Origins of Petroleums; Chemical and Geochemical Aspects

Abstract

The survival of organic material in oil shales of great geological antiquity is evidence against the assumption that petroleums have been formed by the heat decomposition of organic material. Another evidence of the low-temperature history of oil shale is the discovery of Treibs that many shales, asphalts, and asphaltic petroleums contain chlorophyll porphyrins. The fact that many petroleums contain complex labile substances easily decomposed by heat, may indicate that cellulose, peats, and coals are progressively carbonized under pressure and during geologic time at temperatures much lower than has heretofore been assumed. The carbon-ratio theory has only a negative value in relation to petroleum occurrence. Some bottom-hole temperatures in producing fields do not exceed 100°F., and are in strata only slightly arched or otherwise disturbed. The conclusion seems warranted that organic materials as stable as the paraffins and some waxes, once formed and sealed in the sedimentary rocks, normally undergo no further changes due to the influence of heat. The principal types of organic substances which are to be considered as the main source material for petroleum include cellulose, starches, and sugars, proteins, lignins, oleo-resins, waxes, and fatty oils. Fatty oils are to be regarded as the principal source material. Putrefactive decomposition of organic matter is recognized as an initial step in the formation of petroleum. More experimental evidence is needed on the role of bacteria in the formation of petroleum. At present the depth at which bacterial action ceases is not known. The low-temperature history of petroleum favors the view that the hardening of sedimentary rocks is due entirely to compaction and the deposition of cementing material from solution.