Distribution and geochemical significance of biphenyls and related bicyclic aromatic structures in oils from Ancon oilfield (Tertiary Progreso Basin), Ecuador

A set of oils from Ancon oilfield (Neogene Progreso Basin, Ecuador), were studied by GC-MS in terms of the occurrence and distribution of cyclohexylbenzenes, biphenyls, and diphenylmethanes, and their use as indicators of thermal maturity and biodegradation. In the studied oils, biphenyls are more abundant than the other related bicyclics; 3-methylbiphenyl is the most predominant over remnant homologs. A weak correlation between different biphenyl ratios and sensitive parameters for thermal maturity (MPI-1 and %Rc) is observed; probably, the input of terrestrial organic matter could be responsible for the low correlation. The linear correlation between biphenyls and cyclohexylbenzenes is explained through dehydrogenation reactions during oil formation. However, no correlation is observed with diphenylmethanes nor cyclohexylbenzenes. The biodegradation achieved in the studied oils is equal to or less than 3 in the PM scale, according to the negligible effects over the distribution of these compounds, confirmed by aromatic biodegradation indices derived from alkylnaphthalenes.