Phase Composition and Surface Microstructure of Asphaltenes from Heavy Crude Oil

Abstract

Sequential solvent extraction with nheptane and toluene was used to isolate three fractions from asphaltenes of heavy bituminous crude oils produced from Permian deposits in the Republic of Tatarstan, Russia: a heptane fraction, a toluene fraction, and a toluene-insoluble organomineral residue. These fractions differed in elemental and hydrocarbon composition, paramagnetic properties, solubility in organic solvents, and surface microstructure/morphology. The heptane fraction, extracted from the interlayer spaces of asphaltenes, consisted primarily of benzene resins, alcohol–benzene resins, and C₂₀–C₅₂ crystalline paraffins. The toluene-insoluble organomineral residue contained rockforming mineral elements such as Si, Al, Fe, and P. The toluene fraction accounted for the bulk of the asphaltenes (86.36–90.35 wt %) and exhibited a relatively homogeneous elemental composition; the fractions differed in the concentrations of paramagnetic centers derived from free radicals and tetravalent vanadium present in porphyrin complexes. Comparative analysis revealed that hydrocarbons occluded within asphaltenes, as well as organomineral inclusions, are uncharacteristic of the primary asphaltene structure. They likely penetrated asphaltene pores from the liquid dispersion medium during crude oil migration or during the formation and subsequent transformation of oil deposits in agglomeration zones. Crystalline minerals and highmolecularweight paraffins present in the crude oil may serve as a nucleating phase for asphaltene associate formation and, consequently, may affect the stability of petroleum dispersed systems during extraction and processing.