H/D RATIOS IN AUSTRALIAN PETROLEUM SYSTEMS

To examine the processes which influence D/H ratios of petroleum and to better classify terrestrial-sourced Australian oils into oil families, we determined delta-D values of bulk oil, fractions (saturated, aromatic, polar), individual n-alkanes, formation water and non-exchangeable hydrogen in kerogen from source rocks. D/H ratios are a useful discriminator among sources of fossil fuels from terrestrial paleoenvironments where the isotopic heterogeneity is larger than in the marine realm. D/H analyses of 48 source-rock kerogens and 53 bulk oils with matching source ages from four basins indicate that oils are depleted in D by ca. 20 permil on average (and in 13C by ca. 1 permil) relative to source-rock kerogen. Although petroleum is in contact with copious amounts of formation water in reservoirs over geologic time, most hydrogen in hydrocarbons has been isotopically conservative under reservoir conditions, as demonstrated by the lack of hydrogen isotopic correlation between formation waters and associated organic hydrogen in bulk oils and their saturated fractions. In contrast, significant isotopic correlation between water and hydrogen in the aromatic fraction suggests isotopic exchange of about one quarter of hydrogen, in qualitative agreement with published experimental data on aromatic hydrogen exchange. Comparison of 75 bulk oils and fractions indicates a trend delta-D(saturate) < delta-D(polar) ≈ delta-D(bulk oil) < delta-D(aromatic), albeit with strong isotopic overlap. delta-D of bulk oil, its saturate fraction, and the weighted mean values for individual n-alkanes are typically within 10 permil. delta-D of fossil organic matter can be related mainly to the delta-D of the water in paleoenvironments which was available for primary production. Compound-specific hydrocarbon delta-D values suggest that thermal maturation of organic matter in formation waters that are isotopically similar to paleoenvironmental waters tends to equalize delta-D of different hydrocarbon products, without erasing the overall isotopic value of oils. The combined use of hydrogen and carbon stable isotope data shows superior resolving power in characterizing relationships between oils and source rocks.