**6. Conclusions**

In the presented research, the pore structure and its heterogeneity of the Lower Permian Taiyuan Formation tight sandstone in the south China North basin were investigated by a series of experiments and multifractal methods. These conclusions could be obtained:

(1) The tight sandstones from Taiyuan Formation are characterized by a high content of quartz and clay minerals, low porosity (1.95–3.41%) and low permeability (0.037–0.494 mD). Pore types mainly include micropores associated with clay minerals, intergranular and intragranular dissolution pores, whereas primary pores are rarely observed. The NMR T2 distributions of samples mainly show bimodal characteristics with less long T2 components, since clay-dominated micropores associated with short T2 components predominate the whole pore system of the tight sandstones.

(2) The multifractal spectrum, generalized dimensions and mass exponent spectrum demonstrate that pore structures of tight sandstone samples have multifractal characteristics. Meanwhile, the multifractal parameters are useful to quantify the pore heterogeneity in the areas with di fferent probability densities.

(3) Multifractal parameter (*D*−10) of lower probability density areas is more appropriate to determine petrophysical properties. However, the multifractal parameter (*D*10) of higher probability density areas is the available indicators for evaluating pore structure.

(4) Mineral compositions have various effects on pore structure in different probability density areas. The decrease of quartz content and increase in clay mineral content contribute to high degree of pore anisotropy of higher probability density areas, whereas the decrease in clay minerals and the increase of quartz content and feldspar content can result in more irregular and nonhomogeneous pore network in lower probability density areas.

(5) Overall, the pore structure in lower probability density areas consists of larger dissolution pores, which determine the reservoir properties. Gas storage and seepage usually occur at those pores of lower probability density areas. However, micropores dominate pore structure in higher probability density areas, and strong complexity and heterogeneity make pores unfavorable for gas storage and seepage there.

**Author Contributions:** K.Q. conceived of the presented idea, designed the experiments and verified the data along with S.G. The article is originally written by K.Q. and revised by the corresponding author S.G. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was funded by Ministry of Science and Technology of the People's Republic of China (Grant No. 2016ZX05034).

**Acknowledgments:** This study received financial support from the Ministry of Science and Technology of the People's Republic of China (Grant No. 2016ZX05034). We would like to thank the anonymous reviewers for their careful reviews and detailed suggestions.

**Conflicts of Interest:** The authors declare no conflicts of interest.
