Many Tertiary oil basins are convenient for studying the fundamentals of the oil generation process, because the full stratigraphic section from very young to mature source rock commonly is represented, and all stages of the oil generation process can be investigated, except advanced, postmature stages. A study was made of the origin of crude oils in the San Joaquin Valley. Shale cores ranging in age from upper Miocene to Upper Cretaceous were analyzed from depths of 1,000 ft to more than 15,000 ft. Crude oils were analyzed from producing reservoirs ranging in age from Plio-Pleistocene through Late Cretaceous. A gradual change in composition of the shale hydrocarbons was observed from unlike petroleum (immature) in shallow strata, to like petroleum (mature) in deep and wa mer strata (diagenesis of the shale organic matter).

Lower Miocene and upper Eocene shales were identified as the major source rocks of the San Joaquin Valley on the basis of relatively high organic content of the shales, shale hydrocarbon maturity, and the great similarity of shale and crude oil hydrocarbons.

In the San Joaquin Valley, as in the Los Angeles and Ventura basins earlier studied, the bulk of petroleum was formed at subsurface temperatures above 100°C (212°F), where the shales are sterile to bacteria. In the San Joaquin Valley, as in the other two basins, petroleum was formed by a nonbiologic chemical process which is strongly temperature-dependent.

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