The Longmaxi Shale is the most productive stratum for shale gas in China. Although its pores have been investigated extensively, their openness and controlling factors remain poorly understood. This study integrates small-angle x-ray scattering (SAXS), low-pressure N2 adsorption (LPNA) and scanning electron microscopy (SEM) to quantitatively resolve open/closed pore characteristics (1-100 nm) in the bulk and kerogen samples of the Longmaxi shale in the Chongqing area, China, and to identify their geological controls. Results indicate that pore development is primarily governed by TOC, R o and clay mineral content. Closed pores exhibit a significantly high proportion within kerogen, are predominantly mesopores and constitute the main contributor to the total shale porosity. Compared with inorganic minerals, organic matter shows stronger compositional homogeneity, with minimal variability in pore size across samples. For the volume of open pores obtained from N2 gas adsorption, organic pores contribute 2.8%-34.2% (average 9.58%) to total open pore volume, while inorganic pores account for more than 70%. However, SEM imaging reveals abundant organic pores, suggesting that a substantial proportion of these pores may be closed, with gases trapped in closed organic pores being non-producible. Consequently, reliance on SEM evaluation alone may lead to significant overestimation of recoverable shale gas reserves, underscoring the critical importance of quantitative assessment of closed pores.

This study presents an integrated, multi-scale laboratory workflow designed specifically for organic-rich shales using multistage solvent extraction. Applied to oil shales of the Bazhenov Formation of varying maturity and lithology, the key unconventional play in Western Siberia, it enables the construction of a robust, volumetric fluid saturation model. The workflow combines mineralogical characterization, conventional core testing, low-field nuclear magnetic resonance relaxometry, high-resolution X-ray computed microtomography, Rock-Eval pyrolysis, and sequential saturates, aromatics, resins, and asphaltenes fractionation following a three-stage solvent extraction protocol. The core analysis following three-step extraction provides new insights into the interplay between lithology, pore system architecture, and fluid distribution mechanisms within tight, organically heterogeneous media. Key findings highlight that conventional methods often underestimate producible hydrocarbons trapped in kerogen nanopores and asphaltene aggregates, necessitating revised nuclear magnetic resonance interpretation approaches. Mechanically induced porosity, varying with organic matter maturity, is identified and linked to hydrocarbon release and matrix deformation. Combining nuclear magnetic resonance and gas porosity measurements provides a rapid, accurate porosity estimation method with minimal sample alteration. Finally, a conceptual fluid physical model is proposed to better interpret nuclear magnetic resonance data and pore-scale fluid dynamics in similar oil shales. The refined methodology of express core assessment significantly improves industry conventional practices by enabling a more precise and physically meaningful quantification of in-situ fluid saturation, including differentiation between bound heavy hydrocarbons and mobile fractions. Beyond advancing the fundamental understanding of fluid saturation and storage capacity in unconventional systems, this framework supports improved reservoir characterization and modeling efforts.

This study focuses on the investigation of Late Oxfordian-Early Tithonian manganese microbial carbonates (MC) found at the top of the Georgiev regional stage in the Western Siberian oil and gas basin. In some isolated areas, these MC are oil reservoirs. However, these rocks are still one of the least studied sediments in the carbonate interval at the boundary of the Bazhenov and Georgiev subregional stage. We have described their geochemical, lithological and mineralogical composition in detail. Based on the characteristics of MC, we assumed their formation settings. The elevated MnO content, associated with microbial processes, and the high concentration of biophilic elements, both suggest the active involvement of organic matter in the formation and development of microbial sediments. The isotopic composition of carbon in the microbial carbonates further supports a biogenic source of carbon dioxide playing an important role in the formation of these deposits. We have established that the Late Jurassic manganese MC were intensely secondary altered, partially recrystallized and calcified by catagenetic processes. Isometric pyrite crystal aggregates with kutnohorite fragments were formed at the contacts between kutnohorite and calcite zones. We suggest that the presence of caverns and high reservoir potential in the microbial carbonates from the YemEgovskaya field of the Krasnoleninsky arch may be attributed to intense secondary catagenetic alterations.

This study presents the first comprehensive comparison of core sediments (0–155 сm) from an active methane seepage and a background (undisturbed) site on the northern Black Sea shelf (near Cape Martyan). Methods included grain-size analysis, toral organic (TOC) and inorganic carbon (TIC) analysis, gas chromatography–mass spectrometry (GC-MS) based biomarker analysis of non-polar and polar organic matter fractions, pyrolysis-GC-MS (Py-GC-MS) and mineralogical analysis by X-Ray diffraction (XRD) and scanning electron microscopy with energy dispersive-ray analysis (SEM/EDX).,Principal differences have been identified in depositional and early diagenesis. Background sediments are characterized by a diffusion-production methane profile, homogeneous silt-like composition, and stable molecular signatures. Meanwhile, methane seepage is marked by the dominance of an advective methane flow, grain-size heterogeneity, and early pyrite and glauconite formation. The content of TOC and total lipid extract in the seepage column is on average half that of the background, while TIC is 15% higher. Py-GC-MS and biomarker analysis results indicate an increased production of autochthonous organic matter in the seep, but this effect is offset by intensive microbial mineralization during the anaerobic oxidation of methane (AOM). It is shown that even a low-flow methane seep significantly transforms the mineral composition and organic matter of sediments. Results are important for reconstructing palaeoconditions, assessing biogeochemical carbon cycles, and predicting the impact of methane releases under climate change.

The Bazhenov horizon has been studied since the middle of the 20th century and is still a promising prospect for oil and gas exploration. This article presents an overview of the structure of the siliceous radiolarite part of formations revealed in the section and composing the Bazhenov horizon. Three morphological types of radiolarite layers were identified: well-pronounced layers with erosional boundaries, lens-shaped radiolarites with clear boundaries, and poorly traced layers and single radiolarite lenses. The formation of radiolarians of different types is associated with benthic paleoflows. Areal localization of these types of deposits enables us to determine the areas affected by benthic paleoflows in the Volga age and thus to identify the most prospective exploration areas.

To reveal the forming process of organic matter pores in shales, an experiment combining thermal heating and scanning electron microscopy (SEM) was conducted on an oil shale sample with a vitrinite reflectance value of 0.46% from the Huadian Formation in the Huadian Basin, northeastern China. The heating temperatures were from 417.8 °C to 700.8 °C, and the corresponding Easy%Ro values were between 1.00% and 3.70%. Four pieces of macerals in the SEM images, including vitrinite (one piece), funginite (one piece), and solid bitumen (two pieces), were observed during the whole heating process. The results showed that organic pores started to appear and increased in all the studied macerals. Each piece of maceral had two rapid growth points of organic matter pores. During heating, organic pores were initially isolated and then became connected. Among the three types of macerals, solid bitumen was more porous, which may be related to the fact that solid bitumen was more easily thermally degraded. Funginite had more pores than vitrinite at all the heating temperatures. Cracks were observed in vitrinite and funginite during heating, and the vitrinite had more cracks, which may be attributed to its stiffness and brittleness. Almost all the organic matter pores were irregular in this study, but bubble-like or sponge-like organic pores have been reported in natural shales. The difference in shapes of organic matter pores may be derived from our experimental system as it cannot consider pressure. These results provide some implications for the mechanism of formation of organic matter pores.

Microorganisms capable of degrading hydrocarbons are regular components of natural microbial communities and play an important role in self-purification of marine environments from oil contamination. High-throughput sequencing of the 16S rRNA gene V4 variable region was used to analyze microbial communities of the Barents and Pechora seas and of the microcosms with a spectrum of hydrocarbon substrates: oil, n-nonane, n-undecane, and phenanthrene. The Barents Sea communities of hydrocarbon-oxidizing microorganisms were characterized by the predominance of the genera Pseudoalteromonas, Pseudomonas, Porticoccus, and Oleispira, while those of the Pechora Sea contained members of the genera Rhodococcus, Dietzia, Sphingorhabdus, and Hyphomonas. Pure cultures of these microorganisms were shown to utilize the major oil hydrocarbons: n-alkanes, cycloalkanes, and aromatic compounds.

The Bazhenov high-carbon formation is a set of marine pelitomorphic deposits enriched in organic matter, which accumulated under conditions of subsidence of the bottom of the sedimentary basin, which was not compensated by sediments. The formation contains hydrocarbons in its entire volume and is a promising source for expanding the hydrocarbon resource base. This article presents a set of the results of core studies and interpretation of well logging and seismic survey data for structural and facies typification of sections of the Bazhenov high-carbon formation throughout the area of its distribution. A multi-faceted approach to studying rocks has made it possible to divide the West Siberian basin into 13 structural-facies zones, each of which has its own structural features of the Bazhenov high-carbon formation section, including the total thickness, the presence of lithological and geophysical members and lithophysical types of rocks in the members, and the total concentration of organic carbon. In the future, this typification of sections will make it possible to simplify the modeling of the properties of Bazhenov high-carbon formation and the prediction of the most prospective areas.

The paper presents the results of the study of rock samples from Bazhenov deposits before and after laboratory modelling of hydrothermal processes in autoclaves. To evaluate the influence of hydrothermal influence on rocks of different degrees of transformation, 3 samples from the well, the rocks in which are located at the very beginning of the main zone of oil formation (PK3–MK1), and 4 samples from the well, the rocks in which are located in the middle of generation (MK2–MK3) were studied. Comparison of samples before and after heating was carried out by means of pyrolytic, coal petrography studies and analyses of polished section under scanning electron microscope. It is shown that at 350 °C there is a change in pyrolytic characteristics of the rock, which depends on the nature of organic matter and the degree of catagenetic transformation of kerogen at the time of exposure. It was found that as a result of exposure the initial macerals decrease in size and change their luminescence in ultraviolet light, secondary (kerogenic) porosity appears in onychite fragments. New minerals, in particular barium zeolites, are formed in the rocks. Identification of such formations in natural objects may indicate the occurrence of local temperature processes, which may lead to heterogeneity of organic matter transformation in Bazhenov sediments and should be taken into account to identify local zones of increased catagenesis.

The article describes the sedimentation environments of Lower Cretaceous (Aptian–Albian) terrigenous deposits in the North Caspian. These rocks are oil and gas reservoirs and are characterized by an uneven distribution of the pelitic and silty fractions, which leads to a high content of residual water, low permeability, and poor consistency of the filtration-capacity properties with each other. A detailed lithological and facies analysis of the rocks was carried out for correct interpretation of geophysical data. The following marine environments predominated in the studied area in the Early Cretaceous: coastal, shallow marine, relatively shallow shelf with active hydrodynamics; relatively deep-water shelf with low hydrodynamics. The sections show a change from shallow to deeper facies and vice versa, which allows us to conclude periodic fluctuations in sea level. The results of lithological analysis clearly indicate the predominance of shallow marine and coastal sediments in the Aptian and deeper, shelf sediments in the Albian. Thus, we can suggest a sea transgression at the end of the Early Cretaceous and detect signs of it in the sections of Aptian–Albian deposits in the North Caspian.

Both the prolific source rocks of the Bazhenov Formation and the reservoir-prone units of the Abalak Formation cover most of the West Siberian region. Hydrothermal fluids circulation has been suggested to affect hydrocarbon potential and reservoir properties at some localities of this vast hydrocarbon province. However, integrated studies are currently lacking. Here we present an extensive multidisciplinary study combining regional studies with core logging properties and sample analyses (i.e. geochemistry, petrography, organic geochemistry) to reveal processes related to the hydrothermal activity that affected both formations. We also focus on the factors and implications for secondary unconventional reservoir formation within the deposits from the Kamennaya summit of the Frolov oil and gas region. Three boreholes corresponding to type sections located in different parts of the study area (elevated, transitional and deep) were investigated. The elevated and transitional parts of the basin are adjacent to the weakened contact zones of a granite massif. These tectonic discontinuities acted as conductors for hydrothermal fluids circulation on the area. Lithological and stable isotope studies revealed the presence of intensive hydrothermal secondary alterations in various types in rocks, including massive metasomatic carbonatization, sulfatization, sulfide mineralization and leaching processes. This hydrothermal fluid activity resulted in reservoir formation within siliceous radiolarite layers, while in the deep part of the basin potential reservoirs were completely healed. Organic matter maturity varies in different parts of the basin. We identify several mechanisms related to the various hydrothermal processes that influenced rocks composition, structure and organic matter maturity: (1) A weakened zone located on the periphery of the granite massif in the basin basement is characterized by extensive fault damage zones facilitating the migration of high temperature fluids along deposits during post-sedimentation stages; (2) The thickness, mineral composition of the rocks overlying the basement control fault attenuation and fluid workflow; (3) The fluids composition, their acidity/alkalinity, prompted various changes in rock structure, especially in secondary porosity formation. The identification of various hydrothermal processes and their influence on pore space allowed to determine the approach for the spatial distribution prediction of unconventional reservoirs in the studied formations, and subsequently improve the efficiency and effectivity of the exploration process not only at the Kamennaya summit, but also on the other similar West Siberian basin areas.

The results of the studies of organic matter in the rocks of the Bazhenov Formation in the territory of the Frolov oil and gas region of the West Siberian basin are presented. The maceral composition of the sediments is represented by bituminite and alginite, redeposited vitrinite, as well as bioclasts: onychites and calcespheres organic matter. The investigation was focused on bioclasts. Their coal petrographic characteristics and change in the process of catagenesis are described; qualitative parameters to access the degree of bioclast maturity are established; as well, an initial formula for converting the onychite reflectance to the equivalent of vitrinite reflectance is proposed.

Radiolarians are rock-forming constituents of siliceous beds of the Bazhenovo Formation in Western Siberia. Studying radiolarians is necessary for stratigraphy and correlation of the organic-rich Bazhenovo Formation, as they provide reliable dating of productive intervals of the section and detailed stratigraphy and zonation, which is especially important when data on other fossils groups are absent or insufficient. The evolution of the radiolarian fauna in the Bazhenovo Basin of Western Siberia is briefly reviewed. At different times radiolarian paleocommunities were dominated by different morphotypes. Two new species, Orbiculiforma sibirica sp. nov. and Emiluvia retorta sp. nov. are described.

—Geochemical studies of Inikan Formation rocks from the coastal outcrops of the Yudoma River were carried out. All analyzed lithological varieties are characterized by significant enrichment in B, Ni, Mo, Ag, Sb, U (redox-sensitive trace metals and elements associated with biodeposition). The source of the elements was sea water that leached them from the eolian material. It was found that the deposits accumulated in anoxic environments with periods of both euxinic (probably, in the bottom water mass) and suboxic conditions. Such anoxic and euxinic conditions developed due to the stable stratification of the ocean that existed in the early–middle Cambrian in this part of the basin. In addition, the long-term accumulation of organic-rich sediment under conditions of anoxia, periodic euxinia, and stable stratification indicates the presence of a geomorphologically isolated depression in this part of the basin. CIA-Kcorr ranges from 68 to 95. The low values of the CIA may be related to the height of the relief in the sedimentary source area.

The paper presents the results of the closed system hydrous pyrolysis isothermal kinetic experiment on a source rock sample from the Bazhenov shale formation. After 48 hours of thermal exposure at a temperature of 350 °С 80% of the kerogen generation potential relative to the original sample was transformed into hydrocarbons. Organic matter changes during laboratory modeling of maturation, controlled by pyrolytic parameters, follow the same trends as in natural source rocks. The kinetic spectra of kerogen activation energy destruction show a heterogeneity of organic matter inside the rock. At the constant frequency factor, a decrease in low-energy bonds and an increase in the release of hydrocarbon compounds from 53 to 54 kcal/mol are observed. Calculated changes in hydrocarbon compounds formation at different activation energies depending on maturity level are different from experimental ones, which indicates the influence of maceral composition and necessity to take into account a variability of individual components in basin modeling.

Выполнены геохимические исследования пород иниканской свиты из береговых обнажений р. Юдома. Для всех проанализированных литологических разностей характерно значительное обогащение B, Ni, Mo, Ag, Sb, U, т. е. редокс-чувствительными элементами, а также элементами, способными к биоосаждению. Источником элементов являлась морская вода, выщелачивающая их из эолового материала. Установлено, что отложения накапливались в аноксических условиях с периодами возникновения как эвксинных (по-видимому, в придонном слое), так и субоксических условий. Развитие таких аноксических и эвксинных условий вызвано устойчивой стратификацией океана, существовавшей в раннесреднекембрийское время в данной части бассейна. Помимо этого, длительное накопление богатых ОВ осадков в условиях аноксии, периодической эвксинии и устойчивой стратификации указывает на возможное наличие в данной части бассейна геоморфологически обособленной впадины. CIA—Кcorr варьируется от 68 до 95. Невысокие значения индекса могут быть связаны со значительной высотой в области денудации. Geochemical studies of Inikan Formation rocks from the coastal outcrops of the Yudoma River were carried out. All analyzed lithological varieties are characterized by significant enrichment in B, Ni, Mo, Ag, Sb, U (redox-sensitive trace metals and elements associated with biodeposition). The source of the elements was sea water that leached them from the eolian material. It was found that the deposits accumulated in anoxic environments with periods of both euxinic (probably, in the bottom water mass) and suboxic conditions. Such anoxic and euxinic conditions developed due to the stable stratification of the ocean that existed in the early–middle Cambrian in this part of the basin. In addition, the long-term accumulation of organic-rich sediment under conditions of anoxia, periodic euxinia, and stable stratification indicates the presence of a geomorphologically isolated depression in this part of the basin. CIA-Kcorr ranges from 68 to 95. The low values of the CIA may be related to the height of the relief in the sedimentary source area.

Abstract—The geochemical and lithological features of bacterial–algal structures from the top of the Abalak and Georgia Formations in the central part of Western Siberia were studied in order to determine the specific conditions of their formation. The authors compared the element composition of these microbial carbonates with the secondary carbonate rocks of the Abalak and Georgia formations. According to the results of X-ray fluorescence analysis, differences were identified in the contents of MnO, Cr, V, Ni, Cu, and Zn in two types of carbonates. The higher content of MnO in bacterial–algal structures were explained by the ability of bacteria to sorb Mn on the surface of their cells. This process requires oxygen, which suggests the presence of a natural aerobic environment for the development of bacteria during the period of sediment accumulation. According to the results of the study of microbial carbonates under a scanning electron microscope, it was revealed that Mn, for the most part, is concentrated in carbonate minerals, in particular in kutnohorite. The increased content of biophilic elements such as Ni, Cu, Zn, V in microbial carbonates, is probably associated with the transformation of humic organic matter, that was accumulated in shallow water environments and was actively recycled by microbial organisms. Minerals with the high Ba concentration were also found in isolated bacterial–algal structures. According to the authors, such single barium mineralization could be caused by the point effect of both near-surface and deep-seated barium-containing solutions and are not associated with an increased content of manganese in the studied deposits.

Despite the fact that the history of studying the geology of the Achimov Formation spans several decades, the depletion of its reserves is low, and the fields where the Achimov beds have been put into commercial development are few. This is due to the complex geological structure of the reservoirs, their vertical and lateral heterogeneity, low porosity and permeability properties, and low oil saturation factor. This paper presents a generalization and analysis of all petrophysical information and a typification of sections from log data with the possibility of using individual petrophysical relationships to assess the nature of reservoir saturation and the hydrocarbon saturation index. Both historical core data and studies made for this paper by the authors were used to create a petrophysical basis for well log interpretation. The typification of the Achimov sections is based on a detailed lithological core analysis using special tests such as scanning electron microscopy and computed microtomography. The identification of different types of sections will allow adjustment of a further hydrodynamic model for predicting the nature of the influx and will significantly increase the reliability of assessing the nature of saturation and the hydrocarbon saturation index of heterogeneous deposits in the Achimov Formation.