Abstract

Sands of the upper Miocene turbidites account for most of the stratigraphic oil in the southern San Joaquin Valley. Updip, time-equivalent, shallow marine rocks are second in productivity. Third in importance are the deep marine, fractured, siliceous shales deposited beyond distal margins of the fan complex.

Much underexplored area still remains in the San Joaquin Valey despite field development and wildcat drilling spanning a period of 75 years. For this reason, a subsurface study was instituted in anticipation of developing updated concepts of trapping mechanics in turbidite environments that would be useful in evaluating areas of future potential. The Miocene of the Bakersfield arch was selected for a pilot study primarily because it is representative of the classic arcuate type of turbidite deposit that is characteristic of modern submarine fans. The paleogradient of the fans has been accentuated, and not reversed, through geologic time. Stratigraphic oil fields are numerous, and there is dense well control with available electric logs.

Historically, subsurface studies of ancient turbidites in California have been limited to the exploratory potential of facies changes related to the tectonically mobile basin-margins representing only the lateral margin facies of the fan complexes. In most California deep marine troughs, this is the facies most easily reached by the drill and in which most of the oil reserves have been developed. Traps for hydrocarbon accumulation in this environment are related to stratigraphic changes attributed to basin-margin wedging.

In addition to basin-margin wedging and fractured siliceous shale, this study recognizes two additional trapping aspects related primarily to depositional stratigraphy within the midfan facies. They are: 1) Intrabasin contemporaneous faults (analogous to Gulf Coast growth faults) that result in expanded section and reverse drag on the downthrown block; and 2) compaction anticlines caused by post-sedimentary accomodation over submarine channel sands that result in structural inversion with depth.

An extension of this study was the recognition of an earlier (pre-upper Mohnian) turbidite basin, virtually unexplored, south of the Bakersfield arch. Referred to informally as the “Maricopa sub-basin”, depths to potential Miocene pays range from 12,000 to 20,000 feet. Basic stratigraphic relationships recognized in the Bakersfield arch may be applied here to develop new hydrocarbon reserves.