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In order to accelerate the exploration and development of shale gas, this paper elaborates on the micro mechanism of shale gas preservation based on shale self-sealing. The self-sealing capacity of shale formations is mainly related to the low connectivity of nano-throats, low speed diffusion caused by bound water occurrence, capillary force sealing, and breakthrough pressure along the layer direction under burial conditions. Results of analysis based on shale pore morphology and connectivity show that the organic pores in shale are mainly composed of nano-throats with poor connectivity and significant retention effects. At the same time, the stacking of multiple relatively dense sealing layers within the top, bottom, and shale layers is conducive to self-sealing in longitudinal direction. Based on experimental and molecular dynamics simulation analysis of the influence of bound water on the diffusion and breakthrough pressure of the shale matrix, it is revealed that the bound water occurrence significantly reduces the effective diffusion capacity of shale matrix pores and could cause high capillary forces, leading to effective sealing of the gas stored in organic pores. In the study, the permeability-breakthrough pressure evolution relationship was constructed, revealing that under deep burial conditions, the effective closure of shale bedding fractures along the layer direction can form high breakthrough pressure sealing. During the uplift stage, under relatively weak tectonic compression, the shale could still maintain a high sealing capacity along the layer direction, which is conducive to shale gas preservation. However, under strong tectonic compression, the shale bedding fractures open and communicate with open fault surface, causing the failure of preservation capacity and large-scale loss of shale gas. This study elucidates the microscopic mechanism of shale gas preservation through experiments and molecular dynamics simulations, which could provide useful guidance for marine shale gas exploration in complex structural areas.

In January 2023, Well Bazhong 1HF in the northern Sichuan Basin obtained high-yield industrial oil flow of over 100 cubic meters from the Jurassic channel sandstone for the first time, realizing a major breakthrough. In order to provide more support for further oil and gas exploration in this area, this paper analyzes the sedimentary and reservoir characteristics of the Jurassic Lianggaoshan Formation in Bazhong region of northern Sichuan Basin and their control factors based on the exploration achievements of Well Bazhong 1HF. Then, oil and gas reservoir characteristics and oil and gas sources are comparatively analyzed. Finally, the key technologies for the exploration of channel sandstone oil and gas with multi-stage vertical superimposition, lateral migration, thin reservoir and strong heterogeneity are researched and developed, and the next oil and gas exploration direction in the Lianggaoshan Formation of northern Sichuan Basin is pointed out. And the following research results are obtained. First, in the second member of Lianggaoshan Formation (“Liang 2 Member” for short) in Bazhong region, multi-stage underwater distributary sand bodies of delta front are developed with sandstone thickness of about 25 m and average porosity of 5.6%. The pore types are mainly primary intergranular pores and feldspar/debris intragranular dissolved pores, the pore throat structure is good, and the development of good-quality reservoirs is controlled by the sedimentary microfacies of underwater distributary channel. Second, the oil and gas reservoir of Liang 2 Member is a highly oil-bearing condensate gas reservoir/volatile oil reservoir, whose oil and gas is mainly sourced from the semi-deep lacustrine shale of Liang 1 and 2 Members. Third, channel sand bodies are superimposed and developed continuously in the upper part of Liang 2 and Liang 3 Members, and the source-reservoir configuration is good, with the characteristics of near-source hydrocarbon accumulation and overpressure hydrocarbon enrichment. Fourth, for the channel sandstones with multi-stage vertical superimposition, lateral migration, thin reservoir and strong heterogeneity, the reservoir prediction technology of “high-frequency sequence stratigraphy slice, seismic frequency decomposition and facies-constrained seismic waveform indication inversion” is developed to precisely characterize the “sweet spot” target of narrow channel sandstone, and the key fracturing technology of “dense cutting + composite temporary plugging + high-intensity proppant injection + imbibition and oil-increasing” is formed to realize the large-scale reconstruction of channel sandstone reservoir. In conclusion, the breakthrough of Well Bazhong 1HF in the exploration of Lianggaoshan Formation oil and gas in Bazhong region reveals the huge potential of Jurassic oil and gas exploration in the Sichuan Basin, and plays a positive role in promoting the exploration and development of the Jurassic channel sandstone oil and gas in the Sichuan Basin.

Affected by reservoir heterogeneity, developed natural fractures, and bedding fractures, the fracturing pressure curves in fracturing of shale gas horizontal wells present complex shapes. A large amount of information contained in the fracturing curves is still not fully excavated. Based on the theory of shale gas fracture network fracturing, the calculation model of bottom hole net pressure is established by integrating the real-time data such as casing pressure, pump rate, and proppant concentration. Net pressure slope and net pressure index are constructed as key parameters, and the net pressure curve is divided dynamically to describe the mechanical conditions corresponding to the fracture propagation behavior during the fracturing process. Six fracture propagation modes were identified, including fracture network propagation, fracture propagation blockage, normal fracture propagation, fracture propagation long bedding, fracture height growth, and rapid fluid filtration, and then the operation pressure curve diagnosis and identification method were formed for shale gas fracture network fracturing in horizontal wells. The shortcomings of conventional operation curve diagnosis and identification methods are abandoned and the fracture network complexity index is presented. The higher index indicates more time of fracture network propagation and fracture propagation along bedding and the better reservoir stimulation effect. The model is applied to shale gas wells in the southeastern margin of Sichuan Basin, and the average fracture network complexity index of a single well is 0.3, which is in good agreement with the microseismic monitoring results. This proves the good reliability of the method developed. The method is helpful to improve the potential and level of fracturing stimulation of shale reservoirs and is of great significance for improving the post-fracturing evaluation technology of fracture network and guiding the real-time dynamic adjustment of field fracturing operations.

When deep and ultra-deep shale gas well fracturing is carried out, multi-cluster fracturing can hardly realize synchronous initiation and propagation of hydraulic fractures due to the combined effects of heterogeneity of deep in-situ stress and ''dense cluster'' fracture arrangement, and the strong interference between fractures aggravates the unbalanced fracture propagation degree. Field practice proves that the fracture-opening temporary plugging fracturing technology can effectively control the unbalanced propagation of multiple fractures. In addition, the application effect of temporary plugging process can be improved by developing a method for simulating fracture control during fracture-opening temporary plugging fracturing of deep/ultra-deep shale-gas horizontal wells. Based on rock mechanics, elasticity mechanics, fluid mechanics and fracture propagation theory, combined with the flow distribution equation of horizontal-well multi-cluster fracturing and the plugging equation of temporary plugging balls, this paper establishes a fracture propagation model and a fracture control simulation method for the fracture-opening temporary plugging fracturing of deep/ultra-deep shale gas horizontal wells. Then, the influences of the number of temporary plugging balls and the times and timing of temporary plugging on temporary plugging control are simulated, and the influences of temporary plugging balls on fracture propagation morphology and SRV (stimulated reservoir volume) distribution are analyzed by taking Sinopec's one deep shale gas well in Dingshan-Dongxi structure of southeast Sichuan Basin as an example. And the following research results are obtained. First, fracture-opening temporary plugging can significantly promote the balanced propagation of multiple fractures, and the simulation confirms that the number of temporary plugging balls and the times and timing of temporary plugging play an important role in fracture control. Second, as the number of temporary plugging balls increase, the SRV increases firstly and then decreases, so there is an optimal number of temporary plugging balls. Third, increasing the times of temporary plugging can improve the fault tolerance rate of temporary plugging and diverting process, but it is necessary to increase the number of temporary plugging balls appropriately. Fourth, when the timing of temporary plugging is appropriate, the balanced propagation of multiple fractures is achieved and the maximum SRV is reached. In conclusion, this method is of great significance to optimizing the design of temporary plugging fracturing, improve the implementation level of field process and develop deep and ultra-deep shale gas efficiently.

The first fractured shale gas well of China was constructed in 2010. After 10 years of development, China has become the second country that possesses the core technology of shale gas development around the world, realized the shale gas fracturing techniques from zero to one and from lagging to partially leading, and constructed the fracturing theory and technology system suitable for middle–shallow marine shale gas exploitation. In order to provide beneficial guidance for the efficient exploitation of shale gas in China in the future, this paper comprehensively reviews development history and status of domestic fundamental theories, optimized design methods, fluid systems, tools and technologies of shale gas fracturing and summarizes the research results in fundamental theories and optimized design methods, such as fracturing sweet-spot cognition, fracture network propagation simulation and control, rock hydration and flowback control, and SRV (stimulated reservoir volume) evaluation and characterization. The development and application of slick-water fracturing fluid system and new fracturing fluid with little or no water is discussed. The development and independent research & development level of multi-stage fracturing tools are evaluated, including drillable composite plug, soluble plug, large-diameter plug and casing cementing sleeve. The implementation situations of field technologies and processes are illustrated, including the early conventional multi-stage multi-cluster fracturing and the current “dense cluster” fracturing and temporary plugging fracturing. Based on this, the current challenges to domestic shale gas fracturing technologies are analyzed systematically, and the development direction of related technologies is forecast. In conclusion, it is necessary for China to continuously research the fracturing theories, technologies and methods suitable for domestic deep and ultra-deep marine shale gas, terrestrial shale gas and transitional shale gas to facilitate the future efficient development of shale gas in China.

Well Taiye 1 in the Fuling area of the eastern Sichuan Basin has obtained a high-yield industrial gas flow (7.5 × 104 m3/d gas and 9.8 m3/d oil) from the Middle Jurassic Lianggaoshan Formation, presenting a good test production effects, which means the realization of a major breakthrough in the exploration of Jurassic lacustrine shale oil and gas in the Sichuan Basin. In order to further determine the exploration potential of lacustrine shale oil and gas in this area and realize the large-scale efficient development and utilization of lacustrine shale oil and gas, this paper analyzes the geological conditions for the accumulation of lacustrine shale oil and gas in this area by using the drilling data of 10 key wells, such as wells Taiye 1 and Fuye 10. Then, the main factors controlling the enrichment of lacustrine shale oil and gas are discussed, and the exploration potential and favorable target zones of Jurassic lacustrine shale oil and gas in the Fuling area are defined. And the following research results are obtained. First, the quality Jurassic semi-deep lake shale in the Fuling area is characterized by high organic matter abundance, high porosity and high gas content, and it is the geological base of shale oil and gas enrichment. Second, the developed large wide and gentle syncline, good preservation condition and higher pressure coefficient (generally >1.2) are the key to the enrichment and high yield of shale oil and gas. Third, the developed microfractures in lacustrine shale are conducive to the enrichment and later fracturing of shale oil and gas. In conclusion, the Lianggaoshan Formation lacustrine shale in the Fuling area is widely distributed with moderate burial depth, developed microfractures and moderate thermal evolution, and its shale gas resource extent and shale oil resource extent are 1 922 × 108 m3 and 2800 × 104 t, respectively, indicating greater potential of shale oil and gas exploration, so shale oil and gas is the important field of oil and gas reserves and production increase in this area in the following stage.

The Jurassic Dongyuemiao Member is the most promising target for lacustrine shale gas exploration in Sichuan Basin. By integrating SEM, NMR, LTNA, and MICP experiments, and other basic measurements, the nanoscale pore category and structure and the corresponding controlling factors of Dongyuemiao lacustrine shale in Eastern Sichuan Basin are studied. The results denote that organic pores comprise primary pores within plant debris and secondary pores within bitumen. Inorganic pores are composed of intraparticle pores within calcite particles, intercrystalline pores between pyrite crystals, and interparticle pores between different minerals. The 4th Section lacustrine shale of Dongyuemiao Member has the best pore structure, exhibiting high organic pore proportion, large amounts of gas adsorption, and parallel plate-shaped pore morphology. Micropores (2 nm). Quartz does not significantly affect the nanoscale pore formation. The intraparticle pores within calcite particles constitute part of mesopore–macropore but not micropores. Clay minerals are conducive to the formation of micropores but play a negative role in the formation of mesopore–macropore.

Organic-rich continental shale, widespread in the Sichuan Basin during the deposition of the Jurassic Dongyuemiao Member (J1d), is considered the next shale hydrocarbon exploration target in southern China. To identify a shale gas sweetspot and reduce exploration risk, it is of great significance to determine the organic matter (OM) enrichment mechanism of J1d shale. In this study, based on sedimentological characteristics and organic matter content, high-resolution major and trace elements were systematically analyzed to demonstrate terrigenous influx, paleoredox, paleosalinity, paleoproductivity, and paleoclimate. The 1st section interval of the J1d 1st submember is dominated by shallow lake subfacies, while the other intervals have the characteristic of semideep to deep lake subfacies. The 1st submember interval of J1d lacustrine shale is characterized by the warmest-humid paleoclimate, strongest weathering degree, highest terrigenous input, moderate paleoproductivity, and paleoredox condition. Within the Dongyuemiao 1st submember, the 4th section interval has the highest paleoproductivity and the most oxygen-deficient condition in bottom water. During the deposition period of the 2nd submember, the sedimentary environment turned to a cold-dry paleoclimate, weak weathering degree, low terrigenous input, low paleosalinity, and high paleoproductivity. Under the background of semideep and deep lake, the terrigenous OM input plays the most critical role in controlling OM enrichment. Moreover, the high primary productivity of lake surface water and the suboxic condition of lake bottom water contribute to the formation of relatively higher TOC lacustrine shale interval in the 4th section of 1st submember.

A basic research program on deep hydrocarbon resource accumulation mechanisms and key exploitation technologies was initiated by the National Natural Science Foundation of China at the beginning of 2020, to meet the theoretical and technological demands of deep hydrocarbon exploration and development in the Tarim, Sichuan and Ordos basins. This paper provided a review of the recent program progresses on (1) the reconstruction of prototypes and the transformation histories of three ancient cratons in China; (2) the petroleum accumulation and flow mechanisms in deep marine carbonate reservoirs and deep shale gas systems; (3) the geophysical theories and methods for imaging the deep structures and predicting the deep petroleum reservoirs; and (4) the new protocols and downhole tools for deep well drilling and completion. Brief comments were also made on the preliminary application of the recent program results in facilitating several breakthroughs in the deep petroleum exploration of the Tarim and Sichuan basins.

The Upper Permian Linghao Formation marine shale and contemporaneous transitional shale are the most potential shale gas targets in the Nanpanjiang basin, which is characterized by considerable TOC content, wide distribution, and considerable shale thickness. On the basis of division in Linghao Formation, petrographic, mineralogical, and high-resolution geochemical analyses were integrated to reveal the sedimentary environment including paleoproductivity, paleoredox conditions, detrital influx, paleoclimate, and the paleosalinity. There are two organic-rich shale intervals in Linghao Formation, which are Ling 1 member and the lower Ling 3 member. The lower Ling 1 is dominated by deep-water shelf facies, which are characterized by high TOC value (0.93%–6.36%, avg. 2.43%), high detrital influx proxies (Zr, 746–1508 ppm, avg. 1093 ppm; Ti, 19278–128730 ppm, avg. 16091 ppm), relatively warm–humid paleoclimate condition (CIA*, 75.94-91.90, avg. 82.26), low paleosalinity proxies (Sr/Ba, 0.13-0.34, avg. 0.22), and high paleoproductivity (P/Al (10−2), 1.06-2.06, avg. 1.63; Mn/Ca (10−3), 27.37-291.69, avg. 128.07). Detrital influx including gravity flow plays a critical role in the enrichment of organic matter. The sedimentary environment of upper Ling 1 and lower Ling 3 is the same as that of lower Ling 1. Unlike lower Ling 1, these intervals are characterized by low detrital influx proxies, moderate weathering, and relatively high paleosalinity proxies. The volcanic ash of Emei volcanism and felsic volcanism in South China plays a critical role in the enrichment of organic matter in upper Ling 1 and lower Ling 3, respectively. The sedimentary models for Linghao Formation organic-rich shale can reveal factors controlling the enrichment of organic matter.

Carbonate rocks in the first member of the Middle Permian Maokou Formation in the Sichuan Basin (hereinafter “Mao 1 Member” for short) have been taken as a set of carbonate source rocks, and they have not been specifically studied from the aspects of reservoir evaluation and testing. By referring the exploration ideas of unconventional natural gas, the Mao 1 Member in Fuling area of southeastern Sichuan Basin has obtained industrial gas flow in development wells in recent years. In order to further clarify the natural gas exploration potential of the Mao 1 Member in this area, it is necessary to study its sedimentary characteristics, natural gas accumulation conditions and main control factors based on the data of field section measurement, systematic coring in drilling and laboratory testing. The following research results were obtained. First, the gas reservoir in the Mao 1 Member in the Fuling area is of source–reservoir integration, and its natural gas is mainly enriched in blackish gray marlite and nodular marlite. Second, its reservoir spaces are dominated by grain margin pores (fractures), diagenetic shrinkage pores (fractures), organic pores and fractures. Third, the pores are mostly in nanometer scale, and the main pore diameter is in the range of 5–50 nm, which is between shale reservoir and conventional reservoir, with strong heterogeneity. Fourth, the gas reservoir is characterized by source–reservoir coexistence, lithology controlling reservoir and extensive layered distribution, presenting two-stage differential hydrocarbon enrichment, namely intraformational near-source enrichment in the early stage and interformational blowdown adjustment in the late stage. Fifth, the development of blackish gray organic-rich fine marlite which is deposited with the episodic upwelling in the outer ramp facies belt is the foundation for the natural gas accumulation, the transformation of clay minerals controls the development of higher-quality reservoirs, good preservation conditions are the key to the natural gas accumulation, and fracture development is favorable for the enrichment and high yield of natural gas. In conclusion, the Mao 1 Member in this area is a special type of gas reservoir, i.e., carbonate source rock gas reservoir, which has greater potential of natural gas exploration and industrial gas flow have been obtained in several wells. The discovery of such type of gas reservoirs not only expands the domain of natural gas exploration in the Sichuan Basin, but provides the reference for the natural gas exploration in other areas of South China.

In recent years, major breakthroughs have been made in natural gas exploration in deep and ultra-deep strata in the Sichuan Basin, but the overall successful rate is low. To further clarify the prospects there, it is necessary to make an in-depth analysis of the previously discovered large-scale reef-shoal gas fields such as Puguang, Yuanba, Anyue and Longgang and deep shale gas discovery in Dingshan and Dongxi, southern Sichuan Basin. On one hand, large high-energy facies are the basis for controlling the development of large-scale reef-shoal reservoirs in conventional reef-shoal areas. The reservoir original porosity is high. The atmospheric freshwater dissolution in the early diagenetic stage, dolomitization, unconformity karst, and “pore–fracture coupling” mainly control the development of secondary pores. The contribution of hydrothermal fluids to reservoir is double-sided, and such early pores can be preserved till present due to those retention processes such as early hydrocarbon charging. Apart from continuous preservation as the key factor, most gas reservoirs are featured by “near-source enrichment, phase transformation, and dynamic adjustment”. On the other hand, deep shale gas generally has the characteristics of “high pressure, high porosity, and high gas content”, that is, “overpressure and rich gas”. The key to the development of high-quality deep shales with high pores are “quartz compression retaining pore'' and “reservoir fluid overpressure”. The weak tectonic effect in the late stage is the main reason for deep shale gas to maintain the “high pressure and high gas content”. In conclusion, technological advances like geological target identification and “sweet spot” prediction, as well as deep, high-temperature and high-pressure engineering processes, are the guarantee for efficient exploration of conventional and unconventional deep and ultra-deep natural gas, which has great potential in the Sichuan Basin.

Marine strata in South China are mainly distributed in the Paleozoic and Mesozoic. The strata are old and the thermal evolution degree is high. It was controlled by the multi-cycle sedimentary and multi-stage structural evolution history. The petroleum basic geological conditions are superior, but the distribution of oil and gas is complicated. Oil and gas exploration has mainly gone through three stages: general survey, structural reservoir, massive lithologic gas reservoir and shale gas exploration. Although major breakthroughs have been made in many fields, the overall detection rate is low. A systematic summary of the early discovery of large gas fields and the new progress of exploration was made in order to clarify the prospects for oil and gas exploration. There is a large potential for marine petroleum exploration in South China. Deep and ultra-deep strata as well as new areas are important breakthrough directions in the Sichuan Basin. The unconventional shale gas reservoirs in the Upper Ordovician Wufeng-Lower Silurian Longmaxi formations and the conventional gas reservoirs in the platform margin zone of the Upper Sinian Dengying Formation in Langzhong-Yuanba, the platform inner beach facies in the Sinian-Cambrian in Tongnanba, the pre-salt strata in the lower assemblage in Qijiang in the southern Sichuan and the reef beach facies in the Permian-Triassic have a total resource amount to trillions of cubic meters, which is expected to achieve a new round of breakthroughs and discoveries. Besides, unconventional shale gas in the Permian and Jurassic and some new targets for conventional gas such as hydrothermal dolomites, karst fissure group, stucco limestones and sedimentary tuffs in the Permian show a great exploration potential. New discoveries continue to be made in complex basin-margin regions in the Sichuan Basin. Shale gas in the residual syncline area and conventional piedmont areas in the basin margin are the main exploration directions. The periphery of ancient uplifts and the progressive deformation area in the southern periphery have weak structural deformation, and the preservation conditions are favorable with a certain exploration potential, and are expected to become a strategic succession area for oil and gas.

The Upper Permian Linghao Formation shale is the most potential shale gas exploration target in Nanpanjiang Basin. In this study, X-ray diffraction, field emission scanning electron microscopy, CH4 isothermal adsorption, and nuclear magnetic resonance cryoporometry are intergrated to reveal comprehensive characterization of Linghao Formation shale collected from a well in Nanpanjiang Basin. Results indicate that organic-rich shales developed in the Ling 1 member and the lower part of Ling 3 member. The organic-rich shales are predominantly characterized by kerogen type I, with a relatively highly mature to overmature status. The Ling 1 organic-rich shale mainly consists of mixed shale lithofacies, and the organic-rich shale in the lower part of Ling 3 is mainly composed of argillaceous shale. The pore volume in Ling 1 organic-rich shale is mainly contributed by 3- to 6-nm and 8- to 11-nm organic pores. The pore volume of Ling 3 organic-rich shale is mainly contributed by 2- to 3-nm and 4- to 11-nm organic pores. The organic pores between 3 and 10 nm also have a small contribution to the pore volume. The absolute adsorption gas content of Ling 1 and Ling 3 organic-rich shale is 1.21 m3/t and 1.64 m3/t, respectively. The absolute adsorption gas content of Ling 1 and Ling 3 organic-rich shale exceeds the minimum standard for commercial shale gas development in China (1.0 m3/t). According to the adsorption gas ratio of 50%, the total gas content of Ling 1 and Ling 3 organic-rich shale can reach 3.28 m3/t and 2.28 m3/t, respectively. It is suggested that the Upper Permian Linghao Formation shale in the Nanpanjiang Basin has a significant potential for shale gas exploration.

Oil/gas exploration around the world has extended into deep and ultra-deep strata because it is increasingly difficult to find new large-scale oil/gas reservoirs in shallow–middle buried strata. In recent years, China has made remarkable achievements in oil/gas exploration in ultra-deep areas including carbonate and clastic reservoirs. Some (ultra) large-scale oil and gas fields have been discovered. The oil/gas accumulation mechanisms and key technologies of oil/gas reservoir exploration and development are summarized in this study in order to share China’s experiences. Ultra-deep oil/gas originates from numerous sources of hydrocarbons and multiphase charging. Liquid hydrocarbons can form in ultra-deep layers due to low geothermal gradients or overpressures, and the natural gas composition in ultra-deep areas is complicated by the reactions between deep hydrocarbons, water, and rock or by the addition of mantle- or crust-sourced gases. These oils/gases are mainly stored in the original high-energy reef/shoal complexes or in sand body sediments. They usually have high original porosity. Secondary pores are often developed by dissolution, dolomitization, and fracturing in the late stage. The early pores have been preserved by retentive diageneses such as the early charging of hydrocarbons. Oil/gas accumulation in ultra-deep areas generally has the characteristics of near-source accumulation and sustained preservation. The effective exploration and development of ultra-deep oil/gas reservoirs depend on the support of key technologies. Use of the latest technologies such as seismic signal acquisition and processing, low porosity and permeability zone prediction, and gas–water identification has enabled the discovery of ultra-deep oil/gas resources. In addition, advanced technologies for drilling, completion, and oil/gas testing have ensured the effective development of these fields. Keywords: Oil/gas exploration, Ultra-deep, Oil/gas sources, Reservoir, Petroleum accumulation, Exploration and exploitation technologies

The Carboniferous shale in the Guizhong Depression of central Guangxi presents good exploration potential of shale gas, but its exploration effectiveness is restricted by the unclear distribution, quality and potential of high-quality shale. In this paper, the Lower Carboniferous Luzhai shale was taken as the research object in order to guide the exploration of Carboniferous shale gas in the Guizhong Depression. Based on the field outcrops, well drilling, well logging and test data of the Guizhong Depression and its surrounding areas, the shale formation was studied systematically based on the characteristics of distribution, organic geochemistry and reservoir, and the preservation conditions. Then, the development characteristics of black shale were confirmed and the distribution laws of high-quality shale were summarized. Finally, shale gas exploration was analyzed and the favorable exploration areas were defined. And the following research results were obtained. First, the Luzhai shale in the Guizhong Depression is mainly carbonaceous shale and siliceous shale of deepwater continental shelf facies. Second, the high-quality Luzhai shale is 20–60 m thick and it is characterized by high total organic carbon, good type of organic matters, moderate maturity of organic matter, high content of brittle minerals, good reservoir property and developed pores of organic matters. Third, regional caprocks and roofs & floors are well developed in the Liucheng Slope and the Yishan Sag. They are weakly deformed, suffer from less uplift and denudation, and are far away from the strong strike–slip reverse fault, so they have good preservation conditions. Fourth, shallow wells which were drilled in the surface structures of the Guizhong Depression in the early stage present abundant gas shows, and most of them produce gas flow, which reveals a promising prospect of shale gas exploration in this area. In conclusion, the wide and gentle synclines with large burial depth in the Liucheng Slope and the Yishan Sag are the favorable areas for shale gas exploration in the Guizhong Depression. Keywords: Guizhong depression, Luzhai Fm, Lower Carboniferous, Shale, Thickness, Organic geochemical characteristic, Preservation conditions, Shale gas exploration potential, Favorable exploration area

Marine–continent transitional facies shale of the Longtan Fm, Upper Permian is an important source rock stratum in the Sichuan Basin. The previous researches on it mainly focus more on source rock evaluation, but less on shale gas accumulation. In this paper, the test and analysis results of Well DYS1, the coring well of Longtan Fm were dissected. Combined with the drilling results of its adjacent area, the organic rich shale in the Longtan Fm were studied from the aspects of distribution, geochemical, reservoir, gas-bearing characteristics and roof and floor conditions, and then compared with the shale strata with similar sedimentary background and good shale gas shows. And accordingly, its geological conditions for the formation of Longtan shale gas and its exploration potential were made clear. And the following research results were obtained. First, in the southeastern Sichuan Basin, the organic rich shale of transitional facies of Longtan Fm is developed with a thickness of more than 40 m. It is characterized by high brittle mineral content, high porosity, high total organic carbon (TOC), moderate thermal evolution (Ro) and good gas-bearing property. Therefore, it is geologically favorable for the formation of shale gas. Second, compared with the existing transitional facies shales at home and abroad, the Longtan shale is better in terms of porosity, TOC, Ro, gas content and other key parameters. Third, the type of organic matters is the main reason for the low development degree of organic pores in the Longtan shale. And during its shale gas exploration and selection, the coal seam enriched sections shall be avoided. In conclusion, considering the development degree, interlayer thickness, depth and preservation conditions of organic rich shale comprehensively, the Qijiang–Chishui area is the best exploration area for the transitional facies shale gas of Longtan Fm in the Sichuan Basin. Keywords: Sichuan basin, Late Permian, Longtan Fm, Marine–continental transitional facies, Shale, Total organic carbon, Thermal evolution maturity, Gas content, Shale gas exploration potential

Tectonic fractures are a significant factor for production of the Longmaxi formation in Dingshan area. The analyse of the distribution and development degree of tectonic fractures is the main research method in present-day. Based on the data of field outcrops and borehole core, the new method-binary method is put forward by rock fracture and energy value to decide tectonic fracture development. According to the method the fitting formula is offered between the relation of the value of rock fracture, energy and the fracture density. Compared the fracture density predictive value and the observed value of 15 wells, all wells relative error less than 25%. The prediction results are accurate. According to the fitting formula of fracture development and distribution in the study area are obtained. The results show that the fracture density is high as a whole of the Longmaxi formation in Dingshan area. Whole shows by south east to gradually reduce the trend of the north west. Supported the future of the oil and gas exploration to provide geological basis and guidance in Dingshan area.

In addition to marine and marine-continental transitional strata, the continental ones are also widely distributed in various oil and gas-bearing basins in China. The continental shale generally provides favorable material bases for hydrocarbon generation, such as wide distribution, large thickness, multiple series of strata, high TOC content, nice organic matter type, and moderate thermal evolution. Part of such shale contains shale oil, but the pore space characteristics for the occurrence of this oil are not thoroughly studied. In order to accurately and quantitatively characterize the pore space where the continental shale oil in different types of lithofacies occurs, we sampled the rock cores from the Middle Jurassic Lianggaoshan Formation in the southeastern Sichuan Basin of the Upper Yangtze Area. The TOC content and mineral composition were analyzed, and we also carried out experiments on CO2 and N2 adsorptions, high-pressure mercury injection, and wash oil. Results show significant differences in pore space characteristics for the occurrence of shale oil in different types of lithofacies. In organic-rich mixed and clayey mudstones with the highest TOC content, the free shale oil, occupying the largest reservoir space, mainly occurs in macropores and mesopores, and the adsorbed shale oil, occupying the largest reservoir space, mainly occurs in mesopores. In the organic-bearing clayey mudstone, which has a higher TOC content, the free shale oil takes a larger reservoir space and mainly occurs in macropores, followed by mesopores, and the absorbed one, occupying a larger reservoir space, mostly occurs in micropores and then the mesopores. The organic-bearing mixed mudstone has a moderate TOC content, in which the free shale oil occupies a smaller reservoir space and primarily occurs in mesopores, followed by macropores, and the absorbed one, which takes a larger reservoir space, all occurs in mesopores. In the fine sandstone, the free shale oil occupies a smaller reservoir space and primarily occurs in mesopores, while the absorbed one occupies a smaller reservoir space and all occurs in mesopores.

Recently, deep and ultra-deep shales (depth >3500 m) of the Lower Paleozoic Wufeng–Longmaxi formations (WF–LMX) have become attractive targets for shale gas exploration and development in China, and their gas contents may be influenced by the occurrence of water to some extent. However, the water content and its distribution in the different nanopores of the deep and ultra-deep shales have rarely been reported. In this study, a suite of the WF–LMX ultra-deep shale samples (5910–5965 m depth) from the Well PS1 was collected for water content measurements, and low-pressure CO2 and N2 adsorption experiments of both as-received and experimentally dried shale samples were carried out to investigate the distribution of water in the different nanopores. Since the studied ultra-deep shales are characterized by higher thermal maturity (equivalent vitrinite reflectance (EqVRo) > 2.5 %) and ultra-low water saturation, the pore water is generally dominated by irreducible water. The content of irreducible water of the studied shales varies from 1.57 to 13.66 mg/g, averaging 6.74 mg/g. Irreducible water may mainly occur in the clay-hosted pores, while it could also be hosted in parts of organic pores of organic-rich shales. Irreducible water is primarily distributed in non-micropores rather than in micropores of the studied shales, which mainly occurs in micopores with a diameter of 0.4–0.6 nm and mesopores with a diameter of 2–10 nm. Very low contents of irreducible water could reduce the specific surface area and volume of non-micropores of the shales to some extent, but the effect of irreducible water on the specific surface area of non-micropores was more significant than the volume of non-micropores, especially for organic-rich shale samples. The ultra-deep shale gas may be predominately composed of free gas, so low contents of irreducible water may play a limited role in its total gas contents. Overall, our findings can be helpful for a better understanding of water distribution in the highly-matured shales, and provide a scientific basis for ultra-deep shale gas exploration.

The reservoir structural fractures have excellent fractal characteristics, as well as self-similarities. Based on the fractal theory, the surface fractal characteristics of faults and the fractal characteristic of fractures in the core of the Xujiahe Formation in the Fault-Fold Belt of the central Yuanba area were studied, and a quantitative relationship was set up between them. Based on the fractal characteristics of faults, predictions were made of the favorable fracture zone, which provides a new idea for the research of fracture, as well as offers theoretical references for exploring the fracture development zone during oil-gas exploration. The results show the following: the seismic value of reflection fault fractal dimension of the Xujiahe Formation is 1.5284; the correlation coefficient R2 is bigger than 0.9901; the capacity dimension linear regression correlation coefficient of the fracture in core of the Xujiahe Formation is bigger than 0.98; the fractal dimension D can well reflect the fault and fracture development degree, as well as the complexity of the fracture system; it can quantitatively calculate the density of the fracture of the reservoir in the area; the areas of capacity dimension bigger than 1.45 are the fracture development zones in the Fault-Fold Belt of the central Yuanba area; the oil and gas enrichment degree is high; the areas with the fractal dimension value between 0.95 and 1.45 are the fracture relatively-developed zones; the fractal dimension with values smaller than 0.95 are the lack of fracture areas.

To solve the difficulties in exploration and development in Yuanba ultra-deep gas field in Sichuan Basin, SW China, the article studies the mechanism of quality reef reservoirs development and gas accumulation and innovates techniques in ultra-deep seismic exploration, drilling, completion and testing. Through the reconstruction of dynamic depositional evolution process and regional depositional framework of homoclinal ramp-rimmed platform in Upper Permian, three theories are put forward: first, “early beach-late reef, multiple stacking, arrangement in rows and belts” is the sedimentary mode for the reservoirs in the Changxing Formation of Yuanba area; second, “dissolution in early exposure stage and dolomitization during shallow burial giving rise to the pores in matrix, overpressure caused by cracking of liquid hydrocarbon during deep burial inducing fractures” is the reservoirs development mechanisms; third, “coupling of pores and fractures” controls the development of high quality reservoirs in deep formations. From correlation of oil and source rock, it is concluded that the Wujiaping Formation and Dalong Formation of deep-water continental shelf are the major source rocks in the Permian of northern Sichuan Basin. The hydrocarbon accumulation mode in ultra-deep formations of low-deformation zones is characterized by “three-micro (micro-fault, micro-fracture interbed crack) migration, near-source enrichment, and persistent preservation”. Through seismic inversion using the pore structure parameters of pore-fracture diadactic structure model, the high production gas enrichment area in Yuanba gas field is 98.5 km2. Moreover, special well structure and unconventional well structure were used to deal with multiple pressure systems and sealing of complex formations. A kind of integral, high pressure resistant FF-level gas wellhead and ground safety linkage device was developed to accomplish safe and environmentally friendly gas production. Key words: Sichuan Basin, Yuanba gas field, ultra-deep formation, Changxing Formation, reef, seismic exploration, testing technology

Based on the understandings on enrichment rules of marine shale gas in southern China and data obtained from exploration and development in Fuling shale gas field, this article discusses the key controlling factors on shale gas enrichment and their relationships, it also discusses further the theory of Two-Factor Enrichment of marine shale gas in southern China. The bases for shale gas enrichment are shale gas generation and accumulation, the shale gas reservoirs of deep-water shelf are characterized by high TOC, high porosity, high gas contents and high siliceous contents, with high hydrocarbon-generation intensity. The organic pores are rich in shales in favorable for stimulation, so they are the bases for large scale hydrocarbon accumulation. Preservation conditions are vital to the formation and enrichment of shale gas reservoirs, good top and base layers can effectively prevent hydrocarbon from escaping vertically at the beginning of hydrocarbon generation. Shale gas preservation conditions depend on the intensity and duration of tectonic movements, good preservation conditions are key geological factors for shale gas accumulation, shale reservoirs have high gas contents, high porosity and high pressure and are likely to form high yield area of shale gas. Key words: Sichuan Basin, Fuling shale gas field, Lower Silurian, Longmaxi Formation, shale gas, enrichment and high production, controlling factors

The relationship between reservoir tightening time and gas charge period are the key subjects that have not been well solved considering the studies on the tight sand gas accumulation mechanism and enrichment regularity. The diagenetic evolution history, interaction sequence of organic–inorganic in aquiferous rock, gas charge history, the tightening mechanism of tight sandstone reservoir and the relationship between reservoir tightening time and gas accumulation period of the Xujiahe Formation have been analyzed in the Yuanba–Tongnanba area of the Sichuan Basin. It has been confirmed that the main reason for the tight sandstone reservoir formation is the intensive mechanical compaction which has dramatically reduced the sandstone reservoir quality, and it resulted to a semi-closed to a closed diagenetic fluid system formation at the early diagenetic stage. In the semi-closed to a closed diagenetic fluid system, at the later part of the diagenetic stage, the fluid circulation is not smooth, and the migration of the dissolution products are blocked, hence, the dissolution products mainly undergo the in situ precipitation and cementation. Such dissolution products block the dissolution pores and the residual primary pores; and the stronger the dissolution is, the higher the cement content is, which makes the reservoir further tightened. The hydrocarbon generation and expulsion history of source rocks and reservoir fluid inclusion characteristics in the Xujiahe Formation show that the charge of natural gas occurs in the Middle Jurassic–Early Cretaceous (mainly Early Cretaceous). A comprehensive analysis of the reservoir tightening history, gas charge history, and interaction sequence of organic–inorganic aquiferous in rock indicate that the sandstone reservoir experienced a tightening process when gas charging took place in the Xujiahe Formation in the Yuanba–Tongnanba area of the Sichuan Basin.

The lithofacies types of Upper Ordovician Wufeng Formation-Lower Silurian Longmaxi Formation shale, the main producing layer in Fuling gas field, are classified in detail using the modified ternary diagram of siliceous minerals-carbonates minerals-clay minerals. There develop eight lithofacies in the Wufeng-Longmaxi shale: siliceous shale lithofacies (S), mixed siliceous shale lithofacies (S-2), clay-rich siliceous shale lithofacies (S-3), calcareous/siliceous mixed shale lithofacies (M-1), argillaceous/siliceous mixed shale lithofacies (M-2), mixed shale lithofacies (M), silica-rich argillaceous shale lithofacies (CM-1), and argillaceous/calcareous mixed shale lithofacies (M-3). The advantageous shale lithofacies is defined as lithofacies with gas content reaching a specific industrial standard. Based on the current development status of the study area, advantageous shale lithofacies is divided into two classes, namely, Class I with gas content of more than 4.0 m3/t (also known as extra superior), Class II with gas content of 2.0−4.0 m3/t (also known as superior). The correlation between the abundance of organic matter, the content of siliceous mineral, clay content and gas content has been analyzed to establish the classification criteria for advantageous shale lithofacies in the Wufeng-Longmaxi shale. The mixed siliceous shale lithofacies (S-2) and clay-rich siliceous shale lithofacies (S-3) have been identified as Class I advantageous shale lithofacies, and argillaceous/siliceous mixed shale lithofacies (M-2) as Class II. The classification criteria of advantageous shale lithofacies can provide reference for shale gas evaluation in other exploration areas Key words: shale, advantageous shale lithofacies, Ordovician Wufeng Formation, Silurian Longmaxi Formation, Fuling gas field, Sichuan Basin

Complex tectonic movements and high thermal maturity of marine shale dominate South China, where preservation conditions are critical for shale gas enrichment and productivity. Based on the exploration practices of the Silurian shale gas at the southeastern margin of the Sichuan Basin in recent years, conventional gas and shale gas were compared in terms of their preservation conditions. The results revealed that superior roof and floor conditions are indispensable to shale gas preservation. Moreover, the self-sealing ability and the huge gap of up to 2–8 times between vertical and lateral permeability of shale gas reservoirs determine the lateral diffusion as the basic pattern of shale gas migration. The unconformity at the bottom of Lower Cambrian leads to worse preservation conditions in the system, and cutting by faults may accelerate the diffusion of shale gas. Major controlling factors for shale gas preservation and their criteria of discrimination were also investigated. It is suggested that: (1) the strength of tectonic modification is the major factor controlling shale gas preservation. Broad and gentle structures with continuous seals and an anticlinal setting are more favorable for the enrichment of shale gas, and a closed evolutionary environment with late uplifting is more favorable for the preservation of shale gas; (2) shale gas can be preserved well in downdip areas without faults or effectively closed or shielded by faults and areas far away from outcrops or zones with stratigraphic hiatus; (3) pressure coefficient is a comprehensive indicator for discriminating preservation conditions. In the study area, the pressure coefficient is in positive correlation with shale gas production and the high or super-high pressure of reservoir is a signal of good preservation condition for shale gas; and (4) in the areas within the southeastern Sichuan Basin, other than those close to erosion zones or hiatus, the Wufeng Fm. of Upper Ordovician and the Longmaxi Fm. of Lower Silurian present high pressure coefficient (up to 2.25) generally, demonstrating good preservation conditions for shale gas, while the pressure coefficient reduces progressively toward or outside the margin of the basin, corresponding to downgrading preservation conditions.

Abstract Understanding intracellular phase separation is crucial for deciphering transcriptional control, cell fate transitions, and disease mechanisms. However, the key residues, which impact phase separation the most for protein phase separation function have remained elusive. We develop PSPHunter, which can precisely predict these key residues based on machine learning scheme. In vivo and in vitro validations demonstrate that truncating just 6 key residues in GATA3 disrupts phase separation, enhancing tumor cell migration and inhibiting growth. Glycine and its motifs are enriched in spacer and key residues, as revealed by our comprehensive analysis. PSPHunter identifies nearly 80% of disease-associated phase-separating proteins, with frequent mutated pathological residues like glycine and proline often residing in these key residues. PSPHunter thus emerges as a crucial tool to uncover key residues, facilitating insights into phase separation mechanisms governing transcriptional control, cell fate transitions, and disease development.