Home – Home – Jornals – Russian Geology and Geophysics 2019 number 2

A melt inclusion study was carried out in the leucite-bearing tephriphonolite and phonolite lavas of the Bolsena complex in order to obtain direct data on the chemical composition of initial melts, their evolution, and their crystallization temperatures. It has been found that the initial melt for the considered rocks was of tephrite-basanite composition. Its crystallization began with the formation of clinopyroxene phenocrysts at 1205-1100 °C, then leucite and plagioclase crystallization took place at about 1120 °C and 1080-1060 °C, respectively. The initial tephrite-basanite melt was slightly enriched in volatile components (H₂O, F, SO₃, and Cl). During the crystallization of clinopyroxene, leucite, and plagioclase, the composition of the initial magma changed toward an increase in the contents of SiO₂, Al₂O₃, and K₂O and a decrease in the contents of FeO, MgO, and CaO, i.e., evolved toward phonolite. A similar evolution trend is typical of alkaline basic systems. The tephrite-basanite melt was probably the product of the crystallization differentiation of the parental mantle magma similar in composition to the leucite-bearing tephrite-basanite of the Montefiascone complex.

We present data on the mineral and geochemical compositions of metasedimentary Mn-bearing rocks of the Itantsa Formation of the Ikat terrane. According to the mineral composition, the studied quartz-spessartine rocks of the formation are referred to as gondites. The main Mn-concentrating minerals are garnet, pyrophanite, and Mn-ilmenite, and the secondary ones are rhodonite and Mn-amphibole. Two Mn-bearing objects of the Itantsa Formation (Usutai deposit and Almarnatol occurrence) show differences in chemical and mineral compositions, related to different sources of their material, different distances from the mouth of a hydrothermal vent, and different degrees of postsedimentary alteration. The Mn-bearing deposits of the formation accumulated in a sedimentary basin in the immediate vicinity of continental provenance areas in the Late Neoproterozoic (Ediacaran), under synchronous volcanic activity.

The distribution and fractionation of rare-earth elements (REE) in the Northwestern Pacific surface sediments are studied. The REE contents in the sediments were 30-106 ppm, and the Y contents ranged from 9.34 to 24.5 ppm. The bottom sediments located near the Kuril-Kamchatka arc were depleted in REE as compared with the sediments of the abyssal plain of the Pacific, the Kuril basin of the Sea of Okhotsk, and the northwestern Bering Sea. The effect of distributive provinces and lithodynamic setting on the REE composition and REE contents in the sediments was expressed as a positive correlation of the LREE/HREE ratio with the grain composition, Rb/Sr, and Nb/Y and its negative correlation with Zr/Rb. The variations in the bulk REE composition were due to the variations in LREE contents.

The evolution of peat deposits of the Vydrino bog (southern Baikal region) and concentration of elements in them are discussed. The bog peat massif more than 4 m in thickness formed mostly during the Holocene. The beginning of peat formation dates back to the late Allerød (13.1 ka). At present, the Vydrino bog is a biogeocoenosis of the high-moor type with a transitional peat deposit. We have established that the bog nonuniformly accumulated chemical elements during its formation. Concentration of Pb, Sn, Cd, Zn, and Sb in recent vegetation and in the upper layer of the peat bog is mainly due to forest fires and anthropogenic air pollution. The anomalous enrichment of peat with Zn and Cu in the Early Holocene (12.1-8.8 ka) horizons proceeded through the periodic inflow of thermal groundwater into the bottom part of the peat deposit. Authigenic Zn and Cu sulfides formed on the inner membrane of the cell wall of sphagnum moss. Geochemical modeling has shown that Zn and Cu sulfides can form abiotically.

The paper considers present-day rock temperature distribution in the top of the Tyumen and Malyshevka Formations throughout the West Siberian sedimentary basin. A temperature variations map based on earlier and newly obtained data has been compiled for regional hydrocarbon reservoir Yu2. A forecast of the temperature variations in the Malyshevka Formation top has been made for the Arctic regions that have not been well studied with deep drilling. Analysis of hydrocarbon pools distribution within the Bathonian reservoir is presented.

Today there is no verified methodology to calculate oil reserves in the Bazhenov Formation, which makes it impossible to identify any references for the initial oil reserves. Therefore, the analog approach that is usually applied for quantitative assessment of petroleum potential becomes inapplicable. The paper considers different approaches to assessment of the petroleum potential of the Bazhenov Formation in West Siberia. Since oil distribution in the formation is determined by the initial content and catagenetic maturity of organic matter in the rock and controls the formation of an effective reservoir, it becomes apparent that estimating the oil resources and reserves requires geochemical studies. The proposed methodological approaches consider separation of net oil pays using the Rock-Eval data, well log data, and results of oil-promising objects survey and the principles of differentiated assessment of oil resources and territory ranking by the compositions of saturating hydrocarbons.

The paper considers the results of a series of laboratory experiments (more than 100) on the formation of synthetic sand samples containing water/ice and methane or tetrahydrofuran hydrates in the pore space and of the measurement of their acoustic properties (velocities and attenuation of acoustic waves). The main aim of the experiments was to establish the relationship between the velocities of acoustic waves and the ice or hydrate saturation of the samples. An increase in the content of ice and hydrates always leads to a velocity increase. However, the rate of the velocity increase is determined by the localization of ice and hydrates in the samples: at the contacts between the sand grains (“cementing” model) or in the pore space (“filling” model). It has been established that the “cementing” model, characterized by a drastic initial increase in velocities, works for ice or gas hydrates formed from free methane and localized in the pores. On the contrary, tetrahydrofuran hydrates form by the “filling” model and cause a slow increase in velocities.

Examples of two geodynamically active regions in Central Mongolia are used to demonstrate the radon and tectonic activities of faults and find out how their individual parameters affect the radon field. In general, the radon activity of faults increases with the higher contribution of extension in dynamic faulting conditions, with stronger seismic activity, and also in concentrated disjunctive structures at their late evolution stages compared to wide rupture zones at the initial formation stages of their internal structure. Nonuniform structure is an intrinsic property of near-fault radon anomalies, which is defined primarily by nonuniform disruptions of the substrate in the fault zone by ruptures and by spatial variations in the displacement amplitude. Taking the established regularities into account will facilitate a higher efficiency of emanation surveys in studies of seismic hazard associated with crustal faults.

The Carboniferous Donghe sandstone reservoir is the most important target in the Tabei Uplift of the Tarim Basin, which contains a range of hydrocarbon types, including bitumen, heavy oil, condensate oil, light oil, crude oil, and hydrocarbon gas, and has high contents of CO₂ and N₂. The origin of multiple phase hydrocarbons from Carboniferous reservoir rocks in the Donghetang area, Western Tabei Uplift, is documented in this paper based on integral analysis of the geochemistry, pyrolysis, and carbon isotopes of the bulk composition and light composition hydrocarbons. Oil-source correlations determined that the paleoreservoir hydrocarbons that formed from the Permian to the Triassic derived from the Lower Ordovician (O₁) source rocks and that those of the present-day reservoir that formed in the Neogene derived from Middle-Upper Ordovician (O_2-3) source rocks. During the uplift episode lasting from the Permian to the Triassic, the hydrocarbons in the entire paleoreservoir underwent water washing, biodegradation, and bacterial sulfate reduction (BSR), resulting in residual bitumen, heavy oil, H₂S, and pyrites in the paleoreservoir. The high CO₂ and N₂ contents originated from volcanic degassing due to volcanic activity from the Permian to the Early Triassic. The present-day reservoirs underwent gas washing and evaporative fractionation due to natural gas charging that originated from oil cracking and kerogen degradation in the deeper reservoirs; this resulted in fractionation and formed condensate oil and light oil with a high wax content in the residual crude oil. Based on this research, it was concluded that the diverse hydrocarbon phases in the Donghetang area were primarily attributed to water washing, biodegradation, BSR, volcanic degassing, gas washing, and evaporative fractionation.

Local site effect is a significant aspect of seismic hazards, which often causes amplification of ground motions and results in increase in the damage potential of an earthquake. This article displays an experimental study of microtremors data to investigate the dynamic characteristics of the soil and structures in the desert zone of Akhmim City, east of River Nile, Sohag governorate, Egypt. Because of progressive population growth in the Nile valley and delta regions, the Egyptian Governorate considered increasing the number of new communities, especially in the Sohag district, to improve the living conditions. The study area is one of the new urban areas which was suggested by Egyptian Urban Communities Authority (EUCA). For this purpose, the horizontal to vertical (H/V) spectral ratio was used to provide precious information about the soil properties and its engineering features for foundation purposes. The H/V spectral ratio analyses of ambient noise data at the measurement stations were processed and interpreted using Geopsy Software to calculate the amplification factor and fundamental resonance frequency at each observation point. The results indicate that the resonance frequency ranges between 0.21 and 0.28 Hz at the most of the measurement stations, with some irregularities at stations No. 2 and 4 because of the higher softness of sediments in these locations. Most of lower fundamental frequencies are observed in the southern part of the study area, whereas higher values are observed in the northern part of the study area. The amplification factor ranges between 1.38 and 4.15. Higher values were recorded in the northeastern and southern parts, while lower ones were observed in the northwestern and southwestern parts of the mapped area. It can be concluded that the amplification component increases with increasing softness of the sediments. Interpretation of fundamental frequency distribution map is very significant to estimate the building heights and the number of stories, which was estimated to be 35 to 47.