The Cymric oil field lies in the West Side area of the San Joaquin Valley, which includes some of the most important oil fields of the state. It is noteworthy because of the large number of producing zones and because in its geologic history and varied structural features it epitomizes the post-Eocene history of the whole area. Although discovered in 1916, oil was produced entirely from two shallow Pleistocene sand zones until 1945, when deeper drilling discovered oil in Miocene and Oligocene sands. Development of these discoveries led to further discovery of oil in Eocene and Pliocene sands, and eleven separate producing zones are now recognized.

Formations of Miocene age and older show an anticlinal structure strongly faulted on the northeast flank. Oil accumulation in these formations has been influenced by faulting, anticlinal closure, and stratigraphic traps formed by pinch-out of sands and buttressing at unconformities. Pliocene and younger strata lie on a deeply eroded Miocene surface and their structure is monoclinal with little if any faulting. Oil accumulation in these younger strata was controlled by sand pinch-outs and probably also by permeability changes.