25th International Meeting on Organic Geochemistry IMOG 2011

P-282
Land plant markers in Gippsland Basin oils, Australia
Herbert Volk 1 , Manzur Ahmed 1 , Se Gong 1 , Chris Boreham 2 , Peter Tingate 3 , Dianne
Edwards 2
1 CSIRO, North Ryde, Australia, 2 Geoscience Australia, Canberra, Australia, 3 GeoScience Victoria,
Melbourne, Australia (corresponding author:Herbert.Volk@csiro.au)
The Gippsland Basin is one of Australia‘s most prolific
oil provinces and is dominated by oils with strong
terrestrial signatures. A previous study of over 60 oils
used multivariate statistical analyses of aliphatic
biomarkers and stable carbon isotope signatures to
define two main oil families, ‗GA‘ and ‗GB‘ [1]. The
larger and less variable oil family (GA) was further
distinguished into two sub-families, GA1 and GA2
oils, which occur in differnt parts of the basin. GB oils,
and to a lesser degree GA2 oils, have both mature
and immature biomarker features, possibly acquired
during petroleum migration. Some oils remained
unclassified or were classified as vagrant, but all other
oils were suggested to be derived from coals and
carbonaceous shales of the Latrobe Group. This
study investigated the distribution of land plant
markers in a subset of 23 oils representatiing all the
oil families for a more precise understanding on the
source characteristics and migration of petroleum in
the Gippsland Basin. The studied markers include di-,
tri- and tetracyclic diterpanes derived chiefly from
Southern Hemisphere gymnosperms, oleanane ±
lupane and their A-ring contracted counterparts
originating from angiosperm plants, bicadinanes that
are particularly abundant in Dipterocapacea
hardwood trees, and aromatic land plant markers
such as retene and cadalene.
Studies on Cretaceous and Palaeogene sediments of
the Taranaki Basin, [2] defined an age-specific
angiosperm/gynmosperm index (AGI), which when
applied to the Gippsland Basin oil samples analysed
in the present study suggests Late Cretaceous source
rocks, except for a West Seahorse oil which may be
derived from Palaeogene source rocks. The
Gippsland Basin oils are dominated by 16� (H)phyllocladane
and in some cases by 4�(H)-19norisopimarane,
except for an ―unassigned‖ Marlin 1
oil dominated by isopimarane. This oil also contains
the highest amount of A-ring contracted lupane,
indicating that angiosperm contributions are also
distinct (Figure 1). Bicadinanes, often interpreted as
angiosperm markers for Dipterocapacea, were
present in low amounts in all the oils studied. Their
abundance relative to C30 �� hopane falls within a
very narrow range for GA1 oils and is more variable
for other samples. The widespread occurrence of
bicadinanes is noteworthy, since Dipterocarpacea did
not evolve until the Oligocene, whereas the source
rocks for most of the oils are probably hosted in the
Cretaceous Latrobe Group. This observation confirms
that bicadinanes are not exclusive to oils containing
Dipterocarpaceae input. Aromatic land plant marker
input to unassigned Mulloway 1 and Marlin 1 oils is
much greater than for other oils, in particular it is
higher than for the West Seahorse 1 oil and the tightly
clustering oils from Family GA1.
dO
Figure 1. Ternary diagram of de-A-oleanane (dO), de-Alupane
(dL) and de-A-ursane (dU). Oil families after [1].
References
Marlin 1
dU
[1] Summons et al., (2002) The Oils of Eastern Australia,
Geoscience Australia GeoCat# 68754.
[2] Killops, S.D., Raine, J.I., Woolhouse, A.D. and Weston,
R.J., (1995) Chemostratigraphic evidence of higher-plant
evolution in the Taranaki Basin, New Zealand. Organic
Geochemistry 23, 429-445.
dL
GA1 GA2 GB GV Unassigned
415