Water-rock interaction triggered by drilling and fracturing fluid retention in shale gas reservoirs induces secondary processes and the subsequent alteration of rock physical properties, critically modulating reservoir productivity. In this study, X-ray diffraction, nanoindentation, focused ion beam-scanning electron microscopy, and micro-computed tomography were utilized to characterize and analyze the mineral alteration, mechanical weakening and pore-fracture evolution of marine shale in the Longmaxi Formation, Sichuan Basin. The results reveal that the water-rock interaction preferentially dissolves clay minerals (mainly illite), feldspars and pyrite via hydration and redox reactions while promoting quartz and carbonate mineral recrystallization. The hydration-dissolution-precipitation process significantly weakens the rock mass by reducing cohesion and the friction angle. This mechanical degradation is evidenced by a substantial decrease in elastic modulus, exhibiting pronounced anisotropy relative to stratifications. The resultant heterogeneous stress fields initiate and propagate secondary pores and fractures, dramatically increasing the number, volume and surface area of pores. These newly formed structures integrate with pre-existing pore-fracture networks, markedly elevating overall porosity and enhancing interconnectivity, which consequently amplifies permeability by orders of magnitude. Additionally, water preferentially enters the reservoir through stratification, and the associated difference in water-rock interaction strength further enhances the heterogeneity of structural and mechanical heterogeneity. These findings link micro-scale physical-chemical reactions with the meso-scale mechanical properties and macro-scale pore-fracture structures, emphasize the key role of water-rock interaction in reshaping reservoir characteristics, and provide important insights for optimizing hydraulic fracturing strategies and improving shale gas recovery.

Anyue gas field in the Sichuan Basin and Jingbian gas field in the Ordos Basin are two giant marine carbonate gas fields with the largest overall scale and the largest single-layer scale respectively discovered in China so far. Based on the analysis of the source-location structure of gas reservoirs, the process of gas accumulation and the space-time configuration of key play elements during reservoir generation, we consider that Anyue and Jingbian gas fields are characterized by a variety of source-location structure types and source-trap configuration of high efficiency. However, Anyue gas field is of “in-situ” accumulation of pyrolysis gas from paleo-oil reservoirs in inherited paleo-uplift, while Jingbian gas field is of adjusted accumulation in the structural transformation zone of a slope. There are three key factors controlling the large-scale enrichment of Anyue and Jingbian gas fields. First, the hydrocarbon source rocks underwent abnormally thermal events, with the duration of thermal events with a heat flow value over 70 mW/m² being about 70 Myr. Different types of kerogen got fully cracked to generate methane-dominated gases in large amount. Second, the multi-layered reservoirs of large scale and high quality are well developed due to the modification of favorable lithofacies by karstification. Third, many types of large-scale traps are well developed including structural, structural-lithologic, stratigraphic-lithologic and lithologic traps, being favorable for large-scale and effective accumulation of gas. The areas with ideal configuration relationship between high-energy beach body and lithologically tight zone within paleo-oil reservoirs or adjacent to hydrocarbon source rocks are favorable for discovering large-scale gas accumulation in the near future.

In recent years, China has shifted its focus of shale gas exploration to underexplored plays, sequences and new types (collectively referred to as the “three new fields”). This shift has led to intensified efforts in addressing theoretical challenges and enhanced exploration, elevating both the reserves and production of shale gas to new levels. Based on new advances in theoretical research and exploration in the three new fields, we analyze the characteristics, trends, and prospects of shale gas exploration in these fields, explore theories on the unique shale gas geology in China, and identify challenges in shale gas exploration and corresponding countermeasures. The results indicate that China has developed innovative theories on the enrichment of highly-to over-mature marine shale gas with distinct characteristics of shales in the Wufeng-Longmaxi formations within the Sichuan Basin and its surrounding areas. A total of nine shale gas fields have been discovered in China with proven geological reserves of approximately 3×10¹² m³, resulting in a shale gas productivity of 450×10⁸ m³/a, and an annual shale gas production of 250×10⁸ m³. Furthermore, we ascertain three majorcharacteristics of the shale gas exploration in the three new fields in China: (1) significant progress in deep and extremely shallow strata for shale gas exploration of the Wufeng-Longmaxi formations within the Sichuan Basin and its surrounding areas; (2) breakthroughs in multiple underexplored units, such as the Qiongzhusi and Wujiaping formations in the Sichuan Basin; and (3) discoveries and breakthroughs in the Wulalike and Shanxi formations located on the western and eastern margins of the Ordos Basin, respectively. Three major strategic shifts have been achieved: (1) a shift in exploration target from a single type, basin, and unit to multiple types, basins, and units; (2) a shift of target area selection from focusing on the interiors or peripheries of basins to including basins’ exterior with weak tectonic modification; (3) a shift in the exploration philosophy from pure organic-rich shales to organic-rich shale systems. Analyzing challenges in shale gas exploration in the three new fields in China leads to the conclusion that these fields both represent the direction for the sustainable development of shale gas in China and need corresponding countermeasures for their progress.

Source rock strata are filled and aggregated with large-scale continuous hydrocarbon resources, including significant volumes of in-place retained, short-distance migrated and potentially generated hydrocarbons. Source rock strata simultaneously possess the properties of reservoirs and hydrocarbon source rocks, known as source-reservoir coexisting systems. Reservoir properties refer to the physical properties concerning the storage and transmission of oil and gas, while hydrocarbon source rock properties refer to the physicochemical properties related to governing the generation, retention and expulsion of oil and gas in the source rock strata. These properties fundamentally determine the technical path for the successful exploitation of petroleum and natural gas in the source rock strata. With regard to reservoir properties, in-depth research and development of the advanced energy-storing fracturing technology can aid the construction of complex fracture networks to overcome the limitations in the connectivity properties of source rock strata. Focusing on the hydrocarbon source rock properties, an underground in-situ conversion technology should be created and developed to alleviate the shortcomings of organic matter quantity and maturity properties of the source rock strata. Furthermore, selecting the appropriate exploitation path based on the property characteristics can promote the achievement of commercial and sustainable development of oil and gas in the source rock strata.

More than 30 years of continuous development has made onshore deep and ultra-deep conventional and unconventional oil and gas become an integral part of increasing the energy reserves and output by China’s petroleum industry. Based on the deep oil and gas geological conditions in the country, the present study finds that paleo stratum and deep burial are the two basic geological characteristics of deep oil and gas. Furthermore, we put forward the notion that it is necessary to strengthen the fundamental research of theories in four aspects and the core technology in five aspects of deep oil and gas. It is suggested that it is of special importance to promote the scientific and technological research of deep oil and gas through the scientific exploration of "myriameter deep" wells as the starting point, so as to boost the development of deep oil and gas field in China.