The types and configurations of source-reservoir coupling can be identified based on the shale oil and gas source-reservoir coupling, which provides a basis for the determination of ideas about shale oil and gas exploration and the efficient exploration and development of shale oil and gas. However, until now, shale oil and gas have not introduced by any scholars into a unified evaluation system for the classification of source-reservoir coupling types, which to some extent restricts the exploration and development process of shale oil and gas. In view of this, based on analyzing the source-reservoir configuration characteristics of typical marine and terrestrial shale oil and gas reservoirs in China, the source-reservoir coupling relationship of shale oil and gas is divided into three categories. Moreover, this study makes clear the geological connotations of different source-reservoir coupling types and their mechanisms controlling oil and gas enrichment, and proposes an efficient exploration approach based on the overall evaluation of shale oil and gas in China. The research results suggest that the source-reservoir coupling types of shale oil and gas can be divided into three categories: source-reservoir separation, source-reservoir coexistence, and source-reservoir integration. Specifically, the migration distance of source-reservoir separation hydrocarbons is above meter scale, and the near-source oil and gas forms sweet spots, represented by the Lower Cambrian Qiongzhusi Formation in Sichuan Basin, the first and second submembers of Member 7 of Triassic Yanchang Formation in Ordos Basin, and the Permian Lucaogou Formation in Jimusar sag of Junggar Basin. The source-reservoir coexistence is characterized with the multi-source supply of hydrocarbons and the coexistence of source and reservoir, of which hydrocarbons are migrated into the nearby advantageous reservoirs to make them oil-bearing as a whole, represented by the Member 2 of Permian Wujiaping Formation in Sichuan Basin, the Jurassic Lianggaoshan Formation in Sichuan Basin, and the Member 4 of Paleogene Shahejie Formation in Jiyang depression of Bohai Bay Basin. The source-reservoir integration indicates that the source rock and reservoir are in the same stratum, and hydrocarbons undergo micro migration within the stratum, represented by the Ordovician Wufeng Formation and Silurian Longmaxi Formation in Sichuan Basin and the Cretaceous Qingshankou Formation in Songliao Basin. Sedimentary environment, biogenic silica, thermal maturity, and hydrocarbon generation/expulsion efficiency are the core elements that affect the shale oil and gas source-reservoir configuration and furtherly control the enrichment of shale oil and gas. Taking the typical shale oil and gas reservoirs in China as an example, the paper furtherly clarifies the exploration levels and ideas under the vertical multi-type source-reservoir coupling configuration at different levels of maturity. The research results are beneficial for quickly identifying and optimizing favorable intervals of shale oil and gas, providing an important scientific basis for the efficient exploration and development of shale oil and gas in China.

As the super and most petroliferous basin in China, Songliao Basin has achieved strategic breakthroughs in the exploration and evaluation of Gulong shale oil, of which the potential and scale of resources remain unclear. Based on the extensive geochemical data including total organic carbon (TOC), rock pyrolysis, vitrinite reflectance and pressure-reserved core, in combination with logging and production data, a systematic evaluation was conducted on various types of shale oil, primarily in Qijia-Gulong sag. A classification scheme using organic matter maturity and reservoir type as key indicators was developed for shale oil in Songliao Basin. As a result, grading standards for shale oil were established based on the key parameters such as TOC content, oil content, effective porosity, and oil saturation. A shale oil resource evaluation method was created, involving the key technologies such as precise evaluation of oil content, light hydrocarbon recovery and calibration of recoverable coefficient. Based on dynamic production data, the geological resource potential of shale oil under current technological conditions was assessed, achieving the predictive analysis of resource recoverability. The comprehensive evaluation indicates that Qijia-Gulong sag contains medium- to high-maturity shale oil resources of 107.73×10 ⁸t (including 42.08×10 ⁸t of Class Ⅰ resources and 33.67×10 ⁸t of Class Ⅱ resources), with technically recoverable resources exceeding 8×10 ⁸t. Additionally, the geological resources of dissolved gas are estimated to be 1.75×10 ¹²m ³, and the technically recoverable resources amount to 0.13×10 ¹²m ³. The resource evaluation results suggest that the favorable shale oil resources in Songliao Basin are mainly distributed in Qijia-Gulong sag, as being the essential strategic replacement resource. With future advancements in development technologies, the recoverable potential of shale oil is expected to increase significantly.

The Whole Petroleum System theory establishes the unified accumulation mechanism, enrichment regularity and geodynamic control conditions of conventional and unconventional oil and gas. Tight oil and gas are crucial components of the Whole Petroleum System. This paper reviews the development history of tight oil and gas, looks forward to the resource prospect of tight oil and gas, describes the geological characteristics of typical tight oil and gas reservoirs in China, and reveals the accumulation mechanism and enrichment regularity of tight oil and gas from the perspective of the Whole Petroleum System theory. The research results are as follows. (1) China’s tight oil and gas resources have broad prospects and great development potential, and great achievements have been made in the field of exploration and development, but there are still great challenges in the future, including geological theory, engineering technology and enhanced oil recovery technology. (2) Both physical property and accumulation process of tight oil and gas reservoir are between those of conventional oil-gas and shale oil-gas. The complex capillary network composed of pore throats within the tight reservoir is the key to the self-containment of tight oil and gas. (3) Oil and gas resources in different petroliferous basins in China show distinct differential enrichment characteristics. The Ordos Basin is a super tight oil and gas enrichment basin. (4) Based on the source-reservoir coupling relationship, tight oil and gas reservoirs can be classified into far-source type, near-source type, and intra-source type.

In 2024, according to the exploration idea of "fracture-cavity gas reservoirs formed after fracture reconstruction of carbonate rock", three exploration wells were deployed in the Ordovician subsalt area of Ordos Basin to test gas and obtain high-yield industrial gas flow, revealing good exploration potential for the subsalt fracture-cavity gas reservoirs. However, the high-yield and enrichment regularity and comprehensive prediction model of fracture-cavity gas reservoirs are still unclear, making it difficult to determine favorable zones and achieve drilling targets. Therefore, based on cores, thin sections, seismic data, logging data, and production performances, this paper investigates the basic geological conditions and high-yield and enrichment mechanism of the Ordovician subsalt fracture-cavity gas reservoir in Ordos Basin. The results show as follows. (1)The subsalt fracture-cavity gas reservoirs have dual source hydrocarbon supply conditions of the Upper Paleozoic coal measures and Lower Paleozoic marine source rocks, of which the latter are the main source rocks, with the maximum hydrocarbon generation intensity of 1.2×10 ⁸m ³/km ² and sufficient hydrocarbon supply capacity. (2)The reservoir spaces of the subsalt fracture-cavity units are mainly composed of fractures and dissolution pores developed along the fracture zone, mixed with a small amount of matrix intergranular pores. The average porosity can reach more than 10 %, and the average permeability can reach up to 10 mD, demonstrating good reservoir performance. (3)The subsalt faults and associated fracture systems can not only improve the physical properties of dolomite reservoirs and form fracture-cavities with good reservoir performance, but also communicate source rocks with reservoirs, thus providing effective channels for oil and gas to accumulate in fracture-cavity reservoirs. (4)The thick layer of gypsum-salt rock developed in the 6th submember of Member 5 of Majiagou Formation serves as the regional cap rock for the fracture-cavity gas reservoir. The gypsum-salt rock developed in the 10th submember of Member 5 of Majiagou Formation, as well as the Member 3 of Majiagou Formation, serves as the cap rock overlying on the gas reservoir. Moreover, the tight carbonate rocks around the fracture-cavities form lateral sealing, and those good preservation conditions are conducive to natural gas enrichment and formation of fracture-cavity gas reservoirs. Through comprehensive evaluation, it has been preliminarily determined that the favorable exploration area for the Ordovician subsalt fracture-cavity gas reservoirs is about 2.5×10 ⁴km ², and the estimated natural gas reserves can reach 5000×10 ⁸m ³, indicating great exploration potential. It is an important direction and real target for exploration of the Ordovician subsalt natural gas.

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 %.

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.

To make clarify the geological characteristics and resource potentials of tight gas in northern Songliao Basin, the classification and optimization of favorable exploration areas for tight gas were carried out based on comprehensively using the seismic, coal, geological and geochemical data and analyzing the hydrocarbon accumulation conditions in Shahezi Formation, including tectonic evolution, sedimentary system, source rock, favorable reservoir and abnormal pressure. The results show as follows. (1)The tight gas is mainly concentrated in Shahezi Formation in the deep fault depressions. Influenced by paleo-geomorphology and fault activity, fan delta and braided river delta sediments were formed on the steeps and the gentle slopes of deep fault depressions, and huge pebbly sandstone was deposited, which is the main gas-bearing horizon of tight gas. (2)The coal rock and dark mudstone have high organic matter abundance and are in the high mature-over-mature stage, which provides sufficient material guarantee for the formation of tight gas. Under the joint control of sedimentation and dissolution, high quality reservoirs with intragranular and intergranular solution pores of feldspar as the main reservoir space are developed in tight sandstone, which is the sweet spots for tight gas exploration. Hydrocarbon generation from organic matter results in the 29 MPa to 45 MPa pressurization, forming a large-scale continuous overpressure storage chamber in the middle of the depression, which is the main driving force for tight gas charging. (3)Homogenization temperature simulation of hydrocarbon-bearing fluid inclusions and burial history of strata show that the accumulation time of Shahezi Formation tight gas lasted from the end of Qingshankou Formation to the sedimentary period of Nenjiang Formation (91-72 Ma), and there were two accumulation processes in the end of Qingshankou Formation and the Nenjiang Formation. (4)Based on the sedimentary system, reservoir thickness and physical property conditions, the tight gas is divided into two favorable exploration types, i.e., thick-layer and interlayer. Moreover, four favorable exploration zones have been identified, including Anda, Xuxi, Xudong and Xunan, and the amount of tight gas resources is estimated to be about 4 224.10×10 ⁸m ³.

The Member 6 of Triassic Yanchang Formation is one of the key contributors to the oil reserves and crude oil production in Ordos Basin, and it is also the earliest oil production reservoir in the continental areas of China. This study aims at revealing the formation mechanism and distribution pattern of tight oil in the Member 6 of Yanchang Formation. Based on analyzing the data of drilling, mud logging, well logging, and core samples from 1 505 prospecting wells in southeastern Ordos Basin, the paper systematically studies the formation mechanism and enrichment laws of tight oil in the Member 6 of Yanchang Formation, and presents three palaeo-geomorphic units, including steep and gentle paleoslopes, and paleolake bottoms, as well as more than ten types of secondary palaeo-geomorphic units. Based on the comprehensive analysis of palaeo-geomorphology and hydrocarbon accumulation conditions, in combination with the physical simulation experiments of crude oil migration and accumulation, four typical accumulation assemblages have been identified, i.e., the dual-source hydrocarbon supply with the medium to poor reservoirs at the end of distributary channels on gentle slopes, the single-source hydrocarbon supply with the poor to medium reservoirs under scattered gravity flow at the lake bottom, the dual-source hydrocarbon supply with the medium to good reservoir assemblages under gravity flow at the bottom of steep slopes, and the single-source hydrocarbon supply with the damaged and adjusted good to medium reservoirs on steep slopes. The genesis mechanism and accumulation process of tight oil reservoirs in delta front and gravity flow are revealed. Then it is clarified that the scale of high-quality reservoirs in near-source effective traps is the key to the enrichment and high yield of tight oil. Further, the paper proposes the differential distribution and accumulation mode of tight oil be controlled by the heterogeneous source-reservoir-cap assemblages under diverse paleogeomorphology setting, improves the hydrocarbon exploration strategies applied in typical oilfields, and puts forwards the unconventional oil exploration strategies such as the expanding exploration along the bottom of steep slopes, three-dimensional exploration at the lake bottom, detailed exploration of multiple wells in gentle slopes, and effective traps exploration in the structurally adjusted areas of steep slopes. Finally, the research focuses on exploring the four basic geological conditions for the formation of large-scale continental tight oil fields, i.e., tectonic action, provenance, paleo-geomorphic units, and preservation condition. In the Member 6 of Yanchang Formation in southeastern Ordos Basin, the cumulative geological reserves of tight oil amount to 1.885 522×10 ⁸t, the cumulative crude oil production in the past three years is 16.72×10 ⁴t, and the first 100 million-ton-level integrated tight oil field, i.e., Huangling oilfield, has been discovered and established. It is expected that the research results have a positive impact on the development of geological theories in terms of unconventional petroleum accumulation, as well as the exploration and development practices in the terrestrial lacustrine basin of China.

Jizhong depression is the main crude oil production base in Bohai Bay Basin, boasting of rich and diversified oil and gas resources. After more than 50 years of exploration and development, the proved reserves of conventional oil and gas are very considerable. However, there is an increasing difficulty in hydrocarbon exploration. As a result, the unconventional oil and gas resources will become the key target for the next large-scale discovery. Through systematically summarizing the geological conditions for the formation of tight oil and gas in the faulted lacustrine basin of Jizhong depression, it has been determined that there are a number of sedimentary and subsidence centers and oil-generating sags and troughs in the depression, where the high-quality hydrocarbon source rocks together with fan sedimentary sands of the adjacent delta front, fan delta, and nearshore subaqueous fan or lacustrine facies carbonate reservoirs form a dominant configuration, thus providing favorable conditions for the efficient charging and accumulation of tight oil. According to the source-reservoir configuration, the tight oil and gas reservoirs in Jizhong depression can be divided into two types, i.e., extra-source and intra-source hydrocarbon reservoirs, generally showing the characteristics of large-area continuous distribution, complex lithology, non-homogeneity and near-source reservoirs. The study suggests that three types of tight oil and gas reservoirs are mainly developed in the Paleocene of Jizhong depression, including tight oil and gas conglomerate reservoirs in nearshore subaqueous fan of the steep belt (Type 1), and in fan delta of the slope belt (Type 2), as well as delta-front and shore-shallow lake facies tight oil and gas reservoirs at the peripheral of sags and troughs (Type 3), which have superior hydrocarbon accumulation conditions, demonstrating a large resources potential and a broad exploration prospect. Among them, Type 1 is mainly developed in single-faulted dustpan-like sags such as Langgu sag, Wuqing sag, Jinxian sag, and Shulu sag, with the distribution area of 476 km ², the predicted oil resource of 0.53×10 ⁸t, and the estimated natural gas resource of 1 460×10 ⁸ m ³. It is the main zone where breakthroughs have been made in hydrocarbon exploration of deep sags in the steep belt of Jizhong depression. Type 2 is mainly developed in Shuolu sag, Jinxian sag, and Baxian sag with narrow and steep slopes in Jizhong depression, with the distribution area of 700 km ² and the predicted resources of 2.6×10 ⁸t. It demonstrates a realistic battleground for the realization of large-scale efficient discovery. Type 3 is mainly distributed in the lower member of the first Member of Shahejie Formation of Raoyang sag, with the distribution area of 1 214 km ² and the predicted resources of 1.8×10 ⁸t, indicating a favorable direction for the strategic succession.

Tight oil and gas is an important unconventional resource, equivalent to ultra-low permeability and ultra low permeability oil and gas in traditional low-permeability oil and gas. At present, tight gas is the most productive in China, and has achieved an annual gas production of over 600×10 ⁸m ³ in 2023; tight gas are mainly enriched in Ordos Basin and Sichuan Basin. The tight oil production reached 1 193×10 ⁴t in 2023; the main production areas include Member 6 of Yanchang Formation (Chang 6 Member) and Member 8 of Yanchang Formation 8 (Chang 8 Member) in Ordos Basin, the Permian-Triassic tight glutenite reservoir in Mahu sag of Junggar Basin, and Fuyu tight oil reservoir in Songliao Basin. This paper discusses the temporal and spatial distribution characteristics of tight oil and gas resources from the distribution and geological evolution of sedimentary basins in China, tight oil and gas resources are mainly distributed in the Carboniferous-Permian, Triassic-Jurassic-Cretaceous, and Paleogene-Neogene periods. China’s tight oil and gas are mainly accumulated near the source rocks; horizontally, they are primarily distributed within and near source rocks in slope and trough areas; vertically, they are mainly distributed in tight reservoirs above, below, and between source rocks. According to the geological characteristics, reservoir formation mechanisms and conditions, distribution patterns, and main controlling factors of tight oil and gas in China, three accumulation modes of tight sandstone gas are determined as below:continuous tight deep basin gas, quasi continuous tight gas, and tight conventional trap gas; there are three accumulation modes for tight oil:source-reservoir separated and far-source accumulation, source-reservoir connected and near-source accumulation, accumulation in reservoirs alternated with adjacent source rocks. During the 15th Five Year Plan period(2026-2030), in the fields of southern Ordos Basin, central-western Sichuan Basin, deep Songliao Basin, and Ahe Formation in Tarim Basin, the newly proved geological reserves of tight gas are expected to exceed 2×10 ¹²m ³; and in the Chang 6 and Chang 8 members of Yanchang Formation in Ordos Basin, Fuyu oil layer of Quantou Formation in Songliao Basin, Mahu Fengcheng Formation in the western depression of the Junggar Basin and other fields and zones, the proven geological reserves of tight oil is expected to increase by (18-20)×10 ⁸t.

The tight sandstone oil and gas in Tarim Basin is characterized with wide exploration area, large-scale reserves, and low proportion of proved reserves, and also faces the problems of hydrocarbon source distribution, complex reservoir prediction, as well as oil and gas accumulation mode, which restrict the overall profitable exploration and development of tight reservoirs. Based on outcrop profile, experimental analysis, geophysical and well logging data, a detailed analysis is performed on the new fields and resource potential of tight sandstone reservoirs in forelands and basins. The results show that the new fields of tight sandstone oil and gas in Kuqa depression are dominated by the medium to thick gravel sandstones of the Cretaceous Yageliemu Formation and the medium to thick sandstones of the Middle Jurassic Kezilenuer Formation, forming the structural-lithologic reservoirs longitudinally adjacent to high-quality source rocks in the Upper Member of Pusige Formation, and transversely adjacent to structural-lithological hydrocarbon reservoirs in the hydrocarbon generating center of Awati sag. In the north wing of the Kelasu structural belt and the Dongqiu-Dina structural belt in Kuqa depression, the cumulative area of tight oil and gas traps is 1 830 km ² for tight oil and gas, and the predicted natural gas resource is 16 625×10 ⁸m ³. The favorable trap area of tight oil and gas in Kedong structural belt of southwest depression of Tarim Basin is 301 square kilometers, and the natural gas geological resources are estimated to be about 2 930×10 ⁸m ³ and the condensate oil geological resources are about 2×10 ⁸t. The favorable area of tight oil and gas in Kepingtage Formation in the northwest margin of Awati sag is 4 320 km ², the natural gas resources are estimated to be 7 076×10 ⁸m ³, and oil resources are 7 817×10 ⁴t. New fields and resource potential provide a solid foundation for sustained and efficient hydrocarbon exploration.

With the continuous growth in global energy demand, the exploration and development of tight oil and gas resources, as a crucial component of unconventional oil and gas, is increasingly gaining attention. Tight reservoirs have the characteristics of low permeability and complex pore structures, posing high demands on drilling fluid technology. This paper reviews the challenges and current research status of drilling fluid technology for tight oil and gas, elaborats on the action mechanisms and performance characteristics of water-based drilling fluids, oil-based/synthetic-based drilling fluids, and drilling fluid lost circulation prevention and plugging technologies, identifies critical issues in the current drilling fluid technology, and proposes future directions for the technology. For the water-based drilling fluid technology, attention should be paid to improving suspension and carrying capacity, enhancing borehole wall stability, and maintaining rheological stability. For the oil-based/synthetic-based drilling fluid technology, special focus should be paid on improving emulsion stability and wettability, enhancing plugging performance, and addressing issues of the resource utilization or harmless disposal of waste drilling fluids. For the drilling fluid lost circulation prevention and plugging technologies, efforts should be made to develop new materials applicable under various formation pressures and permeability conditions and improve the prediction accuracy of lost circulation location. As one of the key technologies for tight oil and gas exploration and development, the research and advancement of drilling fluid technology is critical for increasing tight oil and gas resources and ensuring national energy security.

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.

Underground coal gasification can achieve the clean development and utilization of coal resources, which is in line with the low-carbon development strategy of petroleum enterprises and the national carbon peaking and carbon neutrality goals. The paper sketches the current research status of the theory and technology of underground coal gasification, and it is considered that the theoretical system of underground coal gasification has been fundamentally established, the shallow coal underground gasification technology is relatively mature and has not yet become industrialized. The middle-deep coal underground gasification has been preliminarily validated and become the development tendency of underground coal gasification. However, it still faces key challenges such as complex technology, poor basic studies, and stability control in the gasification process. Since 2019, China National Petroleum Corporation Limited has implemented a number of major science and technology projects, which aims to tackle key technologies for coal underground gasification in the middle and deep strata deeper than 800 m. From both theoretical and technical perspectives, this paper summarizes and introduces important progresses and understandings achieved from the implementation of major projects. In terms of basic theory, the understandings of underground coal gasification kinetics, gasification cavity expansion mechanism and overburden thermal deformation mechanism is deepened based on a breakthrough in experimental technique and systematic experimental and simulated studies, laying a theoretical basis for the engineering and process design of middle-deep coal underground gasification. In terms of process technology, a breakthrough has been made in the key core technologies, including gasification resource evaluation and site selection evaluation, gasifier integrity design and control as well as gasification operation control, thus forming the gasifier construction equipment system and underground gasification borehole operation equipment system. In addition, it is proposed to continue to deepen the basic research on middle-deep coal underground gasification and simultaneously promote field tests, so as to improve technology maturity and process stability, and push forward the industrialization of underground coal gasification.

Fuyu oil reservoir and Yangdachengzi oil reservoir (referred to as Fuyang oil reservoir)in northern Songliao Basin, located adjacent to the overburden source rocks of Member 1 of Qingshankou Formation, are typical tight oil reservoirs characterized with upper generation and lower storage, where oil and gas have not undergone long-distance migration, large-scale reservoirs are contiguously distributed without obvious reservoir boundaries. Based on a review of the exploration history of Fuyang oil reservoir over the past 60 years, the paper analyzes the key fields and directions of exploration at different stages. By using multi-scale and multi-type experimental testing results combined with logging data and seismic data, it was clarified that the whole-sag distribution and staggered contiguous distribution pattern of Fuyu and Yangdachengzi oil reservoirs are derived from the widely developed high-quality source rocks, dense fault belt transport systems, large-scale distribution of meandering river to shallow water delta sedimentary sand bodies, as well as the ideal "source-fault-reservoir" matching relationship formed under overpressure driving. At present, Fuyang oil reservoir has achieved a high degree of exploration in positive structures including Daqing placanticline, Weixing-Zhaozhou areas of Sanzhao sag, and Longhubao terrace, with large-scale distribution of tertiary reserves, demonstrating good exploration prospects. Two significant directions for the future exploration of tight oil below source rocks include Fuyu reservoir with relatively low exploration degree, represented by the syncline area of Qijia-Gulong sag, and the new tight oil enrichment zone of Yangdachengzi reservoir. Those fields are worthy of increasing exploration efforts, accelerating exploration progress, and strengthening technical breakthroughs. The research results can provide a theoretical basis for increasing reserve and production of tight oil on a large scale in northern Songliao Basin.

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.

With the deepening exploration from conventional to unconventional hydrocarbon, and shallow to deep reservoirs, a batch of new fields of tight oil and gas reservoirs are gradually emerging in southern Songliao Basin. Based on geological theory innovation and key technological breakthroughs, significant breakthroughs and progress have been made in exploration of tight oil and gas in southern Songliao Basin in recent years. By summarizing progress in three major fields of tight oil in Fuyu oil reservoir, deep tight gas in basin, and coalbed methane in fault depression, the paper analyzes the geological conditions, exploration progress, accumulation mechanism and resource potential in southern Songliao Basin, and also proposes the next research and development direction. (1) The study clarifies the coupling accumulation mechanism of "three in one" for Fuyu tight oil reservoir, involving overpressure drive of high-quality source rocks in Qingshankou Formation, transport of intensive interlayer faults, and hydrocarbon accumulation in the thick sand bodies of main channel of Member 4 of Quantou Formation. The tight oil resources in Fuyu oil reservoir amount to 9.72×10 ⁸t, and the undiscovered resources are estimated to be 4.91×10 ⁸t. Three favorable areas are selected, namely Xinbei, Gudian, and Yuzijing-Tahucheng. Moreover, it is pointed out that Fuyu tight oil reservoir plays a key role in increasing reserves and production in Jilin oilfield (PetraChina Jilin Oilfield Company), and efforts should be made to sustainably promote the integration of reserve increase and productivity construction. (2) The study reveals the source-reservoir integrated accumulation mechanism of deep tight gas based on the coupling of high-maturity coal-bearing source rock and volcanic material-rich reservoir. The deep tight gas resources amount to 7.67×10 ¹²m ³, and five favorable zones that can be effectively utilized have been identified, including Wukeshu, Huajia-Guojia, Xiaochengzi, Shenzijing, and Hashituo areas. It is clarified that deep tight gas is an important replacement resource in Jilin oilfield. (3) Based on the parameters of coal rock, coal quality, coal reservoir characteristics, and gas content, the coal bed methane resources are estimated to be 1.11×10 ¹²m ³. Three favorable areas for coalbed methane exploration have been identified, i.e., Wangfu fault depression, Dehui fault depression, and Fulongquan fault depression, with a favorable exploration area of 19 370 km ². In conclusion, the three favorable areas for tight oil and gas exploration provide important resource foundations for stable production in Jilin oilfield, and will provide replacement resources for high-quality development of Jilin oilfield. Relevant exploration experience and understandings are expected to provide reference and inspiration for neighboring areas and similar basins.

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.

A major breakthrough has been made in exploration of deep coalbed methane (CBM) in recent years, leading the CBM industry to enter its best period in history. However, the rapid expansion of the industrial scale still faces many problems and challenges. To promote the comprehensive upgrade of deep CBM exploration and development, implement the development strategies for large-scale CBM industry, guarantee energy security, and achieve the "dual carbon" target, a systematic analysis was conducted on the current status, theoretical and technical progress, industry upgrading strategy, and challenges in the deep CBM industry. The study suggests that the connotation of upgrading the deep CBM industry can be summarized in four aspects, i.e., theoretical and technical maturity, improved development efficiency, strong investor confidence, and rapid scale expansion. The basic conditions for industry upgrading include six aspects, i.e., policy support and guidance, technological innovation and breakthroughs, infrastructure construction and improvement, talent training and introduction, coordinated development of industrial chains, safe production and environmental protection. For deep CBM industry upgrading, the study proposes a "Eight-in-One" technological, economic, and management system integrating exploration and development, geology and engineering, theories and technologies, underground and ground surface, research and production, investment and profitability, big data and artificial intelligence, as well as strategy and tactics. By integrating exploration and development with geology and engineering, it is feasible to achieve an integration of theoretical research and technological innovation, promote the rapid transformation of scientific research achievements and their applications in production, improve the whole technological level, and enhance technological innovative ability. Through the integration of underground and ground surface development, the coordinated development of underground resources and ground facilities can be achieved to ensure safety operation and environmental protection throughout the entire process. By integrating investment efficiency, big data and artificial intelligence, resource allocation can be optimized and development costs are reduced. Through practical applications, the CBM exploration and development efficiency has been significantly improved; an operation mechanism has been formed through government organization under the leadship of enterprises. A set of coordinated and mutually supporting strategic development goals and corresponding strategies have been formulated and implemented, achieving remarkable effects. To achieve the goal of deep CBM industrial upgrading in the future, it is required to strengthen five aspects and implement three programs, i.e., to strengthen the technological innovation and R & D investment, cooperation and coordination between upstream and downstream of the industrial chain, safety and environmental protection management, international cooperation and exchange, as well as policy support and guidance; adhering to the implementation of "technological innovation as the main body, resource, technology, talent, policy and investment as a whole, as well as coordinated and innovative development" in a coordinated and cooperative manner, to effectively implement the "deep coal seam gas revolution" project, gradually implement the "Eight-in-One" strategic system, and promote industrial upgrading and development.