California has occupied a peculiar position among the petroliferous provinces of the United States in having formations associated with the oil-bearing rocks which, from the content of microscopic organisms, obviously might have been the source of the oil. This has led to the general acceptance of the theory that of these organisms, the diatoms, which occur in greatest abundance, provided the greater part of the organic material which was altered to form the petroleum of the major oil fields of the state.

Recent work has shown that the organic shales in other petroliferous provinces do not necessarily contain recognizable fossil remains, and that organic material carried into the basins of deposition by rivers and precipitated by saline waters, is an adequate source for the petroleum of our oil fields. Decay-resistent vestiges of plant and possibly animal remains may have contributed to the supply.

The shales within the oil zones of the fields of southern California have many characteristics which suggest that they may have been the source of the oil now contained in the sandy beds with which they are interbedded. The present position of the oil in the Pliocene section and the distribution of the oil in the anticlinal structures in the Los Angeles basin point to the Pliocene sediments, which are relatively free from diatoms, as the source rocks. This hypothesis seems to fit the observed conditions in southern California fields better than the diatom theory.

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