Sequential Solvent Extraction Reveals Lithofacies-Dependent Pore Accessibility, Wettability, and Hydrocarbon Partitioning in Organic-Rich Lacustrine Shales

Abstract

Understanding the reservoir characteristics of shale oil formations is an essential workflow in petroleum exploration and development because it supports petroleum-reserve evaluation and guides production activities. Few studies have examined the coupled effects of pore structure and oil composition on reservoir characterization, which limits the accuracy of reservoir evaluation. In this work, three representative dolomitic shale and silty shale samples from the Kongdian Formation, Bohai Bay Basin, were studied with X-ray diffraction, petrographic microscopy, and μ-X-ray fluorescence to determine mineral compositions and sedimentary textures. In conjunction with sequential solvent extraction, pore structure, wettability, and oil composition were characterized using mercury intrusion porosimetry, nitrogen physisorption, contrast variation-small angle neutron scattering, gas chromatography, and saturated hydrocarbon, aromatic hydrocarbon, resin, and asphaltene fractionation. After solvent extraction of the as-received core samples, the porosity of the two dolomitic shale samples increased only from 5.36% and 6.34% to 6.46% and 7.28%, in contrast to the dramatic increase from 2.96% to 20.4% for the silty shale. CV–SANS results show a slight increase in water-accessible pore space after solvent extraction, but water-accessible pores still account for less than 30% of the total pores. Oil composition gradually changed from light to heavy fractions, and the presence of light oil indicated that oil charging occurred in both dolomitic shale and silty shale. In addition, a rule-of-thumb extraction duration for pore-structure characterization is proposed as a reference for future studies.