Pore Characteristics and Controlling Factors of the Yanchang Formation Mudstone and Shale in the South of Ordos Basin

Key words: 
• Yanchang shale /
• pore characteristic /
• low-temperature nitrogen adsorption /
• residual hydrocarbon

Abstract: Taking the Yanchang Formation Shale for Well YK-1 in Tongchuan Area of the south of Ordos Basin as the target, pore characteristics and controlling factors were investigated by using Argon-ion Polishing Scanning Electron Microscope(SEM)and low-temperature nitrogen adsorption experiments. The results show that pores of Yanchang Formation shale can be classified into inter-granular pore, intra-granular pore, intra-crystalline pore in pyrite, dissolved pore and micro-cracks, of which inter-granular pore in clay mineral is most developed, but organic pore is basically not developed. The pore characteristics of Yanchang Formation in different members have obvious differences. Chang 9 has larger proportion of micropores along with larger specific surface area, while the pore volume of Chang 8 is occupied by largest mesopores and pore morphology of Chang 8 and Chang 9 is mainly in the shape of column and slit parallel to the wall. Chang 7 has the largest proportion of macropores, smallest proportion of micropores and corresponding smallest specific surface area, of which there are many close pores and wedge-shaped pores with one or both ends open. The proportion of pore volume and specific surface area of Chang 6 are between other members, the pore morphology of which is mainly parallel-plate shaped silt pores. The content of clay is the main controlling factor of pore development, but TOC content shows a negative correlation with the pore volume and the specific surface area, mainly due to residual hydrocarbon which block the pore space. The pore volume, the specific surface area will increase after extraction.