Effects of organic-inorganic transformation and interaction on the occurrence of nanopores within the organic-rich shale during thermal maturation: Insights from the hydrous pyrolysis experiments in a closed system

The organic-inorganic transformation and interaction act as the critical role in the occurrence of nanopores within the organic-rich shales during thermal maturation. Hydrous pyrolysis experiments were conducted on the organic-rich mudrock collected from the Upper Cretaceous Nenjiang Formation of the Songliao Basin, China in a closed system. The pore types and pore network, and organic and inorganic compositions of pyrolyzed shales were detected from the early to over mature stages (%R_o = 0.61–4.01). The experimental results indicate that geochemical transformation of organic matters and minerals and the interaction control the formation and evolution of nanoporosity. In oil window mineral matrix pores are infilled by the generated oil, K-feldspar dissolution by organic acids promotes clay illitization to form illite, and the catalytic effects of clays (e.g. illite) in the complex of organic matter and clays may promote the in-situ retained oil cracking to generate natural gas, resulting in the early occurrence of organic-matter pores in the complex within oil window. Due to significant primary cracking of solid kerogen to generate extractable liquid oil, pore volume for storing fluids presents a persistent increase and approaches the maximum at the end of oil window. In gas window intensive oil cracking facilitates the hydrocarbon migrating out of the source home and pyrobitumen formation, resulting in the significant occurrence of modified mineral matrix pores and organic-matter pores. Pore volume for hosting hydrocarbons presents a slight decrease at %R_o = 1.36–2.47 due to pyrobitumen formation by oil secondary cracking. The organic-inorganic interaction favors clay illitization, quartz dissolution, and pyrite and carbonate decomposition, which facilitate the occurrence of nanoporosity. Pyrobitumen within the complex with illite and organic matters are much more porous than that hosted in modified mineral matrix pores and microfractures. The catalytic effects of clays are supposed to be responsible for this. This study improves our understanding of the formation and evolution pathways of nanoporosity and the underlying controls in organic-rich shales during thermal maturation, and hence should be helpful in evaluating the sweet spots for shale-oil and shale-gas plays in a sedimentary basin.