The Horseshoe atoll is composed of bedded bioclastic limestone and limestone detritus that accumulated in the interior part of a developing intracratonic basin during late Paleozoic time. The reef environment was established early in the basin history and retained because of the lack of significant terrigenous clastic filling in the basin interior. Mixed types of bioclastic debris accumulated cyclically and the upper level of the reef complex was maintained near sea level as basin subsidence continued. About 1,800 ft of limestone accumulated during the Pennsylvanian, and primary dips as great as 8° developed along the margins of the atoll. During early Permian time the reef was restricted to the southwest side of the atoll where more than 1,100 ft of additional limes one accumulated before death of the reef. Continued tilting of the reef complex after burial elevated Pennsylvanian pinnacles along the east side of the atoll 1,400 ft higher than Permian pinnacles along the west side. The updip migration of hydrocarbons was uninhibited, and reef pinnacles along the eastern half of the atoll are full to the spill point. The Scurry reef is the largest single area of closure on the Horseshoe above the oil-water contact. It includes approximately 69,000 productive acres and has a maximum oil column of 765 ft. The reef was discovered in 1948 with reflection-seismic methods. Production from the Scurry reef exceeded 500 million bbl by the end of 1967, and this represents approximately 60 percent of the oil produced from the Horseshoe atoll.

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