Publication Name: PESA Journal No. 24
Authors: P.E. Williamson, G.W.O'Brien and M.G. Swift
Publication Volume: 24
Date Published: December 1996
Number of Pages: 28
Reference Type: Journal Article
Abstract:
The Otway Basin is one of a series of Late Jurassic to midCretaceousdepocentres which developed along the southeastern
Australian margin during the rifting which
ultimately led to the separation of Australia and Antarctica.
This rifting took place in two stages. The first rift (-153 to
120 Ma) is structurally dominant on the near-shore Crayfish
Platform in the western Otway Basin and is characterised by
half-graben development with dominantly east-northeast
trending bounding faults. The second rift (120 to 96 Ma)
largely by-passed the Crayfish Platform and is best
developed in the Voluta Trough and Mussel Platforms of the
central and western basin respectively. Subsidence of more
than 7 krn occurred in the Voluta Trough and the eastern
Mussel Platform.
Geohistory maturation modelling is based on palaeo-heat
flow levels that imply full sediment load subsidence in the
time span of interest. Results of this modelling and
geochemical results are consistent with geochemical
readings for water bottom sediments, and suggest that an
oil-prone source interval may occur in the Early Cretaceous
Pretty Hill Formation. On the Crayfish Platform of the
western basin, this interval first entered the oil window in
the Early Cretaceous, prior to trap formation resulting in the
probable loss of at least some produced oil, as evidenced by
Cretaceous tar-mats. The interval has subsequently moved
into the thermal gas regime over much of the Crayfish
Platform. The reservoir sequences (upper Pretty Hill Formation) are presently immature or within the oil window,
and consequently the relationship between the timing of trap
formation and hydrocarbon generation is critical to the
preservation of oil, gas or gas-condensate accumulations in
that region.
By latest Cretaceous time, source rocks of the mainly gasprone
Early Cretaceous Eumeralla Formation had entered
the oil window over most of the Voluta Trough. The
overlying Late Cretaceous Waarre Formation reservoir,
which underlies the regional Late Cretaceous Belfast
Mudstone seal, thus could have been charged in essentially
its present structural configuration in the eastern and central
Voluta Trough, and on parts of the Mussel Platform. The
recent significant gas discovery at Minerva 1, within the
eastern offshore basin, has confirmed the potential of this
play. In parts of the western Voluta Trough, lean gas-prone
source rocks of the Belfast Mudstone could possibly source
reservoirs in the underlying Waarre Formation and the
overlying Late Cretaceous Paaratte Formation.
Integration of geohistory and geochemical data suggests that
a significant, and as yet untested, Early Cretaceous oil
exploration play may exist on the offshore Mussel Platform,
providing that stratigraphic relationships exist similar to
those of the Crayfish Platform. The play may be present in
structural positions marginward of the Prawn 1 and Pecten
IA exploration wells, where the basal Early Cretaceous is
now within the oil window. This play is analogous to that on
the Crayfish Platform and may extend into the. onshore
Otway Basin in both areas.