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Carbonate-rich lithologies of the gas-producing Upper Mississippian Barnett Shale, Fort Worth Basin, Texas, are diverse and include lithologies with carbonate components that are primarily authigenic, as well as those that have carbonate components dominated by skeletal debris and other allochems such as peloids and intraclasts. Compositionally, carbonate-bearing lithologies of the Barnett Shale (including the informal unit known as the Forestburg Limestone) can be viewed as mixtures of authigenic or allochemical carbonate and siliciclastic sediment derived mostly from outside the basin. With the exception of the Forestburg Limestone, these varied carbonate lithologies dominate only in local zones, at the scale of a hand specimen or thin section, and do not constitute a volumetrically significant part of the gas-producing reservoir rock. Carbonate lithologies are significant, however, for clues they provide on environmental and early diagenetic conditions during accumulation of the Barnett Shale. Carbonate lithologies dominated by skeletal components contain distinct and impoverished marine faunas that are consistent with low oxygenation levels. The generally early timing of carbonate cement precipitation is supported by the reworking of diagenetic carbonate as silt- to sand-size intraclasts, sediment infilling of fractures in cemented beds and concretions, displacive fabrics, and highly random orientations of phyllosilicate grains within carbonate units. In some cases, detrital allochemical carbonates provided nucleation substrates for precipitation of highly displacive authigenic carbonate that was extensively reworked into microspar-size sediment particles. The elemental and isotopic chemistries of authigenic carbonates are consistent with near-sea-floor authigenesis driven by microbial cycling of organic carbon into carbonate minerals under generally reducing and low-temperature conditions.

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