Coalinga Oil Field - Geology and Operations

Geology and Operations

The Coalinga Oil Field is the northernmost of a series of oil fields along anticlines extending along the western margin of the San Joaquin Valley, anticlines which parallel the San Andreas fault and have their origin in compression from associated tectonic processes. Other anticlinal oil fields in the same series include the Lost Hills, South Belridge, Kettleman Hills, and Cymric fields. The southernmost, and largest in the series, is the Midway-Sunset Field in the southwestern corner of the valley.

The eastern part of the Coalinga field is a southeast-plunging anticline. Most of the oil in the Coalinga field comes from a large-scale geologic formation known as the Kregenhagan-Temblor petroleum system, a body of Eocene-age shales rich in organic sediments. Oil in the field is found in both structural traps such as anticlinal folds, in which oil migrates upwards and is trapped beneath an impermeable layer, and stratigraphic traps, where oil is trapped within a rock unit due to changes within the rock itself.

Drillers have found a total of four oil pools in the Coalinga field. The first to be discovered was the "Oil City" pool of Cretaceous age, discovered in 1887 or 1888; however, it had little yield. On the east side of the field, along Anticline Ridge, the huge Temblor pool of Miocene age, at depths of 700 to 4,600 feet (1,400 m), produced an enormous amount of oil in the early 20th century, and is now mostly exhausted. The other large pool is the Etchegoin-Temblor on the west side, of Pliocene-Miocene age, and at depths of 500 to 3,500 feet (1,100 m); this area is currently active and subject to enhanced recovery methods such as steam flooding, fire flooding, and water flooding, methods developed in recent decades in order to extract previously submarginal deposits.

The Coalinga Oil Field is a mature field, and closer to exhaustion than most of the other major fields in California. Its remaining reserves, at around 58,000,000 barrels (9,200,000 m3), are less than 6 percent of its total original capacity; over 912 million barrels (145,000,000 m3) of oil have been pumped from the field since the late 19th century. In the San Joaquin Valley, only the Buena Vista Oil Field and Kettleman North Dome Oil Field are closer to exhaustion, with about one percent and one-half of one percent of their original oil remaining, respectively.

Oil from the Etchegoin-Temblor pool on the westside is heavy crude, with a specific gravity of 11-18 API, and a relatively low sulfur content of 0.75%; oil from the Temblor pools on the Eastside is more variable, ranging from heavy to medium crude, having an API index of 12 to 30. Aera Energy, LLC sends its oil from the Coalinga field to its refinery in Martinez for processing.