Geologic Environment of Cuyama Valley Oil Fields, California1
Cuyama Valley, in the southeastern part of the Salinas-Cuyama Tertiary basin of the southern Coast Ranges of California, contains the major Russell Ranch and South Cuyama oil fields, which have produced respectively over 40 and 67 million barrels of 350 A.P.I.-gravity oil from unitized and pressure-maintained reservoirs since mid-1948 and mid-1949. Production from the fault-trapped reservoirs is derived respectively from 290 and 480 feet of Vaqueros (lower Miocene) sands at depths ranging from 2,800 to 4,300 feet; productive areas comprise 1,400 and 2,500 acres. In addition, there are three minor fields in Cuyama Valley.
San Ardo, the only other major field, is located in the northwestern portion of the Salinas-Cuyama basin. This field produces 11.30 gravity oil from upper Miocene sands at depths of 1,800 to 2,400 feet, from an area of about 4,000 acres. Cumulative production to July 1, 1956 has been 50 million barrels.
The Salinas-Cuyama basin, which is approximately 160 miles long and 28 miles wide, is bounded on the northeast and southwest by the San Andreas and Nacimiento fault zones, respectively. It is underlain by granitic basement and contains up to 7,000 feet of lower Miocene marine clastics. In the Cuyama portion of the Miocene basin a thick continental section grades southwestward into a very thick shallow-water marine section composed principally of deltaic sands. These, in turn, change rapidly southwestward to a thin section of marine organic shales and sands of the oil-producing, moderately shallow-water belt, which thins rapidly southwestward. These sharp thickness and facies changes are accentuated by reverse faults of probable Quaternary age. Locally these faults have a prominent strike-slip component of displacement and conceal at least one important, old and steep lateral fault which further accentuated the stratigraphic changes.
The original Salinas-Cuyama Miocene basin has been elongated many miles by cumulative movements on northwest-trending right lateral-slip faults which now juxtapose formerly widely separated sedimentary facies. In Cuyama Valley, one of these is the buried Russell fault which traps most of the oil, and another probable one, aided by reverse faulting, explains the close proximity of thick deltaic and thin basin facies.