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.
