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A thick shale section cored in the EOG Resources Gordon saltwater disposal (SWD) well in the southern Fort Worth Basin contains six different lithologies. Gamma-ray readings on well logs can be used to distinguish the Barnett Shale (Mississippian) from the overlying Pennsylvanian shales and to divide the Barnett Shale into upper and lower units referred to informally as the Barnett A and Barnett B. Laminated silty claystone to mudstone is the dominant lithology in the Pennsylvanian shales above the Barnett Shale. The relative abundance of this lithology decreases downward in the core. It makes up a significant part of the Barnett A, but only a minor part of the Barnett B. A dark claystone shows the opposite trend, decreasing in relative abundance upward in the core. Sponge spicules are the most common silt- and sand-size grains in both the laminated claystone to mudstone and the dark claystone. Thin shell layers and phosphatic intervals are also found throughout the core. Shell layers are more common in the Pennsylvanian shales. Phosphatic material is most abundant in the Barnett B at the base of the core. Claystones and mudstones, lacking sponge spicules, but containing significant amounts of silt-size quartz are found only in the Barnett B. Isopach maps show that the Barnett B is part of a large shale wedge that prograded into the central and southern parts of the Fort Worth Basin from the northeast and that the Barnett A is part of a smaller shale wedge that prograded from east to west across Johnson County. The upper wedge onlaps and dies out against the flank of the lower wedge. The distribution of lithologies in the Gordon SWD well can be related to the position of the well site on the shale wedges. The site was far removed from areas of active sedimentation during the deposition of the Barnett B and closer to the main sources of sediment and areas of sedimentation during deposition of the Barnett A.

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