Organic-rich marine shale in South China has great potential for gas generation, and the pore structure has a considerable influence on the enrichment of shale gas. The nanoscale pore characteristics and fractal characteristics of the Niutitang shales in the Fenggang block were studied by means of methane adsorption and low-pressure nitrogen adsorption experiments, porosity and permeability tests, X-ray diffraction analyses, organic geochemical analyses, and field emission scanning electron microscopy. The fractal geometry of the pore structure was assessed, and the factors influencing the pore structure were determined. We used a fractal Frenkel–Halsey–Hill-based method to obtain the fractal dimensions D1 and D2 by nitrogen adsorption at relative pressures of 0–0.5 and 0.5–1, respectively. The relationships between the shale pore structure parameters and the fractal dimensions, the mineral composition, the total organic carbon content, and the pore structure parameters of shale are discussed. In addition, the significance of D1 and D2 and the implications of the fractal dimension for the adsorption capacity of shale are investigated. The results show that the two fractal dimensions are positively correlated with total organic carbon content and specific surface area, negatively correlated with quartz content and average pore diameter, but have a weak relationship with clay mineral content and total pore volume. The Niutitang shale samples are dominated by mesopores, and the pore structure is complicated. The results presented here indicate that fractal analyses represent an effective method of characterizing the complexity of pore structure.

There are abundant shale gas resources in the Lower Paleozoic marine organic shale strata of the upper Yangtze region, South China. However, affected by multi-period tectonic activities, leading to poor exploitation of the Lower Paleozoic marine shale gas in the complex tectonic zone. Therefore, this study proposes fracturability index (FI) based on shale brittleness, fracture toughness, and stress decay rate. Therefore, this study combines logging data and rock mechanics experiments to obtain the rock mechanics parameters of the Fenggang block shale reservoir. Based on Young’s modulus, fracture toughness and critical strain energy release rate, combined with the analytic hierarchy process, the shale fracturability index is proposed to guide the exploration and development of shale gas. The fracturability index of the Longmaxi Formation shale in the study area is 0.34 ~ 0.72. The fracturability of the middle and lower part of the Longmaxi Formation shale is obviously better than that of the upper part. In general, shale reservoirs with high brittleness index, low fracture toughness, and low critical strain energy release rate have high fracturability.

Southern China is affected by multi-stage tectonic activities, with strong fold deformation, complex fault systems and poor shale gas preservation conditions. Here, we used shale samples from the lower Silurian Longmaxi shale in the complex tectonic area of Southern China, to study the relationship between differential structural deformation, and pore structure and adsorption capacity. According to the deformation mechanism of the shale, it is further divided into brittle-slip rheological deformation (BD) and ductile-slip rheological deformation (DD). The results show that all micro-fractures can be observed under scanning electron microscopy in deformed shale samples, but in shale samples with different types of rheological deformation, the micro-fractures have large differences in scale, fracture length and lateral connectivity. The micro-fractures developed in DD shales are small in scale and short in fracture length, but have strong local connectivity. In contrast, brittle minerals are more developed in BD shales, and interlayer shearing has formed micro-fractures with large fracture length and good lateral connectivity, which is beneficial for later fracturing. In these two types of deformed shales, pores in organic matter are rare, and sporadic organic pores have small pore size and poor connectivity. The total pore volume (1.8–2.4 × 10⁻² cm³ g^–1) of BD shale samples is higher than that of DD shale samples (0.8–1.6 × 10⁻² cm³ g^–1). There is a positive correlation between total pore volume and quartz content. In addition, the specific surface area (12–18 m² g^–1) of DD shale samples is larger than that of BD shale samples (6–12 m² g^–1).

The marine shale gas in South China has great exploration potential, and exploration in the Sichuan Basin has been successful, but the degree of exploration is low in Guizhou Province. We used organic geochemical analyses (total organic carbon content and kerogen type), ordinary scanning electron microscopy (SEM), field emission scanning electron microscopy (FE-SEM), nuclear magnetic resonance (NMR), X-ray diffraction (XRD) analysis, and low-temperature CO₂ and N₂ adsorption experimental methods to study the micropore types and pore structures and their effects on the methane adsorption capacity of organic-rich shales found in the Fengang block in northern Guizhou Province. The results show that the microscopic surface porosity of the Lower Cambrian Niutitang Formation ranges from 2.88% to 5.34%, and the average value is 3.86%. Based on nitrogen adsorption methods, the range of the average pore size distribution is 4.6 - 9.491 nm, and the average value is 6.68 nm. All of the samples exhibit significant unimodal distributions. The main pore size is less than 10 nm, and these pores account for the majority of the mesopore volume, which is generally consistent with NMR results. The methane adsorption capacity of the shale samples gradually increases in the range of 0-8 MPa at 30 ^°C and reaches a maximum at approximately 10 MPa, which indicates that the methane adsorption capacity increases with pressure. Positive correlations were found between the gas content and specific surface area, total pore volume and micropore volume. These strong correlations indicate that the Niutitang shale has a high specific surface area, a high pore volume and narrow-diameter pores, indicating that it has a high gas adsorption capacity. The results of this study provide valuable information regarding the adsorption characteristics of marine shales and the factors that affect those characteristics.

The marine shale gas in South China has great exploration potential, and exploration in the Sichuan Basin has been successful, but the degree of exploration is low in Guizhou Province. We used organic geochemical analyses, X-ray diffraction (XRD) analysis, and CH₄ adsorption experimental methods to study pore structures and their effects on the methane adsorption capacity of organic-rich shales. According to the measured isothermal adsorption data, and based on the theory of adsorption potential, the calculation model of shale adsorption gas under the influence of temperature and pressure is established, and the model is validated and applied by the measured isothermal adsorption data. The results show that the adsorption performance of shale samples is better, with the average adsorption capacity of 4.38 ml/g. The Langmuir model fits well with the adsorption curves. The adsorption capacity of shale samples increases as total organic carbon (TOC) increases and temperature decreases. The adsorption potential theory was used to explain the influence of shale adsorption factors.

The brittleness of shale determines the fracturability of the shale reservoir and has a great influence on the exploration and development of shale gas. Therefore, prediction of brittleness and evaluation of fracturability of shale are very important in finding favorable areas for shale gas. We have used the mineral composition, rock-mechanics experiment, logging evaluation, 2D seismic data inversion, and fracture development degree to evaluate and analyze the vertical and plane brittleness characteristics and the main controlling factors of Longmaxi shale. In addition, we established the fracturability index of shale based on the brittleness index, critical strain energy release rate, and fracture toughness. The results indicate that the brittle mineral content of Longmaxi shale in the Fenggang block is between 69% and 90%, and shale samples are prone to brittle deformation by microscopic observation. Brittle deformation has a positive effect on the porosity and percolation ability of shale; the fractures in the upper and lower parts of the Longmaxi shale are relatively developed, and the degree of core fracture development is consistent with rock brittleness. The brittleness distribution of 2D seismic data inversion is in good agreement with the brittleness predicted by well logging; shale reservoirs with low fracture toughness, low critical strain energy release rate, and high brittleness index have high fracturability. Therefore, the research on shale brittleness and fracturability of Lower Silurian Longmaxi shale is expected to have important guiding significance for shale-gas exploration and development in southern China.