Pore structure of Triassic Yanchang mudstone, Ordos Basin: Insights into the impact of solvent extraction on porosity in lacustrine mudstone within the oil window

Abstract

Low-pressure nitrogen physisorption was performed on Triassic Yanchang mudstones before and after solvent extraction to characterize the extractable organic matter (EOM) occurrence and ascertain the impact of extracts on the nanoporosity of lacustrine mudstone within the oil window. Organic-rich Yanchang mudstones are currently thermally mature (vitrinite reflectance of 0.48%–0.92%). SEM-observed pore types chiefly comprise interparticle and intraparticle pores related to inorganic minerals, with organic matter pores less developed. Pore volume (PV) of the original mudstones is 0.35–2.15 cm3/100 g and the specific surface area (SSA) is 1.22–7.77 m2/g. Brittle minerals (quartz + feldspar + carbonates) and I/S mixed layer jointly show the positive effect on the pore space development in the Yanchang mudstone. Four groups of mudstones with obvious differences in pore structure properties were identified, which probably result from the various development degrees of interparticle pores at the edge of brittle minerals, and intraparticle pores associated with I/S aggregates. Generally, solvent extraction will result in a noticeable increase in the volume of N2 adsorbed due to the reopening of pore space blocked by EOM, which is related to increases in PV an SSA. A declining case, probably caused by the progressive shift of the enlarged open pore space to the larger macropore range (d > 200 nm) which is not detected by N2 adsorption and counteracts the increased porosity by extraction, however, was also observed in the current study. Overall, the infilling degree of EOM increased with increasing thermal evolution degree of shales within the oil window, which mainly blocks minor pores in lower mature samples but larger pores in higher mature samples.