GEOCHEMICAL SIGNATURES OF FLUID FLOW IN THRUST FAULT ZONES, WESTERN WYOMING

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 times. The major thrust faults in the region are east verging, and become younger from the west toward the east. 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. Homogenization temperatures (minimum trapping temperatures) of aqueous inclusions define three populations: 110-125 C, 130-140 C and 158-163 C degrees. Waters with salinities ranging from about 1 to 21 weight percent NaCl equivalent were trapped. Stable isotope analyses of limestone rocks indicate that they are typical marine limestones (with delta C-13 values mainly in the range of +5 to -5 per mil relative to PDB) altered to various degrees by low delta O-18 waters. Similarities in the delta C-13 values of the host limestone rocks and calcite veins suggest that the carbon was derived from the same limestone formation. In contrast, the delta O-18 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 O-18 composition or higher temperatures. Calculated for fluid inclusion homogenization temperatures of 110-165 C degrees, the composition of vein water (with delta O-18 values of 0 to +14 per mil relative to SMOW) 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. Micro-permeability measurements of calcite veins in fault breccia 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.