Evolution characteristics and controls of shale nanopores during thermal maturation of organic matter

Organic maturity is one of the main factors controlling the formation and evolution of nanopores in shale. The whole process of hydrocarbon generation was modeled for lacustrine shale in the 2nd member of Nenjiang Formation in the Changling Sag of Songliao Basin by using hydrous pyrolysis experiments in a closed system (R_o=0.61%-4.01%). Shale samples at different thermal evolution stages were solvent extracted. Based on geochemical analysis results of organic carbon content, N₂ adsorption and mineral composition, the formation and evolution characteristics and influencing factors of nanopores during thermal maturity of organic matter were systematically studied. The BJH pore volume and BET specific surface area of shale increase greatly after pyrolysis experiments, and the variation ranges are 0.006 73-0.101 61 cm³/g and 0.60-15.75 m²/g, respectively. Thermal degradation of kerogen and cracking of residual hydrocarbons promote the rapid development of nanopores in the mature to high maturity stage, and growth rate of nanopores slows down with weakening of hydrocarbon generation ability of organic matter in the over-mature stage. Liquid hydrocarbon is generated and fills nanopores during peak oil generation, which inhibits the formation of nanopores. Generation and expulsion of oil and gas play a leading role in development of nanopores. Organic nanopores may develop while solid pyrobitumen continuously enriched. Illitization of clay minerals and quartz dissolution are conducive to the development of nanopores.