Home – Home – Jornals – Russian Geology and Geophysics 2023 number 1

We report data on 9 samples of basaltic rocks and a spinel lherzolite xenolith collected during dredging in the area of Alba Guyot (Magellan Seamounts, Pacific Ocean). The data include ⁴⁰Ar-³⁹Ar ages of five samples and mineral chemistry of rock-forming and accessory minerals and glasses, with implications for the formation conditions of Ti-amphibole in basaltic melts. The upper slope parts at Alba Guyot and its satellite Oma Vlinder at sea depths of 3600 to 2200 m, are composed of ~1400 m thick basaltic rocks that formed within the 112 to 86 Ma interval (⁴⁰Ar-³⁹Ar plateau ages). Later, in its 60 Myr history postdating the Cretaceous volcanism, the guyot was exposed to erosion, which produced a plateau-like flat top, and underwent another pulse of volcanism in the Cenozoic. Miocene (<20 Ma) eruptions of basanitic magma on the Alba Gyot plateau were associated with petit-spot volcanoes. Judging by the absence of hyaloclastic deposits around those volcanoes and the age of the lavas, the Cenozoic eruptions occurred in subaerial conditions before the guyot submerged below the sea level. The Alba volcanics have greater contents of incompatible elements than typical oceanic island basalts (OIB), apparently, because they formed by low-degree partial melting of different sources: pargasite-bearing spinel peridotite of the metasomatized oceanic lithosphere, mainly for Cretaceous basaltic rocks, and/or garnet peridotite for Miocene petit-spot basanites. Multiple eruptions at Alba Guyot for 100 Myr may have been maintained either by the Southern Pacific Superplume in the region of the South Pacific Thermal and Isotopic Anomaly (SOPITA) in the Mesozoic or by translithospheric faulting originated at the lithosphere-asthenosphere boundary (LAB) in the Cenozoic.

We present new data on the chemical composition of the late Precambrian rocks in the upper part of the regional Balaganakh stratigraphic horizon (the Nugan Formation at the Baikal segment of the Sayan-Baikal-Patom belt (SBPB) and the Bugarikhta Formation of the Balaganakh Group at the Patom segment of the SBPB) and the U-Pb (LA-ICP-MS) age of detrital zircons from the Bugarikhta Formation. It has been established that the rocks of the Nugan and Bugarikhta formations resulted from the destruction of igneous and metamorphic rocks. The contents and ratios of trace and rare-earth elements in the rocks of the Nugan and Bugarikhta formations indicate a predominance of felsic igneous rocks in the provenance and the presence of rocks of mafic and intermediate compositions at the source of the terrigenous rocks of the Nugan Formation. The U-Pb age of detrital zircons from all rocks of the upper part of the regional Balaganakh Horizon is close to the age of the rocks in the basement of the southern margin of the Siberian Platform, which suggests that this basement was the main source of clastic material in the sedimentation basins. It is argued that the "pre-glacial" (pre-Marinoan) terrigenous deposits of the Nugan and Bugarikhta formations, as well as the deposits of the Ipsit Formation at the Sayan segment of the SBPB, accumulated in the Late Riphean (ca. 720-640 Ma), probably in postrift basins that formed immediately after the separation of Siberia from Laurentia at the early stages of the opening of the Paleoasian Ocean.

Cenozoic alkali basaltoids are developed in southern Transbaikalia (Daur-Hentiyn Range). They are represented by thin flows and nappes. Unlike the volcanic fields of the South Baikal volcanic region (SBVR), the investigated fields are not related to rifting. The volcanics from the Zharnichikha River flow were studied. By composition, they are referred to as basanites. The mineral compositions of the volcanics have been determined for the first time. In geochemical parameters the basanites correspond to mafic volcanics of oceanic islands. The P - T parameters of the equilibrium crystallization of olivine and clinopyroxene phenocrysts have been determined. For the calculated parent melt, the formation temperature was 1536 ºС, and the pressure was 3.28 GPa. The obtained original data on the isotopy of Sr, Nd, and Pb showed that the isotopic composition of the magmatic melt formed from the PREMA source, with a strong subordinance of the latter. Olivine clinopyroxenites (Ol + Cpx + Grt) were the probable mantle source for the studied basanites. The generation of basaltoid melts occurred under the influence of the mantle plume, which caused Cenozoic activation.

We present results of geochemical and geochronological (U-Pb, LA-ICP-MS) studies of plagiogranites of the Mt. Zmeinaya massif in the Nora-Sukhotino terrane. The studied rocks are of calc-alkaline series and peraluminous type. They formed in collision settings at the final stage of evolution of the Nora-Sukhotino terrane. The high Sr/Y ratios and low Y and Yb contents of the plagiogranites indicate that these are adakitic rocks. The trace element composition of the rocks suggests their high-pressure (>15 kbar) genesis through the melting of metabasites. The youngest zircon population from the plagiogranite has a mean weighted concordant age of 295 ± 4 Ma, which corresponds to the early Permian. With regard to the zircon morphology, the obtained age can be considered the age of the plagiogranites.

The crustal stress field of the Baikal Rift System has been reconstructed by tectonophysical inversion of focal mechanisms from the catalog of earthquakes recorded by the regional seismological network. Cataclastic analysis of fault slip data developed at the Shmidt Institute of the Physics of the Earth (Moscow) revealed previously unknown features in the behavior of principal stresses. Namely, the maximum deviatoric stresses diverge off the rift axis while the normalized spherical and deviatoric stress tensor components reach high magnitudes in the crust of the Baikal Basin. The obtained stress pattern of the Baikal Rift System is consistent with the rift origin by a joint action of a vertical mantle flow (upwelling branch of convection) and a horizontal flow in the asthenosphere which drives the NW-SE motion of the Amur plate off Eurasia.

We present a filtration tomography technique for isolating components of the gravity field anomalies generated by inhomogeneities of the horizontally layered density model. The filtration algorithm of field separation relies on the solution of the forward and inverse problems of analytical continuation of harmonic functions through the horizontal boundary plane. We applied the regularizing algorithms to analytical continuation of the gravity field "down" to its generating sources. The fields successively recalculated upward and downward relative to preset depths allowed us to partition the initial (total) field as the sum of the fields generated in the layers based on the properly selected adaptive regularization parameter α. For the sake of stability of the inverse problem solution in the analytical continuation of the observed gravity field to a certain depth, we used the Lavrentiev regularization scheme involving the L -curve method (for selecting the adaptive regularization parameter). The smoothing regularization parameter values obtained from the preset successive depth intervals and grid step for the observed field are shown to be optimal for dividing the observed field into components corresponding to different depths. The developed algorithms for massively parallel computing systems and their application to a group of different heights were numerically implemented on the Uran supercomputer.

The work is concerned with the theoretical substantiation of a new geophysical technology for studying a unique geologic object with unconventional hard-to-recover hydrocarbon reserves. The technology is based on transient electromagnetic sounding from a spatially distributed system of highly deviated wells drilled in target objects near the Bazhenov Formation. The results of computer modeling predetermine a new direction for the geological exploration of unconventional hydrocarbon deposits. We consider a numerical solution to the 3D direct problem of pulsed electromagnetic sounding and, on its basis, develop a computational scheme and a computer program. A mathematical model is constructed, which describes the sensing process through a pulsed source for electromagnetic-field excitation. The Fourier transform is used for time discretization, and the vector finite element method, for spatial discretization. This approach makes it possible to obtain many independent 3D problems and solve them in parallel by applying the modern multiprocessor technology. Using the KNL and Broadwell computing nodes of the Siberian Supercomputer Center SB RAS, we performed calculations of electromagnetic signals, which showed a high efficiency of the devised computing scheme and a high performance of the implemented algorithm. Despite the fact that the total peak performance of the KNL nodes is 2.5 times higher than that of the Broadwell nodes, their practical application for performing large-scale 3D modeling on the cluster shows a high efficiency of the latter. When choosing the most suitable computing architecture for the implementation of mass calculations, one should not rely on their formal characteristics only; significant performance is achieved when taking into account the peculiarities of the computational methods employed for solving a specific problem. The implemented more efficient ways of performing parallel matrix-vector operations did not significantly increase the performance for this computational scheme. The created computational tools form the basis for further design of the configuration of a pulsed electromagnetic sounding system and for identifying the capabilities of the new geophysical technology for examining complex geologic media.

The work deals with the development of methodological and algorithmic tools for the quantitative interpretation of oil well resistivity logs. We review the results of applying the neural-network-based approach to the inversion of resistivity logs measured in thinly bedded high-contrast environments. The capabilities of the proposed approach are demonstrated by the example of the algorithm for noninterative express-inversion of unfocused lateral logs (BKZ). BKZ is the unfocused array logging method widely used in the Commonwealth of Independent States for oil well studies. BKZ logs are known for their complexity: The signals of unfocused gradient probes are highly affected by the medium properties below and above the measuring point. The developed algorithm is based on the inversion of full logs into the parameters of a 2D axisymmetric model of the medium, which allows naturally taking into account the influence of surrounding rocks and borehole conditions. Transition from the «layered» parametrization conventional for BKZ logs interpretation to a quasi-continuous change of properties along the well axis allows extracting meaningful information at every measurement point and constructing high-resolution geoelectric models of the sediments. The noniterative nature of the algorithm provides a high computing efficiency. This opens up the possibility of using the 2D inversion advantages to increase the reliability of the initial express interpretation results. Testing the algorithm on the practical data from West Siberian oilfields has revealed the field of its maximum efficiency, namely, study of impermeable and low-permeability sediments, such as the complex shaly caprocks and bituminous deposits of the Bazhenov Formation. With high-quality input data, the approach is also efficient for studying permeable terrigenous sediments.

Prediction of the open porosity factor below boreholes and in the interwell space, from electromagnetic sounding and electrical logging data, has been studied. Modeling porosity data are synthesized from laboratory studies on the samples of two boreholes drilled in the Bishkek geodynamic site. The porosity prediction is carried out using specific electrical resistivity data obtained from 1D inversion of magnetotelluric sounding data collected in the vicinities of these wells. A new approach to predict porosity is suggested, based on the constructing of electromagnetic resistivity pseudologs at the target location. The comparison of this technique with other options indicates that its application results in substantial improvement of the predicted accuracy (in particular, relative errors of prediction in double depth of the borehole and in the interwell space could be 2 and 8 %, respectively). In general, the porosity predictions, based on Archie formula, give worse results.

The Albian carbonates of the Quissama Formation in the Campos Basin, southeastern Brazil, are important oil reservoirs. They make part of a carbonate platform that formed along the eastern coast of Brazil and the western coast of Africa during the Albian, which resulted in the opening of the South Atlantic Ocean. Subsequently, this reservoir was subjected to different postdepositional diagenetic processes. The present study utilized geophysical well logs to estimate the porosity of this reservoir, based on density, neutron porosity, and sonic logs. The estimates do not show good results when compared with the laboratory measurements. Then, exploring the fact that these logs are obtained with different physical principles, a multiple linear regression and an artificial neural network with Bayesian stochastic approach were applied, which resulted in a better porosity estimate. As porosity is a petrophysical parameter considered significant in the characterization of reservoirs, it was used, hereafter, to estimate permeability and water saturation of the reservoir, applying empirical equations. From there, it was not enough just to estimate the porosity, but was necessary to know what type it is. For this purpose, the concepts of the electrical formation factor, cementation coefficient, tortuosity, and anisotropy were used. With them, the zones with primary intergranular and interparticle porosity as well as secondary porosity, such as fractures, fissures, and vugs, were mapped. It was concluded that, with studies of this type, it is also possible to identify the connected and nonconnected porosities, which permits estimation of the effective porosity along the well.