Hangjinqi area in the Ordos Basin contains abundant natural gas resources. However， the distribution of effective reservoirs is complex. The lack of systematic reservoir evaluation standards limits the evaluation and subsequent development of gas reservoirs. This study takes the first member of the Lower Shihezi Formation （He 1 Member） in the J58 well area of the Dongsheng gas field as the research object. It analyzes the reservoir characteristics， establishes the classification standard for reservoir quality， and clarifies the distribution of favorable reservoirs by integrating the data of core， thin sections and physical properties. The results show that the lithology of the reservoir in He 1 Member is mainly lithic sandstone and feldspar lithic sandstone， and the rounding degree is mainly sub-angular， with medium sorting property. The reservoir type is predominantly characterized by dissolution pores， and four hole-throat combination configuration relationships are developed. The reservoir is classified as low porosity and ultra-low permeability. By optimizing the sedimentary facies， physical properties and microscopic pore throat characteristics， the classification and evaluation criteria were established. The reservoirs of He 1 Member are classified into four types： I， II， III and IV， whith types Ⅱ and Ⅲ being the most prevalent. He 1-1-2 and He 1-2-1 sublayers exhibit a high proportion of I reservoirs， while the He 1-4 sublayer has the highest proportion of type Ⅳ reservoirs. Type I reservoir are predominantly located within the channel bar and in the central portion of the main channel， whereas type Ⅳ reservoirs are typically found in non-major channels or along the sides of the main channel. This classification and evaluation standard can provide a reference for the later exploration and development of He 1 Member in J58 well area.
The Upper Paleozoic tight sandstone gas in the western Ordos Basin has become a key area for the increase of natural gas reserves and production in the basin. The Taiyuan Formation， as a newly explored layer in the western Ordos Basin， has demonstrated significantly superior gas test and production performance compared to the 8th member of the Shihezi Formation and the Shanxi Formation above it. In order to clarify the natural gas enrichment law of Taiyuan Formation， the main controlling factors of natural gas enrichment in Taiyuan Formation were determined by using logging， analytical test and three-dimensional seismic data， and through the study of coal-measure source rock evaluation， sedimentary reservoir characteristics， structural characteristics， etc. Research has shown that the main hydrocarbon source rock of Upper Paleozoic Benxi Formation in the Yanchi area has a hydrocarbon generation intensity of （10-24）×108 m3/km2， and the middle-eastern part of the study area has a high hydrocarbon generation intensity； a new understanding of the development of barrier coastal sedimentation in the Taiyuan Formation has been proposed for the first time. The distribution of barrier sand bars is stable， with an average thickness of 10.2 m. The reservoir is homogeneous， and the rock type is quartz sandstone， with an average porosity of 7.6% and an average permeability of 1.12×10-3 μm2； multi-phase faults are developed， which are not penetrated to the Shiqianfeng Formation. In particular， the Hercynian faults connect the gas source rocks at the bottom， constitute a favorable migration channel for natural gas， and improve the physical properties of the reservoir； the higher the intensity of hydrocarbon generation， the more faults in Hercynian and the closer the distance， the better the physical properties of the reservoir， and the more enriched the natural gas. The research results can provide reference for natural gas exploration with the same geological characteristics in Ordos Basin.
The Daniudi Gas Field in the northern part of the Ordos Basin in China has developed numerous small-scale strike-slip faults， but for a long time， there has been a lack of systematic structural analysis， which seriously restricts the oil and gas exploration process. Based on drilling and 3D seismic data， a strike-slip fault system was established in the Daniudi Gas Field， with the NNE trending Shibantai Fault， NNW trending Tuweihe fault， NEE trending Taigemiao Fault， and NNW trending Xiaohaotu Fault as the framework. The fault-controlled reservoir formation was discussed， and the conclusion was drawn that the strike-slip faults in the Daniudi Gas Field are mainly characterized by vertical type， flower structure， and inverted structure， with combined patterns of linear and echelon in plane distribution. The main fault exhibits vertical layering characteristics， while the Xiaohaotu fault， Shibantai fault， and Xiaohaotu strike slip fault exhibit left step segmented distribution characteristics. The Daniudi Gas Field is characterized by natural gas migration mainly from west to east， with the strike-slip fault uplift section as the oil and gas accumulation area， and the unconformity surface and fault fracture zone as the migration channels. The research results deepen the understanding of strike-slip faults in the Daniudi Gas Field and provide certain theoretical guidance for the exploration and development of Ordovician carbonate reservoirs in the study area.
In order to make sure the genesis and distribution of favorable reservoir in the second member of Xujiahe Formation， we divide the tight gas reservoir into different kinds of diagenesis facies according to Q-cluster analysis of multiple diagenetic parameters， clarify the distribution of different kinds of diagenetic facies based on Matlab analysis， and systematically study the diagenesis and reservoir characteristic， based on diagenesis and reservoir space analysis. The results show that the reservoir space in the second member of Xujiahe Formation is mainly residual intergranular pore and intragranular dissolved pore， followed by intergranular dissolved pore and fissure， while the intergranular micropores characterized by small radius and poor connectivity， are ineffective pores. Pore and throat radii range from 1.61 to 55 μm and 0.05 to 1.14 μm， respectively， classifying them as millimeter-micrometer pores + micrometer throats. The tight gas reservoirs have experienced early cementation， compaction， dissolution and later cementation， the intense compaction and quartz enlargement destroy lots of property， which is the significant reason for the reservoir densification. Although the chlorite occupied some intergranular volume， they enhance the anti-compaction ability and suppressed quartz enlargement， which is beneficial to the preservation of primary pore. The dissolved pores formed by feldspar and debris dissolution before gas accumulation， effectively improved the reservoir quality. The tight gas reservoir can be divided into four diagenetic facies， the reservoirs in chlorite-cemented residual intergranular pore facie and feldspar-dissolved intragranular dissolved pore facie have better reservoir quality and pore structure， which are the main “sweet spot” in the tight gas reservoir， while the reservoirs in quartz-cemented intergranular micropores and matrix-filled intergranular micropores facies were already dense before hydrocarbon accumulation， making them invalid reservoir.
The geological characteristics of the Dalong Formation in the Sichuan Basin vary greatly horizontally. A set of marine black shales is developed in the eastern Sichuan Basin， with a thin stratigraphic thickness and a high average TOC content. The mechanism of organic matter enrichment in the high-organic-rich black shale is still unclear. Selecting the Dalong Formation of Well FT1 in the eastern Sichuan Basin as the research object， this study uses petrological， organic geochemical， and inorganic geochemical methods to investigate the lithological characteristics， organic geochemical characteristics， sedimentary characteristics， and ancient marine environment of the black shale in the Dalong Formation in the eastern Sichuan Basin， and to explore its organic matter enrichment mechanism. The research results indicate that： （1） The first member of the Dalong Formation is mainly composed of thin layers of gray-black siliceous shale and calcareous shale， with calcareous black shale interbedded with mud crystal limestone， exhibiting horizontal bedding， and deposited in deep-water shelf facies； （2） the TOC content of the black shale in the Dalong Formation is relatively high， ranging from 1.84% to 9.4%， with an average of 5.11%. The microscopic components of the kerogen are mainly composed of exinite and vitrinite groups， which are type II1-II2 kerogen， reaching high to over-mature stages. The inorganic geochemical characteristics show that during the sedimentation period of the Dalong Formation black shale， volcanic activity was strong， upwelling ocean currents were developed， ancient productivity was high， and the water body was a limited anaerobic oxygen-poor sulfide environment. It is believed that high productivity and anoxic conditions are conducive to the enrichment of organic matter in the black shale of the Upper Permian Dalong Formation， but the enrichment of high TOC black shale in the Permian Dalong Formation is mainly controlled by the type of kerogen.
In order to determine the pore structure characteristics of porous tight sandstone reservoirs and establish and improve the criteria for reservoir classification and evaluation， taking the tight sandstone gas reservoir in Bashijiqike Formation of Zhongqiu 1 Block in the Tarim Basin as an example， the pore and throat characteristics and inter-well differences of tight sandstone reservoirs were systematically analyzed by using a variety of test methods， such as ordinary thin section， casting thin section， scanning electron microscope observation and mercury injection experiment. The results show that there is no significant difference in the petrological characteristics of Wells Zhongqiu 101， Zhongqiu 102， and Zhongqiu 2， but there are significant differences in the micro-pore structure between the wells. The Well Zhongqiu 101 has the highest porosity and the best physical properties. The pore throat radius distribution is mainly characterized by a multi-peak and coarse-grained state， and the pore structure conditions are the best. The Wells Zhongqiu 102 and Zhonqgiu 2 are mainly characterized by a bimodal coarse-grained shape. The pore structure of the study area is divided into four types， with the pore structure types of Well Zhongqiu 101 mainly being Class A and Class C； Well Zhongqiu 102 is mainly classified as Class B and C， with local development of Class D； Well Zhongqiu 2 is mainly classified as Class C， with the development of Class B and Class D. It is clear that primary intergranular pores contribute the most to reservoir properties， dissolution pores improve properties， micropores have little impact， and primary intergranular pores， dissolution pores， and micropores have the best properties. The four types of pore structure properties deteriorate in sequence， with sedimentation being the basis for differences in reservoir pore structure and diagenesis being the main factor affecting pore structure differences. Based on the correlation between pore throat characteristic parameters and physical properties， the maximum pore throat radius， average pore throat radius， displacement pressure， and sorting coefficient are selected as the main evaluation indicators for reservoir classification. The reservoirs in the study area are divided into four categories： Class I is high-quality reservoirs， Class II is good reservoirs， Class III is medium-grade reservoirs， and Class IV is poor reservoirs. The high-quality reservoirs are located in the Zhongqiu 101 well area.
Sanhu Depression is the most important biogas-producing area of the Quaternary in Qaidam Basin， with huge natural gas resources. In order to clarify the depositional environment for the formation of mudstone reservoirs in gas reservoirs and provide a basis for reservoir sweet spot evaluation， this paper analyzes the elemental geochemical characteristics and sedimentary environment of mudstone core samples from Wells ST1 and ST2 in Sebei area of Sanhu Depression through hand specimen analysis， microscopic observation and elemental testing. The findings reveal that samples from Wells ST1 and ST2 are predominantly composed of dark mudstone and siltstone， interspersed with dolomite in block， band， and laminar forms. They contain a great number of snails and plant fragments， reflecting the sedimentary environment of shallow lakes and semi-deep lakes. It has the characteristics of low silicon， weak supersaturation of aluminum， low potassium sodium， rich in magnesium and calcium， enrichment of Ba， Sr and Rb， and loss of Zr， Hf and Ni. The ICV and CIA indices， along with Th/SC-Zr/Sc discriminant maps， suggest that the Quaternary mudstone in the Sebei area underwent only mild to moderate weathering and remained largely unaffected by sedimentary sorting or recycling processes. Parameters such as Ceanom， &U， and δCe indicate that the deposition period is the environment for the transition from reduction to oxygen deficiency. The ratios of Sr content to Th/U and V/Zr are indicative of a saline water to brackish water environment. Moreover， the MAP and LST parameters， as well as Sr/Cu， Zr/Rb ratios collectively reflect a cold and dry paleoclimatic environment with weak hydrodynamics. The high soluble organic matter content of Quaternary mudstone in the Sebei area of the Sanhu Depression， coupled with frequent changes in sedimentary water environments， has resulted in distinct vertical sand-mud interbedded characteristics， forming a favorable reservoir cap combination and facilitating the formation of mudstone biogas reservoirs.
To investigate the distribution patterns in the degree of gas invasion of oil and gas reservoirs in the Tarim Basin，geochemical analysis methods were employed to delineate the distribution characteristics of biomarkers and carbon isotope composition of individual n-alkane ratios in oil and gas reservoirs across different well areas in this paper.The results indicate significant differences in the physical properties of crude oil， the characteristics of light hydrocarbons， and the carbon isotope composition of individual n-alkanes between the TZ83 well area and the ZG43 well area. The extent of n-alkane loss， the carbon number at the breakpoint， and the content of adamantane series compounds indicate that the crude oil in the TZ83 well area exhibits relatively strong gas invasion， while the crude oil in the ZG43 well area shows relatively weak gas invasion. The study indicates that the variations in gas invasion intensity may be attributed to the differing structural positions of the well areas. The TZ83 well area is situated in the high structural zone at the intersection of the Tazhong Ⅰ fault slope fold belt and the strike-slip fault， where the underlying gas source， adjusted by faults， exerts a strong gas invasion influence， resulting in condensate oil. In contrast， the ZG43 well area is located on the platform zone of the Tazhong 10 fault belt， where the development of deep and large fault systems is less pronounced， leading to weaker gas invasion effects and the formation of waxy oil. In the shallow reservoirs， there may be undiscovered condensate oil reservoirs.
In recent years， the Qixia Formation in central Sichuan Basin has become a hotspot for natural gas exploration in the Sichuan Basin due to its high-yield industrial gas flow. Research indicates that the natural gas in the Qixia Formation exhibits the characteristics of multi-source hydrocarbon supply and mixed-source characteristics， exhibiting distinct geochemical signatures across different tectonic blocks due to variations in the proportion of hydrocarbon source rocks. However， there is currently limited research on the contribution proportions of source rocks from various formations to mixed-source gas reservoirs， and quantifying these contributions is an urgent issue. Through an analysis of the geochemical characteristics of natural gas and the application of a mixed-source contribution model， a comparative analysis was performed on the geochemical signatures of natural gas in the Gaoshiti， Moxi， and Longnvsi tectonic blocks within the Qixia Formation in central Sichuan Basin. This analysis clarified the specific contributions of source rocks from different formations to the gas reservoirs in these three blocks. The results indicate that： （1） The Gaoshiti block has the highest average C2H6 content and the lowest δ13C2， with isotope reversal observed； the Moxi block shows the highest average CH4 content and the lowest δ13C1， with no isotope reversal； the Longnvsi block presents average values between the other two， with the highest δ13C1 and δ13C2， and no isotope reversal. （2） In the Gaoshiti block， natural gas is sourced from both the Qiongzhusi Formation （with a contribution ratio of 18%-98%） and the Longmaxi Formation， with the latter’s contribution decreasing as wells approach the pinch-out line. In the Longnvsi block， natural gas originates from both the Qiongzhusi Formation （with a contribution ratio of 55%-92%） and Middle Permian source rocks， with the latter’s contribution increasing as wells move northward. The Moxi block's natural gas is entirely derived from the Qiongzhusi Formation source rocks. Comprehensive analysis indicates that the variations in mixed-source contributions are the primary factor contributing to the differences in geochemical characteristics across the three blocks. This variation in source contribution also explains the significant differences in individual well production and the complex gas-water distribution observed in the Qixia Formation in the central Sichuan.
Reservoir solid bitumen has been found in many petroliferous basins in the world， and it contains important information such as hydrocarbon generation， expulsion time from source rocks， and timing of paleoreservoir oil cracking. In recent years， Re-Os isotope dating is widely used in the study of solid bitumen. However， the interpretation of Re-Os isotopic ages is often ambiguous， and their geochemical significance remains unclear. This study combines isotope geochemistry and organic petrology to analyze solid bitumen from three different locations in the Upper Yangtze region. The results show that the Lower Cambrian bitumen from Songtao area has a Re-Os age of 195 ± 20 Ma， and it represents the time of bitumen solidification and the upper limit of the time when the shale of Niutitang Formation of Lower Cambrian stops hydrocarbon generation. The Dengying Formation bitumen from Jinsha area has a Re-Os age of 297 ± 20 Ma， which represents the time of bitumen solidification. The Dengying Formation bitumen from the Weiyuan Gas Field has a Re-Os age of 342.8 ± 4.7 Ma， and it also represents the time of petroleum generation. In this research， the relationship between solid bitumen Re-Os data and the thermal evolution of organic matter is discussed. It is considered that solid bitumen Re-Os data may represent the time of petroleum generation， time of hydrocarbon accumulation， and the time of bitumen solidification or the time of thermochemical sulfate reduction （TSR） completion， which ultimately depends on the genesis of solid bitumen and the homogeneity of Os in the system.
Pore structure characteristics are the main factor affecting shale reservoir， and its qualitative and quantitative characterization and main controlling factors are key issues in shale reservoir research. In order to explore the differences in microscopic pore structures and main controlling factors of different sedimentary microfacies of deep shale reservoir， this paper selects the Wufeng-Longmaxi formations in Well Z301 of Zigong area in southern Sichuan Basin as an example， based on systematic experiments such as core， thin section， scanning electron microscopy observations， X-ray diffraction analysis， organic geochemical analysis， N2/CO2 adsorption， high-pressure mercury injection， the vertical heterogeneity of pore structure in the O3 w-S1 l 1 shale reservoir is analyzed. The research results indicate that the sedimentary microfacies of the O3 w-S1 l 1 shale reservoir in the study area can be divided into three categories from bottom to top： strong reducing， high carbon， calcium-rich， and silicon rich deep-water continental shelves （microfacies ①）， weak reducing-medium carbon-calcium containing-silicon mud mixed-deep water continental shelves （microfacies ②）， and weak reducing-weak oxidizing-low carbon-siliceous mud-semi deep water continental shelves （microfacies ③）； among the three types of microfacies， macropores are mainly inorganic pores， while mesopores and micropores are mainly organic pores. Mesopores and micropores are also the main pore types that control the volume and specific surface area of shale pores； the development degree of different pore types varies among the three microfacies； mesopores and micropores are the most important pore type that controls reservoir physical properties and gas content； TOC and the content of clay minerals are the key factors affecting the pore structure of deep shale. Quartz has a slightly weaker controlling effect on nanoscale pores， while carbonate minerals have no significant controlling effect on nanoscale pores； the characteristics of high TOC， low clay minerals， and high brittleness minerals in microfacies ① determine that it is the most commercially valuable lithofacies for mining. The relevant conclusions can provide guidance for enriching the high-yield patterns of deep shale gas enrichment.
Natural fractures play a critical role in controlling the productivity of shale oil reservoirs， and their accurate identification and characterization are essential for the optimal selection of sweet spots and efficient development of shale oil and gas. This study investigates the mixed shale oil reservoirs in the upper member of the Lower Ganchaigou Formation in the Yingxiongling area， Qaidam Basin. Core samples， thin sections， scanning electron microscopy （SEM）， and both conventional and advanced logging data were used to summarize the types and characteristics of natural fractures in the study area， and a comprehensive logging-based fracture identification model was developed. This model enables continuous fracture identification and characterization from well logs. Furthermore， by integrating fracture parameters with core experimental data， the degree of fracture development and their effectiveness were analyzed. The results show that natural fractures in the study area can be classified into three types： tectonic fractures， diagenetic fractures， and abnormal high-pressure fractures. Tectonic fractures are predominantly high-angle and highly filled， diagenetic fractures are typically low-angle horizontal with a low degrees of filling， and abnormal high-pressure fractures exhibit irregular orientations and are often filled. Fractures are most developed in the VI oil layer group of the upper member of the Lower Ganchaigou Formation， followed by the V oil layer group， with the IV oil layer group being relatively less developed. In general， low-angle unfilled fractures are the most abundant， horizontal unfilled fractures exhibit the highest developmental intensity， and high-angle unfilled fractures demonstrate the best effectiveness. The current maximum horizontal principal stress direction in the study area is NE-SW， and most unfilled fractures form angles less than 30° with this direction， which enhances their fracturing efficiency. In contrast， filled fractures have dominant orientations forming angles greater than 40° with the maximum horizontal principal stress， which reduces their fracturing efficiency. The findings of this study are expected to provide theoretical and methodological support for the identification and evaluation of natural fractures， and the optimization of sweet spots in lacustrine mixed shale oil reservoirs.
The shale in the second member of Funing Formation （E1 f 2） from the Gaoyou Sag， Subei Basin has demonstrated significant industrial oil and gas production. The composition and enrichment of organic matter （OM） in the shale play a crucial role in hydrocarbon generation， highlighting the importance of studying its OM characteristics and enrichment mechanisms. This study investigates Well H1 in the Gaoyou Sag， employing organic petrology， X-ray diffraction （XRD）， and organic-inorganic geochemical analyses to systematically investigate the microscopic characteristics of OM， depositional environment， and the main controlling factors of OM enrichment in the E1 f 2 shale， and ultimately reveal its enrichment model. The results show that： （1） The E1 f 2 shale can be classified into three types：Felsic shale， hybrid shale， and calcareous shale， with significant differences in microscopic components. The hybrid shale exhibits a wide range of alginite content （40%-95%） with an average of 72.94%， forming laminar algal with a thickness of over 30 μm and a length of up to 200 μm； （2） The E1 f 2 shale was primarily deposited in an arid climate and high-salinity， anoxic environment， with OM mainly derived from low-level aquatic organisms， predominantly Type I and Type II1 kerogen， reaching the peak oil generation stage of maturity， which is favorable for shale oil formation； （3）The OM enrichment in the E1 f 2 shale is mainly controlled by paleoproductivity and paleosalinity， with preservation conditions having a relatively minor influence. This study provides essential insights for further shale oil exploration and development within OM-lean shale in the Gaoyou Sag， as well as similar areas.
The XJJ Gas Field area in the central part of the Pinghu slope belt in the Xihu Sag of the East China Sea Basin hosts fault steep slope type braided river delta plain distributary channel sandstone structural lithology oil and gas reservoirs. The development of oil and gas reservoirs has entered the late stage， and the identification and evaluation of structural lithology traps in the surrounding areas without wells is urgent. The reservoir in the study area exhibits significant heterogeneity， and the interlayer interference of coal seams is large. Conventional poststack seismic and seismic sedimentology techniques based on this are insufficient for predicting current lithological traps. Prestack inversion also has uncertainty in predicting in remote areas without wells. To address these challenges， seismic sedimentology tech-niques are employed. Conventional seismic sedimentology is based on poststack seismic， while prestack AVO preserves significant sensitive information about lithology and fluids. Therefore， prestack seismic sedimentology research will play a crucial role in evaluating the potential of lithological traps. First， the study aims to identify the favorable conditions for the development of various types of lithological traps in different parts of the study area within different structural sequence stratigraphic styles， and to determine the distribution patterns and controlling factors of sand bodies. Next， based on the optimization of angle gathers， the prestack AVO sensitive seismic attributes are utilized for reservoir prediction and hydrocarbon detection in the non-drilled area around the gas field； the sensitive lithology AVO attributes were shifted by 90 degrees， and on this basis， seismic lithology and seismic geomorphology were studied. Finally， through iterative understanding and correction of geological knowledge and prediction methods， multiple different structural sedimentary lithological traps in the non well area are identified， and the development characteristics and reservoir formation patterns of lithological traps are summarized， providing guidance for future exploration and development evaluation. The research ideas and technical methods provide valuable insights for the evaluation of lithological traps in non-drilled areas of slope zones.
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