Effect of biodegradation on the molecular composition and structure of asphaltenes: Clues from quantitative Py–GC and THM–GC

Abstract

There is an on-going debate about whether asphaltenes can be anaerobically biodegraded in reservoir rocks due to their complex macromolecular structure. In this study, asphaltenes were precipitated from a sequence of bitumens at various degrees of biodegradation. The compositions of alkyl moieties within the asphaltene molecular structure were characterized by on-line flash pyrolysis–gas chromatography (Py–GC), on-line thermally assisted hydrolysis and methylation using tetramethylammonium hydroxide (THM–GC), and Py–GC combined with alkaline hydrolysis. The results indicate that alkyl moieties bonded to asphaltene macromolecules are dominated by linear structures with chain lengths up to C27, mainly bonded through C–C and ether (thioether) bonds but less so through ester and hydrogen bonds. Furthermore, most n-fatty acids and linear aliphatic alcohols bonded to asphaltene structure by ester and hydrogen bonds were altered at slight–moderate biodegradation stages, while linear alkyl moieties attached to the asphaltene core through C–C and ether (thioether) bonds were likely altered at heavy–severe biodegradation stages. This indicates that the linear alkyl moieties bonded to the asphaltene structure by hydrogen and ester bonds are more susceptible to biodegradation than those bonded to the asphaltene core through stronger covalent bonds such as C–C and ether (thioether) bonds. Furthermore, variations in the chemical compositions and molecular structure of asphaltenes during biodegradation may not only be attributed to the alteration of linear alkyl moieties, but also the replacement of linear alkyl moieties bonded to the asphaltene structure by hydrogen and ester bonds with other biodegradation products.