The organic geochemistry of asphaltenes and occluded biomarkers

Asphaltenes are heteroatom rich macromolecules thought to be mainly derived from the early breaking of covalent bonds in kerogen and thus have been considered to be a lower molecular weight analog of the kerogen at the time of early oil generation. This paper reviews information on asphaltene structures and sources derived from a variety of chemical procedures including oxidation, desulfurization and various forms of pyrolysis. An additional focus is on the geochemical information that can be obtained from oxidation of asphaltene with different chemical reagents. This procedure yields not only the chemical units that make up the asphaltene but also the occluded hydrocarbons providing biomarker information on the oil from which the asphaltenes were isolated. The latter is particularly useful when studying biodegraded oils that have received a subsequent charge and where biomarker information on the first, biodegraded phase may have been preserved within the asphaltene structure and/or occlusions. A common observation is that occluded hydrocarbons show distributions of biomarkers having less mature stereochemistry than in the oil maltenes (pentane soluble fraction). Ruthenium ion catalyzed oxidation mineralizes aromatic carbons in asphaltenes leaving a residue of oxidized (acidic) alkyl and alicyclic fragments that show how the aromatic centers were bound together. In addition, occluded hydrocarbons are released with little or no oxidation. Mild oxidation with hydrogen peroxide/acetic acid degrades a limited number of bonds in the asphaltene, releasing occluded or physically trapped species with little contribution from the asphaltene structure itself. Reduction techniques include the use of alkali metal or Raney nickel and generally these are used to desulfurize asphaltenes and release hydrocarbon moieties that were bonded via sulfide linkages. Hydropyrolysis, thermal degradation in the presence of high pressure hydrogen and a Mo catalyst, results in the cleavage of saturated and aromatic asphaltene moieties comprising the asphaltene that are then available for typical biomarker analysis using gas chromatography-mass spectrometry and other approaches. These degradation products include biomarker compounds useful for making genetic correlations among samples, age dating, providing estimates of thermal maturity and deconvoluting mixtures of oils. (C) 2015 Elsevier Ltd. All rights reserved.