Producible petroleum represents only a very small proportion of the source material preserved in the earth. Most of this carbonaceous matter remains in the rocks as a dark-colored, chemically complex, insoluble solid.

The marine environment seems to have been the most favorable for the genesis of accumulatable crude oil, and this apparent association may reflect favorable chemical composition of the source material, a favored path of diagenesis, or perhaps merely a higher incidence in the proximity of suitable traps.

Most investigators believe that plant and animal detritus is the ultimate source material for crude oils and associated fossil organic matter. The chemical composition of the tissues and metabolic products of marine organisms is not proving to be as different from that of terrestrial organisms as has been assumed on occasion in the past.

Present evidence indicates that a long and complex chemistry is involved in the conversion of the constituents of the original plant and animal detritus into crude oils or other fossil organic matter. The conversions, however, proceed along definite reaction pathways which are governed by the subsurface environment.

Data are rapidly accumulating concerning the chemical composition and variability of the source material as seen in recent aquatic sediments, the composition of the fossil organics, including crude oil, and the reactions which lead from one to the other in the earth. Ultimately, the geochemist hopes not only to be able to describe how crude oil forms in a given sedimentary environment but also why it has its particular composition.