Currently, sweet spot evaluation plays an important role in unconventional oil and gas exploration and development, which is of great significance to the large-scale efficient development of unconventional oil and gas. The concept connotation of sweet spot has been increasingly expanded and more diversified and regional the corresponding evaluation parameters and standard values are more diversified and regionally distinctive. At present, the commonly-used sweet spot evaluation and prediction methods include the contour map semi-quantitative plane superimposition evaluation method, the sweet spot quantitative and semi-quantitative evaluation method based on multi-parameter co-constraint, the radar graphic method, and the sweet spots quantitative evaluation method established based on the geological anomaly theory. In the practical application, the applicability evaluation and prediction method should be developed according to the basic data such as tectonic depositional settings, lithological association and resource type of the basin, and the principle of superposed progressive discrimination. Continental shales in China are highly heterogeneous, and there is a significant difference in the enrichment laws and main controlling factors of different types of shale oil. Both interlayer and hybrid shale oil have experienced migration and accumulation in the source. The main lithology of the reservoir is sandstone (siltstone) and carbonate rock (hybrid sedimentary rock). The reservoir property, hydrocarbon potential and brittleness of reservoirs are the key indicators. The pure shale oil intervals in the thick and ultra-thick layers is generally oil-bearing, and the oil is mainly retained in the source. The source rocks are the reservoirs. It is suggested to use the trichotomy method to divide hydrocarbon enrichment layers into Type I, Type II and Type III using 5~8 key parameters based on the data of sedimentary cycle, laminated texture type, lithological association, hydrocarbon potential, reservoir property, compressibility, mobility and recoverability. During the optimization of enrichment layers, zoning should be planned according to the thermal evolution maturity of different basins and zones. In the medium-low maturity zone, the medium-high TOC content shale interval (felsic laminae develop) adjacent to the high TOC content shale interval should be optimized; in the medium-high maturity zone, the high TOC content shale interval should be optimized. As the "gold target layer", Type I oil reservoir should be developed and produced initially, and progressively exploited according to technological maturity, so as to realize the maximum development and utilization of China's continental shale oil resources, and effectively serve to guarantee the national energy security.

The evaluation of oil and gas resources is to predict the potential of oil and gas resources in the future based on the understanding and judgement of the geological conditions of hydrocarbon accumulation,which is of great significance for oil companies to implement the internationalization strategy.According to the evaluation methods,petroleum geological theories,exploration technologies,and types of discovered traps,the development history of global oil and gas resources evaluation can be divided into four stages:start-up (1900-1957),rapid development (1958-1985),stable development (1986-2007) and participation by China (2008-).The evaluation results of global oil and gas resources are affected by the evaluation methods used at different stages,the evaluation scope of resources,petroleum geological theories,exploration technologies such as geophysics and drilling,as well as oil price,and hydrocarbon production.After comparing and analyzing the evaluation or statistical results of global oil and gas resources of China National Petroleum Corporation (CNPC) in 2020,the United States Geological Survey (USGS) in 2012 and the International Energy Agency (IEA) in 2019,it is considered that the evaluation results of CNPC (2020) is more comprehensive,and the evaluation results of recoverable resources to be discovered are lower than those of IEA (2019).With the development of petroleum geological theories and the progress of science and technology,oil and gas resources will be continuously transformed into petroleum reserves and production in the future.The evaluation methods of global oil and gas resources will gradually focus on the overall evaluation of source rock as the core object,the evaluation results will be presented in a three-dimensional way,the evaluation objects will continuously expand to deep water,deep play and unconventional resources,the evaluation process will highlight the requirements in terms of "economy" and "low carbon" ;big data and artificial intelligence technology will also play an important role in the future evaluation of global oil and gas resources.

As a type of unconventional oil and gas resources, shale oil and gas are self-generated and self-preserved. Divided according to the thermal maturity, medium-high maturity and medium-low maturity shale oil can be obtained; when divided according to the depositional environment of shale, marine, transitional and lacustrine facies shale gas are obtained. China is one of the most successful countries that have achieved large-scale commercial development of continental shale oil in the world. Significant discoveries of continental shale oil have been made successively in the Ordos Basin, Junggar Basin, Songliao Basin and Bohai Bay Basin. The marine shale gas industry has achieved breakthroughs and rapid scale development in the Sichuan Basin and surrounding areas. By the end of 2021, a total of 8 shale gas fields had been found in southern China, with the proved geological reserves totaling 2.74×10 ¹²m ³. In 2021, the annual production capacity of shale gas was 230×10 ⁸m ³, bringing the total production capacity to 924×10 ⁸m ³. The shale oil and gas resources in China have great potential, but there are many challenges during the large-scale exploration and beneficial development. Based on the development experience of shale oil and gas in foreign countries and relevant inspirations, it is suggested that the state takes the lead in evaluating and implementing the technologies that can convert black shales or high-carbon coals into oil and gas, estimating the total amount of economic resources involved, and formulating corresponding development plans. Additionally, national pilot test areas should be set up for in-situ conversion of black shales and high-carbon coals into oil and gas, and fiscal incentives and tax support policies for the conversion of shales and coals into oil and gas should be introduced and implemented to promote the maturity and conversion of underground shales and the heated conversion of underground coals into oil and gas. Moreover, national top-level design and coordinated investment should be implemented to establish the base of "worldwide super energy basins" represented by Ordos Basin and Sichuan Basin, so as to achieve the collaborative and integrated development of underground and overground resources such as renewable energy sources on the ground and oil, gas, coal, heat, lithium and uranium resource under the ground, involving CO ₂ capture and storage (CCS) and CO ₂ capture, utilization and storage (CCUS).

Continental lacustrine shale developed in the Lower Jurassic of Sichuan Basin has high-quality source rock, high-abundance organic matter, and rich shale oil resources. Previous studies mainly focused on the Da'anzhai Member of Ziliujing Formation, and less on other horizons. The geological characteristics and resource potential of lacustrine shale oil in the Lower Jurassic were determined by analyzing the geochemical characteristics of Jurassic shale such as depositional environment, organic matter abundance, type, and maturity, as well as the geological conditions such as shale reservoir properties and reservoir space types. The results show that for the three sets of lacustrine shales in Dongyuemiao Member and Da'anzhai Member of Ziliujing Formation and Lianggaoshan Formation developed in Sichuan Basin, TOC content is generally greater than 1.0%; the organic matter of type Ⅱ ₁-Ⅱ ₂ indicate that these three sets of shale have good hydrocarbon potential; R _o is within the range of 1.00% to 1.82%, indicating a medium to high degree of thermal evolution. The shale is thick and widely distributed, with an average porosity of 4% -9% and the development of shale bedding fractures. Dongyuemiao Member has a gentle broad basin still-water deposit environment; Da'anzhai Member was formed in the largest lake flooding period in the Jurassic System, shown as a deep basin deep-water environment; Lianggaoshan Formation is dominated by a broad basin shallow-water environment. There are three combination patterns of shale development:pure shale type, shale-carbonate rock interbedding type and shale-sandstone interbedding type. Based on the evaluation of enriched intervals, five sweet spots are clearly divided vertically and the zones of thin oil, light oil and condensate oil and gas are divided horizontally. In particular, major discoveries such as Well Ping'an 1 have demonstrated that there is great exploration and development potential for shale oil in the Jurassic System of Sichuan Basin.

In terms of the"dual carbon"target, green, pollution-free and high-energy density hydrogen energy has become an important trend for the future development of energy industry. Underground hydrogen storage is a promising technology for large-scale hydrogen storage. This paper describes the concept, field practice and theoretical research status of underground hydrogen storage. A deep analysis is performed on the modes and characteristics of hydrogen storage in the salt cavern, aquifer, depleted oil and gas reservoirs, as well as abandoned coal mine, of which the advantages and disadvantages are compared from multiple perspectives. Underground natural gas storage provides technical experience for hydrogen storage, but there are obvious differences between the both. Further, the paper systematically analyzes the feasibility of underground hydrogen storage, and elaborates the difficulties and challenge in terms of caprock, wellbore integrity and chemical reaction in reservoirs. Based on the above research and analysis, the difficulties in the development of underground hydrogen storage technology are clarified; the future development prospects of underground hydrogen storage have been predicted; the corresponding countermeasures and suggestions are also put forward, such as strengthening the research on the geomechanical integrity of caprock and the integrity of wellbore, and facilitating the assessment of the geochemical and microbial reactions of reservoir rocks, fluids and hydrogen, which provide a significant reference for the underground hydrogen storage in China.

Based on analyzing the history of shale revolution in the United States and the differences in geological and engineering characteristics of shale oil between continental basins in China and marine basins in the United States, the paper proposes ten noteworthy issues on continental shale oil revolution in China, including the application and evolution of the nomenclatures of shale oil and tight oil in U.S., the development process of shale oil/tight oil in U.S., the proposal and connotation of shale revolution, and the experience in system and mechanism that can be learned from successful shale revolution in U.S., the work pattern of shale oil in U.S., the relationship between the profit model of shale oil and investment channels, the relationship between production declines of shale oil, the concrete time when a breakthrough is made in shale oil exploration in China, the principles and standards for classification of shale oil, and the continental shale oil revolution in China. Research suggests that the concepts of shale oil and tight oil in North America are identical to some extent. The core of "shale revolution" in U.S. is to improve drilling and completion efficiency, reduce well construction costs, and increase single well production; the development stages of its work pattern is divided according to the changes in both well type and horizontal section length of horizontal well, as well as the development of hydraulic fracturing. The number of drilled and completed wells is an important indicator reflecting shale oil exploration and development. The profit models and investment channels of shale oil extraction are closely related. American companies’ pursuit of recovering investment as soon as possible to obtain profits leads to the general adoption of a production model based on pressure release, with a rapid decline in yield and an L-shaped production curve. In terms of system and mechanism, we should draw on the experience from the application of market mechanisms, the project operation model of "oil companies+lean management", as well as the establishment of shared and open databases. From the perspective of resource base, engineering and technological capabilities, and production expectations, China has the basic conditions for the success in the continental shale oil revolution. All efforts should be made to promote the marketization, technological and management transformation of shale oil exploration and development, highlight the "qualitative development and quantitative breakthrough" of shale oil, and effectively transform resources into reserves and then into beneficial production, which can ensure the victory of shale oil revolution.

Heavy oil is an important type of oil resources. Sustainable and efficient development of heavy oil resources have great significance to national energy security. The main characteristics of thermal recovery of heavy oil in China are summarized as below:as viewed from geological and oil-reservoir characteristics, China boasts various types of heavy oil reservoirs with deep burial, thin bed, strong heterogeneity and complex oil-water system; as for composition, the spatial reticular structure formed from the interaction between colloid and asphaltene molecules leads to high viscosity of heavy oil; as for rheological properties, there is a critical temperature. When the temperature is higher than the critical temperature, heavy oil displays the properties of Newtonian fluids; when the temperature is lower than the critical temperature, heavy oil displays the rheological properties of Bingham fluids with yield value; as for percolation characteristics, heavy oil possesses the properties of underground non-Darcy flow, with a starting pressure gradient, and it is subject to the influences of temperature, reservoir permeability, crude oil viscosity and asphaltene content. This paper summarizes the status quo of heavy oil development technology at home and abroad, and elaborates the main mechanism, applicable conditions, application examples, current problems and development direction of steam huff and puff, steam flooding, steam assisted gravity drainage (SAGD), in situ combustion and thermal composite development. Steam huff and puff is still the main method for thermal recovery of heavy oil and steam flooding is one of the effective substituted techniques to steam huff and puff; SAGD has made important progress in technology introduction and absorption; in situ combustion has become an important technology of greatly improving recovery efficiency; thermal composite development technology has realized efficient production of the marginal heavy oil. It is indicated that heavy oil requires high-quality and efficient thermal recovery technology in the future. In line with the maximization of "double objectives" for recovery efficiency and oil steam ratio, it is required to continuously strengthen reservoir description, dynamic monitoring and injection-production regulation, and actively explore the transformation of heat generation mode, so as to realize efficient development of heavy oil and green low-carbon development.

The production measures of oil and gas field development is severely restricted by the degree of understanding of subsurface reservoirs and fluids; reservoir description plays an important role in oil and gas field development business, and the main goal of reservoir description is to construct the digital twin system of reservoirs. Based on analyzing the current research status and development trend of oil and gas field development and the demands for digital transformation-based production, the paper proposes the digital twin theory of oil and gas reservoir, which defines the reservoir digital twin as "building a digital twin simulation model of oil and gas reservoirs to maximize the quantitative approximation of the real reservoir, characterizing the dynamic changes of the whole life cycle of reservoirs in a timely and accurate manner, and simulating the development behavior of reservoir entities from the physical world in a real environment". This points out the direction for the integrated research of reservoir description and reservoir engineering. Further, the paper presents the key directions and contents of smart oil and gas field construction based on the digital twin system of oil and gas reservoirs, and it is pointed out that by establishing a digital copy of oil and gas reservoir development system, i.e., the digital twin system of oil and gas reservoirs, as a basis for optimal development and scientific management of oil and gas fields in the cloud, a new technological revolution in the field of oil and gas field development will emerge, and the digital twin system of oil and gas reservoirs will give new connotation to the construction of smart oil and gas fields.

Shale reservoirs are characterized by their nanometer pore size. At the nanoscale, fluid flow mechanisms and phase behaviors are significantly influenced by the size and surface effects, resulting in deviations from classical fluid theories. Conventional oil and gas reservoir engineering theory is not fully applicable to shale reservoirs, restricting the efficient development of shale oil and gas. It is thus of both scientific significance and engineering value to clarify fluid transport and phase properties at the nanometer pore scale of shale. Nanofluidics, with the capabilities of precisely manufacturing pore structure and observing in-situ fluid behaviors at the nanoscale, gives new experimental insights into microscopic seepage and phase behavior of shale oil and gas, and provides essential validation for theoretical studies. This paper reviews recent research progress on the nanofluidic study of the nanoscale single- and two-phase flow of oil, gas and water, phase behavior of single- and multi-component hydrocarbons, diffusion and mixing process, as well as microphysical model of shale reservoirs. We focus on introducing nanofluidic methods to detect fluid characteristics, and the differences between experimental results and theoretical descriptions. The current limitations of nanofluidic studies of shale reservoir fluids are discussed in the end, and future directions in this field are foreseen.

China has a complete range of coalbed methane (CBM) resources, which are extremely rich and relatively reliable. The resource foundation can completely form an emerging industry with an annual gas production of hundreds of billions of cubic meters. With strong national support, after 30 years of arduous efforts, China has made significant progresses in the exploration and development of coalbed methane, forming an industry with an annual production of 10 billion cubic meters. However, this is too far from the goal of 100 billion cubic meters of coalbed methane annually, and the national task has not been completed for three consecutive Five-year plans. At the same time, China CBM industry has lost its clear development direction. Only a small contribution can be made to the urgently needed natural gas industry in China. The fundamental reason is that the theory and technology of CBM exploration and development established domestically and internationally over the past 30 years can not fully reflect the composition and pore structure characteristics of coal, as well as the occurrence state of methane mainly in an adsorbed state, which is not fully in line with the mechanism of CBM production, of which applicability is too small to be universal. On the basis of in-depth analysis of the mechanism of coalbed methane production, it is proposed that only the integration of coal mine gas and natural gas development disciplines can establish scientific and practical theories and technologies for CBM exploration and development. Then, four types of CBM resources are divided. Moreover, each CBM resource can be established with scientific and practical theories and technologies to achieve efficient exploration and effective development. This article discusses the possibility and implementation path of building a 100 billion-cubic-meter-scale CBM industry in China from aspects such as the status of CBM resources, progress in oil and gas exploration and development technology, occurrence and migration laws of methane in coal. This article proposes to rely on the cross integration of coal and oil and gas industries, strengthen basic research, establish a theoretical and technical system suitable for efficient exploration and effective development of various types of coalbed methane reservoirs, generate a new discipline (direction), form a new production, technology, and industry field, and build a path of a large industry, To achieve the development strategy of forming an emerging coalbed methane industry supported by original theories and technologies of coalbed methane exploration and development, and to ensure the rapid formation of China's annual production of a 100 billion-cubic-meter-scale CBM industry, in order to significantly reduce China's dependence on external natural gas.

In recent 3 years, breakthroughs have been made in exploration of Member 2 and 4 of Dengying Formation of Upper Sinian in Sichuan Basin, Member 1 of Canglangpu Formation of Lower Cambrian, and Member 2 of Maokou Formation of Permian in the north slope of the central Sichuan paleo-uplift. After the discovery of Anyue gas area, a new large gas area of trillion cubic meters, namely Penglai gas area, has been formed. Under the joint control of large extensional strike-slip faults, paleo-taphrogenic trough and platform margin zones, multi-layer large-scale source rocks, reservoirs and fault-controlled lithologic traps are vertically developed upwards in Penglai gas area; the good source-reservoir configuration and vertical transport pathways promote the efficient filling of multi-layer oil and gas; the tectonic evolution of "ancient uplift and present slope" ensures the efficient accumulation of oil and gas; the development of direct cap rocks and regional cap rocks in the multilayer system provides good conditions for hydrocarbon preservation. Thanks to the good spatio-temporal collocation of accumulation elements, the Sinian-Permian system in Penglai gas area has formed a unique three-dimensional accumulation model of deep, ultra-deep, multi-layer and marine carbonate rocks in the slope area, i.e., "three-dimensional hydrocarbon supply, three-dimensional reservoir formation, three-dimensional transportation, early oil and late gas, three-dimensional accumulation". The breakthrough in the natural gas exploration in multi-layer system of Penglai gas area can not only greatly expand the exploration of deep and ultra-deep natural gas in Sichuan Basin, but also prove that after multi-period tectonic movements, the ultra-deep and ultra-old strata at the slope of the ancient uplift still have the potential to form large-scale natural gas accumulation, which is of great significance for guiding the further exploration of ultra-deep natural gas in Sichuan Basin.

Oil and gas are generated from organic matters in the rocks of sedimentary basins. Through an intensive and systematic study of global petroliferous basins, it is recognized that the distribution of global oil and gas fields is highly uneven, and most of oil and gas are enriched and accumulated in a few strata of sedimentary rocks. The distribution of oil and gas is significantly controlled by source rock, so that it is necessary to search for the source rocks initially before discovering new petroliferous basins. The nutrients required for biological growth in the sedimentary basins primarily come from rivers, and the nutrients flowing from rivers into the sedimentary basins control the degree of biological reproduction, and then control the abundance of organic matters in the source rocks, which decides the amount of oil and gas generated and the degree of enrichment of oil and gas resources in the sedimentary basins. Oil and gas are mainly distributed in the three systems on the earth, i.e., the river-lake system, river-gulf system and river-delta system. Specifically, the river-lake system is an important oil-bearing area on the earth. Lacustrine oil is mainly produced by sedimentary organic matters from the algae died in lakes. The growth of algae depends mainly on the nutrients that come from the rivers and flow into the lakes, and these nutrients can facilitate the growth of algae in the rift period and provide a guarantee for the formation of high-quality source rocks. The river-gulf system is the main distribution location of global marine oil. Gulfs are the estuary of rivers, which brings abundant minerals for rivers to promote the growth and proliferation of algae and other aquatic organisms; moreover, the gulfs are relatively isolated, which are conducive to the preservation of organic matters. In fact, boasting of the biggest reserves, the coal-type gas generated from coal-measure source rocks is the most widely distributed in the world and is mainly distributed in the river-delta system. The sediments brought by the river provide fertile soil for the growth of higher plants, and the native higher plants on the river-delta plain provide a solid material basis for the formation of coal-measure gas source rocks. The delta stratum reservoir is well developed with good reservoir-caprock configuration, which is beneficial for natural gas enrichment and accumulation.

Positive structures with low amplitude and linear distribution are generally developed in Songliao Basin and Bohai Bay Basin,which are attached to faults,slopes and large anticlinal flanks.This kind of positive structures cannot form a trap by itself as a whole,but it has a great control over the oil and gas distribution,so it is an important target for oil and gas exploration.Based on the analysis of structural characteristics,the positive structures are classified as structural ridge and subdivided into three types,i.e.,faulted anticline ridge,nose-like ridge and fault block ridge.Five geometric elements,i.e.,the ridge line,length,amplitude,width and area of the structural ridge are determined,thus enriching the connotation of structural ridge.Through studying the control effect of structural ridge on hydrocarbon accumulation,the paper explores the accumulation control theory of structural ridge,points out that structural ridge is the predominant pathway for hydrocarbon transportation,and establishes a hydrocarbon accumulation mode based on the configuration of structural ridges and thin reservoirs or narrow river courses.By applying seismic identification technology of the structural ridge to hydrocarbon exploration in the middle-shallow strata in Xingxi-Harwen area and Jiangqiao area of Songliao Basin,a number of exploration targets related to structural ridge have been discovered and determined,and a number of wells have been achieved high yield industrial oil flow.That has expanded the oil and gas exploration fields,and supported Daqing oilfield to achieve remarkable results in middle-shallow oil and gas exploration.

Mudstone and shale are developed in 7th Member of the Triassic Yanchang Formation in Ordos Basin, and there is no progress in the exploration of pure shale-type shale oil enriched inside. Oil has been produced only from a few vertical wells in the well testing stage, and the overall pilot production effect is poor. The paper systematically summarizes the petrological, geochemical and reservoir characteristics of mud shale in the 3rd sub-member of 7th Member of Yanchang Formation (Chang 7 ₃ sub-member), optimizes the favorable parameters for the enrichment of shale type shale oil, and determines its hydrocarbon accumulation conditions, exploration direction and potential. Compared with dark mudstone, the black shale in Chang 7 ₃ sub-member is characterized by frequent development of multi-type laminae, high abundance of organic matters, good type of parent material of source rock, early main hydrocarbon generation stage and high oil content, which are favorable lithologies for the exploration of pure shale-type shale oil. The horizontal permeability of black shale is generally higher than its vertical permeability due to the complex effect of horizontal lamellation fractures and high porosity and high permeability laminae such as terrigenous felsic and tuffaceous sandstone. The reservoir space of mud shale in Chang 7 ₃ sub-member is mainly composed of intergranular pores, intragranular denudation pores, clay mineral intercrystalline pores and pyrite intercrystalline pores supported by rigid minerals; lamellation fractures and overpressure microfractures are developed. The composite element composed of the binary laminae combination of "organic matters+terrigenous felsic lamina" and "organic matters + tuffaceous lamina" and lamellation fractures is the most favorable for the occurrence of shale oil. Black felsic shale with the total organic carbon (TOC) content of 4 % ~14 % , vitrinite reflectance R _o>0.8 % , development of lamellation and laminae as well as high content of brittle minerals is a favorable exploration target for pure-shale type shale oil in Chang 7 ₃ sub-member. The Maling-Huachi deep depression area with thick black shale and moderate organic matter abundance is a Class I favorable area for exploration of pure shale-type shale oil. The Chang 7 ₃ sub-member in Ordos Basin is rich in pure-shale type shale oil resources, which is expected to actually become a new major strategic replacement area for unconventional oil exploration and development in the basin.

Analyzing the successful experience of the shale gas revolution in the United States is of great significance for promoting the development of China’s shale gas industry. A comparative study is performed on the exploration and development history, geological conditions for accumulation, development and utilization conditions and resource development status of shale gas in China and the United States. The results show that although both China and the United States have abundant shale gas resources, China’s shale gas output is far lower than that of the United States due to late beginning of exploration and development, low proved resource rate, and low development degree. The shale gas exploration and development in China is still in the early stage of rapid development. The United States has unique geological conditions for shale gas accumulation, superior geological conditions for development, and complete development infrastructure. Moreover, the United States has such advantages as strong technological innovation capability, world-leading process technology, and industrial policy support, which jointly promote the leap in shale gas production. Compared with the United States, China has several disadvantages such as multiple stages of tectonic evolution, multiple types of sedimentary basins, and three types of shales developing in marine, transitional and continental facies. Different types of shales varies greatly in quality. In addition, due to the strong tectonic transformation in the late period, the shale gas preservation conditions are very different in various structural units. The main distribution areas of shale gas in China have complex landforms and subsurface geological conditions, and relatively weak development infrastructure. At present, China is still far behind the United States in terms of key technologies and processes such as three-dimensional and efficient development. As the main body of natural gas production growth in China and the United States and an important field of natural gas production growth in China in the future, this paper makes the following suggestions. (1) The exploration and development of shale gas in Wufeng-Longmaxi formations in Sichuan Basin should be further accelerated to increase reserves and production. (2) Public oil and gas survey should further play a leading role, strengthen the geological survey and evaluation of shale gas in new areas and new strata, and set up new shale gas production bases. (3) It should strengthen independent innovation, enhance theoretical research on shale gas accumulation, tackle key technologies suitable for China’s landform and geological conditions, and promote low-cost and effective development of deep, normal pressured, continental and transitional shale gas. (4) Policy support should be continued to promote the rapid development of shale gas industry.

After more than ten years of exploration and practice, a series of progress has been made in marine shale gas exploration in Sichuan Basin. During that period, focusing on Wufeng-Longmaxi formations as the most favorable pay zone, a few theories for the enrichment and high production of shale gas were proposed, the exploration and development technologies integrating geological evaluation, development optimization, optimal-fast drilling, volume fracturing, factory operation and clean exploitation was formed, and a large shale gas field with reserves of one trillion cubic meters was proved. At an important time for the development of shale gas exploration in China, it is of great significance to promote the exploration and development of marine shale gas by reviewing the exploration and development history of marine shale gas, summarizing the achievements and knowledge of shale gas in Wufeng-Longmaxi formations in terms of geological conditions and enrichment laws, and making an outlook on the key replacement fields for marine shale gas exploration in Sichuan Basin. (1) The exploration and development of marine shale gas in Sichuan Basin have gone through four stages:layer and zone evaluation stage to find the target; pilot test stage of shallow to medium-deep reservoir; demonstration zone construction stage of shallow to medium-deep reservoir; stage of shallow to medium-deep reservoir production, deep reservoir evaluation, non-pressurized shale gas evaluation, three-dimensional development evaluation, ultra-deep and new reservoir exploration. (2) The sedimentary conditions of the Wufeng-Longmaxi formations in Sichuan Basin are superior, in which the deepwater shelf shales with high organic content are continuously and stably distributed. Shale gas mainly occurs in organic matter pores, and its reservoirs are concentrated longitudinally with large continuous thickness. The structures are relatively simple in the southern Sichuan Basin and the Fuling block of southeastern Sichuan Basin. The conditions for shale gas enrichment and preservation include sustained gas supply and reservoirs far away from the ancient/present denudation areas and oil-gas escape areas in large faults. At present, the shale gas resources in Wufeng-Longmaxi formations are 33 19×10 ¹²m ³ with a proved rate of 9.4%, showing great exploration potential. (3) There are rich marine shale gas resources in Sichuan Basin. In addition to Wufeng-Longmaxi formations, there are several sets of marine shale gas reservoirs as reserves in the horizons shallower than 4 500 m. The Cambrian Qiongzhusi Formation, Permian Wujiaming Formation and Dalong Formation as the strategic breakthrough strata, the favorable shale gas reservoirs mainly lie in Weiyuan-Ziyang area inside and around aulacogen for Qiongzhusi Formation, with the predicted resource potential of 1.40×10 ¹²m ³, and in Jiange-Nanjiang area and Dazhu-Kaijiang area of northern Sichuan Basin, and Lichuan area of southeastern margin of Sichuan Basin for Wujiaping Formation and Dalong Formation, with the predicted resource potential of 0.91×10 ¹² m ³. The Member 1 of Xujiahe Formation as the strategic preparation layer, its favorable shale gas reservoirs mainly lie in Ya'an-Qionglai area, with the predicted resource potential of 0.88×10 ¹² m ³.

Archean buried-hill zone in the western section of Bonan low salient of Bohai Bay Basin has good conditions for hydrocarbon accumulation. Bozhong26-6 oilfield is an Archaean integrated oilfield with proven reserves of crude oil exceeding 100 million tons. Based on a large number of core, thin section, well logging and geochemical data, a systematical study was performed on Bozhong26-6 oilfield. The analysis suggests that the Archean buried-hill reservoirs can be vertically divided into weathered conglomerate zone, weathered fracture zone and bedrock zone, among which the weathered fracture zone is the key reservoir development zone. The superimposed fractures formed by the Indosinian, Yanshanian and Himalayan movements provided the foundation for the development of Archaean buried-hill reservoirs. The Indosinian compression and collision and the Yanshanian strike-slip thrust were the main driving forces for the formation of fractures, and the south-north extension of the Himalayan epoch maintained the validity of earlier fractures. Under the communication of fractures, a wide area of high-quality buried-hill reservoirs is formed by the dissolution of atmospheric fresh water, and the high-quality reservoirs are developed in the zone within 420 m away from the unconformity. The mudstone of Dongying Formation with weak overpressure and strong stability overlying buried hill provides good sealing conditions for the preservation of large-scale oil reservoirs. The Archean buried hills are in direct contact with the source rocks of Huanghekou sag in the south, and are connected with the source rocks of Bozhong sag in the north by the unconformity, thus forming a multi-dimensional oil-gas migration and charging mode. In conclusion, the above findings provide a guidance for the efficient exploration of Archean high-abundance oil reservoirs in Bozhong26-6 oilfield, further improve the hydrocarbon accumulation and reservoir mode of deep Archean buried hills in Bohai Bay Basin, and are of important guiding significance for the oil and gas exploration of the Archean buried hill zone around the southwest Bozhong sag.

After nearly 40 years of exploration in Kaiping sag of Pearl River Mouth Basin, a medium to large light oil field has been discovered in deep-water Paleogene strata of Kaiping11-4 structure. To further guide oil-gas exploration in Kaiping sag, the paper deeply investigates the petroleum geological conditions such as hydrocarbon sources, reservoirs, traps, preservation and migration conditions in Kaiping11-4 structure, and summarizes the oil and gas accumulation mode. The geological structure of Kaiping sag is a detachment-type compound half-graben with faults in the north and overlaps in the south. In Kaiping sag, Wenchang Formation develops high quality semi-deep to deep lacustrine source rocks with great hydrocarbon generation potential, and the shallow-water braided delta front deposits of Enping Formation develop a favorable reservoir-cap assemblage. The traps formed under the early tectonic activities are various, including faulted block, faulted anticline and plunging anticline, while the weak tectonic activity in the late period is conducive to oil and gas preservation. The inheritance faults that cut through the source rocks provide vertical migration channels so that oil and gas can be transported through the "source-fault-reservoir" system. The reservoirs in Kaiping11-4 structure have undergone multi-phase continuous charging since 17 Ma, presenting a hydrocarbon accumulation mode with the characteristics of "hydrocarbon supplying from high-quality lacustrine sources, strongly charging through near-source faults, and enriching in multiple stages and levels". The discovery of medium to large light oil field in Kaiping11-4 structure is not only a breakthrough of the new area exploration in Kaiping sag, but also a breakthrough of the Paleogene crude oil exploration in the eastern deep-water area of the South China Sea. It shows that the deep-water area of Pearl River Mouth Basin has a broad prospect for oil and gas exploration, and is of great significance to the exploration of petroliferous basins in the northern deep-water area of the South China Sea.

The recovery of shale gas is generally low,and achieving a significant increase in the recovery of marine shale gas in Sichuan Basin has become a key technical issue to be tackled during the 14th Five-Year Plan (2021-2025).Based on the development characteristics of shale gas platform cluster wells,long horizontal sections and multi-cluster volume fracturing,this paper systematically summarizes the domestic and international research progress of shale gas recovery enhancement technology from three aspects:horizontal well control area,fracture control volume and matrix recovery degree,analyzes the main control factors affecting recovery,sorts out the technical and scientific issues regarding recovery enhancement,and gives corresponding suggestions for tackling the problem.The study shows that the single well production of shale gas in southern Sichuan area is affected by the horizontal section length,well spacing,fracture parameters,and production system,with the characteristics of low planar and longitudinal reserve utilization rate,limited fracture control volume and low utilization of matrix.Shale gas recovery enhancement should be oriented to maximize the elastic energy utilization efficiency,significantly improve the fracture control volume by optimizing the cluster spacing,fracture construction parameters and soaking time,and enhance the matrix recovery degree by reducing the bottom hole pressure,optimizing the drainage system and injecting CO ₂.The authors suggest to pay attention to the key issues such as recovery enhancement mechanism and evaluation model,matrix-fracture coupled flow mechanism and evaluation model,repeated fracturing optimization process parameters and the mechanism of recovery enhancement by CO ₂ injection,thus providing theoretical guidance and technical support to significantly improve the recovery rate of shale gas in southern Sichuan area.

Large tight sandstone gas reservoirs in Tianfu gas field of Sichuan Basin were discovered in the Member 2 of Shaximiao Formation in 2019 and the Member 1 of Shaximiao Formation in 2021, respectively. The proven reserves are 1 349×10 ⁸m ³ and the production is 15.7×10 ⁸m ³ in 2022. Based on the core and geochemical analysis data, the paper investigates the sedimentary reservoir characteristics, natural gas geochemical characteristics, gas reservoir types and gas reservoir formation conditions of Shaximiao Formation in Tianfu gas field. In the study area, shallow-water deltaic and lacustrine deposits are found in the Member 1 of Shaximiao Formation, while fluvial deposits are found in the Member 2 of Shaximiao Formation. The lithologies of the reservoir are mainly composed of feldspathic litharenite and lithic arkose, and the reservoir spaces are mainly occupied by residual intergranular pores, followed by feldspar dissolution pores. The natural gas source of reservoir is mainly from the Triassic Xujiahe Formation. The natural gas in the Member 1 of Shaximiao Formation and the 6th, 8th and 9th sand groups of 1st submember of Member 2 of Shaximiao Formation is dominated by coal-type gas with a small amount of mixed gas, while the gas in the 7th sand group of 1st submember of Member 2 of Shaximiao Formation is characterized by the occurrence of both mixed gas and oil-type gas. The gas reservoir in Shaximiao Formation of Tianfu gas field is a large lithologic gas reservoir with accumulation regularity of dual-source and multi-phase charging, fault and sandbody transport, accumulation around the source, and differential enrichment at the channel. A series of key exploration and development technologies have been developed by tackling the key exploitation problems of large tight sandstone gas reservoirs in Shaximiao Formation of Tianfu gas field, i.e., (1) technology of precisely choosing layers under the constraint of high-precision isochronous stratigraphic framework, (2) technology of finely characterizing sand bodies and precisely predicting target based on 3D seismic survey, (3) supporting technology to accelerate exploration and production based on horizontal well and volume fracturing, (4) processing technology of exploitation and transportation on the ground for the purpose of fast construction, investment, optimization and simplification, (5) integrated technical and economic template for scale and cost-effective development. The discovery of Tianfu gas field has improved the exploration and development of tight sandstone gas in China, and enriched the exploration methods of tight sandstone gas, and effectively promoted the exploration and development process of tight sandstone gas in Sichuan Basin.