Micro-pore structure and fractal characteristics of terrestrial shales: A case study of Shahezi Formation in Xujiaweizi Fault Depression

The pore structure and fractal characteristics of shale are analyzed, and their impacting factors are also revealed using the low temperature nitrogen adsorption-desorption method, combined with scanning electron microscopy, C-S analysis and XRD. The results showed that the mineral compositions of Shahezi Formation continental shales were dominated by clay minerals, quartz and feldspar. The pore types in shales were mainly clay mineral intragranular pores, feldspar dissolution pores and grain edge pore, while the organic pores were not developed. The adsorption isotherms of shales belonged to the type Ⅳ isotherm according to the IUPAC, which develop two types of hysteresis loop, H2 and H3. The type H3 shales are characterized by a lower specific surface area, a larger average pore size, and a higher macro-pore content. Meso- and macro-pores mainly contribute to the total pore volume, while the meso-pores mainly contribute to the specific surface area. The pore size distribution was bimodal, characterized by a left peak at about 2.7 nm and the right peak at 20-70 nm. The shales showed two stages of fractal characteristics and the pore structures of type H3 shales were more homogeneous and simpler. The pore structures of Shahezi Formation shales were mainly controlled by the mineral composition rather than TOC. The higher the micropore content and the specific surface area value, the higher the macropore content and the average pore size, the more complex the shale pore structure, which is not conducive to the migration and production of shale gas. The differences between marine and terrestrial shale reservoirs are constrained by the differential sedimentary environments and the organic matter types, which fundamentally affected their pore structures and shale gas enrichment.