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.
Figures & Tables
Structure of Typical American Oil Fields: A Symposium of the Relation of Oil Accumulation to Structure
Modern petroleum geology in the United States had its beginning in the first decade of the 20th Century when the U.S. Geological Survey began mapping the structure of the rocks in and near old fields in order to discover the various types of structural conditions under which oil and gas are trapped. Structural geology has evolved as a branch of the broader science far more rapidly than have methods of mapping the attitude of rocks at the surface. This volume, published in the late 1920s, was designed to afford authoritative and modern descriptions of the structure of typical oil fields in the United States. Each of the 39 fields contained here is described by an author who is intimately familiar with the available data. The relationship of structure at the surface and at depth for different terranes is clearly set forth wherever the strata are not parallel. The volume concludes with a summary paper on the role of geologic structure in the accumulation of petroleum. Fields include: Florence, Colorado; Stephens, Arkansas; Kevin-Sunburst, Montana; Bradford Pennsylvania; and Salt Creek, Wyoming.