What controls porosity in cherty fine-grained carbonate reservoir rocks? Impact of stratigraphy, unconformities, structural setting and hydrothermal fluid flow: Mississippian, SE Kansas

Abstract

Abstract The localization and heterogeneity of carbonate oil and gas reservoirs are commonly controlled by extensive diagenetic alteration. Mississippian (Osagean–Meramecian) strata in SE Kansas are investigated to determine structural, relative sea-level, diagenetic and depositional controls on stratigraphy, lithofacies distribution and reservoir character. This project shows how karst horizons and fractured zones can provide preferred conduits for hydrothermal porosity enhancement. Thus, enhanced porosity in karst horizons may have a late origin, with chemically aggressive hydrothermal fluids following preferred pathways of fluid flow. Lithofacies include echinoderm-rich bioclastic wacke–packstone, sponge-spicule-rich packstone, dolomitic bioclastic wackestone, argillaceous dolomite, tripolitic chert and chert breccia. Four cores are used to construct a 10 mile-long SW–NE-trending cross-section, showing three genetic units deposited on a mostly south-facing distally steepened ramp, with periods of upwelling. Paragenesis reveals that early and late dissolution enhances porosity in chert and carbonate facies. Fluid inclusion microthermometry from megaquartz and baroque dolomite reveals variable but increasing homogenization temperatures (70–160 °C) and increasing salinity through time. The best reservoirs may be controlled by depositional setting that led to large amounts of chert, alteration associated with subaerial exposure, and a hydrological and structural setting that led to enhanced hydrothermal fluid flow for later dissolution.