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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. [ABSTRACT FROM AUTHOR]

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). [ABSTRACT FROM AUTHOR]

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 CH4adsorption 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. [ABSTRACT FROM AUTHOR]

The Lower Cambrian organic-rich marine shale, which is a significant source of China’s shale gas, is widely distributed in southern China. An integrated characterization of the Niutitang shale is provided in this study in terms of organic geochemistry, mineralogy, pore characterization, methane sorption capacity, rock mechanical properties, fractures and gas content based on samples from three wells. The results indicate that the Lower Cambrian Niutitang shale is thermally over-mature and has rich shale gas resources, with a total organic carbon content (TOC) between 0.51% and 10.49% and a high quartz content between 35.3% and 78.5%. Compared to the major gas-producing shales in the U.S. and China, most of the organic matter (OM)-hosted pores in the Niutitang shale are generally smaller than 5 nm, significantly affecting the methane sorption capacity. The inter-particle and intra-particle pores and fractures are the primary sources of storage space, especially for free gas. For samples with TOC values less than 6.5%, TOC is positively correlated with the total porosity, total pore volume, brittleness (Young’s modulus), core fracture density, free gas content and Langmuir pressure; however, for samples with TOC values greater than 6.5%, the positive correlations become negative. These characteristics are due to the ductility and low hydrocarbon generation potential of organic matter in high thermal maturity shales that are vulnerable to compaction. Thus, TOC has a significant impact on the macroscopic (e.g., brittleness) and microscopic (e.g., pore structure and sorption capacity) properties of shale reservoirs, potentially controlling the enrichment and productivity of shale gas. These results can be used to optimize drilling and fracturing stimulation intervals during shale gas exploration and development. [ABSTRACT FROM AUTHOR]

Natural fractures are an important storage space and main seepage channel of shale gas reservoirs, controlling the permeability of shale gas reservoirs and the migration, enrichment, preservation conditions and single-well productivity of shale gas. The successful production of shale gas has shown that the development of natural fractures in shale is beneficial, and a high degree of natural fracture development can allow a large-scale fracture network to form in the process of hydraulic fracturing, which can effectively improve the shale reservoir conditions. The development of fractures in shale reservoirs not only effectively improves the permeability of shale reservoirs but also increases the gas content, improving the productivity of shale reservoirs. Therefore, the degree of shale fracture development is the key to the production of shale gas reservoirs. Through the outcrop, shale core, thin section and scanning electron microscopy observations of the Longmaxi shale in the Fenggang block of Guizhou Province and the experimental data analysis of the corresponding samples, the fracture characteristics are statistically analyzed, and the fracture development characteristics and dominant factors are discussed in depth. The study area is located in the complex tectonic area of southern China, which experienced a strong tectonic deformation and a high degree of fracture development. The multiscale characterization of Longmaxi shale fractures indicates the following: the fractures observable in the field are mainly regional structural fractures, fold-related fractures, fault-related fractures and bedding fractures; additionally, the statistical analysis of core fractures shows that the structural fractures, including high-angle shear fractures, tension-shear fractures, and low-angle slip fractures are dominant and that most of them are filled with calcite; the micro-fractures are mainly intragranular fractures and intergranular fractures, and the fracture aperture is 0.01–2 μm. The degree of fracture development is controlled by structural factors, lithology, mineral composition and content, TOC, rock mechanical properties, paleoenvironment and diagenesis, and the structural factors are the most influential. The gentle transition zone of a fold limb is a favorable position for shale gas structure preservation. Fractures significantly impact the formation of seepage channels and storage space and the connection of isolated micro-pores, so the degree of fracture development has a positive relationship with the total and analytical gas contents. • The developmental characteristics of reservoir and fractures in Longmaxi shale. • The effects of fractures on shale reservoirs and gas contents. • The controlling factors and their interrelations of fractures in shales. • Summarized the favorable position for shale gas structure preservation and fracture development model of Longmaxi shale. [ABSTRACT FROM AUTHOR]

The great potential that southern China has for marine shale gas resources is well known, yet shale gas production is very small. Exploration and development of southern China has lagged because of the complicated structural history of this region. Thus, it is important to recognize the tectonic history of this region and to carefully describe the developmental characteristics of natural fractures associated with the tectonic evolution of southern China. In this paper, we discuss the developmental characteristics of natural fractures based on the description and statistical analysis of natural fractures in outcrops, cores and thin sections, and from several experimental tests. Fractures examined in outcrop appear to be principally fold-fault related, regional tectonic, bedding or weathering fractures. Fractures observed in core are mainly shear fractures, tension fractures, and shear-tension fractures. Most of these fractures are sealed by calcite. Micro fractures and pores are well developed. Analysis of the cross-cutting relation of micro fractures suggests that the study area has been subject to at least two tectonic movements, consistent with our investigation of core. Fracture development appears to have been strongly influenced by structural evolution. However, fracture density displays a weak negative correlation with organic matter content. Further, fracture density appears to correlate positively with quartz content. Vertical variations of mineralogy appear to have influenced fracture generation and, therefore, hydrocarbon migration within the Niutitang Shale. [ABSTRACT FROM AUTHOR]