Area Summary L12-14 - Department of Mines and Petroleum

Topography
Release Area L12-14 extends eastward over a coastal
system of dunes, limestone ridges, and wetlands, and
spans the relatively featureless Swan Coastal Plain, which
is bounded by the Whicher Range in the eastern and
southeastern parts of the area. The range contains eroded
Lower Cretaceous rocks, rising to a maximum elevation of
160 m, and is bounded by the Whicher Scarp. The scarp is
a curved erosional feature extending parallel to the coast
about 15 km inland. The more prominent Darling Scarp
is located farther east and joins the Whicher Range to the
northeast of the area (Freeman and Donaldson, 2006). The
Whicher Range is a part of the Blackwood Plateau, covered
with native eucalypt forest. The Darling Scarp is associated
with the position of the Darling Fault, which defines the
eastern boundary of the Perth Basin (Figs 1 and 2).
Wells and seismic coverage
Twelve deep (>1000 m) stratigraphic and exploration
wells have been drilled within the Bunbury Trough, all to
the south of the Release Area (Table 1). Fair to good gas
shows were encountered within the Willespie Formation
in most wells, including Sabina River 1. The latter
was drilled 7 km southwest of Wonnerup 1 to test the
hydrocarbon potential of Permian sandstones within the
Sue Group, though the well may not have tested a valid
trap (Crostella and Backhouse, 2000).
Some stratigraphic data are available from hydrogeological
bores within the Release Area (Fig. 3; Commander, 1982).
Four of these are between 1000 m and 1200 m deep. Five
others are located close to the southern border of the area,
four of which are over 1000 m deep (Wharton, 1981).
Several of the bores appear to have terminated within the
Cattamarra Coal Measures, although only two (Q2 and
Q3) intersected the Sue Group. They are located to the
west of the Wirring Fault in the Vasse Shelf, between 10
and 15 km west of Busselton (Wharton, 1981).
Seismic coverage is sparse, except for the southwestern
part of the Release Area (Fig. 3). Line H69-7 (Harvey
SS, 1969) extends into the Release Area from the north
and terminates at its southern border. Line A61-049
(Ambergate SS, 1980–81) also extends from southwest
into the area. Three lines of the Margaret River seismic
survey (1968) lie entirely within the area. From north to
south, these are MR68-AM, MR68-AL, and MR68-AN.
The quality of vintage data can be an issue for effective
exploration (Luck, 1990; WA:ERA, 2012), although
reprocessing may significantly improve the data quality.
Regional geology and
prospectivity
The Bunbury Trough is an asymmetrical full-graben,
nearly 150 km long and up to 50 km wide, extending
offshore (Fig. 2). It is tilted toward the east across a series
of longitudinal normal faults between its bounding faults,
Geology and petroleum prospectivity of State Acreage Release Area L12-14, Southern Perth Basin
5
the Darling and the Busselton Faults (Iasky, 1993, fig. 10).
The Darling Fault has an estimated cumulative throw of
up to about 11 km in the eastern part of the Release Area
(Iasky, 1993). It has had a longer history of reactivation
than the Busselton Fault, probably going back to early
Proterozoic (Wilde and Nelson, 2001). The Darling Fault
has also exerted fundamental control on the development
of the Perth Basin and its pattern of subsidence (Dentith
et al., 1994). By comparison, the Busselton Fault is a
Middle–Late Mesozoic normal fault.
The structural configuration of the Perth Basin is a
product of a multi-phase process of oblique rifting, which
commenced in the Early Permian and terminated with the
breakup of Australia from Greater India during the Early
Cretaceous (Harris, 1994). In the southern Perth Basin,
two phases of transtension and transpression followed by
another phase of extension have been recognized between
the Dunsborough and the Darling Faults (Iasky et al.,
1991; Iasky, 1993, fig. 14). In the first phase, right-lateral
movement along the Darling Fault along with northeast–
southwest compression resulted in north-northwest
trending folds during the Late Permian to the Early
Triassic. In the second phase, left-lateral movement along
the Dunsborough Fault took place along with northwest–
southeast compression during the Jurassic. Finally, normal
faulting along the Darling and the Dunsborough Faults
and right-lateral movement along the Harvey Ridge led
to continental breakup during the Early Cretaceous.
Therefore, wrench-induced en echelon folds — such as
those drilled in the Whicher Range Gasfield and Wonnerup
1 (Crostella and Backhouse, 2000) — may have been
dissected by the dominantly extensional phase of faulting
during the Early Cretaceous.
Folding may have been augmented along the Bunbury
Trough by the presence of two rigid crystalline blocks
to the east and west (Iasky, 1993). The Leeuwin Inlier
extends northward offshore and joins the Southern Turtle
Dove Ridge, which defines the western boundary of the
central Perth Basin (Iasky and Lockwood, 2004). Further
faulting and fault reactivation during the Early Cretaceous
and the absence of regional or effective marine seals can
have serious implications for seal integrity in the sandrich
and faulted Upper Permian to Lower Cretaceous
succession of the Bunbury Trough. However, not all of the
folds have been dissected and other types of traps are also
possible (Crostella and Backhouse, 2000).
The northern and southern parts of the basin have
similar stratigraphic histories (Fig. 4). For example, both
parts contain thick intervals of coal measures within
the Permian and Jurassic sections. However, viable
objectives younger than Permian may be less likely in
the Bunbury Trough (Crostella and Backhouse, 2000),
as thermal maturity modelling suggests that only the
Permian–Triassic section is within the oil window
(Iasky, 1993). This is probably because of a lower rate
of heat flow, which might be due the thick and relatively
‘cool’ continental crust of the trough (Iasky, 1993). The
Dandaragan Trough, which is the main depocentre and
kitchen of the northern part of the basin with a comparable
sedimentary fill to that of Bunbury Trough, has an
average geothermal gradient of 2.5°C/100 m (Crostella,