San Ardo Oil Field - Geology

Geology

The San Ardo Oil Field is the farthest north of the major oil fields west of the San Andreas Fault in California; most of the other large fields are east of the fault. As is common with California oil fields, the San Ardo Field is an anticlinal structure. The productive units are the Aurignac Sands, which are a portion of the huge Monterey Formation, a sedimentary rock unit which underlies much of coastal California. These sands are an average of about 700 feet (210 m) thick, contain an abundance of heavy crude oil, and overlie granodioritic basement rocks. Above the Aurignac sands are the thinner but also productive Lombardi sands, which can be found at about 1,800 feet (550 m) below ground surface. The granodioritic basement is generally around 2,500 feet (760 m) below ground surface.

Rock formations above the highest productive unit include the Paso Robles, Pancho Rita, and Santa Margarita Formations, of Pleistocene, Pliocene, and Miocene age, respectively. All of the productive units are of Miocene age, and the underlying basement rocks date to the Jurassic period. The Los Lobos thrust fault complex demarcates the western boundary of the field.