Effects of solvent extraction on pore structure properties and oil distribution in shales of alkaline lacustrine basins

Abstract

Shale contains numerous nano-scale pores, whose pore structure property changes affect petroleum flow, complicating shale oil accumulation and exploration. Twelve shale samples from the Permian Fengcheng Formation in the Mahu Sag and the Permian Lucaogou Formation in the Jimusar Sag in northwestern China were analyzed to investigate the coupled oil distribution and pore structure in shales from alkaline lacustrine basins. Shale samples were comprehensively analyzed before and after solvent extraction using X-ray diffraction, total organic carbon measurement, Rock-Eval analyses, field emission-scanning electron microscopy, nitrogen physisorption (NP), and (ultra) small-angle X-ray scattering [(U)SAXS] to assess nanoscale pore structure (2–300 nm in diameter) and oil distribution. Solvent extraction increased total pore volume and specific surface area (SSA). However, the accessibility of nanoscale pores remains limited. Additionally, even after retained oil removal, (U)SAXS-derived total pore volumes are 1–10.4 times larger than NP-derived connected pore volumes. Complex variations in pore volume and SSA mainly result from the removal of extractable organic matter (EOM) and the refilling of small pores by organic matter. Despite the relatively small pore volume of mesopores (2–50 nm), the amount of EOM distribution in mesopores is comparable to that in macropores (50–300 nm); therefore, it is crucial not to overlook the retention capacity of mesopores for EOM. Macropores, particularly interparticle pores associated with quartz and feldspar, play a crucial role in oil mobility. The quantity and composition of EOM, along with other factors, can alter pore structure before and after solvent extraction and should be considered in evaluating the distribution and content of free oil.