Pore structure characterization of different lithofacies in marine shale: A case study of the Upper Ordovician Wufeng-Lower Silurian Longmaxi formation in the Sichuan Basin, SW China

Pore structure has distinct features in different shale lithofacies. This paper presents a case study of the Upper Ordovician Wufeng-Lower Silurian Longmaxi formation in southeast Sichuan Basin to fully describe the characteristics of shale pore structure in different lithofacies. 16 types of shale lithofacies were classified using ternary diagram of siliceous minerals (quartz + feldspar, QF), carbonate minerals (Ca) and clay minerals (Cl). Among all, argillaceous-rich siliceous shale lithofacies (S-3), argillaceous/siliceous mixed shale lithofacies (M-2) and siliceous-rich argillaceous shale lithofacies (CM-1) are the predominant lithofacies. According to scanning electron microscope (SEM) images, combined with focused ion beam-scanning electron microscope (FIB-SEM), mercury intrusion porosimetry (MIP), and gas adsorption (N2 and CO2), the pore structure characteristics and its influencing factors were systematically analyzed. The results show that: Illite + illite/smectite minerals primarily influence the development of the inorganic pores in mixed clay and silica lithofacies (CM-1). Clay minerals are protected from compaction by the rigid minerals, which significantly favors the development of mesopores (2-50 nm) and macropores (>50 nm). The abundance of organic matter is an important determinant for the organic pores in the siliceous lithofacies (S-3), and TOC has a positive contribution to the development of micropores (<2 nm) in this lithofacies. Shale with high TOC content tends to be easily affected by compaction, which results in reduction of some organic pores. Even though the siliceous S-3 reservoir has slightly lower porosity than CM-1, it is currently the primary reservoir target due to its brittle response to hydraulic fracturing.