METAMORPHIC HYPOTHESIS OF THE ORIGIN OF OIL

L. A. PUKHLYAKOV

(Presented by Professor A. G. Sivov)

There is no single opinion among scholars on the question of the origin of oil. Some connect it with the transformation of organic matter in special strata, which have been called oil-source rocks and are located at depths of 2–3 km, at temperatures of no more than 100–120°C [1, p. 9] (the organic hypothesis). Others believe that oil forms in magma under conditions of high temperatures and pressures (the inorganic hypothesis).

The strong side of the inorganic hypothesis is that it explains the mechanism of oil formation. Indeed, it is known that in installations for obtaining artificial liquid fuel, where separate constituent parts of oil are obtained from coal, pressures and temperatures reach very high values. In any case, they are quite close to those temperatures and pressures that prevail at depths on the order of 15–20 km and clearly exceed those observed at depths of 2–3 km.

The weak side of this hypothesis is that it cannot explain where the material for the formation of oil comes from in magma. After all, carbon and hydrogen, of which it is mainly composed, are encountered extremely rarely among magmatic rocks.

The organic hypothesis, on the contrary, has all the data needed to explain the question of where the material for the formation of oil comes from, since carbon and hydrogen are always found in large quantities in sedimentary rocks. However, the mechanism, or process, of its formation remains unclear, since under the conditions of the temperatures and pressures that prevail in the upper layers of the Earth’s crust, only methane is formed. Compounds with a large number of carbon atoms, of which oil proper consists, have not been obtained at low temperatures and pressures.

A comparison of the data considered involuntarily suggests the idea that oil forms in sedimentary rocks which, as a result of folding or isostatic subsidence, find themselves in the deep zones of the Earth. Indeed, during the formation of mountain structures, enormous masses of sedimentary rocks occur at depths of 50–70 km and more, where temperatures may reach on the order of 500–1000°C, and pressures 10–20 thousand atm. As a result of isostatic subsidence—po-

subsidence caused by the weight of accumulating sediments, the depth of burial may reach 15–20 km. Thus, in the Cordilleras the thickness of shallow-water deposits, dislocated in the process of formation of this mountain system and subsequently exposed by erosion, reaches 19 km. Approximately the same magnitude, according to seismic data, is attained by the thickness of sediments in the northern part of the Caspian Sea and in some other basins.

Together with other sedimentary rocks, at depths of 15–20 km and more there must be much dispersed organic matter and even whole beds of coal, as well as a large quantity of minerals that contain water, for example kaolin, gypsum, etc. In the process of metamorphism these minerals must be transformed into minerals that do not contain water. For example, kaolin must be transformed into feldspar, gypsum into various compounds of sulfur and calcium, etc. The water must be released as a free substance. According to V. A. Sulin [2] and other authors, the amount of such water may amount by volume to as much as 20% of the volume of the rocks undergoing metamorphism. Of course, the greater part of this water must reach the Earth’s surface and replenish its hydrosphere. But some quantity of it could have come into contact with dispersed particles and beds of coal heated to 500–1000°C. Naturally, in this process water must decompose into oxygen and hydrogen, each of which must enter into combination with coal and form, in the first case, carbon monoxide, and in the second, various hydrocarbons.

The resulting mixture of gases must continue its path upward. In this process carbon monoxide, as the more active substance, must enter into reaction with the most diverse rocks; hence its amount should decrease as it rises. As for the hydrocarbons, these substances are more inert. They react actively only with oxygen. Therefore their percentage content in the rising mixture must gradually increase.

Hydrocarbons of the petroleum series may form from carbonized organic residues (fossil coals) even without the participation of water, since any fossil coal contains not only carbon but also hydrogen. True, the amount of coal that has reacted in this case must be much smaller than under analogous conditions but with the participation of water. The participation in oil formation of various sulfur compounds and many other substances, including catalysts, is quite probable; however, to deny the role in this process of high temperatures and pressures, as many adherents of the organic hypothesis do, seems illogical to the author.

Oil formed in the deep zones of our planet must in part remain in place (in the loci of its formation), and in part must migrate along fractures into the surface zones and there accumulate in traps of various kinds, from which it is extracted at the present time.

The proposed hypothesis explains well the presence of oil fields near young folded structures or in zones of intensive isostatic subsidence, where the thickness of sediments reaches 15–20 km. But deposits of this useful mineral also occur on platforms, where the thickness of the sedimentary cover barely exceeds 2 km. This fact requires some explanation. Obviously, here the principal role belongs to the rocks of the basement. In fact, no one denies the universally known fact that the rocks of the folded basement of any platform were formed from ordinary sedimentary rocks: sands, clays, limestones, etc., among which at one time there were...

there must have been much dispersed organic matter. Gathered into folds, which were then denuded to a considerable extent, these rocks underwent metamorphism and subsequently were covered by a mantle of sedimentary rocks. The oil that arose in the process of their metamorphism must gradually— as fractures appeared in these rocks—have migrated toward the surface of the Earth. This process, apparently, is continuing at the present time as well.

If oil had formed in the same beds from which it is now produced, then its accumulations would have to be confined to every structure favorable for its accumulation, and the size of these accumulations would be the greater, the larger the structures themselves. In reality, however, structures of comparatively small size are sometimes encountered that are accompanied by accumulations, while at other times structures of large size are encountered that contain no oil accumulations. From the standpoint of the proposed hypothesis, this is explained by the fact that the formation of oil accumulations requires not only structures favorable for its accumulation, but also fractures in the basement along which it can rise from the deep zones of the Earth. In an analogous way one can also explain the fact that, in sections of different fields within the same provinces, oil accumulations are confined now to one bed or series, now to another.

LITERATURE

1. H. D. Hedberg. Geological Aspects of the Origin of Petroleum (trans. from English). “Nedra,” 1966.

2. V. A. Sulin. Hydrogeology of Oil Fields. Gostoptekhizdat, 1948.