Nanoscale pore structure and fractal characteristics of lacustrine shale: A case study of the Upper Cretaceous Qingshankou shales, Southern Songliao Basin, China

by Changjun Ji, Tianfu Liu, Yun Chen, Qian Wang, Peng Sun, Lei Sun, Taohua He

The Upper Cretaceous Qingshankou Formation’s lacustrine shales in the Songliao Basin are among China’s most promising shale oil reservoirs. To elucidate their pore and fractal characteristics, a comprehensive set of analyses encompassing total organic carbon (TOC), X-ray diffraction (XRD), and low-temperature N₂ adsorption (LTNA), Rock-Eval pyrolysis experiments and two-dimensional nuclear magnetic resonance (2D-NMR) were conducted. Using the Frenkel-Halsey-Hill (FHH) method, fractal dimensions (D) were calculated, and their relationship with pore metrics and shale compositions were explored. Two distinct fractal dimensions, D₁ (0 < P/P₀ < 0.5) and D₂ (0.5

0 <1.0), were derived from LTNA isotherms via the FHH approach. D₁ values fluctuated between 2.5715 and 2.7551 (mean 2.6564), while D₂ spanned from 2.3247 to 2.4209 (mean 2.3653). Notably, D₁ consistently surpassed D₂, signifying that smaller pores exhibit greater homogeneity compared to their larger counterparts. D₁ gradually increases with the increase of clay content. A direct correlation was observed between pore volume (PV), specific surface area (SSA), and D (both D₁ and D₂), whereas the association between average pore diameter (APD) and D was inverse. Both D₁ and D₂ escalated with diminishing TOC, 2D-NMR solid organic matter (OM), S₁ content and 2D-NMR light oil. Intriguingly, D₁ showed a stronger association with key pore and "sweet spot" parameters, highlighting its utility in assessing pore structural complexity and shale oil potential. This study illustrates how fractal theory enhances our understanding of pore structures and the shale oil enrichment process for the lacustrine shale.