Genetic mechanism and evolutionary pattern of pore structure in deep marine shales in Sichuan Basin, South China: Insights from SEM observation and quantitative characterization

Abstract This study addresses gaps in research on the pore structures of deep, organic-rich marine shales, focusing on the lower Silurian Longmaxi Formation in Sichuan's Luzhou Block, using wells L1 and L2. Employing techniques like organic geochemistry, mineral composition analysis, CO2 and N2 adsorption-desorption, high-pressure mercury intrusion, FIB-SEM, and NMR, the study identifies pyrobitumen as the main pore contributor, accounting for approximately 59.95%-62.00% of pores. It documents a reduction in pore storage and connectivity from the over-mature to semi-graphitised stages, with mesopores showing a smaller decline (8.37%-9.14%) compared to micropores and macropores (25.02%-52.86%). Additionally, connectivity of 100–1000 nm pores decreased significantly (33.46%-62.09%). Changes in the stages of maturity affected the pore characteristics: retained oil in mature stages transformed into pyrobitumen, reducing surface porosity. In the over-mature stage, diagenesis and thermal evolution led to gas-filled pyrobitumen pores which balanced with formation pressures to preserve pore structure. The semi-graphitised stage saw a significant reduction in pore volume and surface porosity due to compaction and carbonization.