Using neo-pentane to probe the source of gases in accumulations of the Browse and Perth Basins

For Further Information: Dr Emmanuelle Grosjean Email: emmanuelle.grosjean@ga.gov.au Ph: +61 2 6249 9017 Web: www.ga.gov.au

eCat 101680

Introduction The origin of gases in Australia’s Browse and Perth basins is deciphered using the molecular and isotopic compositions of individual C1–C5 hydrocarbons. As neo-pentane is strongly resistant to biodegradation and least influenced by thermal maturation (Boreham and Edwards, 2008), its carbon isotopic composition provides a powerful correlation tool for biodegraded and high maturity gases. The molecular compositions of the gases and carbon isotopic compositions were obtained following methods described in Boreham and Edwards (2008).

Emmanuelle Grosjean*, Dianne Edwards, Chris Boreham, Ziqing Hong, Junhong Chen, Jacob Sohn

Source of gases in the Browse Basin Four gas families are identified based on the carbon isotopic compositions of C1-C5 alkanes supporting the occurrence of four Mesozoic petroleum systems in the Browse Basin (Figs. 1a; 2) :

• Gases found in the Plover reservoirs of the Brecknock-Scott Reef Trend and Ichthys fields (displayed in orange in Fig. 2) are sourced by the fluvio-deltaic sediments of the Lower to Middle Jurassic Plover Formation.

• Relatively wetter gas accumulations reservoired within the Upper Jurassic Brewster Member of the Ichthys and Prelude/Concerto fields (displayed in pink in Fig. 2) are likely derived from the Upper Jurassic to Lower Cretaceous Vulcan Formation.

• The 13C-enriched gases from the Crux field in the Heywood Graben (displayed in blue in Fig. 2) are thought to derive from coal-rich facies within thick Jurassic syn-rift sediments.

• The 13C-depleted gases at Adele and Kalyptea (displayed in green in Fig. 2) are sourced by marine source rocks within the Lower Cretaceous Echuca Shoals Formation.

Browse Basin The Browse Basin on Australia’s North West Shelf hosts considerable undeveloped gas accumulations in the central Caswell Sub-basin (Ichthys, Prelude and Concerto), along the Brecknock-Scott Reef Trend (Calliance, Brecknock, Torosa and Poseidon) and in the Heywood Graben (Crux). Oil discoveries are restricted to the central Caswell Sub-basin and to the Yampi Shelf.

Source of gases in the Perth Basin Compared to the gases from the Browse Basin, the majority of gases from the Perth Basin show depleted values in 13C (Fig. 4), consistent with sourcing from the marine Lower Triassic Kockatea Shale (Boreham et al., 2011). Only three gases are isotopically enriched in 13C, pointing to 13C-enriched Permian (Elegans 1) and Jurassic (Gingin West 1 and Warro 3) sources.

The Kockatea Shale-sourced gases have been generated from a wide range of thermal maturities. As thermal maturity drives carbon isotopic compositions towards more 13C-enriched values, the ability of discriminating between an overmature Kockatea Shale-sourced gas and the Permian and Jurassic-sourced gases comes into question. However, thermal maturity exerts only a weak control on the carbon isotopic composition of neo-pentane (Boreham and Edwards, 2008). Although Redback South 1 and Redback 2 represent dry mature gases, their neo-pentane is still strongly depleted in 13C, unambiguously attesting to a Kockatea Shale source (Fig. 4).

Figure 2. Carbon isotopic composition of C1-C5 alkanes highlighting the occurrence of four gas families in the Browse Basin.

References Boreham, C.J., Hope J.M., Hartung-Kagi, B., van Aarssen, B.J.K., 2000. More sources for gas and oil in Perth Basin. AGSO Research Newsletter, December 2000, 5-9. Boreham, C.J., Edwards, D.S., 2008. Abundance and carbon isotopic composition of neo-pentane in Australian natural gases. Organic Geochemistry 39, 550-566. Boreham, C.J., Chen, J., Grosjean, E., Poreda, R., 2011. AAPG Hedberg Conference Natural Gas Geochemistry, Beijing, China, 9-12 May 2011. Rollet, N., Pfahl, M., Jones, A.T., Kennard, J.M., Nicholson, C., Grosjean, E., Mantle, D., Jorgensen, D.C., Bernardel, G., Kempton, R.H., Langhi, L., Zhang, Y., Hall, L., Hackney, R., Johnston, S., Boreham, C.J., Robertson, D., Petkovic, P. & Lech, M., 2013. Northern extension of active petroleum systems in the offshore Perth Basin—an integrated stratigraphic, geochemical, geomechanical and seepage study. In: Proceedings of the West Australian Basins Symposium 3, Perth, WA, 18–21 August 2013. Rollet, N., Abbott, S.T., Lech, M.E., Romeyn, R., Grosjean, E., Edwards, D.S., Totterdell, J.M., Nicholson, C.J., Khider, K., Nguyen, D., Bernardel, G., Tenthorey, E., Orlov, C., Wang, L., 2016. A regional assessment of CO2 storage potential in the Browse Basin: results of a study undertaken as part of the National CO2 Infrastructure Plan. Geoscience Australia Record 2016/17.

Acknowledgements We thank Prince Palatty and Neel Jinadasa for laboratory technical assistance, Irina Borissova for internal review and David Arnold for help with figure production. This poster is published with the permission of the CEO, Geoscience Australia.

Figure 1. a) Distribution of the four known Mesozoic petroleum systems and of oil and gas accumulations in the Browse Basin and b) stratigraphy from Rollet et al. (2016)

Figure 3. a) Oil and gas fields, and selected hydrocarbon discoveries and occurrences coloured by age of source rock, as interpreted from geochemical evidence in the Perth Basin and b) stratigraphy from Rollet et al. (2013)

Comparison with the Perth Basin The Perth Basin is a proven petroleum basin with oil and gas production onshore and oil production offshore at the Cliff Head oil field. Oil is sourced primarily from the Hovea Member of the Lower Triassic Kockatea Shale and gas originates from the Kockatea Shale, Permian Irwin River Coal Measures (Boreham et al., 2000; Rollet et al., 2013) and overlying Jurassic sources (Boreham et al., 2011).

Figure 4. Carbon isotopic composition of C1-C5 alkanes from gases of the Browse and Perth basins.

Source of gases on the Yampi Shelf The origin of gases on the Yampi Shelf is challenging to establish due to the impact of biodegradation occurring in these shallow Cretaceous reservoirs. The gases are typically dry as a result of the preferential removal of wet gases by microbes and the residual C2-C5 hydrocarbons are significantly enriched in 13C due to the bacterial selectivity towards 12C during biodegradation. However, neo-pentane, thanks to its high resistance to biodegradation, retains a strong source signature and can be applied here as a correlation tool.

The carbon isotopic compositions of neo-pentane for the Yampi Shelf gases in Caspar 1A, Cornea South 2 and Macula 1 all fall within the range expected for Plover-sourced gases (Fig. 2), supporting the fact that these gases have been charged by the Lower–Middle Jurassic Plover Formation. As the co-reservoired oils are derived from marine organic matter within the Echuca Shoals Formation (Rollet et al., 2016), this highlights the existence of multiple hydrocarbon charges towards the basin margins.

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