Chemical structural variations of asphaltene during solvent swelling characterized by 13C nuclear magnetic resonance and X-ray diffraction

The asphaltene plays an important role in hydrocarbon retention and expulsion processes. In this study, the ¹³C nuclear magnetic resonance and X-ray diffraction were employed to investigate adsorption capacity and chemical structural variations of the asphaltene during the solvent swelling. The saturates and aromatics mainly accumulate between the aliphatic carbon chains in asphaltenes with both long and short aliphatic chains. However, it is difficult for the aromatics to accumulate between aromatic carbon chains in short aliphatic chain asphaltene. The aliphaticity of kerogen, solid bitumen and asphaltene primarily controls their adsorption capacity of saturates, whereas their adsorption capacity of aromatics is influenced by the complex interplay of aliphaticity, aromaticity, and thermal maturity. With increasing of thermal maturity, the structures of three organic macromolecules evolve to be more stable structures, resulting in a smaller and more consistent saturation swelling ratio for aromatics. Furthermore, two new methods are established to calculate the masses of saturates and aromatics adsorbed by asphaltene and kerogen. Asphaltene is the main product of kerogen cracking in the low-to-moderate maturity stage and has a strong adsorption capacity on liquid hydrocarbons. Its adsorption capacity needs to be incorporated into future studies on the hydrocarbon expulsion from shale.