Origin of Petroleum—A Review

Few problems in petroleum technology have aroused more vigorous and controversial discussion than has the origin of petroleum. The complex chemical composition of oil; its fluid, hence migratory nature; and the multiplicity of geologic variables involved present a formidable challenge to the investigator. Many and conflicting theories have been developed over the past century. In this paper, the more significant contributions are reviewed and evaluated in the light of present-day knowledge. There is now no reasonable doubt concerning the organic origin of oil. As a whole, the evidence indicates that oil is formed slowly, in a marine or brackish-water environment, from both marine and non-marine organic matter. The latter source material may be of much greater importance than once thought. There is no convincing evidence for the biochemical synthesis of oil as such, although there is undoubtedly some direct contribution of hydrocarbon components by plants and animals. Significant quantities of solid paraffin hydrocarbons were found in extracts of recent marine sediments by Trask and Wu in 1930. Complex mixtures of hydrocarbons were found by Whitmore and Oakwood to be a natural product of both marine and non-marine plants and animals. More recently, P. V. Smith, using chromatography, separated organic extracts of recent marine sediments into liquid saturated hydrocarbons, aromatic hydrocarbons, and a fraction containing oxygen, nitrogen, and/or sulphur derivatives of hydrocarbons. Similar studies by the present writer and co-workers show that such mixtures are common in the organic matrix of a wide variety of soils and sediments, although they do not, in general, have the composition of those comprising petroleum. The mechanisms involved in the transformation of this organic debris to petroleum have not been definitely established, nor is the time required known, since the various steps in the process from the initial accumulation of organic matter in sediments to the first appearance of liquid oil have not been traced. Modern techniques of chromatography, infrared spectrometry, and high mass spectrometry are providing new approaches to the problem.