Siberian Research Institute of Geology, Geophysics and Mineral Resources

Over the past two decades, significant geological exploration work, including seismic exploration, has been carried out within the junction zone of the Vilyui syneclise and the Aldan anteclise. In addition, oil and gas companies are also interested in this territory, acquiring licenses for the search and exploration for hydrocarbon deposits. The object of the study is the terrigenous deposits of the Permian system. The main oil and gas potential of this territory is associated with these Permian deposits. Within the Vilyui syneclise, there are gas fields in the Permian deposits, which provide gas to the central regions of Yakutia. Thus, it is necessary to update the oil and gas potential of the area. The study was based on seismic exploration data, deep well data, including well logging curves, core descriptions, and Permian sediment test results. Previously published literature on the geological structure of Permian rocks was also analyzed. Our seismic interpretation and analysis of well data resulted in structural surfaces. The structural surfaces characterize Permian, Triassic and Jurassic sediments. The thickness maps have been used to determine the distribution areas of Permian deposits, Triassic and Jurassic fluid-sealing rocks. Using structural maps, thickness maps, faults and well data, the most oil and gas promising zones and objects of Permian deposits were identified. The promising zones are associated with a reduction in the thickness of Permian deposits, and identified near the wedge line. The fault-limited positive structures are promising objects.

In Western Siberia, the zone of abnormally high reservoir pressures (AHRP) covers an area of more than 500 thousand km2 in the north of the basin. It begins with a clay layer above the Achimov formation of Neocomian sandy-silty formation, covers the Achimov formation, Upper–Lower Jurassic, Triassic and partially Paleozoic and is subject to tectonic control, which indicates the deep origin of this phenomenon. Gas-pressure, or gas-dynamic, theory of AHRP, proposed by K.A. Anikiev in the 70s of the 20th century, allows us to assess the prospects for oil and gas content of the AHRP zone higher than it is commonly-accepted. Analysis of the results of previously completed geological exploration work on the deep horizons of the Yamalo-Nenets Autonomous Okrug indicates that their relatively low efficiency (50–60%) is associated with the insufficiently high quality of well operations, primarily cementing, which is also due to the influence of AHRP. In all wells drilled to deep horizons, direct signs of oil and gas potential were obtained and cementing defects were identified. It is concluded that deep fluid-dynamic processes (active, pressure degassing of the Earth’s interior) are responsible both for the saturation of reservoir rocks with hydrocarbons and for the dynamics of their filling (ultra-high pressures and velocities), which determine the main characteristics of reservoir rocks. Recognition of a deep source of hydrocarbons will not only make it possible to fundamentally increase the resource base of the AHRP zone, but will also require a revision of ideas about the formation and structure of hydrocarbon deposits in this zone and the petrophysical substantiation of their models. However, to realize the unique hydrocarbon potential of the AHRP zone, it is necessary, first of all, to improve the quality of deep wells construction and appropriate information content.

Результаты количественной оценки ресурсов нефти

This study is concerned with an analysis of seismicity and deep structure in the Transbaikalia along the 1-SB reference geophysical traverse. We have found a complex inhomogeneous structure of the crust and upper mantle. The crustal thickness varies between 40 km in the southeastern part of the traverse and in intermontane troughs in its northwestern part on the one hand and 48 km in mountain ranges. Strong variation also affects boundary velocities at the Moho, ranging from 8.4‒8.5 km/s for compressional waves and 4.9‒4.95 km/s for shear waves (especially in the southeastern part of the traverse) to lower values of 7.8‒8.0 km/s for compressional waves and 4.6‒4.7 km/s for shear waves in the area of the Baikal Rift Zone in the northwestern part of the traverse. A strongly inhomogeneous earth structure based on elastic wave velocities, Vp/Vs velocity ratios, and Poisson’s ratio was found for upper and middle crust at depths of 8‒20 km. It was also found that zones of higher seismicity tend to coincide with crustal blocks with inhomogeneous velocity structure based on differently polarized compressional and shear waves. Higher inhomogeneity in upper crust as inferred from elastic wave velocities and secondary earth parameters (Vp/Vs velocity ratios, the parameter K* = Vp/(γ ‒ 1), where γ = Vp/Vs, and Poisson’s ratio (σ)) characterize the area of the Baikal Rift Zone in an immediate vicinity of the great Muya earthquake of 1957 with М = 7.6. As well, several other deep zones of inhomogeneity have been identified along the traverse line based on anomalies of Р and S velocities and secondary earth parameters that correlate to varying degrees with seismically active areas based on multiyear instrumental observations. We have identified an unambiguous relationship of large inhomogeneous zones in the Transbaikalia crust with stress buildup and stress release in the shape of large earthquakes, thus substantiating the intermediate-term prediction of catastrophic events.

Abstract —The Tuvinian trough is one of the large grabens of the rift system formed in the Devonian–Carboniferous in the eastern part of the Altai–Sayan fold area. Based on the results of comprehensive studies, the age was refined, and the geochemical features of igneous rocks formed during two stages of tectonic and magmatic activity within the Tuvinian trough were studied. In the Early Devonian (397 Ma, Emsian), at the stage of the initiation of the Tuvinian trough in the stretching setting, the volcanic and subvolcanic rocks of the Kendei Formation formed, which make a bimodal series. The Early Devonian igneous rocks of mafic composition have geochemical features of both intraplate (low values of Mg#, high contents of K2O (up to 2.9 wt.%) and TiO2 (up to 2.2 wt.%), and enrichment in LREE relative to HREE) and suprasubductional (enrichment in Pb and Sr and depletion in Ta and Nb) formations and are characterized by high values of εNd(T) (+5.9 to +8.0). They are assumed to have formed from a mixed source including the depleted mantle and components modified by subduction. The Early Devonian felsic volcanic rocks, which are the extreme member of the bimodal sequence, also combine the geochemical features of rocks of intraplate (high Fe, low Sr, P, and Ti contents, Zr and Hf enrichment) and island arc (Ta and Nb depletion) origin. These rocks with εNd(T) values from +4.0 to +7.0 resulted from the melting of a heterogeneous source corresponding in composition to the lower continental crust. In the Middle Devonian–early Carboniferous (390–350 Ma), the Tuvinian rift trough evolved into a mature stage, at which the mafic rocks of the Torgalyk complex were intruded. The Middle Devonian–early Carboniferous mafic rocks are similar in isotope and geochemical characteristics, including the Nd isotopic composition (εNd(T) = +6.7), to the Early Devonian formations. In contrast to the Early Devonian rockes, the magmas for the Middle Devonian–early Carboniferous mafic rocks were generated a relatively homogeneous mantle source without significant metasomatic transformations, the features of which are better manifested in the Kendei rocks.