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.

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.

—The age of the Turga Formation in the Middendorf’s outcrop is determined as latest Barremian–earliest Aptian based on fauna and microfossil assemblages. The composition of the fauna and palynomorph assemblages allows correlations between the Turga Formation and the Yixian Formation and lower part of the Jiufotang Formation of Northeast China. Results of the petrographic and geochemical analysis as well as microfossil studies showed that sediments accumulated in a shallow lake with low-energy hydrodynamics, sometimes under low-oxygen conditions. Climate features (warm temperate climate transitional to subtropical) and type of vegetation (coniferous forests with admixture of hardwoods) are reconstructed taking into consideration the composition of spore-pollen assemblages.

На основании изучения фауны и микрофоссилий возраст тургинской свиты в обнажении Миддендорфа определен как конец баррема-начало апта. Состав комплексов фауны и палиноморф позволяет коррелировать тургинскую свиту с формацией Yixian и низами формации Jiufotang Северо-Восточного Китая. Результаты петрографического и геохимического анализов, а также изучения микрофоссилий показали, что отложения накапливались в мелководном озере со спокойной гидродинамикой, иногда в условиях дефицита кислорода. По составу спорово-пыльцевых комплексов восстановлены климатические условия (умеренно теплый климат, переходный к субтропическому) и тип растительности (хвойные леса с примесью лиственных пород). The age of the Turga Formation in the Middendorf’s outcrop is determined as latest Barremian-earliest Aptian based on fauna and microfossil assemblages. The composition of the fauna and palynomorph assemblages allows correlations between the Turga Formation and the Yixian Formation and lower part of the Jiufotang Formation of Northeast China. Results of the petrographic and geochemical analysis as well as microfossil studies showed that sediments accumulated in a shallow lake with low-energy hydrodynamics, sometimes under low-oxygen conditions. Climate features (warm temperate climate transitional to subtropical) and type of vegetation (coniferous forests with admixture of hardwoods) are reconstructed taking into consideration the composition of spore-pollen assemblages.

Kerogens of the Upper Jurassic Gol'chikha, Yanovstan, Sigov and Tochin formations in the northeastern West Siberia have been studied using modern analytical methods. Quantitative characteristics of samples (ash content, moisture), elemental composition (contents of carbon, hydrogen, sulfur, nitrogen, and oxygen), carbon isotope composition, and pyrolytic characteristics of kerogens were determined. The type and catagenesis of organic matter (OM) and generation potential of oil source rocks were estimated. It was found that the oxidizing conditions of accumulation of organic matter had a significant effect on the composition of kerogens in the northeastern West Siberia. Compositional variations of kerogens depending on the OM type and catagenesis have been revealed. The proportions of biochemical components (lipids, proteins and carbohydrates) in the initial OM in rocks and their influence on the elemental composition of kerogens have been discussed.

The paper presents an analytical review of studies that discuss issues concerning the geochemistry of organic matter in Jurassic–Cretaceous rocks in the Arctic regions of Western Siberia and genetically related oils, gas condensates, and hydrocarbon gases. The Late Jurassic Bazhenov Formation and the coeval Golchikha (upper part) and Yanovstan (middle part) Formations are regarded as the major oil-source rocks in this region. Unlike the classic Bazhenov Formation in the central areas of Western Siberia, the Late Jurassic oil-source marine organic matter (OM) in the Arctic region is characterized by an admixture of terrestrial biomaterial and oxidizing conditions during diagenesis, with these features reflected in its composition of the OM and the related oils and gas condensates. These hydrocarbon fluids were accumulated mostly in Early Cretaceous rocks. The other oil-source rocks in the area are those of Early–Middle Jurassic formations enriched in mixed highly mature organic matter: the Zimnyaya, Sharapovo, Kiterbyut, Laida, and Malyshev. Oil and gas condensates generated in these formations were detected in the Middle Jurassic rocks (in the Yenisei–Khatanga area and in the eastern part of the Gydan Peninsula), and in the Jurassic and Cretaceous rocks (in the Yamal Peninsula and in the western Gydan Peninsula). Other rocks thought to have been able to generate oil in the area are the shale beds enriched in marine organic matter of the poorly studied Late Jurassic Sigov and Abalak Formations and the lower strata of the Early Cretaceous sedimentary sequences (Nizhnyaya Kheta, Shuratov, and Akh Formations). The almost whole Jurassic–Cretaceous rock complex is thought to be gas-producing. However, the main gas-generating rocks are thought to be those of Early–Middle Jurassic age and the Late Jurassic rocks with highly mature organic matter. The source of isotopically light dry gases of the Aptian–Albian–Cenomanian formations are reportedly the terrestrial (carbonaceous) organic matter in the Cretaceous rocks affected by early catagenic transformations. The contribution of the early catagenic gases to the gas potential of this region is debatable. Oil and gas generation centers in Arctic Western Siberia are localized in the most strongly subsided depression zones of the area and south of it. It should be noted that much published data on the geochemistry of organic matter are unsystematic and heterogeneous, and hence, detailed geochemical studies of the Jurassic–Cretaceous rocks of this area as a whole shall be carried out, with primarily focus on its northernmost parts.

—The uppermost Bathonian–lowermost Boreal Berriasian clay horizons (Gol’chikha Formation) of the Yenisei–Khatanga regional depression are regarded as probable oil source strata. Considerable core recovery in the Middle Jurassic to Lower Cretaceous sections from the boreholes drilled in the Paiyakhskaya well site and the presence of oils in the overlying strata of the Shuratovo Formation permit us to carry out integrated stratigraphic (bio-, litho-, chemo-, and seismostratigraphic) and geochemical (organic matter and oils) studies of the entire section of the Gol’chikha Formation and boundary beds, to reveal oil-producing horizons, and to compare the genotype and maturation level of their oils with those of the potentially oil source organic matter (OM) of the rocks. A detailed biostratigraphic zonation of the sections of the Gol’chikha Formation based on microfossils has been carried out. Comparison of δ13Corg variations in the Volgian and in the lower beds of the Boreal Berriasian with those in the Barents Sea shelf and in the northeast of East Siberia provided the basis for more accurate definition of the boundaries of stages and substages in the intervals free of fossils in the Paiyakhskaya area. The studied section of the Gol’chikha Formation has been divided into eight lithologic members calibrated with bio- and seismostratigraphic units. The distinctive features allowing the definition of the upper boundary of the Gol’chikha Formation are proposed using GIS data. Analysis of bio- and chemostratigraphic data allowed the correlation of the seismic reflecting horizons defined in the Gol’chikha Formation and its boundaries with the geologic section and relevant litho- and biostratigraphic units. It has been established that the strata with the highest content of organic matter consist of the upper part of the Gol’chikha Formation (the Upper Volgian and basal Boreal Berriasian). According to the vitrinite reflectance data, the OM catagenesis in the Upper Volgian interval corresponds to the oil window, which is confirmed by pyrolysis data. Thus, these strata can be considered oil-producing. The low δ13Corg values confirm the predominantly marine OM composition. Analysis of oils from the Cretaceous productive strata of the Paiyakhskaya area shows that they formed from the marine OM of the upper part of the Gol’chikha Formation at the same accumulation stage. The comprehensive studies of the Gol’chikha Formation in the Paiyakhskaya well site and complete stratigraphic coverage of the sections confirm that they can be considered a hypostratotype.

We studied the distribution of organic carbon in rocks of the Bazhenov horizon, a unique object of predominantly biogenic sedimentation in the West Siberian sedimentary basin. The contents of organic carbon in the rocks were determined using the data from 4094 core analyses and core-log relationships derived from 48,500 radioactive- and electrical-log measurements. For the Bazhenov and Tutleima Formations, both approaches gave the same results. The average content of organic carbon in the rocks is 7.7%. These data were used to compile a detailed map of the distribution of organic carbon contents in sedimentary rocks of the basin. It was shown that the average organic carbon content in the rocks increases from 2-4% on the periphery of the basin to 10-12% in its central, deepest part. The distribution of C-org values in the basin is highly asymmetric. The highest C-org values are observed in the southwestern part of the basin interior, where beds with >10% C-org range in thickness from 5 to 12-15 m. In sections, the highest C-org values are observed in their middle and upper parts, composed predominantly of silicites and mixtites enriched in biogenic silica. (C) 2018, V.S. Sobolev IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved.

This study presents the results of geochemical analysis of organic matter and paleogeographic reconstructions supplemented by data from well log interpretation and cyclostratigraphic analysis for the Vasyugan, Georgiev, and Bazhenov Horizons of the Callovian-Lower Berriasian section in the western part of Yenisei-Khatanga regional trough and adjacent areas of the West Siberian geosyneclise. It was found that each horizon contains zones dominated by terrigenous, mixed, and aquatic organic matter. The distribution of different types of organic matter over the area and throughout the section has been examined. It was shown that the accumulation of aquatic organic matter took place in the deepest parts of the trough during the Callovian-Oxfordian. The area of accumulation of aquatic organic matter expanded considerably and reached its maximum extent within the Bolshaya Kheta megasyneclise during the Kimmeridgian and Early Volgian and in the west Yenisei-Khatanga regional trough during the Volgian and Early Berriasian.

We performed a chromato-mass-spectrometric study of the distribution of hopane hydrocarbons in the organic matter of Jurassic–Lower Cretaceous mudstones from wells drilled in the western Yenisei–Khatanga regional trough. A series of 25-norhopanes has been first identified; these hydrocarbons, often found in naphthides, are commonly related to biodegradation processes. We consider other hypotheses of formation of 25-norhopanes. Based on the examined distribution of rare rearranged hopanes, new biomarker parameters have been introduced, which ensure a more reliable estimation of the conditions of accumulation of organic matter in rocks and of its thermal maturity.

We present results of geochemical studies of organic matter of the Jurassic–Cretaceous deposits in the west of the Yenisei–Khatanga regional trough. The studies were carried out on a representative set of well cores by a complex of modern organic-geochemistry methods (determination of organic-carbon content in rocks, pyrolysis, estimation of the carbon isotope composition in the kerogen of rocks, extraction, liquid and gas–liquid chromatography, and chromato-mass spectrometry). Based on the distribution of biomarkers in the studied bitumens and pyrolysis of rocks, two groups of the samples were recognized: with terrigenous (type III) and marine (type II) organic matter. The terrigenous bitumens are characterized by a low hydrogen index (HI) and a predominance of hydrocarbons C29 among steranes and C19 and C20 among tricyclanes. The marine bitumens, revealed in stratigraphic analogs of the Bazhenovo Formation and in the Malyshevka, Nizhnyaya Kheta, and Shuratovka Formations, show an even distribution of sterane homologues and a predominance of medium-molecular tricyclanes. The Pr/Ph and C35/C34 ratios and the presence of diahopanes testify to the burial of organic matter in suboxidizing sea coast environments. In the Yanov Stan (J3–K1), Gol’chikha (J2–K1), and, to a lesser extent, Malyshevka (J2), Nizhnyaya Kheta, and Shuratovka (K1) Formations, we have recognized widespread stratigraphic levels with marine organic matter of rocks. Its contents and degree of maturity permit these rocks to be considered oil-generating.