Characterization of pore structure before and after solvent extraction: A case study of lacustrine shales in the Triassic Chang 7 Formation, Ordos Basin, Northern China

Abstract

The presence of hydrocarbons/bitumen in shales makes accurate characterization of reservoir properties challenging. Solvent extraction helps reveal the true pore structures and assess the movable oil distributions in early mature lacustrine shale oil reservoirs. In the present study, total organic carbon, optical microscopy, Rock-Eval pyrolysis, X-ray diffraction, scanning electron microscope, gas adsorption, and high-pressure mercury injection pressure experiments were performed to investigate the petrographic, geochemical and petrophysical characteristics of extracted and nonextracted shale samples that were collected from the Triassic Chang 7 Member in the Ordos Basin, China. The results revealed that changes in the micro-, meso-, and macropore volumes; specific surface area; average pore diameters and porosities of the shales significantly differ after solvent extraction compared with their original states, which indicates that the pore structures of early-mature, organic-rich shales are largely concealed by the retained oil and residual bitumen. Solvent extraction generally increases porosity by removing extractable organic matter (EOM), which occupies a sizeable pore space. Solvent extraction may, however, also be affected by several factors, resulting in reduced porosity. The main factors influencing the solvent extraction process are the removal of EOM, solvent–kerogen interactions, clay swelling, and the coupling effects of the sedimentary structure and pore system. In addition, the use of changes in porosity to indicate the movable oil content in shales is more meaningful than the changes in S₁ and total pore volume are. This study is beneficial for the accurate characterization of shale pore structures and precise assessments of the movable oil contents of lacustrine shales.