Effect of extractable organic matter on nanopore structure and heterogeneity in Triassic Yanchang lacustrine shale of the Ordos basin, China

Abstract

The low-pressure nitrogen adsorption and multifractal analysis of Triassic Yanchang Chang 7 shale samples, before and after solvent extraction, were conducted on the effect of extractable organic matter (EOM) on pore microstructure, porosity, and heterogeneity. Chang 7 Shale, primarily composed of type II₁-II₂ organic matter (OM) and argillaceous-siliceous lithofacies, is characterized by interparticle and intraparticle pores in inorganic minerals, with limited development of OM pores. After extraction, pore volume and specific surface area significantly increased from 2.91 cm³/1000 g to 4.43 cm³/1000 g and 0.61 m²/g to 1.91 m²/g, respectively. This increase is attributed to the reopening of minor pores (<23 nm) previously occupied by saturated hydrocarbons, which enhanced the proportion of minor pores and reduced overall pore heterogeneity. Saturated hydrocarbons in EOM were mainly enriched in carbonate interparticle pores, while aromatic hydrocarbons were concentrated in pyrite intraparticle pores. Resins and asphaltenes were predominantly adsorbed onto OM and inorganic mineral surfaces. Clay and pyrite intraparticle pores were nearly completely filled with EOM, reducing heterogeneity, whereas detrital mineral interparticle pores, partially filled with EOM, increased heterogeneity. A linear correlation between EOM content and Rock-Eval S₁ suggests that sample crushing may lead to an underestimation of saturated hydrocarbon content in S₁. These findings provide critical insights into the pore structure evolution and shale oil enrichment mechanisms in Yanchang Shale.