Geochemical Signatures of Fluid Flow in Thrust Sheets: Fluid-inclusion and Stable Isotope Studies of Calcite Veins in Western Wyoming
Rasoul Sorkhabi, 2005. "Geochemical Signatures of Fluid Flow in Thrust Sheets: Fluid-inclusion and Stable Isotope Studies of Calcite Veins in Western Wyoming", Faults, Fluid Flow, and Petroleum Traps, Rasoul Sorkhabi, Yoshihiro Tsuji
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The fold and thrust belt of western Wyoming consists of a series of thrust sheets formed in the Sevier-Laramide orogenies of Late Cretaceous-early Eocene. Cambrian to Eocene rocks form thrust packages that have been transported several kilometers in a thin-skinned (basement-detached) tectonic regime. The major thrust faults in the region are east verging and become younger from the west toward the east with a decrease in displacement and an increase in thrust velocity. The thrust sheets contain calcite veins of various styles, including those in fractured limestones, conjugate veins, en echelon veins, and fracture fills of fault breccias. Calcite veins provide geochemical signatures of fluid flow during the development of the thrust faults. Fluid-inclusion data from the calcite veins demonstrate the flow of various generations of aqueous fluid and liquid hydrocarbons. Homogenization temperatures (minimum trapping temperatures) of aqueous inclusions define three populations: 110–125, 130–140, and 158–163°C. Waters with salinities ranging from about 1 to 21 wt. % NaCl equivalent were trapped. Petroleum-bearing inclusions record relatively lower homogenization temperatures (<100°C) compared to aqueous inclusions. Stable isotope analyses of limestone rocks indicate that they are typical marine limestones (with δ13C values mainly in the range of +5 to −5% relative to Peedee belemnite) altered to various degrees by low δ18O waters. Similarities in the δ13C values of the host limestone rocks and calcite veins suggest that the carbon was derived from the same limestone formation. In contrast, the δ18O values of calcite veins were found to be more negative than those of host limestones, indicating that vein calcite precipitated from water with either lower δ18O composition or higher temperatures. Calculated for temperatures of 110–165°C (based on homogenization temperatures of aqueous inclusion obtained from fluid-inclusion microthermometry), the composition of vein water (with δ18Owater values of 0 to +14% relative to the standard mean ocean water) appears to have been similar to diagenetic or formation water in the marine basin. This interpretation is also supported by the existence of highly saline aqueous inclusions in calcite veins. Micropermeability measurements of calcite veins in a fault breccia and in a calcite-cemented sandstone yielded a similar range of gas permeabilities between the host rock fragments and the calcite veins, indicating that mineralized veins do not seem to reduce single-phase permeability in carbonate reservoirs.
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Faults, Fluid Flow, and Petroleum Traps
Japan National Oil Corporation (JNOC) (presently Japan Oil, Gas and Metals National Corporation) launched a multidisciplinary and international project on the Evaluation of Traps and Seals in 1997. The project ended in 2003. This publication resulted from that project and includes JNOC research articles as well as contributions from industry and academia. The 17 papers in this volume cover topics such as a method to estimate the amount of oil/gas accumulation using the concept of equivalent grain size in seal rock, and oil/gas migration to and spill-point geometry of petroleum traps; two case studies of fault seal assessment applied to normal faults in Tertiary clastic reservoirs in offshore Sarawak and offshore Gulf of Thailand; and physical analog studies of the development of extensional faults. This publication also contains a valuable bibliography of nearly 1000 additional articles and books published on fault traps, fault seal processes, and fault-related fluid flow in sedimentary basins, for use as a reference tool to delve into publications preceding this volume.