Characteristics and Controlling Factors of Nanopores of the Niutitang Formation Shale from Jiumen Outcrop, Guizhou Province.

Abstract

This paper studies the characteristics and controlling factors of nanopores in organic-rich shale in the Niutitang Formation. Six samples were collected from the bottom of the formation at the Jiumen Outcrop, Guizhou Province. Experiments were conducted to investigate the pore structures of these high-maturity shale samples. The TOC contents vary between 4.81-17.51% with an average of 10.18%. The XRD data show that these samples are dominated by quartz (44%-71%), with a significant amount of clay minerals, such as illite, with a content of 8%-27.5%. Based on the low-pressure liquid N₂ sorption measurements, the pore structures can be divided into two groups. Group A including samples of N-2, N-3 and N-4, mainly develop slit-shaped pores, mesopores and macropores. Group B shown from samples N-1, N-5 and N-6, are mainly composed of narrow slitlike pores, which may provide more space for shale gas than slit-shaped pores. The mesopores, macropores, porosity and specific surface areas of group B are more developed than those of group A. With the comparison of pore structures in shales with various organic matter and mineral contents, the dissolution of quartz and feldspar can be the important factor controlling pore development. The evolution of diagenesis is closely related to pore evolution. This diagenesis has various types and complex effects on the pores, mainly including compaction, dissolution and cementation.