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Five lithologies are present in the Barnett Shale (Mississippian) in a core taken in Johnson County, Texas, in the southern part of the Fort Worth Basin. Dark claystone to mudstone makes up 86% of the cored interval. Sponge spicules are the most common silt-size grain in this lithology. The clay-size material comprising the matrix is a mixture of cryptocrystalline quartz, probably derived from radiolarian tests, and clay minerals. The rock is highly siliceous, hard, dense, and brittle. Three calcareous lithologies are present in the core: limy layers, shell layers, and concretions. Together, these lithologies make up only 7% of the cored interval. The limy layers and concretions consist almost entirely of micrite. The shell layers contain gravel-size fragments of brachiopods, pelecypods, and cephalopods. The calcareous lithologies are found as thin interbeds in the dark claystone to mudstone throughout the core. A laminated siltstone to mudstone containing abundant sponge spicules is found only at the top of the cored interval. Glauconite and phosphatic material are conspicuous components of this lithology. The phosphatic material includes phosphate-coated grains of glauconite, quartz, and fossil fragments. The lithologies in the core resemble those described in the core from the northern part of the basin. However, the relative abundance of the various lithologies changes greatly from the northern part to the southern part of the basin. Understanding lithologic variation within the Barnett Shale is key to locating sweet spots within the play and then selecting intervals within the reservoir in which to land horizontals wells.

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