Fractal Characteristics of the Middle-Upper Ordovician Marine Shale Nano-Scale Porous Structure from the Ordos Basin, Northeast China

The fractal characteristics of marine shale from the Middle-Upper Ordovician Wulalike Formation (O(2)w) in the southwest margin of the Ordos Basin are studied. Based on low-temperature nitrogen adsorption experiments, the FHH (Frenkel-Halsey-Hill) model was employed to investigate the relationship between the marine shale composition, such as TOC, mineral content and shale gas content, and pore structure parameters, such as BET specific surface area, average pore diameter, porosity and fractal dimension. The results show that the pore size distribution curve of shale slowly decreased after the pore size was greater than 50 nm, the pore size distribution showed multiple peaks, and the peak value was mainly in the range of 2-10 nm. Most pores are nanopores, although the pore type and shape are different. Two different fractal dimensions D-1 and D-2 are obtained from the two segments with relative pressures of 0-0.5 and 0.5-1.0, respectively: the D-1 range is 2.77-2.82, and the D-2 range is 2.63-2.66. As D-1 is larger than D-2, the pore structure of small pores is more uniform than that of large pores in the shale samples. The relationship between the fractal dimensions D-1 and D-2 and the total organic carbon (TOC) content is a convex curve. Fractal dimension D reaches its maximum when TOC is 0.53 wt.%. Fractal dimension D decreases with increasing specific surface area, porosity and average pore size. The fractal dimension has a different influence on the gas storage and migration in shale; the larger the fractal dimension is, the stronger the heterogeneity and the more complex the pore structure, and this outcome is conducive to the storage of gas in shale but not beneficial to the permeability and production of gas.