Nondestructive investigation on hydrocarbons occluded in asphaltene matrix: The evidence from the dispersive solid-phase extraction

Abstract

Asphaltenes are the heaviest compound group in crude oil. The complex and porous macromolecular network of asphaltenes both adsorbs and occludes small hydrocarbon molecules, which are considered source-related petroleum biomarkers as they are shielded from secondary alterations. The current method to isolate the occluded content involves mild oxidative degradation of asphaltenes with CH3COOH/H2O2, but it is generally not suitable for quantitative studies, as side reactions that chemically alter the bound hydrocarbons are difficult to mitigate. In the current study, we compared the performance of dispersive solid-phase extraction (DSPE) and mild oxidative degradation in isolating asphaltene-occluded hydrocarbons, using Lower Cambrian solid bitumen from northwestern Sichuan and Ordovician crude oil from the Tazhong area of Xinjiang, China. We demonstrated that DSPE was effective in extracting occluded hydrocarbons from asphaltene aggregates without undesirable chemical alterations, thereby allowing subsequent quantitative analysis. Further investigations revealed that the chemical composition of the occluded n-alkanes provided useful information with regard to the origin of the petroleum. Importantly, the enrichment of aromatic hydrocarbons inside the asphaltene aggregates, particularly the relative abundance of different methylphenanthrene isomers, provided crucial experimental evidence in support of our earlier hypothesis that the occluded hydrocarbons were protected from secondary alterations, and therefore had remained relatively stable over a long geologic period.