^2H/^1H ratio of hopanes, tricyclic and tetracyclic terpanes in oils and source rocks from the Potiguar Basin, Brazil

Abstract

We report the hydrogen isotope ratio ^2H/^1H (expressed as δ^2H values) of 8 selected hopanes, tricyclic and tetracyclic terpanes from oils and source rocks in the Potiguar Basin, and of associated formation water. Hopanes ranged in δ^2H value from −79‰ to −142‰, whereas tri- and tetracyclic terpanes (TTTs) ranged from −137‰ to −225‰. Formation water δ^2H values ranged from −23‰ to −32‰. The most significant pattern in the data is the systematic ^2H enrichment of hopanes relative to TTTs, by an average of 45‰ in oils and 78‰ in source rock extracts. The hopanes appear close to hydrogen isotopic equilibrium with coeval formation water, whereas TTTs are significantly more ^2H depleted. Given the similarities in structure between the two compound classes, it is unlikely that hopanes would be exchanged completely while the others would not. More likely, both classes have undergone a limited extent of exchange, but with the hopanes being biosynthesized with δ^2H values closer to equilibrium. Our data suggest that at least some primary environmental and/or biotic information can be retained in the δ^2H values of biomarkers in oils and extracts, and is not completely obscured by hydrogen exchange.

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