The Browse Basin, situated in the offshore Northwest region of Australia, is part of a world class hydrocarbon province hosting vast reserves of gas and condensate (Le Poidevin et al., 2015). In addition, both non-biodegraded (Caswell) and biodegraded (Cornea and Gwydion) oil fields are present. The primary source of gas is thought to be a Lower-Middle Jurassic fluviodeltaic sequence whereas Upper Jurassic to Lower Cretaceous marine sequences are the most likely source of liquid hydrocarbons (Rollet et al., 2016). Complex fill histories, mixed marine and terrestrial biomarker signatures and, on the Yampi Shelf, the addition of biogenic methane, have made it difficult to understand the charge history of accumulations in the basin.
This study combines comprehensive two dimensional gas chromatography coupled to time-of-flight mass spectrometry (GCxGC-TOFMS) and compound specific isotope analyses (CSIA) of n-alkanes, aromatic hydrocarbons and diamondoids to identify numerous hydrocarbon contributions to the accumulations in the Browse Basin. The absolute concentrations, ratios and isotopic composition of diamondoids have been shown to be source-specific and highly resistant to both thermal maturity and biodegradation (e.g Dahl et al., 1999; Grice et al., 2000; Moldowan et al., 2015). Diamondoid analysis and quantitation was performed on GCxGC-TOFMS as it minimises interference of co-eluting compounds and allows whole oil injection, eliminating potential losses from sample preparation (Silva et al., 2013; Wang et al., 2013).
The non-biodegraded Caswell oils contained high diamondoid concentrations, well above diamondoid yields recovered from laboratory oil cracking experiments (Dahl et al., 1999; Fang et al., 2012), indicating the contribution of a high maturity fluid (wet gas - early dry gas). Contrastingly, typical biomarker parameters indicate that these fluids have been generated from a marine source rock within the oil window (methylphenanthrene index = 0.4). Thefore, by combining the information from routine biomarker analyses with quantitative diamondoid analysis, the Caswell accumulation can be demonstrated to consist of a mixture of hydrocarbons. Furthermore no gas was recovered from this field, which is in disagreement with the high diamondoid concentrations found in the fluids. This indicates that gas has escaped from the structure, leaving behind a hydrocarbon field that initially seems to consist of marine oil but actually contains a mixture of hydrocarbons.
Although the biodegraded oils displayed even higher diamondoid concentrations than the Caswell oils, a similar mixed hydrocarbon signature cannot be confirmed from quantitative diamondoid analysis as biodegradation increases the concentration of these compounds. However due to their high abundance, carbon isotopic composition of individial diamondoids could be measured in the biodegraded oils. The δC values of methyladamantanes in biodegraded oils become progressively more depleted with increasing biodegradation and the depletion is more pronounced for dimethyladamantane. As biogenic methane is isotopically depleted compared to thermogenic methane, these results suggest that alteration of adamantanes could have occurred during the biodegradation process.
Diamondoid concentrations and their stable isotopic signatures provide further insight into the multiple sources of hydrocarbons that are contained within the Browse Basin acumulations and furthermore provide insight into the formation and occurrence of diamondoids in biodegraded oil fields.
References
Dahl, J.E., Moldowan, J.M., Peters, K.E., Claypool, G.E., Rooney, M.A., Michael, G.E., Mello, M.R., Kohnen, M.L., 1999. Diamondoid hydrocarbons as indicators of natural oil cracking. Nature 399.
Fang, C., Xiong, Y., Liang, Q., Li, Y., 2012. Variation in abundance and distribution of diamondoids during oil cracking. Organic Geochemistry 47, 1-8.
Grice, K., Alexander, R., Kagi, R.I., 2000. Diamondoid hydrocarbon ratios as indicators of biodegradation in Australian crude oils. Organic Geochemistry 31, 67-73.
Le Poidevin, S.R., Kuske, T.J., Edwards, D.S., Temple, P.R., 2015. Australian Petroleum Accumulations Report 7 Browse Basin: Western Australia and Territory of Ashmore and Cartier Islands adjacent area, 2nd edition. Record 2015/10. Geoscience Australia, Canberra.
Moldowan, J.M.M., Dahl, J., Zinniker, D., Barbanti, S.M., 2015. Underutilized advanced geochemical technologies for oil and gas exploration and production-1 . The diamondoids. Journal of Petroleum Science and Engineering 126, 87-96.
Rollet, N., Grosjean, E., Edwards, D., Palu, T., Abbott, S., Totterdell, J., Lech, M.E., Khider, K., Hall, L., Oolov, C., Nguyen, D., Nicholson, C., Higgins, K. and Mclennen, S., 2016. New insights into the petroleum prospectivity of the Browse Basin: results of a multi-disciplinary study. The APPEA Journal, 483-494.
Silva, R.C., Silva, R.S.F., Castro, E.V.R. De, Peters, K.E., Azevedo, D.A., 2013. Extended diamondoid assessment in crude oil using comprehensive two-dimensional gas chromatography coupled to time-of-flight mass spectrometry. Fuel 112, 125-133.
Wang, G., Shi, S., Wang, P., Wang, T., 2013. Analysis of diamondoids in crude oils using comprehensive two-dimensional gas chromatography / time-of-flight mass spectrometry. Fuel 107, 706-714.
