Diagenetic history of Eocene Wilcox sandstones and associated formation waters, South-Central Texas




Fisher, Robert Stephen

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The Lower Eocene Wilcox Group of Texas is a wedge of terrigenous clastic sediment which extends from outcrop to depths greater than 5 km where temperatures exceed 200°C. Wilcox sandstones in South-Central Texas are feldspathic litharenites, sublitharenites, lithic arkoses, litharenites, and subarkoses. Authigenic quartz, kaolinite, calcite, ankerite and albitized plagioclase are volumetrically important, whereas dolomite, chlorite, illite and albite overgrowths are present in trace amounts. Petrographic and geochemical evidence indicate that the order of cementation was: chlorite and albite overgrowths, quartz, kaolinite, calcite and ankerite. The degree of albitization of detrital plagioclase increases with burial depth and temperature. Chemical and oxygen isotopic compositions of quartz, kaolinite, calcite and ankerite indicate that each mineral formed during a single precipitation event. None of these phases are in isotopic equilibrium with modern formation water. Chemical and oxygen isotopic compositions of formation water samples reflect chemical equilibrium with authigenic minerals in sandstones and approach isotopic equilibrium with smectite-illite in enclosing shale. Ca, Mg, and Fe concentrations are determined by equilibrium between calcite and ankerite; Na and K values are controlled by feldspar stabilization reactions. Formation water δ¹⁸O compositions increase with depth, reflecting the conversion of smectite to illite in Wilcox shales