Importance of Unconformities to Oil Production in the San Joaquin Valley, California
Discoveries of new fields have occurred intermittently throughout the 35–40 years that prospecting has been in progress in the southern end of the San Joaquin Valley, California. The early operators were attracted by the seepages, and oil discoveries were made adjacent to them. By 1910, the value of isolated anticlines as prospective oil fields was recognized. Discoveries of new fields and extensions of old fields have continued to the present day due to the gradual improvement of drilling equipment and technique, permitting progressively deeper prospecting, and due to the more or less accidental discoveries in areas lacking favorable surface indications.
The discussion of the stratigraphic and structural history of the region is necessarily brief. To deal fully with the many complexities of the area and to attempt to review thoroughly the changes in stratigraphic nomenclature during the past 20 years are beyond the scope of this paper. The writers have attempted to avoid controversial matters that do not affect the understanding of the general relation of oil accumulation to the presence of unconformities.
The major producing areas are discussed for the purpose of pointing out the dominating influence of unconformities, even in the presence of anticlinal or other favorable structure, on the accumulation of, or the availability of, the oil of the region.