Study of the porphyrins released from the Messel oil shale kerogen by hydrous pyrolysis experiments

Abstract

Eight samples of pre-extracted Messel oil shale have been submitted to hydrous pyrolysis at different temperatures between 225°C and 350°C. The porphyrins released were evaluated, fourteen isolated, and seven of them characterized (from the sample heated at 250°C) by comparison with geological and/or synthetic standards (UV-visible, HPLC, MS and NMR). We followed the thermal release of porphyrins by HPLC and MS to examine changes in the distributions and, to get a better idea of the porphyrins bound to the macromolecular kerogen network of the Messel shale. The structures of the released porphyrins indicate that dealkylation reactions occur during the thermal stress. The presence of acid porphyrins, in the case of the lowest temperature experiments (225°C and 250°C), suggests that at least part of the released alkyl porphyrins are formed by the decarboxylation of acid porphyrins present in the kerogen. Etioporphyrins are more abundant and they are released later (at higher temperature) than DPEP's. This observation may indicate that ETIO's are more strongly attached to the macromolecular network than DPEP's. The results obtained seem to indicate that, during these maturation experiments, the DPEP's are destroyed by the thermal stress rather than being transformed into ETIO's.