The occurrence and geochemical origin of 9-alkyl HDBTs substituted by long isoprenoid alkyl chains in crude oil

1,1,4a,6-tetramethyl-9-alkyl-1,2,3,4,4a,9b-hexahydrodibenzo[b,d]thiophene (9-alkyl HDBT) was initially found in highly desulfurized diesel oils. This and related organic sulfur compounds (OSCs) have a special shielding structure that make them extremely resistant to hydrodesulfurization. The structure of 9-alkyl HDBTs is likely directly inherited from biological precursors related to terpenoids, but their origins have not been determined. In this study, OSCs were isolated from a low sulfur crude oil from the Junggar Basin in China by the methylation/demethylation method and characterized by gas chromatography-mass spectrometry. A series of 9-alkyl HDBTs substituted with long isoprenoid alkyl chains were identified in the OSCs. Raney nickel desulfurization was conducted to assist the structural identification. The presence of 9-alkyl HDBTs with a β-carotene carbon skeleton and the predominance in specific carbon numbers of alkyl-substituted 9-alkyl HDBTs support a carotenoid origin of such OSCs. Possible geochemical pathways are proposed for the formation of these OSCs via β-carotene, involving processes such as sulfur incorporation, cyclization, aromatization, hydrogenation, and cleavage.