Abstract

LOPEZ, Liliana  and  LO MONACO, Salvador. GEOCHEMISTRY OF CRUDE OILS FROM THE ORINOCO OIL BELT, VENEZUELA. Rev. Fac. Ing. UCV [online]. 2010, vol.25, n.2, pp.41-50. ISSN 0798-4065.

This work presents the study of biomarkers, sulfur and trace elements V, Ni, Mo, Cr in crude oils from the Orinoco Oil Belt, Venezuela. The results are interpreted based on the identification of the origin of the oils, their correlation and secondary processes in the reservoir. All oils are biodegraded, with the absence of n-alkanes and isoprenoids phytane and pristane. Based on the scale of biodegradation, three oil groups can be distinguished: Group A: level of biodegradation 4 with abundance of tricyclic terpane (C_23-3), without evidence of alteration of hopanes (C₂₉ and C₃₀), homohopanes (C₃₁ to C₃₅) and steranes (C₂₇-C₂₉); and absence of demethylated hopanes. Group B: level of biodegradation 6 to 7, with abundance of the tricyclic terpane (C_23-3), presence of hopanes (C₂₉ and C₃₀), homohopanes (C₃₁ to C₃₅), and steranes (C₂₇-C₂₉), presence of demethylated hopanes. Group C: level of biodegradation 7 characterized by the abundant tricyclic terpane (C_23-3), alteration of hopanes (C₂₉ and C₃₀), homohopanes (C₃₁ to C₃₅), and steranes (C₂₇-C₂₉), and the presence of demethylated hopanes. Based on the distribution pattern of methyl the dibenzothiophene isomers, all oils were generated by a carbonatic source rock, with the exception of the oils from the Carabobo area, which present evidence of generation by a siliciclastic source rock. These crude oils have been generated by a mature source rock, based on the methylphenanthrene index and the calculated vitrinite reflectance (Rc). The V/Ni and Mo/(Mo + Cr) ratios and the S concentration indicate the presence of three oil families (Groups I, II, and III). Nevertheless, the possibility that these oils have been generated by the same source rock deposited under variable redox (Eh) conditions cannot be ruled out

Keywords : Crude oils; Biomarkers; Trace elements; Alteration; Reservoir; Orinoco Oil Belt; Venezuela.