Comparative study of nanoscale pore structure of Lower Palaeozoic marine shales in the Middle-Upper Yangtze area, China: Implications for gas production potential

摘要

The Lower Cambrian Niutitang and Lower Silurian Longmaxi shales in the Middle-Upper Yangtze area are considered the primary shale gas units targeted for development in China. To shed some light on the difference in nanopore structures between Niutitang and Longmaxi shales, systematic comparative investigations were conducted using various techniques, including geochemical analyses, field emission scanning electron microscopy (FE-SEM), high-pressure mercury intrusion porosimetry (MIP), and low-pressure N-2/CO2 adsorption techniques. The results show that both Niutitang and Longmaxi shales have high total organic carbon (TOC) content and complex mineral compositions. The porosity of Longmaxi shales is higher than that of Niutitang shales, with an average value of 3.26% and 2.04%, respectively. Interestingly, for both shale formations, the mesopores (2-50 nm) are the major contributors to pore volumes, whereas the specific surface area is dominated by micropores (2 nm). For the mesopore size distributions (PSDs) calculated from the N-2 adsorption, the Longmaxi shales have a dominant pore size ranging from 10 to 60 nm. In contrast, there are more fine mesopores (2-8 nm) in the Niutitang shales. Furthermore, we found that numerous nanoscale pores are well-developed within graptolite-derived organic matter (OM) in the Longmaxi shales. These interconnected graptolite periderm pore systems may not only provide a storage space for both adsorbed and free gas but also serve as pathways for gas transport. The Niutitang shales developed relatively fewer OM pores with smaller diameters, lower OM surface porosity, and lower connectivity compared to the Longmaxi shales. The differences in OM pore structure partly explain why there is a large production difference between these two formations.