Delta-beach bar composite sand bodies represent a unique sedimentary architecture formed through the interplay of multiple factors. These include reworking by the hydrodynamic force of marine/lacustrine waters and shoreline migration caused by frequent sea/lake level fluctuations,which collectively act on deltaic sand bodies at river-sea/lake interfaces. Their complex distribution pattern cannot be characterized by a single-origin sedimentary model. However,these composite sand bodies are widely developed within clastic reservoirs of hydrocarbon-bearing basins. Through comprehensive analysis of numerous case studies,modern sedimentary investigations,and review of relevant research literatures on composite sand bodies,this paper systematically examines research advances in sedimentary models and genetic mechanisms of delta-beach bar composite sand bodies. Key findings are as follows: (1)Primary controlling factors on composite sand bodies development include sedimentary cyclicity,composite hydrodynamic interactions,topographic factors,mixed sedimentation,and event sedimentation,etc..(2)There are four types of distribution models for delta-bar complex sand bodies,namely: shoreline migration-delta lateral margin bar complex model,delta front estuarine bar-clastic bar complex model,delta distributary channel-coastal bar complex model,and delta front estuarine bar-carbonate bar superposition complex model.

The Mesoproterozoic Wumishan Formation in the central Jizhong Depression hosts abundant hydrocarbon and geothermal resources. However,studies on the hydrothermal processes affecting Wumishan reservoirs remain limited. Based on field and core observations,this study integrates electron microprobe analysis,carbon and oxygen stable isotope analysis,strontium isotope ratios,and fluid inclusion thermometry to investigate the characteristics of hydrothermal activity and its impacts on reservoir evolution within the Wumishan Formation. Results reveal the presence of diverse hydrothermal minerals,and the hydrothermal processes can be classified into three categories: (1)magmatic-related activity,(2)carbonate-dominated hydrothermal activity,and(3)siliceous hydrothermal activity. These are evidenced by dolomite undergoing marbleization,hydrothermal dolostones showing more negative δ¹³C and δ¹⁸O values and higher⁸⁷Sr/⁸⁶Sr ratios compared to matrix micritic dolomite,and anomalously elevated homogenization temperatures of fluid inclusions in dolomite,quartz,and fluorite cements. Variations in hydrothermal intensity are primarily controlled by regional magmatism and fluid migration pathways. Hydrothermal activity is strongest when faulting and magmatic events are temporally coupled,with greater intensity in the northwest compared to the southeast. More intense hydrothermal activity is also observed near higher-order,large-scale faults. Hydrothermal processes such as dissolution,recrystallization,and hydraulic fracturing enhance reservoir quality,while cementation,mineral filling,and silicification reduce porosity and act destructively. In contrast,marbleization has minimal influence on reservoir performance and largely preserves the original rock storage properties.

A set of marine strata dominated by clastic rocks was deposited during the Changchengian period in the Zhongtiao Mountain region of Yongji,Shanxi. These strata are the earliest sedimentary strata in the Ordos region,recording a complete history of the rifting activities during the Changchengian period. The strata contain four lithostratigraphic units including,from bottom to top,the Baicaoping,Beidajian,Cuizhuang,and Luoyukou Formations. Among them,the Baicaoping and Beidajian Formations are dominated by quartzose sandstones,the Cuizhuang Formation by shale,and the Luoyukou Formation by dolomite. In this study,various mixed sedimentary deposits were discovered in the Taojiayao section of the Zhongtiao Mountains. This study combines field sedimentary structures with microscopic observations to clarify the characteristics and types of mixed sedimentary deposits and further reveals their origins and patterns. The result shows that the Baicaoping Formation is a mixed-facies deposit with massive structures;the Beidajian Formation contains in-place mixing,phase mixing,and intermittent mixing deposits,presented as medium-to-thin layers interspersed in cross-bedded quartzose sandstone;the Cuizhuang Formation develops a small amount of mixed-facies deposition;the Luoyukou Formation mainly contains source-mixed deposits. The mixed sedimentary processes were controlled by rifting: in the early stage with intense rifting,the steep slope landform was dominated by mixed facies;during the Beidajian period with weakened rifting,a gentle slope formed,leading to the development of mixed-facies,and in-place and intermittent mixing deposits;during the Luoyukou depositional period when rifting ended and sea-level dropped significantly,the carbonate rocks deposited earlier were exposed and eroded,and transported into the basin interior as internal debris forming source-mixed deposits. Overall,mixed deposits appeared at the top of each formation during the period when the sea-level began to accelerate,suggesting that sea-level rise was a fundamental condition for determining mixed deposition to a certain extent.

The phosphate ore of Meishucun Formation in northeastern Yunnan Province represent an important component in sedimentary phosphate ore along the southwestern margin of the Yangtze block. While their formation is commonly attributed to upwelling currents,the relationship between these upwelling processes and oceanic circultion patterns remains insufficiently studied. This study focuses on the Meishucn Formation in Well ZK0816 from the Yangchang phosphate ore area,Northeastern Yunnan Province. Core data were systematically collected,including detailed lithological descriptions bed by bed,sedimentary successions,structure features,and environmental and facies analysis. The results show that: (1)The Meishucun Formation can be divided into three members and seven submembers from bottom to top. The first,second and third members are composed of phosphorous rock,phosphorite and phosphorous rock,respectively;(2)The sedimentary succession is dominated by thinning-upward pattern,with subordinate bidirectional and waving-upward trends;(3)Overall,the sedimentary structure is dominated by stripped and banded sedimentary structure,showing a transition from regular to irregular and back to regular layering upward;(4)The typical irregular stripped and banded structures forming coarsening-up and then fining-up succession is developed in the first submember of the second member. Additionally,small-scale bidirectional cross-lamination and wavy lamination are developed in the second submember. These findings suggest that: (1)the first,second and third members of the Meishucun Formation were probably deposited in subtidal zone,slope and shelf environments,respectively;(2)Deep-water traction current deposits are developed in the second member which corresponds to the main industrial phosphate ore layer(KT2)in study area. Contour current deposits are identified in the upper slope setting within the first submember,while internal-wave and internal-tide deposits are recognized in the lower slope setting of the second submember;(3)Upwelling currents may have been induced by deep-water traction currents,contributing to vertical mixing of oceanic water mass.

A significant breakthrough in the exploration of Jurassic sandstone oil and gas was achieved in well Fusha 8,located in the piedmont area of the southwestern Tarim Basin. However,the overall exploration level of the Jurassic remains low. The characteristics of the provenance and the sedimentary systems within the basin are still unclear,which hinders the process of exploration. This paper uses logging data,core samples,and seismic data to clarify the types and distribution of sedimentary facies through a systematic analysis of the Jurassic provenance characteristics in the Fusha fault depression. It is concluded that there are two major provenance systems in the Jurassic strata of the Fusha fault depression: the western Kunlun Mountains in the south exhibit a strong provenance supply capacity,while the northern slope area displays a weaker provenance. The provenance areas are mainly composed of Proterozoic metamorphic rocks,Paleozoic magmatic rocks and Permian sedimentary rocks. The Jurassic basin underwent a complete rifting cycle,which can be categorized into three evolution stages: the initial rifting stage,the intense rifting stage,and the rifting-depression transition stage. In the study area,three sedimentary facies were identified: fan delta,braided river delta and lacustrine facies. The southern steep slope zone mainly develops an alluvial fan-fan delta-lacustrine sedimentary system,while the northern gentle slope zone is mainly characterized by a braided river delta-lacustrine sedimentary system. The distribution of the Jurassic sedimentary system in the Fusha fault depression,located in the piedmont area of the southwestern Tarim Basin,exhibits vertical inheritance development,and is influenced by factors such as palaeogeomorphology,provenance supply,and basin rifting activities. Based on this research,it is proposed that the braided river delta sandbodies controlled by the southern tectonic transition zone during the sedimentary period of the Shalitashi Formation(initial rifting stage)represent favorable exploration targets within the region.

The Middle Jurassic Lianggaoshan Formation in the Sichuan Basin is one of the key hydrocarbon-bearing strata;however,its macro-scale sedimentary systems and evolutionary processes remain poorly understood,and a comprehensive basin-scale sedimentary model is lacking. This study integrates data from heavy and light mineral analyses,core and well logs,palynology,trace elements,and field outcrops to investigate the provenance systems,paleoclimatic conditions,sedimentary microfacies types,sedimentary systems,and their temporal evolution across different depositional intervals of the Lianggaoshan Formation. The results identify six primary provenance areas active during the deposition of the Lianggaoshan Formation: the central and northern Longmen Mountains,the Micang Mountains,the southern Qinling Mountains,the Daba Mountains,and the ancient Xuefeng Uplift. The basin experienced a climatic transition from hot and arid conditions in the early stage,to warm and humid in the middle stage,and a return to hot and arid conditions in the late stage. Nine types of sedimentary microfacies were recognized,including braided channels,interdistributary areas,subaqueous distributary channels and interdistributary zones,estuarine bars,sheet sands,coastal and shallow lacustrine bars,shallow lacustrine mud,and semi-deep lacustrine mud. During the Lianggaoshan depositional period,alternating uplift of surrounding mountain ranges led to flexural subsidence of the eastern and northeastern basin basement,forming a foreland basin in front of the Daba Mountains. This process established a paleo-tectonic framework characterized by a steep eastern and gentle western margin,with multiple sediment source systems. In the early stage,both the depositional and subsidence centers were concentrated in eastern and northeastern Sichuan due to active tectonism,climate variation,and paleotopography. In the middle stage,increased humidity led to enhanced terrigenous input,and the depositional center shifted toward central Sichuan,resulting in a spatial offset between deposition and subsidence centers. By the late stage,the lake basin was filled and gradually contracted.

Clastic rock reservoirs serve as critical carriers of hydrocarbon resources both in China and around the world. However,due to inherent limitations such as strong heterogeneity and insufficient subsurface characterization data,traditional methods of reservoir characterization and modeling have struggled to fulfill the demands for high-resolution exploration and efficient development. Since the 21th century,researchers have progressively integrated artificial intelligence(AI)techniques into the field of clastic reservoir characterization and modeling,resulting in significant advancements over the past decade. These innovations have significantly improved both the accuracy and efficiency of reservoir characterization. In this context,this paper systematically reviews the development history and current research status of intelligent technologies in clastic reservoir characterization and modeling. It highlights recent progress and application outcomes in areas such as intelligent well log interpretation for reservoir parameters,AI-based fault and stratigraphic framework analysis,intelligent reservoir prediction through well-seismic integration,and intelligent 3D geological modeling. Furthermore,we discuss the challenges faced by various intelligent approaches and outlines future directions for their development. Overall,these intelligent characterization techniques have made significant advances and demonstrated positive outcomes in practical applications. Nevertheless,they also face multiple challenges,including a lack of high-quality training samples,suboptimal generalization capabilities of learning models,and inadequate coupling of knowledge-driven with data-driven approaches. Despite these limitations,there remains significant potential for advancement,with promising application prospects emerging across reservoir characterization workflows.

Bioturbation refers to various sedimentary textures or structures formed on sediment surfaces or within sediments due to biological activity. It plays a crucial role in analyzing paleoenvironmental conditions in sedimentary strata,predicting distribution patterns,evaluating the hydrocarbon generation capacity of source rocks,assessing the sealing capacity of caprocks,and revealing the mechanisms and effects of bioturbation on hydrocarbon reservoirs. Traditional methods for analyzing bioturbation intensity mainly relies on manual identification,followed by semi-quantitative classification using bioturbation index charts. This approach is highly subjective,inefficient,and prone to large errors. In this paper,we proposed a residual network model that incorporates an attention mechanism(Res-EMANet)by integrating the Efficient Multi-Scale Attention(EMA)mechanism into the ResNet-50 model. During training,the model employs stochastic gradient descent(SGD)with an initial learning rate of 0.01,a weight decay parameter of 0.0001,a batch size of 16,and a total of 300 epochs. Model performance improvements are evaluated based on five aspects: accuracy,precision,recall,F1-score,and the confusion matrix. We validated the model using a dataset of 3,028 core images from 16 wells of the Ordovician in the Tarim Basin,which contain various levels of bioturbation. The results show that: (1)The model can accurately classify bioturbation intensities ranging from level 0 to 5 in digital core images,achieving an accuracy of up to 91%. This significantly outperforms traditional manual methods as well as the original ResNet-50 model. (2)The model not only improves the accuracy of bioturbation grade recognition but also effectively reduces dependence on expert knowledge,as well as the labor intensity and subjectivity associated with manual bioturbation assessments. It demonstrates significant advantages in the automation,intelligence,and quantification of bioturbation feature analysis. This research offers an efficient and reliable quantitative analysis tool for the automated processing of bioturbation degree assessment and identification,which is of great significance to the sedimentology and paleontology studies in the field of oil and gas exploration.

The spatial distribution of lithofacies of fine-grained sedimentary rocks is a critical research focus in shale oil exploration. Due to the high cost and scarcity of core wells,which constrain direct lithofacies analysis,logging-based prediction has become increasingly essential. Taking the First Member of the Qingshankou Formation in the Songliao Basin as a case study,this research establishes a lithofacies classification scheme integrating lithology,mineral composition,total organic carbon(TOC),and sedimentary structures,resulting in the identification of seven distinct lithofacies types. A lithofacies-well log dataset was constructed for Well X8 using six conventional logging curves. Three machine learning algorithms—Random Forest(RF),eXtreme Gradient Boosting(XGBoost),and Support Vector Machine(SVM)—were employed to evaluate classification performance,with SVM identified as the optimal model. To address class imbalance in the training data,the BSMOTE(Borderline Synthetic Minority Oversampling Technique)algorithm was applied. The balanced dataset was then used to develop a hybrid lithofacies prediction model: BSMOTE-SVM. The BSMOTE-SVM model demonstrated the best predictive performance,achieving an accuracy of 86.49%,precision of 86.60%,recall of 86.49%,and F1-score of 86.31%. This integrated model enables rapid and accurate lithofacies prediction across multiple wells and delineates the lithofacies distribution in Member 1 of the Qingshankou Formation in the Changling sag,offering a robust foundation for selecting favorable shale oil enrichment zones in future exploration.

The application of the source-to-sink methodology in predicting reservoir occurrence in deep-water hydrocarbon exploration is frequently impeded by the difficulty of quantifying deep-water source-to-sink(S2S)parameters,such as sea-level fluctuations,sediment supply(Q_s),and accommodation space(δ_a). This study establishes a method for quantitatively characterizing these three S2S parameters and identifies suitable deep-water hydrocarbon reservoirs based on the classification of shelf margins. Our findings indicate that sea-level fluctuations can be categorized as either icehouse or greenhouse based on geological age,sediment supply can be distinguished as high or low based on the rates of shelf-edge movement,and accommodation space can be classified as low,medium,or high based on the angles of shelf-edge trajectories. The interaction of these three S2S parameters results in 10 distinct types of shelf margins,each associated with a specific risk level for finding suitable hydrocarbon reservoirs in deep-water areas. Applying these quantitative methods to the Pearl River margin of the SQ13.8 age has led to the identification of three primary types of icehouse margins: high Q_s-medium δ_a,high Q_s-low δ_a,and high Q_s-high δ_a. Shelf-edge trajectories that are slightly falling or rising(-2°<T_se<1°)can transport coarse-grained clastics to the deep-water slope,forming sand-rich submarine fans regardless of whether the system is controlled by icehouse or greenhouse sea-level conditions and whether sediment supply is high or low. Moderately rising shelf-edge trajectories(1°<T_se<4°)promote the formation of sandy highstand submarine fans under high sediment supply conditions but are associated with muddy deep-water systems under low sediment supply scenarios. Steeply rising or backstepping shelf-edge trajectories(1°<T_se<4° and 90°<T_se<180°,respectively)tend to form small-scale sand-rich submarine fans under high sediment supply,while under low sediment supply they are commonly associated with large-scale mud-rich mass-transport depositional systems. According to this shelf-margin classification-based prediction model,icehouse shelf margins with high Q_s and low δ_a show the highest sand-rich potential in deep-water settings. This conclusion is supported by borehole data,which reveal the presence of tens of meters thick sandstones in channelized submarine fans,exhibiting block well-log patterns. These findings validate the effectiveness of the proposed new methods for identifying suitable deepwater reservoirs.

The meandering river sand bodies exhibit rapid lateral variability and feature complex and heterogeneous internal architectures. In this study,ground-penetrating radar(GPR)was employed in conjunction with a combined plan-view and cross-sectional approach to investigate sedimentary patterns in gravel-dominated meandering river segments of the Hutubi and Irtysh Rivers in Xinjiang. Interpretations of GPR profiles were supported by modern sedimentological observations,UAV imagery,and remote sensing data. Four distinct GPR facies were identified: (1)continuous subparallel low-to-medium amplitude reflections corresponding to floodplain deposits;(2)continuous flat-topped to convex-based low-to-medium amplitude reflections indicative of abandoned channels;(3)continuous inclined medium-to-high amplitude reflections characteristic of point bars;and(4)discontinuous inclined high-amplitude reflections associated with lag deposits. Through an integrated analysis of the vertical and lateral sedimentary evolution and their interactions in gravel-bed meandering systems,a multi-phase depositional model was developed. This model supports the construction of a three-dimensional geological framework,providing refined architectural configurations and quantitative geological information that offer valuable guidance for reservoir prediction and modeling in meandering fluvial systems.

The Member 2 of the Paleogene Funing Formation in the Gaoyou and Jinhu sags of the Subei Basin is not only the primary hydrocarbon source rocks, but also develops complex fine-grained mixed rocks. It has now become a key target for unconventional oil-gas exploration in continental lacustrine basins. Research on sedimentary environments of these facies plays a crucial role in both understanding their depositional processes and lithofacies characteristics,and predicting sweet spot intervals. Based on integrated analyses including whole-rock X-ray diffraction(XRD)analysis,X-ray clay mineral diffraction analysis,trace element geochemistry,and total organic carbon(TOC)content,this study reveals that the Member 2 of the Funing Formation was deposited in a non-sulfidic,anoxic water environment. The climate was generally arid and unstable during deposition of the “Shanzixing Unit”. The “Four-Seven Peaks Units” depositional period witnessed a more humid and still unstable climate. The climate during deposition of the “Wangbagai Unit” exhibited relative humidity and stability. In contrast,the “Nibozi Unit” depositional period experienced significantly more humid climatic conditions compared to other units. During the deposition of the “Shanzixing-Wangbagai units”,the lake level remained generally low but gradually rising with strong fluctuations. The “Nibozi Unit” depositional period experienced a major lacustrine transgression event. The Member 2 of the Funing Formation was deposited under saline water,with palaeosalinity consistently exceeding 10‰,which can be vertically subdivided into two intervals: the “Shanzixing-Four Peaks Units” formed in saline to hypersaline environments,and the “Seven Peaks-Wangbagai-Nibozi Units” developed in brackish to saline environments. This study proposes that the brackish water environment controlled by palaeoclimate constitutes the fundamental prerequisite for fine-grained mixed facies,thereby establishing mixed sedimentary models dominated by arid conditions in the early stage and humid conditions in the late stage.

With the continuous warming of the global climate,the frequency and extent of wildfires have increased significantly. To gain a deeper understanding of the interactions between climate change and wildfires,it is crucial to study wildfire activity patterns throughout geological history. The Permian marks a pivotal transition from an icehouse to a greenhouse climate. Although extensive research has reported evidence of the Permian wildfires,the global spatiotemporal distribution and controlling factors of the Early Permian wildfire activity remain unclear. This study presents the first discovery of abundant fossil charcoal in the Lower Permian Taiyuan and Shanxi Formations in eastern North China,indicating frequent wildfire occurrences during this period. Fossil charcoal reflectance ranges from 0.61% to 2.76%,suggesting that wildfires were predominantly surface and ground fires. Additionally,this research systematically compiled and analyzed 174 globally reported records of the Early Permian wildfire activity. The evidence includes fossil charcoal,inertinite,and pyrolytic polycyclic aromatic hydrocarbons. Spatially,the Early Permian wildfire activity was concentrated in tropical and cool temperate climate zones at mid- to high latitudes in the southern hemisphere,primarily influenced by regional climate conditions and vegetation distribution. Temporally,the frequency of the Early Permian wildfire events exhibited an initial ascending trend followed by subsequent decline,potentially influenced by variations in atmospheric CO₂ concentraions and modulated by fluctuations in atmospheric O₂ levels. This study provides critical insights into the global distribution and controlling factors of wildfire events during the Permian,contributing to a more comprehensive understanding of their patterns and influences.

The grain size data of sediments can quantitatively reflect hydrodynamic characteristics and terrestrial material composition,providing comprehensive insights into sea-level fluctuations,regional sources,and climate environmental changes. Among these data,environmentally sensitive factors in the particle size data serve as excellent indicators of sedimentary environment and monsoon intensity. The abundant drilling data from the northwestern shallow water shelf of the South China Sea provide an excellent research platform for studying hydrodynamics and climate change. Based on the particle size analysis of 52 Quaternary samples in the continental shelf area,it is concluded that the continental shelf sediments in the northwest of the South China Sea are primarily composed of clay and silt. From bottom to top,hydrodynamic activity gradually intensifies,resulting in a relatively turbulent overall sedimentary environment,with variations in source input and hydrodynamic activity at different times. To explore the response relationship between the East Asian monsoon intensity and the response of sensitive components,environmentally sensitive components in sediment samples were extracted using the grain-size-standard deviation method,the end-member component analysis,and the principal component factor analysis. The results show that the contents of fine and coarse grain components extracted by different methods exhibit the same trends. Therefore,it is considered that all three methods are suitable for analyzing environmental sensitive components in the northwestern shelf area of the South China Sea. The ratio of EM1/(EM1+EM2+EM3+EM4)obtained by end-member component analysis and the comprehensive index GS calculated from the four particle size main components separated by principal component factor analysis show the same trend and it thus can be used as alternative indicators of the relative intensity of the East Asian winter and summer monsoons. The content variation of 4.03~8.68 μm fine fractions obtained by the three methods can also indicate the intensity variation of the East Asian summer monsoon. After the climate transition in the Mid-Pleistocene Climate Transition(around 0.8-0.9 Ma),the amplitude of East Asian monsoon variations intensified,with a significant strengthening of the winter monsoon. After 0.9 Ma,the shelf is exposed as a result of sea level fall,with a subsequent increase in coarse-grained sediments.

Steroids in the geological records is generally relevant to the organic matter source;in return,the distributions of steroids would provide unique insights to the organic source in sediments. Organic matter input in the alkaline sediments is complex,which can form various steranes. In this study,a total of 17 cores were sampled to carry out GC-MS and GC-IR MS to investigate the distributions of steranes and their controlling factors by combination of previous reports. Low Pr/Ph combined with high content β-carotane,C₂₃ tricyclic terpane and gammacerane indicates a high-salinity water body during deposition of the Permian Fengcheng Formation. The samples are characterized by low abundance of C₂₇ regular sterane and high abundance of C₂₈ and C₂₉ regular steranes,which is due to the contributions from some special organic matter input such as green algae. The content of diasteranes in the selected samples is very low due to the low clay content. A low alkyl-steranes present in the Fengcheng Formation source rocks,which may be affected by organic matter source as well as strongly reducing environment as shown a low C₂₈ 4-methyl sterane relative to C₂₉ 4-methyl steranes and C₃₀ sterane. Occurrence of C₃₀ 24-n-propyl cholestane in the Fengcheng Formation samples located in edge of paleo-lake may imply the organic matter contribution from red algae. Moreover,high contents of lanosteranes have been detected in the samples,which suggests a reducing sedimentary environment and a green-algal source as supported by abundant C₂₈ and C₂₉ regular steranes and β-carotane. The absence of triaryldinosterane in the samples is likely ascribed to some special organic matter source and a low contribution of dinoflagellates. In summary,it is the strongly reducing environment,distinct organic matter input,as well as low clay content in mineral compositions that lead to the various and complex steranes distributions pattern in alkaline sediments.

The Middle Permian represents a crucial phase of structural transformation in the Sichuan Basin. Under the cumulative effects of various structural activities,the lithofacies palaeogeographic attributes of the Maokou Formation remain ambiguous,thereby hindering the prediction of favorable reservoir facies belts. By analyzing outcrop,drilling and seismic data,the sedimentary facies types of the Maokou Formation in the Sichuan Basin are clarified and the tectonic-lithofacies palaeogeographic pattern along with its evolution process are reconstructed. The results show that: (1)Eight types of rock microfacies and nine typical microfacies assemblage sequences have been identified in the Maokou Formation. Two sedimentary systems,specifically carbonate gentle slopes and marginal carbonate platforms,have been distinguished by integrating palaeontological and sedimentary tectonic markers. (2)The Maokou Formation underwent three stages of tectono-sedimentary evolution: the early Hercynian inheritable palaeouplift controlled the “southwest high and northeast low” landform pattern of the early Maokou(SQ1-SQ2),leading to the development of a stable carbonate gentle slope sedimentary environment,and the formation of a multi-zone migration thin beach body around the southwest Sichuan palaeouplift. Continuous extension in northern Sichuan resulted in the formation of a recessional multistage slope break zone,which led to significant platform-to-basin differentiation in the northern and southern parts of the basin,thereby directly controlling sedimentary differentiation during the middle and late Maokou(SQ3-SQ5). The sedimentary system transitioned into a marginal carbonate platform,with sedimentary facies zones exhibiting distinct northeast zoning characteristics. The uplift and stretching of the Emei plume led to the development of the southwest Sichuan depression belt,further intensifying the differentiation of the late Maokou(SQ6)platform,and causing the southwest Sichuan region to evolve into an inner depression. (3)The distribution of cavern dolomite reservoirs and karst pore limestone reservoirs in the Maokou Formation is controlled by sedimentary palaeogeomorphology and high-energy beach facies. The platform edge beach and inner beach facies represent favorable zones for reservoir development within the Maokou Formation. Key regions for the development of beach facies reservoirs include the platform margin beach zone in the Maokou Formation controlled by the retreating multistage slope break zone;the belt beach body surrounding the early inheriting paleo-uplift in the western rim;and the inner platform beach zone,which is distributed in parallel belts under an extensional setting. This study reconstructs the tectono-lithofacies palaeogeographic pattern of the Maokou Formation within a chronostratigraphic sequence framework,reveals the distribution patterns of favorable reservoir facies belts,and provides guidance for large-scale oil and gas discoveries in the Maokou Formation.

Several hundred meters of organic-rich shale are developed in the upper 4th Member of the Shahejie Formation in the Niuzhuang sub-sag of the Dongying sag,Bohai Bay Basin. This shale formation occurred during a critical period of climate fluctuations in the Middle Eocene in East Asia. However,the sedimentary evolution of this shale and its organic matter enrichment mechanisms in response to climate changes remain insufficiently studied. Through core and thin section observations,as well as organic and elemental geochemical analyses,we explore the mechanisms of organic matter enrichment under climatic constraints. Based on lithofacies assemblages and geochemical parameters,the study interval is divided into three stages: The first stage was characterized by a relatively dry and cold climate,with an average Total Organic Carbon(TOC)content of 2.52%. This stage featured rhythmically layered,dolomitic,fine-grained sedimentary rocks with moderate organic matter content and high-organic-matter rhythmic layered mixed fine-grained sedimentary rocks. The second stage experienced a relatively warm and humid climate,with an average TOC of 3.29%. It was marked by the development of high-organic-matter rhythmically layered quartz-containing calcareous fine-grained sedimentary rocks and high-organic-matter rhythmically layered mixed fine-grained sedimentary rocks. The third stage was a warm and humid period,with an average TOC of 2.78%. This stage was characterized by the development of high-organic-matter non-rhythmically layered mixed fine-grained sedimentary rocks and medium-to high-organic-matter rhythmically layered quartz-containing fine-grained calcareous sedimentary rocks. A sedimentary model for the Upper Fourth Member of the Shahejie Formation in the Niuzhuang sub-sag has been established based on lithofacies assemblages and trends in sedimentary environment changes. Under relatively warm and humid climates,moderate terrestrial input can bring terrestrial detrital organic matter and nutrients to the lake basin,which is beneficial for organic matter enrichment. Conversely,under relatively dry and cold climates,shallow water with high salinity can inhibit the growth and reproduction of aquatic organisms. Under warm and humid climates,excessive terrestrial input can dilute the organic matter concentration in water and introduce large amounts of oxygen,both of which are unfavorable for organic matter enrichment. This study provides valuable insights into the analysis of geological sweet spot analysis of organic-rich shale in continental faulted lake basins in eastern China.