D/H ratios in terrestrially sourced petroleum systems

Abstract

D/H ratios of terrestrially-sourced whole oils and their respective saturated, aromatic, and polar fractions, individual n-alkanes, formation waters and non-exchangeable hydrogen in kerogen were measured in potential source rocks from seven Australian petroleum basins. Data for 75 oils and condensates, their sub-fractions and 52 kerogens indicate that oil sub-fractions have δD values comparable to δD_(oil), with a ΔδD offset (δD_(kerogen)−δD_(oil)) averaging ca. 23‰. The weighted-average δD of individual n-alkanes is usually identical to δD_(oil) and δD_(saturate). A trend of increasing δD with n-alkane chain length in most oils causes individual n-alkanes from an oil to vary in δD by 30‰ or more. A modest correlation between δD for aromatic sub-fractions and formation waters indicates that about 50% of aromatic C-bound H has exchanged with water. In contrast, δD_(oil) and δD_(saturated) show no evidence for H-exchange with formation water under reservoir conditions at temperatures up to 150 °C. Acyclic isoprenoids and n-alkanes show essentially indistinguishable δD, indicating that primary isotopic differences from biosynthesis have been erased. Overall, extensive exchange of C-bound H in petroleum with other hydrogen is apparent, but seems to have affected most hydrocarbons only during their chemical genesis from precursor molecules. Our isotopic findings from terrestrially-sourced oils should be qualitatively relevant for marine oils as well.

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