Understanding multiscale migration differentiation within shale oil systems is critical for identifying high-mobility migration sweet spots that facilitate the sustainable shale oil recovery. However, owing to geological complexity and technical uncertainty, the chemical differentiation and mobility effects of shale oil migration have not been clarified. This study is focused on elucidating the μm–m-scale migration differentiation and mobility effects in shale oil systems by combining multiple methods, including laser scanning confocal microscopy (LSCM), nuclear magnetic resonance (NMR), multi-temperature pyrolysis, and extract geochemical analysis. Core samples collected nearly equidistantly from the Triassic Chang 7 Member shale system in the Ordos Basin are employed for investigation. The LSCM results reveal that interlaminar shale oil migration on the μm–mm scale manifests in the form of light/heavy component differentiation. However, cm–m-scale migration across laminae assemblages (or lithofacies) corresponds to pyrolysis parameter anomalies, original–true hydrocarbon generation potential differences, chemical compositions and n-alkane differentiation. The migration hydrocarbon inputs can improve the in situ oil composition and movable oil content, thus affecting shale oil mobility. The mobility disparities between source–reservoir units in a shale oil system are primarily controlled by the degree of migration differentiation. Notably, the good source–good reservoir laminated units at the μm–mm scale and the thick source–thick reservoir configurations at the cm–m scale exhibit relatively strong migration differentiation and favorable mobility improvement. These mobility patterns associated with μm–m migration differentiation are critical for evaluating high-mobility sweet spots and developing cost-effective and sustainable shale oil systems.

Abstract Widespread organic‐rich shales are associated with perturbations in productivity or preservation conditions; however, the major controlling factors, especially the phosphorus (P) and iron (Fe) interactions and their impacts on organic carbon burial in the paleolake ecosystem remain poorly understood. Here, we report iron and phosphorus speciation data from the Chang 7 Member of Ordos Basin to elucidate iron–phosphorus–carbon cycling (Fe–P–C) in the freshwater lacustrine system during the Middle Triassic. Our results show that the redox conditions and phosphorus cycling exhibit strong spatiotemporal heterogeneity. During Interval I (0–140 kyr), oxic–ferruginous conditions were dominant in the water column, and abundant Fe (oxyhydr)oxides promoted sedimentary P retention and sustained oligotrophic conditions. During Interval II (140–405 kyr), the widespread development of euxinic water bodies expanded from the basin floor to the slope belt of the lake basin, leading to extensive P recycling back to the photic zone, thereby increasing primary productivity and organic matter production. This transition is probably attributed to enhanced volcanic activity and a significant shift in redox conditions, which play an important role in regulating the concentration of bioavailable P in the water column. This study sheds new light on Fe–P–C cycling in an ancient freshwater lake under volcanic perturbations, providing guidance on the link between biogeochemical feedback and carbon burial.

With the continuous advancement in lacustrine shale oil exploration and development in China,traditional reservoir evaluation methods are facing a series of challenges in microscale characterization.In this study,we analyze the advantages and limitations of existing reservoir evaluation techniques and methods.Accordingly,a more intelligent,comprehensive shale reservoir evaluation method that integrates multidimensional data is proposed.Based on the Adaptive Pyramid Context Network(APCNet)for semantic segmentation,combined with the previously independently developed shale pore-fracture segmentation network(ShaleSeger),this method enables intelligent segmentation of minerals and pore structures within reservoirs.By further integrating image processing techniques with mathematical statistics,the method allows for both the quantitative calculation of the shale brittleness index and the fine-scale characterization of pore structures.The analytical results indicate that this proposed method serves to provide more specific,comprehensive,and quantitative analytical data for shale oil and gas exploration and development,as well as sweet spot identification.These analytical data in turn facilitate the quantitative evaluation of resource potential in shale hydrocarbon reservoirs and assist in the comprehensive assessment of relevant technical difficulties and economic benefits.The systematic solution established based on the proposed method offers a reliable basis for intelligent decision-making in the efficient exploration of lacustrine shale oil and gas.Additionally,this study presents a thorough analysis of challenges associated with current intelligent analysis techniques for lacustrine shale reservoirs and points out focus for future research.

Through systematic comparison of the geological characteristics, resource distribution, and exploration and development status of global marine and continental shale oil, this paper deeply analyzes the key theoretical and technical issues that restrict the development of continental shale oil in China. It points out that the basic theoretical research areas, such as the enrichment and accumulation mechanisms of shale oil with different lithological combinations, and the multi-scale and multiphase flow mechanisms in nanoscale confined spaces, are relatively underdeveloped; the accuracy of sweet spot prediction cannot effectively guide the selection of target layers and the positioning of horizontal well trajectories, and geology-engineering integration practices remain insufficient. All these factors severely restrict the large-scale utilization of shale oil resources. Focusing on the progress in the study of continental shale oil in the Songliao Basin, Ordos Basin, Junggar Basin and Bohai Bay Basin of China, this paper systematically analyzes six bottleneck issues (genetic models of fine-grained sedimentary rocks, types and distribution of hydrocarbon-generating organic matter, hydrocarbon generation-expulsion models and potential, types and performance of reservoir spaces, parameter selection and evaluation techniques for sweet spots, and productivity laws and enhanced oil recovery), summarizes progress in theoretical and technological research, and discusses examples and directions for tackling key issues. It identifies six major challenges on geological theory and engineering technology confronting the shale oil revolution in China: hydrocarbon accumulation mechanisms, sweet spot identification, seepage law, fracturing modification, drainage and production technology and recovery enhancement. To address these, the study proposes to establish a shale oil classification scheme based on source-reservoir configuration, to promote the refined development model of “geology-engineering-geology spiral integration”, and build an efficient shale oil development technology system tailored to the continental geological conditions in China, providing theoretical and technical support for achieving large-scale and beneficial development.

Characterizing the in-situ distribution of fluids within crustal rocks is a fundamental challenge, as changes in external conditions and conventional analysis can introduce significant artifacts. Shale oil systems provide a critical example where post-retrieval alteration has obscured the primary controls on hydrocarbon storage. Here, we present an integrated cryogenic workflow employing cryo-focused ion beam scanning electron microscopy (Cryo-FIB-SEM) to examine pressure-preserved organic-rich shale cores, enabling direct visualization of microscale fluid migration phenomena during the simulated transition from subsurface conditions (fresh samples rapidly frozen at −150 °C) to surface conditions (+25 °C). Our results provide direct, time-resolved evidence of hydrocarbon redistribution. In-situ, immature heavy oil is predominantly hosted and immobilized within solid organic matter. Upon warming to ambient conditions, we observe significant micro-migration of hydrocarbons into inorganic mineral domains (clays and quartz), accompanied by the volatilization of light components (C4–C8). Room-temperature SEM induces pervasive artifacts, including the collapse of organic pore networks and the formation of new fractures at organic-quartz interfaces (approximately 27

Biomarkers in source rocks are crucial in uncovering biological source and depositional environment. However, the detection of critical biomarkers such as steranes and terpanes in high-maturity source rocks is often challenging. On one hand, biomarkers in high-maturity source rocks are inherently present at low concentrations; on the other hand, the detection of these low-concentration biomarkers can be hampered by the high concentrations of interfering compounds, such as n-alkanes and other branched/cyclic alkanes. In this study, we developed a sample pretreatment protocol of stepwise extraction, using the combination of solvent in varying polarities (i.e., n-hexane and dichloromethane) and samples of different sizes (i.e., blocks >3 cm, 2.5-5 mesh, 10-20 mesh). The specific procedures are as follows: (1) removing the high amounts of interfering compounds during the first-step extraction to increase the relative proportion of biomarkers in saturated fraction by extracting coarse samples with relatively weak-polar solvents; (2) analyzing the soluble organic matter extracted in the second step by Gas chromatography-mass spectrometry (GC-MS) for biomarker (i.e., steranes and terpanes) detection by extracting samples of 100-200 mesh with solvents of stronger polarity. This method was demonstrated using organic-rich shales with high maturity (R-o = 1.42%) from the Qingshankou Formation (Fm.) in the Gulong Sag, and it was validated using the medium maturity (R-o = 1.0%) shales from the Qingshankou Fm., Sanzhao Sag, Songliao Basin. The results show that under routine Soxhlet extraction combining GC-MS analysis, sterane and terpane biomarkers in the Qingshankou shales are unidentifiable, regardless of whether urea adduction is applied or not. In contrast, these biomarkers can be successfully identified utilizing stepwise extraction coupled with GC-MS analysis. The effectiveness of biomarker identification is influenced by the interplay of both the shale particle size and organic solvent polarity. The integration of biomarker indexes was then employed to interpret the biological source and the depositional environment of the organic matter, providing detailed insights of the hydrocarbon generation potential of shales. This information can further provide additional guidance for selecting favorable resource zones for shale oil exploration in the Gulong Sag, Songliao Basin.

The Middle Triassic Ordos Basin witnessed the earliest rehabilitation of complex lacustrine ecosystems after the end-Permian mass extinction (EPME). The specific challenges faced by freshwater ecosystems during this interval remain unclear, however, owing to the limited spatiotemporal coverage of integrated biogeochemical studies. Here, we combine high-resolution geochronology, mineralogical and multi-proxy geochemical data from the mid-Triassic Ordos Basin with temporal and spatial biogeochemical modelling to reconstruct lake redox structure and nutrient dynamics. Our results indicate that a transient increase in external sulfate input strengthened endogenous phosphorus recycling and eutrophication, promoting shoaling and intensification of a metastable sulfidic zone at mid-depths. This shoaling would have led to poisoning of benthic habitats, causing a collapse of the oldest known Mesozoic lacustrine ecosystem. We propose that sulfate loading prolonged anoxia and ecological stress by extending the residence time of phosphorus, a mechanism that may be relevant to deoxygenation events and resulting biocrises in both ancient and modern lacustrine ecosystems.

Through tracing the background and customary usage of classification of fine-grained sedimentary rocks and terminology, and comparing current “sedimentary petrology” textbooks and monographs, this paper proposes a classification scheme for fine-grained sedimentary rocks and clarifies related terminology. The comprehensive analysis indicates that the classification of clastic rocks, volcanic clastic rocks, chemical rocks, and biogenic (carbonate) rocks is unified, and the definitions of terms such as lamination, bedding and beds are consistent. However, there is a disagreement on the definition of “mud”. European and American scholars commonly use the term “mud” to include silt and clay (particle size less than 0.062 5 mm). Chinese scholars equate the term “mud” to “clay” (particle size less than 0.003 9 mm or less than 0.01 mm). Combined with the discussion on terms such as sedimentary structures (bedding, lamination and lamellation), shale, mudstone, mudrocks/argillaceous rocks and mud shale, it is recommended to use “fine-grained sedimentary rocks” as the general term for all sedimentary rocks composed of fine-grained materials with particle size less than 0.062 5 mm, including claystone/mudrocks and siltstone. Claystone/mudrocks are further classified into argillaceous (or clayey) mudstone/shale, calcareous mudstone/shale, siliceous mudstone/shale, silty mudstone/shale and silt-containing mudstone/shale. Argillaceous (or clayey) mudstone/shale emphasizes a content of clay minerals or clay-sized particles exceeding 50%. Other mudstones/shales emphasize a content of particles (particle size less than 0.062 5 mm) exceeding 50%. The commonly referred term “shale” should not include siltstone. It is necessary to establish a reasonable, standardized, and applicable classification scheme for fine-grained sedimentary rocks in the future. An integrated shale microfacies research at the thin-section scale should be carried out, and combined with well logging data interpretation and seismic attribute analysis, a geological model of lithology/lithofacies will be iteratively upgraded to accurately determine sweet layer, locate target layer, and evaluate favorable area.

Currently,significant hurdles lie ahead in China's lacustrine shale oil exploration and exploitation,particularly due to the high clay mineral content,which severely impacts pore develop-ment in organic-rich shales.In this study,we take the medium-to high-maturity shale of the Qing-shankou Formation(K2qn)in the Gulong sag,Songliao Basin,China,as an example and analyze the dynamic evolution of the clay minerals and their effects on pore development by core observa-tions,scanning electron microscopy,X-ray diffraction,physical property and nitrogen adsorption to provide a valuable reference for the effective evaluation of lacustrine clay-rich shale oil reservoirs.Our findings reveal that the dynamic transformation of clay minerals in the Qingshankou Formation is classified into three stages based on diagenesis,thermal maturity,and mineral characteristics:the initial stage(vitrinite reflectivity(Ro)<0.7%),the rapid transformation stage(Ro=0.7%-1.3%),and the late diagenesis stage(Ro>1.3%).Clay minerals play crucial roles in controlling shale reser-voirs during the rapid transformation process,which is evidenced by the formation of shrinkage frac-tures,enhanced pore connectivity,increased porosity,and expanded space for shale oil adsorption and storage.In addition,authigenic quartz can form during the transformation of clay minerals,con-tributing to increased reservoir brittleness and improving the potential for reservoir fracturing.

The sulphur (S) cycle is important for reconstructing paleoenvironmental evolution and organic matter enrichment. Compared with research on marine environment, studies on the terrestrial sulphur cycle and its relationship with key geological events, such as volcanic activity or hydrothermal fluids, are more limited. The Fengcheng Formation in the Mahu Sag of the Junggar Basin in northwestern China, which was deposited during the Carboniferous to early Permian in an alkaline lake, is an ideal research object for studying the relationship between the terrestrial sulphur cycle and geological events. Therefore, in this work, we established a link between volcanic activity and the alkaline lacustrine carbon-sulphur cycle during the Carboniferous-Permian Fengcheng Formation through petrologic, geochemical, and geophysical data from the MY1 Well in the Mahu Sag. The results revealed that (1) multiple volcanic episodes occurred during the deposition of the Fengcheng Formation, as evidenced by high mercury (Hg) concentrations, high Hg/S ratios, increased sulphate concentrations and large negative pyrite sulphur isotope (delta 34Spy) values (to-20.52 parts per thousand); (2) long-term ferruginous bottom water conditions may have been conducive to the preservation of organic matter; however, sulphate from volcanic activity promoted bacterial sulphate reduction, resulting in intermittent alternating euxinic conditions, as evidenced by iron speciation, molybdenum concentrations, and framboid and euhedral pyrite morphologies, which may have resulted in some consumption of organic matter; and (3) after volcanic activity, most of the sulphate in the lake water was depleted, and the bottom water gradually closed and was continuously enriched with positive delta 34Spy. Therefore, volcanic activity appears to have been the key factor controlling the sulphur cycle and organic matter enrichment through increased sulphate fluctuations in the oldest alkaline lake during the deposition of the Fengcheng Formation. This study sheds new light on the sulphur cycle of ancient alkaline lakes and can serve as a reference for organic matter enrichment under different mechanisms in shale.

The evolution of the Triassic megamonsoon was closely linked to Earth's orbital variations. Despite recognizing secular orbital cycles as a fundamental pacemaker of the megamonsoon, the driving mechanisms remain unclear. Here, we use data-model synthesis to study orbital-scale megamonsoon variability during the Middle Triassic (~ 246-239 Ma). By integrating high-resolution reconstructions of hydrologic fluctuations, obtained from lithological and magnetic susceptibility data series in the lacustrine sediments of the Ordos Basin (Northeast Tethys), with the climate simulations, we identify monsoon cycles in the ~ 20, 100, and 405 kyr Milankovitch bands. Comparisons with other records further reveal an additional eccentricity-related ~ 3.3 Myr orbital cycle in monsoon variabilities, temperature oscillations, carbon cycles, and sea-level changes. Earth system models show the effects of orbital configurations and atmospheric CO₂ concentrations on megamonsoon dynamics, implying threshold responses to solar radiation and the impacts of temperature and sea-level fluctuations on long-term megamonsoon variability. These findings improve our understanding of the interplay between astronomical forcing and feedbacks in shaping orbital-scale monsoon dynamics.

Bedding-parallel fractures play a critical role in enhancing storage capacity and horizontal permeability in tight shale reservoirs, significantly influencing the enrichment and productivity of shale oil. This study focuses on the continental shale of the Qingshankou Formation in the Songliao Basin, China, and systematically investigates the development characteristics and controlling factors of bedding-parallel fractures using a multi-scale dataset including core observations, image logs, scanning electron microscopy (SEM), and optical microscopy. Results show that bedding-parallel fractures are extensively developed across nano-to macro-scales, with nano-scale fractures reaching densities of up to 10(5) fractures/m. Fracture density is negatively correlated with both fracture scale and aperture. These fractures are generally sub-parallel to bedding or exhibit low-angle orientations (mostly <10 degrees), and are typically discontinuous with limited lateral connectivity. Quantitative analyses indicate that fracture density is positively correlated with the contents of clay minerals, pyrite, and carbonate minerals, while showing weak or negative correlations with brittle minerals such as quartz and feldspar. Higher fracture densities are also observed in shales with elevated TOC content and higher thermal maturity. Furthermore, the type, thickness, and density of lamination significantly influence fracture development. At micro- and nano-scales, bedding-parallel fractures primarily act as hydrocarbon storage spaces and micro-flow conduits, whereas at the macro-scale, they mainly serve as horizontal flow pathways. These findings elucidate the multi-scale development mechanisms of bedding-parallel fractures in continental shale and underscore their dominant role in improving reservoir performance.

IntroductionClay minerals are critical components of lacustrine shale systems, where their associated pores govern reservoir properties. The diagenetic transformation of these minerals, particularly illitization, profoundly impacts reservoir quality, yet the mechanisms driving illitization and its influence on pore evolution remain poorly understood. This study investigates the illitization processes and pore characteristics in the Cretaceous Qingshankou Shale (QSK shale) to elucidate their implications for unconventional hydrocarbon storage.MethodsCore samples from the QSK shale in the Gulong Sag, Songliao Basin, were analyzed using a multi-method approach: field-emission scanning electron microscopy (FE-SEM) for pore morphology, QEMSCAN® for mineralogical mapping, X-ray diffraction (XRD) for clay mineral quantification, and nitrogen adsorption for pore-size distribution analysis.ResultsKey findings include: (i) Clay minerals in the QSK shale are dominated by illite and illite/smectite mixed layers (average >40% content), with intense illitization yielding >60% illite in clay fractions. (ii) Pores within clay minerals (micropores and mesopores <50 nm in illite/illite-smectite; macropores 50–100nm and elongated pores >800nm in chlorite) constitute primary storage spaces, interconnected by microcracks. (iii) Illitization occurs via two pathways: transformation of illite/smectite mixed layers and albite alteration, with the former dominating. Porosity initially increases with illite content (up to ∼30%) but declines progressively beyond this threshold.DiscussionThe threshold effect of illitization on porosity highlights a critical balance between pore generation (via mineral dissolution) and occlusion (from authigenic illite precipitation). These findings provide a mechanistic framework for predicting reservoir quality in lacustrine shale systems, emphasizing the dual role of illitization in enhancing or degrading storage capacity depending on diagenetic maturity.

The successful exploration and development of shale oil in the clay-rich Gulong shale have sparked increased research into the influence of clay minerals on shale reservoirs. However, compared to chlorite in sandstones, limited studies have focused on the occurrence of chlorite in continental shales and its effects on shale reservoir properties. This study offers a comprehensive analysis of chlorite in Gulong shale samples from three wells at different diagenetic stages. Four primary chlorite occurrences are identified in the Gulong shale: Type I, which is chlorite filling dissolved pores in carbonate; Type II, which is isolated chlorite; Type III, which is chlorite filling organic matter; and Type IV, which is chlorite filling authigenic microquartz. Types I and III chlorites exhibit higher porosity, offering more storage space for shale reservoirs. Chlorites of Types I, III, and IV, filled with other substances, display higher fractal dimensions, indicating more complex pore structures. These complex pores are favorable for oil adsorption but hinder oil seepage. The processes of organic matter expulsion and dissolution, which intensify with increasing diagenesis, promote the development of Types I and III chlorites, thereby positively influencing the shale reservoir porosity of Gulong shale. This study underscores the influence of chlorite occurrences on shale reservoir properties, providing valuable insights for the future exploration and development of shale oil and gas.

The presence of hydrocarbons/bitumen in shales makes accurate characterization of reservoir properties challenging. Solvent extraction helps reveal the true pore structures and assess the movable oil distributions in early mature lacustrine shale oil reservoirs. In the present study, total organic carbon, optical microscopy, Rock-Eval pyrolysis, X-ray diffraction, scanning electron microscope, gas adsorption, and high-pressure mercury injection pressure experiments were performed to investigate the petrographic, geochemical and petrophysical characteristics of extracted and nonextracted shale samples that were collected from the Triassic Chang 7 Member in the Ordos Basin, China. The results revealed that changes in the micro-, meso-, and macropore volumes; specific surface area; average pore diameters and porosities of the shales significantly differ after solvent extraction compared with their original states, which indicates that the pore structures of early-mature, organic-rich shales are largely concealed by the retained oil and residual bitumen. Solvent extraction generally increases porosity by removing extractable organic matter (EOM), which occupies a sizeable pore space. Solvent extraction may, however, also be affected by several factors, resulting in reduced porosity. The main factors influencing the solvent extraction process are the removal of EOM, solvent–kerogen interactions, clay swelling, and the coupling effects of the sedimentary structure and pore system. In addition, the use of changes in porosity to indicate the movable oil content in shales is more meaningful than the changes in S1 and total pore volume are. This study is beneficial for the accurate characterization of shale pore structures and precise assessments of the movable oil contents of lacustrine shales.

Organic matter abundance is an important material basis for shale oil enrichment and a key indicator for shale oil sweet spot prediction and resource evaluation. Compared to marine sediments, in continental lake basins the organic matter abundance of shale intervals varies more frequently and is highly heterogeneous, the organic matter formation and preservation are significantly affected by water salinity. Therefore, it is important to investigate water conditions and the factors that influence them to understand the differential enrichment of organic matter in different continental lake facies. In this study, we compared the sedimentary and geochemical characteristics of typical freshwater (Chang 7 Member in the Ordos Basin) and saline alkaline lakes (Fengcheng Formation in the Mahu Sag in the Junggar Basin), and found that the salinity of water bodies in both basins is dynamic: In the freshwater lake basin, there was a trend towards saltier conditions during organic- rich intervals, while in the saline-alkali lake basin, there was a trend towards less salty. By comparing the paleoclimate at the global scale, we propose that the warmth and cold of the climate affects the lacustrine salinity and therefore the paleoproductivity, which is the main factor controlling the deposition of organic-rich shale in continental lake basins. Freshwater lake basins are more developed in the greenhouse climate, and the organic matter in the water is mainly from plankton, benthos and bacteria, the dissolved oxygen content in the surface water of lake basins is higher, and therefore its paleoproductivity is higher than that of saline lake basins. After the decomposition and degradation of organic matter in the shallow water layer, an anoxic ferruginous or euxinic environment is conducive to the preservation of organic matter at the bottom of the water body. In contrast, due to the high salinity in the alkali water in the glacial period, the environment is relatively stable, the dissolved oxygen content is low, and the organic matter may be accumulated and enriched for a relatively long time only in its deep water. Meanwhile, organic matter in lacustrine basins mostly forms in anoxic ferruginous water conditions, the saline intervals in freshwater lake basins are more conducive to the preservation of organic matter than that in alkaline lakes: the stratification in the water in the Chang 7 section is subtle, and the sulfate resulting from key geological events such as volcanic activity can cause rapid changes in water salinity, causing the water body at the bottom of the lake from long-term ferruginous quickly changing to euxinic conditions, which is conducive to the preservation of organic matter; the stratification of the water in the Fengcheng Formation in the Mahu alkaline lake is clear, volcanic or hydrothermal activity can frequently cause the salinity at the bottom of the water body to become euxinic even to the stage of methane anaerobic oxidation, which may have a consuming effect on organic matter, but the mechanism of organic matter enrichment in alkali lake still remains to be explored. Future research is suggested to strengthen the study of organic-inorganic geochemical characteristics in different shale intervals, especially the analysis of stable isotopes of hydrocarbon, oxygen, nitrogen, sulfur and key metal isotopes (Fe, Cu, Hg et al. ), to quantitatively reconstruct the spatial and temporal distribution of primary productivity, finely characterize the degree of hypoxia at the bottom of sedimentary water bodies during the formation of organic-rich shale, highlight the role of clay minerals and microorganisms, and establish a quantitative characterization model, to deepen the understanding the enrichment of continental shale oil and to give the theoretical guidance for shale oil exploration.

Shale oil exploration in China's clay-rich shale formations has gained attention, but the origin and impact of clay minerals on reservoirs remain unclear. This study examines the Qingshankou Formation in the Songliao Basin using Nanomin analysis, X-ray diffraction, X-ray fluorescence analysis, TOC analysis, rock pyrolysis, and FE-SEM to investigate the origin, the evolution, and the pore structures of clay mineral. Illite and I/S mixed layers dominate the matrix, while compaction disrupts their authigenic structures. Illite and I/S mixed layers primarily originate from detrital deposition and transformation of smectite. Chlorite is mostly authigenic and increases with thermal maturity. Illite and I/S mixed layers form elongated intergranular pores and microfractures, which significantly contribute to pore space, with their aligned axes pores playing a key role in microfracture formation. Authigenic chlorite primarily fills pores, while detrital chlorite exhibits discrete boundaries and alignment within the matrix. In comparison, authigenic chlorite pores are larger and more significant for reservoir porosity than detrital chlorite pores. Detrital clay mineral deposition promotes organic matter enrichment, while feldspar and quartz detritus deposition hinder it. These findings enhance the understanding of clay mineral evolution and the mechanisms of continental shale oil reservoir formation and organic matter enrichment.

The presence of hydrocarbons/bitumen in shales makes accurate characterization of reservoir properties challenging. Solvent extraction helps reveal the true pore structures and assess the movable oil distributions in early mature lacustrine shale oil reservoirs. In the present study, total organic carbon, optical microscopy, Rock-Eval pyrolysis, X-ray diffraction, scanning electron microscope, gas adsorption, and high-pressure mercury injection pressure experiments were performed to investigate the petrographic, geochemical and petrophysical characteristics of extracted and nonextracted shale samples that were collected from the Triassic Chang 7 Member in the Ordos Basin, China. The results revealed that changes in the micro-, meso-, and macropore volumes; specific surface area; average pore diameters and porosities of the shales significantly differ after solvent extraction compared with their original states, which indicates that the pore structures of early-mature, organic-rich shales are largely concealed by the retained oil and residual bitumen. Solvent extraction generally increases porosity by removing extractable organic matter (EOM), which occupies a sizeable pore space. Solvent extraction may, however, also be affected by several factors, resulting in reduced porosity. The main factors influencing the solvent extraction process are the removal of EOM, solvent-kerogen interactions, clay swelling, and the coupling effects of the sedimentary structure and pore system. In addition, the use of changes in porosity to indicate the movable oil content in shales is more meaningful than the changes in S1 and total pore volume are. This study is beneficial for the accurate characterization of shale pore structures and precise assessments of the movable oil contents of lacustrine shales.

Characterizing diverse pore types is essential for optimizing resource exploration in shale formations. Scanning electron microscopy (SEM) is the primary tool for this analysis; however, its practical pore resolution (PPR) limit of 30 nm hinders the quantitative analysis of smaller pores. To address this challenge, we introduced a method for characterizing mesopores below the SEM's PPR threshold, focusing on the interclay mineral mesopores in Chang-7 shale as a case study. We employed low-pressure nitrogen adsorption and SEM to analyze the overall mesopore structures, confirming their self-similarity and dominance of interclay mineral mesopores. SEM digital analysis was used for the quantitative examination of interclay mesopores exceeding the PPR threshold. Additionally, a fractal model was developed using data from these larger mesopores and their fractal properties to predict the size distribution of super-PPR mesopores (2-30 nm). To validate this approach, we applied a same modeling method to predict the pore size distribution in the 30-60 nm range, and compared these predictions with measured values. With relative root-mean-square error (RRMSE) values ranging from 9.98 to 19.80%, our approach demonstrates high accuracy. This research advances mesopore characterization methods and offers deeper insights into hydrocarbon mobility and pore genesis in shale formations.

Accurately predicting relative permeability is an important issue in the research of multiphase flow in tight reservoirs. Existing predictive models typically rely on the capillary tube bundle model featuring circular cross-sections, often overlooking the impact of pore geometry on fluid flow behavior within reservoirs. In this work, the intermingled fractal theory of porous media is introduced to characterize the intricate local features within the internal space of tight rocks. Initially, iterative rules for diverse fractal units are skillfully designed to capture the actual characteristics of pore cross-sectional shapes. Subsequently, analytical relationships are derived between the iterative parameters and the area, wetted perimeter, and hydraulic diameter of pores generated by these units, followed by the establishment of a relative permeability model that considers pore geometry. The model's validity is confirmed through comparisons with experimental data and published relative permeability models, with correlation coefficients exceeding 0.996. Finally, various factors affecting two-phase flow characteristics are analyzed. The results reveal that pore geometry has a significant impact on flow behavior in porous media. Assuming that the flow channels are cylindrical typically leads to an overestimation of permeability, with the maximum relative error reaching 46.91%. Additionally, the tortuosity fractal dimension is a determinant factor influencing the relative permeability of both wetting and non-wetting fluids, and the phase permeability is sensitive to variations in solid particle size and porosity. The proposed intermingled fractal model enhances the accuracy of evaluating fluid flow characteristics in microscale pore channels and offers a novel framework for simulating porous media with complex geometries.