Corroborative insights into genetically related kerogen, asphaltenes and hydrocarbons using free and bound biomarkers

Abstract

Unlike the free biomarkers, which can be routinely recovered via solvent extraction, the bound biomarkers are covalently linked to the kerogen or asphaltene macrostructures and are more resistant to secondary alteration processes. Although similarities have been identified between these two types of biomarkers using different techniques, it remains unclear how the kerogen- and asphaltene-bound biomarkers are complicatedly corelated with their free counterparts, thus limiting the applications of bound biomarkers in geoscience research. In this study, the characteristics of free biomarkers recovered from shale samples, from 10 sedimentary basin around the world, and petroleum samples, from the North Sea, have been systematically compared with their kerogen- and asphaltene-bound biomarkers, which were released by the newly developed Microscale Sealed Vessel Catalytic Hydrogenation (MSSV-Hy) method. Although the kerogen-bound biomarkers are characterized by the absence of rearranged biomarkers (e.g., Ts and diasteranes) and elevated C29/C30 hopane ratios, many key source-related parameters show strong positive correlations between free and bound fractions, including the steranes/hopane, C27/C29 sterane, C30/C29 sterane, C29/C30 hopane (αβ + βα), C35/C34 homohopanes and C24 tetracyclic terpane/C26 tricyclic terpane ratios. These results suggest that bound biomarkers can reveal critical information about deposits and organic matter precursors of source rocks. Thermal maturity indicators, such as C29 sterane ββ/(αα + ββ), C29 sterane S/(S + R), C31 hopane S/R, and Tβ/Tm ratios, also display excellent correlations between the free and bound counterparts in the source rocks. Although the correlations are not 1:1, empirical formulae, which are based on the correlations between free and bound biomarkers, can be used to calibrate bound biomarkers into values that can be directly compared to routine free biomarkers. Importantly, asphaltene-bound biomarkers of North Sea samples exhibit similar characteristics to kerogen-bound biomarkers, and the conversion between free and bound biomarkers of the source rock and reservoir samples is the same. In summary, the bound biomarkers are found highly effective in determining the organic facies, lithology, and thermal maturity of the parent kerogen and/or asphaltene, and, thus, have a great potential to be widely used in geoscience research when information from free biomarkers is hampered by oil mixing, contamination or biodegradation.