The Qiongzhusi Formation represents the second strata in China to achieve commercial shale gas development, following the Longmaxi Formation. However, a limited understanding persists regarding the pore structure of its shales, particularly concerning the distinctions between its shale and silty shale. This study comprehensively analyzed 25 samples of shale and silty shale from the Sichuan Basin. These analyses included total organic carbon content, X-ray diffraction, porosity measurements, field emission scanning electron microscopy, low-temperature CO2 adsorption, low-pressure N2 adsorption, and high-pressure mercury intrusion capillary pressure. Utilizing multifractal theory, the study investigated the pore structure differences between Qiongzhusi Formation shales and silty shales and their implications for shale gas storage and migration within the formation. The results indicated that the pore volume and specific surface area distribution trends for both shale and silty shale were generally consistent, with mesopores being the primary contributors to both pore volume and specific surface area. However, the shales exhibited superior characteristics, with pore volume of 0.0139 cm3/g and specific surface area of 10.305 m2/g, compared to silty shales, which showed pore volume of 0.0126 cm3/g and specific surface area of 8.184 m2/g. Moreover, the pore size distributions of both shale and silty shale displayed multifractal characteristics. In micropores and mesopores, the shales exhibited weaker heterogeneity in pore size distribution than the silty shales, while both possess higher correlation dimension (D2) and Hurst exponent (H). Conversely, regarding macropores, the silty shales demonstrated weaker heterogeneity in pore size distribution compared to the shales, but the former’s D2 and H were higher than those of the latter. This suggests that micropores and mesopores in the shales are clustered and have better connectivity than those in the silty shales. Conversely, macropores in the silty shales are densely clustered and show better connectivity than those in the shales. Based on the analysis of pore structure, organic geochemical, and mineralogical characteristics, the shales demonstrated better gas-bearing potential. Therefore, shale gas exploration and development in the Qiongzhusi Formation should prioritize shale reservoirs.

Gas content and its occurrence characteristics are two key factors affecting the evaluation of shale gas reservoirs' exploration and development potential. Conventional analysis methods, however, are often costly and generate only discrete data. To address this, this study highlights the Qiongzhusi Formation shale in the Sichuan Basin, developing logging-based prediction models targeting adsorbed and free gas content by integrating well logging data with results from 30 methane isothermal adsorption tests and 3 variable-temperature adsorption experiments. These models were used to characterize the formation's gas content. According to the results, the model-predicted total gas content deviates slightly from measurements via preserved core testing, with an average error rate of 11.89%. Logging interpretation indicates the Qiongzhusi Formation shale has a gas content of 1.56 to 18.55 cm(3)/g, dominated by free gas. The ratio of free to adsorbed gas varies from 0.001 to 8.55, averaging 3.78. By layer, gas content ranks as: (5) > (4) > (7) > (3) > (6) > (1) > (2) > (8) . Notably, layer (4) has the highest free-to-adsorbed gas ratio, suggesting exploration should not be limited to layers (5) and (6) -layers (4) , (7), and (3) , especially layer (4) , also have significant potential.

Clay minerals are crucial components of shale, this study focuses ons the Ordovician-Silurian Wufeng and Longmaxi Shale in the Sichuan Basin, aiming to clarify the genesis and diagenetic evolution of clay minerals, as well as their impacts on reservoir quality. To characterize the composition and morphological features of clay minerals, a combination of analytical techniques was employed, including X-ray diffraction, transmission electron microscopy and scanning electron microscopy. The results show that the degree conversion of illite/smectite in bulk shale is lower than that in bentonite interlayers. Compare the initial reactant composition, temperature, water-rock ratio and fluid composition between shale and bentonite, it is inferred that the limited K+ content in shale is the key factor responsible for the lower I/S conversion degree. For the quantitative evaluation of pore systems, scanning electron microscopy coupled with image analysis and mercury intrusion porosimetry was used to statistically determine the visible porosity of different pores types and characterize the pore structure parameters. Diagenetic analysis reveals that clay minerals conversion promotes the formation of shrinkage joints and micro-fractures, while compaction reduces intergranular porosity but tends to preserve pores adjacent to rigid solid grains. With respect to organic matte pores, further analysis indicates that the development of organic matter pores in the organic matter-clay composite is related to hydrocarbon gas generation, and the catalytic effect of clay minerals plays a significant role in regulating hydrocarbon gas generation. Specifically, a clay mineral content of 10%-15% in composites provides the most favorable condition for the development of OM pores.

The shale reservoirs of the Lower Cambrian Qiongzhusi Formation in the Sichuan Basin exhibit complex pore structures,which provide important spaces for pore water occurrence.However,the specific occurrence characteristics and their controlling factors remain poorly understood.In this study,we investigate deep marine shales from the Qiongzhusi Formation in the basin.By integrating a series of experiments,including water saturation centrifugation-nuclear magnetic resonance(NMR)combined tests,scanning electron microscopy(SEM),nitrogen adsorption,and molecular dynamics simulation,we systematically reveal the microscopic occurrence characteristics of pore water in the shale reservoirs and their primary controlling factors.Accordingly,the corresponding pore water occurrence patterns are established.The results indicate that in deep marine shale samples from the Qiongzhusi Formation,totally irreducible fluids occur primarily in pores with a diameter smaller than 4.98 nm.In contrast,partly movable fluids are distributed in pores with sizes ranging from 4.98 nm to 8.54 nm,while pores larger than 8.54 nm mainly contain totally movable fluids.Molecular dynamics simulations reveal that water molecules are preferentially adsorbed on the surface of inorganic minerals,followed by methane molecules.With the increase of temperature and pressure(corresponding to the increase of buried depth of strata),the adsorption capacity of both increases,but the presence of water molecules has a significant inhibitory effect on methane adsorption.Under high water saturation,the proportion of movable water increases significantly,while tectonism-induced microfractures further enlarge its occurrence space.Concurrently,the increased proportion of large pores reduces capillary pressure,leading to a decreased proportion of irreducible water which tend to primarily concentrate in small pores.Mineral composition,pore structure,tectonism,and the water saturation of reservoirs jointly constitute an interconnected system that governs the occurrence state of pore water in shales of the Qiongzhusi Formation.These findings serve to lay an important basis for advancing geological theories on deep shale gas accumulation.

The black shale strata in the Talung Formation of the Upper Permian is a new target for marine shale gas exploration breakthroughs in South China. Through petrological and elemental geochemical analyses of black shales in the Talung Formation, the sedimentary environment during the late Wuchiapingian to Changhsingian stage was clarified, and an enrichment model of extraordinarily high organic matter (EHOM) was established. The shales in the Kaijiang-Liangping Trough are mainly composed of siliceous calcareous shales, with a total organic carbon content ranging from 5.45% to 20.15%, and the kerogen type is predominantly Type II1. During the deposition of the Talung Formation, the margin of the Kaijiang-Liangping Trough was characterised by a shallow to semi-deep water shelf environment, with a warm, humid climate and anoxic reducing water conditions. The water at the margin of the marine trough was slightly turbulent, with a low degree of stagnation. The primary productivity proxies established using trace elements Cu, Zn, and Ni reveal a high level of primary productivity in the water during the Late Permian. The widespread upwelling activity and suitable terrigenous clastic input brought abundant nutrients into the waters, significantly enhancing primary productivity. However, hydrothermal and volcanic activities had no significant contribution to primary productivity. Thus, the high primary productivity brought by upwelling and terrigenous clastic input significantly promoted the production of EHOM, and the reducing water conditions facilitated the preservation of EHOM. The excellent production and favorable preservation jointly ensured the accumulaiton of EHOM in the Talung Formation shale of the Kaijiang-Liangping Trough.

The deep and ultra-deep Lower Cambrian Qiongzhusi (DUDQ) shales have abundant gas resources. However, the strong heterogeneity of the DUDQ shale makes it complex to accurately predict the gas-in-place (GIP) content and resource potential. The first high-yield DUDQ shale well Z201 in the southern Sichuan Basin, offering a crucial opportunity to investigate their gas-bearing characteristics. This study focuses on the Qiongzhusi Formation Q12 submember (Q12) of well Z201, analyzes their geological characteristics, and measures its adsorption parameters through high-temperatures (60-150 degrees C) and high-pressures (0.01-35 MPa) methane adsorption experiments, and a geological GIP model is established to predict the shale gas resources of the DUDQ shales. The maximum absolute adsorption gas content (n0) and adsorbed phase methane density (rho ads) of the DUDQ shales range from 1.90 to 4.91 cm3/g and 0.28-0.47 g/cm3, respectively. Both n0 and rho ads are positively correlated with total organic carbon (TOC) content and geopressures, but negatively correlated with temperature. The average adsorption capacity of inorganic matter (IM) in the DUDQ shales is 1.99 cm3/g, which, together with high TOC content, are the main important factors contributing to the high GIP contents. With increasing burial depth, the formation temperature and pressure gradually increase. Meanwhile, the adsorbed gas content (nads) and total gas content (ntot) initially tend to increase followed by a subsequent decrease, whereas the free gas content (nfree) tends to increase. Moreover, ntot is also affected by the TOC content and effective porosity. The most promising areas for the DUDQ shale gas exploration and development are the central area of the intracratonic sag and the surrounding regions of the Weiyuan anticline, with estimated shale gas resources exceeding 2.52 x 1012 m3.

The types and structures of inorganic pores are key factors in evaluations of the reservoir space and distribution characteristics of shale oil and gas. However, quantitative identification methods for pores of different inorganic components have not yet been fully developed. For this reason, a quantitative characterization method of inorganic pores using pixel information was proposed in this study. A machine learning algorithm was used to assist the field emission scanning electron microscopy (FE-SEM) image processing of shale to realize the accurate identification and quantitative characterization of inorganic pores on the surface of high-precision images of shale with a small view. Moreover, large-view image splicing technology, combined with quantitative evaluation of minerals by scanning electron microscopy (QEMSCAN) image joint characterization technology, was used to accurately analyze the distribution characteristics of inorganic pores under different mineral components. The quantitative methods of pore characteristics of different inorganic components under the pixel information of shale were studied. The results showed that (1) the Waikato Environment for Knowledge Analysis (WEKA) machine learning model can effectively identify and extract shale mineral components and inorganic pore distribution, and the large-view FE-SEM images are representative of samples at the 200 μm × 200 μm view scale, meeting statistical requirements and eliminating the influence of heterogeneity; (2) the pores developed by different mineral components of shale had obvious differences, indicating that the development of inorganic pores is highly correlated with the properties of shale minerals themselves; and (3) the pore-forming ability of different mineral components is calculated by the quantitative method of single component pore-forming coefficient. Chlorite showed the highest pore-forming ability, followed by (in descending order) illite, pyrite, calcite, dolomite, albite, orthoclase, quartz, and apatite. This study contributes to advancing our understanding of inorganic pore characteristics in shale.

The deep Qiongzhusi (QZS) shales in the Deyang–Anyue Aulacogen, Sichuan Basin, are important shale gas targets in China. However, the differences in pore development of the shales in the marginal areas of the aulacogen and its main controlling factors remain unclear. In this study, the organic and inorganic compositions, water-bearing characteristics, and porosities of the sublayer 7 (SL7) and sublayer 5 (SL5) deep QZS shales collected from Well Z204 were systematically investigated. The results indicate that, compared with the SL5 shales, the SL7 shales have higher total organic carbon (TOC) and clay mineral contents, lower brittle mineral contents, greater porosities, and water content and saturation. The TOC content of the deep QZS shale has a strong positive correlation with porosity, and a negative correlation with water saturation. Therefore, the TOC content is the main controlling factor for the development of effective porosity. The TOC content of the SL7 shale is higher than that of the SL5 shale, so the SL7 shale has a larger effective porosity. Therefore, the margin of the Deyang–Anyue Aulacogen is a new exploration area for the deep QZS shales, and the shale gas accumulate conditions of the SL7 shales are better than that of the SL5 shales, especially in the lower section of the SL7 shales. This new understanding provides an important geological basis for the next exploration of the Deyang–Anyue Aulacogen.

The Emeishan Large Igneous Province (ELIP) substantially influenced Permian environmental conditions. This study focused on Middle-Late Permian strata in the Kaijiang and Liangping regions, located similar to 800 km northeast of the ELIP center, integrating petrology and geochemistry to investigate the influence of the ELIP on the sedimentary environment and organic matter enrichment. Results indicate that the stratigraphic records preserve eruptive products from different developmental stages of the ELIP. The Kaijiang-Liangping Trough underwent initial uplift followed by subsidence from the late Capitanian to the Wuchiapingian, with organic enrichment patterns differing between northern and southern sections owing to tectonic and volcanic activity. In the north, organic matter is enriched in the siliceous mudstones of the Gufeng Member in the late Capitanian and the upper Wujiaping Formation of the Wuchiapingian age, whereas the south additionally contains coastal swamp facies shales at the base of the Wujiaping Formation. Organic matter enrichment was controlled by high productivity and anoxic conditions during the late Capitanian, whereas elevated sedimentation rates and enhanced terrigenous input attributable to volcanic activity became dominant factors in the early Wuchiapingian. Late Wuchiapingian Emeishan felsic volcanic eruptions might have triggered climatic cooling, promoting upwelling restoration and biotic recovery, with anoxic-euxinic conditions and high productivity jointly governing organic matter enrichment. The negative delta C-13(org) excursion during this period might reflect reduced terrestrial organic matter input caused by sea level rise. This study revealed the spatiotemporal coupling of volcanic and sedimentary processes, offering new insights into hydrocarbon source rock formation in large igneous provinces.

The Lower Cambrian shales in the Sichuan Basin are considered one of the most promising shale gas resources in China.However,large-scale commercial development has not been achieved due to the relatively low and significantly variable gas contents of the drilled shales.Excitingly,the first major breakthrough in deep and ultra-deep Lower Cambrian shale gas was made recently in the well Z201 in the southern Sichuan Basin,with a gas yield exceeding 73 × 104 m3/d.The success of well Z201 provides a favorable geological case to reveal the distinct enrichment mechanism of deep and ultra-deep Lower Cambrian shale gas.In this study,at drilling site of well Z201,fresh shale core samples with different gas-in-place contents were collected,and their geochemical,pore development and water-bearing charac-teristics were analyzed systematically.The results showed that the Z201 organic-rich shales reached an overmature stage,with an average Raman maturity of 3.70％.The Z201 shales with high gas-in-place contents are mainly located in the Qiongzhusi 12 section and the upper Qiongzhusi 11 section,with an average gas-in-place content of 10.08 cm3/g.Compared to the shales with low gas-in-place contents,the shales with high gas-in-place contents exhibit higher total organic carbon contents,greater porosities,and lower water saturations,providing more effective pore spaces for shale gas enrichment.The effective pore structures of the deep and ultra-deep Lower Cambrian shales are the primary factors affecting their gas-in-place contents.Similar to the shales with high gas-in-place contents of well Z201,the deep and ultra-deep Lower Cambrian shales in the Mianyang-Changning intracratonic sag,especially in the Ziyang area,generally developed in deep-water shelf facies with high total organic carbon contents and thick sedimentary thickness,providing favorable conditions for the development and preservation of effective pores.Therefore,they are the most promising targets for Lower Cambrian shale gas exploration.

Based on the basic data of drilling, logging, testing and geological experiments, the geological characteristics of the Permian Dalong Formation marine shales in the northern Sichuan Basin and the factors controlling shale gas enrichment and high yield are studied. The results are obtained in four aspects. First, the high-quality shale of the Dalong Formation was formed after the deposition of the Permian Wujiaping Formation, and it is developed in the Kaijiang-Liangping trough in the northern part of Sichuan Basin, where deep-water continental shelf facies and deep-water reduction environment with thriving siliceous organisms have formed the black siliceous shale rich in organic matter. Second, the Dalong Formation shale contains both organic and inorganic pores, with stratification of alternated brittle and plastic minerals. In addition to organic pores, a large number of inorganic pores are developed even in ultra-deep (deeper than 4 500 m) layers, contributing a total porosity of more than 5%, which significantly expands the storage space for shale gas. Third, the limestone at the roof and floor of the Dalong Formation acted as seal rock in the early burial and hydrocarbon generation stage, providing favorable conditions for the continuous hydrocarbon generation and rich gas preservation in shale interval. In the later reservoir stimulation process, it was beneficial to the lateral extension of the fractures, so as to achieve the optimal stimulation performance and increase the well-controlled resources. Combining the geological, engineering and economic conditions, the favorable area with depth less than 5 500 m is determined to be 1 800 km2, with resources of 5 400×108 m3. Fourth, the shale reservoirs of the Dalong Formation are thin but rich in shale gas. The syncline zone far away from the main faults in the high and steep tectonic zone, eastern Sichuan Basin, with depth less than 5 500 m, is the most favorable target for producing the Permian shale gas under the current engineering and technical conditions. It mainly includes the Nanya syncline, Tanmuchang syncline and Liangping syncline.

Reservoir properties and their evolutionary history are of paramount importance for elucidating the formation mechanisms of oil and gas reservoirs and for the selection of prospective exploration areas. However, such investigations remain relatively limited within the realm of shale reservoir research, particularly concerning the quantitative evolution of shale porosity. In this study, we incorporate shale-specific initial porosity and organic matter (OM) pore development models into the conventional backstripping method. A case study of the Longmaxi Formation shale in the Luzhou block was conducted, using total organic carbon (TOC) analysis, X-ray diffraction (XRD), porosity measurements, large-area scanning electron microscopy (Maps-SEM), wide-field scanning electron microscopy (WSEM) coupled with energy-dispersive X-ray spectroscopy (EDS), quantitative evaluation of minerals by scanning electron microscopy (QEMSCAN), and basin modeling. These methods were used to elucidate the pressure evolution, diagenetic processes, and their intensity within the deep Longmaxi Formation shale reservoir of the Sichuan Basin. Based on these findings, a quantitative shale porosity restoration model was developed, explicitly accounting for pore pressure, diagenetic alterations, and OM maturation. The results reveal the following: (1) Longmaxi Formation shale can be divided into siliceous shale, argillaceous shale, and mixed shale. The pores in all three types of shale are mainly OM pores. Inorganic pores are mainly intergranular pores and intergranular fractures, with a low proportion of dissolution pores. The pore composition varies among different shale lithofacies. (2) Differences in the intensity of diagenesis are the cause of differences in pore type composition among different shale lithofacies. Furthermore, compaction is the main factor leading to the decrease in porosity of Longmaxi Formation shale, while OM hydrocarbon generation is the main reason for the increase in porosity. (3) The evolution of porosity in the deep Longmaxi Formation shale can be divided into four stages: During the early diagenetic stage, porosity significantly decreases due to compaction and cementation. In the middle diagenetic stage, vertical effective pressure initially increases and then decreases, while porosity fluctuates significantly due to hydrocarbon generation from OM, reaching a minimum in the Late Triassic (approximately 207 Ma). In the late diagenetic stage, the influence of OM hydrocarbon generation results in a gradual increase in porosity, which subsequently decreases again due to recompaction during the stratigraphic uplift stage. This research can provide insights into the accumulation process and favorable area prediction of deep shale gas.

Currently, the Longmaxi shale in the Sichuan Basin is the most successful stratum of shale gas production in China. However, because Longmaxi shale mostly has high over-maturity, a low-maturity sample cannot be obtained for gas generation thermal simulations, and as a result, a gas generation model has not yet been established for it. Therefore, models of other shales are usually used to calculate the amount of gas generated from Longmaxi shale, but they may produce inaccurate results. In this study, a Longmaxi shale sample with an equivalent vitrinite reflectance calculated from Raman spectroscopy (EqVRo) of 1.26% was obtained from Well Yucan 1 in the Chengkou area, northeast Sichuan Province. This Longmaxi shale may have the lowest maturity in nature. Pyrolysis simulations based on gold tubes were performed on this sample, and the gas generation line was obtained. The amount of gas generated during the low-maturity stage was compensated by referring to gas generation data obtained from Lower Silurian black shale in western Lithuania. Thus, a gas generation model of the Longmaxi shale was built. The model showed that the gas generation process of Longmaxi shale could be divided into three stages: (1) First, there is the quick generation stage (EqVRo 0.5–3.0%), where hydrocarbon gases were generated quickly and constantly, and the generation rate was steady. A maximum of 458 mL/g TOC was reached at a maturity of 3.0% EqVRo. (2) Second, there is the stable stage (EqVRo 3.0–3.25%), where the amount of generated gas reached a plateau of 453–458 mL/g TOC. (3) Third, there is the rapid descent stage (EqVRo > 3.25%), where the amount of generated gas started to decrease, and it was 393 mL/g TOC at an EqVRo of 3.34%. This model allows us to more accurately calculate the amount of gas generated from the Longmaxi shale in the Sichuan Basin.

Taking the Lower Silurian Longmaxi Formation shale in the Sichuan Basin as an example, this study employs atomic force microscopy-based infrared (AFM-IR) spectroscopy to analyze the submicron-scale molecular functional groups of different types and occurrences of organic matter. Combined with the quantitative evaluation of pore development via scanning electron microscopy (SEM), the response of organic pore formation and evolution mechanisms to chemical composition and structural evolution of organic matter in overmature marine shale is investigated. The results indicate that the AFM-IR spectra of graptolite periderms and pyrobitumen in shale are dominated by the stretching vibrations of conjugated C=C bonds in aromatic compounds at approximately 1 600 cm-1, with weak absorption peaks near 1 375, 1 450 and 1 720 cm-1, corresponding to aliphatic chains and carbonyl/carboxyl functional groups. Overall, the AFM-IR structural indices (A and C factors) of organic matter show a strong correlation with visible porosity in shales of equivalent maturity. Lower A and C factor values correlate with enhanced development of organic pores, which is associated with the detachment of more aliphatic chains and oxygen-containing functional groups during thermal evolution. Pyrobitumen-clay mineral composites generally exhibit superior pore development, likely attributable to clay mineral dehydration participating in hydrocarbon generation reactions that promote the removal of more functional groups. Additionally, hydrocarbon generation within organic-clay composites during high–over mature stages may induce volumetric expansion, resulting in microfracturing and hydrocarbon expulsion. The associated higher hydrocarbon expulsion rates promote the formation of larger pores and fracture-shaped pores along the flake-shaped clay minerals. This study highlights that the research of submicron-scale molecular functional groups provides a deeper understanding of organic matter evolution and pores development mechanisms in overmature shales, thereby offering critical theoretical parameters for reservoir evaluation in shale oil and gas exploration.

Although pores in solid bitumen are crucial for shale reservoirs, their development mechanisms have not been fully understood partly due to the small size of solid bitumen in shales and the difficulty in its discrimination from residual kerogen. However, solid bitumen in conventional reservoirs generally has a larger size and definite origin, and therefore the examination of their pores can provide some insights into the mechanisms of pore formation in shale solid bitumen. In this study, solid bitumen samples from conventional reservoirs of the Ediacaran Dengying Formation in the central Sichuan Basin were collected for petrographic and geochemical analyses to investigate their pore characteristics and formation mechanisms. The results indicate that pores are commonly identified in solid bitumen grains that are derived from the cracking of tar pads and in solid bitumen grains that have experienced gravity-driven component separation during their formation. The development of pores in solid bitumen is controlled by the combined effect of gas expulsion and the viscosity of solid bitumen during its movable stage. The gas expulsion from movable bitumen creates pores, and they can be preserved or healed depending on the component-controlled viscosity of movable bitumen. The enrichment of sulfur- and oxygen-containing compounds and heavy components such as asphaltenes in movable bitumen can enhance its viscosity and thus inhibit the healing of pores, ultimately facilitating pore development in the present overmature solid bitumen grains.

The study of shale reservoirs is a crucial aspect of shale gas exploration and development. With China’s continued breakthroughs in deep and ultra-deep shale gas, the traditional view that reservoir quality deteriorates with increasing depth faces significant challenges. Currently, there remains a limited understanding of the pore development characteristics and controlling factors in shallow, deep, and ultra-deep shales, which affects the effectiveness of shale gas production. Taking the Longmaxi Formation shale in southern Sichuan Basin as a case study, this study conducted a series of experimental analyses, including TOC, XRD, low-temperature N2 adsorption, CO2 adsorption, SEM, and large-view FE-SEM splicing technology (Maps-SEM). The results reveal that: (1) the porosity of the Longmaxi Formation shale in southern Sichuan ranges from 3.37

Helium is a valuable natural resource used widely in high-tech industries because of its unique physical and chemical properties. The study of helium in shale gas is still in its infancy, and the content, genesis, and enrichment patterns of helium in shale gas are not yet clear. In this paper, the concentrations and isotopic characteristics of helium were investigated in the Wufeng-Longmaxi shale gas in the Sichuan Basin and the periphery areas. The analytical results show that the concentrations of helium in the southern Sichuan shale gas fall in the range of 0.018-0.051 vol% with an average of 0.029 vol%. The helium abundance in Weiyuan shale gas are relatively low compared to those in conventional natural gas pools from the same area (generally greater than 0.20 vol%), reflecting the significance of long distance migration to the enrichment of helium in gas pools. The relatively low ratios of 3He and 4He in shale gas indicate that most of the helium are crustal derived helium. Further quantitative estimate based on helium, neon, and argon isotopic ratios suggest almost 100% crustal helium source. The helium residing in shale reservoirs can be deconvoluted into the indigenous helium generated in-situ by shale and exogenous helium generated from external helium source rocks and charged through faults and/or fractures networks. According to preliminary calculations, external helium source is required to meet the threshold of an economic helium-rich field of helium concentration of 0.1 vol% except for particular areas with extraordinarily high uranium and thorium concentration. Based on detailed study on typical helium-rich shale gas reservoirs, major advantageous features for helium's enrichment in shale gas include: (1) high-quality helium source rocks, (2) effective migration paths, and (3) diminished dilution effects of shale gas. Shale gas plays with underlying ancient cratonic basement, well developed source-connecting faults, and moderate pressure coefficient are potential targets for helium exploration. (c) 2025 The Authors. Publishing services by Elsevier B.V. on behalf of KeAi Communications Co. Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

The marine shale within the Sichuan Basin constitutes China’s significant shale gas production, featuring old formation age, high degree of thermal evolution, multiple tectonic movements, and complex structural conditions. However, there are significant differences in the shale gas preservation conditions and reservoir quality in different areas, limiting future large-scale exploration and development. Pore structure significantly influences shale reservoir quality, gas content, and exploration of shale gas occurrence, migration, and enrichment mechanisms. The influence of structural-dominated preservation conditions on shale pore structures is essential to comprehend for effective shale gas exploitation. This study employs field-emission scanning electron microscopy in conjunction with other techniques (low-temperature N2 adsorption, low-temperature CO2 adsorption, and nuclear magnetic resonance) for detailed analyses of the pore structure across varied structural zones, revealing the influence of structural attributes, fault systems, depth of burial, and formation pressure on pore architecture, and examining the relationship between pore structure and shale gas preservation conditions. The results show that stable structural condition is conducive to the development and preservation of shale pores. Structural compression causes inorganic and organic pores to become narrow and elongated due to shrinkage, with a significant increase in microfractures. The porosity of shale with stable structural conditions exhibits markedly increased porosity compared to samples under structural compressions. Under conditions of similar TOC and mineral composition, the pore size distribution (PSD), pore volume (PV), and specific surface area (SSA) of shale after structural compression are significantly lower than those of samples with stable structural conditions. As the burial depth increases, the shale porosity shows a decreasing trend, but the decrease is limited. Burial depth significantly impacts the SSA and PV of high-TOC samples (3%–6%). As the burial depth increases, both SSA and PV show a significant decreasing trend. When the burial depth reaches 4000 m, SSA and PV tend to concentrate. The formation pressure coefficient is an important factor for the development and preservation of shale pores, and porosity is positively correlated with the formation pressure coefficient. Increased formation pressure coefficient indicates superior preservation conditions and enhanced pore development.

Pore water occupying pore surfaces and filling pore spaces significantly impacts the occurrence and exploration of shale gas. Shale gas exploration and development in China have shifted toward deep to ultradeep marine shale formations in the high-maturity to the overmaturity stage. However, the existing research on shale pore water mainly focuses on shallow to medium-deep shale, and the understanding of the wetting characteristics and pore water evolution of high-maturity-overmaturity marine shale is insufficient. Therefore, this study takes the Longmaxi Formation shale in the Sichuan Basin as an example to carry out contact angles, spontaneous imbibition, and drying-NMR coupling experiments to explore the wettability characteristics and pore water occurrence conditions of shale in the overmaturity stage. Subsequently, an evolution model of wettability characteristics and pore water occurrence characteristics in the overmaturity stage of shale was established. The results show that the maturity degree controls the shale wettability significantly. With increasing maturity, the Longmaxi Formation shale reservoir transforms from water-wet to weakly water-wet. The pore size range of movable water, capillary-bound water, clay-bound water, and hydration water are 6.48-8.47, 2.28-7.12, 1.07-2.28, and 1.07 nm or less, respectively. The controlling model with two stages of wettability on the occurrence of pore water was established. In stage I (R-0 = 3.0%-3.8%), due to the effect of organic matter graphitization, graphitized organic matter pores collapsed under the action of ground stress. This leads to a decrease in the area available for fluid adsorption and the percentage of micropores, and the shale water-wettability is slowly weakened, so the percentage of movable water increases and then slowly decreases. In stage II (R-0 = 3.8%-4.2%), with a further increase in graphitization, graphite crystals occupy the original microporous volume, resulting in a rapid weakening of shale wettability and a rapid increase in the percentage of movable water.

The shale of the Cambrian Qiongzhusi Formation in the Sichuan Basin is characterized by large burial depth and high maturity, but the shale gas enrichment pattern is still unclear. Based on the detailed characterization of Deyang-Anyue aulacogen, analysis of its depositional environments, together with reconstruction of shale gas generation and enrichment evolution against the background of the Leshan-Longnüsi paleouplift, the aulacogen-uplift enrichment pattern was elucidated. It is revealed that the Deyang-Anyue aulacogen controls the depositional environment of the Qiongzhusi Formation, where high-quality sedimentary facies and thick strata are observed. Meanwhile, the Leshan-Longnüsi paleouplift controls the maturity evolution of the shale in the Qiongzhusi Formation, with the uplift located in a high position and exhibiting a moderate degree of thermal evolution and a high resistivity. The aulacogen-uplift overlap area is conducive to the enrichment of shale gas during the deposition, oil generation, gas generation, and oil-gas adjustment stage, which also has a joint control on the development of reservoirs, resulting in multiple reservoirs of high quality and large thickness. Based on the aulacogen-uplift enrichment pattern and combination, four types of shale gas play are identified, and the sweet spot evaluation criteria for the Qiongzhusi Formation is established. Accordingly, a sweet spot area of 8 200 km2 in the aulacogen is determined, successfully guiding the deployment of Well Zi 201 with a high-yield industrial gas flow of 73.88×104 m3/d. The new geological insights on the aulacogen-uplift enrichment pattern provide a significant theoretical basis for the exploration and breakthrough of deep to ultra-deep Cambrian shale gas, highlighting the promising exploration prospect in this domain.