THE KINETICS OF STERANE BIOLOGICAL MARKER RELEASE AND DEGRADATION PROCESSES DURING THE HYDROUS PYROLYSIS OF VITRINITE KEROGEN

The hydrous pyrolysis of a mineral-free vitrinite kerogen (Dinantian coal Lower Carboniferous, North East England) has been carried out at four temperatures (270, 300, 330, and 350°C) for heating times ranging from 2 to 648 h. No significant differences in the epimer-based maturation parameters 20S (20S + 20R)-5α(H),14α(H),17α(H) C29 non-rearranged steranes and 22S (22S+22R)-17α(H), 21β(H) homohopanes were found for a comparison between "expelled oil" and "bitumen" fractions in the resulting pyrolysates. A deuterated model compound ((20R)-5α(H),14α(H),17α(H)-[2,2,4,4-d4] cholestane) was added to a number of preextracted kerogens (vitrinite, Kimmeridge, Messel and Monterey) and the mixtures were heated under typical hydrous pyrolysis conditions. These experiments showed that direct chiral isomerisation at C-20 in the non-rearranged steranes appears to be relatively unimportant during hydrous pyrolysis which has also been suggested by other recent studies on geological samples.A kinetic model comprising consecutive release and degradation processes was derived to measure first-order rate coefficients from the bi-exponential concentration-time functions of both the (20R)-and (20S)-5α(H),14α(H),17α(H) C29 "free" steranes in the vitrinite kerogen pyrolysates. This data was then used to calculate preliminary Arrhenius parameters for release ((20S): ΔEa = 125 ± 30 kJ mol-1, A ≈ 4.7 × 105 s-1;(20R): ΔEa = 151 ± 39 kJ mol-1, A ≈ 2.7 × 109 s-1) and degradation ((20S): ΔEa = 104 ± 22 kJ mol-1, A ≈ 5.8 × 103 s-1; (20R): Δa = 87 ± 6 kJ mol-1, A ≈ 2.2 × 102 s-1) of the above individual isomers and the values were found to be consistent with a free-radical chain mechanism. This work helps in the greater understanding of the important biomarker reactions that prevail in hydrous pyrolysis experiments. © 1990.