The gas-liquid-solid mixture which we know as producible petroleum represents only a small proportion of all fossil organics including coals, oil shales, natural asphalts, etc. In past ages, as today, plant and animal detritus was deposited and preserved in fine-grained sediments in environments ranging from fresh-water swamps to marine slopes and basins. Why do only certain of these environments favor the genesis of accumulable oil? What are the controlling ecological and chemical factors? By what mechanisms do the constituents of petroleum migrate into the reservoir? To what extent does fractionation occur during migration? Does chemical alteration occur during migration and after accumulation in the reservoir? Today, answers to these questions are being sought in many aboratories throughout the world.

Living organisms do not generate, as part of their life processes, many of the hydrocarbons and other chemical compounds characteristic of petroleum. Further, the components are not in thermodynamic equilibrium under earth conditions and, hence, independent of the structure of the source material. The nature of the source material derived from living organisms and its quantitative variation as a function of environment is being determined through study of Recent sediments. Changes in composition with age and depth of burial are being observed and the mechanisms of the reactions elucidated. Good progress has been made toward recognizing the sources for constituents of petroleum ranging from the hydrocarbon gases to the asphaltic residues. In several instances reactions involved in thei genesis have been duplicated.

Fractionation of the organic matter begins with deposition and continues throughout subsequent geologic time. Physical processes are interrelated with the chemical and the two must be considered together.

No satisfactory mechanism has been proposed for the migration of petroleum out of the fine-grained source rocks into the reservoir. Mechanisms involving oil as a separate phase, as a soapy colloid, or as a solution in water, all seem to fail under experimental scrutiny.

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