Differences in the thermal evolution of hopanes and steranes in free and bound fractions

Hopanes and steranes are the two of the most commonly used biomarker classes in the application of organic geochemistry to petroleum exploration. The same carbon skeletons also occur as a bound fraction, and can be used in a relatively high maturity range compared to their extractable (free) counterparts as a result of protection by the kerogen macromolecular structure. The pools of free and bound biomarkers are expected to be thermally degraded over geological time. There has been little work to address the chemical stabilities of hopanes and steranes in both free and bound forms. This study uses anhydrous pyrolysis to simulate the thermal evolutions of biomarkers from two Type II kerogens. The bound biomarkers within the above thermally altered kerogen residues were also released by catalytic hydropyrolysis and discussed in this study. The anhydrous pyrolysis results show that source-related parameters based on hopanes are more stable than those based on steranes. The hydropyrolysis results show that the bound hopane distributions are quite stable even at 460 °C (Easy%Ro = 2.86), while the bound sterane distributions are changed beginning at 430 °C (Easy%Ro = 2.27). This indicates that the thermal stability of steranes is lower than that of hopanes in both free and bound fraction which can be explained by the different chemical structure and mode of incorporation of their precursors into kerogen.