Differentiation and quantification of pore-fracture structures in marine and transitional shales

Marine shales are the primary targets for commercial shale gas development in China, while transitional shales represent a significant exploration potential. Understanding the pore and fracture structures in these shales is crucial for China's shale gas resource strategy. This study systematically analyzes the development characteristics of pores and fractures in marine and transitional shales. Using the hysteresis loops from low-temperature N2 adsorption–desorption curves, combined with previous classifications of pore morphology, the “visual slope” parameter is proposed to differentiate and quantify the proportions of pores and fractures. The visual slope correlates with pore and fracture distribution: higher values indicate a dominance of pores, while lower values indicate a prevalence of fractures. When the visual slope is <0.4, fractures dominate; between 0.4 and 0.8, both pores and fractures are well developed; and >0.8, pores are more prevalent. Marine shales typically exhibit higher visual slopes, indicating well-developed pores and fractures, whereas transitional shales show lower slopes, reflecting fracture dominance. Further analysis of the controlling factors revealed that both burial depth and thermal maturity negatively impact the development of micropores and mesopores in both shale types, while total organic carbon has a positive influence, depending on the type of organic matter. In marine shales, the high quartz content, as a brittle framework mineral, promotes macropore development. Pyrite contributes positively to pores and fractures development across various scales due to its irregular crystal growth, which facilitates intercrystalline pores formation, and postgrowth crystal disintegration, which creates larger mold pores. While clay typically hinders pores and fractures development in marine shales, they promote pore and fracture formation in the more clay-rich transitional shales.