This paper tries to determine the key evaluation parameters of shale reservoirs in the complex tectonic provinces outside the Sichuan Basin in South China, and also to target the sweet spots of shale reservoirs accurately. The pore-structure characteristics of the Lower Silurian Longmaxi shale gas reservoirs in Well LD1 of the Laifeng–Xianfeng Block, Upper Yangtze region, were evaluated. N₂ adsorption and helium ion microscope (HIM) were used to investigate the pore features including pore volume, pore surface area, and pore size distribution. The calculated results show good hydrocarbon storage capacity and development potential of the shale samples. Meanwhile, the reservoir space and migration pathways may be affected by the small pore size. As the main carrier of pores in shale, organic matter contributes significantly to the pore volume and surface area. Samples with higher total organic carbon (TOC) content generally have higher porosity. Based on the Frenkel–Halsey–Hill equation (FHH model), two different fractal dimensions, D₁ and D₂, were observed through the N₂ adsorption experiment. By analyzing the data, we found that large pores usually have large values of fractal dimension, owing to their complex pore structure and rough surface. In addition, there exists a good positive correlation between fractal dimension and pore volume as well as pore surface area. The fractal dimension can be taken as a visual indicator that represents the degree of development of the pore structure in shale.

The characteristics of the nanopore structure in shale play a crucial role in methane adsorption and in determining the occurrence and migration of shale gas. In this study, using an integrated approach of X-ray diffraction (XRD), N₂ adsorption, and field emission scanning electron microscopy (FE-SEM), we systematically focused on eight drilling samples of marine Taiyuan shale from well ZK1 in southern North China to study the characteristics and heterogeneity of their nanopore structure. The results indicated that different sedimentary environments may control the precipitation of clay and quartz between transitional shale and marine shale, leading to different organic matter (OM)–clay relationships and different correlations between total organic carbon (TOC) and mineral content. The shale with high TOC content tended to have more heterogeneous micropores, leading to a higher fractal dimension and a more complex nanopore structure. With the increase of TOC content and thermal evolution of OM, the heterogeneity of the pore structure became more significant. Quartz from marine shale possessed abundant macropores, resulting in a decrease of the Brunauere–Emmette–Teller (BET) surface area (SA) and an increase of the average pore size (APS), while clay minerals developed a large number of micropores which worked together with OM to influence the nanopore structure of shale, leading to the increase of the SA and the decrease of the APS. The spatial order of interlayer pores increased with the increase of mixed-layer illite–smectite (MLIS) content, which naturally reduced the fractal dimensions. In contrast, kaolinite, chlorite, and illite have a small number of nanopores, which might enhance the complexity and reduce the connectivity of the nanopore system by mean of pore-blocking. Taiyuan shale with higher heterogeneity is highly fractal, and its fractal dimensions are principally related to the micropores. The fractal dimensions correlate positively with the SA and total pore volume, suggesting that marine shale with higher heterogeneity may possess a larger SA and a higher total pore volume.

Until recently, the characteristics of nano-microscale structures in the naturally deformed, overmature, marine shales were poorly known. Thermally overmature Lujiaping shales in the complex tectonic area of the northeast part of the upper Yangtze area, China have experienced strong tectonic deformation and are considered as potentially important strata for shale gas exploration. Naturally deformed samples from the main source rocks are selected from the Lower Cambrian Lujiaping Formation in the Dabashan Thrust-fold Belt to investigate nanometer- to micrometer-sized structures. A combination of scanning electron microscope (SEM), low-pressure nitrogen adsorption (LPNA), and low-field nuclear magnetic resonance (NMR) suggests that the pore types are dominantly fracture-related pores with a lesser abundance of mineral-hosted pores. These two pore types account for the 90% of total pore space. Organic matter (OM)-hosted pores are rare and make up a small part of the pore systems (less than 10%) due to high thermal maturity and intensive tectonic compression. Overall, the Lujiaping deformed, overmature samples have abundant nanometer- to micrometer-sized inorganic pores. High-resolution SEM images provide direct evidence of the formation of nano- and microsized structures such as OM–clay aggregates and silica nanograins. OM–clay aggregates are commonly observed in samples, which also exhibit abundant open microfractures and interparticle pores. Quartz can occur as silica nanograins and botryoids typically 20–100 nm in size, which may influence porosity through the creation or occupying interparticle pore space.