25th International Meeting on Organic Geochemistry IMOG 2011

P-069
The age and palaeoenvironmental conditions spanning the
Permian/Triassic boundary in the northern onshore Perth Basin
by using biomarker distributions and stable isotopes (C, H)
Mojgan Ladjavardi 1 , Kliti Grice 1 , Chris Boreham 2 , Dianne Edwards 2 , Ian Metcalfe 3 , Roger
Summons 4
1 WA Organic and Isotope geochemistry centre, Department of Chemistry, Curtin University of Technology,
Perth, WA, Australia, 2 Geoscience Australia, GPO BOX 378, Canberra, Australia, 3 Schools of environmental
and Rural science, University of New England, NSW, Australia, 4 Massachussetts Institue of Technology,
Earth and Planetary Sciences, Cambridge, United States of America
The Perth Basin petroleum system has been
intermittently explored for the last few decades,
resulting in the production of gas and oil from several
onshore fields (e.g. Summons et al., 1995). The
effective source rock for petroleum in the Perth Basin
is the marine Kockatea Shale, with the hydrogenrichest
interval being the Sapropelic Unit of the Hovea
Member (Thomas et al., 2004). The Perth Basin in
southwest Western Australia (WA) is a deep, north to
south trending basin, extending over 1,000 km from
Geraldton to the north of Perth. The Perth Basin
sediments comprise of rocks of Permian−Early
Cretaceous in age. The current perception is that the
area is gas prone and has been challenged with the
recent discovery of the near-shore Cliff Head oil field.
The Dongara gas field in the onshore Perth Basin
contains more than half the oil and gas reserves. The
Perth Basin sediments used in this study are from the
Senecio-1 core which is located approximately 15.5
km from the north of Dongara. Stable carbon and
hydrogen isotopic composition of biomarkers
measured by compound specific isotopic analysis
(CSIA) has been shown to be an effective tool for
establishing biogeochemical changes across the
Permian/Triassic boundary (e.g. Nabbefeld et al.,
2010). In the present study bulk geochemical,
biomarker and CSIA of biomarkers are used to
restrain the age and palaeoenvironmental conditions
spanning the Permian/Triassic in the northern
onshore Perth Basin and to compare these results
with the Hovea-3 drill core ‗type-section‘ (Grice et al.,
2005a). For this purpose 31 samples from the
Senecio-1 cored including Permian/Triassic interval at
1 m spacing were selected. The ages of the samples
have been determined by conodont biostratigraphy.
Rock-Eval & TOC analyses have been performed to
identify the type and maturity of organic matter and to
evaluate the petroleum potential of these samples.
The samples were analysed following the
methodology of Grice et al. (2005b). Each sample
was ground to a fine powder and extracted using an
Accelerated solvent extractor. The extracts were
separated into 6 fractions by liquid chromatography.
Saturate and aromatic hydrocarbon fractions were
characterised by GC-MS. The saturated hydrocarbon
fractions were separated from branched and cyclic
hydrocarbons by treating with 5A molecular sieves
and CSIA of biomarkers was performed for these
fractions. Bulk stable isotopic compositions were
measured on the kerogens isolated from the extracted
powders. The data is consistent with Hovea-3 for both
Rock-Eval and kerogen type. � 13 C of the bulk organic
matter is consistent with the Permian interval of the
core being mainly land plant derived, whereas the
Triassic interval is mainly phytoplanktonic, the change
in stable isotopes is not as abrupt as shown in Hovea-
3.
References
Grice, K., Cao C., Love G.D., Bottcher M.E., Twitchett R.,
Grosiean E., Summons R., Turgeon S., Dunning W. J., Jin
Y., 2005a. Photoc Zone Euxinia during the Permian-Triassic
Superanoxic Event. Science 307,706-709.
Grice, K., Summons, R.E., Grosjean, E., Twitchett, R.J.,
Dunning, W., E., Wang, S.X., Boettcher, M.E., 2005b.
Depositional conditions of the northern onshore Perth Basin
(Basal Triassic). Australian Petroleum Production and
Exploration Association Journal 45, 263-273.
Nabbefeld, B., Grice, K., Twitchett, R.J., Summons, R.E.,
Hays, L., Boettcher, M.E., Asif, M., 2010. An integrated
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Summons R.E., Boreham, C.J., Foster, C.B., Murray, A.P.,
Gorter, J.D., 1995. Chemostratigraphy and the composition
of oils in the Perth Basin, Western Australia. Australian
Petroleum Exploration Association Journal 35, 613-632.
Thomas, B.M., Willink, R.J., Grice, K., Twitchett, R.J.,
Purcell, R.R., Archbold, N.W., George, A.D., Tye, S.,
Alexander, R., Foster, C.B., Barber, C.J., 2004. Unique
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