Thermal stability of thiophene biomarkers as studied by hydrous pyrolysis

An immature (Ro = 0.25%) sulphur-rich calcareous shale from the Gessoso-solfifera Formation (Messinian) in the Vena del Gesso Basin (northern Italy) was artificially matured by hydrous pyrolysis at constant temperatures ranging from 160 to 330°C for 72 h to study the applicability of alkylthiophenes as molecular indicators during progressive diagenesis and early catagenesis. Artificial maturation at temperatures up to 330°C generated large amounts of alkylthiophenes from the thermal degradation of kerogen. The importance of the kerogen in the production of alkylthiophenes is supported by the observation that the isomer distributions of alkylthiophenes at high temperatures coincide with the isomer distributions of alkylthiophenes in the Curie-point pyrolysate of the isolated kerogen of the original sample. The generation profiles of the thiophenes with a phytane carbon skeleton and the 3,4-di-n-alkylthiophenes suggest that continuous degradation of alkylthiophenes is also taking place, so that the overall effect is alkylthiophene concentrations with intermediate values. The structural isomer distribution of linear alkylthiophenes does not change significantly when these compounds are generated at different temperatures. This implies that important information regarding the original positions of functionalities in the precursor molecules is retained and, thus, that alkylthiophenes are useful biomarkers at relatively high levels of thermal maturity (up to Ro ≈ 0.95–1.1%).