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Petrographic and geochemical studies of igneous and volcaniclastic rocks of the middle part of the Pronchishchev Ridge and the Terpei lowland provided new data on their composition and structure. The subdivision of the Permian-Lower Triassic strata enclosing igneous and volcanicalstic bodies has been clarified by lithological/stratigraphic methods. The Lower Triassic Ulakhan-Yuryakh, Chekanovsky, Ystanakh, and Pastakh formations have been traced for the first time in the ridge area. It is shown that the studied dolerite body and adjacent breccias in the field of middle-upper Permian rocks are part of the volcanic pipe. The tuffite sand gritstones in the field of Lower Triassic rocks are parts of an eroded tephra ring around this diatreme. The confinedness of the tuffite bodies to the Ystanakh Formation makes it possible to determine the age of the pipe as the beginning of the late Olenekian. The obtained ⁴⁰Ar/³⁹Ar dates for the dolerites of the volcanic pipe and one of the dikes of the Pronchishchev dike belt indicate the repeated tectonomagmatic activity of the studied area in the Triassic.

We document the evolution of A-type granitoid magmatism during the Cryogenian tectonic transformation of the Yenisei Ridge from a postcollisional mode to the early stage of development of an active continental margin. We illustrate the A-type granitoid magmatism evolution in a model for the emplacement and cooling of the intrusions of the Strelka pluton, reflecting the final stage of magmatism during the formation of the postcollisional Glushikha complex (719-702 Ma). These processes took place at the same time as the formation of mantle, mantle-crustal and crustal rocks of the Tatarka complex (711-683 Ma), including the Yagodka pluton A-type granites (711-705 Ma) during the early stage of active continental margin development. During this period of tectonic transition, both convergent events involved the continuous formation of felsic intrusions corresponding to oxidized A-type granites.

The paper presents data on the composition and age of mafic rocks of the shoshonitic series in the Irkut block of the Sharyzhalgai uplift (southwest of the Siberian craton). According to the U-Pb dating of magmatic zircon, monzodiorites of the Poludennyi massif and gabbro-dolerites in the endo- and exocontact zones of the Toisuk pluton formed 1.87 and 1.86-1.85 Ga, respectively. The intrusion of mafic magmas and their underplating into the basement of the crust under postcollisional extension resulted in the near-coeval mafic and granitoid magmatism in the Irkut block between 1.87 and 1.84 Ga. The Paleoproterozoic mafic associations belong to the shoshonitic series and are characterized by enrichment in incompatible elements, including Zr, and low negative ε_Nd(T) values. These geochemical and isotopic characteristics point to the magma derivation from a long-lived enriched mantle source, such as the subcontinental lithospheric mantle. The crystallization of zircon from the last portions of the evolved mafic melt is evidenced by low zirconium saturation temperatures (710-965 °C) and zircon enrichment in U and Th with increasing Th/U, reflecting the increase in the concentrations of highly incompatible elements in the residual melt.

The Precambrian-Cambrian transition is one of the most fundamental evolutionary turnovers in the Earth’s history. However, in many cases precise identification and correlation of this crucial boundary in lithologically contrasting sections is complicated, even if the whole spectrum of biostratigraphic and chemostratigraphic methods is applied. The Precambrian-Cambrian transitional strata of the Igarka Uplift (Sukharikha Formation) perfectly illustrate this problem. This unit has arguably one of the most detailed carbonate carbon isotope curves for this time interval (at least within the Siberian Platform). However, an extremely poor paleontological record previously reported from the Sukharikha Formation makes identification of major stratigraphic boundaries highly debatable. We present a detailed study of the Sukharikha and overlying Krasnyi Porog formations in three sections (Sukharikha River, Kulyumbe River, and Khantaisko-Sukhotungusskaya-1 well). Our chemostratigraphic and biostratigraphic data provide a correlation basis for these sections and identify the Cambrian boundary in terms of both the International Chronostratigraphic Chart and General Stratigraphic Scale of Russia. We show that lithologic boundaries and levels of the local first appearance of Tommotian small skeletal fossils are diachronous within the Igarka paleobasin. The latter is putatively caused by paleoecology of the early Cambrian biota and by taphonomic factors. Our data specify the location of the Tommotian Stage (sensu lato)/Cambrian Stage 2 boundary in other key Precambrian-Cambrian transitional sections of the Siberian Platform.

The paper presents the results of the study of organic geochemistry of carbonate rocks, silicites, and calcareous mudstones of the Vendian (Ediacaran) Khatyspyt Formation in the northeast of the Siberian Platform. The distribution of dispersed organic matter (OM) is shown to be uneven. The formation comprises carbonate-dominated intervals depleted in OM, with total organic carbon content (TOC) < 0.1%, or weakly enriched in OM (TOC up to 0.4%), interstratified with carbonate-shale, carbonate-shale-siliceous, carbonate-siliceous and siliceous intervals enriched in OM (TOC up to 1-4%). There is also a thin-layered calcareous mudstones anomalously enriched in OM (black shales with TOC at 10%) cropping out in the Khorbusuonka River valley (unit thickness 10 cm). The redox conditions in the water column varied during deposition of the Khatyspyt Formation. Certain intervals characterize a stratified water column and euxinic bottom water conditions. During the study, the assessment was conducted of the OM catagenesis and the generative potential of the Khatyspyt Formation. Based on the results of geochemical study and the synthesis of information on the geological structure of sedimentary succession in the northeastern Siberian Platform, the Khatyspyt Formation has been evaluated as one of the hydrocarbon sources for the territory.

We present results of measurements of the physical properties (thermal conductivity, porosity, permeability, and density) of 65 air-dry sedimentary-rock samples from the cores of six deep wells drilled in the Lena-Anabar interfluve. The rocks are compact low-porosity, almost impermeable siltstones, sandstones, and dolomites mainly of Paleozoic and Precambrian ages. Correlations of thermal conductivity with porosity and bulk density have been established. The available information about the thermal conductivity of rocks as well as the production thermograms recorded after drilling made it possible to estimate the geothermal gradient and heat flow ( q ) for the Ust’-Olenekskaya-2370, Charchykskaya-1, Khastakhskaya-930, and D’yappal’skaya-1 wells. The gradient was calculated from the temperature values at the lower boundary of the permafrost (0 ºC) and at the bottom-hole. The determined heat flow varies from 37 to 70 mW/m². These q estimates are consistent with the available data on the distribution of heat flow in the north of the Siberian Platform. The proposed method for heat flow estimation is worthy of use in other northern regions of Siberia for obtaining more geothermal data.

The work considered an integrated approach to the study of deep-lying Riphean strata of the Buolkalakh-Lena region, which have certain prospects in terms of oil and gas potential. The method of revision and synthesis of acoustic and density logs is described in detail using the example of wells drilled in 80-90 for the purpose of correct reference to seismic sections. It has been shown that the Khaipakh, Debengdin, Arymas, Kyutingdinsky, and Sygynakhtakh formations of the Early and Middle Riphean eras overlook the pre-Permian surface in the middle part of the Lena-Anabar trough. The region also developed older Riphean deposits that are not exposed on the Olenek uplift and are not studied by deep drilling. The prospects for the oil and gas content of Riphean strata in the region are mainly associated with carbonate reservoirs formed in the top of the Riphean sequence and cropping out on the pre-Permian erosive surface, the most promising rocks are those constituting large carbonate platforms that were partially eroded and subjected to processes of hypergenesis and karst formation.

To study the effect of tilted uniaxial electrical anisotropy parameters on electric and electromagnetic logging signals, their numerical simulation and comparative analysis in homogeneous and layered models have been performed. An algorithm for three-dimensional numerical simulation of resistivity logging signals in spatially heterogeneous models with tilted principal axes of the electrical conductivity tensor, based on the finite-element method, was used in the calculations. The obtained results indicate the potential for a more accurate estimation of electrophysical properties of fractured carbonate reservoirs of the pre-Jurassic basement of West Siberia, which are characterized by the tilt of the electrical conductivity tensor principal axes.

The integral Sumudu transform was established as an alternative to the Laplace transform in the 1990s. The Sumudu transform fundamental properties include preservation of the dimensionality of a function, ensuring measurement units both in the function and its image to be equivalent. Among its disadvantages is the absence of an explicit formula for calculating the inverse transform. The transforms are inverted by solving the corresponding Fredholm integral equation of the first kind, which is reduced to solving an ill-conditioned system of linear algebraic equations. We apply Tikhonov’s method for regularization of this type system. The paper proposes a technique for constructing a parametrized regularizing matrix that takes into account the properties of the Sumudu images obtained by modeling the electromagnetic sounding process of the Earth’s interior. A method for choosing the Tikhonov regularization parameters and the regularizing matrix is considered. The effectiveness of the proposed method for the Sumudu transform inversion is examined on a model problem of electromagnetic sounding of the Earth’s interior by a measurement system consisting of two loops spaced apart.

This paper describes a three-dimensional inversion of magnetovariational tippers calculated for the Dalat observatory in South Vietnam on the basis of the geomagnetic variation records obtained from the INTERMAGNET network. The inversion of the frequency dependences of the tippers is performed using the ModEM software, which makes it possible to construct a geoelectric model of the Dalat zone in a 300 × 300 × 150-km spatial region along the x , y , and z axes, with its center located at the observatory. The resulting model of the geoelectric section contains both surface and deep conductive blocks located at the top and bottom of the Earth’s crust and in the upper mantle. The central part of the upper local conducting block with a resistivity of 0.5-1.0 Ohm·m is located near the observatory at a depth of 12-14 km. A more massive conductive block is located to the east of the observatory in the coastal region. Its central part with a resistivity of 0.3 Ohm·m is located at a depth of 24-28 km and extends under the continent along the coast for a distance of about 80 km. The roots of this block are visible to depths of ~100 km in the South China Sea shelf region. It is assumed that the high electrical conductivity of these blocks is provided by melting in the presence of aqueous fluids and granite series rocks, which are widely distributed in the Dalat zone. The crystallization depths of the samples of these granites estimated from the geothermobarometry data are close to the depth intervals of conductive blocks in the model section, which was also observed in the interpretation of magnetotelluric soundings in Southern Tibet, where granitoid rocks are widespread.