The Horseshoe atoll is an arcuate chain of reef mounds, composed of mixed types of bioclastic debris, that accumulated in the interior part of the developing intracratonic Midland basin during late Paleozoic time. The atoll is 175 mi (282 km) long and locally is almost 3,000 ft (914 m) thick. The reef environment was established early in basin history and retained because of the limited amount of terrigenous clastic material transported to the basin interior. About 1,800 ft (549 m) of limestone accumulated during the Pennsylvanian, and primary dips commonly as great as 8° developed along the margins of the atoll. During earliest Permian time the reef was restricted to the southwest side of the complex, where more than 1,100 ft (335 m) of additional limestone accumulat d before death of the reef. Reef mounds were buried by prograding Early Permian terrigenous clastic material which progressively covered the atoll from northeast to southwest. Westward tilting of the reef complex after burial elevated Pennsylvanian mounds along the east side of the atoll 1,400 ft (428 m) higher than Permian mounds along the southwest side. The updip migration of hydrocarbons was uninhibited in the lower part of the reef, and most mounds along the eastern half of the atoll are full to the spill point. Some mounds along the trend are not productive because of Wolfcamp sandstone contacts with the upper surface of the mounds. Fifteen individual fields, containing 2.54 billion bbl of recoverable oil, are present along the crest of the atoll. The Scurry field is the giant of t e trend. It includes approximately 73,000 productive acres (295 km²), has a maximum oil column of 765 ft (233 m), and ultimately will yield 1.72 billion bbl. This field has no active water drive, so pressure maintenance was initiated early to achieve maximum efficient recovery. Scurry field has produced 521 million of the 857 million bbl of oil produced from reef rocks along the crest of the atoll. Scurry was discovered in 1948 with reflection-seismic methods, but only a small part of the field was mapped before the drilling of the discovery well.