Evolution of asphaltenes during artificial maturation: A record of the chemical processes

Type II kerogens have been artificially matured by confined and hydrous pyrolysis. The asphaltenes recovered from the experiments were characterized by FT-IR, synchronous UV fluorescence, Py-GC-MS, and elemental analysis. The results show that important modifications related to the physicochemical maturation conditions affect the asphaltenes: (i) The asphaltenes yields are modified by temperature, pressure and by the presence of water. (ii) A progressive defunctionalization and increased aromatic content during maturation is recorded by the asphaltenes. This phenomenon is retarded by the presence of water in hydrous conditions. (iii) The asphaltenes expelled during hydrous pyrolysis have a different composition than the asphaltenes remaining in the bitumen. (iv) Data on the oxygen content of the asphaltenes strongly support the occurrence of interactions between asphaltenes and water, both in confined and hydrous pyrolysis. Therefore, it is suggested that hydrogen transfer reactions between water and the newly formed hydrocarbons are catalyzed by the asphaltenes and are based on a combination of reduction and oxidation reactions through a radical ion mechanism. (v) A comparison with the elemental composition of the residual kerogen leads to the conclusion that the excess of CO2 generated during hydrous pyrolysis is related to the oxidation of the asphaltenes. The study of the asphaltenes can bring important information concerning the chemical processes taking place during artificial maturation. It appears that the asphaltenes are not simple intermediates between the kerogen and the hydrocarbons during oil generation, but they actively participate to the chemical reactions involved in maturation.