Home – Home – Jornals – Russian Geology and Geophysics 2010 number 4

We have studied Jurassic sections in the Predyenisei subprovince of the West Siberian petroleum basin, which were penetrated in the Vostok-1, Vostok-3, and Vostok-4 stratigraphic wells. The Urman, Togur, Ilan, Peshkovo, Tyumen’, Naunak, and Mar’yanovka Formations are described from a detailed comprehensive core analysis and log data. The depositional environment for these sediments was predominantly continental. There is evidence for short transgressions in the Ilan (Lower Toarcian) and Peshkovo (Upper Toarcian) Formations, as well as the Upper Urman (Upper Pliensbachian) and the Upper Tyumen’ (Bajocian) Subformations. In the Upper Naunak Subformation (Oxfordian), there was a change of facies from continental to littoral continental and littoral marine. The Mar’yanovka Formation developed in normal marine shallow- or moderately deep-water environments. Although good reservoirs are common throughout the Jurassic section in the southeast of West Siberia, only small, lithologically screened deposits are predicted here.

New data have been obtained on the paleogeographic and stratigraphic assignment of Neopleistocene morpholithogenesis in the Chuya basin. They were derived from analysis of topographical maps, digital elevation models, and medium- and high-resolution satellite images as well as a textural and facies characteristic of Quaternary sections. Near the basin side, a paragenetic association of sediments is widespread. These sediments, laid down by fluid debris flows and mudflows, formed a ridge terrain during the emptying of a Late Quaternary paleolake. A diluvial–erosional origin of a series of small parallel scarps has been suggested. They are usually interpreted as wave-cut benches, which mark temporary levels of a glacier-dammed lake. We have examined geological evidence for a high glacial dam with an age of maximum glaciation inside the Chuya basin. Because of the dam, glacial floods during the Middle Neopleistocene were larger than those during the Late Neopleistocene.

The phase state of fluid in the system H₃BO₃–NaF–SiO₂-H₂O was studied at 350–800 °C and 1–2 kbar by the method of synthetic fluid inclusions. The increase in the solubility of quartz and the high reciprocal solubility of H₃BO₃ and NaF in water fluid at high temperatures are due to the formation of complexes containing B, F, Si, and Na. At 800 °C and 2 kbar, both liquid and gas immiscible phases (viscous silicate-water-salt liquid and three water fluids with different contents of B and F) are dispersed within each other. The Raman spectra of aqueous solutions and viscous liquid show not only a peak of [B(OH)₃]⁰ but also peaks of complexes [B(OH)₄]^–, polyborates [B₄O₅ (OH)₄]^2-, [B₃O₃ (OH)₄]^–, and [B₅O₆ (OH)₄]^–, and/or fluorborates [B₃F₆O₃]^3-, [BF₂ (OH)₂]^–, [BF₃ (OH)]^–, and [BF₄]^–. The high viscosity of nonfreezing liquid is due to the polymerization of complexes of polyborates and fluorine-substituted polyborates containing Si and Na. Solutions in fluid inclusions belong to P–Q type complicated by a metastable or stable immiscibility region. Metastable fluid equilibria transform into stable ones owing to the formation of new complexes at 800 °C and 2 kbar as a result of the interaction of quartz with B-F-containing fluid.
At high concentrations of F and B in natural fluids, complexes containing B, F, Si, and alkaline metals and silicate-water-salt dispersed phases might be produced and concentrate many elements, including ore-forming ones. Their transformation into vitreous masses or viscous liquids (gels, jellies) during cooling and the subsequent crystallization at low temperatures (300–400 °C) should lead to the release of fluid enriched in these elements.

⁴⁰Ar/³⁹Ar dating yielded the reliable ages of andesite from the Unerikan complex (102.1 ± 1.4 Ma) and basaltic andesite from the Burunda complex (107.3 ± 2.4 Ma). The established age of volcanism is close to one of the stages of formation of the Khingan-Okhotsk volcanoplutonic belt. The petrography and geochemistry of basic, normal-basic, and normal rocks point to their dual character: They combine features specific for tholeiitic and calc-alkalic rocks. Most likely, these rocks formed in the setting of transform continental margin.

Published and new data on the Earth’s past magnetic field have been interpreted in terms of its links with the frequency of magnetic polarity reversals and with tectonic events such as plume-related eruptions and rifting. The paleointensity and reversal frequency variations show an antiphase correlation between 0 and 160 Ma, and the same tendency, likely, holds for the past 400 Myr. The geomagnetic field intensity averaged over geological ages (stages) appears to evolve in a linearly increasing trend, while its variations increase proportionally in amplitude and change in structure. Both paleointensity and reversal frequency patterns correlate with rifting and eruption events. In periods of high rifting activity, the geomagnetic field increases (15 to 30%), and the reversals become about 40% less frequent. Large eruption events between 0 and 150 Ma have been preceded by notable changes in magnetic intensity, which decreases and then increases, the lead being most often within a few million years.

Rock complexes in Mongolia experienced two remagnetization events. Almost all secondary remanence components of normal polarity were acquired apparently in the Cenozoic, after major deformation events, and those of reverse polarity were associated with intrusion of bimodal magmas during the Late Carboniferous–Permian reverse superchron. Active continental-margin sequences in some areas of Mongolia were folded prior to the Late Carboniferous–Permian magnetic event. The primary origin of magnetization in Late Paleozoic and Mesozoic rocks has been inferred to different degrees of reliability. According to paleolatitudes derived from most reliable paleomagnetic data, the analyzed rocks were located far north of the North China block throughout the Late Paleozoic and Early Mesozoic. Mongolia, as well as Siberia, moved from the south to the north in the Paleozoic, back from the north to the south between the latest Triassic and the latest Jurassic, and remained almost within the same latitudes in Cretaceous and Cenozoic time. These paleolatitudes show no statistical difference from those for the Siberian craton at least since the latest Permian (275–250 Ma). Older Mongolian complexes (with ages of 290, 316, and 330 Ma) likewise might have formed within the Siberian continent, which makes their paleomagnetic determinations applicable to calculate the polar wander path for Siberia. The paleolatitudes of Early Carboniferous sediments in Mongolia differ significantly from those of Siberia, either because of overprints from the reverse superchron or because they were deposited away from the Siberian margin.

Chloroform extracts of coals of different genetic groups and different ages have been studied by chromato-mass-spectrometry, namely, Devonian liptobioliths from the Barzas region (Kuznetsk Basin) and Lower Cretaceous humites and sapropelites from the Kangalas and Taimylyr deposits, respectively (Lena Coal Basin). It has been established that the most ancient Devonian liptobioliths formed in coastal environments. Lipids of different biotas of marine and continental genesis, including resins of early Conifers, were the source of biomarker molecules. The Mesozoic humic and sapropelic coals differ little in chemofossil biomarkers, which might be related to their significant bacterial transformation and biosynthesis of chemofossils mainly by prokaryotes.

It is suggested to estimate the Pliocene–Quaternary fault activity in a formalized way from synthesis of different data. The respective database consists of two main sections: general information and basic fault parameters and geomorphic, structural, paleoseismic, seismological, geophysical, geodetic, engineering-geological, hydrological, and meteorological data. The fault characteristics are scored according to their significance, and the cumulative score measures the fault activity. With this approach, the faults in the Barguzin and Tunka rift basins and in the northeastern flank of the Baikal Rift system have been divided into five activity classes (low, medium, relatively high, high, and very high activity) and mapped correspondingly. It has been recommended that the concept of a hazardous fault, as updated with regard to the activity rating, refer to faults of relatively high, high, and very high activity. Thus identified hazardous faults within the study area are quite few (4–8 %), though this percent may increase slightly as more input data become available. The underestimation cannot be dramatic because all known seismological and structural characteristics of faults essential for the activity rating have been already taken into account. The new approach may be useful in seismic risk assessment and in choice of sites for instrumental monitoring of seismicity.

The article discusses the effect on the measured signals of a multicoil electromagnetic tool offset with respect to the borehole axis. The tool is a nonconductive cylindrical body with several coaxial coils. It is assumed that the borehole is vertical and the medium is axisymmetric with respect to the borehole axis.
A number of computations based on the finite-element method were carried out. Simulations took into account the tool body and finite sizes of coils. A wide range of transmitter-receiver spacings (0.18–1.0 m) was studied. The operating frequency was 1.75 MHz. Different drilling muds and values of the tool offset were examined. It was shown that, for highly conductive drilling muds, the type of dependence between apparent resistivity and eccentricity varies greatly with the array length. It was discovered that increasing the contrast between invaded zone resistivity and mud resistivity also increases the eccentricity effect. The eccentricity effect increases with the decrease in mud resistivity and decreases as the array length increases. Also analyzed was the effect of the tool offset on electromagnetic field pattern.