Pore Structure and Multifractal Characteristics of Overmature Continental Shale: A Case Study from the Xujiaweizi Fault Depression, Songliao Basin, China

Overmature continental shales are widely distributed in China, while few investigations have been conducted. The pore structure is a critical parameter controlling the resource potential of shale gas. However, the pore structure and heterogeneity of continental shales from the Shahezi Formation, Xujiaweizi Fault, Songliao Basin are not well revealed. In this study, helium porosity and permeability, low-temperature N2 adsorption (NGA), mercury intrusion capillary pressure (MICP), and nuclear magnetic resonance (NMR) were applied and characterized to the pore structures of continental shales. Moreover, the heterogeneity and complexity of the pore structure were revealed by the multifractal based on the NMR $T_2$ spectrum. The results showed that clay minerals, quartz, and feldspar are the dominant minerals in the continental shales, and the most content of the clay minerals is the illite-smectite. The studied shales are the low porosity (mean 1.73%) and the ultralow permeability (mean 0.0707 mD) tight reservoirs. The hysteresis loops of ten shales belong to Types H2 and H3, characterized by high special surface area (mean 5.28 m2/g) and pore volume (mean $14.15\times {10}^{-3}\text{c}{\text{m}}^3/\text{g}$ ). The pore size distributions are unimodal, and Type H3 shales have more larger pores than Type H2 shales. MICP results indicate that the pore-throats are almost less than 20 nm. NMR $T_2$ spectra commonly show three peeks, i.e., p1 (<1 ms), p2 (1~20 ms), and p3 (>20 ms) with the small $T_{2,gm}$ values, ranging from 0.18 ms to 1.36 ms (0.69 ms), which suggests that more nanopores are in the continental shales. Moreover, the average movable fluid percentage is low, ranging from 1.22% to 15.08% (mean 6.84%). The singularity strength range ( $\Delta \alpha$ ) shows that pore structures are heterogeneous. And the heterogeneity and complexity can be better revealed by the multifractal spectra rather than a monofractal model.