FIELD STUDY OF THE SUSAN PEAK OIL FIELD, EASTERN SHELF OF THE PERMIAN BASIN, TOM GREEN COUNTY, TEXAS

The Susan Peak Oil Field, approximately 20 miles southeast of San Angelo, Texas, lies in the Eastern Shelf of the Permian basin within the north-trending Fort Chadbourne fault zone. From 1948 through 1982 the field produced 16 million barrels of oil from three major reservoirs of Pennsylvanian age: Lower Strawn Group limestone, Upper Strawn Group limestone, and Canyon Group calcareous sandstone (Holmberg, 1987). The field was discovered by mapping a northeast-trending anticline in surface exposures of Cretaceous Edwards Group limestone (Graham, 1953). Cretaceous, Permian, and Pennsylvanian horizons were correlated in electric logs from 62 wells in and near the field. Cross-sections and structure maps confirm the geometry of a northeast-trending doubly plunging anticline described by Graham (1953) and Holmberg (1987), and add new details that indicate timing and origin of the structure. First, Pennsylvanian Canyon sandstone beds thin and pinch-out on the crest of the anticline, documenting that the structure was active in the late Pennsylvanian. Second, a pre-Cretaceous Edwards Group unconformity indicates uplift after Early Permian and before the early Cretaceous. Third, the unconformity at the base of the Cretaceous section is broadly folded, which indicates a third deformation phase no older than Cretaceous. This third phase could be a northwest-striking Laramide folding event that produced double plunges in northeast-trending Paleozoic structures. Alternatively, the third phase may result from reactivation of the strike-slip Fort Chadbourne fault zone, producing en echelon anticlines above this basement fault. The Bronte, Otto, and O’Harrow Fields also along the Fort Chadbourne fault zone, contain similar structures.