Micron-Nano Pore Structures and Microscopic Pore Distribution of Oil Shale in the Shahejie Formation of the Dongying Depression, Bohai Bay Basin, Using the Argon-Scanning Electron Microscope Method

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To investigate the pore structure of shale oil reservoirs, seven organic-rich shales from the Shahejie Formation in the Dongying Depression were studied by rock pyrolysis analysis, X-ray diffraction analysis and scanning electron microscopy with argon ion beam polishing. The proportions of the different types of pores at different scales, the statistical relationships between the mineral contents and pore development, and the development of pores in the mineral laminae combination were discussed. The results demonstrated that the nanometer to micron pore structures were divided into intraparticle pores, interparticle pores, intercrystalline pores, organic pores, microfissures and diagenetic contraction fractures. Interparticle pores and intraparticle pores are both essential components of the sample pores. The original residual pores are mainly small pores and mesopores, while the secondary dissolution pores are primarily mesopores and macropores. For different pore sizes, there are small surface pore contributions of diagenetic contraction fractures, microfissures, intercrystalline pores, and organic pores. The carbonate minerals content controls the oil storage capacity of shale by dominating the development of the various pore types, while the clay mineral bands content can affect the permeability of shale by influencing the development of large-scale microfissures. In the laminae development sample, surface pore development is closely related to the sedimentary mineral microcircle, which consists of a falling semicycle and a rising semicircle. The total surface porosity, including microfissures, diagenetic contraction fractures, interparticle pores and intraparticle pores, mainly developed at the intersection of the rising semicircle and the falling semicircle, and this development corresponds to the highest level of a cycle. In summary, interparticle pores and intraparticle pores are the main components of pores developed in low thermal maturity shale and shale laminae where their heterogeneity is influenced by mineral composition and laminae microcircles.