Origin of micro-rhombic calcite matrix within Cretaceous reservoir rock, West Stuart City Trend, Texas

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Cores from four wells from the West Stuart City Trend in La Salle and Webb Counties, Texas, were analyzed to define depositional and diagenetic facies and to determine factors controlling porosity distribution. In all, 1187 ft (362 m) of core and 220 thin sections were examined in detail and supplemented by SEM analyses of fractured surfaces and plastic casts of pore systems. A comparison of lithofacies and ecologic facies to permeability and porosity values compiled from core data revealed that zones with permeabilities greater than 0.1 md and porosities of at least 6% were associated with rudistid grainstones cemented by isopachous, submarine cement and packstones with a finely crystalline rhombic calcite matrix. Isotopic and trace element analyses of both isopachous submarine cements and micro-rhombic matrix suggest a common origin. The precursor to the micro-rhombic calcite is believed to have been peloidal, high-magnesian calcite. This internal marine sediment may be analogous to the peloidal fabrics that have been reported from Holocene and Pleistocene carbonates. Diagenetic equilibration of both submarine cements and peloidal infill is believed to have occurred during burial either in marine pore waters at elevated temperatures or in restricted flow, phreatic freshwaters.