Pore Heterogeneity in Deep Marine Longmaxi Shales: Insights from Organic–Mineral Coupling

Deep marine Longmaxi shales in the southern Sichuan Basin display pronounced pore heterogeneity even within a single stratigraphic target, but the pore-scale controls remain uncertain. We integrated large-area, high-resolution scanning electron microscopy (SEM), automated mineral mapping, and quantitative image analysis for 17 Long 111 core samples from four deep wells to evaluate relationships among organic matter (OM) populations, OM abundance, mineral fabric, and SEM-visible pore development. OM-hosted pores account for 70–97% of the segmented pore area. The total SEM-derived pore-area ratio ranges from 0.23% to 2.39%, and the image-derived apparent transformation ratio (ATR) of OM ranges from 2.40% to 18.78%. Migrated pyrobitumen (PB2) generally contains coarser pores and has a higher ATR than in-place pyrobitumen (PB1), although this contrast diminishes in some high-TOC shales. OM-hosted pore development varies nonlinearly with total organic carbon (TOC) and is commonly most favorable at intermediate TOC. Bulk quartz content is not systematically related to pore development, whereas carbonate-rich fabrics are associated with restricted OM occurrence and poorer pore preservation. These observations indicate that pore development reflects the coupled effects of OM abundance and occurrence, pyrobitumen population, available accommodation, and mineral framework support. We propose three end-member configurations, including framework-supported, OM-connected, and carbonate-restricted configurations, to explain pore heterogeneity within the target interval. The results show that OM–mineral spatial relationships provide additional information beyond bulk TOC and mineral composition for understanding pore heterogeneity in deep shales.