The Wilmington oil field in the south-central Los Angeles basin has produced oil from fractured basement rocks since 1945. Oil was discovered in a fractured upper or middle Miocene shale and basement reservoir by THUMS in the Long Beach Unit in March 1968. Oil production is from a southeasterly thickening prism of fractured, black, micaceous shale, siltstone, cherty shale, and marly limestone up to 1,385 ft (421 m) thick, and from fractured Franciscan schist. The black shale is correlated with the Palos Verdes peninsula Altamira Shale Member of early Mohnian and Luisian age. No definite unconformity has been recognized in the black shale member in contrast to the marked hiatus commonly seen between the upper and middle Miocene in the Los Angeles basin.
The absence in Long Beach Unit wells of the thick schist breccia and associated volcanic rocks cored at Seal Beach, 3 mi (5 km) northeast, indicates a northeasterly source of sediment. Correlation with similar rocks across the Newport-Inglewood fault zone suggests right-lateral displacement of up to possibly 15 mi (24 km).
The oil is believed to have originated in the shale and to have migrated into an interconnected network of vertical, high-angle, and horizontal fractures with migration occurring during several geologic episodes. The fracture system was produced by breakage of the brittle rocks along fold axes and adjacent to the larger faults.