Characterization of Pore Structures of Jiulaodong Formation Shale in the Sichuan Basin by SEM with Ar-ion Milling

Microscopic pore structure is the key to studying shale gas reservoirs. Traditional Scanning Electron Microscope cannot be used to identify topographic irregularities caused by differential hardness of components during mechanical polishing. Microscopic pore structures of Jiulaodong formation shale in the Weiyuan area of the Sichuan basin were investigated using scanning electron microscope coupled with an Ar-ion milling technique. Three types of pores were identified: (1) mineral matrix pores (pore diameters are mostly 100-500 nm), mainly including interparticle pores and intraplatelet pores within clay aggregates, and intercrystalline pores within pyrite/anatase and biological pores; (2) organic matter pores (pore diameters range from tens of nanometers to several micrometers), either controlled by thermal evolution of kerogen or clay minerals coated by organic matter; (3) micro fractures (with length of dozens of micrometers and width of a few micrometers), including shrinkage fracture, tectonic fracture, overpressure fracture and artificial fracture. In conclusion, micro-nano pores in Jiulaodong shale are mainly composed of mineral matrix pores and micro fractures which provide the main storage space and flow channel for shale gas.