Comparison of asphaltene structures from oils with different levels of biodegradation using X-ray diffraction, FTIR and NMR spectroscopy

Petroleum production in the Campos Basin, Brazil, faces significant challenges related to oil compositional heterogeneity and asphaltene precipitation, in which biodegradation acts as one of the main factors altering petroleum quality. The main objective of this work comprises the geochemical characterization of asphaltenes from A and B oils, originating from the same source rock but with different levels of biodegradation, with emphasis on investigating the modifications on its molecular and supramolecular architecture. The methodology adopted integrated  techniques, including gas chromatography/mass spectrometry (GC/MS),  X-ray Diffraction (XRD), Infrared Spectroscopy (FT-IR), ¹H and ¹³C Nuclear Magnetic Resonance (NMR) analysis. As GC/MS results and according to the PM scale, the A oil is slightly biodegraded (level 1), whereas the B oil shows moderate biodegradation (level 3). Structural analysis via XRD established a correlation between supramolecular organization and microbial attack. It was observed that the asphaltene from B oil presented a reduction in the distance between saturated chains, accompanied by an increase in the stacking height and in the diameter of the aromatic sheet. These results indicated that biodegradation acted selectively in the consumption of peripheral aliphatic chains, reducing the steric hindrance that kept the sheets apart. Unlike thermal evolution, which tends to fragment the clusters, the biological process preserved the integrity of the polyaromatic core, consolidating a predominantly island-type molecular architecture. FT-IR analysis showed that the selectivity of the biological attack was diagnosed by the decrease in the long chain index observed in B’ asphaltene, as well as a relative increase in the branching index, confirming the preferential removal of linear chains. Additionally, biodegradation imprinted a distinct signature on polar functionality by a reduction in the carbonyl index in the asphaltene of B oil, corroborating the consumption of oxygenated groups. NMR (¹H and ¹³C) characterization was crucial in elucidating the molecular topology and confirming the biodegradation mechanism. The data proved that the microbial attack on B asphaltene occurred predominantly via chain shortening, evidenced by the drastic reduction of terminal hydrogens (15%) when compared with A asphaltene (34.0%). This selective consumption of the linear aliphatic periphery of the B asphaltene resulted in an increase in the aromaticity factor. Structurally, ¹³C NMR validated the 'Island' model (Yen-Mullins) for both asphaltenes, highlighting that the polyaromatic core remained intact during biodegradation. This was demonstrated by the increase in bridgehead carbons (Car-b) in B asphaltene (28.8%) compared to those in A asphaltene (18.7%). This work contributed to elucidate that biodegradation reshapes the asphaltenes molecular and supramolecular architectures.