The potential occurrence modes of hydrocarbons in asphaltene matrix and its geochemical implications

Abstract

As the most polar component of crude oils, asphaltenes are characterized by their complex molecular structures and their sequestration of small organic molecules. These organic compounds are often protected from various secondary alteration processes, and thus might serve as useful molecular markers. However, little is presently known about the mode of occurrence of hydrocarbons isolated from asphaltenes by conventional approaches. In the current study, we collected veined bitumen samples from the Changjianggou Formation of the Longmen Mountains on the northwestern rim of the Sichuan Basin, extracted soluble organic matter with dichloromethane, precipitated asphaltenes with petroleum ether, and isolated hydrocarbons from the asphaltenes by centrifugation and hydrous pyrolysis. The results indicated that the n-alkane profiles of saturated hydrocarbons obtained by centrifugation and by hydrous pyrolysis at 300 °C not only were similar to each other, but also resembled those of typical crude oils more closely than the n-alkane profiles of the saturates fraction in veined bitumen extracts did. The asphaltene-bound aromatic hydrocarbons and resins were found to be too difficult to extract thoroughly by the current separation methods. A possible explanation is that saturated hydrocarbons are mostly associated with the peripheral substituents of asphaltenes via hydrophobic association, whereas aromatics and resins tend to interact with the asphaltene cores through π-π stacking, which is much stronger. Therefore, the dissociation or diffusion of the loosely bound saturated hydrocarbons would alter the geochemical information that they had originally recorded, whereas the covalently bonded or more tightly associated hydrocarbons are better shielded from secondary alteration processes.