Home – Home – Jornals – Russian Geology and Geophysics 2021 number 12

We present results of U-Pb (LA-ICP-MS) dating of detrital zircons from the alluvial deposits of the Angarakan River (North Muya Ridge, northern Baikal region), whose drainage basin is composed mainly of granitoids of the Barguzin Complex, typomorphic for the late Paleozoic Angara-Vitim batholith (AVB). Three age clusters with peaks at 728, 423, and 314 Ma have been identified in the studied population of detrital zircons. It is shown that small outliers of igneous and metamorphic rocks, probably similar to the large AVB roof pendants mapped beyond the drainage basin, are the source of Neoproterozoic and early Paleozoic zircons. The late Paleozoic cluster comprises two close peaks at 314 and 285 Ma, which totally «overlap» with the time of the AVB formation and mark a granitoid source of the zircons. The results of detrital geochronology, together with the data on bedrocks, point to the prolonged (~40 Myr) formation of the AVB, but the intensity of magmatism during this period calls for additional study. Based on the analysis of published geological, geochemical, and geochronological data, we assume that the AVB resulted from the plume-lithosphere interaction that began in the compression setting and gave way to extension 305-300 Ma (the Carboniferous-Permian boundary), which caused replacement of «crustal» granitoids by granitoids formed from a mixed mantle-crustal source.

Based on the materials collected by L.N. Repina in the southern Hövsgöl area (Mongolia) in 1986-1988, we describe three sections and identify lower Cambrian trilobites. The sections of Mt. Hurtel Harna and along the Ujigin Gol River were studied earlier. We consider trilobites that have been found in these sections for the first time: Redlihia zharkovi , Lermontoviella shanganica , Erbia granulosa , Kootenia siberica , and Parapoulsenia lata . The section along the ravine of the Zuun-Shuvuutyn-Sair Brook and the findings of trilobites in it are described for the first time. The found new trilobites make it possible to correlate the coeval lower Cambrian strata in the southern Hövsgöl area, Altai-Sayan folded area, and western Siberian Platform and refine the age of the certain parts of the Egyin Gol, Ukhaa Tolgoi, and Ujigin Gol formations.

The Quaternary stratigraphy of the southern West Siberian Plain is considered in the context of the updated International Chronostratigraphic Scale, with the Neogene/Quarternary boundary at 2.588 Ma. New geological, lithological, paleontological, and paleomagnetic data from a reference Quaternary section in the Irtysh River valley near Isakovka Village provide more rigorous constraints on the Lower-Middle Pleistocene stratigraphy and the respective deposition conditions. The Isakovka outcrop exposes two stratigraphically expressed and paleontologically characterized units that were deposited during regional Pleistocene interglacial events of different ages: the Strunino and Serebryanoe alluvium beds lying over eroded surfaces. The species of the Corbicula genus coexisting with numerous species of Palearctic molluscs, small mammals Allophaiomys deucalion, Mimomys reidi , and last rooted lagurids Borsodia found in the Strunino alluvium are known also from continental equivalents of the warm upper Gelasian Stage in different parts of northern Eurasia. Judging by the presence of extant corbicules, the Strunino alluvium was deposited in a warm climate, with a mean annual air temperature above +16 °C and a mean winter temperature no colder than -8 °C, in rivers that remained free from ice all year round. On the basis of fauna constraints for the Serebryanoe alluvium, along with the first appearance of the European Pisidium clessini molluscs in West Siberia, the unit can be correlated with one of the earliest Middle Pleistocene interglacial events. The faunal assemblages and spore-pollen patterns of the two units indicate that the climate during the Serebryanoe deposition was slightly cooler and wetter than during the Strunino deposition, though steppe landscapes predominated on watersheds in both events. The two alluvial beds, which are traceable in river bluffs, at 30 m above the modern water level in a tectonically stable part of West Siberia, mark the hypsometrically higher position of the river network during the Early Pleistocene and early Middle Pleistocene interglacials.

Using image recognition methods (principal component method (PCM) and discriminant analysis) made it possible to group and identify more than 500 research objects developed in five oil and gas areas of the West Siberian oil and gas province (WSOGP), which are confined to 13 large tectonic structures and 10 productive horizons. The grouping was made according to 19 parameters characterizing the mode of oil and gas occurrence and the geophysical and physicochemical properties of the reservoirs and hosted fluids exerting a prevailing influence on the recovery of oil reserves and used on projecting the development of research objects. The performed study has identified 19 relatively homogeneous groups of objects, each having a specific set of geophysical properties. It is shown that the parameters reflecting the geophysical and physicochemical properties of the reservoirs and fluids within the identified groups of objects exert different effects on the recovery of oil reserves. This requires differentiation and grouping of the objects during the solution of various development problems. It has been established that the specific features of groups of objects are determined primarily by areal, tectonic, and stratigraphic factors and that grouping must be performed separately in each stratigraphic system. Algorithms are proposed for grouping the developed oil and gas fields and for searching for groups of analogous objects in fields out of exploration that are most similar to the developed ones. The performed grouping and the results obtained provide the necessary information about the research objects and increase its reliability, thus making possible to improve the efficiency of managing the oil company assets, i.e., the WSOGP oil fields.

We present results of an integrated research into the spatial distribution of methane in the area of the northern closure of the Central Basin of the Sea of Japan and in the southern part of the Tatar Trough. Methane emissions have been revealed in the study area. The methane fluxes are distributed unevenly within the area (1 to 23 mol/(km² day)). The discrete high-frequency measurements and calculation of methane fluxes at the water-atmosphere interface, combined with the study of the content of natural gases and microbiologic parameters in sediment cores, allow us to explain the formation of local methane emission zones in the water area. Despite the great sea depths, there are sources and fluid-conducting zones that determine methane concentrations (exceeding the equilibrium ones) and high methane emissions from the water area. The data obtained provide new information and suggest the presence of deep gas sources, which determine gas dispersion in the bottom sediments, the methane content in the surface water layer, and the distribution of methane flows at the water-atmosphere interface in the study area. This study is part of the integrated program of geological and geophysical expeditionary research performed by V.I. Il’ichev Pacific Oceanological Institute (Vladivostok) in the northern part of the Sea of Japan.

Results of study of natural waters of the Baidar valley (southwestern Crimean Peninsula) obtained during the 2018-2019 field works are presented. Major groundwater resources of the study area are confined to the Upper Jurassic aquifer complex, which serves as a recharge source for the aquifer systems of the Plain Crimean and the Azov-Kuban’ artesian basins and hydrogeologic folded region of the Crimean Mountains mega-anticlinorium. The regional waters are fresh and ultrafresh, predominantly of calcium bicarbonate composition, with TDS varying from 208 to 1269 mg/dm³. The study enabled their classification into eight geochemical groups: (1) waters of a regional fracture zone in carbonate-terrigenous rocks affected by continental salinization; (2) waters of a regional fracture zone affected by leaching of aluminosilicates and sulfide oxidation; (3) waters of a regional fracture zone dominated by sodium aluminosilicates in the fracture filling (long-term interaction in the water-rock system), affected by continental salinization; (4) regional fracture zones dominated by sodium aluminosilicates affected by anthropogenic continental salinization; (5) groundwaters in fracture-vein aquifers affected by leaching of aluminosilicates and sulfide oxidation; (6) fracture-vein aquifers affected by leaching of sodium aluminosilicates (long-term interaction in the water-rock system); (7) waters in fractured karst aquifers; and (8) surface waters subjected to continental salinization. Fracture karst waters, which were found to be most protected against human impact and continental salinization processes, are slightly alkaline (pH = 7.7), fresh (with average TDS = 444 mg/dm³), with low silicon concentrations (2.23 mg/dm³), and of calcium bicarbonate composition. Waters residing in regional fracture and fracture-vein zones are affected by continental salinization and anthropogenic load and are neutral to alkaline (pH = 7.1-8.6), predominantly fresh (TDS = 269-1269 mg/dm³), with average silicon concentrations of 4.61-4.70 mg/dm³, of calcium bicarbonate composition, with high concentrations of sulfate ion, magnesium, and sodium. The waters of the Chernaya River, Chernorechensk reservoir, and lakes, which are also affected by continental salinization, are slightly alkaline (pH = 8.3), brackish (TDS = 207-364 mg/dm³), with an average silicon concentration of 1.18 mg/dm³, of calcium bicarbonate composition, with high concentrations of chlorine ion, magnesium, and sodium. The calculated intensity of chemical-element migration in waters of the background composition follows the descending order: very strong, I_17.7 > Br_14.4; strong, Se_2.83 > B_2.22 > Sr_1.46 > Sb_1.12 > Be_1.07 > Hg_1.06; moderately strong, Zn_0.74 > Mo_0.50 > Li_0.46 > Sc_0.41 > Ag_0.18 > As_0.16 > Si_0.123 > Ba_0.122; weak, Cr_0.10 > Cu_0.096 > Bi_0.080 > Sn_0.068 > Tl_0.067 > P_0.062 > Ni_0.043 > Ta_0.040 > Ge_0.034 > Cd_0.028 > Fe_0.026 > Rb_0.024 > >Co_0.023 > Pb_0.020 > W_0.017 > V_0.012; very weak (inert), Nb_0.008 > Hf_0.0033 > Mn_0.0031 > La_0.0029 > Cs_0.0022 > Ti_0.0018 > >Ga_0.0016 > Y_0.0013 > Al_0.0008 > Zr_0.0008. All the studied waters are found to be of atmospheric origin and located along the global (GMWL) and local (LMWL) meteoric water lines. Their δ¹⁸O value varies from -9.9 to -3.3‰, and δD value, from -64.2 to -32.5‰. Sedimentary carbonate rocks, atmospheric carbon dioxide, organic compounds, and hydrolysis of aluminosilicate minerals serve as the source of δ¹³C bicarbonate ion in natural waters of the Baidar valley. Surface waters have a heavier carbon isotope composition (δ¹³C = -9.2 to -6.2‰), which is due to atmospheric CO₂, plant growth, and associated microbial activity. Fracture karst waters are characterized by a lighter carbon isotope composition (δ¹³C = -12.8 to -11.0‰) because of their interaction with dispersed organic matter. Waters of the regional fracture and fracture-vein zones display the widest variation in δ¹³C (-15.5 to -6.9‰), which is associated with a mixed type of «isotope supply» to the waters. A complex hydrogeochemical field that has formed in the Baidar valley tends to be increasingly affected by the anthropogenic factor.

We consider results of measurements of thermal conductivity, porosity, and permeability for 780 samples of Mesozoic sandstones and siltstones from the cores of 50 wells drilled in three southern regions of the West Siberian Plate (Novosibirsk and Tomsk regions, Surgut region of the Khanty-Mansi Autonomous Area). The thermal conductivity of the samples was measured twice: in dry and in water-saturated states. It has been established that the thermal conductivity of water-saturated rocks is on average 20-40 % higher. The thermal conductivities of dry and water-saturated samples show stable correlations between each other and with the sample porosity and permeability. These correlations can be used for the approximate estimation of the thermal conductivity of water-saturated rocks from the measured thermal conductivity of dry samples or even from the porosity values. The relationship between thermal conductivity and porosity can be used for the rapid assessment of rock porosity from the measured thermal conductivity of the core.

A mathematical model of the influence of the Earth’s magnetic field (the Hall effect) on results of the controlled source transient electromagnetic (TEM) method has been elaborated. For identification of this effect, we propose a schematic layout of the experimental grounded system with a pulsed loop source and signals recording by radial receive lines equally spaced relative to the loop. The 2018-2019 special field experiments were conducted in the Tatar region of the West Siberian Lowland with an aim to estimate the Hall effect contributions to the TEM method. To detect the Hall effect, transient electromagnetic responses were measured mainly by four receive lines radiating from a 500×500 m square loop. Analysis of the TEM results processing aimed at improving the signal quality and reducing the interference revealed a great similarity in signals from the radial lines, which is theoretically possible only under the Hall effect. Comparison of the field signals with the theoretical ones enabled estimation of the components caused by the Hall effect, in particular, conductivity at ~0.002 S/m.

Inductively induced electric polarization (IIP) is one of the aftereffects inherent in the geologic materials and affecting results of the transient electromagnetic method. Its effect on the inductive transient response manifests itself as a nonmonotonic EMF decay, including the polarity reversal. The dependence of IIP on many conditions makes it difficult to study the basic regularities in its manifestation. One of the ways to address this problem is to present the simulation results as a normalized transient response. From the most general point of view, the intensity and time range of the IIP manifestation are controlled by the competition between induction and induced polarization phenomena. Induced polarization manifests itself differently, depending on the transmitter used for the excitation of the ground response. Therefore, when studying polarizable ground, the results of the conventional IP method and those of the TEM method do not always correlate.