West Siberian Platform Research Articles

Strike-slip faulting in the West Siberian Platform: Insights from 3D seismic imagery

The 3D seismic exploration, actively deployed in recent years on much of the West Siberian Platform – one of the world's largest oil and gas bearing basins – has brought out extensive development of a system of strike-slip faults within the basement. The fault system causes local structural and fluid flow anomalies within the Jurassic-Cretaceous sedimentary section, which is known to accommodate a multitude of large oil and gas deposits. This article will show the distribution geography and the scale of the basement strike-slip tectonics, and explain why this phenomenon, so clearly highlighted by 3D seismic, was not discovered earlier. The article will also consider how strike-slip faults are detected in the sedimentary cover and how they impact the characteristics of oil and gas fields located in their vicinity.

Genesis of ultramafite complexes of the basement of the West Siberian Platform

86 In recent years, the widespread occurrence of ultramafic rocks in the basement of the West Siberian Platform has been established [1–5, etc.]. Ultramafic complexes of the basement can be divided into two groups. The first group, which includes the vast major ity of the studied samples, is characterized by almost complete serpentinization with the presence of only Cr spinel as a primary mineral. The second small group is represented by peridotites containing chromites, olivines, orthopyroxenes, and clinopy roxenes. The compositions of the rocks were deter mined by the X ray spectral fluorescence (XRF) method at the Zavaritskii Institute of Geology and Geochemistry, Ural Branch, Russian Academy of Sci ences (Yekaterinburg). The compositions of minerals were studied with an X ray scanning electron micro probe analyzer CAMEBAX MICRO at the Sobolev Institute of Geology and Mineralogy, Siberian Branch, Russian Academy of Sciences (Novosibirsk).

The structure of the tectonosphere of the Urals and West Siberian Platform by electromagnetic data

Advances in magnetotelluric sounding (MTS) have provided the possibility to study deep parts of the planet and obtaining information on electroconductivity of the Earth’s tectonosphere, which is very sensitive to the lithology of rocks in the upper part of the crust, as well as to the phase state of substance, the temperature, concentration, and mineralization of fluids. Introduction of modern broadband digital measuring and computation equipment, as well as programs of computational modeling allowed us to realize more completely the possibilities of geoelectric methods. The technical approach including the integration of geoelectric methods, the processing of experimental materials, and interpretation of data with construction of geoelectric models of the medium was considered in detail by the example of geotraverses through the Northern and Southern Urals [1, 2]. The material on the study of fields of natural sources of geomagnetic variations of the ionospheric and magnetospheric origin (AMT‐MTS‐DMTS methods) and the induction electromagnetic sounding (IEMS) method with a controlled source along a traverse about 1000 km long from the Settlement of Askino (Bashkortostan) in the west to Tyukalinsk (Omsk oblast) in the east with an a posteriori test of the previously obtained results [3] was generalized for the first time in this work; the geotraverse partially coincides with the Sverdlovsk profile of deep seismic sounding (DSS). In this case, major faults and zones of higher fracturing in the crust, as well as relationships between electroconductivity and the tectonic structure of regions, were studied. The originality of the studies performed lies in the fact that peculiarities of tectonosphere stratification were traced by electric parameters within a depth from 10 m to 600 km, and traces of events that proceeded there over a large time interval were revealed. The results are presented in Fig. 1. According to the distribution of total longitudinal conductance of the lithosphere ( S ), the territory under consideration can be subdivided into three sectors, the boundaries of which mark zones of abnormally high conductivity, which

Loz’va dislocations of the north Transural region: A response to neotectonic underthrusting of the West Siberian platform beneath the Urals

Loz’va dislocations of the north Transural region: A response to neotectonic underthrusting of the West Siberian platform beneath the Urals

Upper mantle structure of the Northern Eurasia from peaceful nuclear explosion data

Several long-range seismic profiles were carried out in Russia with Peaceful Nuclear Explosions (PNE). The data from 25 PNEs recorded along these profiles were used to compile a 3-D upper mantle velocity model for the central part of the Northern Eurasia. 2-D crust and upper mantle models were also constructed for all profiles using a common methodology for wavefield interpretation. Five basic boundaries were traced over the study area: N1 boundary (velocity level, V = 8.35 km/s; depth interval, D = 60–130 km), N2 ( V = 8.4 km/s; D = 100–140 km), L ( V = 8.5 km/s; D = 180–240 km) and H ( V = 8.6 km/s; D = 300–330 km) and structural maps were compiled for each boundary. Together these boundaries describe a 3-D upper mantle model for northern Eurasia. A map characterised the velocity distribution in the uppermost mantle down to a depth of 60 km is also presented. Mostly horizontal inhomogeneity is observed in the uppermost mantle, and the velocities range from the average 8.0–8.1 km/s to 8.3–8.4 km/s in some blocks of the Siberian Craton. At a depth of 100–200 km, the local high velocity blocks disappear and only three large anomalies are observed: lower velocities in West Siberia and higher velocities in the East-European platform and in the central part of the Siberian Craton. In contrast, the depths to the H boundary are greater beneath the craton and lower beneath in the West Siberian Platform. A correlation between tectonics, geophysical fields and crustal structure is observed. In general, the old and cold cratons have higher velocities in the mantle than the young platforms with higher heat flows. Structural peculiarities of the upper mantle are difficult to describe in form of classical lithosphere–asthenosphere system. The asthenosphere cannot be traced from the seismic data; in contrary the lithosphere is suggested to be rheologically stratified. All the lithospheric boundaries are not simple discontinuities, they are heterogeneous (thin layering) zones which generate multiphase reflections. Many of them may be a result of fluids concentrated at some critical P– T conditions which produce rheologically weak zones. The most visible rheological variations are observed at depths of around 100 and 250 km.

A variable resolution surface wave dispersion study of Eurasia, North Africa, and surrounding regions

This paper presents the results of a large‐scale study of surface wave dispersion performed across Eurasia and North Africa. Improvements were made to previous surface wave work by enlarging the study region, increasing path density, improving spatial resolution, and expanding the period range. This study expands the coverage area northward and eastward relative to a previous dispersion analysis, which covered only North Africa and the Middle East. We have significantly increased the number of seismograms examined and group velocity measurements made. We have now made good quality dispersion measurements for about 30,000 Rayleigh wave and 20,000 Love wave paths and have incorporated measurements from several other researchers into the study. We have improved the inversion from the previous study by adopting a variable smoothness with the conjugate gradient method for the group velocity tomography. This technique produces higher‐resolution models where the concentration of data allows it without producing artifacts. The current results include both Love and Rayleigh wave inversions across the region for periods from 7 to 100 s on a 1° × 1° grid and at resolutions approaching 1° under some conditions. Short‐period group velocities are sensitive to slow velocities associated with large sedimentary features such as the Caspian Sea, West Siberian Platform, Mediterranean Sea, Bay of Bengal, Tarim Basin, and Persian Gulf. Intermediate periods are sensitive to differences in crustal thickness, such as those between oceanic and continental crust or along orogenic zones and continental plateaus. At longer periods, fast velocities are consistently found beneath cratons, while slow upper mantle velocities occur along rift systems, subduction zones, and collision zones such as the Tethys Belt. We have compared the group velocities at various periods with features such as sediment thickness, topographic height, crustal thickness, proximity to plate boundaries, lithospheric age, and lithospheric thickness, and we find significant correlations. We have developed an empirical relationship between long‐period group velocity and lithospheric thickness that works well for both oceanic and continental lithosphere. We do not find any similar correlation between the longest‐period surface waves and hot spots.

2D model of the crust and uppermost mantle along rift profile, Siberian craton

A two-dimensional model of the crust and uppermost mantle for the western Siberian craton and the adjoining areas of the Pur-Gedan basin to the north and Baikal Rift zone to the south is determined from travel time data from recordings of 30 chemical explosions and three nuclear explosions along the RIFT deep seismic sounding profile. This velocity model shows strong lateral variations in the crust and sub-Moho structure both within the craton and between the craton and the surrounding region. The Pur-Gedan basin has a 15-km thick, low-velocity sediment layer overlying a 25-km thick, high-velocity crystalline crustal layer. A paleo-rift zone with a graben-like structure in the basement and a high-velocity crustal intrusion or mantle upward exists beneath the southern part of the Pur-Gedan basin. The sedimentary layer is thin or non-existent and there is a velocity reversal in the upper crust beneath the Yenisey Zone. The Siberian craton has nearly uniform crustal thickness of 40–43 km but the average velocity in the lower crust in the north is higher (6.8–6.9 km/s) than in the south (6.6 km/s). The crust beneath the Baikal Rift zone is 35 km thick and has an average crustal velocity similar to that observed beneath the southern part of craton. The uppermost mantle velocity varies from 8.0 to 8.1 km/s beneath the young West Siberian platform and Baikal Rift zone to 8.1–8.5 km/s beneath the Siberian craton. Anomalous high Pn velocities (8.4–8.5 km/s) are observed beneath the western Tunguss basin in the northern part of the craton and beneath the southern part of the Siberian craton, but lower Pn velocities (8.1 km/s) are observed beneath the Low Angara basin in the central part of the craton. At about 100 km depth beneath the craton, there is a velocity inversion with a strong reflecting interface at its base. Some reflectors are also distinguished within the upper mantle at depth between 230 and 350 km.

Lateral variations of the upper mantle structure in Eurasia from group velocities of surface waves

Group velocities of both Rayleigh and Love waves in the 20–70 s period range along paths across Asia and the Eastern Europe are used to obtain group velocity maps of the region. For this purpose, we developed a method for tomographic inversion on spherical surface, which is practically free of a priori assumptions. Checker-board test shows that velocity anomalies of 10°×10° size can be resolved. The ‘local’ dispersion curves are inverted to velocity-depth curves up to 160 km depth along three profiles: (1) Tibet, the South China Platform and the southern part of the Japan Sea; (2) the West Siberian Platform, the Tienshan and the Pamir; (3) the East European Platform and the Black Sea Basin. Lateral S-wave velocity variations are presented at the depths of 60, 90 and 120 km. A low velocity zone in asthenosphere under Japan Sea is extended under the South China and the North China–Korean Platforms. The upper mantle structure of the Russian (East European) and the West Siberian Platforms is different: a low-velocity zone at 120–140 km depths is observed under the West Siberian Platform, whereas the velocity under the Russian Platform increases monotonously with depth. A low velocity layer appears under the Black Sea indicating an oceanic structure of the Basin. Low surface wave velocities in Tibet are caused not only by a thick crust, but also by low S-wave velocities in the upper mantle. The Precambrian West Siberian, North Indian and North China–Korean Platforms are characterized by high velocities at the depth of 120 km unlike the younger tectonic units.

Arctic and Asia lithospheric satellite magnetic anomalies

The polar regions present special problems in magnetic studies because they are largely inaccessible and contain the most disturbed magnetic fields on earth (e.g., effects from auroral phenomena). A processing method, previously developed for south polar satellite magnetic data (e.g., Magsat), is applied to north polar data to separate the core and external fields from the lithospheric anomalies. The core field is removed with a least-squares procedure and Fourier correlation coefficient filtering effectively reduces the external field signatures. As demonstrated by improved correlation coefficients and reduced sum-of-squared differences between passes and between dawn and dusk maps, the processing effectively extracts the static geologic signal from the dynamic noise (e.g., external fields). Quantitative comparisons of the resulting lithospheric anomaly map with previous maps suggest that the processing maintains anomaly amplitude while significantly reducing non-lithospheric energy. The map shows a long-wavelength contrast across the Ural mountains, the suture zone between the east European craton (+4 nT) and the west Siberian platform (−4 nT). This contrast might be indicative of significant petrologic differences between these large crustal blocks. If the steepest portion of the magnetic gradient is marking the main Uralian fault, then the fault may extend southward to a latitude coincident with the middle of the Aral sea. The easternmost section of the intraplate Tien Shan is marked by a +3 nT anomaly whereas the remainder of the mountain belt is not associated with a significant anomaly. This pattern agrees with previous interpretations that suggest the eastern Tien Shan is supported by a lithospheric root and the rest of the belt is held up by a positive buoyancy force, i.e., it lacks a root.

Abstract: Intraplatformal Troughs of the North of the West Siberian Platform, Their Structure and the Prospects for Gas

Abstract: Intraplatformal Troughs of the North of the West Siberian Platform, Their Structure and the Prospects for Gas

Low geothermal heat flow of the Urals fold belt — implication of low heat production, fluid circulation or palaeoclimate?

The Urals are characterized by extremely low heat flow density (HFD). We present a discussion on the relevant factors which may contribute to the observed distribution of heat flow values. The available heat flow data in the Urals and surrounding East European and West Siberian platforms are based on borehole measurements at about 300 sites which range from the Arctic Sea coast to the Kazakshtan Plain. Along the Urals fold belt heat flow density is 30 mW m −2 or less, whereas the platform areas are characterized by 20–40 mW m −2 higher values. The low heat flow density zone is 50–100 km wide. Its extension to the north is not exactly known, but the minimum extends at least to the latitude 61°N. We present new results of heat flow and heat production measurements, Peclet number analyses on advective heat transfer by groundwater flow, as well as numerical conductive models of heat transfer in the lithosphere in the Troitsk DSS transect. The most important factor contributing to the low heat flow density in the Urals seems to be the low level of radiogenic heat production in the crust in the Tagil-Magnitogorsk Zone. The minimum is apparently enhanced by the palaeoclimatically induced vertical variation in HFD produced by the periglacial climatic conditions during the latest glaciation epoch 70,000–10,000 years B.P. The boreholes used for HFD measurements are shallower (500–1500 m) in the Tagil-Magnitogorsk Zone than in the adjoining areas in the west (1000–3000 m) or in the east (300–3000 m), and therefore the palaeoclimatic disturbance is more pronounced in these boreholes.

Preliminary study of the uppermantle P wave velocity structure beneath central Asia

The travel-times of multi-bounce P waves have been used to investigate the lateral heterogeneity of the uppermantle P wave velocity structure beneath north-west part of China and middle Asia. The synthetic seismograms of P and PP waves for seven large Chinese continental earthquakes were calculated by using the WKBJ method. The PP-P residuals, which reflect the P wave structure of the midpoints of the seismic rays, were calculated by using the WKBJ method, The results show that: the uppermantle wave structure beneath Russian platform in the middle Asia area, the Seytho-Turanian and the west Siberian platform is similar to that of Canadian shield; the uppermantle P wave velocity in the fold system of west China, the center Asia fold system in Russia, and Kazak fold system is lower than that of Canadian sheild. It seems that the velocity structure is related to the tectonic structure.

Reduced geothermal gradients in the shallow West-Siberian platform

Reduced geothermal gradients in the shallow West-Siberian platform

Reduced geothermal gradients in the shallow West-Siberian platform

Reduced geothermal gradients in the shallow West-Siberian platform

Geophysical investigation of deep crustal structure of the Siberian Platform region

A sparse network of regional profiles developed in Siberia using deep seismic sounding methods (DSS) does not allow a valid three dimensional model of Siberian deep crustal structure to be constructed. To create such a model, interpretation methods of available geophysical and geological data are suggested. These are based on modelling of geological objects using potential fields. Information on DSS key profiles is considered to be key data.The results of such interpretation are shown in the crustal thickness map and in that of crystalline basement, as well as on a deep geological-geophysical profile across the West Siberian Plate and Siberian Platform. The crustal thickness in Siberia varies from 30 to 55 km. It is on average 4?6 km thicker on the Siberian Platform than the West Siberian Plate. The least crustal thickness of 30 to 33 km appears to be beneath regions with maximum thickness of Mesozoic deposits. Modern and Early Mesozoic rift zones are characterized by decreased crustal thickness of 36?39 km. Ancient shields of the Siberian Platform and adjacent folded mountain units are up to 45?55 km thick.The geometry of the top of crystalline basement in the young West Siberian Plate is sharply different from that of the ancient Siberian Platform. Within the West Siberian Plate structural features with 5 to 11 km of relief dominate, while on the Siberian Platform the structural relief varies from 2?3 to 14 km.The peculiarities of deep crustal structure have greatly influenced the formation and distribution of sedimentary units. The constructed section shows the depths and thicknesses of the Riphean, Vendian?Silurian, Devonian?Middle Triassic and Mesozoic?Cenozoic megacomplexes, most promising for oil and gas in Siberia. Data obtained on the deep structure of Siberian Platform regions are thus of great practical importance. They can be used to predict areas of petroleum potential and determine optimal trends of further oil and gas exploration by geophysical methods.

THE BAZHENOVO FORMATION OF THE MIDDLE OB' REGION

The cherty-clay rocks of the Bazhenovo formation offer prospects for finding a new type of petroleum deposit, as indicated by the flows of oil recovered from it in western Siberia. Jurassic Neocomian bituminous sediments occur in an area of more than one million km/sup 2/ on the West Siberian platform. In the Middle Ob' region, they have been assigned to the Bazhenovo formation. The Bazhenovo lies 2.5-3 km beneath the surface, its average thickness is 30 m, and the formation temperature is 80-100/sup 0/C. Over the greater part of the area, the Bazhenovo is overlain and underlain by argillaceous units, which are fluid-impervious units of the third class, that is, they are virtually impervious to oil. They isolate the Bazhenovo from permeable sandy beds and make it a closed system with anomalously high formation pressures. The paper describes the texture of the rocks, lithology, forms of organic matter, geochemistry, physical properties, and overall geology. 8 references, 5 tables.

Upper jurassic black bituminous shales in Western Siberia

The Bazhenovskaya Formation, of Volzkski (Tithonian)-Berriasian age is one of the most interesting and unique sedimentary formations of the Mesozoic section of the western Siberian platform. It is a shaly sequence characterized by a uniformity of lithology, high content of organic matter, broad regional distribution (in excess of 1 × 10 6 km 2), and relatively uniform thickness. The formation shows high electrical resistivity (up to 5000 ω), and its natural radioactivity exceeds background radiations by 10 mes or more. Because of these features, the Bazhenovskaya Formation is the leading marker horizon in the sedimentary cover on the west Siberian platform.

Lower Cretaceous stratigraphic oil- and gas-bearing complexes of the West Siberian Platform

Lower Cretaceous stratigraphic oil- and gas-bearing complexes of the West Siberian Platform

Deep-seated structure of the north of the West Siberian platform based on seismic data

Deep-seated structure of the north of the West Siberian platform based on seismic data

Influence of the thermal field on oil-gas content of Mesozoic rocks of the West Siberian Platform

Results of field and experimental investigations of the thermal field of oil-gas regions of the West Siberian platform are discussed and its relations with the distribution pattern of hydrocarbon pools and the physicochemical properties of the oil in the beds are examined. A schematic map of the distribution of the heat flux in West Siberia from data of 98 determinations was prepared. Five zones exhibiting different values of the abyssal heat flux density are distinguished. Gas or gas-condensate pools are confined to the zones of heat flux regional maxima in the field. Oil pools are confined to medium values of the heat flux, and regions with poor prospects of oil and gas finds have low values. The geothermal data show that the northern regions of the West Siberian platform remain promising sites for oil strikes. An analysis is made of the influence of the crustal energy relations of specific tectonic structures of the West Siberian Platform on the geochemical appearance of the oil in traps in the sedimentary mantle. The thermal field overall, not merely the temperature in the pools, governs the physicochemical properties of the oils.