The effect of the variation in material composition on the heterogeneous pore structure of high-maturity shale of the Silurian Longmaxi formation in the southeastern Sichuan Basin, China

Abstract

Studying complex pore structures is the key to understanding the mechanism of shale gas accumulation. The effect of the variation in material composition on the heterogeneous pore structure of high-maturity shale of the Silurian Longmaxi formation was analyzed based on fractal geometry using field emission scanning electron microscopy (FE-SEM), low-pressure N2 adsorption, and high-pressure mercury intrusion porosimetry (MIP). The results show that abundant micro- and nano-scale pores were distributed in all shale samples, with the majority of pores composed of ink-bottle-shaped organic pores and slit-shaped mineral matrix pores. Samples with different total organic carbon (TOC) contents and mineral compositions showed distinct differences in pore structure. Organic matter and clay mineral primarily affected the mesopore structure, and the heterogeneity of the mesopore structure increased with the clay mineral content. Plastic minerals, such as quartz and carbonate, primarily affected the macropore structure. The increase in the quartz content could reduce the heterogeneity of the macropore structure, and an increase in the carbonate mineral content could increase the heterogeneity of the macropore structure. Finally, organic matter could strengthen the heterogeneity of the macropore structure only when the TOC content was greater than 1.1 wt. %.