Home – Home – Jornals – Russian Geology and Geophysics 2003 number 6

The main factors that contribute to postsedimentary salinization of reservoirs are: contact, hydrochemical, and pressure-thermal metamorphism of underground waters and brines; an increase in mineralization of brines on evaporation, freezing, gas hydration, and membrane processes; role of connate waters and mother brine of halmeic basins; effect of trap magmatism; man-induced salinization. They have been discussed and quantitatively estimated.
It has been established that the main and virtually the only reason for regional salinization of subsalt terrigene reservoirs is pressure-thermal metamorphism of brines (decrease in pressure and temperature). All the above-mentioned natural processes contributed to the regional salinization of carbonate basins, and the main one was contact metamorphism of underground waters and brines (their interaction with halmeic-carbonate rocks). The buried brine of halmeic basins and other connate waters had no effect on salinization of reservoirs. The disjunctive activity of trap magmatism accelerated these processes, but thermal action of traps and their subsequent cooling promoted only slight salinization of reservoirs. Man-induced salinization can occur in reservoirs of any type, restricted to local zones around absorbing wells. Some recommendations are given with respect to further study.

We describe the technique and results of gas resources estimation in discovered and predicted gas pools in gaize (silica clay) and siliceous mudstones of the Lower Berezovskaya subformation of Cenomanian age in West Siberia. The estimation technique is based on complex geophysical studies of wells, data on stony material, and results of sampling from the study section in the Komsomol'skoe field. Also, data on the physical properties of gaize and gaize-like clays sampled from stratigraphic wells are analyzed. The results obtained show low coefficients of gas saturation, thus pointing to a low gas potential of gaize.

Infauna trails (trace fossils) have been identified in rocks of the Bazhenov and Georgiev Formations of the Upper Jurassic of the West Siberian Plate. Two types of trace fossils, Zoophycos and Chondrites, are found in deposits of the Bazhenov Formation, and three types of trails, Chondrites, Thalassinoides, and Skolithos, are recognized in the Georgiev Formation. Conditions of formation have been determined for each type of trace fossils.

Early Cretaceous palynological assemblages of West Siberia were compared by statistical processing of numerical data. Modern methods of biometry permitted us to analyze a great body of information and to recognize palynological associations within West Siberia on the basis not only of qualitative but also quantitative criteria. Lateral distribution of these associations is controlled by a combination of both paleoclimatic and paleogeographical factors. As a result, the interpretation of palynological material is rather ambiguous. Reasons for the differentiation of the Early Cretaceous palynoflora are discussed in terms of the obtained results and existing paleofloristic models.

Alkali granites with abnormally high contents of Na-pyroxenes and Na-amphiboles (rockallite, lindinosite, etc.) have been found in deeply differentiated alkaline complexes all over the world. Dikes of rare-metal taxitic or foliated aegirine-riebeckite granites developed in the Ampasindava province of Madagascar are called fasibitikites. These are intricate obliquely cutting veined bodies, each consisting of alternating interbeds parallel to the plane of contact and made up of pegmatoid riebeckite granites and fine-grained quartz-aegirine rock poor in feldspar. The thickness of ultimately veined fasibitikite bodies varies from 20 cm to 2.5 m. The proximal veins are grouped into veined series, which form a 300 m wide and 2 km long zone. Fasibitikites have commercially significant contents of Ta, Nb, Zr, and REE; the average contents are as follows (%): Ta₂O₅ - 0.037, Nb₂O₅ - 0.34, ZrO₂- 2.31, and REE₂O₃ - 0.6%. The aegirine granites have extremely inhomogeneous composition, structure, and texture and are ultimately enriched in rare-metal minerals such as eudialyte, zircon, pyrochlore, and chevkinite. The content of commercially important components in them is 1.5-2 times higher than that in the riebeckite varieties. The REE pattern of fasibitikites shows the maximum enrichment in light lanthanides and a serious Eu deficit.
The extremely high content of modal silica in fasibitikites is inconsistent with the hypothesis of their fenite nature, though these rocks are likely to be related to an undersaturated foid source. Obviously, they resulted from a residual Si-oversaturated melt rich in superstoichiometric Na and incompatible Fe, Zr, Ti, Nb, Ta, and Y with participation of the fluid phase. By analogy with the foliated aplite-pegmatite bodies, we suggest that the interbeds of pegmatoid riebeckite granites crystallized through separation of melt portions enriched in volatiles. Crystallization from disequilibrated overcooled melt with possible precrystallization segregation is not ruled out either.

The Tuva-Mongolia block has become commonly recognized and is often invoked in palinspastic reconstructions as a separate terrane. However, its position has been doubted recently despite abundant geological evidence providing new details of its origin and evolution.
We analyze the available geological data to reconsider the structure and outlines of the Tuva-Mongolia terrane and justify its existence as an independent tectonic unit in the Central Asian belt of Paleozoides. The terrane has a pre-Vendian accretionary basement overlain by Vendian-Cambrian carbonates. In Vendian-Cambrian time it was a microcontinent drifting in the Paleoasian ocean, and platform carbonate deposition within its limits was synchronous with the formation of ophiolites and island arcs in the surrounding oceanic space. Paleozoic plutonism and metamorphism record rather the Ordovician collision than the preorogenic tectonic history of the terrane.

Upper mantle beneath the Alpine-Himalayan orogen (AHO) at depths from 100 to 500 km has been studied using ITS inversion of teleseismic P travel times from earthquakes that occurred in the region and were recorded by the worldwide seismological network (ISC bulletins). 3D velocity maps were obtained as a sum of independent inversions in about sixty overlapping blocks 600-1000 km across. This approach is similar to high-frequency spatial filtration and provides a much higher resolution than the global tomography. High-velocity zones within the orogen have been interpreted as signature of continental or oceanic lithosphere sinking to the upper mantle under active regional compression. The maps show well pronounced traces of subduction in the regions of Cretan arc, Hindukush, and Burma, also confirmed by independent data, as well as other less certain evidence of subduction. Positive anomalies in the western part of the orogen are attributed to subduction in the area of Cyprus and along the Caucasus-Kopet Dagh-Lut belt. The lithosphere of the Indian plate subducted beneath the Himalayas and Tibet has a rather complicated structure. Zones of sinking lithosphere are traceable around the Tarim block. Distinct low-velocity anomalies in Mongolia, Tibet, and Southern Caspian basin may be produced by active mantle plumes.

Decade-long paleoseismic trenching (30 trenches) across the Arshan and Tory fault scarps in the southwestern end of the Baikal basin furnished new details of seismic rupture and highlighted the geometry of different segments of the Tunka fault. It is mostly a reverse-oblique fault within its W-E part and a normal fault in NE-trending segments. The estimated Holocene left-lateral strike-slip offset is 3.5-8 m in single events, 12-16 m for single- or multiple-event displacement, and up to 22-35 m in two and more events. Trenching revealed six large prehistoric earthquakes with their ages bracketed between 1315 and 1742 (M 7.3), 2464-2809 (M 7.4), 5257-5907 (M 7.2), 7091-7385 (M 7.3), 9214-9902 (M 8), and 10386-11187 (M 8) years BP. Rupture during the 9-10 Ka event may have reactivated the Tunka and Main Sayan faults along 100 km in total.

Formation of residual strain in solids was distinguished into a separate problem during studies of spontaneous conversion in a constrained cylinder when it was found out that this form associated with earthquakes, shocks and rock ejecta into mines is not the only species of athermal conversion of elastic strain.
Two more species are the so-called forced volume conversion (FVC) and forced frontal conversion (FFC) manifested, respectively, in plastic flow and in explosive failure of brittle rocks under uniaxial compression on a common press and, presumably, under monotonic hard loading. Forced frontal conversion in metals manifests itself in incremental deformation (stepwise deformation, or Savart-Masson effect, and gearwise deformation, or Portevin-Le Chatelier effect).
Of the four elementary forms of conversion, two volume species (forced volume conversion and thermal conversion) provide monotonic formation of residual strain and two frontal species (forced frontal conversion and spontaneous conversion) cause incremental deformation.
Investigation into the origin and species of residual strain provides additional support for the idea of spontaneous conversion but shows that the SC hypothesis is only one aspect of the problem.
The conversion approach to residual strain can be useful for understanding deformation in rocks and mitigation of related hazard.

The first absolute Ar/Ar dates have been obtained for buried basalts from the pre-Jurassic basement of the West Siberian Plate, 249-250 Ma, which correspond to the Permian-Triassic boundary. The dating shows that magmatism in East and West Siberia proceeded synchronously. This is consistent with the concepts of superplume activity just in this time span.