Based on the development practice of volumetric fracturing in shale oil horizontal wells in Ordos Basin, and the method of combining laboratory research and field production data analysis, the paper systematically summarizes the overall production decline law of shale oil horizontal wells and differences in the estimated ultimate recovery (EUR)of single well in different types of reservoirs, as well as the variation patterns of water content, salinity, pressure, gas-oil ratio and liquid production in horizontal well intervals during different development stages. On this basis, a production system featuring "reasonable shut-in periods to promote reservoir equilibrium, controlled flowback for sand control in early stage, and pressure-stabilized production to preserve reservoir energy" has been developed, along with a suit of remediation technologies and processes including "sand washing, wax removal, scale removal, gas content control, and eccentric wear prevention". Under the primary energy replenishment development in shale oil horizontal wells, Type Ⅰ and Ⅱ ₁ reservoirs demonstrate the average estimated ultimate recovery (EUR)of over 3.0×10 ⁴ t and approximately 2.4×10 ⁴ t per well, respectively, achieving economically viable large-scale development. Besides, in response to the problem of low single-well production in mid-to-late development stages of shale oil horizontal wells and the demand for enhanced oil recovery (EOR), the method of refracturing in existing horizontal wells in mature areas to create new fractures is proposed to increase EUR and EOR of single well, which is built based on preliminary field implementation analysis in combination with numerical simulation studies on reservoirs, and is predicted to increase the recovery rate by more than 5 %.

In 2023, a major breakthrough was achieved in exploration of the Member 2 of Maokou Formation in Longnüsi area of central Sichuan Basin. The proven reserves submitted to Ministry of Natural Resources of the People’s Republic of China exceeded 460×10 ⁸m ³, marking the first large-scale gas reservoir discovered in the 60-year exploration history of Maokou Formation in Sichuan Basin. As of 2024, PetroChina Company Limited has proven geological reserves of natural gas exceeding 1 500×10 ⁸m ³ in Maokou Formation of central Sichuan Basin, making the Maokou Formation an important new field for increasing reserves and production in the Sichuan Basin. The main understandings obtained are as follows. (1) It is innovatively proposed that the paleo-uplift of central Sichuan Basin has a significant controlling effect on the differentiation of ancient landforms in the early sedimentary period of the Member 2 of Maokou Formation. The slope break zone developed at the edge of the paleo-uplift is a favorable area for the development of high-energy shoals. The shoal thickness is large in Longnüsi-Hechuan-Nanchong area, which is a favorable breakthrough area for exploration. (2) The high-energy shoal facies in Longnüsi area of central Sichuan Basin has undergone multiple dolomitization processes such as early exposure, dissolution and shallow burial, resulting in the formation of high-quality dolomite reservoirs in the Member 2 of Maokou Formation and providing reservoir space conditions for large-scale accumulation. The lower submember of Member 2 of Maokou Formation in the study area develops fracture-pore type high-quality reservoirs, with thickness from 5 m to 30 m, core porosity from 2 % to 8 %, and permeability from 0.01 mD to 1.00 mD. (3) The large-scale hydrocarbon accumulation in Maokou Formation of the Longnüsi area was mainly controlled by multi-source relay hydrocarbon supply, early reservoir formation and trap formation. High-quality source rocks from the Cambrian Qiongzhusi Formation, the Silurian Longmaxi Formation, and the Member 1 of Permian Maokou Formation successively played an important role in sustained hydrocarbon supply after maturation, providing four-stage continuous oil and gas charging from Middle Triassic to Late Cretaceous in Maokou Formation. Early-stage hydrocarbon accumulation occurred in high-energy shoal facies reservoirs, which were laterally sealed by tight limestone, forming early lithologic traps favorable for the formation of large-scale gas reservoirs. (4) The Longnüsi-Hechuan-Nanchong area is the most favorable exploration area for the lower submember of Member 2 of Maokou Formation. The Ya’an-Dujiangyan area in western Sichuan Basin, the Yanting area in northern Sichuan Basin, and the Luzhou area in southern Sichuan Basin have similar sedimentary backgrounds and favorable accumulation conditions, and are potential favorable areas for the next exploration.

Numerical simulation technologies faces new challenges from the development of ultra-low permeability and tight oil reservoirs by large-scale fracturing and water/chemical injection for enhanced recovery. A discrete fracture model is used to characterize the complex fracture network; on this basis, a multiphase and multicomponent flow mathematical model has been established when considering reservoir stress sensitivity and nonlinear flow characteristics, and coupling the machanisms of the surfactants/salts adsorption and diffusion effects and their impacts on capillary pressure, relative permeability curves, and osmotic pressure variations. The explicit characterization of fractures is achieved using an adaptive grid refinement method, and the mode is solved by the finite volume method. The simulation results of the "vertical well injection with fractured horizontal well production" test model are consistent with the results from commercial software. The multiphase and multicomponent flow model established based on the discrete fracture model can successfully simulate the development of ultra-low permeability and tight oil reservoirs under the influence of complex fracture networks. The results show that when the matrix and fractures exhibit high stress sensitivity, a significant drop in reservoir pressure will lead to a substantial decline in well productivity. The development of ultra-low permeability and tight oil reservoirs has to consider the nonlinear flow characteristics of reservoirs, so as to accurately evaluate the development range and well productivity. To appropriately reduce the oil-water interfacial tension through surfactant addition can improve the energy-enhanced imbibition efficiency. The osmotic pressure effect induced by low salinity can improve the energy-enhanced imbibition to a certain extent, whereas the incremental oil recovery is limited.

Coal-rock gas is a new type of natural gas discovered in recent years, generally buried deeper, and some scholars also call it as deep coalbed methane. The exploration and development of this type of natural gas has made rapid progress, with significantly increasing production. Preliminary research indicates that deep coal-rock gas differs significantly from traditional coalbed methane, shale gas, tight gas, and other gas reservoirs in terms of reservoir characteristics and development methods, and has the potential to become a strategic replacement resource for natural gas in China. Therefore, a new round of national science and technology major projects for oil-gas exploration and development will focus on a series of scientific and technological breakthroughs in deep coal-rock gas. Based on systematical analysis, this paper summarizes the three major scientific challenges and ten key research directions for coal-rock gas exploration and development as follows. (1)The key fundamental geological issues are the accumulation mechanism of coal-rock gas and the construction of coal-measure whole petroleum system. The main research directions include the whole process mechanism of hydrocarbon generation/expulsion and enrichment lower limit of coal rocks, sedimentation and source-reservoir coupling effect of coal measures, multiple reservoir types and structures of coal rocks, and coal-measure whole petroleum system and oil-gas distribution. (2)The key fundamental development issues are the flow mechanism and migration regularities of fluid in coal beds. The main research direction include the occurrence state and migration mechanism of coal-rock gas, controlling factors of fluid co-production in coal reservoirs, and stereo development models of complex fluid systems in multiple (thin)reservoirs. (3)The key fundamental engineering issues are the mechanical characteristics and fracture propagation regularities of coal-rocks. The main research directions include the mechanical characteristics and fracturing mechanism of coal rocks, the interaction mechanism between coal rock and fracturing media, and the instability mechanism of coal reservoirs. The analysis of the above-mentioned major scientific issues and key research directions will provide important support for the efficient exploration and development of deep coal-rock gas, and lay the foundation for improving the geological theory of coal-rock gas with Chinese characteristics and coal measure whole petroleum system.

Hydrocarbon exploration has been carried out in the lower assemblage (P and F formations) of the western Bongor Basin for many years without breakthrough. Through in-depth studies of the hydrocarbon accumulation patterns in western Bongor Basin, Well D-2 was deployed and drilled by China National Petroleum Corporation in 2024; oil reservoirs were encountered during drilling in the lower assemblage of Bongor Basin, and the well production during well testing exceeded 200 tons per day, thus determining the exploration potential in the study area. To better summarize this discovery and guide further exploration, a systematic study, which is based on analysis of regional geological setting and exploration history, has been conducted on the tectonic evolution, stratigraphy, sedimentation, and source-reservoir-seal assemblages of the western Bongor Basin. On this basis, the hydrocarbon accumulation models and exploration deployment strategies have been clarified. The results show as follows. (1) The lower assemblage of Bongor Basin developed in the fault depression period. Thick lacustrine mudstones and near-source deltaic sand bodies were developed in the deep depression area, forming favorable lithological combinations. (2) The thick lacustrine mudstones of P and M formations, with high total organic carbon content and good organic matter type, have already entered into the oil generation window. They served as not only the excellent source rocks but also regional seals in western Bongor Basin. (3) Due to tectonic inversion, uplift and denudation during late stages, the deltaic sand bodies of P Formation had a certain porosity despite large burial depths, thus being considered as good reservoirs. The underlying buried hills experienced long-term structural fracture, weathering and denudation, and formed composite reservoirs with the sandstones of P Formation. (4) The Bongor Basin underwent multiple stages of tectonic evolution and inversion, leading to extensive fault development, which allowed oil and gas to migrate along the faults and accumulate in both upper and lower assemblages and buried hills. (5) Based on the characteristics of reservoirs in the lower assemblage, a comprehensive three-dimensional exploration deployment strategy was recommended to explore both shallow and deep formations, structural and non-structural traps, which can achieve breakthroughs in exploration of new strata and new fields. (6) The quartz sandstone above the basement is speculated to be a set of older formation than P Formation, and widely developed in western Bongor Basin, indicating good hydrocarbon accumulation conditions. The exploration breakthrough of Well D-2 has confirmed the resource potential in western Bongor Basin and exploited new reservoirs in Bongor Basin. Moreover, the three-dimensional exploration deployment strategy can provide guidance for oversea risk exploration in the future.

In 2023, a major breakthrough of hydrocarbon exploration had been made in Triassic Yanchang Formation of Hongde area, southwestern Ordos Basin, where Hongde oilfield was discovered with 100-million-ton crude oil reserves. To clarify the geological characteristics and accumulation conditions of Yanchang Formation in Hongde area, the factors and patterns of hydrocarbon accumulation were systematically sorted out by integrating core, well logging, 3D seismic and analytical test data. Moreover, the key technologies for hydrocarbon exploration and development were summarized. The research results show that the braided river delta plain subfacies with distributary channel sand bodies are developed in the Member 8 of Yanchang Formation (Chang 8) in Hongde area, where the reservoirs have large thickness and good physical properties, demonstrating excellent conditions for oil accumulation. The source rock in the Member 7 of Yanchang Formation (Chang 7) in Hongde area is characterized with thin layer, and its total organic carbon (TOC) content is 1.16 % on average, thus indicating a low potential for supplying hydrocarbons. The crude oil in Chang 8 of Hongde area is mainly originated from the high-quality source rock of Chang 7 near the center of lake basin in the eastern part of Ordos Basin. The oil migrates laterally through the three-dimensional transport system composed of faults, fractures and high-quality reservoir sandbodies developed in Yanshanian period, and accumulates in the high parts of paleo-structures. Horizontally, the structural and structural-lithologic reservoirs developed in the west of Hongde area are characterized with the hydrocarbon accumulation mode of "lateral migration and accumulation, reservoirs controlled by fault and uplift, and enrichment characteristics controlled by physical properties of reservoirs". In contrast, the large scale of lithologic reservoirs are developed in the east of Hongde area, with the characteristic of being close to source rocks. During the petroleum exploration and development in Hongde area, the study establishes a series of key technologies focusing on 3D seismic processing and interpretation of depth migration, evaluation of reservoir fluid properties based on the integration of well logging and mud logging, and fracturing transformation for fracture controlling and reservoir stimulation. Those technologies have provided strong supports for new oil and gas discoveries. The breakthrough of Hongde oilfield proves that the area far from the oil source of Tianhuan depression still has the potential for large-scale accumulation. The western margin of Ordos Basin is expected to further implement the petroleum geological reserves of more than 2×10 ⁸t, which is a key field for expanding the extra-source oil and gas exploration.

Sanmenxia Basin is a Mesozoic-Cenozoic fault basin located on the western Henan uplift in the southern margin of the North China block. No petroleum resources and effective source rocks were discovered during previous exploration activities. In recent years, non-profit oil and gas surveys have confirmed the presence of the Paleogene source rocks in the southern margin of the basin and have gained new insights into oil and gas accumulation. To verify the hydrocarbon potential of the basin, Well Yuxiadi 1 was drilled at Hanguguan structural belt. Drilling data of Paleogene Xiao’an Formation reveal that the porosity ranges from 13.43 % to 20.60 %, and the permeability varies from 35.1 mD to 215.5 mD. The drill stem test (DST) results of the lower oil layer of Xiaoan Formation demonstrate a wellhead oil production of 17.52 m ³ under 24-hour intermittent flow conditions (water-free). Through formation testing by layer, combined with mechanical pumping production, the upper, middle, and lower oil layers of Xiaoan Formation have achieved the stable daily oil production of 4.79 m ³, 6.79 m ³, and 15.83 m ³ (water-free), respectively. These results indicate that the Hanguguan structural belt develops the water-free wax-bearing light oil reservoirs characterized with medium-high temperature, medium porosity, medium permeability, medium-shallow depth, and normal pressure. A comprehensive research shows that the oil source of Well Yuxiadi 1 may be derived from the lower Member of Xiaoan Formation and the upper Member of Podi Formation, the mudstone in Liulinhe Formation and its overlying strata serve as regional cap rocks, and normal faults act as the primary hydrocarbon transportation system. Sanmenxia Basin develops four sets of potential source-reservoir-cap assemblages, and it is inferred that its hydrocarbons have the characteristics of "short-distance migration, multiple hydrocarbon accumulation types, and forming reservoir in late stage", and the main accumulation stage is in the Himalayan period. The oil and gas breakthrough in Sanmenxia Basin signifies the emergence of a new petroliferous basin, which is expected to re-attract attention from the industry on medium- to small-sized basins, such as Southern North China Basin and Weihe Basin. This is of certain reference value and guiding significance to the exploration of oil and gas resources in these basins.

A great breakthrough has been made in the exploration and development of coalbed methane in Ordos Basin. To futher clarify the geological regularities governing deep coalbed methane enrichment, a systematic analysis was conducted on the hydrocarbon generation mechanism, hydrocarbon accumulation evolution, lithological assemblages, and differential migration-accumulation characteristics of the No.8 coal seam of Carboniferous Benxi Formation. Moreover, the exploration prospects of coalbed methane and favorable areas for next hydrocarbon exploration were also pointed out. The results show as follows. (1)The No.8 coal seam of Benxi Formation is mainly deposited in swamps covered with mixed vegetation and water, and has been stably developed in the basin. It exhibits the hydrocarbon generation and evolution characteristics of "high gas generation intensity and long duration", and lays a material foundation for coalbed methane accumulation. (2)Deep coal reservoirs are characterized with dumbbell-shaped pore size distribution, of which the reservoir spaces mainly consist of organic matter pores such as cell tissue pores and bubble pores, as well as a large number of well-developed cleavage-fracture networks. Micropores account for more than 60 % of the total porosity, and the pore size distribution is obviously affected by reservoir maturity. The study has clarified the four-stage evolution mechanism of coal reservoirs. (3)Deep coal reservoirs have abundant gas, with the average gas content of 21.7 m ³/t, and the free gas content of 24.48 % on average. The adsorbed gas occurs mainly in micropores, and free gas is stored in macropores and fractures. (4)The lithological assemblages control the three-dimensional differential migration and accumulation of coalbed methane. The coal-mudstone assemblage and the coal-limestone assemblage show good sealing properties, which are conducive to the in-situ retention and enrichment of coalbed methane. On this basis, the paper establishes a deep coalbed methane accumulation mode of "continuous hydrocarbon generation, source-reservoir-accumulation integration, three-dimensional hydrocarbon expulsion, and differential enrichment". Based on the geological characteristics of deep coalbed methane, three favorable exploration areas have been determined in the study area. Additionally, the paper establishes a comprehensive theoretical and technical system encompassing gas enrichment geological theory, geophysical exploration and prediction, full through-type fracture network fracturing technology, and efficient development of geological engineering, which lays a solid foundation for the effective utilization of deep coalbed methane. This understanding is expected to provide significant references for pioneering large-scale exploration and development of deep coalbed methane, promoting exploration breakthroughs in coal reservoirs of other basins in China, and leading the rapid development of the deep coalbed methane industry.

For the current deep shale reservoirs in southeastern Sichuan, the variables such as geostress magnitude, orientation and structure are complex and changeable, and their changing laws are still unclear, thus severely restricting the deployment, implementation, and production efficiency of shale gas exploration and development. This study targets at Longmaxi Formation in key blocks of southeastern Sichuan Basin. Based on the core analyses and the multi-source and multi-dimensional stress responses data from wells, as well as geomechanical analyses and numerical simulations, the study identifies the key geological processes causing geostress disturbances, reveals the mechanical mechanisms and patterns of stress variations and further clarifies their impacts on shale gas enrichment and high production. Results indicate that the southeastern Sichuan Basin can be divided into five regions according to the current geostress, and the stress machanism is mainly presented as strike-slip stress regime. However, in complex marginal zones, the stress mechanism transitions from reverse faulting at shallow depths to strike-slip or normal faulting at greater depths. Folds and faults are identified as the critical external factors causing stress field deflections. Above the neutral surface of the folds at the first and second submembers of Member 1 of Longmaxi Formation, the stress orientation deflects along the fold axis, with the deflection angle controlled by the mechanical properties and deformation intensity of the rock layers, and stress magnitudes decrease. Below the neutral surface, the opposite trend from stress magnitudes and orientation is observed. Moreover, the stress orientation deflects along the fault strike as the distance from the fault decreases, with the stress magnitude decreasing and the differential stress between principal directions increasing. Pore pressure variations mainly influence the minimum horizontal principal stress, with the deflection angles reaching up to 35°. Vertically, as influenced by lithological disturbances, stress values are low for siliceous shales from Wufeng Formation to the first sub-layer of the third submember of Member 1 of Longmaxi Formation, and these weak-stress layers are favorable for fracturing. Comprehensive analysis suggests that below the neutral surface, syncline zones or areas near low-level (Grade Ⅳ or below)NE-trending faults exhibit good fracture sealing and high gas content. The tensile stress disturbance areas exhibit low stress magnitudes and small differential stresses, facilitating the formation of complex fracture networks with high fracture heights and high production rates. These areas are highlighted as priority zones for future shale gas exploration and development. The research results are expected to provide important insights and guidance for the accurate evaluation of geostress fields and optimal selection of sweet spots in deep and structurally complex shale reservoirs.

Significant breakthroughs have been made in exploration of deep coalbed methane (CBM) in China, demonstrating promising prospects for future development. However, due to the complex geological conditions and the high difficulty in CBM development, the existing basic theoretical researches cannot fully explain the deep-seated issues such as the enrichment mechanism and development prospects of CBM. The research progress of global CBM exploration and development shows that the enrichment and accumulation conditions for CBM in deep reservoirs are superior to those in shallow layers. However, the deepening research on deep CBM faces a series of challenges, including multiple genetic types with unclear genetic relationship, undetermined mechanism and key controlling factors of deep CBM accumulation, lack of discrimination criteria for enrichment modes and critical accumulation conditions of CBM, difficulties in predicting and evaluating high-yield sweet spots and fully applying the exploration experience of deep CBM in the eastern Ordos Basin to other regions. To solve the problems, the Whole Petroleum System (WPS) theory and the hydrocarbon accumulation model with dynamic field are introduced to expound the differences, correlations, and united symbiotic relationships between conventional and unconventional coal-formed gas reservoirs in petroliferous basin, in an attempt to provide new theoretical and methodological guidance for the prediction and evaluation of high-yield and rich CBM areas. The preliminary research results on CBM in major petroliferous basins of China indicate that during the evolution process of the Coalbed Whole Petroleum System (CWPS), the free hydrocarbon dynamic field is conducive to the enrichment and accumulation of CBM in an adsorbed state, whereas the confined hydrocarbon dynamic field is conducive to the enrichment of free gas in coal seams. In the free hydrocarbon dynamic field of coal seams, the amount of adsorbed gases increased with the increasing of burial depth and decreased after reaching a peak, while the free gas content has begun to increase. In the confined hydrocarbon dynamic field, the amount of gas trapped in coal seams in a free state continues to increase with the increasing of burial depth, and then decreases until it tends to disappear after reaching its peak. Vertically, the lower part of the free hydrocarbon dynamic field and the upper part of the confined hydrocarbon dynamic field (organic matter accumulation degree ranges from 0.50 % to 2.75 % ) are most favorable for multiphase enrichment and high production of CBM. The buried depths of the high heat flow basins of the eastern China, medium heat flow basins of the central China, and low heat flow basins of the western China are from 1 000 m to 3 600 m, 1 500 m to 7 500 m, and 3 000 m to 8 500 m, respectively. In these favorable fields, the total amount of coal resources is about 80 596×10 ⁸t, and the in-situ and recoverable resources of CBM are 115.91×10 ¹²m ³ and 56.5×10 ¹²m ³ respectively, showing broad prospects for development.

Coupled thermo-hydro-mechanical flows in complex fractured rock occur widely in unconventional and deep reservoir development scenarios. Coupled simulations of complex fracture networks will produce huge computational cost. Parallel computing technology is the effective method to achieve high-resolution simulations of complex discrete fractures. In this paper, massively parallel numerical simulation technology for thermo-hydro-mechanical coupling using general embedded discrete fracture model on unstructured grids is introduced. Firstly, a parallel solution of embedded discrete fracture model is achieved based on the domain decomposition method; and the way to decompose two independent matrix and fracture systems is introduced for unstructured grids. the existing embedded discrete fracture model by using two independent matrix and facture grid systems, and this approach significantly enhance the simulation ability of the EDFM for a complex 3D discrete fractures system. Secondly, for the thermo-hydro-mechanical coupling problem, the finite volume method is adopted to discretize compositional flow, heat transform and poro-mechanical equations uniformly, a parallel sequential implicit method is employed to solve the nonlinear coupling problem. Finally, this simulator is validated against two analytical model, and it is used for a multi-layer shale gas reservoir three-dimensional simulation and a deep high-temperature fractured reservoir simulation, and the parallel computational performance and scalability are analyzed at different parallel scales. The proposed methods can achieve high-resolution simulations of discrete fracture networks in practical engineering, and we obtain a great parallel performance and scalability at different scales, this simulator can be an efficient tool for the design and analysis of energy development in fractured rocks.

The research of Whole Petroleum System (WPS) theory, emerging as a frontier field in the petroleum and natural gas geology at present, has attracted extensive attention from both the petroleum industry and academic community since its proposal. Expanding beyond the traditional concepts of petroleum systems, the research of WPS theory involves the geological regularities and resource development of unconventional oil and gas, representing an evolution and development of the petroleum system theory. This paper elucidates the fundamental principles of WPS theory, and summarizes the research progress during its development, including:(1) research advances and exploration practices of the Permian WPS in Junggar Basin; (2) the differential enrichment in the sequential distribution pattern of conventional and unconventional oil and gas, as well as the hydrocarbon accumulation and enrichment mechanism controlled by "source-reservoir coupling"; (3) hydrocarbon migration models in WPS; (4) analysis of WPS in sedimentary basins with the development of multiple sets of source rocks; (5) research and development of the new-generation basin modeling technology and genesis-based hydrocarbon resource appraisal methods on the basis of WPS theory.

Intelligent drill bits are one of the core components of future intelligent drilling systems. By integrating sensors capable of stable performance under high-temperature and high-pressure conditions, adaptive control algorithms, and optimized drill bit structures, these advanced drill bits enable real-time downhole condition monitoring, dynamic adjustment, and precise execution, thereby improving drilling efficiency. As oil and gas exploration expands to ultra-deep formations and complex, unconventional reservoirs, drill bits face increasingly harsh geological conditions. Traditional drill bits, under high-temperature and high-pressure environments and complex geological settings, suffer from challenges such as rapid wear, short service life, and high drilling costs that urgently demand the development and application of high-performance drilling technology. This further highlights the importance of intelligent drill bits as a pivotal future direction for improving drilling speed and efficiency. By systematically summarizing and analyzing the current research, core technologies, and product advancements in intelligent drill bits, such as multi-parameter sensing technology capable of withstanding extreme downhole environments, machine learning- and deep learning-based adaptive control algorithms, and progress in drill bit structure and control mechanisms, this work reveals the potential of intelligent drill bits in modern drilling operations. Typical case studies from both domestic and international projects demonstrate the advantages of intelligent drill bits in enhancing rate of penetration, extending bit life, and reducing unplanned downtime. Looking ahead, research on intelligent drill bits will move toward higher autonomy, broader adaptability, and deeper integration with digital platforms. They are poised to play an even greater role within the intelligent drilling technology framework.

Marine carbonate rocks are an important part of onshore oil-gas exploration in China. A series of exploration breakthroughs have been made in recent years, which play an important role in ensuring energy security. Based on the recent achievements of petroleum exploration in the three craton basins of China, this paper systematically reviews the latest advances in the theory and technology of marine petroleum accumulation in China, and forms the following understandings and advances. (1)Significant achievements have been made in marine oil and gas exploration in three major cratons. For example, two ultra-deep fault-controlled giant oil-gas fields with 1 billion tons of geological reserves have been discovered in in Fuman and Shunbei areas of Tarim Basin, the trillion cubic meters of large gas field has been initially established in the northern slope of central Sichuan Basin, and several new exploration zones have been discovered in the dolomites from the Ordovician pre-salt submember 6 of Member 5 of Majiagou Formation to Member 4 of Majiagou Formation in Ordos Basin. (2)A series of important advances have been made in reservoir prediction and reservoir tracing. For example, the study of Mg isotope tracing dolomitization process has promoted the development of genetic mechanism analysis of dolomite. By analyzing the spatial characteristics and retention mechanism of carbonate reservoirs such as deep fault solution, it is clear that the lower limit of exploration depth of carbonate cavern reservoir is far beyond the drilling depth. Advances in fault solution characterization, reef bank characterization, reservoir transparency and constant volume characterization have promoted efficient exploration and significantly improved drilling success. (3)Remarkable progress has been made in hydrocarbon accumulation process and fluid tracing. For example, technological innovations such as light hydrocarbons, isotopes and biomarkers have been used to improve the effectiveness of hydrocarbon source correlation. Important progress has been made in the dating techniques of accumulation with various methods, which can effectively guide the determination of favorable accumulation areas. With the development of hydrocarbon reservoir evolution and reconstruction technology, the marine carbonate reservoir formation process under complex tectonic background has been transformed from qualitative analysis to quantitative study. (4)By systematically analyzeing the accumulation law and main controlling factors of marine carbonate oil and gas in China, it is clear that marine oil and gas reservoirs in China are characterized by large area distribution, large amplitude of oil and gas column, high production well controlled by source rock and large-scale reservoir, oil and gas richment controlled by strike-slip fault, and ultra-long life of hydrocarbon reservoirs. (5)The deep and ultra-deep resource potential of marine basins in China is huge, with the potential development of 5 trillion cubic meter gas fields and 1 billion-ton oilfield. The development of marine oil and gas theory enriches the accumulation theory of small craton basins, promotes the progress of petroleum geology theory, and plays an important role in the practice of oil and gas exploration.

The flow field and pressure field of low-permeability reservoirs are changed by dynamic fractures, significantly affecting the swept volume of water flooding. Comprehensively using the data of imaging logging, core analysis, rock mechanics experiments, production performance, and dynamic monitoring, the paper analyzes the characteristics and controlling factors of dynamic fractures in four typical ultra-low permeability reservoirs in the Ordos Basin, including Jing’an, Ansai, Xifeng, and Huaqing. Based on their geneses, dynamic fractures can be classified into two types, i.e., being formed by exceeding the rock formation’s fracture pressure and activated by natural fractures. According to impacts on the water cut of oil wells, they can be divided into unidirectional and multidirectional dynamic fractures. The evolution of dynamic fractures undergoes four stages:rapid growth to a certain scale, a fixed length, and a maximum length, and contraction stage; the extension direction of fractures is primarily controlled by the current maximum horizontal principal stress direction. The development intensity of dynamic fractures is influenced by factors such as matrix permeability, natural fractures, rock brittleness index, as well as development technology policies. The lower matrix permeability, more developed natural fractures, higher brittleness index, and stronger water injection development technology policies are easier for the formation of dynamic fractures, and the transition from unidirectional to multidirectional fractures. Dynamic fractures significantly affect the water cut increase regularity of oil wells and the distribution of remaining oil.

The Sinopec "Shendi-1" project in Tarim Basin is characterized by the exploration and development of ultra-deep strike-slip fault-controlled oil and gas reservoirs. Yuejin block, located on the southern margin of Shaya uplift and adjacent to the northern part of the Shuntuoguole low uplift, develops three oil and gas enrichment belts primarily controlled by NE-trending strike-slip faults. This region is distinguished by its ultra-deep target formations (>7 000 m), high reservoir heterogeneity within fault zones, and surface coverage by poplar forest reserves. Despite breakthroughs in exploration since 2012 and more than a decade of production, there are still a large scale of untapped reserves. Great challenges are encountered in the geological engineering practices, and it is required to deepen the understandings of hydrocarbon accumulation and distribution patterns and achieve the efficient production of ultra-deep reservoirs beneath protected areas. Based on analyses of the regional fault-controlled hydrocarbon accumulation conditions and faults, as well as three-dimensional reservoir characterization in the well area, the geological study has identified the distribution of oil and gas reservoirs and untapped reserves, thus optimizing target selection. From an engineering perspective, solutions to the difficulties of ultra-deep layer and ultra-long displacement during well completion mainly include optimization of borehole trajectory design and control, borehole cleaning, stable large-inclination and long-open borehole, as well as the technologies of accelerating drilling operations and safe casing running. These techniques can enable the successful completion of Well Yuejin 3-3XC, which has obtained high-yeld oil and gas flow during testing. The well has set a domestic record with a measured depth of 9 432.55 m(slant)/ 7 212.53 m (vertical)and a horizontal displacement of 3 439.34 m, marking it as a landmark case in geological and engineering practices for complex ultra-deep reservoirs.

In the central part of southern margin of Junggar Basin, Well Hutan1 at the Huxi anticline and Well Tianwan1 at the Dongwan anticline have obtained high-yield oil-gas flows from the Lower Cretaceous to Upper Jurassic reservoirs at a burial depth from 7 300 m to 8 100 m, which has made a major breakthrough in ultra-deep oil-gas exploration, showing a good prospect for hydrocarbon exploration in the ultra-deep lower assemblage of the thrust belt in the southern margin of Junggar Basin. The source and genesis of oil and gas in reservoirs are vital for understanding the hydrocarbon accumulation regularity and controlling factors as well as hydrocarbon resource potential in the ultra-deep lower assemblage. Based on the fine analysis and study of physicochemical and geochemical characteristics of ultra-deep condensate oil at the Huxi anticline and Dongwan anticline, the source and genesis of condensate oil was explored. The results show that the condensate oils in Well Hutan1 and Well Tianwan1 are characterized by low densities, low viscosities, low wax contents and freezing points, and high contents of alkanes, naphthenes and aromatics with low carbon numbers, as well as rich contents of n-alkanes with middle-high carbon numbers, and thus they are classified as light oils with low densities, low wax contents and low freezing points. The condensate oils have a light carbon isotope composition, the δ ¹³C value of whole oil is from -30.7 ‰ to -28.6 ‰, and nC ₉-nC ₃₀ n-alkanes have light carbon isotope compositions, especially nC ₁₅-nC ₃₀ n-alkanes, and the corresponding δ ¹³C value is from -29 ‰ to -33 ‰ . The condensate oils have a relatively high abundance of isoprenoids which show equalizing pristine (Pr)and phytane (Ph)contents or having a dominant Ph content, with Pr/Ph ratio from 0.69 to 1.27. In the compositions of biomarkers, C ₂₇, C ₂₈ and C ₂₉ steranes and diasteranes are abundant, C ₃₀ methylsteranes are relatively abundant, and C ₂₇, C ₂₈ and C ₂₉ ααα-20R steranes show a V-shaped distribution. The 20S/(20S+20R)ratio and ββ/(αα＋ββ)ratio of C ₂₉ sterane are greater than 0.45 and 0.60, respectively. The condensate oils have abundant tricyclic terpenes showed a near-normal distribution with C ₂₁ terpane as the highest, have very abundant 18α(H)- trisnorneohopane (Ts), C ₂₉Ts, C ₂₉ diahopane and C ₃₀ diahopane, and have a high content of gammacerane with an isomers, indicating that the condensate oil is generated by organic matters deposited in semi-saline to saline lacustrine clay-rich environment. As a whole, the condensate oils from Well Hutan1 and Well Titanwan1 in the ultra-deep reservoir of the central part of southern margin of Junggar Basin have very similar geochemical characteristics with the crude oil/condensate oil from the middle to shallow reservoirs at the Horgos anticline. The condensate oils are derived from the lacustrine source rocks of the Lower Cretaceous Qingshuihe Formation, and are formed by later gas invasion/washing in the early oil reservoirs.

Continental shale oil reservoirs in China are characterized with abundant clay minerals, well-developed laminae, and complex and diverse lithologies. The crude oil is accumulated in organic pores, inorganic pores, and bedding fractures, featuring multi-scale distributions. Therefore, it is very difficult to accurately characterize the flow mechanism of shale oil reservoirs and precisely predict their apparent permeability. Based on the nano-confinement effects and different flow boundary conditions, flow velocity equations were derived for organic pores, inorganic pores, and micro-fractures. Meanwhile, considering the distribution characteristics and arrangement-combination patterns of pores and fractures, a comprehensive apparent permeability prediction model for the fractured continental shale reservoirs was established. Taking the Jiyang depression shale oil reservoir as a case study, a systematical analysis was conducted on the effects of pore-fracture combination patterns, total organic carbon (TOC)content, nano-confinement effects, and fracture distribution characteristics on the apparent permeability of shale reservoirs. The results show that in continental shale oil, the low-velocity nonlinear flow behavior is mainly controlled by the liquid-solid interfacial slippage effects and adsorption boundary layer effects. The slippage effects in organic pores help increase the flow velocity, whereas the adsorption boundary layer effects in inorganic pores play a role in flow inhibition. The arrangement patterns of pores and fractures exert significant influences on the apparent permeability, with particularly pronounced nonlinear characteristics observed in serial arrangement, indicating the great impact of TOC content on flow behavior. Furthermore, under low-pressure gradient conditions, macropores such as inorganic pores and micro-fractures are the main flow channels, and the apparent permeability of micro-fractures increases nonlinearly with the increase in fracture density. The increase in fracture aperture significantly enhances its permeability, especially in laminated shale reservoirs.

With the continuous advancement of oil and gas exploration, the reserves of medium-shallow reservoirs are gradually decreasing, as a result of which deep-layer oil and gas development has become a critical strategic direction for China’s energy succession. However, deep drilling operations are subjected to the high-temperature and ultra-high-temperature environments, which can induce failures in drilling fluids, drilling tools, and logging-while-drilling equipment, thereby severely compromising operational safety and efficiency. To address the multiple challenges posed by high temperatures, wellbore cooling technology has emerged as a focal point in the research and development of deep drilling technologies. By lowering the borehole temperature, this technology can improve drilling fluid performance, ensure equipment stability and extend service life of critical components. This review summarizes latest advancements in the research of wellbore cooling, involving drilling fluid cooling technologies, wellbore thermal insulation techniques, and drill pipe thermal insulation technologies, and further systematically analyzes their working principles, application scenarios, cooling effects and mechanisms. Additionally, the challenges and future development directions of these technologies are explored in depth, aiming to provide technical references for safe and efficient deep drilling operations and support the sustainable development of deep oil and gas resources.

The water depth achieved by deepwater drilling in the South China Sea has been increasing annually, and three wells have been already drilled at a depth of more than 2000 meters, and the maximum depth exceeds 2600 meters. The increasing water depth results in the prolonged tripping time for drilling tools during operations in deep water, particularly ultra-deep water, and operational safety risks are increased due to the long-time exposure of drilling tools to deep water for extended periods. To reduce the frequency of tripping in deep water and enhance the efficiency of operations in shallow formations, the paper proposes an efficient "three-in-one" drilling technology for shallow formations in deep water, which is based on the analysis of regional shallow geological and mechanical parameters and practical experience from over 40 deepwater wells drilled in the South China Sea. Through the optimization of wellbore structure design, drill string and tool optimization, jet bit design, and subsea wellhead stability analysis, the "three-in-one" efficient drilling operation mode for shallow formations in deep water has been developed. This technology allows for the completion of the whole wellbore comprising surface conductor, surface casing, and intermediate casing in a single trip through innovative designs of drilling tools and tubular structure, thus simplifying operational procedures, significantly enhancing operational efficiency, and achieving the goal of deepwater drilling. The technology was applied in 10 deepwater wells in the deepwater areas of the South China Sea, such as Baiyun, Liuhua, Liwan, and Kaiping. The application results indicate that the "three-in-one" efficient drilling technology for deepwater shallow formations can reduce tripping operations by two trips compared to conventional jetting methods, by which the drilling efficiency in deepwater shallow formations can be increased by over 50 % , and the exposure time and frequency of drilling tools in deep water is also reduced.