Abstract The upper Miocene Stevens Sandstone is a prolific oil producer in the San Joaquin Basin of California. Stevens production is mainly from deep water sandstones which were most commonly deposited by turbidite flows. Although the Stevens has produced for forty years, a resurgence of activity by Tenneco, Gulf, Texaco, and Arco, as well as many independents, has greatly increased the reserves in the Stevens in recent years. Production from the Stevens interval is primarily from turbidite sandstones deposited as part of submarine fan complexes in fan channels and fan lobes and from sands deposited in topographically lower areas on the sea floor. Fractured siliceous shales of the Stevens interval also contribute to production. These shales, also of deep water origin, are laterally-equivalent or slightly younger than the Stevens sandstones. These shales were deposited on the fringes of the fan, on the basin plain, or as drapes on bathymetrically-high areas of the sea floor. Along the eastern margin of the basin where deposition occurred on a relatively-undeformed homoclinal surface, patterns of turbidite sedimentation and facies associations generally conform to the Mutti and Ricci Lucchi or other submarine fan models. However, in the central and western portions of the basin, fan models seem to be inappropriate. Observed relationships between Facies Associations, sandbody geometries and submarine fan subenvironments often appear anomalous when facies interpreted from cores are compared with relationships described by some currently popular fan models. Such anomalous relationships were observed in cores from several fields producing from the Stevens Sandstone. To explain these inconsistencies, an “on-lap” model and a “confinement” model are proposed for some of the observed depositional patterns of the Upper Miocene Stevens Sandstones in the San Joaquin Basin. Cores from Paloma, North Coles Levee, Rio Viejo and Tule Elk Fields demonstrate the generally thin bedded nature of Stevens tur-bidites deposited in the western portion of the basin. Fining and thinning upward cycles, as well as coarsening and thickening upward cycles, are observed in the cores. Upward variation in the frequency of interbedded shales within the overall sandstone cycles is demonstrated to be the major cuase of apparent “fining” or “coarsening” upward as observed on logs. Complete and incomplete Bouma sequences and relatively thin massive-appearing to graded sandstones are observed in the cores. Amalgamation of sandstones is common. At Tule Elk Field a significant thickness of trough-cross-bedded sandstones show the effect of deep water traction type currents, a phenomena that has rarely been documented. Superimposed on the facies analyses are the effects of basin bottom topography. An “on-lap” model is defined to describe turbi-dite deposits which lap onto and stack vertically against contemporaneously rising anticlinal structures. Internally these sand-bodies exhibit distinct sedimentation cycles and facies associations characteristic of fan progradation. Externally these sandbodies pinch out crestward, may or may not be lobate- or fan-shaped, and tend to be abnormally thick. The Paloma Field is an example of sediments that fit the “on-lap” model. A “confinement” model is defined to describe deposits of turbidity flows which are confined to bathymetric lows between adjacent (en echelon) anticlines. These deposits, which accumulated in synclinal lows, tend to have an external channel-like morphology but do not necessarily exhibit facies associations commonly ascribed to channels in fan models. Deep-water sediments from Yowlumne Field, Tule Elk Field, and some of the production Elk Hills Field are best explained by the “confinement” model.

Abstract More than 6 ½ billion barrels of oil have been discovered in the San Joaquin Valley. Approximately 4 ¼ billion barrels have been produced and there is a reserve of 2 ¼ . billion barrels. San Joaquin Valley oil fields account for 42 per cent of the annual production, and contain 45 per cent of the reserves of the state. Ninety per cent of the oil produced to date has come from rocks which are Miocene or younger in age. Nonmarine sediments, largely of Miocene or later age, have accounted for 1 ½ billion barrels of oil, or 35 per cent of the total production of the valley. Oligocene and Eocene reservoirs are assuming importance as deeper drilling is done and new areas are explored. The pre-Tertiary sediments have accounted for less than one per cent of the oil produced. However, Cretaceous rocks are now being actively prospected. The San Joaquin Valley is a synclinorium some 250 miles long and 50–60 miles wide, lying between the Sierra Nevada Mountains to the east and the Coast Ranges to the west. The maximum depth to the basement is estimated to be in excess of 30,000 feet with the thickest sedimentary section being on the western side of the valley. Oil production is confined mainly to the southern portion which contains a thick section of organic Tertiary sediments. The San Joaquin Valley does not lend itself well to the basin analysis suggested by authors. The geosynclinal trough is present, as is the Sierra Nevada foreland to the east. Numerous faults of minor displacement along the east side of the valley might be termed a "hingebelt" but they interrupt the regional dip in only a minor way. The San Andreas and other faults of large lateral displacement have probably altered or removed the "geanticlinal welt," if it ever existed, on the west side of the valley. Furthermore, the western boundary of the valley has been transgressed many times by Tertiary seas making it difficult to determine the exact outline of the San Joaquin "basin." In the San Joaquin Valley, oil is found in almost every type of trap. Anticlinal structures with complications either by faulting or stratigraphic changes have accounted for the major volume of production. Examples of this type of accumulation are fields on the Coalinga-Kettleman Hills anticline, the Elk Hills-Coles Levee anticline, the Rio Bravo-Greeley trend, the Wheeler Ridge-Tejon Ranch anticline and the Belridge anticline. Oil occurs in faulted homoclines on the east side of the valley at Round Mountain, Kern Front, Fruitvale, Mount Poso, and Mountain View. Sands open to outcrop but with low fluid level produce at East Coalinga. The Lakeview area of the Midway-Sunset field appears to be synclinal. Although sand is the reservoir rock for 95 per cent or more of the fields, significant accumulations also occur in fractured shales at Elk Hills and Buena Vista Hills and in fractured schist basement at Edison. Inclined water tables have been noted in the Coalinga Nose, North Dome of Kettleman Hills, Paloma, Coles Levee, Elk Hills, and other fields. Advocates of the hydrodynamic theory are opposed by those who consider such inclined water surfaces due to differences in permeability. In some cases water encroachment is opposed to direction of inclination. In the future, stratigraphic and fault traps should account for a larger percentage of oil discovered in the San Joaquin Valley than in the past. Obscure structural highs revealed by subsurface geology, detailed geophysics, or other methods will undoubtedly contribute future discoveries. Recently, extensions and deeper drilling of known structures have accounted for substantial amounts of new oil and should continue to do so in the future.