Home – Home – Jornals – Russian Geology and Geophysics 2023 number 6

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.

Interpretation of magnetotelluric data within the North Caucasus marginal massif, the uplift of the Main Range, and the Stavropol Arch, performed on the basis of one- and two-dimensional inversions of the impedance tensor components and three-dimensional mathematical modeling, makes it possible to compile the starting and test models that are necessary for a three-dimensional inversion. It is shown by the three-dimensional inversion of all the impedance tensor components, carried out on a test three-dimensional model, that it is possible to estimate the parameters of conducting model blocks. The three-dimensional inversion of all the experimental impedance matrix components, performed with account for the results obtained on the test three-dimensional model, significantly corrects the parameters of the conducting blocks identified at the stages of one- and two-dimensional inversions, as well as the three-dimensional mathematical modeling of magnetotelluric fields in the central part of the North Caucasus. In the resulting three-dimensional geoelectrical model of the region, the position of low-resistance blocks correlates with the location of suture zones, deep faults, volcanic chambers, and domains with converted earthquake waves, whose velocities are reduced and whose absorption is increased. This can be explained by the dependence of the conductivity of crustal blocks on water saturation. Earthquake hypocenters are grouped near low-resistance anomalies.

We provide an analytical assessment of electrical and seismic signals of the seismoelectrical effect over a hydrocarbon deposit in the sea with seismic wave excitation using pulsed nonexplosive sources and reception by automatic underwater vehicles. The hardware complex required for the implementation of the method is described. Methods for suppressing reverberation interference are considered.