Title: Defining the Transition Between the Horseshoe Atoll and the Eastern Shelf of the Permian Basin; A Geologic Characterization of Eastern Scurry and Western Fisher Counties, Texas, Including the Ocho Juan (Pennsylvanian) Reef Abstract: The Horseshoe Atoll and Eastern Shelf are both well-known hydrocarbon-producing regions within the Permian Basin. However, the transition between the two features has never been analyzed. By mapping on the top of the Pennsylvanian Limestone, a highly diachronous surface that encompasses multiple periods of weathering and deposition, a northeast–southwest trending structural and depositional trough between the Horseshoe Atoll and Eastern Shelf is revealed. Sitting on the edge of the Eastern Shelf and trough is Ocho Juan Field, a Pennsylvanian age reefal buildup consisting of 2 main lobes and a subsidiary buildup that includes the adjacent Caballos Field. The western flank of Ocho Juan, adjacent to the trough, displays steep structural contours indicative of high amounts of weathering…