Hopanoid breakdown products (HBPs) and their influence on tetracyclic and tricyclic terpane (cheilanthane) oil-source rock parameters

Abstract

Tricyclic and tetracyclic terpane ratios are used as biomarker proxies for depositional environments and to classify crude oils and source rocks. However, the biological precursors and diagenetic drivers of these proxies are not well understood. This study reports on the discovery of tricyclic and tetracyclic breakdown products of pentacyclic hopanoids that co-elute with 14α-C₁₉ and βα-C₂₀–C₂₂ cheilanthanes and C₂₄ tetracyclic terpane in gas chromatograms. This includes hopanoid breakdown products (HBPs) that are identical to 14α-C₁₉ cheilanthane and C₂₄ tetracyclic terpane, biomarkers that are commonly used as indicators of terrigenous organic matter in source rocks and their derived hydrocarbon products. These HBPs were generated by pyrolysis of C₃₀ diplopterol, a common compound found in the cell membranes of almost all hopanoid-producing bacteria, some lichens and higher plants. HBPs are common in rock-extracted bitumens and crude oils from the Neoproterozoic to the Cenozoic and across a wide range of thermal maturities. Comparison of the relative abundance of HBPs and cheilanthanes against commonly used tricyclic and tetracyclic terpane parameters shows that HBPs can drive or strongly influence terpane-based paleoenvironmental proxies. This study begins to rationalise why some of these parameters indicate specific depositional environments and proposes new ratios that quantify the impact of HBPs on biomarker proxies.