The temporal distributions of seed-plant terpanes in petroleum samples: Implications as oil age-diagnostic biomarkers for crude oils

Abstract

Gymnosperm and angiosperm are sister clades of seed-plants thriving on land since the Carboniferous and Cretaceous, respectively. Both biosynthesize specific terpenoids whose diagenetic hydrocarbon skeletons are preserved in geological samples as molecular fossils of seed-plants. Although the radiation of seed-plants has been well established, the temporal distributions of their biomarkers in geological samples remain poorly characterized. We defined angiosperm and gymnosperm ratios to quantify the relative abundances of seed-plant terpanes and examined the relationship between these ratios and geological age via well-studied, world-wide, petroleum samples deposited in various environments. Although angiosperm ratios show broad ranges in the Cretaceous – Tertiary samples, the Epoch-maxima increase from Early Cretaceous to Paleocene, and from Paleocene to Eocene-Miocene. Gymnosperm ratios remain low pre-Jurassic; however, the Period-maxima significantly increase from pre-Jurassic to Jurassic, and further increase from Jurassic to Cretaceous-Tertiary. These patterns are consistent with seed-plant evolution. We propose three age indicators: 1) O/(O + 29Ts) − an oleanane-based angiosperm ratio to distinguish age of Early Cretaceous or younger vs. Paleocene or younger vs. Eocene or younger; 2) Retene/(Retene + DBT) − a retene-based gymnosperm ratio to differentiate Jurassic or younger vs. Cretaceous or younger; and 3) under marine influence (DBT/P > 0.05), enrichment of retene [Retene/(Retene + DBT) > 0.3] and absence of oleanane [O/(O + 29Ts) < 0.04] indicative of Jurassic – Early Cretaceous. The three new age indicators improve age resolution of published age biomarkers from Era to Period level for oil age differentiation, a critical factor in de-risking petroleum systems.