Landes et al (1960) and Hubbert & Willis (1955) state that oil has been produced from fractured basement metamorphic rocks in five fields of the Pacific Coast province, California. These are Edison and Mountain View in the San Joaquin Valley, and Wilmington, El Segundo, and Playa del Rey in the Los Angeles Basin. Few wells produce oil from the basement rocks alone. Most are multiple completions in the basement schist and the overlying schist conglomerate.

McNaughton (1953) states that the first commercial oil production from basement metamorphic rocks in California was probably in Placerita Canyon near Newhall. According to Brown & Kew (1932, after McNaughton, 1953), small quantities of light-gravity oil were produced from 5 wells drilled between 1899 and 1901. The basement complex in the vicinity of the wells consisted mainly of schists.

The next discovery of oil in the basement complex was made in the Playa del Rey field at Venice in 1929. Available descriptions of the Playa del Rey field have been limited. Production was from a fractured schist reservoir with relatively low rates, approximately 400 bbl/day (Eggleston, 1948).

An important basement discovery was made in El Segundo field in 1937. It was accidental in that the objective of the test well was the schist conglomerate capping the basement. Examination of cores revealed that the well stopped in fractured schist containing oil. The well was highly productive and the discovery was followed by intensive development of the basement reservoir in the field.

Another basement pool was discovered in 1942 in the Santa Maria Valley field. The discovery was also accidental, a fortunate consequence of ignorance concerning the exact depth of the basement in the central part of the field (McNaughton, 1953). Similar exploration in the Edison field southeast of Bakersfield disclosed oil in fractured metamorphic rocks in 1945. Intensive development of the basement reservoir followed this discovery with 103 wells being drilled into the basement and all but 6 were completed as commercial producers. Oil production from these wells in the first 18 months after discovery amounted to 4,500,000 barrels. In 1947, the estimated reserve in the basement pool was 25 million barrels (McNaughton, 1953).

Another basement oil reservoir that was discovered accidentally was in the Wilmington field in 1945. Apart from the Santa Maria field (in which the oil is produced from sandstone), oil from basement rocks was produced from fractures in schists (Aguilera & van Poollen, 1979). Most of the oil-producing schists are in a relatively high position. They have usually undergone weathering and erosion that has increased porosity.

Located in the southeastern San Joaquin Valley 18 km east of Bakersfield, Kern County, California, the Edison Field was discovered in 1931. From 1931 to 1945 production from the field was exclusively from two separate sands in the Tertiary sediments (Hubbert & Willis 1955). In June 1945, one well was drilled through the entire sedimentary section, penetrating 100 ft (30 m) into the schist and encountered the basement reservoir complex. Slight traces of oil were found in the sediments but more promising showings were observed in the fractures of cores from the basement. A test was made and the well was completed in the metamorphic rocks of the basement complex with the well initially flowing oil at 528 bbl/day.

Rapid development of the basement followed. Initial daily production in other wells varied greatly. A few wells produced more than 2,000 bbl/day, most produced about 1,000 bbl/day. The height of the oil column in the schist gradually increased from the southwest to the northeast, the maximum being 1,000 ft (305 m). Wells with daily production of more than 1,000 barrels of oil were concentrated in the north-central part of the Edison field. By the end of 1955, the cumulative oil production from the fractured schist was more than 20 million barrels (Eggleston, 1948; P'An, 1992). The estimated ultimate production was around 50 million barrels (Hubbert & Willis 1955).

El Segundo Field

El Segundo oil field, which was discovered in 1935, is located along the coast of Santa Monica Bay, southwest of Los Angeles County. The field is divided into two areas by a northwest trending zone of faulting. The eastern part produces from a basal conglomerate made up of schist pebbles with the western part producing directly from fractures in the schist itself.

The first well was drilled to the east of the faulted fracture zone. The daily production of oil from the basal conglomerate was about 600 barrels. The western part of the field began producing in 1937 from the fractured schist reservoir. The production rate was 4,563 bbl/day at a depth of 7,253 ft (2,210 m). This discovery was accidental as the well was exploring for the schist conglomerate. 66 wells were drilled in the development of the El Segundo field with a wide variation in production from adjacent wells (Eggleston, 1948; Landes et al, 1960; P'An, 1982).

Wilmington Field

The Wilmington oil field is located in the city of Wilmington and includes the harbour area of Long Beach (the Los Angeles basin). The discovery well was completed in January 1932 with oil production from the Union Pacific E-47 well during May 1945. The schist reservoir was encountered at a depth of 5,787 ft (1,764 m). Production from well E-47 was initially 387 bbl/day. However, in 1946 rates of 1,200 bbl/day to 2,000 bbl/day were established from wells producing from the schist reservoir. Wilmington has produced more than 22 million barrels of oil from the basement reservoir (Cabeen & Sullwood, 1946; Landes et al, 1960; P'An, 1982).

Eight productive zones are present in the Wilmington field. Of these, seven are in Pliocene and Miocene sediments with the eighth in the fractured basement schist, known as 237 Basement (Robertson et al, 1987). The productive zone in the schist does not extend below 6,200 ft (1,890 m). The Wilmington field is the third largest oil field in the USA in terms of cumulative production, at 2.8 billion barrels (Gibson, 1997).

USA, Central Kansas Uplift