The Horseshoe atoll is an arcuate mass of deeply buried fossiliferous limestone of Pennsylvanian and early Permian age in the Midland basin of west Texas. The southern part of the atoll is in southern Borden and northern Howard Counties. From early Strawn until early Wolf camp time this atoll, which had some of the characteristics of a reef, was a prominent feature on the floor of the Midland basin. The crest of the atoll is a series of buried hills and saddles. In the area of this report, 10 oil fields have been found in the hills along the crest, 2 on spurs on the flank of the atoll, 2 in the distant seaward area, and 2 on the seaward flank occupied by the central lagoon.
Calcilutite, calcarenite, and calcirudite comprise most of the reef rock, but there are a few thin beds of shale. The complex facies relations and stratigraphy of this reef are best explained as effects of oscillation of sea level during the time of reef growth. The cementation of the reef and subsequent development of porosity can be explained by the same oscillations of sea level, plus leaching by ground water during the periods of emergence. Reef growth was terminated in early Wolfcamp time when large volumes of fine-grained terrigenous sediments were deposited in the area.
The oil probably accumulated during Wolfcamp time shortly after the atoll was covered by black shale. Some peculiarities in the observed distribution of oil in the atoll may be explained by the apparent stratification of porosity.
