ABSTRACT

The Horseshoe Atoll, a horseshoe-shaped accumulation of Pennsylvanian and lower Permian limestone in the Midland Basin in West Texas, is the largest carbonate petroleum reservoir in North America. This carbonate mass is 70 to 90 miles across and as much as 3,000 feet thick. From 1947, when the discovery well was drilled in the southeastern part of the atoll, to the end of 1953, the atoll yielded 244,066,994 barrels of oil, and is currently being subjected "to extensive water-flooding in an effort to increase further the ultimate yield of petroleum.

By March 1, 1954, more than 5,00 wells had been drilled into the Horseshoe Atoll, and the logs of these wells, together with cores from more than 100 of the wells, were used by the Geological Survey and the Texas Bureau of Economic Geology in a cooperative study of the characteristics and significance of this vast and unusual limestone mass. The availability of this large amount of subsurface data has made possible an analysis of the configuration, stratigraphy, and porosity of the atoll that may aid in understanding other reef-like carbonate reservoirs. Although it is not

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certain that he Horseshoe Atoll originated within the same environments that control the development of modern reefs, the terms "reef" and "atoll" are applied to this limestine mass because it apparently had most of the characteristics of a reef during its growth.

The origin of the atoll is open to debate. The atoll may, however, have accumulated in a steadily subsiding basn, shifts in sea level having produced the several unconformities found in the reef.

The entire faunal assemblage suggests that the waters in whch the atoll accumulated were marine and were probably of normal or near-normal salinity. Apparently there was little turbidity during periods of maximum atoll growth. It is suggested that the upwelling of nutrient-rich sea water from adjacent deep water contributed food to organisms growing on the atoll. In early Woilfcamp time large amounts of clay and silt filled the eastern and northern parts of the Midland Basin. These muds encroached to the south and west, smothering the atoll. The deposition of the Dean sand of Rodin (1950) marks the death of the Horseshoe Aoll

The investigation of the Horseshoe Atoll was carried on cooperatively by the U. S. Geological Survey and Texas Bureau of Economic Geology from 1949 until 1955. During the early part of the study as many as 8 geologists participated, but the present discussion is based on the work of the authors, who carried the investigation to a conclusion.

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