Bazhenov horizon is the main source rock stratum of the West Siberian petroleum province, which possesses significant resources of hard -to -recover hydrocarbons, making it a unique exploration target. The study summarizes geochemical data on crude oils (39 samples) collected from pools within the Bazhenov and Tutleima formations. A comparative analysis of the carbon isotope composition and geochemical parameters calculated from the composition of the identified saturated and aromatic compounds (pristane/phytane, C29/C27 steranes, C35/C34 homohopanes, tricyclane index, relative concentrations of dibenzothiophenes) confirm a common aquatic genotype of the studied oils and their genetic affinity to the organic matter of the same age. At the same time, significant variations in the main physical and chemical characteristics of the studied Bazhenov oils may be caused their fractionation during migration processes.

The results of integrated isotope-geochemical and microbiological studies of reservoir waters from oilfields under development in the southern part of the West Siberian petroleum province are reported for the first time. The waters have mainly Cl Na chemistry, with salinity 13–72 g/L TDS, pH between 6.9 and 8.5, and reductive geochemical environment with Eh ranging from − 166.3 to − 90.1 mV. Salinity variations are due to the injection of waters from the Aptian–Albian–Cenomanian aquifer to maintain the reservoir pressure early during the development, and subsequent use of closed-cycle produced water, at increased water cutting of the production well stock. The presence of two main genetic types of groundwaters in the hydrogeological section is revealed: the ancient waters of infiltration genesis, and the waters of sediment genesis. In most of the sampled waters, the δD–δ 18 O pair indicates climate-induced 4–5‰ 16 O depletion prior to burial, combined with later 2–3‰ 18 O enrichment during prolonged water–rock interaction. In some water samples, dissolved inorganic carbon is of bacterial origin, as suggested by its isotope composition (δ 13 C DIC ). The waters hotter than 90°Cshow an evident positive δ 13 C DIC shift to a range of − 8 to + 4‰ VPDB, which is common to thermal water values. The analyzed waters are characterized by lower 87 Sr/ 86 Sr ratios than those in the present-day seawater. A substantial Sr input from mantle sources is assumed, manifested up to different degrees in the waters of the studied oilfields. Microorganisms involved in the nitrogen, sulfur, iron, and carbon cycles are detected in the studied water samples. The products of the vital activities of nitrifying, thionic, and sulfate-reducing bacteria are nitric and sulfuric acids, and hydrogen sulfide. The joint action of these microbial groups poses large-scale corrosion risks to the oilfield facilities.

Баженовский горизонт является основной нефтепроизводящей толщей Западно-Сибирской нефтегазоносной провинции, в которой одновременно аккумулированы значительные ресурсы трудноизвлекаемого углеводородного сырья, что делает эту толщу уникальным объектом для исследований. В работе обобщена геохимическая информация о нефтях (39 проб) из залежей в баженовской и тутлеймской свитах. Сравнительный анализ изотопного состава углерода и геохимических показателей, рассчитанных по составу идентифицированных насыщенных и ароматических соединений (пристан/ фитан, стераны С29/С27, гомогопаны С35/С34, трициклановый индекс, относительные концентрации дибензтиофенов), подтверждают единый аквагенный генотип исследованных нефтей и их генетическое родство с одновозрастным органическим веществом. Вместе с тем, отмечается широкий диапазон изменения базовых физико-химических характеристик исследованных баженовских нефтей, которые обусловлены влиянием их фракционирования в ходе миграционных процессов. The Bazhenov horizon is the main source rock stratum of the West Siberian petroleum province, which possesses significant resources of hard-to-recover hydrocarbons, making it a unique exploration target. The study summarizes geochemical data on crude oils (39 samples) collected from pools within the Bazhenov and Tutleima formations. A comparative analysis of the carbon isotope composition and geochemical parameters calculated from the composition of the identified saturated and aromatic compounds (pristane/phytane, C29/C27 steranes, C35/C34 homohopanes, tricyclane index, relative concentrations of dibenzothiophenes) confirm a common aquatic genotype of the studied oils and their genetic affinity to the organic matter of the same age. At the same time, significant variations in the main physical and chemical characteristics of the studied Bazhenov oils may be caused their fractionation during migration processes.

Баженовская свита Западно-Сибирской нефтегазоносной провинции в настоящее время выступает в качестве одного из ключевых объектов потенциального прироста запасов и добычи нефти. В статье представлены результаты исследования геохимии органического вещества баженовской свиты по методике, разработанной в ИНГГ СО РАН для высокообогащенных органическим веществом пород. Рассмотрены особенности группового и углеводородного состава битумоидов открытых и закрытых пор баженовской свиты Северо-Сургутского района. Установлено, что основная часть новообразованных нафтидов находится в свободном (фазово-обособленном) и сорбированном состояниях в закрытом поровом пространстве пород. В открытых порах находятся наиболее подвижные компоненты битумоидов, перераспределенные из закрытых пор в ходе первичной миграции и оставшиеся после эмиграции нафтидов из баженовской свиты в берриас-нижнеаптские (пласты групп А и Б) резервуары. Используемая в работе методика позволила по объему порового пространства, занимаемому битумоидами открытых пор, выделить интервалы с повышенной открытой пористостью и нефтенасыщенностью в разрезе баженовской свиты (силициты и карбонаты). The Bazhenov Formation of the Western Siberian oil-and-gas province is currently one of the key objects of potential growth in oil reserves and production. Here we present results of a geochemical study of the Bazhenov Formation according to the methodology developed at Trofimuk Institute of Petroleum Geology and Geophysics, Novosibirsk (IPGG SB RAS) for rocks highly enriched in organic matter. The features of the group and hydrocarbon compositions of the bitumoids that are filling open and closed pores of the Bazhenov Formation rocks from the North-Surgut region were considered. It has been established that most of the naphthides are in a free (phase-segregated) or sorbed state in a closed pore space. The open pores contain the most mobile components of bitumoids, redistributed from closed pores during the primary migration and remained after the secondary migration of naphtides from the Bazhenov Formation to the Berriasian-lower Aptian reservoirs (A and B strata groups). The methodology used in this work allows determining intervals with increased open porosity and oil saturation in the section of the Bazhenov Formation (silicites and carbonates) by the volume of pore space occupied by open-pore bitumoids.

The distribution regularities of the main components and types of rocks of the Bazhenov Formation are investigated. It is one of the largest black shale formations in the world. The results of the lithological and geochemical study revealed that the brittlest rocks of the formation are widely distributed in the Central region and are considered as potential reservoirs: siliceous mudstones, kerogenic siliceous mudstones, carbonated radiolarites (limestones and dolomites). In the South-Eastern region, thicknesses of silicites are noticeably reduced; kerogen-clay-siliceous mudstones (similar in silica content to siliceous mudstones) prevail, which are also reservoirs in a number of areas. These types of rocks are practically not typical for the Northern and Southern regions. Detailed distribution maps of the main components of rocks for the lower, middle and upper parts, as well as for the Formation as a whole, were constructed. It was revealed that the highest concentrations of siliceous material are observed in the lower and to a lesser extent in the middle parts of the Bazhenov Formation section in the Central region. By the nature of distribution, composition and thickness of potential reservoirs, the Central region can be considered as a highly promising region in terms of development for hydrocarbon raw materials, the South-Eastern is medium-promising one, the Northern and Southern are low-promising ones.

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 sections of the boundary Jurassic–Cretaceous black shale sediments (Bazhenov Formation) of the West Siberian sedimentary basin in conditional Central, Southeastern, and Northern regions are compared on the basis of a complex lithological-geochemical study, as well as geophysical and paleontological data. The studies are conducted using a common classification scheme of rocks and common principles of recognition of members in the structure of the formation. The elaborated sedimentation models reflect the spatiotemporal principles of the origination of the Bazhenov Formation depending on the features of paleotopography, proximity to the provenances, climatic fluctuations, and changes in sea level. The Central region was mostly characterized by biogenic siliceous sedimentation (radiolarian or similar sediments) during the entire Volgian Stage (locally, to the beginning of the Ryazanian Stage) and by carbonate-siliceous sedimentation (coccolith-rich sediments) from the end of the Volgian Stage to the beginning of the early Valanginian, which was favorable for the accumulation of a significant amount of organic matter. The accumulation of carbonate-siliceous sediments finished by the middle of the Ryazanian Stage within the Khantei Hemianteclise and the southeastern part of the South Nadym Megamonoclise of the Central region. In the Southeastern region, the formation of organic-rich sediments terminated at the stage of accumulation of biogenic-siliceous silts and, from the beginning of the Ryazanian Stage, was followed by sedimentation of organic-poor clays of the Megion and Kulomzino formations. The Northern region was characterized by the accumulation of relatively organic-poor siliceous-argillaceous sediments with a dominant terrigenous (over biogenic) component by means of more intense gain of clay material to this zone of the paleobasin. The sedimentation of the Bazhenov Formation here was probably terminated in the first half of the Ryazanian Stage.

The paper presents the results of a study of the structure and stages of accumulation of the Bazhenovo Formation in the area of the Koltogorsk-Nyurol’ka trench and adjacent positive structures of the Southeastern region of Western Siberia. Based on the use of unified lithological criteria, taking into account geophysical and paleontological-stratigraphic data, the lower four out of six members identified within the Bazhenovo Formation in the Central Region of Western Siberia were traced in the studied area. In the Southeastern region, members 1-4 constitute the entire Bazhenovo Formation, which corresponds here to most of the Volgian-lower part of the Ryazanian. At the end of the Middle Volgian time, an increase in the supply of clay material to the study area is noted due to the proximity of this area to sediment sources and some decrease in global sea level. The depositional environments of the Bazhenovo Formation in the Southeastern region were suboxic during the Middle Volgian and changed to anoxic in the Late Volgian-first half of the Ryazanian. The development of suboxic conditions in the second half of the Ryazanian led to the cessation of the accumulation of Bazhenovo deposits in the studied area.

The article discusses the concept of Bazhenov reservoir rocks exemplified by a oil productive interval of the Bazhenov Formation cross-section in the West-Kvenzer area, southeastern Western Siberia. Characteristics of the analyzed interval are derived from the interpretation of the results obtained by the geophysics, lithology, rock physics and organic geochemistry integrating methods. It has been shown that the studied oil saturated interval is represented by kerogen-siliceous and siliceous mixtites. The studied rocks are found to be identical in terms of the lithological composition, organic matter geochemistry, geophysical and petrophysical characteristics. Most of the mobile (allochthonous) bitumoids (chloroform extractable bitumens) generated by the Bazhenov Formation within the West-Kvenzer area are concentrated in open pore space, rather than being fracture-filling type.

The results of a comprehensive analysis of geological and field information are presented in order to assess changes in the hydrogeochemical field of the oil reservoir of the U horizon of the Verkh-Tarskoye oil field during its development in the period from 1994 to 2021. The main production facility is at the IV stage of declining production. The water cut of the production wells stock reaches 98% with cumulative production of 14.86 million tons (as of May 1, 2021). Since 2015, there has been an increase in the TDS of produced water, which is explained by the processes of their mixing with circuit waters along the periphery of the reservoir with a decrease in reservoir pressure and more saline waters of the U horizon, supplied during joint operation.

Summary An important feature of the Bazhenov Formation is the heterogeneity of its lithological composition along the section. This article presents the results for one of the wells from the Druzhnaya area, located on the northern slope of the Khantheyskaya hemianteclise in the central part of the Khanty-Mansi Autonomous Okrug. For each lithological type of rocks, core-GIS dependencies were found that best describe its distribution over the section. The organic matter content correlates best with the radioactive log values. The concentrations of siliceous material and pyrite have a pronounced relationship with the electrical resistivity of rocks. The concentration of carbonate material correlates with the values of neutron logs, and the content of clay matter - with the density of the rocks and their electrical resistivity. After that, the multidimensional dependences of the content of each component on several types of logging were analyzed, which served as the basis for calculating the curves of their content in the rock. The calculated curves correlate well with the core data, which is confirmed by the high values of the reliable approximation (R2), which varies from 0.72 to 0.89 for different components. The resulting volumetric model clearly shows the relative content of the main rock-forming components in the Bazhenov Formation and their distribution over the section. The obtained dependences can be used to calculate the lithological composition of the Bazhenov formation in closely spaced wells that are not characterized by core material.

The article presents the results of revealing the relationship of siliceous, clay, carbonate and organic substances and pyrite from the types of electrical and radioactive logging. The influence of the content of the listed rock-forming components of rocks on the geophysical characteristics is described. The volume lithological model of the Archinskaya-47 well, constructed on the basis of established correlations, is presented.

Conceptual models are formulated and quantitative predictive functions of reservoir porosity, fluid saturation, effective thickness rate of the Bazhenov Formation and its geological and geochemical characteristics are obtained. These parameters were used to predict oil and gas potential and quantitative assessment of oil resources in the Bazhenov Formation in the southeastern regions of the Western Siberian Basin.

The paper provides results of the field-geological information analysis in order to evaluate the current development stage of reservoir Yu1 of the Verkh-Tarskoe oilfield. The main recoverableasset is currently indevelopment phase III (declining production). The total well stock consists of 175 wells, of which 134 are producing.The monthly volume of water injection into the reservoir pressure maintenance (RPM) system reaches 100, 000 m3, and fluidproductionis about 170, 000 m3 with averagewater flooding of producing wells is90.4%. The structures with complex geometry thatformedwithin thehydrodynamic field where the deep pressure sinks are associated withproduction wells, and piezomaximawith injection wells.Given the multi-facies pattern of thesandstone depositional environmentsand secondary processes ofrocks metamorphismduringcatagenesis, thetarget reservoirhas a complex structure and its study in greater detail willrequirebuilding a new geological modelof the oilfieldand more precisedefinition engineeringto achieve maximumrecovery.

The relevance of the research lies in solving the issues of isotope geochemistry of groundwater in oil and gas deposits of large artesian basins. The waters of oil and gas bearing sediments of the region under consideration are practically not studied from the point of view of the distribution of stable isotopes delta D, delta O-18, delta C-13 and isotopic ratios Sr-87(86)/Sr and Rb-87/Sr-86. The aim of the research is to identify the features of the genesis of groundwater and the interaction in the "water-rock" system from the standpoint of isotopic geochemistry. Methods. The analysis of the complex of delta D, delta O-18, delta C-13(DIC) of waters and dissolved inorganic carbon was carried out at the Center for Collective Use of the IGM SB RAS using the Isotope Ratio Mass Spectrometer FinniganTM MAT 253. The Sr-87(86)/Sr. Rb-87/Sr-86 isotopic ratios were studied on an MI 1201T mass spectrometer in a two-tape mode with registration on one collector. Results. The authors have studied the stable isotope composition (delta D, delta O-18, delta C-13(DIC)) and isotopic ratios of Sr-87(86)/Sr and Rb-87/Sr-86 of the waters of the developed oil fields of the Novosibirsk region. The hydrogeological section shows the presence of two main genetic types of groundwater: ancient infiltrogenic and sedimentogenic. For most of the studied samples, the delta D-delta O-18 indicates a combination of two determining factors: initial depletion of water in the light isotope O-16 as a result of climatic impact during the period "before disposal" up to 4-5 parts per thousand; further saturation of waters with the heavy isotope O-18 during a long WRI, which led to additional 2-3 parts per thousand of heavier isotopic composition. The delta C-13(DIC) values for a number of samples suggested bacterial origin of dissolved inorganic carbon. There is a clear increase in the delta C-13(DIC) in waters which temperature exceeds 90 degrees C, to the values corresponding to the "normal" ones in thermal waters: from -8 up to +4 parts per thousand VPDB. The waters are characterized by low Sr-87(86)/Sr ratios relative to ocean waters. A significant contribution of mantle sources of strontium is assumed, which manifests itself to different degrees for the waters of various deposits.