Carbonate rock-water diagenesis lower cretaceous, Stuart City Trend, South Texas

Prezbindowski, Dennis Robert
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The Stuart City Trend in south Texas is a Lower Cretaceous shelf-edge buildup of bioclastic and reefal carbonates. This carbonate system is currently buried to depths of between 3,300 and 5,000 meters. Combined with the equivalent updip shallow water, shelf carbonates of the Edwards and Glen Rose Formations, these carbonates form an extensive Gulfward dipping carbonate wedge. The whole-rock chemical and isotopic characteristics of the limestones of the Stuart City are the product of their initial composition and early surface and near-surface diagenetic history. Burial diagenesis has not significantly altered the chemical and isotopic character of these limestones. Marine diagenesis was volumetrically important and consisted of micritization and the precipitation of fine to very coarsely crystalline, fibrous to bladed, isopachous, Mg-calcite cements. These cements have been neomorphically altered to calcite, while still retaining a Mg⁺⁺ memory and a marine-like isotopic character. Diagenesis by meteoric water was volumetrically important in altering the depositional character of the carbonate sediments associated with the topographic highs along the Stuart City Trend. Secondary porosity formation, Mg-calcite and aragonite stabilization and equant spar calcite cementation are important products of this diagenesis. The equant spar calcite cements are poor in iron and manganese. They make up approximately 16 percent by volume of the limestones studied. The majority of these cements have δ¹³C compositions which fall in the range of modern marine carbonates [...]. Oxygen isotopic variability of the equant spar calcites is controlled by the δ¹⁸O composition of the diagenetic fluids and the degree of openness of the carbonate sediments to these fluids. Thermally induced ¹⁸O depletion in the equant spar calcites was of secondary importance. Pyrobitumen pore fillings and inclusions in the outer one millimeter rims of the very coarsely crystalline, equant spar calcite cements indicate that only minor amounts of calcite cementation have occurred since hydrocarbon migration. These hydrocarbons are believed to have originated from Jurassic and Cretaceous source rocks. Deep burial diagenesis, i.e., post hydrocarbon migration, consisted of the precipitation of minor amounts of galena, fluorite and Sr⁺⁺-rich equant spar calcites. These diagenetic events can be directly related to the chemistry of present-day formation waters. The densities of sodium-calcium-chloride brines of the Edwards Formation and of the Stuart City Trend range from 1.018 grams per cm³ in the shallow, updip hydrocarbon fields to more than 1.21 grams per cm³ in the deepest hydrocarbon fields. The oxygen isotopic composition of the formation waters becomes increasingly enriched in ¹⁸O with increasing depth. Interaction of the formation waters with the carbonate country rock at elevated temperatures is responsible for this enrichment trend. The Mg⁺⁺/Ca⁺⁺ ratio of the formation waters increases with decreasing depth. Dedolomitization and dolomite dissolution are the major diagenetic processes responsible for Mg⁺⁺ enrichment. Movement of deeper basinal waters, potentially of Jurassic origin, into the Stuart City Trend and Edwards Formation is occurring via major faults