Mineralogical, microstructural and physiochemical characteristics of organic-rich shales in the Sichuan Basin, China

Although the Lower Silurian Longmaxi Formation shales in the Sichuan Basin have been widely investigated for their shale gas potential, detailed research on the differences between the Upper Longmaxi Formation (ULF) shales and the Lower Longmaxi Formation (LLF) shales is still scarce. In this study, the divergent properties of the LLF and ULF shales in the Sichuan Basin of China were investigated, including mineralogical characteristics, microstructural characteristics and physiochemical characteristics. The mineralogical characteristics and their influences, the relationships between the specific surface area, pore volume and TOC, mineralogical compositions, and the influences of microstructure on the development of the Longmaxi Formation (LF) shale gas reservoir were explored and discussed. The results indicate that the LF shales mainly consist of quartz and clay minerals. The ULF shales have higher clay minerals contents and lower quartz contents, whereas the LLF shales have lower clay minerals contents and higher quartz contents. Compared with the ULF shales, the LLF shales have higher TOC contents, which results in higher specific surface area and total pore volume, a larger fractal dimension, a more complex pore structure and thus a greater adsorption capacity. The LLF shales have weaker hydration swelling and stronger brittle features, indicating that the LLF shales would have good wellbore stability and good hydraulic fracturing. Because of fractures in the LF shale formation, designs for oil-based or water-based drilling fluid systems need to consider sealing technologies. For water-based drilling systems, hydration inhibition should be considered, especially for the ULF shale formation.