Abstract 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 unconformity 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 mi 2 . 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 wafer flooding, if 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 Wilming- tion) within the next 35—40 years under a pressure-main- tenance 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 zone in the eastern area.

The Los Angeles Basin is a densely populated coastal area that significantly depends on groundwater. A part of this groundwater supply is at risk from saltwater intrusion—the impetus for this study. High-resolution seismic-reflection data collected from the Los Angeles–Long Beach Harbor Complex have been combined with borehole geophysical and descriptive geological data from four nearby ~400-m-deep continuously cored wells and with borehole geophysical data from adjacent water and oil wells to characterize the Pliocene to Holocene stratigraphy of the Dominguez Gap coastal aquifer system. The new data are shown as a north-south, two- dimensional, sequence-stratigraphic model that is compared to existing lithostratigraphic models of the Los Angeles Basin in an attempt to better understand pathways of saltwater intrusion into coastal aquifers. Intrusion of saltwater into the coastal aquifer system generally is attributed to over-pumping that caused the hydraulic gradient to reverse during the mid-1920s. Local water managers have used the existing lithostratigraphic model to site closely spaced injection wells of freshwater (barrier projects) attempting to hydraulically control the saltwater intrusion. Improved understanding of the stratigraphic relationships can guide modifications to barrier design that will allow more efficient operation. Allostratigraphic nomenclature is used to define a new sequence-stratigraphic model for the area because the existing lithostratigraphic correlations that have been used to define aquifer systems are shown not to be time-correlative. The youngest sequence, the Holocene Dominguez sequence, contains the Gaspur aquifer at its base. The Gaspur aquifer is intruded with saltwater and consists of essentially flat-lying gravelly sands deposited by the ancestral Los Angeles River as broad channels that occupied a paleovalley incised into the coastal plain during the last glacio-eustatic highstand. The underlying sequences are deformed into a broad anticlinal fold that occurs parallel to, but ~2 km north of, the axis of the Pliocene Wilmington anticline. The Dominguez sequence breaches the crest of the young anticline, cuts through the upper Pleistocene Mesa and Pacific sequences, and into the middle Pleistocene Harbor sequence. Saltwater migrates along channels within the Dominguez sequence and into the underlying sequences (composed mostly of shallow marine and tidal sands, silts, and clays) that contain the classically defined Gage and Lynwood aquifers. The newly recognized Pacific Coast Highway fault cuts through the core of this young fold and is downthrown on the northern side, thereby creating accommodation space for a thick succession of middle Pleistocene sediments that constitute the Upper Wilmington sequence. North of the Pacific Coast Highway fault, the Upper Wilmington sequence contains the classic Silverado aquifer (composed of fluviodeltaic deposits); the Silverado is the primary freshwater aquifer for the West Coast and Central Los Angeles Groundwater Basins. Pore fluid and electric log analyses show the upper part of this aquifer to be saline-intruded near the crest of the young fold. This relationship implies that some saltwater is migrating into deeper aquifers from above, across the regional unconformity that marks the base of the Harbor sequence (ca. 240–270 ka). This sequence-stratigraphic model provides new insight into the potential flow paths for saltwater intrusion, and as such, should allow improved characterization of fluid flow that will aid in transport model studies and in managing groundwater resources.