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Geology of the South of the Siberian Platform and the Oil Capability of the Cambrian

Considerable oil and gas exploration has been carried out in recent years in the south part of the Siberian Platform. These operations were accompanied by a large volume of geological and geophysical study and by drilling of structural and exploration wells. As a result, new data were obtained on the stratigraphy, tectonics, and oil capability of a vast area of the south of the Siberian Platform.

A group of geologists and geophysicists of the East Siberian Geological Prospecting Trust, of the All-Union Science Research Institute of Geophysical Methods of Prospecting, and the Vostsibneftegeofizika - V. G. Vasil’ev, E. N. Kalenov, I. P. Karasev, E. V. Kravchenko, M. M. Mandel’baum, S. N. Gushkovich, B. A Fuks, V. N. Troitskiy, M. A. Tsakhnovskiy, and S. V. Vertov - took upon themselves the task of synthesizing and analyzing the collected data in order to clarify the oil and gas prospects of the area under consideration.

The results of this work has been published in a book “Geology of the South of the Siberian Platform and the Oil Capability of the Cambrian,” published by the Gostoptekhizdat in 1957; A. A. Borisov, Editor.

The publishing of similar works, which summarize large and costly operations and which clear the way for increased efficiency at exploration, must be encouraged in every way.

In contrast to many books devoted to the geology of an area, the work being reviewed differs in that it illuminates the results not only of the geological but also of the geophysical studies. Such an approach permitted the authors to give a broader incite into the geology of the south part of the Siberian Platform and to formulate basic ideas concerning the practical solution of the problem of the oil and gas capability of the Siberian Platform.

The book treats problems of the stratigraphy and physical properties of rocks, results of gravity, magnetic, electrical, and seismic exploration, and questions of the oil-gas capability of the Cambrian sediments. A description of the tectonic structure of the south part of the Siberian Platform is also given. The book ends with recommendations for the regional complex of exploration operations.

In the section “Stratigraphy” are presented general data that characterize the sediments of the older units of this region including the Proterozoic and Lower Paleozoic; the Mesozoic sediments are described less fully. Use was made of the results of deep drilling data and also of the most recent paleontological data.

It should be noted that in describing the stratigraphy of the older units, the authors subdivided the Upper Proterozoic into stages and substages and gave new names. Such a subdivision is not sufficiently grounded within the text, and the introduction of such a subdivision does not follow from the factual data.

In the sections devoted to a description of the gravity and magnetic fields, the authors give a very interesting comparison of the characteristics of the gravity fields of the Siberian and Russian platforms. They relate features of the gravity characteristics of these platforms with their historical development. In clarification of the sharp difference of the gravity characteristic of East Sayan and also of the Siberian and Russian platform, the authors proceed from the position of large shifts of subsurface material and of masses of the crust. Processes of uplift of mountains and of large tectonic elements of the platforms are ascribed to gravity factors - with the lifting power of light sialic material concentrated in them for some reason or another. This leads to growth of the roots of the mountains, an increase of their thickness. It is natural that flow of this material from adjacent areas leads to decrease in the thickness of the sial cover and to its replacement by sima.

The authors demonstrate quite reliably that the gravity field on the south of the Siberian platform reflects the structure and the distribution of the Lower Cambrian and Proterozoic sediments. They conclude that gravimetry is for this region one of the principal methods of geotectonic subdivision of the lower structural stage, which embraces sediments from Proterozoic to Lower Cambrian inclusively.

The authors relate the large belt magnetic anomalies present in this region with disjunctive faults and in part with the presence of large deep faults that bound the Siberian platform.

In describing the electrical prospecting work the authors point out that this method was used in the south part of the Siberian platform for study of the structure of the sediments above the salt and also for study of the zone of active water exchange in this same stratigraphic interval. In the Kansko-Taseev and Rybin depressions, electrical prospecting is very effective for the tectonic subdivision of large areas and for exploration of structures along a marker horizon of high resistivity. In depressions of the Baykal type the electrical method can be very useful for study of the surface of the basement. Very interesting electrical data are presented that establish the presence of a thick sedimentary section under the bottom of Lake Baykal.

The section devoted to the results of seismic exploration is poorly written, although this method has turned out to be the principal one for revealing and preparation of structures for deep prospecting drilling in the Mot suite, which is the main object of exploration.

In the section “Structure of the southern part of the Siberian platform and its framework” the authors in compilation of the geotectonic outline of the south of the Siberian platform proceed from several unusual ideas on the platform and surrounding geologic structure.

Under platform the authors understand a large downwarp of the crust that rises in different stages of its geologic development. Surrounding positive structural elements (shields of old platforms) are distinguished in an independent category regardless of the age of their formation; these are called geanticlines. These and the other forms develop in conjunction but in opposite directions. As a result, a sharply expressed negative gravity field is characteristic of the geanticlines, and geologically they are areas predominately of upwarp. The platforms, on the other hand, have relatively negative fields and are areas predominately of downwarp.

The authors draw the boundary of the East Siberian platform along the margin of greatest extent of the sedimentary cover. They also consider that the platform is not constant in the process of development but changes greatly and passes into other tectonic forms. In particular, the authors think that the marginal parts of the Siberian platform were earlier parts of a regional downwarp but are now uplifted and are part of a geanticline. To such a type of region, which they designate as an area of a platform involved in geanticlinal uplift, is referred the East Sayan, for example. This point of view deserves attention for clarifying its applicability to geotectonic subdivision.

On the basis of analysis of the gravity field and also of the geologic data, three large tectonic elements are distinguished in the platform: the Pri-Baykal - Lensk synclease, the Angaro - Irkutsk anticlease, and the Pri-Sayan - Yenisei synclease.

The revealing of a large tectonic element in the form of an anticlease has great significance for orientation of further oil and gas exploration. It should be noted that the first two structures in the Mot sediments have recently been discovered within this anticlease. Moreover, the closure of one of these - Atov - is 120 m.

Within the anticlease and the syncleases the authors distinguish a whole series of structural elements of a lesser order. They describe all the local structures distinguished by geological and geophysical methods within the southern part of the Siberian platform. The chapter “Tectonics,” in spite of several debatable points, gives a clear idea on the structure of the southern part of the Siberian platform.

The book ends with a section devoted to the efficient organization of oil and gas exploration in the southern part of the Siberian platform. We must agree with the practical recommendations for simplification.

In the nature of shortcomings of this part of the work, the absence should be noted of clear recommendations of the feasibility of the use of structural drilling. There is also an underestimation of the possibilities of the seismic method for distinguishing structures along the top of the Mot sediments.

The book on the whole, in spite of a number of shortcomings, is certainly a valuable summary of recent data obtained during study of the geology of the southern part of the Siberian platform. This work will contribute to an increased efficiency of oil and gas exploration.

Akad. A. A. Trofimuk



Sedimentology and Stratigraphy,Structure

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