Short chain tricyclic terpanes as organic proxies for paleo-depositional conditions

Abstract

Reconstructing the paleo-environmental conditions that prevailed during the deposition of sediments provides invaluable information for paleo-ecological and evolutionary reconstructions and petroleum exploration. However, reconstructions of depositional environments based on sedimentary structures are often ambiguous, in particular if only core or mud chip samples are available. Organic proxies provide another avenue for paleo-depositional reconstruction of sedimentary rocks and related crude oils, but there is only a limited number of proxies and these rarely allow for unambiguous interpretations. Although tricyclic terpanes are among the most widely used depositional proxies, few studies calibrated terpane proxies with a large variety of samples. Here we show based on analyses of 485 oil and sediment samples from a variety of formations with well characterized depositional environments spanning in age from the late Paleozoic to Neogene, that the distribution of C19 + C20, C21 and C23 tricyclic terpanes in ternary diagram has high predictive power to differentiate between marine/saline lacustrine, freshwater lacustrine, swamp and fluvial/deltaic depositional environments. Our analyses indicate that the tricyclic terpanes related parameters are not adulterated by even severe biodegradation or thermal maturation processes throughout the whole oil window, and our analyses of the largest terpane compilation to date encompassing >700 samples indicate robust proxy potential in post-Ordovician settings around the globe. This facile and widely applicable method for ecosystem definition can enhance a variety of petroleum, sedimentary and paleo-ecological studies and should best be used in concert with other organic and inorganic proxies for a detailed reconstruction of paleo-ecological conditions.