Area Summary L12-14 - Department of Mines and Petroleum

Geology and petroleum prospectivity
of State Acreage Release Area L12-14
Southern Perth Basin, Western Australia
by
Geological Survey of Western Australia
Abstract
State Acreage Release Area L12-14 covers about 10.5 graticular blocks (716.8 km 2 ) of the northern part of the
Bunbury Trough with excellent access to infrastructure. The area spans the Swan Coastal Plain to the Whicher
Range, and is bounded by the Darling Fault to the east. There are no petroleum exploration wells within the area
and the seismic coverage is sparse. Fair to good gas shows are known from several wells within the Bunbury Trough
to the south of the area, including Wonnerup 1, which is the closest. The Whicher Range Gasfield is also 30 km
south of this well. The Permian section is estimated to be up to 6.5 km thick in the area, and the top of the Sue Coal
Measures varies from a depth of 4200 m in the southwest to 8200 m in the northeast. The Lower Cretaceous – Upper
Jurassic section gently dips eastward and becomes thicker toward the Darling Fault. Various trapping mechanisms
are possible, although mature source rocks younger than Permian are unlikely to be present. The main risks are
likely to be reservoir quality and trap integrity.
KEYWORDS: State Acreage Release, Bunbury Trough, Southern Perth Basin, Western Australia, Petroleum
prospects, tight gas
Introduction
The Perth Basin covers about 100 000 km 2 and contains
a number of proven Permian and Mesozoic petroleum
systems. The onshore part of the basin covers approximately
45 000 km 2 and extends 650 km along the southwest coast
of Western Australia, mainly to the south of Geraldton
(Hocking, 1994). In total, 300 wells have been drilled in
the basin, onshore and offshore (as of December 2011),
resulting in the discovery of 31 hydrocarbon fields and
several significant accumulations (GSWA, 2011), mostly
within the northern onshore part of the basin.
The southern Perth Basin extends from the Harvey Ridge
southwards across the south coast of the State, consisting
of the Bunbury Trough and the Vasse Shelf (Figs 1 and 2;
Crostella and Backhouse, 2000, fig. 4). Hydrocarbon
shows have been reported from several wells within
the Bunbury Trough, including those close to the State
Acreage Release Area L12-14 (Fig. 2). Wonnerup 1 is the
closest well. It was drilled on the crest of a north–south
trending anticline (Crostella and Backhouse, 2000, figs 62
and 69). This structure extends northward into the Release
Area, although there were no successful mechanical tests.
The location has been ranked as the Wonnerup prospect
(Fig. 2; Ferdinando and Dedman, 2003).
1
The only major discovery within the Bunbury Trough has
been the Whicher Range Gasfield, where significant gas
flows from the Upper Permian Willespie Formation were
encountered in five wells (Table 1; Sharif, 2007). This is
a much larger structure than the one at the Wonnerup 1
site, and it is estimated to contain an original gas in place
volume of up to 142.5 Gm 3 (5 Tcf), although attempts to
develop the field have been unsuccessful so far. A major
research project has analyzed causes of failure, and has
proposed new drilling methods and approaches to reservoir
stimulation (WA:ERA, 2012).
Location and infrastructure
State Acreage Release Area L12-14 (716.8 km 2 ) covers
almost 10.5 graticular blocks within the poorly explored
northern part of the Bunbury Trough, nearly 170 km
south of Perth (Figs 1–3). The port city of Bunbury is
partially within the Release Area, and Busselton is about
13 km to the southwest (Fig. 3). The Dampier–Bunbury
Gas Pipeline extends to 7 km north of the area, and the
BP Kwinana Refinery is 30 km south of Perth. Rising
demand and proximity to potential industrial consumers
could facilitate commercialization of even a relatively
small discovery.