Molecular Characterization of Remnant Polarizable Asphaltene Fractions Upon Bitumen Upgrading and Possible Implications in Petroleum Viscosity

Thermal processing and hydrotreatment are used to decrease the viscosity of Alberta bitumen. However, changes in bulk properties, such as API gravity and viscosity, do not correlate to the gravimetric content of maltenes and asphaltenes. Thus, the work herein employs an extrography separation that yields asphaltene fractions enriched with distinct structural motifs and aggregation tendencies to investigate if changes in viscosity could be linked to the transformation or survival of specific asphaltene compounds and/or extrography fractions. Samples with limited change in viscosity upon thermal processing display minor changes in the gravimetric distribution of the extrography fractions, specifically polarizable species (Tol/THF/MeOH fraction). Ultrahigh-resolution mass spectrometry analysis demonstrates that such samples reveal neither a significant decrease in chemical polydispersity nor a change in the relative content of multicore/archipelago structural motifs post thermal treatment. Conversely, hydroprocessed samples with a pronounced viscosity reduction feature a remarkably lower chemical polydispersity and increased content of single-core (island) structural motifs. Polarizable asphaltene fractions from severely hydrotreated samples feature S-containing species with a low aromaticity, which on the basis of their molecular composition suggests that they are composed of the expected, alkyl substituted, geologically stable thiophenic cores (e.g., benzothiophene) as well as “unexpected” sulfides and sulfoxides. Collectively, the results suggest that the high viscosity of thermally upgraded samples could be correlated to the survival of asphaltene species with high heteroatom content (up to five heteroatoms per molecule) and persistent, high abundance of archipelago structural motifs. Thus, it is suspected that nanoaggregation of such fractions prevents their transformation into lighter products.