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Petroleum, as discussed here, is that naturally occurring and usually complex mixture of dominantly hydrocarbon substances--liquid, gas, and solid--which constitutes the commercial crude oil, natural gas, and natural asphalt of the petroleum industry. The problem of its origin is complicated by its capacity to migrate and its susceptibility to change in state and composition.
It is generally accepted that petroleum is derived from the remains of organic life, but many uncertainties exist concerning the processes involved. These are in large part problems of chemistry and physics, but reasoning based on the geology of petroleum occurrences constitutes an essential guide and a critical control on all hypotheses.
Discrete occurrences of petroleum are rare. Typically, petroleum is found within other rocks--filling pore space or fractures. Most occurrences are in the pore space of sedimentary rocks, and this relation is in itself of major genetic significance. Occurrences in fractured igneous and metamorphic rocks are also common but the proximity of all such occurrences to areas of sedimentary rocks serves further to reinforce the genetic relation of petroleum to sediments. Some minor occurrences of petroleum-like hydrocarbons may be of primary igneous origin, and locally some oil has been distilled out of sediments by igneous intrusions. Also, hydrocarbons frequently have been reported from volcanic gases although the evidence is somewhat uncertain.
Probably all living organisms contain hydrocarbons and supposedly indigenous petroleum-like hydrocarbons have now been found widespread in Recent marine sediments and in soils. However, these hydrocarbon assemblages still differ significantly from those of crude oils or those extracted from older rocks and it seems evident that the organic matter of Recent sediments must undergo a further evolution before it becomes true petroleum. Gaseous and liquid hydrocarbons are no doubt present in immense aggregate quantity in surficial and subsurface waters but quantitative information on their concentrations and physical state is very inadequate.
Geographically, petroleum is common and widespread throughout the world. However, conditions for combined large-scale genesis, preservation, and accumulation have been very selective so that large accumulations are known from only a very tiny part of the total world area. With respect to geologic age, petroleum in at least some quantity appears to be indigenous to rocks of Precambrian to Pleistocene age, inclusive. Petroleum has been found to depths of nearly 25,000 feet and at temperatures as high as about 170°C. With respect to tectonic environment, the earth's mobile belts appear to have been particularly favorable to petroleum generation and accumulation. Hydrocarbon gas appears to make up much of the atmosphere of several of the planets of the solar system.
A favorable sedimentational setting for large-scale petroleum genesis appears to require the abundant production of organic matter of the right kind, and conditions under which this source material and the resulting petroleum may be preserved. Such conditions appear to include a reducing environment, the absence of destructive organisms, active deposition of fine-grained sediment, and readily available trap reservoirs. Aquatic plant and animal life offers the most promising source material, although this may have been substantially supplemented by land-derived colloidal, dissolved, or clay-complexed organic matter. It appears that petroleum can not be merely a concentration and accumulation of products generated in living organisms, but requires the contribution of secondarily develop d hydrocarbons.
Restricted depositional basins have been a favorable element in petroleum genesis because they are commonly conducive to reducing conditions and because the evaporite deposits which they commonly generate provide excellent seals for petroleum accumulations. Marine conditions appear to have produced the bulk of the world's oil but much petroleum was also formed under non-marine conditions.
The role of microorganisms in petroleum generation has long been the subject of much speculation but thus far there is little conclusive direct evidence for the microbial origin of any petroleum hydrocarbon except methane. The importance of the action of certain natural catalysts has been emphasized by many. Radioactivity has frequently been considered as an agency in petroleum origin but while its effects may have been important there is little evidence that it directly produced much petroleum. Hypotheses involving the solution (or colloidal solution) of petroleum hydrocarbons in water are geologically attractive because they provide a means for movement of petroleum from fine-grained source rocks to reservoirs and also allow for long continuing contribution of petroleum from such so rces, pari passu with the common and universal process of compaction. However, adequate observational support for the quantitatively effective operation of solution mechanisms has not yet been satisfactorily demonstrated.
In spite of good reasons for believing that most petroleums have originated at temperatures well below 100°C., and that relatively few petroleums have ever been exposed to temperatures of more than 120°C., the effect of increasing temperature with increasing depth of burial is invoked by many as an essential
factor in petroleum generation. This finds support in some recent laboratory experiments which have produced light saturated hydrocarbons at an appreciable rate by heating shales to no more than 185°C. Geologic time may have allowed nature to attain the same results at even more moderate temperatures.
Changes in petroleums with respect to optical activity, density, paraffinicity, carbon isotope composition, and other properties, have been attributed to increasing geologic age, but the evidence is frequently inconsistent. Many of these changes may rather be related to variations in the catalytic activity of associated rocks and minerals.
The hypothesis that petroleum accumulations generally originate in situ in the reservoir traps in which they are now found has been examined but appears very improbable.
The element of timing is critical with respect to hypotheses of petroleum genesis and accumulation. It appears that such hypotheses must provide not only for early origin and migration from source rocks, but also to some degree for late origin or at least late migration. Specific instances of postulated late origin or late primary migration are cited.
Variations in the composition and properties of petroleums with depth or geographic position may provide helpful clues to their mode of origin. Many of these variations are no doubt due to secondary alterations; others appear to be the result of original local variations in source material or genetic processes or both. The distribution of petroleum gas versus crude oil may be particularly significant in this respect.
A number of dominantly chemical methods for the identification of commercially important petroleum source beds have been proposed and are being applied to some extent at the present time. Their effectiveness, however, is rather questionable, particularly with respect to determining the quantitative richness of a source. Perhaps the determination of source-water is as pertinent as the determination of source-rock.
Purely geological evidence may be very useful as a means of narrowing down petroleum source possibilities. Several specific cases are cited where geological reasoning has been or can be invoked to advantage. The desirability of close coordination of analytical chemical attack with geological data and geological reasoning is emphasized and suggestions for such research investigations are made.
In conclusion, Cox's "geological fence" is reviewed and revised in the light of present information.
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