Home – Home – Jornals – Russian Geology and Geophysics 2018 number Неопубликованное

There is the Tira Horizon, represented by the Chistyakov, Omoro, and Oskoba Formations in different facies zones, widespread in the central and southwestern parts of the Siberian Platform. Deposits of the Tira Horizon in most sections of the Southwestern Siberian Platform are differ significantly in lithology from the underlying Nepa and overlying Danilov Horizons. In this paper considers the most complete section in the eastern part of the Angara Folded Zone, which serves as a key section. There are three main transgressive-regressive cycles, separated by sequence boundaries, are noted in the Tira interval. Three members correspond to these cycles. The first member records an abrupt change in facies from coastal to deep-sea, containing organic-walled microfossils of the first assemblage with Vanavarataenia insolita. The second member, formed in a coastal marine environment, exhibits an increase in carbonate and sulfate content, the appearance of chert lenses and nodules with microfossils, and the second assemblage with Tinnajaphyton amplum (lower submember). The third member is characterized by upper littoral, sabkha, and continental-deltaic deposits. The identified associations of microfossils, along with the features of the lithological composition and isotopic characteristics, allow them to be used as benchmarks in regional constructions on the Siberian platform, and possibly also beyond its borders. A carbon isotope anomaly of up to -11.3‰ (EPDB) was detected in dolomites of the Oskoba Formation at the Kamo Arch of the Baikit Anteclise. This may indicate a restructuring of the ecosystem in the paleobasin, accompanied by the appearance of certain microbiota groups and points to global climate change.

The problem of forecasting the filtration-capacitance properties (FCP) of target horizons (porosity, effective thickness, oil saturation) using methods capable of learning on large volumes of heterogeneous data is considered. 3D seismic data, both full-stack (seismic attributes extracted from a seismic cube) and partial-stacks (synchronous inversion), are used, limited to the target horizons interval, obtained at one of the offshore fields in the north of Sakhalin. The large data volume and the strong interdependence of the extracted features required dimensionality reduction of the seismic parameters. The novelty of the developed approach lies in the application of factor analysis prior to performing FCP forecasting. As a result, 92% of the data variations associated with reservoir properties were represented by five main factors. Subsequent application of neural networks to the forecasting problem demonstrated that the identified factors are significant predictors of porosity, oil saturation, and effective thickness. Cross-validation identified zones with low accuracy in FCP estimation – resulting from the strong influence of gas on seismic data – as well as zones with high accuracy in FCP estimation, which were subsequently used to determine optimal weights and the final forecasts. Based on the results of FCP forecasting using factor analysis and neural networks, correlation coefficients at test well locations ranged from 0.92 to 0.93, and forecast accuracy – verified following the drilling of two production wells – fell within the 85–93% range, thereby demonstrating the effectiveness of the proposed technology.

The article presents new data on a complex study of the composition of quartzites and gneisses of the Olkhon terrane. It has been established that the quartzites are typical metasedimentary rocks characterized by Cr–V mineralization—derived from volcanic exhalations—that is unevenly distributed throughout the stratigraphic section. The presence of organic matter in the sedimentary basin facilitated the precipitation of vanadium and chromium, as well as their subsequent sorption by clayey-organic material. The presence of quartzites with Cr-V and manganese mineralization in a single section may indicate alternation of oxidizing and reducing conditions in the sedimentation basin and the active tectonic regime during precipitation accumulation.
The morphological features of zircon from quartzite and its rare element composition suggest its age ~516 Ma as a result of a tectonothermal event, which contributed to the recrystallization of siliceous sediments with the formation of Cr-V mineralization (garnets, mica-roscoelites, chromium-vanadium oxides). The youngest population of detrital zircon from a biotite gneiss sample is 538 Ma, indicating that the sedimentary sequence formed within a narrow time of 538-516 Ma.

Quartz gabbroic rocks are generally considered as a rare variety of mafic plutonic rocks. Their occurrences have been reported, among other settings, in Precambrian gneiss complexes. This study presents the first detailed petrological investigation of quartz gabbronorites from two small intrusive bodies hosted within the gneissic sequence of the Kem-Ludy area of the Archean-Paleoproterozoic Belomorian Mobile Belt (BMB), Karelia. The studied quartz gabbronorites exhibit variable degrees of metamorphic overprint, ranging from low to complete. In two samples, primary magmatic minerals (clinopyroxene, orthopyroxene, plagioclase, and quartz) are preserved in significant amounts, while metamorphic alteration is locally developed. In contrast, garnet amphibolite from the marginal zone of the quartz gabbronorite body is completely devoid of primary magmatic minerals; only inclusions of relic clinopyroxene within garnet are observed. Geothermobarometric data and phase equilibrium modelling indicate that crystallization of the quartz gabbronorites occurred at temperatures of approximately 1100 °C and pressures of ~0.7 GPa, followed by subsequent subisobaric cooling to ~850°С. These estimates suggest that the rocks formed under lower- to mid-crustal conditions, either through fractional crystallization or in its absence. The results imply that the distribution of quartz gabbronorites within the Belomorian Mobile Belt may be significantly underestimated.

More than 30 years have passed since the current Working Stratigraphic Framework for the Kuril Islands was approved. Over this period, extensive age data (from diatom analysis, K-Ar dating, and other methods) have been obtained for the rocks composing the islands of the Greater Kuril Ridge, through both regional geological mapping and targeted research projects. In addition, the upper boundary of the Neogene was revised in 2011. Consequently, to synthesize these data, it became necessary to draw up an updated Neogene-Quaternary stratigraphic framework and a correlative intraregional chart for the magmatic rocks of the Greater Kuril Ridge. Based on comprehensive diatom biostratigraphy, radioisotope dating, and geological correlation of volcanic complexes, the following have been developed: (1) a new Unified Stratigraphic Framework for the Neogene-Quaternary deposits, and (2) a new intraregional correlation chart for the magmatic assemblages. The diatom and radioisotope data confirmed the absence of a stratified Paleogene–Early Miocene "Green Tuff Complex" previously recognized by researchers at the base of the exposed section of the Greater Kuril Ridge. Diatom analysis of the oldest stratigraphic subdivisions (the Rybakovsky Volcanic Complex and the Shumnovskaya Formation) of the Greater Kuril Ridge – specifically, the identification of the Thalassionema schraderi and Neodenticula kamtschatica diatom zones – indicates that the rocks composing the islands of the Greater Kuril Ridge began to form in the Tortonian (Late Miocene).