The Wilmington oil field is near the southwestern margin of the Los Angeles basin of southern California, one of the most prolific oil-producing basins of the world and considered to be an example of optimum conditions in the habitat of oil. The Wilmington structure, discovered in 1936, is a broad, asymmetric anticline broken by a series of transverse normal faults which divided the producing reservoirs into many separate pools. The seven major producing zones range in age from late Miocene (Puente) to early Pliocene (Repetto). Production is primarily from sandstone beds of varied thickness and character, but some production is obtained from the basement schist and overlying conglomerate beds. Approximately 1,800-2,000 ft of nearly horizontal beds on top of the unconformi y between the lower Pliocene Repetto Formation and the upper Pliocene middle Pico Formation conceals the Wilmington anticline from the surface. The effectiveness of the faults as barriers to communication between fault blocks is shown by significant variations in edgewater conditions, subsurface pressure, gas-oil ratio, and oil gravity from one fault block to another. Generally the development program in the field has been based primarily on segregation of the pools by fault blocks and zones.

The problem of land subsidence in the Wilmington oil field has been attributed by many investigators to the reduction of pressures in the reservoirs as a result of the withdrawal of oil and gas. Total subsidence to date in the center of the bowl of subsidence is 29 ft. A massive water-injection program has increased oil recovery and reduced subsidence at the center of the bowl from an annual rate of 2.37 ft in 1951 to 0.1 ft in 1967. The area of subsidence has been reduced from 20 to less than 4 mi².

By April 1, 1968, the Wilmington oil field had produced more than 1.156 billion bbl of oil, primarily from the old developed area. With water flooding, it is estimated that 500-700 million bbl will be recovered from the old area of the field. An estimated 1.0-1.2 billion bbl will be produced from the new area on the east (known as the Long Beach Unit or East Wilmingtion) within the next 35-40 years under a pressure-maintenance program. Recent developments in the eastern area revealed horizontal lithologic changes in the strata. To date, six of the seven known productive zones in the old area are also productive in the new area, but are somewhat limited in extent. Recent discovery of production from sandstones and fractured shale above the basement schist could add another commercial z ne in the eastern area.