The present status of chemical, geological, and biological information on the origin of petroleum strongly favors the theory of organic (or biogenic origin over that of inorganic origin. There is no general agreement, however, on the mechanism or mechanisms by which biologic products become petroleum. Among the various hypotheses offered to explain this conversion, two have received most attention and occupy the forefront of present-day theories of petroleum origin. One, which may be designated as the "Direct-Accumulation" hypothesis, contends that petroleum consists of a selective accumulation of relatively unchanged organic compounds produced by living organisms. Opposed to this explanation are the so-called "Transformationists" who believe that biologic source material must undergo marked chemical transformations, under the influences of temperature, pressure, and lithologic environment before they are converted into petroleum. A critical review of available chemical and geologic information on petroleum and other organic materials in ancient and modern sediments suggests that chemical transformations play an important role in petroleum formation.

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