The Horseshoe Atoll is a westward-tilting arcuate chain of reef mounds 175 miles (282 km) long located in the Midland Basin, consisting of 1,804 feet (550 m) of limestone accumulated in the Pennsylvanian and 1,099 feet (335 m) in the Permian, with 15 significant reservoirs from 6,099 feet (1,859 m) to 9,902 feet (3,018 m) in depth.
The reef complex consists of Upper Pennsylvanian Strawn, Canyon and Cisco limestones, overlain by Lower Permian Wolfcamp sandstones and shales of terrigenous origin prograding northeast to southwest.
The first production well, Seabird Oil Company of Delaware No.
 1-B J. C. Caldwell, was completed in 1948.
The Horseshoe Atoll is a westward-tilting arcuate chain of reef mounds 175 miles (282 km) long located in the Midland Basin, consisting of 1,804 feet (550 m) of limestone accumulated in the Pennsylvanian and 1,099 feet (335 m) in the Permian, with 15 significant reservoirs from 6,099 feet (1,859 m) to 9,902 feet (3,018 m) in depth. The reef complex consists of Upper Pennsylvanian Strawn, Canyon and Cisco limestones, overlain by Lower Permian Wolfcamp sandstones and shales of terrigenous origin prograding northeast to southwest. The first production well, Seabird Oil Company of Delaware No. 1-B J. C. Caldwell, was completed in 1948.
Permian Basin (North America)