Petrography and paragenesis of diagenetic mineral phases in cherty and dolomitic spiculite strata, Mississippian, South-central Kansas

Abstract

Mississippian strata in four cores from three different locations in southcentral Kansas (Rhodes field, Wellington field, and a "wildcat" well in Comanche county) were used to investigate the depositional environment, and the diagenetic history in variable cherty and dolomitic sponge-spicule-rich facies. The cores described comprises a series of lithofacies that are representative of inner-, mid and outer-ramp depositional environments, and depict conditions in which colonization of siliceous-sponges suppressed the deposition of carbonates. Paragenesis reveals that the most important and abundant diagenetic mineral phases are chert and dolomite, which partially to pervasively replaced the sediments early after they were deposited. Chertification was triggered in part by remobilization of silica from dissolution of siliceous sponge-spicule. Salinity measurements on fluid inclusions of early megaquartz revealed values consistent with seawater and evaporated seawater (early hypersalinity). This suggests that silicification processes (including chertification) began shortly after deposition of the sediments, and before complete lithification. Multiple phases of dolomite were described and are texturally different in the locations studied. Petrography and geochemistry data lead to the conclusions that primary dolomite (cD1 and wD1a) precipitation occurred in near-surface environments from potentially seawater-derived fluids and temporally associated with nodular cherts. Elevated concentrations of iron and manganese and void-filling textures characteristic of secondary dolomite phases indicate that secondary dolomitization occurred in the meteoric or subsurface realms. Two cases of regional scale events were described and include 1) an early event of nodular evaporite precipitation in the sediment column, and 2) a mineral assemblage of late stage cements comprising megaquartz, chalcedony (zebraic), baroque dolomite and calcite which has been indicated as a predictive mineral series to indicate hydrothermal fluid migration on Mississippian and adjacent units in the mid-continent by previous authors. Prospective hydrocarbon reservoirs involve facies rich in sponge-spicule debris that had undergone dissolution (either meteoric or hydrothermal), or pervasively dolomitized facies where intercrystaline porosity dominates.