Geochemical Signatures of Fluid Flow in Thrust Sheets: Fluid-inclusion and Stable Isotope Studies of Calcite Veins in Western Wyoming

Abstract

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 158–163C. Waters with salinities ranging from about 1 to 21 wt. % NaCl equivalent were trapped. Petroleum-bearing inclusions record relatively lower homogenization temperatures (100C) compared to aqueous inclusions. Stable isotope analyses of limestone rocks indicate that they are typical marine limestones (with 13C values mainly in the range of +5 to 5 relative to Peedee belemnite) altered to various degrees by low 18O waters. Similarities in the 13C values of the host limestone rocks and calcite veins suggest that the carbon was derived from the same limestone formation. In contrast, the 18O 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 18O composition or higher temperatures. Calculated for temperatures of 110–165C (based on homogenization temperatures of aqueous inclusion obtained from fluid-inclusion microthermometry), the composition of vein water (with 18Owater 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.