Insights into shale pore structure and microscopic fluid occurrence characteristics in the Paleogene Funing Formation, Subei Basin, China

Microscopic pore structure and mineral composition play a critical role in controlling the distribution and occurrence of shale fluids. This study examines shale samples from the Paleogene Funing Formation in the Gaoyou Sag of the Subei Basin using an integrated approach, including nuclear magnetic resonance (NMR) T₂ and T₁–T₂ tests, Rock-Eval, scanning electron microscopy (SEM), and low-temperature nitrogen adsorption/desorption (LTNA/D) analyses. This study focuses on the influence of organic matter, mineral composition, and pore structure on the occurrence and distribution of fluids. Results indicate that the pore types primarily consist of intergranular and grain-edge pores amidst quartz and calcite, with secondary contributions from intragranular pores associated with clay minerals. The NMR T₂ spectra are dominated by p2 (1–30 ms) and p1 (<1 ms) peaks, corresponding to mesopores (100–1000 nm), micro- and minipores (<100 nm), respectively. The NMR T₁–T₂ technique effectively quantifies pore fluid distributions and contents in both the as-received (AR) and water-oil restoration (WOR) states. The loss of original pore oil is mainly due to the migration of movable oil. The abundance of organic matter in shale affects its oil content, with excessive organic matter accumulation tending to restrict shale oil mobility. Pore fluid distribution is predominantly determined by pore structure. Capillary-bound water and adsorbed oil primarily occur in micro- and minipores, while the presence of pore water further influences the spatial distribution of adsorbed oil. Conversely, capillary-bound oil is primarily concentrated in mesopores, whereas movable oil is mostly stored in macropores (>1000 nm). Mineral contents in different lithologies exert a significant influence on the distribution of shale oil and water contents. Clay-rich shale typically exhibits higher amounts of capillary-bound water. Felsic-argillaceous mixed (FAM) shales, show that an increase in quartz content further enhances the occurrence and mobility of shale oil, while higher calcite content has the opposite effect. Conversely, in argillaceous-calcareous mixed (ACM) shales, which are dominated by calcite, higher calcite content is associated with larger accumulations of free oil. These findings may provide valuable insights into the mechanisms of shale oil enrichment and offer practical guidance for identifying sweet spots in the Gaoyou Sag, Subei Basin.