Progress and prospects in the research on pore structures of organic-rich mud shales

Exploring the division scheme, frontier characterization method, evolution and influence factors of pore structures of organic-rich shales is important for the evaluation and development strategy of shale oil reserves. The classification scheme of the pore structure and the advantages of different modern testing tools in characterizing the pore structure of mud shale were summarized by investigating the research progress on the pore structures of organic-rich shales. The evolutionary patterns of pore structures and the main controlling factors of organic pores and inorganic pores in marine shales and continental shales were discussed. The future development trend of pore research in organic-rich shales was foreseen. Our results indicate that the means of shale reservoir characterization can be divided into four main categories: imaging method, fluid intrusion method, adsorption method, and scattering method. Infrared-linked atomic force microscopy (AFM-IR) can reveal non-homogeneity in the chemical and rock mechanics of microscopic components in mud shales. The application of small-angle neutron scattering (SANS), nuclear magnetic resonance (NMR), and nano-CT is an important way to reveal pore connectivity. The sedimentary environment controls the lithofacies of shale and the source of organic matter. Diagenesis, hydrocarbon generation and their interaction are the main controlling factors for pore evolution of shale. The porosity of marine and terrestrial mud shale exhibits a pattern of decreasing, subsequently increasing, once again decreasing, then increasing again, and finally decreasing, with time and depth. However, in the immature to mature stage of continental shale, the porosity changes occur more frequently compared with marine shale. The development potential of organic pores of type Ⅰ kerogen is much higher than that of type Ⅲ kerogen, and abundant organic pores can develop in sapropelic macerals with strong hydrocarbon generation capacity. Pyrobitumen, formed by secondary cracking of migratory liquid hydrocarbon, can provide a more efficient and continuous infiltration path. The interconversion between different inorganic minerals during diagenetic processes, the differential dissolution of feldspar, carbonate rock and other minerals, as well as the compaction, pressolution, and cementation of minerals, all complicate the inorganic pore network. Organic-inorganic interactions and the rock mechanical properties of minerals are also important influences on the development of inorganic pores. The environments in which cores are located at the surface and in the subsurface vary greatly, and future research needs to establish a feedback mechanism and correction mechanism for pore structure between surface and subsurface realities of the core, in order to further restore the real state of shale oil and gas in the pore structures of the subsurface.