to 1‰, and the δ13C values vary between approximately 6‰ and 11‰.Clumped isotope analysis of beef in the Vaca Muerta Formation indicates temperatures between approximately 140°C and 195°C, whereas the surrounding mudstones vary from approximately 120°C to 150°C. The corresponding formation fluid δ18Ow values range from 8.5 to 14.5‰. These temperature data are higher than the maximum temperatures suggested by published studies modeling the basin’s thermal and burial histories. If these models are correct, the clumped isotope data indicate that the growth of beef in the Vaca Muerta Formation required the input of hydrothermal fluids from greater depths. Alternatively, the geothermal gradient or burial depth was underestimated in these models.

Description: We present a geochemical profile through a 445-m (1459.9-ft) section of shallow-water carbonate platform strata in the upper part of the Khuff Formation. The Permian–Triassic boundary (PTB) is recognized in this section based on the immediately preceding negative shift in bulk-rock carbonate carbon isotope composition (equivalent to the end-Permian extinction horizon), combined with biostratigraphic control from nearby wells. These strata show an abrupt and long-lasting decrease in bulk-rock uranium (U) content coincident with the carbon isotope shift. Because of low siliciclastic content and the consequently low potassium and thorium of these carbonates, the decrease in U is clearly reflected in the total gamma-ray (GR) profile recorded by wire-line logging. Published log curves show similar distinctive GR profiles throughout a large area of the Middle East, indicating that U depletion across the PTB is a regional characteristic. This feature cannot be explained as diagenetic and is not related to the organic matter content of the host sediments, but it is suggested to reflect the global depletion of U in earliest Triassic seawater, caused by the abrupt onset of deep-ocean anoxia and the resulting increase in U precipitation in oxygen-poor sediments. This explanation carries the implication that similar U depletion should be characteristic of lowermost Triassic carbonates from shallow-water (oxygenated) settings worldwide. Analogous signatures of U depletion should also have developed in shallow-water carbonates deposited contemporaneously with episodes of deep-ocean anoxia during other periods of geological time. These predictions can be tested by high-accuracy U profiling of other well-characterized carbonate successions, potentially yielding a new approach for tracking the degree of oceanic circulation throughout Earth's history. Steve has a Ph.D. from the University of California at Los Angeles. He works on sandstone and carbonate reservoir studies for exploration and production projects. Tore received his Candidatus Scientiarium degree in geology at the University of Oslo, Norway, in 1984. He began his professional career as a well-site and operations geologist, first at Norsk Hydro and thereafter at Statoil. He has worked on Barents Sea exploration and Mideastern carbonate reservoirs. Peter received his Ph.D. from the University of London in 1980 for his 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.

Description: 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 δ18O 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 δ18O values across even large calcite veins, indicates that the calcite precipitated rapidly after brecciation. Values for δ13C (∼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 CO3 2- 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. 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.