Characterization on Structure and Fractal of Shale Nanopore: A Case Study of Fengcheng Formation in Hashan Area, Junggar Basin, China

The Lower Permian Fengcheng Formation in Halaalate Mountain in the Junggar Basin has enormous potential for shale oil, while few investigations on quantifying pore structure heterogeneity have been conducted. Thus, total organic carbon (TOC), X-ray diffraction (XRD), scanning electron microscopy (SEM), and low-temperature N-2 adsorption tests were conducted on the shales collected from the HSX1 well in the Hashan region to disclose the microscopic pore structure and its heterogeneity. Results show that the selected shales mainly consist of quartz, plagioclase, calcite, and clay minerals. The primary pore types are intergranular pores in quartz and carbonate and intragranular pores in clays, while organic matter (OM) pores are rare. Typical types of H2 and mixed H2-H3 were observed. Type H3 shale pore size distributions (PSD) are unimodal, with a peak at about 70 nm, while Type H2-H3 shales are bimodal, with peaks at about 70 nm and 3 nm, respectively. Type H3 shales have lower D-2 than Type H2-H3 shale, corresponding to weaker pore structure heterogeneity. Multifractal analyses indicate that macropores in Type H3 shales have stronger heterogeneity with large D-10-D-0 ranges, while minor D-10-D-0 ranges mean weaker heterogeneity of micro- and mesopores, and so do Types H2-H3 shales. The higher the contents of plagioclase and clay minerals, the more heterogeneous the micro- and mesopores are; a larger content of quartz leads to more heterogeneous macropores. Specific surface area, micro-, and mesopores contents positively correlate to D-2, while average pore diameter and macropores are on the contrary, thus the higher the content of micro- and mesopores and the specific surface area, the lower the content of macropores and average pore diameter, the more complex the microscopic pore structure of shale. Micro- and mesopores control the heterogeneity of shale pore development with a great correlation of D-10-D-0 and D-10-D-10, and D-2 can effectively characterize the heterogeneity of a high porosity area with a strong correlation of D-2 and D-0-D-10.