EVOLUTION OF ASPHALTENES DURING ARTIFICIAL MATURATION - A RECORD OF THE CHEMICAL PROCESSES

Type II kerogens have been artificially matured by confined and hydrou s pyrolysis. The asphaltenes recovered from the experiments were chara cterized by FT-IR, synchronous UV fluorescence, Py-GC-MS, and elementa l analysis. The results show that important modifications related to t he physicochemical maturation conditions affect the asphaltenes: (i) T he asphaltenes yields are modified by temperature, pressure and by the presence of water. (ii) A progressive defunctionalization and increas ed aromatic content during maturation is recorded by the asphaltenes. This phenomenon is retarded by the presence of water in hydrous condit ions. (iii) The asphaltenes expelled during hydrous pyrolysis have a d ifferent composition than the asphaltenes remaining in the bitumen. (i v) Data on the oxygen content of the asphaltenes strongly support the occurrence of interactions between asphaltenes and water, both in conf ined and hydrous pyrolysis. Therefore, it is suggested that hydrogen t ransfer reactions between water and the newly formed hydrocarbons are catalyzed by the asphaltenes and are based on a combination of reducti on and oxidation reactions through a radical ion mechanism. (v) A comp arison with the elemental composition of the residual kerogen leads to the conclusion that the excess of CO2 generated during hydrous pyroly sis is related to the oxidation of the asphaltenes. The study of the a sphaltenes can bring important information concerning the chemical pro cesses taking place during artificial maturation. It appears that the asphaltenes are not simple intermediates between the kerogen and the h ydrocarbons during oil generation, but they actively participate to th e chemical reactions involved in maturation.