In order to figure out the redox conditions and paleo-sedimentary environment of the Middle Devonian shales in the northwest of Guizhong Depression, the trace element analysis was conducted on the Middle Devonian cores (320.35–938.50 m) of the typical shale gas investigation well (GY-1) at a 1.50 m sampling interval through X-ray fluorescence spectroscopy (XRF) and inductively coupled plasma mass spectrometry (ICP-MS). According to the test result, the average values of V/(V+Ni), V/Cr and Ni/Co in Nabiao formation (Fm.) are larger than 0.67, 4.65 and 7.71 respectively, and Nabiao Fm. is rich in biological assemblages such as tabasheer, ammonite, etc. These evidences indicate the rising sea level rose relatively in the sedimentation period of Nabiao Fm. and a deepwater shelf environment, which was favorable for the preservation of organic matters. The V/(V+Ni), V/Cr and Ni/Co in Luofu Fm. and Tangting Fm. are 0.38–0.65, 0.73–4.10 and 3.70–6.72 respectively, indicating that the sea level dropped relatively in their sedimentation period, during which the water bodies became shallow, and the sedimentary environment was a weak oxidizing shallow water shelf environment. In addition, the variation of TOC has a high correlation with the enrichment degree of Ba element, indicating the favorable conditions for the enrichment and preservation of organic matters under an oxygen-deficient environment. Moreover, according to the identification of trace element indexes, the northwest of Guizhong Depression experienced the sedimentary cycle of relative rise to relative fall of sea level from bottom to top in the Middle Devonian sedimentation period. The relative sea level rose to the highest in the sedimentation period of Nabiao Fm., in which the organic-rich shales with stable thickness and high organic content were deposited. Hence, the Nabiao Fm. could be regarded as the favorable exploration target interval in this area.

Since shale gas mainly occurs in shale pores, research on pore structure characteristics is the key to understanding the shale gas accumulation mechanism. The pore structure of the Lower Carboniferous Dawuba Formation shale in the Qianxi area, represented by the well QSD-1, which obtains a daily shale gas flow of 10,000 m³ and represents an important breakthrough in the investigation of marine shale gas in the Upper Paleozoic region of southern China, is characterized by high-pressure mercury compression experiments and low-temperature gas adsorption (N₂ and CO₂) experiments with whole pore size. The main controlling factors affecting the pore development of the shale are discussed. (i) The micropores and mesopores are more developed in the shale, and the macropores are the second most developed in the shales of the Dawuba Formation in the Qianxi area, among which the mesopores and macropores contribute most to the pore volume and the micropores and mesopores contribute most to the pore-specific surface area. (ii) The microfractures and interlayer pores of clay minerals are developed in the shales of the Dawuba Formation, which are the main storage spaces for hydrocarbon gases. (iii) The main factors affecting the adsorption capacity of the shales of the Dawuba Formation in the Qianxi area are the organic carbon content and clay mineral content of the shales, both of which have an obvious positive correlation with the variation of pore structure.

A series of qualitative descriptions and quantitative analyses was used to determine the lithofacies characteristics and recognize the favorable shale intervals of Dawuba Formation in the Ziyun area of South Qian Depression. The qualitative descriptions include core description and scanning electron microscope (SEM) observation. The quantitative analyses include X-ray diffraction, total organic content analysis, vitrinite reflectance, maceral composition, porosity, and permeability, as well as gas and element composition. The Dawuba Formation could be divided into four members. In general, the shale in the first and third members showed similarly high organic matter content with most samples in range of 2–2.5% and higher brittle mineral content with a content of quartz 40.53% and 33.21%, respectively, compared with the other two members, as well as high gas content. However, the first member shale samples exhibited much higher porosity and permeability than the third member shale samples. Furthermore, the shale gas in the first member was chiefly composed of methane (average: 83.63%), while that in the third member mainly consisted of nitrogen (average: 79.92%). Hence, the first member should be regarded as the most favorable target.

Investigating shale pore characteristics has deepened our understanding of shale reservoir, while that of postmature-overmature shales is yet to be revealed, which is especially critical for shale gas evaluation in southern China. Ten Middle-Upper Devonian organic-rich shale samples were collected from well GY-1 in the Guizhong Depression, and the paleoenvironment, geochemistry, and pore system were analyzed with a series of experiments, including trace element analysis, X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), low-pressure N₂ adsorption, and source rock geochemistry. Results show that the Middle-Upper Devonian shales in the Guizhong Depression are organic-rich mudstones with TOC ranging from 0.14% to 6.21%, which is highest in the Nabiao Formation () and Lower Luofu Formation () that were deposited in the anoxic and weak hydrodynamic deep-water shelf. They are thermally postmature to overmature with equivalent vitrinite reflectance () of 3.40%~3.76% and type I kerogen. The lithofacies in and are primarily siliceous/argillaceous mixed shale as well as a few siliceous argillaceous shales and argillaceous siliceous shales as well. Organic matter- (OM-) hosted pores within bitumen are primary storage volume, rather than inorganic pores (interparticle and intraparticle) which are rare. The total helium porosity of samples varies between 1.20% and 4.49%, while total surface area and pore volume are 2.39-14.22 m²/g and 0.0036-0.0171 ml/g, respectively. Porosity, pore surface area, and pore volume are in accordance with increasing TOC, , and siliceous mineral contents. Considerable OM-macropores are found in shales with in our study which demonstrates that the porosity at postmature to overmature stage ( ) does not change fundamentally. The high level of maturity is not considered the main controlling factor that affects shale gas content, and more attention should be paid to preservation conditions in this area.

The Early Carboniferous Dawuba Formation within the Yadu-Ziyun-Luodian aulacogen (YZLA), southern China, represents a significant shale gas play formed within a dynamic platform-basin transitional sedimentary system influenced by multi-sphere coupling (the dynamic interaction of tectonic, climatic, and oceanic spheres). This setting generated pronounced heterogeneities in lithology and gas content. Integrating field geological surveys and well data, this study comprehensively evaluates the mineralogy, geochemistry, structural features, and gas content of these shales to decipher the key factors controlling gas enrichment, particularly in this tectonically complex area. A crucial finding is the distinction between low-energy, “thick-bedded” deposits and higher-energy, “thin interbedded” sections characterized by frequent alternations of mudstone and argillaceous limestone (0.5–5 m single layers, mudstone:limestone ratio 1:1–1:2). Significantly, these thin interbedded sections demonstrate superior reservoir quality, exhibiting higher porosity (∼2.9%) and notably higher gas content (∼2.5 m³/t). We reveal that in the YZLA’s complex structural setting, shale gas content is primarily governed by preservation conditions dictated by the unique litho-mechanical properties of the interbedded strata, rather than traditional controlling factors like TOC abundance or matrix porosity. The frequent interlayering promotes stable micro-fractures, supported by clay-rich horizons and effectively sealed by compact limestone, thereby enhancing secondary storage capacity and permeability for free gas. Consequently, these thin interbedded sections represent the most favorable intervals for shale gas accumulation, identifying them as the prime targets for achieving Early Carboniferous shale gas breakthroughs in the YZLA.

As a non-renewable strategic resource, trace helium (He < 0.1%) is critical for sustainable industrialization and innovation. However, its precise assessment in natural gas reservoirs presents significant challenges due to its extremely small kinetic diameter (0.26 nm) and high diffusivity, which lead to severe escape risks and detection uncertainty. To quantify systematic errors introduced by sampling containers and elucidate the underlying physical mechanisms, this study established a rigorous comparative framework to evaluate the immediate capture efficiency and long-term stability (30 days) of trace helium in double-valve stainless steel cylinders versus aluminum-plastic composite bags (total n = 24). Results demonstrate fundamental disparities in the barrier mechanisms and mass transfer characteristics of the two containers. Immediate analysis reveals that composite bags, limited by a low-pressure laminar purging mode, fail to eliminate residual air within the dead volume. This results in a marked background dilution effect and anomalous initial O₂ concentrations. Long-term monitoring indicates that trace helium in composite bags follows the polymeric “solution-diffusion” mechanism, exhibiting severe non-linear decay (up to 78.7% loss over 30 days) accompanied by bi-directional permeation and irreversible component exchange with atmospheric air. In contrast, stainless steel cylinders effectively block physical permeation pathways due to their dense face-centered cubic (FCC) lattice and metal-to-metal sealing structure. Observed data fluctuations (Δ < 0.008 mol%) fall entirely within the reproducibility limits defined by gas analysis standards, confirming a metrologically zero-loss characteristic. Error analysis further reveals that systematic bias introduced by bag permeability (~ 79%) exceeds random instrumental error (< 2%) by a factor of approximately 40, establishing storage container integrity as the decisive limiting factor for detection accuracy. Consequently, the applicability boundary for flexible bags is strictly confined to immediate, on-site rapid screening (< 24 h) under mild environmental conditions. We recommend establishing a hierarchical container selection protocol based on “component characteristics, storage duration, and logistical operational windows.” Furthermore, current sampling guidelines should be revised to mandate the use of high-pressure metal-sealed cylinders for any medium-to-long-term archiving of trace noble gases. Ensuring geological data fidelity through such stringent metrological controls is essential to prevent resource misallocation, thereby promoting responsible exploration and production practices.