The subsurface Horseshoe atoll is an arcuate accumulation of fossiliferous limestone 70 to 90 miles across in the northern part of the Midland basin, in western Texas. The stratigraphy, the lithologic character of the rocks, and the petroleum reservoirs of the southeastern part of the atoll in Scurry County and parts of adjacent counties are described herein.

Rocks of Strawn, Canyon, and Cisco ages, belonging to the Pennsylvanian system, and rocks of Wolfcamp age, belonging to the Permian system, form the Horseshoe atoll. Reworking of these rocks at several times during the growth of the atoll has resulted in the formation of large amounts of limestone breccia, in the mixing of the fusulinid faunas used to date the rocks, and in the complex age relations between rocks in different parts of the atoll. Thin beds of shale within this limestone mass are the only lithologic units that can be correlated for any appreciable distance in the atoll, but studies of micrologs from wells penetrating the atoll have revealed many zones of low porosity that can be correlated with reasonable certainty over much of the area described in this report.

The atoll rests on a platform of bedded limestone and shale, which has been designated as equivalent in age to the Bend and Strawn groups of the Pennsylvanian system. It is covered mainly by shale, which has been designated as equivalent in age to the Wolfcamp series of the Permian system and is partly equivalent in age to the youngest rocks within the atoll.

Reworking of the rocks, the complex distribution of rocks of different ages, the presence of thin beds of shale, and the stratification of porosity in the atoll suggest that this structure has many of the characteristics of a reef. Its growth in the Midland basin was apparently cyclic and may have been related to changes of sea level during the Late Pennsylvanian and early Permian periods.

Oil is contained in porous zones within the atoll, mainly in reservoirs in hills along the crest of the structure where the overlying shale formed an impervious cap, but some is found in the lower part of the limestone mass where the reasons for the oil traps are not as apparent. The source of the oil was probably the shale of Wolfcamp age that surrounds the atoll.