D/H ratios in terrestrially sourced petroleum systems

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 deltaD values comparable to deltaD(oil), with a DeltadeltaD offset (deltaD(kerogen) - deltaD(oil)) averaging ca. 23parts per thousand. The weighted-average deltaD of individual n-alkanes is usually identical to deltaD(oil) and deltaD(saturate). A trend of increasing deltaD with nalkane chain length in most oils causes individual n-alkanes from an oil to vary in deltaD by 30% or more. A modest correlation between deltaD for aromatic sub-fractions and formation waters indicates that about 50% of aromatic C-bound H has exchanged with water. In contrast, deltaD(oil) and deltaD(saturated) show no evidence for H-exchange with formation water under reservoir conditions at temperatures up to 150degreesC. Acyclic isoprenoids and n-alkanes show essentially indistinguishable deltaD, 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. (C) 2004 Elsevier Ltd. All rights reserved.