Home – Home – Jornals – Russian Geology and Geophysics 2005 number 2

The study further details the formation and subformation stratigraphy of the Silurian section of the northwestern Siberian Platform in terms of the distribution of ecosystems correlated to chronozones. A lithified fossil ecosystem is represented by a genetically uniform dominant lithology and the hosted fossil biocenosis, which together make an ecostratigraphic unit (lithified biocenosis or biolithocenosis). The ecostratigraphic structure of Silurian sediments in the region is described according to the ages, locations, and dominant lithologies and fossil fauna assemblages of 105 units distinguished in the section.

Principles of medium-scale neotectonic mapping have been considered by the example of junction of the old Siberian Platform and young West Siberian Plate. Instead of neotectonic systems commonly used as a basic unit in teotectonic mapping, the fault-block structure of terranes and its morphotectonic analysis are proposed. The role of faults in the formation of contrasting relief features increases in the transition from global through regional to local level of structures. Being a derivative of neotectonic movements, the relief mirrors neotectonic structures. The more contrasting relief, the younger it is and the faster neotectonic movements are. A schematic succession of neotectonic analysis is as follows: morphotectonic analysis of relief - detailed mapping of network of fault structures - recognition and ranking of neotectonic blocks - characteristic of neotectonic relief within the blocks and estimation of its contrast - calculation of velocities of neotectonic movements - construction of a neotectonic map.At the junction of the Siberian Platform and West Siberian Plate in the lower reaches of the Kan River, the neotectonic structure is controlled by the Baikal-Yenisei fault zone, represented by a dense network of submeridional reverse-fault-nappe structures having distinct traces of Pleistocene rejuvenation. The zone of the Baikal-Yenisei fault is crossed by young diagonal strike-slip structures, of which the northwestern ones are dextral, and northeastern, sinistral.This system of faults cuts out a series of macro- and microblocks. West of the Baikal-Yenisei fault, the block structures tend to subside, and east of it, they tend to rise. The velocities of subsiding and rising are quite close, which is inferred from the compensated filling of basins with young sediments in the immediate vicinity of the rising block. Vertical and horizontal displacements along young faults have been recognized for the study area, their kinematic type has been ascertained, and the character of lithosphere blocking has been defined as a function of the type of its stress state. It has been established that the territory is most fragmented where fields of compressive and shear stresses are superposed.

Natural bitumens are widespread in the sedimentary cover of the Yakutian diamond province. They occur within a broad stratigraphic range and differ in size and character. A number of oil and gas fields have been discovered in the province. Numerous natural bitumen occurrences are found in kimberlite pipes and host rocks. Geochemical characteristics of the samples taken from different sites in and off the body of kimberlite pipe - kimberlite, xenolith, host rocks - display a rather intricate pattern. Their composition can be considered as a result of superposition of two processes: thermal destruction and deasphaltization of oils. The calculated temperatures of host rock heating and the time of cooling of kimberlite pipe suggest weak thermal destruction in both xenoliths and host rocks.

Increasingly more attention is paid to the problem of hydrocarbon production from reservoirs with a complex geologic structure. In particular, these beds can be of lenticular structure, with separate sandstone lenses distributed in a shale matrix. A stochastic approach seems to be quite natural to describe such deposits.The probabilistic description of a system of lenses involves such parameters as: shape and size of deposit, shape and size of lens, probability density of distribution of lenses, and number of lenses in the study area.Mathematical tools have been devised to describe relationships between a system of lenses and a system of wells. Formulas for the probabilities of the relevant events have been derived and used for solving an inverse problem. The parameters of the system of lenses are numerically estimated by the method of maximum likelihood using data of exploratory drilling.Some results of practical significance have been obtained. The proposed approach has been used to analyze the uncertainty and risk of well location and to estimate the effectiveness of horizontal well drilling and hydraulic fracturing in lenticular reservoirs. The described approach is illustrated by its application to the estimation of oil field parameters. An example was a field in West Siberia.

The northern zone of West Sayan, made up of Vendian, Early Cambrian, and Middle Cambrian volcanosedimentary rocks, is a fragment of a large island-arc system on the southwestern periphery of the Siberian craton. The specifics of this zone is a wide occurrence of plagiogranitoids of the Maina tonalite-plagiogranite complex, which are spatially associated only with the deposits of the Early Cambrian Nizhny (Lower) Monok Formation.By now, the nature and age of the Maina plagiogranites have been debatable because of the absence of reliable isotope-geochronological data and drastic diversity of opinions regarding the nature of pebble of plagiogranitoids from conglomerates of the Lower and Middle Cambrian volcanosedimentary deposits.Plagiogranitoids of the Maina complex were compared with plagiogranitoid pebble from the lower beds of the Verkhny (Upper) Monok Formation, which contain paleontologic remains of Early Cambrian age. It has been established that some pebble varieties do not differ in petrographic, petrochemical, and trace-element compositions from plagiogranites of the Yenisei and Tabat plutons. Results of U-Pb and Ar-Ar isotope dating of the plagiogranitoids point to their Early Cambrian age (523.8±2.1 Ma). Isotope-geochemical modeling of the composition of the source of plagiogranites showed that their primary melts were produced from metabasic substrates corresponding in trace-element composition to island-arc tholeiitic basalts at low (<10 kbar) pressures.

The geologic structure and lithology of the Sobolev kimberlite pipe and some other nearby pipes are considered. These pipes are buried beneath Upper Paleozoic terrigenous-sedimentary rocks intruded by trap sills. The total thickness of the overlying rocks reaches 80 m. The pipe kimberlites belong to a magnesian group. They are composed of autolithic breccias typical of kimberlites of the Alakit-Markha field and have a low diamond potential. In addition to indicator minerals, the heavy fraction of kimberlites has elevated contents of almandine and ferriferous ilmenite, which were transported from schists of the platform basement.Despite the low diamond potential, the kimberlites often bear not only magnesian garnets and chromites but also subcalcic knorringite-containing pyropes and chromites of diamond association. This is a typomorphic feature of garnets and chromites of kimberlites of the Daldyn-Alakit region.

Using a modern microprobe, we have thoroughly studied the structure and phase composition of polymictic rims on picroilmenites sampled from modern alluvial deposits and from eluvium of the Khorkich pipe in the Taigikun-Nemba kimberlite field. Data obtained helped to refine the composition and genesis of similar rims. We have established that their formation was a long process, which included both reactionary replacement of picroilmenite by neogenic mineral phases at the magmatic stage as a result of the long-term interaction of its grains with kimberlitic melt and the subsequent postmagmatic replacement (corrosion) of these phases by secondary minerals. At the magmatic stage, perovskite and titanomagnetite were the major minerals that developed after the picroilmenite grains. At the postmagmatic stage, both picroilmenite and neogenic mineral phases were replaced by leucoxene and serpentine. The formed surfaces are primary in the context of the processes of formation of heavy-concentrate aureoles, and the relief itself is more correct to be called reactionary-corrosive. We suggested that the development of a rather intensive reactionary-corrosive relief on picroilmenites can exert a negative effect on the diamond potential of kimberlite bodies.

Lichens are widespread in the vegetative cover of West Siberia, particularly in its north, where they serve as one of the links of the natives' food chain. They play an important role in the migration and transformation of chemical pollutants. This paper presents results of studying the effect of some factors on the accumulation of chemical elements in the lichen cover.Throughout the study area, lichens have background average contents of trace elements close to those in the Arctic regions of Eurasia. Elevated contents have been established only for Cd and Mn in the Altai Territory and for Pb and Cr throughout the West Siberian region. We studied the ratios of concentrations of man-induced radionuclides (¹³⁷Cs, ⁹⁰Sr, ^238,239,240Pu) and trace elements (Cd, Pb, Cu, Zn, Mn, Cr, Ni, Co, Hg) in the upper and basal parts of the lichen and in the underlying soil substrate. The average specific activity of ¹³⁷Cs in the lichen cover of northern West Siberia is higher than that in southern West Siberia - 153 and 44 Bq/kg, respectively.

Theoretical probability of resonance is tested against experimental seismic measurements in a layer of frozen coarse sediments overlying frozen bedrock. Resonance frequencies are estimated relative to the frequencies of P and S waves from 14 earthquakes recorded at three stations. P and S velocities in the sediments are predicted proceeding from the equality of resonance frequency/velocity ratios using integrate values of water content- and temperature-dependent velocities in sedimentary frozen ground in the Baikal and Transbaikal regions. Thus seismic velocities in unconsolidated sediments can be predicted from earthquake records using integrate laboratory and field velocity data.

It is suggested to use indirect information in machine contouring by means of power potential polynomials as a basic geometrical tool. Unlike other methods, the problem in the new approach is formulated in terms of an information-geometric model free from the mechanical analogies. Interpolation of scattered data does not require approximating procedures, and the basis functions intrinsically minimize the complexity of the contoured surfaces. This allows avoiding labor-consuming explicit variational solutions to ill-posed problems. The new algorithm was successfully used in structural mapping of buried surfaces.

Signature of main latest Late Glacial (Sartan) geological events within West Siberia is found in sections of exposed fluvial sediments in the Ob' valley floor near Surgut.