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.

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.

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.

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).

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

This paper presents a new method for predicting water injectivity of the well formation based on dynamic and static data of oilfield exploration and development. Initially, the water injectivity was classified using Gaussian Mixture Model (GMM). On this basis, machine learning samples were generated using dynamic and static data, and further a prediction model of water injectivity was established using the random forest algorithm. Moreover, the importance of geological factors and development factors affecting water injectivity was also analyzed. Finally, the model has been applied to the study area, providing the water injectivity of all wells in different sections and different periods. The predicted results are consistent with the tracer monitoring results and the water absorption profile test results, thus verifying the feasibility of the method proposed in this study. The method overcomes the disadvantages of traditional methods, such as the hypotheses of ideal conditions, experience and parameters, and explores the water injectivity by historical data mining. It has an important guiding significance for profile control and water plugging in water injection development oilfield.

In recent years, the annual incremental proved reserves of petroleum in China has decreased significantly. As a result, the incremental recoverable reserves are less than the output of the year, and the annual output has also fallen below 2×10 ⁸ t for 5 years. The average investment in exploration and development from 2015 to 2019 was at rock bottom, which is one of the reasons for the decline in oil reserves and production. The rapid development period of gas in China is a phase later than that of oil, and now it is still during a fast-developing period. China's shale gas has made a good beginning in the past 10 years, but it should be noted that the proved reserves obtained are limited to the shale in the Wufeng-Longmaxi formations in the southeastern Sichuan Basin. The proved reserves of shale oil have been obtained in the 7th oil layer of Yanchang Formation, and the construction of pilot development area has been started in the southwest of Ordos Basin. A major breakthrough of shale oil in the Lucaogou Formation has also been made in Jimsar sag of Junggar Basin, which is expected to achieve significant progress in reserves and production. However, it should be noted that the development of shale oil in China faces great difficulty and the threshold for economic development is high. Tight (sandstone) oil and gas is a field in which rapid progress is expected for the development of unconventional oil and gas in China in the near future. Of these, the Ordos Basin and the Sichuan Basin are the main areas for increasing reserves and production. The progress in coalbed methane development is far from the initial expectations, and careful researches are required for its occurrence conditions and development technology. This paper suggests that a new round of evaluation for strategic succession of oil and gas production should be carried out as soon as possible, proposes 5 preferred areas and fields, sponsors the establishment of an exploration fund to support pioneering exploration projects, and proposes to supplement and revise the relevant norms of reserve appraisal based on market orientation.

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.

The genetic method with basin modeling as the core has been widely used in hydrocarbon exploration and production. China’s basin modeling theory and research are in the front ranks of countries of the world, but the application software development lags far behind other countries. According to statistics, 75 % of the 39 pieces of basin modeling software or algorithms available in China are developed by foreign companies, and 100 % of the popular software on the international market are developed by overseas companies. At present, the main challenge faced by basin modeling research is that the classical petroleum system theory widely applied in the past cannot effectively evaluate unconventional oil-gas resources. In this study, whole petroleum system theory is adopted to address the current challenges. Firstly, the basic characteristics, genetic mechanism and distribution regularities for the joint coexistence of conventional and unconventional oil-gas resources are elaborated based on the whole petroleum system theory, providing theoretical and methodological guidance for the prediction and assessment of conventional and unconventional oil-gas resources. Secondly, key technologies for predicting and assessing conventional, tight and shale oil-gas resources have been developed by establishing a combined genetic model for conventional and unconventional oil-gas reservoirs and a material balance relationship between total hydrocarbon generation and hydrocarbon resources. Thirdly, this paper establishes five sets of material balance equations between the total hydrocarbon production and the evolution and productivity of the whole petroleum system, analyzes the key geological parameters such as primitive hydrocarbon production ratio, migration-accumulation coefficient, movable hydrocarbon ratio and oil recovery, and proposes 12 key technologies of three categories for basin modeling. The new generation basin modeling system based on the whole petroleum system theory and modern information technology is expected to achieve quantitative, automated and intelligent assessment of oil-gas resources, increase the total amount of resources as the study objects by 5 to 8 times, deepen the research of predicted and assessed resources by more than 3 times, significantly improve the reliability of the predicted and assessed results, and solve the existing bottleneck problem.