Kettleman North Dome Oil Field - Geology

Geology

The oil field is one of a long line of similar long, narrow, anticlinal fields paralleling the San Andreas Fault, where tectonic forces squeezed the rock formations into anticlinal structures, trapping large quantities of petroleum. To the northwest is the large Coalinga Oil Field; to the southeast are the Lost Hills Oil Field, Cymric, McKittrick, North Belridge, South Belridge, Elk Hills, Buena Vista, and finally the largest of all, the enormous Midway-Sunset field in the southwestern corner of the San Joaquin Valley. The total productive area of the Kettleman Hills North Dome is 13,700 acres (55 km2).

Within the Kettleman Hills, oil is found in the large structural trap represented by the anticline. The large McLure Formation, of upper Miocene age, forms an impermeable cap beneath which enormous quantities of oil have pooled over millions of years, principally in the Temblor Formation, which is of Middle and Lower Miocene age. Within the Temblor Formation, five different zones are identified in cross section, with a total thickness of petroleum-bearing rock approaching 2,000 feet (610 m). Underneath the Temblor is yet another series of impermeable and permeable strata, like layers on a cake: the Vaqueros Sandstone and Kreyenhagen Formation are impermeable units beneath the Temblor, and underneath them yet another large pool of oil is found in the Upper McAdams Formation, of Eocene age, at a depth of around 10,000 feet (3,000 m). This formation was not discovered until 1940. A few other smaller oil pools were discovered later, such as one in the Kreyenhagen in 1957, and the Whepley, a small additional pool at great depth in the Temblor Formation discovered in 1976. While the Kreyenhagen is normally an impermeable shale body, where it is highly fractured it is a productive unit in its own right, as oil then collects in the fractures.

Removal of oil from the large Kettleman Hills fields, mainly the North Dome, has been suggested as a cause of the 1985 M6.1 earthquake, since the total deformation caused by the quake was exactly that necessary to compensate for the oil removed historically, i.e. by filling the vacated space. The same mechanism has been suggested for the 1983 Coalinga earthquake and the 1987 Whittier Narrows earthquake, both of which occurred near the centers of mature oil fields.