Pore structure heterogeneity of Chang 7 shale in Ordos Basin: insights from NMR and multifractal theory

Quantitative characterization of heterogeneity of shale nanopores and its influencing factors are of significant impact on the occurrence quantity, pore size, and mobility of shale oil. 25 Chang 7 shale samples were analyzed for geochemical properties, mineral composition, FE-SEM, and NMR. Combined with multifractal theory, the heterogeneity characteristics and influencing factors of shale pore structure were studied, and the reservoir classification was carried out. The results show that: pore types include microfracture, intercrystalline pores, intergranular pores, intragranular dissolved pores and organic matter shrinkage pore. The pore size distribution of shale samples is divided into 4 groups according to T_{2, gm} and T_{2, 50}, in which the pore-scale composition gradually transitions to micropores (<100 nm), while the heterogeneity gradually diminishes. TOC, the content of clay matrix and pyrite are positively correlated with multifractal parameters, while T_max, the content of quartz, feldspar, and carbonates show opposite trends. T₂ spectrum parameters T_{2, gm} and multifractal parameters  and D₁/D₂ can effectively classify shale oil reservoir quality. This study provides insights and reference for the characterization of pore heterogeneity and reservoir classification of shale oil reservoirs.