Polar heterocompounds in oil-source rock correlation studies

Christian Hallmann, Geological Institute, University of Cologne, Cologne, Germany, Centre for Applied Organic Geochemistry, Curtin University of Technology, Perth, Australia, Email: [email protected]

The purpose of this study is to monitor carbazole distributions in oils, source rock extracts and source rock pyrolysates in order to establish a molecular marker in the polar petroleum fraction, indicative for source facies.

The Cooper/Eromanga Basins, Australia, host oil and gas condensates that were sourced by at least three discrete terrestrial source rocks. Their highly similar source facies precludes a detailed oil-source rock correlation. Jurassic-sourced oils can be discriminated from Paleozoic-sourced oils by aromatic hydrocarbon biomarkers that were produced exclusively by post-Triassic Araucariacea conifers. Preliminary results indicate that distribution patterns of nitrogen-containing carbazole compounds allow a precise discrimination of all three source rock units (Jurassic/Permian/pre-Permian) present in the study area.

We plan to subject immature, unaltered source rock samples from all three aforementioned source units to hydrous pyrolysis. Pyrolysates will be analysed for carbazole and hydrocarbon biomarker compositions by molecular geochemistry. Molecular compositions of hydrous pyrolysates are comparable to those of naturally generated oils. Pyrolysates, however, are unaffected by secondary alteration and oil mixing processes. Thus they should exhibit pristine end-member oil compositions. If the molecular composition of source rock pyrolysates supports our earlier observations, we can provide a powerful new tool for correlating oils and source rocks based on heterocompound compositions.

The anticipated results will not only enhance our understanding of the Cooper/Eromanga Basin oil play, but also aid in future exploration activities, elsewhere. Furthermore, the finding of facies-dependent markers in polar petroleum fractions is a positive motivation for further research into the geochemistry of petroleum heterocompounds.