The petroleum-producing strata of California are found for the most part in the Tertiary system. Only a comparatively small production of oil has been obtained from the Cretaceous and upper Jurassic. Older rocks contiguous to the oil-bearing strata are either igneous or a complex of igneous and metamorphic sedimentary rocks.

The natural history of petroleum from its mother organic substance is briefly discussed in this paper under the generally accepted classes of (1) biochemical, (2) geochemical, and (3) geophysical, processes or forces, according to the general consensus of literature on the origin of petroleum.

The remains of micro-organisms from which the principal mother substances of petroleum in California were derived are widely distributed through the silt and clay sediments. They are in two general classes, (1) animal, foraminifers, and (2) plant, diatoms. The fossil remains of the latter are massed in enormous volume in the middle and upper Miocene and lower Pliocene and locally so in the top of the Cretaceous and in the Oligocene. The foraminifers, though more widely distributed, are locally massed in the upper Eocene and Oligocene.

It is pointed out that these two classes of organic mother substances appear to give rise to oils of two divergent bases and gravities under similar structural conditions. The oil derived from the foraminiferal animal life in prevailing quantity produces a naphthene oil containing little or no tar with more or less wax or paraffine. The oils having their origin in a prevailing abundance of diatomaceous or plant life produce a naphthene oil having a tar base and generally free from wax or paraffine.

A comparative study of the oil fields, therefore, in the light of the oil source materials, permits their classification and orderly discussion under the two groups indicated.

The influence of the numerous and varied types of structure in the California fields on the character and gravity of oils is briefly discussed under the four general classes as follows: (1) exposed monoclinal oil pools in the process of depletion by natural wastage; (2) broken structures due to faulting or shearing being depleted by a leakage or drainage; (3) closed structures having porous or incomplete cap-rock seals which permit some wastage of gas and lighter components of petroleum; and (4) closed anticlinal structures with practically complete cap-rock seals which permit little or no waste. These structures and their varied combinations are pointed out in the discussion and summary of California fields.