Home – Home – Jornals – Russian Geology and Geophysics 2004 number 6

On the basis of new data on melt inclusions in clinopyroxenes, we have established the physicochemical conditions (magma compositions, temperature, and pressure) of formation of boninites in ophiolites of the Dzhida zone (northern Mongolia) and have determined the parameters of generation of primary magmas (T = 1400-1500

The teschenite-syenite mass of Mt. Chernaya Sopka was studied by a complex of petrological and paleomagnetic methods. It was shown by ⁴⁰Ar/39Ar age method that the mass dates from the Lower Devonian (406-402 Ma). There are several mineral parageneses in the rocks: high-temperature assemblage of phenocrysts, low-temperature assemblage of groundmass, and interstitial assemblage of anorthoclase with analcite. Lateral changes in composition of rocks and minerals have been established, which can be related to flow differentiation. It has been substantiated that the gabbroids of the first phase of intrusion and syenites of the second phase are two discrete groups of rocks and have different sources. The data obtained suggest that the mass is a feeding conduit for the Lower Devonian volcanic rocks of the Upper Karym Subformation and that the subalkalic gabbroids of the massif are of plume nature.

The Paleozoic-Mesozoic history of the Kuznetsk sedimentary basin was studied in terms of a one-dimensional two-layer model for lithosphere extension. The reconstructed curves of subsidence demonstrate two stages of "rapid" downwarping of the basement of depression for about 10-20 Myr, followed by slow subsidence as a result of thermal contraction of the lithosphere for 100-120 Myr. Average horizontal extensions were estimated as 2-fold for the crust, and 5-fold, for the mantle. The maximum paleotemperatures in sedimentary rocks of the depression were estimated at under 300-400

The mesoabyssal Kubadru batholith resulted from collisional incorporation of the Chulyshman terrane in the West Sayan structure. The turbidite strata making up the basement of the Teletskoe block were, most likely, a protolith for the batholith granitoids. The U-Pb age of zircons from granitoids of major phase is 373.3 6.5 Ma, which permits correlation of the Kubadru batholith with abundant Late Devonian-Early Carboniferous granitoid plutons in Gorny Altai (Yaloman, Talitsa, etc.) and in the zone of its junction with the West Sayans (Altyntau and Teletskoe-Bashkaus belts). The Kubadru mass is made up of bimodal calc-alkalic rock series. The intrusion and formation of some melt portions (from gabbroids to granite-leucogranites) took place against the background of intense sinistral shearing strains. The mass has a strongly asymmetric internal structure: The western flank is an exposed root zone, which formed during viscoplastic sinistral shearing strains, and the eastern flank is composed of intrusive granitoids, which, most likely, resulted from intense shear compression and extrusion of magma accompanied by the formation of a gentle granitoid body.

Field data and physical modeling show an along-strike regularity as the most common feature of fracturing in crustal fault zones which include the major fault plane and all related subsidiary faults. Fault zones of different evolution stages, geometries, and stress environments are marked by chains of fracture density peaks spaced at roughly equal distances. This regularity, manifested at all fault pattern hierarchies, is especially prominent at the final evolution stage when a fault zone becomes divided along its strike into segments of the major plane, each containing a single large fault, joined at duplexes or accommodation zones. Spacing between fracture density peaks is greater at lower strain rates and in thicker and more viscous crust layers. The peaks are spaced less closely in strike-slip faults than in normal, reverse or thrust faults. The revealed regularity can be used for reference in seismic and metallogenic predictions, as the fracture pattern correlates with the distribution of fault-related earthquakes and mineral deposits.

On the basis of geochronological and palynological materials, 25 sections of Holocene deposits and soils of northern Central Siberia were studied by paleocarpological methods. Special attention was given to the reconstruction of the dynamics of speciation of forest cover in time and space. As a result, climatic and ecological settings have been dynamically portrayed for each kind of landscape, and quantitative parameters of paleoclimates have been calculated. The main peaks of climatic changes of the postglacial history have been detected in the ranges 8.5-8.0 ka (thermal maximum) and 2.5-2.0 ka (thermal minimum). Importantly, the thermal maximum is characterized by warming up by 3-9

Allochthonous bitumens of the Abalak Formation and oils of the Shaim petroliferous area (PA) were studied in detail at the molecular level. They are similar in some geochemical parameters to autochthonous bitumens from mudstones and bazhenovites of the Bazhenov Horizon of the West Siberian megabasin.
The heterochronous reservoir beds of the Upper-Middle Jurassic deposits and weathering crusts in the Shaim PA form fluid-dynamic complexes related to each other and to the source of naphthides and thus combined into a single system.

Integral representations of high-resolution TEM fields in layered media with weakly nonhorizontal layer boundaries are obtained using the perturbation method. In a general case, the model parameters (electrical conductivity and magnetic permeability) are complex frequency functions. According to the results of mathematical modeling for loop-loop and loop-line recording, thin low-conductive layers are detectable from qualitative changes in conductivity curves caused by frequency dependence of conductivity and magnetic permeability and by dipping of layer boundaries. The latter effect on the TEM signal is especially informative.

A new method of fluid-stress modeling on the basis of 3D seismic and drilling data provided new information on the stress conditions and the hydrodynamics of the U₁ Upper Jurassic sand reservoir in the Arigol field. It is suggested to detect and outline oil fields and divide them into isolated traps each marked by low overburden pressure and fluid-trapping properties on the basis of correlation between the structural pattern and mapped stress. In the new fluid-stress model, contour lines of constant anomalous overburden pressure ( = const) are used as outer hydrodynamic barriers for hydrocarbon migration and the geometry of fluid flow lines (horizontal pressure gradients) corresponds to barriers between traps. The areas of the detected fluid-stress traps depend on the choice of the boundary values of constant overburden pressure and its horizontal gradients. It is suggested to map fluid-stress traps and to predict their sizes in a way similar to that for structural-depositional and structural-stratigraphic traps. Deep drilling within the hydrodynamically screened traps should be undertaken if they fall into the most contrasting and unexplored _min anomalies, with the aim to estimate the fluid-stress properties of the section. Unlike the traditional methods of hydrodynamic modeling, the new method is based on 3D multicomponent seismic exploration data (V_P, V_S, ).

Splitting and polarization of converted PS waves are used to investigate azimuthal anisotropy in sections containing vertically fractured reservoirs. The known Alford pseudo rotation algorithm for polarization analysis of 2C x 2C marine VSP data has been adapted to the specific conditions of the Yurubchen-Tokhomo petroleum province (Siberian craton) where deployment of a regular shot network is difficult because of a rugged surface topography. The 11-layer seismic model of the province includes two azimuthally anisotropic layers, the lower one corresponding to the Riphean fractured reservoir. 3C VSP and surface CMP synthetic seismograms computed for this model are described together with the procedures and results of testing of the modified algorithm. The suggested algorithm for the polarization analysis of surface shot gathers and CMP stacks includes cross correlation of traces for orthogonal components and stripping of upper anisotropic layers. All tests demonstrate high quality of the estimated anisotropy parameters.

Mineralized tubular capsules are widespread in glauconites of the Georgievka Horizon (West Siberian Plate), which dates from the upper Oxfordian, Kimmeridgian, and lower Volgian Stages. These capsules, which are supposed to be sheaths of cyanobacterial threads and coccoid forms, make up the glauconite grains (microconcretions). Earlier, mineralized (phosphatized) capsules of cyanobacterial threads were found in phosphorites of the Georgievka Horizon. It is likely that the mineralization of cyanobacterial mats led to the formation of stromatolites and oncolites of carbonate-manganese composition, found in the eastern West Siberian Plate. Thus, a conclusion is possible that the microbial forms, bacteria and cyanobacteria, played a leading role in the formation of authigenic minerals and rocks of the Georgievka Horizon of the West Siberian Plate.