Quantitative analysis of triterpane and sterane biomarkers: methodology and applications in molecular maturity studies

Abstract

With the increasing commercial availability of high purity, authentic steroid and triterpenoid hydrocarbon standards, the ability to accurately quantify the concentrations of biological markers in geologic samples is being facilitated. This chapter discusses aspects of the analytical methodology employed in the quantitative analysis and its application in the study of sterane-derived parameters for thermal maturity assessment in oils and source rocks. Quantitative biomarker data suggest that, while sterane aromatization appears to involve direct conversion of ring-C monoaromatic to triaromatic compounds, little evidence exists for epimerization of saturated steranes at C-20. It is possible that the latter reaction does not occur to a significant extent in natural systems, which is contrary to the commonly held view that steranes isomerize at the C-20 position with increasing thermal maturity. The absolute concentrations of steranes and hopanes in two families of oils generated from marine source facies (Type II and II-S) were found to be highly correlated with the degree of sterane aromatization. A generalized reaction scheme linking absolute concentrations to aromatization is proposed to account for these observations.