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

Abstract In this study, the ¹³C nuclear magnetic resonance (NMR) and X-ray diffraction (XRD) were employed to investigate adsorption capacity and chemical structural variations of the asphaltene during the solvent swelling. The results show that the saturates and aromatics mainly accumulate between the aliphatic carbon chains both for the asphaltenes with 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-f(al) of kerogen, asphaltene, and solid bitumen shows an increasing trend with the saturation swelling ratio of saturates (R² = 0.80). By comparison, the saturation swelling ratio of aromatics for them with the aromaticity-f(ar) shows three different evolution trends, which can be attributed to their differences of chemical structures. Three types of organic macromolecules evolve more stable structures at higher thermal maturity, so that the saturation swelling ratio of aromatics for them gradually become smaller and tend to be the same. Furthermore, two new methods are established to calculate the amounts of saturates and aromatics adsorbed by asphaltene and kerogen. The asphaltene is the main fraction of kerogen cracking during oil generation and has strong adsorption capacity on liquid hydrocarbons, so its adsorption capacity for liquid hydrocarbons should be also considered in the future studies about hydrocarbon expulsion of the shale.