The relevance of the study is associated with the need to develop and improve technological methods for increasing oil production from the Bazhenov Formation in relation to the depletion of reserves in traditional granular reservoirs in the West Siberian oil and gas province. The features of the industrial development of these low-permeability rocks largely depend on their mineral-component composition. To identify different types of reservoirs of the Bazhenov Formation in wells that are not characterized by core material, it is necessary to develop a method for predicting its lithological composition. The main aim: to identify and study the relationships between the mineral component composition, and electrical rocks and radioactive properties of Bazhenov Formation in the central part of the West Siberian sedimentary basin for developing methods of calculating the content of basic rock-forming components in this formation section. The object of the research is the carbonate-argillaceous-siliceous rocks of the Bazhenov Formation, characterized by a large heterogeneity of the mineral-component composition along the section. Methods: identification of relationships between the mineral-component composition of the rocks of the Bazhenov Formation and its electrical and radioactive properties; statistical analysis of the obtained relations; calculation on their basis of the lithological composition of the Bazhenov rocks in the base, as well as in adjacent wells. Results. The article presents the results of an integrated lithological and geophysical interpretation of data from a wide range of electrical and radioactive logging and the results of analytical core studies, on the basis of which a methodological approach to calculating the concentrations of the main rock-forming components of the Bazhenov Formation has been developed. It is based on the identified relationships between the mineral-component composition of rocks and their physical properties. Satisfactory convergence of the calculated values and core data is shown, volumetric lithological models are built for the wells of the Druzhnaya and Povkhovskaya areas. It is shown that the obtained dependences can be used to reliably calculate the lithological composition of the Bazhenov Formation in closely spaced wells that are not characterized by core material. The impossibility of using the proposed methodological approach in thin-layered sections is substantiated due to the fact that the thickness of the interlayers is much less than the resolving power of geophysical borehole probes.

The article presents a methodology developed by the authors for calculating the lithological composition of the Bazhenov Formation in Western Siberia. It is based on the identified “core-logging” interconnections between the mineral-component composition of rocks and the physical properties of the section. The convergence of experimental data and calculated values is shown. The proposed technique was tested. The conditions of its applicability have been substantiated.

The article presents a methodology developed by the authors for calculating the lithological composition of the Bazhenov Formation in Western Siberia. It is based on the identified relationships between the mineral-component composition and physical properties of rocks. The physical meaning of the identified connections has been substantiated. Satisfactory convergence of the calculated values and core data is shown. Possibilities of using the developed technique in wells not characterized by core material and limitations for its application are described.

Manganese (Mn) carbonate rocks are a common lithological constituent of the Upper Oxfordian to Lower Tithonian (Volgian) Georgiev Formation of the Western Siberian marine basin (WSMB). The Mn carbonates in the Georgiev Formation are present in the form of massive sediments, stromatolites, and oncolites, and are associated with glauconite and partly also phosphate-rich clay- and siltstones. Unlike most Mn carbonates, they are not directly associated with organic-rich sediments, but occur below an organic-rich succession (Bazhenov Formation). The Mn carbonate occurrences can be traced from the western central area of the WSMB to its center along a distance of at least 750km. The thickness of the Mn carbonates and their Mn contents becomes reduced in an eastward direction, related to increased detrital input. The geochemical and mineralogical heterogeneity within the Mn carbonates indicates that they were deposited stepwise in a diagenetic regime characterized by steep gradients in Mn, Ca, and Mg. A first step consisted in the replacement of initial sediments within the microbialites during an early diagenetic stage, followed by a second step where massive sediments were transformed into Mn carbonate. During both steps, the decomposition of organic matter was an important source of the newly formed carbonate. During a further step, voids were cemented by Mn carbonates, which are rich in pyrite. This last generation may only have formed once the organic-rich sediments of the overlying Bazhenov Formation were deposited. Accumulation of the Mn carbonates in the Upper Jurassic WSMB was controlled by the proximity of Mn-enriched parent rocks, likely in the Ural, which were subjected to intense geochemical weathering during the Late Jurassic.

Rare-earth elements abundance in black shales of the Upper Jurassic (Tithonian Stage)-Lower Cretaceous (Berriassian Stage) Bazhenov Formation is discussed. This formation is the principal oil source rocks of West Siberia. The deposits within the formation can be subdivided into two main marine groups: (a) moderately hemipelagic deposits (clayey-siliceous, including phosphatic and carbonate rocks) and low-density distal clayey turbidites (argillites), both are considered as normal and (b) silty argillites and clayey-silt rocks, which are channel deposits and considered as anomalous. The hemipelagic rocks of normal sections, which are enriched in the rare-earth elements (REE), accumulated under both slow rates of sedimentation (clayey-siliceous rocks) and faster rates of sedimentation (argillites). The channel deposits of anomalous sections, which are impoverished in the REE, accumulated exclusively under fast rates of sedimentation.Within the hemipelagic group the rate of sedimentation of the argillites was faster than of the clayey-siliceous rocks, but the REE concentration in the former rocks (140.4 ppm) is higher than in the latter group (97.4 ppm). The argillites are more than twice enriched in clayey material than clayey-siliceous rocks. It is likely that the clay fraction was the main carrier of REE in these rocks. In the channel group of rocks, the REE abundance in clayey-silt rocks (21.2 ppm) is lower than in the silty argillite (84.6 ppm), in which the clay content is elevated.With respect to redox potential the Bazhenov Formation rocks can be subdivided further into three groups, based on the degree of pyritization index (DOP); (1) the highly reducing clayey-siliceous rocks of normal sections, with high DOP; (2) the substantially reducing argillites and carbonate rocks of normal sections, with intermediate DOP; (3) the moderately reducing rocks of anomalous sections with low DOP. The rocks with the high DOP (group 1) are characterized by Sigma LREE/Sigma(M+H)REE ratios between 7.37 and 7.5, whereas the rocks with the lower DOP (group 2 and 3) are characterized by Sigma LREE/Sigma(M+H)REE ratios between 12.8 and 13.5. Negative Ce anomalies are either small or absent in all deposits, which is typical for reducing conditions.Thus, the Bazhenov Formation exemplifies the complex depositional conditions that influence the REE concentration in black shale. However, it is this very complexity that has contributed to the development of six separate depositional models (REE contents in ppm are given in brackets). (1) Phosphatic clayey-siliceous rocks of normal sections (367.95); (2) argillites of normal sections (130.73); (3) clayey-siliceous rocks of normal sections (85.97); (4) carbonate rocks, largely dolomites of normal sections (23.23); (5) silty argillites of anomalous sections (78.7) and (6) clayey-silt rocks of anomalous sections (19.66). (C) 2010 Elsevier GmbH. All rights reserved.

Changes in color of Upper Jurassic glauconite of the Georgiev Formation, in the West Siberian Basin, in Russia, are related to changes in physicochemical conditions that caused glauconite maturation and alteration, driven by regional paleoenvironmental evolution. Maturation produces dark green (bluish) glauconite formed from Fe-rich smectite by increasing the content of Fe2+ together with K. Alteration produces brown rims and cracks that are enriched in Al and depleted in Fe and K with respect to the glauconite cores. The change from a yellowish green to dark green color in progressively more mature glauconite is explained by light absorption induced by enrichment in octahedrally coordinated Fe2+ relative to the total Fe, associated with the progressive decrease in the proportion of Fe3+-rich smectite interleaved with glauconite. The brown color in alteration rims is due essentially to light scattered by nanometric inclusions of Fe oxyhydroxides. These, together with residual Al-rich glauconite and a subordinate Fe-rich smectite, constitute reaction products formed by leaching of K and Fe2+, and by the oxidation of yellowish green glauconite cores. Berthierine formed later in the brown rims on Al-rich glauconite, and pyrite formed as a result of drowning of the platform during the latest Jurassic -earliest Cretaceous and sedimentation of black shales under increasingly reducing conditions.

This study examines textural inhomogeneity and variable chemical composition of Upper Jurassic glaucony in relation to small-scale synsedimentary and postsedimentary authigenic processes controlled by the palaeonvironmental and palaeogeographical context. Four glaucony types with complex textural and compositional features have been recognized in cores of the Georgiev Formation of the West Siberian Basin. Samples exclusively made of light green type 1 glaucony (K2O < 6.5%: the less mature type, richer in glauconite-smectite mixed layer) formed under dysoxic conditions in the deepest distal marine environments of the northern sectors of the West Siberian Basin. Dark green type 2 glaucony is the most mature (richest in glauconitic mica: K2O up to 8.5%), is sometimes associated with type 1 glaucony, and is typical of high bottom areas with a low sedimentation rate within the central sectors of the basin. Type 3 glaucony is formed by brown grains, poorer in K and Fe but richer in Al and Si than type 2 glaucony, and is only present in strongly condensed successions of the central-eastern sectors of the West Siberian Basin. Type 4 glaucony is much richer in Fe than any other type, shows fresh yellowish green cores slightly less mature than type 2 glaucony, and brown rims and cracks with composition similar to that of type 3 grains; it was formed in western sectors of the West Siberian Basin, close to Urals. Weathering under a subtropical to temperate climate, and erosion of badly drained peneplaned lowland areas around the basin, provided Al-rich terrigenous clays as substratum for glauconitization, which explains Al and Si enrichment in Siberian glaucony. Maturation from glauconite-smectite to glauconitic mica is monitored by a change from light to dark green colour related to decrease in Al, Si, Mg, Ca and Na, and to increase in K and Fe. Brown rims of type 4 glaucony, and brown type 3 grains formed after leaching of Fe and K from mature glauconite, with formation of clays and Fe oxyhydroxides as reaction products, as a result of free oxygen exposure related to a hydrodynamic regime and temporary sea-level fall. Glauconitization stopped and diagenetic pyrite formed due to basin deepening and burial under black shales during the latest Jurassic-earliest Cretaceous transgression. This study demonstrates that, due to the complex nature of glaucony, the authigenesis of glauconitic minerals in the rock record cannot be correctly understood if the palaeoenvironmental context and the palaeogeographical context of glaucony-bearing sediments are not considered.

Sections of the Bazhenov and Georgiev Formations in the northern Ob'-Irtysh interfluve were studied. Four lithologic types of sections of the Bazhenov Formation and three types of sections of the Georgiev Formation have been recognized, and their relationship with the petroleum potential of the Jurassic-Cretaceous deposits has been established. It is shown that the settings in which sediments accumulated on positive structures at the bottom of the Georgiev paleosea were more favorable for the formation of phosphorite concretions and intense concentration of glauconite grains than the settings in syneclises. The conditions of accumulation of both formations have been refined.