The Wilmington field has the largest production in net barrels of oil of any field in California. The field extends southeast from approximately the northwest corner of Wilmington to a point beneath the tidelands southeast of Long Beach, in the vicinity of Humble's Monterey Island. It consists of about 7,825 proved acres and about 6,500 acres offshore, undeveloped until July 1965, when THUMS Long Beach Company began a development program. In area, Wilmington is potentially the state's fifth largest field.

Cumulative production to January 1, 1965, was 1,013,500,000 bbls of oil and about 843 billion cu ft of natural gas. Peak production was reached in 1951. The discovery well was completed in 1932, and by the end of 1964, 3,280 wells had been drilled in the field, 2,583 of which still were considered active.

The Wilmington field is on a well-developed, highly faulted anticline which trends northwest-southeast. Five major north-south-trending faults divide the field into six structural blocks. The faults have their major displacement at or near the axis of the anticline. All of the larger faults are normal.

The basement rock in the field is Jurassic schist. It is overlain by about 4,000 ft of Miocene rock; 2,000-2,500 ft of Pliocene rock; and 1,000 ft of sand, gravel, and clay of Pleistocene and Recent ages. Unconformities are present between the middle Pico and Repetto (both Pliocene) and between the Puente (Miocene) and the basement rock.

Upward migration of hydrocarbons is controlled by impervious shale beds which cap or separate the producing sandstone beds. Horizontal migration is controlled by the major faults, which act as barriers. Oil production comes from seven major producing zones of Pliocene and Miocene ages and from the Jurassic basement. Porosity ranges from about 36 per cent in the uppermost producing zone to about 20 per cent in the deep zones. Permeability ranges from about 2,000 md in the uppermost zone to less than 100 md in the deep zones. The oil ranges from 13° to 32° API gravity. The field produces wet gas.

Wilmington is among the few huge fields with initial gas reserves of more than 1 trillion cu ft; however, it is essentially an oil field. Average gas-oil ratio for 1964 was 216 cu ft/bbl. Remaining gas reserves at the end of 1964 were estimated at 195 billion cu ft.