Pore Structure Heterogeneity Controlled by Mineral-Organic Associations in the Wufeng-Longmaxi Shale, Upper Yangtze...

Mineral-organic matter associations (MOMA) critically influence organic matter (OM) distribution and pore development during kerogen pyrolysis and hydrocarbon generation. However, their impact on shale pore systems is controversial. In the Wufeng-Longmaxi Formation shale, seven MOMA types were identified through associations of detrital quartz, authigenic quartz, clay minerals, pyrite, and OM. Combining scanning electron microscopy (SEM) with energy dispersive spectroscopy (EDS), cathodoluminescence (CL) images, and trace elements geochemistry, we analyzed the characteristics, formation mechanisms, and distribution of the MOMA across different lithofacies. Quartz can be divided into detrital quartz and diagenetic quartz. Geochemical indicators (Al-Fe-Mn ternary plots, Rb/K₂O ratios) and petrographic analyses confirm that the primary sources of authigenic quartz are the dissolution of siliceous organisms and the clay mineral transformation. Authigenic quartz primarily originates from siliceous plankton, whose habitats promoted OM accumulation. AuthQ2 is mainly composed of authigenic quartz (average 36.4%), with the highest TOC (5.79%). The MOMA mainly composed of detrital quartz (DQ2) is characterized by low TOC (average 0.91%), resulting from dilution by terrigenous input. The OM has reached the overmature stage, and organic pores contribute substantially to the total porosity. MOMA control both OM distribution and pore characteristics. MOMA composed of both brittle and plastic minerals in association with OM (AuthQ2 and Clay2) are more conducive to preserving OM and pores than those formed by a single mineral type in association with OM (AuthQ1 and Clay1). AuthQ2 correlates well with bulk porosity, with pore sizes of 10-50 nm, surface porosity of 6.71%, and pore density >32 pores/μm². DQ2 exhibits a weak negative correlation with bulk porosity due to low TOC, with surface porosity of 3.14%, pore density of 3.3 pores/μm², and pores predominantly >50 nm. With AuthQ2 and AuthQ1 as the dominant MOMA, siliceous shale is the most favorable lithofacies, followed by mixed shale, argillaceous shale, and silty shale.