As the formation of asphaltene aggregates causes its deposition in various processes, exact determination of the molecular and aggregate structure and mechanism of aggregate formation is very important in order to prevent asphaltene deposit formation. The aim of this paper is to review the molecular and aggregate structure of asphaltene using various techniques. The other purpose is to specify and compare different mechanisms of aggregate formation, suggested by researchers. These mechanisms are proposed by researchers according to the molecular characteristics of asphaltene and the structure of aggregates. Researchers have considered four general models for the formation mechanism of asphaltene aggregates: the micellar/colloidal, polymeric, solubility, and modified Yen models. In comparison with other models, the modified Yen model shows more consistency with new studies, which determine the island molecular structure and small nano-aggregates for asphaltene. Experimental studies show that the dominant mechanism of aggregate formation depends on asphaltene architecture, intermolecular forces, and the solubility definition of asphaltene. Depending on the asphaltene source, solvent, thermodynamic conditions, and various techniques such as near-infrared spectroscopy, nuclear magnetic resonance, small-angle neutron scattering, and small-angle X-ray scattering, different structural information on the shape (spheres, discoid, oblate, membrane-like, prolate cylinders, and mass fractal aggregates) and size of aggregates has been reported.

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