The Ordovician subsalt in the Ordos Basin has a good reservoir-cap assemblage， with an exploration area of nearly 70 000 km2. Recently， high-yield wells represented by J41 and HT8 have been drilled in the subsalt area， which confirms that faults and fractures play an obvious role in controlling the Ordovician subsalt oil and gas enrichment accumulation in the basin. Using a large number of two-dimensional seismic data and drilling data， the subsalt faults in the central and eastern parts of the basin were systematically described， and the main controlling factors of the accumulation of subsalt gas reservoirs were clarified. The following understandings have been obtained： （1） Large-scale faults subsalt development， which have the characteristics of transverse partitioning and longitudinal stratification. （2） The overall subsalt reservoir is relatively compact， and the fault has a strong effect on the reservoir. Drilling core and imaging logging revealed that the fractures and pores of the near-fault exploration core were more developed， and the core of the far-fault exploration well was denser and the fractures were basically not developed. （3） Most of the high-yield wells subsalt are located on or near fault zones， and the reservoir types are mainly fractured type， and the structure has different controlling effects on the high-yield enrichment of oil and gas in the central and eastern basins， and the structural control characteristics in the eastern basin are more significant than those in the central basin. （4） The prediction results of structural tensor attributes for subsalt fractured reservoirs are in good agreement with those of drilled wells， which confirms that the fault zone is an efficient target for subsalt exploration， and high-yield wells are near faults， reservoir modification is strong， and oil and gas migration is strong.
Using micro-zone temperature measurement technology， in-situ microthermometric analysis of chlorite， illite and carbonate cement in Chang 82 reservoir in Huan County was carried out. Combined with the contact relationship between diagenetic minerals， the filling sequence of Chang 82 reservoir interstitial materials was quantitatively determined as follows： Phase I chlorite coating （52-94℃） → Phase I carbonate cementation （62-86 ℃）→ Phase I authigenic quartz（54-91 ℃） → Kaolinite + Phase II authigenic quartz （90-115 ℃） → Phase I illite （96-116 ℃） → Phase II chlorite （106-138 ℃） + phase II illite （121-139 ℃） + phase II carbonate （97-114 ℃） → Phase III authigenic quartz （131-140 ℃） → phase III illite （143-162 ℃） → phase III carbonate （117-122 ℃） → phase III chlorite （134-179℃）. Combined with the fluid inclusion temperature measurement technology， the quantitative coupling relationship between cements filling， pore evolution and hydrocarbon charging was established. During the diagenetic evolution of the reservoir， the porosity was reduced by 20.1% in compaction， 5.0% in carbonate cement， 1.7% in quartz， 3.1% in chlorite， 0.7% in kaolinite and 2.4% in illite. In the first phase （60-80 ℃）， the amount of oil and gas injection accounted for 3.6% and the porosity of the reservoir was 22.31%. In the second phase （80-120 ℃）， the main oil and gas charge accounted for 96.4%， the porosity of the reservoir was 18.06%， the porosity after the end of the oil and gas charge was 11.8%， and the final porosity was 7.33%. The Chang 82 reservoir in the study area exhibited the most significant porosity reduction during the main oil and gas charge period， and had the characteristics of “reservoir formation accompanied by tightening”.
The first member of Maokou Formation of the Middle Permian （Mao 1 Member） in Da’an block is a key area for exploration of tight limestone gas in Sichuan Basin. Through the identification and quantitative evaluation of fracture pores by two evaluation wells in Da'an block， this paper explores and reveals the development characteristics， distribution laws and main controlling factors of fractures in Mao 1 Member in southern Sichuan Basin. The study reveals that： （1） In the high part of the structure， low-angle bedding fractures， high-angle and echelon fractures and reticulated fractures are generally developed. Fractures are densely distributed and mostly filled with calcite. Fractures are 1-25 mm wide， and some are filled with asphalt locally. In the low part of the structure， fractures are not well developed or locally developed. （2） The pore system is complex and diverse， mainly including intercrystalline pores of clay minerals， calcite， dolomite， quartz， pyrite and other inorganic mineral grains （crystals）， intra-grain （erosion） pores， organic matter pores and fractures， etc.， and the nuclear magnetic resonance （NMR） T 2 spectrum generally has multi-peak or double peak characteristics. The volume of reservoir space is mainly composed of inorganic mineral pores and fractures， with an average proportion of 47.6%-71.6% and 11.5%-40.3%， respectively， and the volume of organic pores is only 16.5%-26.8%. The average porosity of fractures is 0.23%-1.00%， and the regional variation is large. It is higher in the high part of thrust zone， but relatively lower in the low part of structural zone or syncline area. （3） The thickness of fractured favorable reservoirs is 2-24 m and varies greatly in the region. The high value area is located in the high part of the fold belt or the anticline core， and is distributed in a strip-like distribution from northeast to southwest. The low value area is distributed in the low part of the fold belt or the broad syncline zone. （4） The highly brittle argillaceous limestone rich in siliceous and dolomitic components， combined with the three-stage compressional folding and detachment in the Yinzhi， Yanshan and Himalayan periods， is the key controlling factor for the development of large-scale fracture zones in the area， and the middle and late periods of Yanshan are the peak stages of fracture development.
The Upper Sinian Dengying Formation in the western margin of the Sichuan Basin is characterized by deep burial and relatively complete development. However， current levels of research and exploration remain low， with limited breakthroughs in oil and gas exploration to date. This study focuses on a typical outcrop profile at Yanziyan in Mianzhu， located in the northern segment of the Mianyang-Changning trough， to investigate the sedimentary sequence and reservoir characteristics of the Dengying Formation. The first member of Dengying Formation is dominated by mud-microcrystalline dolomites， while the second member of Dengying Formation is dominated by botryoidal structure， algal-laminated， and algal-striped dolomites. The major reservoirs develop within the second member of the Dengying Formation， with reservoir spaces primarily consisting of algal lattice pores， grape edge dissolution pores， caves， and fractures. Locally， bitumen is developed， reaching a thickness of up to 330 m. The Dengying Formation strata become more complete and thicker toward both northern and southern flanks of the Mianyang-Changning “inner trough zone”（Qingping and Yanziyan）. Carbon and oxygen isotopes indicate that the Dengying Formation sedimentation period was overall in a warm and humid marine tidal flat-lagoon （restricted platform） environment， mainly developing muddy limestone， bioclastic limestone， and dolomite， etc. in the gentle slope type carbonate rocks. The Dengying Formation reservoirs in the study area underwent three main evolutionary stages： the sedimentary period of the microbial reef， the near-simultaneous period of frequent exposure and dissolution， and the supergene karst period with atmospheric freshwater infiltration and dissolution. The repeated leaching and dissolution of atmospheric fresh water in the quasi-contemporaneous period are the key factors for the formation of high-quality reservoirs such as Dengying algal lamination and botryoidal structure algal dolomites. Subsequently， during the deep burial period， the dolomite reservoir of the Dengying Formation in the study area underwent at least three diagenetic fluid transformations， and during the late-stage diagenesis， quartz-rich fluid primarily transformed and filled the dissolution pores， caves， and fractures formed during the contemporaneous and supergene karst periods. The Dengying Formation profile at Yanziyan in Mianzhu， located in the favorable lower intertidal zone of the northern segment of the Mianyang-Changning trough， features thick and high-energy algal mound facies. The algal mound dolomite reservoirs are thick and exhibit good porosity and permeability. Combined with high-quality Cambrian source rocks， the middle segment of the Longmen Mountains in the northern segment of the Mianyang-Changning trough can be considered a key target area for future natural gas exploration.
Due to limited drilling data， the distribution patterns and controlling factors of grain shoals in the Longwangmiao Formation remain poorly understood. The characteristics， distribution， main controlling factors and petroleum geological significance of grain shoal in Longwangmiao Formation are comprehensively analyzed in this study by observation of core and thin section， analysis of log facies analysis， seismic forward modeling and analysis of seismic phase. There are many rock types of grain shoal in the study area， such as oolite dolostone， dolarenite and crystal grain dolostone with residual granular structure. Vertically， grain shoals exhibit a shallowing-upward coarsening sequence， two kinds of sedimentary sequences can be identified： inter-shoal sea-grain shoal and lagoon-grain shoal. Laterally， grain shoals show high continuity， and their planar distribution was delineated using seismic waveform clustering attributes. Regionally， grain shoal distributes around Leshan-Longnvsi paleo-uplift in a ring band， and the development of it in the northern slope is lower than grain shoal in the Gaoshiti-Moxi area. The vertical superposition pattern and development characteristics of the grain shoal in Longwangmiao Formation are determined by the change of relative sea level and the variation of sedimentary energy. The paleo-geomorphology of sedimentary period determines the regularity and difference of grain shoal's planar distribution. In the southern part of the study area， grain shoals experience minimal admixture of terrigenous clastics， significant supergene karstification， and occupy high structural positions during tectonic evolution， ensuring adequate oil supply. Compared to northern shoals， these conditions make them more favorable for oil and gas accumulation.
The western Hubei and eastern Chongqing area is one of the most active shale gas exploration areas in the periphery of Sichuan Basin， and several exploratory wells have obtained Permian shale gas discoveries， showing good prospects for shale gas exploration and development. Previous exploration shows that the shale gas enrichment characteristics of Dalong Formation， the main producing layer in this area， are quite different from those of Wufeng-Longmaxi formations in the Sichuan Basin， and the shale reservoir classification and evaluation standards established for the latter are difficult to meet the new shale gas exploration and development needs， so it is necessary to put forward an applicable shale reservoir evaluation standard system. Based on the existing exploration data， we systematically analyzed the shale gas enrichment conditions of Dalong Formation in the western Hubei and eastern Chongqing area， and compared that with the Wufeng-Longmaxi formations in the Sichuan Basin， so as to clarify the shale gas enrichment characteristics of Dalong Formation. On this basis， with the shale gas richness as the core， taking into account the compressibility of shale and production potential， we carried out the research on the evaluation parameter system of the Permian shale reservoirs， and established the shale gas reservoir classification and evaluation standard and evaluation method system applicable to the Permian shale of the western Hubei and eastern Chongqing area.Exploration and development practice has proved that the established shale gas reservoir evaluation system can be better applied to the evaluation of Permian shale gas exploration and development in western Hubei and eastern Chongqing area， which can provide effective guidance for the regional shale gas large-scale exploration and beneficial development.
The Ordos Basin has entered an unconventional intra-source exploration phase， with multiple high-yield industrial wells drilled in the Chang 73 sub-member of Longdong area marking a strategic breakthrough in new-type shale oil exploration. It is of great significance to deepen the exploration and deployment to clarify the lithofacies assemblage characteristics and distribution patterns of shale oil. Taking the shale oil reservoir of the Chang 73 sub-member as the research object， qualitative description and quantitative characterization of the facies types and facies assemblages of shale oil have been carried out through methods such as core observation， thin-section identification， X-ray diffraction analysis and TOC analysis. Further comprehensive research on the spatial distribution of the facies assemblages has been carried out， so as to ultimately establish the deposition patterns of facies assemblages of different sedimentary genesis. Based on macroscopic sedimentary characteristics， microscopic mineral composition characteristics， and organic matter content characteristics， six lithofacies types can be identified in Chang 73 sub-member， and four lithofacies assemblages of different sedimentary origins can be divided vertically. Under the joint control of factors such as paleo-monsoon， tectonic movement， provenance supply， topographic slope and gravity flow transformation， the spatial spreading characteristics of different lithofacies assemblages are significantly different. Under the background of frequent tectonic movements， different types of lithofacies assemblage exhibit specific distribution patterns. Three distinct lithofacies assemblages （Types Ⅰ-Ⅲ） exhibit a proximal-to-distal zonation from the lake basin slope break to deep lacustrine zones in sand-rich steep slopes， while Type Ⅳ dominates tuff-rich and deep lacustrine areas under SW paleo-monsoon influence. Facies association Type Ⅳ is also easy to form in the deep lake area.
Pore structure heterogeneity between marine-continental transitional shale and coal reservoirs fundamentally plays a crucial role in unconventional oil and gas exploration and development. This research uses the Shan2 3 sub-member in the Danning-Jixian area of the Ordos Basin as a case study， systematically conducting characterization and comparative analysis of the full-scale pore structure of shale and coal reservoirs. Using field-emission scanning electron microscopy （FE-SEM）， mercury intrusion porosimetry （MIP）， N₂ and CO₂ adsorption experiments， along with TOC and XRD analysis， the study investigates the control of organic matter and inorganic minerals on the pore structures at different scales. The results show that： the average TOC value of the shale is 4.69%， with the development of organic matter pores， inorganic pores， and microfractures， among which organic matter pores are the most common， often densely and clustered； the average TOC value of the coal is 74.22%， with organic matter pores being the dominant pore type， and the pore diameter is significantly larger than that of marine-continental transitional shale and marine shale. In the shale， the micropores， mesopores， and macropores all contribute to the total pore volume， with organic matter serving as the material foundation for micropore development， and the clay mineral diagenesis playing an important role in promoting mesopore and macropore development. In the coal， micropores and macropores are the main types， with organic matter being the most significant factor influencing pore development， and a higher TOC supporting the development of larger organic pores. This study comprehensively reveals the similarities and differences in the pore structures of marine-continental transitional shale and coal reservoirs at the micro scale， providing a scientific basis for the precise evaluation and development of unconventional oil and gas resources.
The competitive adsorption behavior of CO2/CH4 binary mixtures in shale was investigated to optimize CO2-enhanced shale gas recovery （CO2-ESGR） injection parameters were optimized. On the basis of deriving the conversion method between the excess adsorption and the absolute adsorption quantity， isothermal adsorption experiments of pure CO2，pure CH4，and CO2/CH4 gas mixtures with different molar ratios were carried out for different types of shale. The adsorption quantities of the CO2/CH4 gas mixtures with different molar ratios were carried out for different types of shale. The adsorption quantities of the CO2/CH4 gas mixtures with different molar ratios in different types of shale were determined， and the relationship between the selective adsorption factor（ ） of shale for CO2/CH4 and pressure and the CO2/CH4 ratio was clarified. Results indicate that adsorption capacity of CO2/CH4 mixtures scales positively with both CO2 molar fraction and pressure. Clay minerals exhibit lower adsorption contributions than organic matter， with mineral-specific effects-higher TOC， illite and smectite contents correlate with enhanced CO2 affinity. The value of the experimental shale is all greater than 1. When the gas mixture ratio is 50%CH4+50%CO2， decreases with the increase of pressure. When the gas mixture ratio is 20%CH4+80%CO2 and 80%CH4+20%CO2， first decreases and then increases with the increase of pressure. SCO2/CH4 first increases and then decreases with the increase of the CO2 ratio in the gas mixture. A lower reservoir pressure and a smaller CO2 injection rate can ensure that CO2 fully diffuses in the shale reservoir at a lower pressure， keep the CO2/CH4 ratio within an appropriate range， and are beneficial for CO2-ESGR. With the increase of the CO2 ratio and pressure， although the preferential adsorption behavior of CO2 in shale weakens， the storage capacity of CO2 increases. The research results provide theoretical guidance and reference basis for the field implementation of CO2-ESGR and CO2 storage.
Micropores are key controls on CO2 sequestration in shale reservoirs， yet their evolution under supercritical CO2-water-rock interactions remains poorly understood across sedimentary facies. This study systematically investigates the effect mechanism of supercritical CO2-water-rock interaction on micropores of different sedimentary shales. The results show that supercritical CO2 injection will dissolve primary minerals and produce secondary minerals， which will alter pore connectivity. After supercritical CO2-water-rock interaction， marine shales show decreased total specific surface area （TSSA） and total pore volume （TPV） in both micropores and mesopores， whereas continental shales exhibit increased TSSA and TPV in micropores despite similar mesopore reductions. In addition， after supercritical CO2-water-rock interaction， the pore fractal dimension of marine shales decreases， while that of continental shales shows the opposite trend. The influence mechanism of supercritical CO2-water-rock interaction on microscopic pores of shale mainly includes organic matter extraction， primary mineral dissolution， secondary mineral precipitation and clay mineral adsorption expansions. The extraction of organic matter and the dissolution of primary minerals can widen the pores， while precipitation of secondary minerals and adsorption expansion of clay minerals will promote the contraction of shale pores. The research results can provide support for CO2 geological storage of exhausted shale gas reservoirs in China.
The study of pore structure and heterogeneity in shales is a key focus in unconventional reservoir research. Transitional shales are widely distributed in China and exhibit significant exploration potential. However， research on the pore structures of transitional shales significantly lags behind that of marine shales， particularly in the northern part of North China Basin， where resource potential is substantial but related studies are limited. This paper investigates the pore structures and heterogeneity of transitional facies shale samples from the Shanxi Formation， using core samples from Well KP1 in the Kaiping Syncline area of the northern North China Basin， employing various experimental methods. The results reveal that the shale is primarily composed of quartz and clay minerals. The total organic carbon （TOC） content ranges from 0.3% to 3.43%， with vitrinite reflectance（R O）values exceeding 1.6%， indicating a high level of thermal maturity. Pores in the shale are predominantly intergranular clay mineral pores， mainly in the form of flat， slit-shaped， with most pore sizes being smaller than 100 nm. Fractal dimensions for ultra-large pores， macropores， mesopores， and micropores range of 2.29-2.34， 2.25-2.69， 2.69-2.76， and 2.36-2.56， respectively， while the overall fractal dimension （D s） ranges from 2.60 to 2.63. The high clay content and low abundance of Type I kerogen in the study area result in poorly developed organic pores （micropores）， which is the main reason for the absence of significant correlations between TOC， R O， and fractal dimensions. The quartz-associated intergranular pores exhibit a highly complex pore structure， characterized by distinctly irregular pore and fracture shapes. This irregularity significantly increases the complexity of the overall pore system， influencing the storage and permeability characteristics of the reservoir. Consequently， higher clay mineral content contributes to a more complex overall pore structure. The fractal dimensions show a positive correlation with pore surface area and porosity， indicating that greater pore structure complexity enhances the internal pore space of the shale， improving its capacity to store both adsorbed and free gas. The impact of fractal dimensions on permeability varies with pore size： complex mesoporous systems increase flow resistance， hindering gas desorption and diffusion， while complex micropore structures enhance connectivity between larger pores， improving the flow capacity of the shale reservoir.
Bright spot technology， which has played a vital role in tight gas reservoir prediction for the second member of Jurassic Shaximiao Formation （Sha2 Member） in the Sichuan Basin， faces limitations in the exploration of the Sha1 Member. Based on the analysis of sedimentary environments in the Sha1 and Sha2 members and the log and seismic responses of reservoirs， multi-component seismic data are found to be a good alternative and applied to Sha1 channel sands and gas prediction after multi-component volume registration in the time domain. The results show that （1） less “bright spots” in Sha1 Member than in Sha2 Member could be attributed to smaller porosities of Sha1 channel sands， resulting in low P-impedance contrasts between Sha1 sandstones and mudstones， and （2） high-graded Sha1 channel sands exhibit weak PP responses and medium to strong PS responses， leading to more reservoirs discovered using PS-amplitude attributes. At the drilling sites deployed recently， reservoir penetration rate accounts for 90% of horizontal section length. Our research findings propel multi-component data acquisition of scale and offer support to Sha1 reserves estimation in northwestern Sichuan.
Permian lacustrine calcareous mudstone is one of the main source rocks in the southwestern Tarim piedmont. Taking shallow Well YQ1 as the main research object， this paper preliminarily expounds the organic matter abundance， types， maturity and parent material source of lacustrine calcareous mudstone by means of classical organic geochemical methods such as Rock-Eval， gas chromatography （GC） and gas chromatography-mass spectrometry （GC-MS）， and further reveals the hydrocarbon generation environment and source rock. The experimental results show that the calcareous mudstone of Permian Pusige Formation in the piedmont of Well YQ1 in southwestern Tarim Basin is generally low in organic matter abundance， and the type of organic matter is mainly type Ⅱ， followed by type Ⅲ， and the thermal evolution is in a mature stage.Normal alkanes have nC21 and nC23 as the main peaks， tricyclic terpanes have a nearly normal distribution with C21 as the main peak， steranes are dominated by C29 regular steranes， showing the characteristics of typical lacustrine source rocks， and the biological sources of organic matter are characterized by the dual contributions of lower aquatic organisms and higher plants. The Pr/Ph value is generally less than 0.8， and the contents of gammacerane， rearranged hopane， rearranged sterane and other compounds increase and decrease. Comprehensive analysis shows that the source rocks were formed in a reduction-strong reduction environment， and the salinity of the upper and lower water bodies is different. The specific sedimentary environment of Permian Pusige Formation provides high-quality soil for halophilic archaea to flourish， which can improve the types of organic matter， prolong the peak period of oil generation， greatly improve the conversion rate of hydrocarbon generation， and greatly reduce the residual organic carbon after hydrocarbon generation， which is one of the reasons why the organic carbon content of source rocks of Permian Pusige Formation is generally low. The unique hydrocarbon generation potential characteristics of Permian Pusige Formation source rocks provide a good material basis for the formation of large and medium-sized oil and gas fields in the Kekeya area.
In order to clarify the geochemical characteristics of Paleogene source rocks and the source of crude oil in Shunde Sag of Pearl River Mouth Basin， the geochemical characteristics of Wenchang Formation source rocks were analyzed systematically， followed by oil-source correlation by combining the total organic carbon content of source rocks， Rock-Eval pyrolysis and biomarkers of source rocks and crude oil. The abundance of organic matter in the E2 w 1 and E2 w 3 source rocks is low， and the type of organic matter is mainly type Ⅱ2 and type Ⅱ1.The abundance of organic matter of E2 w 2 source rocks is high， and the type of organic matter is mainly type I， which enters the mature stage as a whole. The medium Pr/Ph value of the source rocks of the Wenchang Formation indicates that they were deposited in a weak reduction-weak oxidation environment， and the low gammacerane value indicates that they were deposited in a freshwater environment. The C27-C28-C29 regular steranes of the E2 w 1 source rocks are anti-“L” type， indicating that the source of terrestrial higher plants is dominant， while the regular steranes of the source rocks in the other layers are mainly “V” and “L” -shaped patterns， indicating the mixed source of lower aquatic organisms and terrestrial higher plants. The oil shale of E2 w 2 source rocks has high C30 4-methyl sterane content. E2 e 2 crude oil has low Pr/Ph value， low C19+20TT/C23TT value， C27-C28-C29 regular steranes exhibiting a V-shaped pattern， and high abundance of C30 4-methyl steranes. The results of oil source comparison analysis show that the E2 e 2 crude oil comes from E2 w 2 source rocks.
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