Lithofacies Characteristics of Marine Shale and Its Influence on Pore Structure: A Case Study of Lower Silurian Longmaxi Formation in Changning Area, Southern Sichuan Basin, China

ABSTRACT Research on shale pore structure has advanced the understanding of shale gas occurrence, migration and enrichment mechanisms. However, the factors affecting shale pore structure are complex and contentious, limiting a comprehensive understanding of shale gas accumulation. This study focuses on the Longmaxi (LMX) Formation shale in the Changning area of southern Sichuan, utilising X‐ray diffraction (XRD), organic geochemistry, thin section and scanning electron microscope (SEM) observation and nitrogen adsorption experiments to classify lithofacies types and examine how mineral composition and organic matter (OM) control pore structure and identify favourable shale reservoirs. The results show that the LMX Formation shale can be categorised into four different lithofacies: organic‐rich siliceous shale (RS), organic‐rich mixed shale (RM), organic‐lean siliceous shale (LS) and organic‐lean mixed shale (LM). The predominant pore types include OM pores and inorganic pores (mainly intergranular and intragranular pores), with mesopores contributing the majority of the total pore volume (PV) and specific surface area (SSA), accounting for approximately 70% and 80%, respectively. Characterised by high total organic carbon (TOC) content and well‐developed OM pores, RS and RM exhibit larger PV and SSA, suggesting better resource potential compared to LS and LM. In these high‐TOC shales, both SSA and PV show a strong positive correlation with TOC content and a negative correlation with clay mineral content. Conversely, in low‐TOC shales, SSA and PV are positively correlated with clay mineral content while the correlation with TOC diminishes. There is a tight intrinsic relationship between lithofacies characteristics and pore structure.