Interaction of oil components and clay minerals in reservoir sandstones

The free oil (first Soxhlet extract) and adsorbed oil (Soxhlet extract after the removal of minerals) obtained from the clay minerals in the <2 μm size fraction as separated from eight hydrocarbon reservoir sandstone samples, and oil inclusions obtained from the grains of seven of these eight samples were studied via GC, GC–MS and elemental analyses. The free oil is dominated by saturated hydrocarbons (61.4–87.5%) with a low content of resins and asphaltenes (6.0–22.0% in total) while the adsorbed oil is dominated by resins and asphaltenes (84.8–98.5% in total) with a low content of saturated hydrocarbons (0.6–9.5%). The inclusion oil is similar to the adsorbed oil in gross composition, but contains relatively more saturated hydrocarbons (16.87–31.88%) and less resins and asphaltenes (62.30–78.01% in total) as compared to the latter. Although the amounts of both free and adsorbed oils per gram of clay minerals varies substantially, the residual organic carbon content in the clay minerals of the eight samples, after the free oil extraction, is in a narrow range between 0.537% and 1.614%. From the decrease of the percentage of the extractable to the total of this residual organic matter of the clay minerals with burial depth it can be inferred that polymerization of the adsorbed polar components occurs with the increase of the reservoir temperature. The terpane and sterane compositions indicate that the oil adsorbed onto the clay surfaces appears to be more representative of the initial oil charging the reservoir than do the oil inclusions. This phenomenon could possibly demonstrate that the first oil charge preferentially interacts with the clay minerals occurring in the pores and as coatings around the grains. Although the variation of biomarker parameters between the free and adsorbed oils could be ascribed to the compositional changes of oil charges during the filling process and/or the differential maturation behaviors of these two types of oils after oil filling, the fractionation of the ratios of αββ/(αββ + ααα) regular steranes and C27 diasteranes/C27 regular steranes between these two types of oils can be unambiguously ascribed to the selective adsorption effects by the polar components and the active clay surfaces during the interaction of oil phase and clay surfaces.