Division of diagenesis and pore evolution of a Permian Shanxi shale in the Ordos Basin, China

Abstract

This research analyzed the pore evolution and diagenesis in a Permian Shanxi shale from the Ordos Basin. Thermal simulation method simulated the thermal evolution of organic matter in a sample from mature (Ro = 0.96%) to overmature (Ro = 3.05%). Gas adsorption and mercury intrusion capillary pressure techniques were used to analyze pore-size distribution across a maturation gradient. X-ray diffraction, Focused Ion Beam-Scanning Electron Microscopy and rock pyrolysis techniques were used to divide the stages of diagenesis and pore evolution. Results showed that mesopores and macropores dominate pore volume while micropores and mesopores dominate surface area. Pore volume and surface area initially decreased and then increased with maturity. Kaolinite is converted to illite and illite-smectite mixedlayer in the middle to late diagenesis stage. Pore evolution and diagenesis can be divided into four stages in a sample from mature to overmature. Factors influencing pore-size distribution are different in four stages.