Review on molecular structures of asphaltene macromolecules and instrumental analytical approaches

Abstract

As one of the major fractions of petroleum and/or source rock extracts, asphaltene is commonly characte-rized by complicate molecular structures as well as relatively higher molecular weight (MW). The asphaltene occluded biomarkers have been well introduced to be valuable for geochemical research whilst the actual molecular weight and molecular structures of asphaltene macromolecules studies show value in petroleum exploration and production, laboratory treatments as well as petroleum chemical storage and transportation etc. The structure, molecular weight and related experimental measurement methods of asphaltenes reported in this paper are reviewed, in an attempt to provide a more comprehensive understanding on the molecular structure, molecular weight and physicochemical properties of asphaltenes, so as to provide basic data for applying asphaltene geochemical research in the exploration and development study of conventional and unconventional reservoirs. In view of the different views on the structure and molecular weight of asphaltene molecules, the results are reviewed, the main reasons for the controversy are discussed based on the current status of experimental measurement technology, and the future development trend of related researches is pointed out. The current understanding of the structure of asphaltene macromolecules is mainly based on two completely different models, i.e., the island model and archipelago model. The former suggests that asphaltenes have a single aromatic nucleus and the molecular weight characterized is relatively lower. The latter believes that asphaltenes have multiple aromatic nuclei and the molecular weight characterized is relatively higher. The above two models of asphaltene molecular structure are supported by their own experimental measurement results, among which the evidences of the "island" model mainly come from fluorescence spectroscopy, laser vaporization/laser ionization mass spectrometry (L2MS) analysis and stem from various ionization high-resolution (Fourier transform ion cyclotron resonance mass spectrometry, FT-ICR-MS) analysis technologies. The evidences for the archipelago model mainly come from gas phase permeation measurement, size exclusion chromatography and chemical degradation.

Keywords

• asphaltene
• macro-molecular structure
• archipelago model
• island model
• molecular weight
• polarity