The Origin of Asphaltenes and Main Trends in Evolution of Their Composition During Lithogenesis

Resumo

The composition and structure of bitumenoid asphaltenes of modern and fossil sediments has been studied using a set of instrumental methods (elemental and X-ray diffraction analyses, visible spectroscopy, NMR spectroscopy, EPR, electron microscopy), which made it possible to trace the evolution of the asphaltenes from their generation in diagenesis to degradation and transformation into kerogen under high catagenesis conditions. In diagenesis, protoasphaltenes are formed, which turn into asphaltenes as a result of catagenetic transformations. In apocatagenesis under severe thermobaric conditions at great depths, despite an overall decrease in bituminosity, the amount of hydrocarbons increases as a result of degradation of asphaltenes: molecules of organic compounds (occluded hydrocarbons) could be trapped and preserved in large cavities of the macromolecular structure of asphaltenes. Three generations of asphaltenes have been distinguished: early diagenetic (protoasphaltenes of modern sediments), mesocatagenetic (asphaltenes formed in the main catagenesis zone), and late catagenetic (asphaltenes produced under conditions of high temperatures and pressures).