The primary source rock of the Bend Arch-Fort Worth Basin is Mississippian Chester-age Barnett Shale, perhaps including the overlying Chesterian Forestburg Formation. The Barnett commonly exhibits an uncommonly high gamma-ray log response. Other potential source rocks of secondary importance are Early Pennsylvanian and include dark fine-grained carbonate rock and shale units within the Marble Falls Limestone and the black shale facies of the Smithwick/Atoka Shale. The Barnett Shale was deposited over much of North Central Texas; however, because of post-depositional erosion, the present distribution of Barnett is limited to Province 045. The Barnett/Forestburg Chesterian interval is over thick along the southwest flank of the Muenster Arch. It is eroded in areas along the Red River-Electra and Muenster Arches to the north, the Llano uplift to the south where it outcrops, and the easternmost portion of the province where the Barnett laps onto the Eastern Shelf-Concho Platform.