The Wilmington oil field is in 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, asymmetrical anticline broken by a series of transverse normal faults which divide the producing beds into many separate pools. The seven major producing zones range in age from middle Miocene (Topanga) to early Pliocene (Repetto). Deposition of approximately 1,800 to 2,000 ft of nearly horizontal beds on top of the unconformity between the lower Pliocene Repetto beds and the upper Pliocene middle Pico Formation conceals the Wilmington anticline from the surface. The effectiveness of the faults as barriers to oil and gas accumulation in the field is shown by signifuant variations in edgewater conditions, subsurface pressure, gas-oil ratio, and oil gravity from one fault block io another. In general, the development profjram 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 due to the production 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 reduced the subsidence in the area and increased oil recovery. The rate of subsidence at the center of the bowl has been reduced from an annual rate of 2.4 ft in 1951 to 0.1 ft in 1967. The area of subsidence has been reduced from 20 sq mi to less than 4 sq mi.
The Wilmington oil field has produced more than 1.1 billion bbl of oil, primarily from the old developed area. With waterflooding, it is estimated that another 700 million bbl of oil will be recovered from the old area of the field and an estimated 1.2 billion bbl of oil will be produced from the new area on the east (known as the Long Beach Unit or East Wilmington) within the next 35–40 yr under a pressure-maintenance program. Recent developments in the eastern area of the field revealed some lateral lithologic changes in the formations. To date, six of the seven known productive zones in the old area are also productive in the new area, but somewhat limited in extent.