China's continental basins are rich in shale oil resources and thus are an important strategic replacement field for increasing oil and gas reserves and production. Limited by the special and complex geological conditions of continental basins, the enrichment rules and main controlling factors of continental shale oil are not clear enough, and the key scientific issues faced in continental shale oil exploration still need to be further studied. Through a thorough investigation of the latest developments in the foreign and domestic researches focusing on the fine-grained sedimentation, hydrocarbon occurrence, and fluid migration of shale oil, in combination with the progress of China's continental shale oil exploration, this paper summarizes the basic geological characteristics and differences in continental and marine shale oil, and proposes the key scientific issues faced in continental shale oil exploration in China. The research results show that there are many sets of shale oil developed in the continental basins of China; the continental shale series of strata are characterized by rapid changes in sedimentary facies, large deposition thickness, low maturity, and high clay mineral content; compared with marine strata, the sedimentary structure background is relatively unstable, the sedimentation age is newer, the heterogeneity is stronger, the stratum energy and geothermal gradient are lower, and the viscosity and density of hydrocarbon fluid are higher. The formation of fine-grained components and the genetic mechanism of geological events are not only the event information of the source formation and the evolution of earth system that sedimentary geology pays attention to, but also the key content of the study of reservoir formation and source-reservoir coupling mechanism for shale. The occurrence mechanism of shale oil is closely related to its mobility, and clarifying the occurrence mechanism is the key to optimizing sweet spots. The micro-migration mechanism is the basis of shale reservoir development. With the development of the micro fluid flow theory, computer molecular simulation and experimental technology, the migration mechanism will be further revealed. The three key scientific issues are the formation mechanism of fine-grained sedimentary rocks, the occurrence mechanism of continental shale oil and the micro-migration mechanism of continental shale oil. It is suggested that these three scientific issues should be taken as the guide, and the integration of geology-engineering research and technical research should be strengthened to make it a guarantee for the success of China's continental shale oil revolution.

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 subdivision of hydrocarbon evolution stage and the resource potential evaluation of source rocks are of great significance for deep conventional and unconventional petroleum exploration and studies on deep basic petroleum geology. The hydrocarbon evolution of deep source rocks can be divided into four stages, i.e., light oil (volatile oil), condensate, wet gas and dry gas, corresponding to four types of deep hydrocarbons that may be sourced from the source rocks in crude oil reservoirs. Using thermal simulation, this paper evaluates the hydrocarbon generation potential of deep source rocks, and then puts forward indexes for the division of four hydrocarbon evolution stages. In view of the fact that the evaluation of resource potential of deep source rocks needs to consider whether normal crude oil is expelled and how much the expulsion is, an experimental scheme of first performing oil expulsion at peak of normal oil generation and then starting heating of oil expelled source rock in a confined pyrolysis system is adopted to establish the hydrocarbon evolution mode of deep source rocks. This mode can be used to roughly evaluate the hydrocarbon resource potential of deep source rocks. Based on the experience of reservoir classification according to the gas over oil ratio in reservoir (GORr), the gas over oil ratio in source rock (GORs) and the methane content of source rock pyrolysis simulation products are used as the classification indexes of hydrocarbon evolution stage under laboratory thermal simulation conditions. The rapidly rising GORs values, i.e., 142 m ³/m ³ (800 scf/bbl), 890 m ³/m ³ (5 000 scf/bbl) and 3 562 m ³/m ³ (20 000 scf/bbl), and 95% methane content were taken as the upper GORs limits of light oil, condensate, wet gas and dry gas, respectively. Considering that GORs cannot be obtained directly from component analysis of core samples, these limit values cannot be used for dividing the hydrocarbon evolution stages of actual sections. Since the vitrinite reflectance ( R _o) or equivalent vitrinite reflectance ( R _oE) is commonly used by explorers to classify hydrocarbon generation stages of source rocks, the Ro range of the above GORs limits can be obtained by converting the laboratory temperature into R _o using the oil inhibition R _o model. It is worth noting that the R _o value obtained from the thermal simulation experiment in a confined pyrolysis system is higher than the R _oE value measured in the actual formation. Light oil and condensate can be divided into four categories according to their geneses. Among them, type A is formed by type Ⅰ to type Ⅱ organic matter after oil expulsion, type B is formed by type Ⅱ to type Ⅲ organic matter without hydrocarbon expulsion, type C is formed by crude oil cracking, and type D is formed by secondary alteration. At present, the researches on primary light oil and condensates (type A, B and C oil and gas) are insufficient and need to be strengthened. Deep light oil and condensate resources are not only affected by the organic matter content, type and maturity of source rocks, but also related to the following geological factors of deep strata:(1) hydrocarbon expulsion efficiency of normal oil (black oil); (2) large-scale oil cracking in reservoir; (3) mixture of oil and gas from different sources. Various genetic types of light oils and condensates are found in China, showing broad exploration prospects in the fields of light oil and condensate resources.

Tarim Basin has enriched oil and gas resources in ultra-deep marine carbonate rocks, where China's largest weathering crust-type oil reservoir and largest condensate gas reservoir have been discovered. However, the oil and gas exploration and development have been conducted only in palaeohighs and slopes for a long time. In recent years, with breakthroughs in the traditional theoretical understanding of "oil accumulation controlled by palaeohighs and enrichment in slopes" and advances in the supporting technologies of seismic exploration, drilling and development, Fuman oilfield, the ultra-deep (over 7 500 m) strike-slip fault-controlling oilfield, has been discovered in the depression area. Exploration and development practice and studies of the Fuman oilfield show the following results. First, the developed strike-slip faults not only cut through the deep Lower Cambrian source rocks, but also control the development of Middle Ordovician carbonate cavity-fracture reservoirs, and constitute a superior petroleum accumulation system under the control of strike-slip faults together with the huge thick Upper Ordovician mudstone. Second, the oil and gas show a stripe-shaped differential distribution along the strike-slip fractured zone, characterized by special trap type, large oil column height, good oil property, and high single-well production. Third, the predicted geological reserves of oil are up to 4×10 ⁸t, the amount of oil resources is 10×10 ⁸t, and there are many efficient development wells. The oil productivity of 160×10 ⁴t/a has been rapidly achieved. Through continuous research, the supporting technologies such as ultra-deep, high-density and wide-azimuth 3D seismic acquisition, characterization of weak strike-slip faults, evaluation of traps in carbonate fractured zone, efficient well location deployment, ultra-deep drilling and completions have been gradually established in the desert area, providing supports for the large-scale and efficient exploration and development of the ultra-deep complex fault-controlling carbonate reservoirs.

In 2019, PetroChina Changqing Oilfield Company discovered the Qingcheng shale oil field with 1 billion tons of estimated oil resources in Ordos Basin. However, it remains controversial whether its petroleum resources are tight oil or shale oil. Based on systematically analyzing the geological characteristics of unconventional reservoirs in the Member 7 of Yanchang Formation, it is believed that the entire Member 7 of Yanchang Formation is a set of fine-grained sediments, characterized by thin sand body for a single stage, low ratio of total sandstone thickness to formation thickness and source-reservoir integration, with the development of typical shale oil inside. The in-depth understanding of enrichment mechanism and characteristics of shale oil has driven the Changqing oilfield to improve the exploration and development methods and management thinking, and implement integrated operation, platform management, factory operations, information support, full life cycle assessment and other measures. By striving to make breakthroughs in multi-disciplinary technical researches, this study develops key technologies for "sweet spot" evaluation, high-quality fast drilling and completion technologies for horizontal wells, volume fracturing technology for subdivision and cutting of horizontal wells and other special technologies, thus promoting the exploration and development of shale oil in the Member 7 of Yanchang Formation in Ordos Basin to achieve important breakthroughs; meanwhile, a million-ton-level shale oil demonstration plot has been set up. Ordos Basin will be one of the most important areas for increasing oil production in China. The shale oil in the Member 7 of Yanchang Formation is determined as the most practical field. It is planned that the production of the shale oil in the Member 7 of Yanchang Formation will reach 500×10 ⁴ t in 2030, which is of great strategic significance to support China's stable crude oil production and guarantee the national energy security.

The deep and ultra-deep reservoirs of petroliferous basin are one of the three new fields for global oil and gas exploration. In the context of low permeability and densification on the whole, the high-quality reservoirs with relatively high porosity/permeability are the sweet spots for deep-ultra-deep exploration. Exploration practices and researches show that high-quality reservoirs dominated by primary porosity, secondary porosity and pore-fracture assemblage and fractures can also be developed in the deep-ultra-deep clastic rocks of petroliferous basin (at the depth of 4-8 km); in the deep-ultra-deep high-quality clastic reservoirs, Paleozoic reservoirs are dominated by secondary pores, supplemented by primary pores; Mesozoic Jurassic-Cretaceous reservoirs are dominated by primary pores, supplemented by secondary pores; Cenozoic reservoirs are dominated by the massive development of primary and secondary pores. Moreover, the exploration depth in the Mesozoic reservoir is generally greater than that in the Paleozoic and Cenozoic reservoirs. The development of secondary pores in deep-ultra-deep reservoirs is controlled by shallow-deep polygenetic dissolution pores, the development of cracks is controlled by tectonism, and the effective preservation of pores and fractures in the deep-ultra-deep layers under different geological background condtions is controlled by early cementation (chlorite cladding and carbonate cement crust), shallow fluid overpressure, early hydrocarbon charging and burial-thermal evolution history with low time-temperature index. The development of high-quality oil and gas reservoirs in deep-ultra-deep reservoirs attributes to the inter-coupling of different geological factors, including favorable sedimentation, burial-thermal evolution history, fluid pressure history, diagenetic evolution history-pore evolution history and oil-gas charging history, and are classified into 5 typical types as follows:(1) mainly controlled by fluid overpressure and oil-gas charging in shallow-medium layers, dominated by primary pores; (2) mainly controlled by chlorite cladding in shallow layers and oil-gas charging in medium-shallow/deep layers, dominated by primary pores; (3) mainly under the control of overpressure formed by early long-term shallow burial and late rapid deep burial as well as oil-gas charging in medium-deep layers, dominated by primary pores; (4) mainly controlled by surface leaching poers and oil-gas charging in medium-shallow layers, dominated by secondary pores; (5) mainly controlled by polygenetic dissolution and late hydrocarbon charging, dominated by secondary pores. In the absence of connection between large-scale fracture systems, the porosity increased by burial dissolution is limited, and various types of "shallow accumulation-deep preservation" are the key to the development of deep-ultra-deep high-quality oil-gas reservoirs. In addition, the preservation limit of clastic oil-gas reservoir porosity determines the lower limit for the oil-gas exploration in deep-ultra-deep layers; organic-inorganic interaction of "hydrocarbon-water-rock" in deep-ultra-deep oil-gas reservoirs continues to affect reservoir quality and oil-gas quality evolution; the organic-inorganic interaction mechanism under high temperature and high pressure and the depth limit for the preservation of reservoir pores constrained by it are the focus of further study.

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.

Ordos Basin is a typical intracratonic basin, rich in mineral resources such as oil and gas, coal, salt, and uranium. Studying the periods, sequences and attributes of tectonic movement of the basin will not only lay the foundation for revealing the origin and evolution process of the craton basin, but also provide a basis for exploring the internal occurrence mechanism of the multiple energy and mineral resources. Based on high-resolution reflection seismic profile and deep-well data in recent years, in combination with the analysis of peripheral geological outcrops, this paper establishes a spatio-temporal framework of basin evolution by determining the key tectonic evolution periods of Ordos Basin. Studies have shown that there are 10 regional unconformities developed from bottom to top in the Ordos Basin, namely the basal unconformities in the Changchengnian, the Jixianian, the Sinian, the Cambrian, the Ordovician, the Carboniferous, the Triassic, the Jurassic, the Cretaceous and the Quaternary. Six tectono-stratigraphic sequences developed in the Mesoproterozoic, the Cambrian to Ordovician, the Upper Carboniferous to Triassic, the Jurassic, the Lower Cretaceous and the Cenozoic in the basin. The formation and evolution of Ordos Basin was controlled by the tectonization of the peripheral plates. It experienced the continental breakup in the early and middle period of the Mesoproterozoic, the development of passive continental margin during the Cambrian to the Middle Ordovician, the formation of active continental margin and collision orogeny in the Late Ordovician, the periphery breakup during the Late Carboniferous to the end of the Permian, the development of large-scale intracontinental depressions during the Early Mesozoic and intracontinental foreland basins during the Middle to Late Mesozoic, and peripheral fault depressions during the Cenozoic and other evolution processes. Tectonism in the deep lithosphere of Ordos Basin is relatively active. The basin is subjected to four periods of intermediate-acid or mafic-intermediate volcanic activities in the Middle Ordovician, the Middle and Late Triassic, the Early Cretaceous, and the Late Miocene, especially much extensive in the late period of the Early Cretaceous. Further, through analyzing the tectonic events of peripheral plates, intrabasin magmatic activity and basin subsidence-uplifting process, it is believed that Ordos Basin has experienced five key tectonic modification periods of the Neoproterozoic, the Late Ordovician, the Middle to Late Triassic, the Late Jurassic to Early Cretaceous, and the Cenozoic. These tectonic modification periods control the tectonic evolution and geological architecture of the basin, and have a profound impact on the distribution of oil and gas in Ordos 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.

Shale oil resources are rich in Member 1 and 2 of Qingshankou Formation in the Gulong area of Songliao Basin; a breakthrough of strategic importance has been achieved in oil exploration, and preliminary achievements have been made in the pilot test of reservoir development, showing a broad prospect for shale oil exploration and development. Gulong shale oil is a typical primary reservoir in source strata, characterized with source-reservoir assemblage; it is obviously different from tight oil, sandwich-type and hybrid depositional shale oil in terms of the reservoiring characteristics such as source-reservoir relationship, migration features, accumulation dynamics and boundary conditions, as well as the seepage characteristics involving the phase change mechanism of fluid phase behavior and the staged transportation mechanism of the micro-nano fracture-pore system. In order to effectively achieve the target, a series of key exploration and development technologies with a focus on "box-type reservoir development" has been established, and initial success has been achieved. After hydrocarbon generation of the lamalginite associated with clay minerals in shale oil, the fracture-pore system dominated by organic pores has been formed. Under the joint action of capillary force and viscous force, oil and gas molecules can not migrate, and is preserved in situ under overpressure. Thus, the micro-nano oil-bearing pore aggregates that are distributed in a large scale, relatively independent and have different pressure systems and fluid properties have been formed, thus constituting the primary reservoir in source strata. The key to achieve the development of primary reservoir in the source strata of Gulong shale and maintain stable production of oil well for a long time is to increase the complexity of artificial fractures, and make the oil-bearing pores of different pressure systems gradually reach the fluid start-up pressure and the crude oil occurred in pores flow to the fracture network step by step. Actually, the exploration and development practice has promoted the formation of box division technology with "organic phase + stress step" as the core, the gold target optimization technology with "movable oil content" as the core, and the integrated optimization technology of "one-time well layout, multi-target stacking, three-dimensional interleaving and overall employment", which has achieved obvious application results. The theoretical understanding of oil reservoir formation in the source strata of Gulong shale enriches the traditional petroleum geology theory, which will inevitably lead the development of theories regarding shale oil in source strata in the whole world and push the shale oil revolution to a new height. Furthermore, the large-scale efficient development of Gulong shale oil is of strategic significance for safeguarding the national energy strategy, promoting the construction of comprehensive international energy companies in China and boosting regional economic and social development.

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.

Based on the new technical data of high-precision petrophysical experiment, digital core and logging, this paper systematically describes the shale lithofacies characteristics of continental shale oil from the submember 3 of Member 7 of Triassic Yanchang Formation in Ordos Basin, as well as its identification method. The favorable lithofacies interval and distribution of "oil and gas sweet spots" were determined according to the internal characteristics of each category of shale lithofacies. The research suggests that the shale lithofacies with high TOC content are foundational for the development of source rock of continental shale oil, but not the main controlling factor for the production. The shale lithofacies with good pore structure and developed reservoir space is deemed as favorable lithofacies for "oil and gas sweet spots". The compressibility of favorable shale lithofacies is closely related to the particle size and distribution pattern of brittle minerals, as well as its contact relation with flexible minerals. Favorable lithofacies and good source-reservoir configuration are the main controlling factors of continental shale oil production. Fracturing classification and perforation cluster layout can be optimized based on the evaluation results of the reservoir quality, source quality and engineering quality ("three qualities"). The research results and understandings effectively supports the risk exploration and deployment of two horizontal wells in C80 well area of Ordos Basin, and provides effective technical support for making significant breakthroughs.

The Jurassic reservoir in the eastern slope belt of western Sichuan depression is generally characterized by "dual-source" hydrocarbon supply of the high-quality source rocks from the Member 5 of Upper Triassic Xujiahe Formation and the Lower Jurassic, development of multi-stage, multi-set, superimposed channel sandstones in a large scale, hydrocarbon supply owing to the continuous activity of faults connecting source rock, and favorable conditions for configuration of the ancient and present faults and sand bodies. Thus, it has the geological conditions and temporal-spatial combinations of source and reservoir for forming a tight sandstone gas field. By summarizing and analyzing the geological conditions of the reservoir in Zhongjiang gas field, this paper proposes a new understanding that the reservoir in Middle Jurassic Shaximiao Formation is characterized by development of channel-controlled sand bodies, hydrocarbon transportation through faults and sand bodies, and accumulation under superposition and compound condition, and establishes the enrichment pattern with the characteristics of continuous fault activity, multi-period hydrocarbon charging, lithologic gas reservoir development and hydrocarbon enrichment far from faults. Based on the exploration and development of Zhongjiang gas field, key supporting technologies have been formed, such as the fine characterization technology of "sweet spots" of tight sandstone gas, high-efficient development technology for tight sandstone gas reservoir in complex narrow river channel, and dense cutting and strong-support volume fracturing technology of horizontal well. Through the integrated operation of geology and engineering, Zhongjiang gas field, as a tight sandstone gas field in the narrow river channel of Shaximiao Formation, has provided a cumulative proved reserve of natural gas of more than 100 billion cubic meters, with the gas output of over 20×10 ⁸m ³/a.

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.