Geochemical typification of oils and condensates of the Geophizicheskoye and Soletsko-Khanaveyskoye (Khanaveyskaya area) fields (The Gydan Peninsula, West Siberia) was carried out. According to the obtained geochemical information (carbon isotope composition, molecular parameters of genotype and maturity) and its comparison with published materials on the geochemistry of organic matter, it is shown that hydrocarbon accumulations of these fields were formed due to the oil-and-gas generation in the Jurassic source rock strata. The oil of the Geophizicheskoye field from the Akh Formation is generated by the mature organic matter of the Bazhenov Formation. Other samples of the studied collection from Cretaceous accumulations are characterized by mixed facies-genetic type, i.e. they are formed both due to the mixed organic matter of the Lower – Middle Jurassic oil-and-gas source rock and by mixture of hydrocarbon fluids migrating into traps from various Jurassic oil-and-gas source rocks. Judging by genetic indicators of the С5–C8 hydrocarbons composition, at least the light (gasoline) fraction of the condensate from Middle Jurassic strata of the Geophizicheskoye field was generated predominantly by the terragenic organic matter of the Lower – Middle Jurassic. Low concentrations or absence of n-alkanes and acyclic isoprenanes indicate the development of biodegradation processes of the initial and middle stages in shallow Aptian-Albian-Cenomanian accumulations.

The paper presents the results of the geochemical study performed on the core material (30 samples) selected from the intervals of the Akh (K1), Malyshev (J2) and Vym (J2) Formations in the Shtormovaya-125 well. Based on the results of pyrolytic studies of the rocks, distribution of hydrocarbon biomarkers in the composition of bitumoids and data on the isotopic composition of organic carbon, the homogeneous terrestrial genotype of the organic matter contained in the studied samples was determined; the accumulation of this organic matter occurred in environments from weakly reducing (Akh Formation) to suboxidizing (Malyshev and Vym Formations). Moreover, the concentration and catagenetic transformation of organic matter in the studied rocks, on average, are shown to increase from top to bottom along the well section. The values of pyrolytic parameters and the Corg index allow us to estimate generation potential of the studied rocks from low (Akh Formation) to moderate and high (separate intervals of the Malyshev and Vym Formations). At the same time,taking into account the terrestrial genotype, the OM of these strata may be a source of gaseous hydrocarbons. In addition, the revealed features of individual and group distribution of components in the composition of saturated and aromatic fractions of bitumoids enable considering the studied section as a more transformedanalogue of the “terpane geochemical horizon” distinguished in the lower part of the Oxfordian-Valanginian Nordvik section (Anabar Bay).

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 work is devoted to the analysis of the geological structure and assessment of the oil and gas potential of the Anabar-Khatanga and Lena-Anabar oil and gas areas (OGA) of the Lena-Tunguska oil and gas province, which are located on the continental margin of the Siberian Platform, on the coast of the Laptev Sea. Administratively, the study area includes the northeastern regions of the Krasnoyarsk Territory, the northwest of the Republic of Sakha (Yakutia), the offshore of the Khatanga Bay and the southern, transit part of the Laptev Sea (Khatanga-Lena interfluve). Based on integrated interpretation of geological and geophysical materials, we present regional-zonal models of the geology of the upper Proterozoic-Phanerozoic deposits of the Arctic regions of the Siberian Platform, sets of seismogeological sections, structural and parametric maps, results of structural-tectonic analysis, potential estimates of oil-producing formations; analysis of reservoir properties and types of oil and gas promising objects, and a quantitative assessment of hydrocarbon resources. The study area is of interest in terms of oil and gas potential. In this region, the Central Olgin field was discovered, numerous bitumen, oil and gas shows were noted. A series of oil-producing formations enriched in organic matter occurs in the sedimentary cover, and in the sections of all sedimentary complexes, high-capacity reservoirs are distinguished. There are prerequisites for the formation of both anticlinal and complex oil and gas promising objects. The prospects for the oil and gas potential of the Anabar-Khatanga OGA are mainly associated with Permian terrigenous deposits and Cambrian-Devonian carbonates. In the Lena-Anabar OGA, all upper Proterozoic-Paleozoic sedimentary complexes from the Riphean to the Permian level are of interest in terms of oil and gas potential.

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.

Проведенные исследования и анализ опубликованных данных нацелены на совершенствование высокоразрешающего биостратиграфического расчленения верхней юры и нижнего мела эталонного разреза на п-ове Нордвик, Анабарский залив, море Лаптевых. Это позволило установить в разрезе последовательности из 13 биостратонов по фораминиферам, 9 биостратонов по диноцистам и 8 с палинофлорой в ранге зон и слоев. Полученные новые данные дали возможность обосновать стратиграфическую непрерывность этого разреза в интервале нижнего и средневолжского подъярусов, где ранее предполагался крупный стратиграфический перерыв. Установленные зональные подразделения имеют разный корреляционный потенциал. Рассмотрены разные версии расчленения разреза по аммонитам и обоснована используемая шкала. По кальцисферам/известковым диноцистам выявлен реперный уровень, позволяющий осуществлять глобальные корреляции и увязывать приграничные толщи юры и мела Тетиса и арктических регионов. Обсуждено магнитостратиграфическое расчленение изученного разреза и его сопоставление с тетическими регионами. Показано, что кривая вариаций δ13Cорг в верхней юре и нижней части валанжина отражает фациальные изменения и с учетом биостратиграфических данных может служить инструментом для корреляций в разных регионах Северного полушария. Для верхневолжского подъяруса и бореального берриаса установлена цикличность изменений изотопного состава углерода. В исследованном разрезе определено распределение значений геохимических параметров (Сорг, δ13Cорг и водородного индекса), показана геохимическая стратификация разреза и вариации породообразующих оксидов. В основании паксинской свиты (верхневолжский подъярус) установлена резкая смена значений геохимических параметров. Определен интервал распространения «подповерхностного максимума хлорофилла». Проведенные исследования показывают, что высокие значения Сорг с легким изотопным составом первого диастеренового геохимического подгоризонта формировались за счет высокой биопродуктивности фитопланктона и накопления органического вещества в благоприятных для его сохранности восстановительных условиях. Анализ содержания основных породообразующих оксидов в изученных породах позволил выявить определенные вариации химического состава изученных отложений, отражающие изменения минералого-петрографических характеристик. The goal of this study was to improve the high-resolution biostratigraphy of the Upper Jurassic and Lower Cretaceous in the type section on the Nordvik Peninsula, Anabar Bay, Laptev Sea. The results were used to identify a succession of 13 foraminiferal biostratigraphic units, 9 dinocyst units, and 8 palynofloral biostratigraphic units in the rank of zones and local zones. Based on new data, the stratigraphic continuity of this section is proved for the Lower and Middle Volgian, where a major stratigraphic hiatus was previously assumed. The established zones have variable correlation potential. Different versions of the subdivision of the section based on ammonites are discussed and the proposed scale is justified. A reference level based on calcispheres/calcareous dinocysts is defined providing global correlations and calibration of beds near Jurassic-Cretaceous boundary in the Tethyan and Arctic regions. The magnetostratigraphic subdivision of the studied section and its comparison with the Tethyan regions are discussed. Together with biostratigraphic data, the proposed δ13Corg curve in the Upper Jurassic and lower part of the Valanginian can be a tool for detailed correlations in different regions of the Northern Hemisphere. The cyclicity of carbon isotope composition in the Upper Volgian and Boreal Berriasian and isotope events in the Volgian and Lower Valanginian are determined within the Arctic region. The distribution of geochemical parameters (Corg, δ13Corg and hydrogen index) is determined in studied section, geochemical stratification of the section and variations in major oxide compositions are demonstrated. A sharp change in geochemical parameters is identified at the base of the Paksa Formation (Upper Volgian). The distribution interval of the “subsurface chlorophyll maximum” is determined. The study shows that high concentrations of isotopically light organic carbon in the first diasterene geochemical subhorizon were caused by high phytoplankton productivity and were deposited under reducing conditions favorable for preservation of organic matter. The analysis of major oxide compositions of the studied rocks revealed that variations in the chemical composition of the studied rocks reflect changes in their mineralogy and petrography.

Results obtained in the studies of gas composition in the hydrocarbon pools of the Jurassic–Cretaceous sediments in the Arctic regions of West Siberia are reported. The specific features of gas composition in hydrocarbon pools are shown. The main features of the vertical and lateral gas zoning in the Aptian–Albian–Cenomanian, Berriasian–Hauterivian and Jurassic oil and gas bearing complexes are revealed. Generalization of the data on the geochemistry of hydrocarbon gases (the features of component composition, the isotope composition of methane), along with the published data of the geological and geochemical investigation of the Arctic regions of West Siberia, allows us to consider the highly mature Triassic–Middle Jurassic coaly matter as the source of the dry hydrocarbon gases of the Lower and Middle Jurassic complex. Either the Triassic–Middle Jurassic sediments or the Upper Jurassic beds, enriched with the aquatic organic matter, could be involved in the formation of Upper Jurassic and Lower Cretaceous pools. The low-maturity terrestrial (carbonaceous) organic matter of the Cretaceous sediments generated the major amount of light-isotope gases in the Aptian–Albian–Cenomanian sediments. In addition, the source of those gases could be methane formed during oil biodegradation.

goal of this study was to improve the high-resolution biostratigraphy of the Upper Jurassic and Lower Cretaceous in the type section on the Nordvik Peninsula, Anabar Bay, Laptev Sea. The results were used to identify a succession of 13 foraminiferal biostratigraphic units, 9 dinocyst units, and 8 palynofloral biostratigraphic units in the rank of zones and local zones. Based on new data, the stratigraphic continuity of this section is proved for the Lower and Middle Volgian, where a major stratigraphic hiatus was previously assumed. The established zones have variable correlation potential. Different versions of the subdivision of the section based on ammonites are discussed and the proposed scale is justified. A reference level based on calcispheres/calcareous dinocysts is defined providing global correlations and calibration of beds near Jurassic-Cretaceous boundary in the Tethyan and Arctic regions. The magnetostratigraphic subdivision of the studied section and its comparison with the Tethyan regions are discussed. Together with biostratigraphic data, the proposed delta 13Corg curve in the Upper Jurassic and lower part of the Valanginian can be a tool for detailed correlations in different regions of the Northern Hemisphere. The cyclicity of carbon isotope composition in the Upper Volgian and Boreal Berriasian and isotope events in the Volgian and Lower Valanginian are determined within the Arctic region. The distribution of geochemical parameters (Corg, delta 13Corg and hydrogen index) is determined in studied section, geochemical stratification of the section and variations in major oxide compositions are demonstrated. A sharp change in geochemical parameters is identified at the base of the Paksa Formation (Upper Volgian). The distribution interval of the "subsurface chlorophyll maximum" is determined. The study shows that high concentrations of isotopically light organic carbon in the first diasterene geochemical subhorizon were caused by high phytoplankton productivity and were deposited under reducing conditions favorable for preservation of organic matter. The analysis of major oxide compositions of the studied rocks revealed that variations in the chemical composition of the studied rocks reflect changes in their mineralogy and petrography.

—Concepts of the stratigraphy of the coastal and shallow-sea sections of the uppermost Middle Jurassic and Lower Cretaceous, exposed on the right bank of the Anabar River, have radically changed many times. The performed study and analysis of the published data are aimed at refining the bio- and lithostratigraphic subdivision of the section, substantiating its stratigraphic completeness, and describing the identified stratigraphic units in detail. Comprehensive biostratigraphic studies of the uppermost Bathonian — the lowermost Boreal Berriasian reveal a sequence of nine biostratigraphic units with dinocysts and palynoflora in the ranks of zones and local zones. Some of the palynological biostratigraphic units have been identified for the first time. Foraminiferal assemblages of the boreal standard zones are recorded in the sections under study. The obtained biostratigraphic data and analysis of all known ammonite occurrences make it possible to demonstrate the stratigraphic continuity of the section in the Anabar River region, despite the reduced thickness of the stratigraphic units. The stratigraphic position of the boundary between the Sodiemykha and Buolkalakh formations is accurately defined. According to the interpretation proposed, the basal horizon of the Buolkalakh Formation is associated with a beginning of a new major sedimentation stage in the late Oxfordian–early Kimmeridgian, identified as the lower boundary of lithostratigraphic units of different ranks throughout the entire Arctic Region and partially in the Boreal Region. The lithogeochemical parameters of the studied formations are obtained for the first time. The content of Corg in the studied samples does not exceed its Clarke values, and the pyrolytic parameter Tmax indicates that the organic matter is immature and the petroleum potential is low. The considered organic matter of the rocks is characterized by a heavy isotope composition of carbon, suggesting its mostly terrigenous genesis.

For geochemical typification of oils and condensates from the fields of the Tambey group (the Yamal Peninsula) and determination of their assumed sources, the data on the organic geochemistry of the major oil-gas-source rock masses were generalized and compared with the results of geochemical interpretation of the analytical data on the composition of the studied naphthides. In accordance with the genetic hydrocarbon indicators (concentration ratios based on the composition of n-alkanes, acyclic isoprenanes, steranes, terpanes) and with the isotopic composition of carbon, three genetic types of naphtides are distinguished in the considered collection. The mixed genotype predominates, one sample corresponds to the aquagene genotype, and several samples correspond mainly to the terragenic genotype. Maturity indices with respect to the composition of steranes, terpanes and arenes provide evidence of the formation of the analysed samples under the conditions of the main oil generation zone. Relying on gas-liquid chromatographic data (the naphthenic hump, the absence of n-alkanes or their lower content compared to acyclic isoprenanes), condensates from the Albian-Cenomanian and, partially, from the Aptian deposits underwent microbial oxidation of the moderate and initial stages, respectively. The features of the physicochemical properties indicate the condensation type of most of the studied samples. This is consistent with the localization of the deposits in the zone of predominant gas-condensate accumulation.

This work is devoted to the geochemistry of oils and condensates from the Jurassic-Cretaceous deposits of the Bovanenkovskoye oil-gas-condensate field. The data on the distribution of low-boiling hydrocarbons (HC) in the crudes composition were interpreted and the results obtained were compared with the information on GLC and GC-MS of their saturated and aromatic fractions. It is shown that the studied set of oils and condensates can be divided into two groups of samples in accordance with the predominant type of their source organic matter (OM). The influence of biodegradation processes on the composition of condensates from the Aptian and Albian deposits was also determined.

—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.

The paper deals with analysis of the geochemical and NMR characteristics of oils from a number of fields in the West Siberian oil and gas province. The group composition of the studied samples was estimated by NMR data. The correlation with the results of geochemical analysis was established.

—The organic-geochemistry data reveal two levels in the reference Upper Jurassic–Lower Cretaceous section of the lower reaches of the Olenek River: lower (Volgian–lower Boreal Berriasian (diasterene)) and upper (Boreal Berriasian–Valanginian (hopane)). The Volgian beds are composed of clays with abundant prasinophytes Leiosphaeridia and Tasmanites and various dinocyst assemblages and have the highest content of organic carbon (Corg), up to 9%. Isoprenoids, in particular, pristane and phytane, are highly predominant among aliphatic hydrocarbons; their content is more than three times higher than that of coeluting n-alkanes, which is typical of buried chlorophyll-containing plankton (dinocysts and prasinophytes). Sedimentological, biofacies, and paleoecological analyses show that the highly carbonaceous beds of the Buolkalakh Formation formed under oxygen deficit conditions. An integrated analysis demonstrated that the pristane/phytane ratio does not always reliably reflect the reducing or oxidizing conditions of organic-matter accumulation and diagenesis. The discrepancy between the geochemical identification of organic matter according to the pristane/phytane ratio and the biofacies and sedimentological data is due to the low catagenetic maturity of OM. The Volgian was marked by a significant transgression of the Anabar–Lena sea, which was gradually changed by a successive regression of its basin at the end of this stage and in the Boreal Berriasian. The Corg contents in the coastal and subcontinental sediments decreased. Diasterenes and 4-methyldiasterenes disappeared from the balance of biomarker molecules, and the portion of hopanoids increased. Aerobic environments prevailed in the subbottom waters. Earlier, three biomarker horizons were identified according to geochemical criteria in the synchronous sections of Anabar Bay (Laptev Sea coast): terpane, diasterene, and hopane ones. In the section of the Olenek basin, the upper two horizons are well identified by specific biomarkers, and the lower one is absent because of the sedimentation break. Stratigraphic analysis of the location of these geochemical levels in different parts (and bathymetric zones) of the Anabar–Lena basin shows their diachronous formation. According to all geological and geochemical criteria, the Volgian Stage and the lower beds of the Boreal Berriasian Stage of this basin have a high petroleum potential. In the axial zone of the basin and, especially, on the Laptev Sea shelf, there were probably favorable conditions for the generation and accumulation of hydrocarbons genetically related to the Upper Jurassic highly carbonaceous rocks.

Представления о стратиграфии прибрежнои мелководно-морских разрезов верхов средней юры и низов мела, вскрытых на правобережье р. Анабар неоднократно кардинально изменялись. Проведенные исследования и анализ опубликованных данных нацелены на уточнение биои литостратиграфического расчленения разреза, обоснование его стратиграфической полноты, детальное и комплексное описание выделенных стратонов. Комплексные биостратиграфические исследования разреза верхов бата – низов бореального берриаса позволили выявить последовательности из девяти биостратонов в ранге зон и слоев с диноцистами и с палинофлорой. Часть палинологических биостратонов установлена впервые. В исследованных разрезах прослежены фораминиферовые комплексы зон бореального стандарта. Полученные новые биостратиграфические данные и анализ всех известных находок аммонитов позволили обосновать стратиграфическую непрерывность Анабарского разреза, несмотря на сокращенную мощность стратиграфических подразделений. Уточнено стратиграфическое положение границы между содиемыхаинской и буолкалахской свитами. В предлагаемой интерпретации базальный горизонт буолкалахской свиты фиксирует новый крупный этап осадконакопления в конце оксфорда – начале кимериджа, прослеживаемый как нижняя граница разноранговых литостратонов по всему Арктическому и частично Бореальному регионам. Впервые толща охарактризована литогеохимическими параметрами. Содержание Сорг в исследованных образцах свит не превышает его кларковых значений, а пиролитический показатель Tmax свидетельствует о низкой зрелости органического вещества и невысоком нефтегазогенерационном потенциале. Исследованное органическое вещество пород характеризуется тяжелым изотопным составом углерода, что позволяет предполагать его преимущественно террагенный генотип.

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