Home – Home – Jornals – Russian Geology and Geophysics 2022 number 7

We present results of new mineralogical, geochemical, geochronological, and isotope (Sm-Nd, Rb-Sr, and O) studies of the volcanic rocks of the Tyya complex in the Olokit trough. These are normal tholeiitic basalts and scarcer basaltic andesites forming a fractionated series with mg# = 45-65, medium TiO2 contents (0.73-1.62 wt. %), low P2O5 contents (0.04-0.25 wt. %), and a significant domination of Na over K (Na2O/K2O = 2.1-50.0). The rocks are metamorphosed to greenschists, which are composed of chlorite, actinolite, epidote, and albite with quartz, titanite, ilmenite, and magnetite impurity. The metabasalts have is an age of 915 ± 5 Ma (zircon U-Pb dating) and are characterized by wide variations in εNd(T) (-3.5 to -11.9) and 87Sr/86Sr (0.70602-0.70732) and high δ18O values (9.0-15.2 ‰) as compared to the mantle ones. According to the isotope-geochemical characteristics, the studied metabasalts have features of both IAB and E-MORB. The Tyya metabasalts might have resulted from the melting of the lithospheric mantle with a subductional component. Comparison of the studied rocks with volcanic rocks of recent geodynamic settings shows their similarity to basalts of back-arc basins. The Tyya metabasalts might belong to a back-arc basin of the late Mesoproterozoic Nyurundukan island arc system.

In the Central Asian Orogenic Belt, Li-F granites formed in magmatic areas of different ages within a wide time interval, 321-134 Ma. The Li-F granites compose large multiphase plutons (Janchivlan and Baga-Gazriin Chuluu) and small intrusions, have specific mineralogic and geochemical characteristics, and show enrichment in Sn, W, Li, Rb, Ta, and Nb, thus forming concentrated mineralization at the late magmatic and postmagmatic stages. The late Paleozoic and Mesozoic small intrusions (Kharagul, Urugudei, Bezymyanskii, Henteyn, and Turga) are high-alkali Li-F granites enriched in Zr, Nb, Hf, Th, U, and REE, which differ from ore-bearing Li-F granites in mineral assemblages and geochemical features. Such granites can be classified as an individual subtype of rare-metal granites. Irrespective of geochemical characteristics, the Li-F granites of the studied provinces in Central Asia are geochemically different from typical collision-related granites resulting from the melting of the upper continental crust. It is shown that the formation of rare-metal magmas with different geochemical characteristics is related to the mantle-crust interaction. The geochemical evolution of Li-F granites is significantly contributed to by the activity of mantle fluids containing trace elements and by the differentiation of granitic magma in the crustal intermediate chambers, which is favorable for the formation of associated rare-metal mineralization.

The Kopto deposit (northeastern Tuva) is assigned to gold ore objects with a combination of the Au-Cu-skarn and superposed quartz-gold-sulfide stockwork types of mineralization. From the surface, the ores underwent intense oxidation, which formed a zone of secondary gold enrichment, containing a supergene paragenesis with gold and silver chalcogenides and newly formed gold. The depth of distribution of oxidized ores from the surface is 80-90 m. The Au content varies from fractions of ppm to 150 ppm (on average, 30.8 ppm). Using computer thermodynamic modeling, it is shown how the ore gold-sulfide-quartz association transformed under oxidizing conditions with a decrease in the pH of solutions. Gold becomes more and more high-grade; acanthite appears and disappears; limonite prevails (pH = 1.65; Eh = 0.69 V). The conditions for the stability of pyrite, iron hydroxides, and gold and silver chalcogenides (petrovskaite (AgAuS) and uytenbogaardtite (Ag₃AuS₂)) have been estimated. It requires weakly acidic solutions with pH = 5-6 and Eh values close to zero, which ensures the stability of thiosulfate and hydrosulfide complexes of noble metals. The main difference between solutions in equilibrium with petrovskaite and uytenbogaardtite is the Ag/Au ratios, which are maximum in the first case and approximately equal in the second. The paper is concerned with a comparative analysis of the morphologic features of gold from primary and oxidized ores of the Kopto gold deposit. The aim of this work is to identify a set of signs of the supergene nature of gold and to assess the extent of its redistribution.

Electrical measurements in water areas, as well as on land, are carried out to study the electromagnetic (EM) properties of geologic objects. The specifics of aquatic electrical prospecting are associated with the specific influence of a water column. We present numerical calculations of the EM signal of the transient process for electrical lines in the axial region of the source in 50-250 m deep sea waters in order to demonstrate the manifestation of induced polarization (IP) in the transient-process signal at different setups, to reveal the differences in the manifestation of galvanic (GP) and double induced (DIP) polarization in the transient-process signal, and to explain the causes of these differences. To study the influence of the size of the setup on the manifestation of IP during the transient process, we analyzed the changes in the signal of the transient process (Δ U ( t )), in the finite difference between the signals of the transient process (Δ² U ( t )), and in the transform P1( t ) (the ratio of these values) for a horizontal electrical setup with a 50 to 2000 m long source ( AB ) and a 50 to 2000 m long three-electrode measuring line ( M ₁ M ₂ M ₃) and with the distance between the centers of the source and the measuring line M ₁ M ₃ (spacing, r ) from 100 to 4000 m. Some of these parameters are used in differential-normalized electrical prospecting (DNEP). Comparison is made for Δ U ( t ), Δ² U ( t ), and their transforms in the conducting and conducting polarizable models under the same conditions. The setup was placed on the surface and inside a conducting medium (a sea shelf water column) with a conducting polarizable base (geologic medium (earth) covered with a water layer). The polarizability of the base was taken into account by introducing a frequency-dependent resistivity, using the Cole-Cole formula. The performed calculations show that the components of the transient process that are associated with the formation of the EM field and with GP and DIP manifest themselves in different ways at setups of different sizes at different depths. In water area, IP manifests itself in two ways, being associated with both galvanic and eddy currents. In previous practical measurements, DIP was considered to be associated with interference, but this signal is simulated and can be regarded as the information about IP. The factor influencing the IP manifestation in the transient-process signal is the reduced setup height ( h _Δ), i.e., the distance between the setup and the sea bottom (the polarizable base of the model) referred to the AB length. Depending on the reduced setup height, the IP signal in the transform P 1( t ) can manifest itself as an ascending branch at later times or can appear as a descending branch passing into the negative values of P 1. The duration of the pulse impact and the measurements of the transient process affect the contrasting manifestation of the polarizable base in the signal, but the measurements under setup towing impose certain restrictions. The optimal parameters of EM survey for studying IP should ensure a sufficient polarization range and the proper quality of measurements. The software used in the calculations was developed by OOO Sibirskaya Geofizicheskaya Nauchno-Proizvodstvennaya Kompaniya.

We present results of experimental measurements of the complex dielectric permittivity (CDP) of sand and powdered quartz granules with mass-averaged particle sizes of 5 to 250 μm, saturated with distilled water and NaCl solutions with a conductivity of up to 0.77 S/m. The CDP spectra were measured in the frequency range from 1 kHz to 8.5 GHz. The spectra at frequencies above 1 GHz were simulated using a refractive three-component mixture model. In the frequency range from 10 kHz to 1 GHz, the CDP spectra were simulated by three relaxation processes, using the Debye and Cole-Cole formulas. It is shown that the conductivity of the sample at a frequency of ~5 kHz is not proportional to the conductivity of the saturating solution. The intensities of two low-frequency processes are more intimately correlated with the conductivity of the sample than with the volume fraction of the solution. The relaxation times of the low-frequency and high-frequency processes are statistically correlated with the average particle size, and the relaxation time of the low-frequency process is additionally correlated with the specific surface area of the particles. We estimated the conductivity of the saturating solution at which the correlation is the strongest.

Calibration experiments of magnetic resonance sounding (MRS) were carried out on the ice of the Ob’ reservoir. For the first time, a comparative quantitative analysis of the contribution of electromagnetic shielding and the Bloch-Siegert effect to the magnetic-resonance sounding signal was carried out, and the asymmetry of this contribution when detuning from the resonant frequency was investigated. The results of theoretical calculations without regard to the Bloch-Siegert effect differ about three times from the experimental data at the maximum intensity of the radio frequency pulse for the experimental data. At the same time, when taking into account the interference of electromagnetic shielding and the Bloch-Siegert effect, the results of calculating the amplitude and phase of the signal best approximate the field experimental data. Based on the good agreement between the experiment and the theoretical calculations taking into account the interference of electromagnetic shielding and the Bloch-Siegert effect, it is demonstrated that this model should be used to solve the direct and inverse problems of magnetic-resonance sounding of underground aquifers. The results obtained allow us to calibrate the MRS method and use it not only as an indicator of underground water but also as a means of quantitative measurement. The necessity of taking into account the Bloch-Siegert effect in real field conditions is demonstrated by the example of MRS studies in the Ebro River valley (Spain). When taking into account only electromagnetic shielding (without regard to the Bloch-Siegert effect), the signal amplitude differs nearly twice at the maximum pulse intensity for experimental data, and the signal phase differs 12 times at the maximum pulse intensity. Therefore, in previously published works, approximation of the experimental data using the model without regard to the Bloch-Siegert effect was made by adding layers at depths close to the maximum for this type of antenna (approximately 50 m). In reality, however, these layers do not exist; they appear only as an artifact of the model without regard to the Bloch-Siegert effect. The best approximation of the experimental data can be achieved by taking into account the interference of electromagnetic shielding and the Bloch-Siegert effect.

This paper presents a schematic summary of comprehensive analysis of seismic, reflection profiling, and hydroacoustic data on faults which caused sediment deformation in the central segment of the Central Baikal basin. According to the tectonophysical analysis results, the fault pattern within sediment fill has been recognized as zone-block, i.e., it represents a network of high-density fracture zones limiting weakly deformed blocks. The structure of large NE-trending fault zones (Olkhon, Beregovoy, Gydratny, and Svyatoy Nos) is controlled by main fault planes (or their segments) bounded by subsidiary faults. Geomorphic expression of NW cross faults in the sedimentary cover as broad zones of smaller-scale fractures accounts for early stages of the evolution of basement faults. In a longitudinal direction, they divide the basin into large fragments. The zone-block structure of the sedimentary strata was developed in different stress regimes: strike-slip and extension at the early and late orogenic rifting stages, respectively. At the modern stage of tectogenesis, the established network of fault zones controls the gaseous (including hydrate formation) and seismic activity expression in the subsurface. Hydrate-bearing mud volcanoes and seeps are confined to major faults, while earthquake epicenters are confined to fault zones and form clusters at junctions of large NE-trending faults with NW-oriented extension zones and E-W left-lateral strike-slip faults.

Dynamic models of the seismic, geological, and flow characteristics of a reservoir are the main tool used to evaluate the potential of drilling new infill wells. Static geological models are mainly based on borehole data combined with dynamic analyses of production dynamics. They are used to determine the redevelopment of and adjustments to new drilling locations; however, such models rarely incorporate seismic data. Consequently, it is difficult to control the changes in geological models between wells, which results in the configuration of well positions and predicted results being less than ideal. To improve the development of adjusted areas in terms of their remaining oil contents, we developed a new integrated analysis that combines static sediment modelling, including microfacies analysis (among other reservoir and seismic properties), with the production behaviour. Here, we illustrate this new process by (1) establishing favourable areas for static geological analysis; (2) studying well recompletion potential and the condition of nonproducing wells; (3) conducting interwell analyses with seismic and sedimentary data; (4) identifying potential sites constrained by seismic and geological studies, as well as initial oilfield production; (5) providing suggestions in a new well development plan.