Characterizing Shale Oil Occurrence in the Yanchang Formation of the Ordos Basin, Assisted by Petrophysical and Geochemical Approaches

The occurrence and distribution of hydrocarbons exert fundamental control on the mobility and “sweet spot” identification of shale oil systems. Here, we report both quantitative and visual qualitative analyses of shale oil occurrence assisted by petrophysical and geochemical approaches. Oil-bearing shales were extracted using a ternary azeotropic solvent extraction system and then were characterized for pore structure, fractal characteristics, and chemical compositions. The results found that soluble organic matter is mainly controlled by clay minerals because of the strong interaction between the interparticle pores of clays and the extracted hydrocarbons. After solvent extraction, the pore volume, specific surface area, and heterogeneity of shales are all significantly enhanced. The light and heavy hydrocarbons of the extracted soluble organic matter reside in pores with different scales. The light hydrocarbons (aliphatic and aromatic hydrocarbons) are mainly stored in fine mesopores with pore diameters less than 20 nm, while the heavy hydrocarbons (resins and asphaltenes) reside in macropores. New evidence from field emission scanning electron microscopy images shows that native hydrocarbons are expulsed from kerogen–clay aggregates and provide rare in situ information about the microscopic distribution of moveable hydrocarbons in shale oil systems. This study improves the understanding of the occurrence of hydrocarbons in shale oil.