Saturated hydrocarbons occluded inside asphaltene structures and their geochemical significance, as exemplified by two Venezuelan oils

Z. Liao (1, 2) , A. Geng (1) , Alain Graciaa (2) , Patrice Creux (2) , Anna Chrostowska (3) , Y. Zhang (1)

1 State Key Laboratory of Organic Geochemistry
2 TEFC - Thermodynamique et Energétique des fluides complexes
3 IPREM - Institut pluridisciplinaire de recherche sur l'environnement et les matériaux

Inside asphaltene aggregates in crude oils, the adsorbed hydrocarbons exist in the packed periphery while the occluded hydrocarbons exist inside the core of the asphaltene structures. Occluded hydrocarbons may be well protected from secondary alteration events in the reservoir due to the colloidal nature of asphaltenes, and can be representative of older oil than the reservoir oil, and so may be very useful for reservoir geochemical studies. In this work, two Venezuelan oils derived from the same source rock have been studied. The results indicate that the oxidative reagent H2O2/CH 3COOH can release the occluded hydrocarbons from the asphaltene aggregates, making it possible to selectively study these compounds without interference from the moieties that are covalent-bonded to the asphaltene structures. In order to obtain the precise information from the occluded hydrocarbons, prior to the oxidation processes, acetone extraction of the asphaltenes has been applied to rule out interferences from the adsorbed compounds. Furthermore, pyrolysis of asphaltenes has been performed for comparison with the occluded saturated hydrocarbons. From the oxidation products of asphaltenes from these two crude oils, the saturated hydrocarbons (from the occluded original oil) have very similar distributions, including terpanes and steranes, and almost the same biomarker parameters. These results are in accord with the fact that oil from the same source rock was occluded by both Ven1 and Ven2 asphaltenes. The geochemical information obtained from the occluded saturated hydrocarbons suggests that the source rock depositional setting for these two crude oil samples was an anoxic, hypersaline/evaporitic environment.

Domaines: Chimie-Physique [physics.chem-ph]

Z. Liao, A. Geng, Alain Graciaa, Patrice Creux, Anna Chrostowska, et al.. Saturated hydrocarbons occluded inside asphaltene structures and their geochemical significance, as exemplified by two Venezuelan oils. Organic Geochemistry, 2005, 37 (3), pp.291-303. ⟨10.1016/j.orggeochem.2005.10.010⟩. ⟨hal-00342942⟩