Carbonate diagenesis of Pennsylvanian and Permian cyclic strata from the Hugoton embayment, western Kansas, United States of America

Depositional facies and diagenesis are two of the main controls on character of porosity and permeability in carbonate hydrocarbon reservoirs. This study isolates some of the variables that affected diagenesis of Pennsylvanian and Permian carbonates from the Hugoton embayment in western Kansas. It concentrates on diagenesis related to subaerial exposure, brine reflux, and hydrothermal processes and evaluates the potential of a new class of fluid inclusion in studies of microbial paleoecology and diagenesis.; The basis for this study is carbonate diagenesis of samples from two cores of Pennsylvanian and Permian strata from western Kansas. Carbonate diagenetic fabrics are analyzed using detailed petrography, cement stratigraphy, fluid-inclusion microthermometry, carbon, oxygen and strontium isotope geochemistry. Carbonate-diagenetic fabrics are a record of fluid-flow history and understanding this fluid-flow history helps in understanding the evolution of porosity and permeability of oil and gas reservoirs.; In brachiopods, some endolithic microbial borings are preserved as a new class of aqueous fluid inclusion. Nearly all of these inclusions have changed their shapes by open- or closed-system precipitation of calcite. The techniques applied to study this new type of fluid inclusion can be used to identify and determine the timing of entrapment of microbial trace fossils in ancient Earth and extraterrestrial materials. These inclusions may not preserve the best records of early diagenesis because homogenization and ice melting temperatures indicate that they tend to reequilibrate.; Subaerial exposure of Pennsylvanian and Permian sequences resulted in predictable patterns of carbonate diagenesis controlled by sequence architecture, and to a lesser extent, by climate present during subaerial exposure. Fluid inclusion and stable isotope data indicate the prevalence of cementation in mixing zones. Shelf positions closest to sea-level highstand and humid climates produce the most calcite cement. Interbedded aquitards and low paleoslope lead to very little flux of meteoric water and very little carbonate diagenesis.; Cement stratigraphy, fluid inclusion, and stable isotope data indicate that intermediate- and late-stage calcite and dolomite precipitation resulted from heated-brine reflux, which was followed by more hydrothermal circulation. Hydrothermal circulation from brine reflux may have been important in many other basins.