Fractal characteristics of nanopores in lacustrine shales of the Triassic Yanchang Formation, Ordos Basin, NW China

Abstract

To further understand fractal characteristics of nanopores in lacustrine shales, core samples from Chang-7 and Chang-9 members of the Triassic Yanchang Formation, southern Ordos Basin were investigated. Total organic carbon content, vitrinite reflectance, X-ray diffraction, field emission-scanning electron microscopy and low pressure N2 physisorption (LPNP) experiments were performed. The Frenkel-Halsey-Hill (FHH) method was applied to characterize the fractal dimensions using LPNP isotherms. Two fractal dimensions (D1 and D2) were determined at relative pressures of 0-0.45 and 0.45-1, respectively. The Chang-7 shales have a D1 and D2 range of 2.17-2.36 and 2.46-2.63, while the Chang-9 shales have D1values of 2.23-2.40 and D2 values of 2.46-2.64. Fractal dimensions of the selected lacustrine shales are affected by shale mineral compositions and pore structure parameters. Positive correlations of D1 and D2 with clay minerals and quartz contents, and negative correlations of D1 and D2 with TOC contents were presented in the present study. Observations of few organic matter pores and abundant inorganic pores hosted in the Yanchang shales may contribute to these correlations. In addition, comparisons of matrix composition, nanopore fractal characteristics between the Chang-7 shales and the Chang-9 shales suggest that the latter may have more irregular and heterogeneous pore structure.

Keywords

• lacustrine shales
• fractal dimensions
• pore structure
• chang-7 shales
• chang-9 shales
• yanchang formation
• ordos basin