Abstract
We report the hydrogen isotope ratio 2H/1H (expressed as δ2H 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 δ2H value from −79‰ to −142‰, whereas tri- and tetracyclic terpanes (TTTs) ranged from −137‰ to −225‰. Formation water δ2H 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 δ2H values closer to equilibrium. Our data suggest that at least some primary environmental and/or biotic information can be retained in the δ2H values of biomarkers in oils and extracts, and is not completely obscured by hydrogen exchange.
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