PROVINCE GEOLOGY
- continued
Permian sediments directly overlie eroded Precambrian
basement along the eastern flank of the Browse Basin (Stephenson and others,
1994). Deposition in this area ceased with uplift that was associated with
the Late Permian Bedout Movement. The Triassic section is approximately
2 km thick in the center of the basin, and consists mainly of Lower Triassic
(Scythian) to Middle Triassic (Anisian) marine claystones. Sedimentary
rocks on the eastern margin originated as fluvial sands, and deltaic sands
and clays. Rocks in the northern areas of the basin include shelf carbonates,
shales, and deltaic sandstones. Rocks to the west originated as clays,
limes, sandy dolomites, and deltaic sands deposited on a marine shelf.
A widespread erosional unconformity formed during Middle to Late Triassic
(Ladinian-Carnian) time resulted from tectonic reactivation in the Rowley
sub-basin to the south of this province. Transpressional uplift, faulting
and block rotation occurred in the Late Triassic-Early Jurassic. Lower-Middle
Jurassic sediments are approximately l km thick. Uplift associated with
formation of the Argo Abyssal Plain is represented by a Middle to Upper
Jurassic (Callovian-Oxfordian) unconformity. Middle and Late Jurassic time
was marked by volcanics, which in places overlie the unconformity. A regional
claystone, about 1.5 km thick, was deposited during Early to early Late
Cretaceous (Valaginian-Cenomanian) time and sealed Jurassic rocks of the
Scott Reef area. Regional deposition of carbonates began, in earnest, in
the Late Cretaceous (Turonian) across the entire Browse Basin. These marls,
calcarenties and calcilutites, deposited in the center of the basin to
approximately 3.5 km thick, interfingered with clastics at the northern
and northeastern margins. The present carbonate shelf edge lies approximately
at the Scott Reef trend.
The southern portions of the Vulcan sub-basin are included
in Province 3913. The structure of this sub-basin is traced, from seismic,
under the Browse basin where resolution becomes poor (Pattillo and Nicholls,
1990). The Vulcan graben lies between the Ashmore Platform and the Londonderry
High (Fig. 4b). The Permian seismic reflector
|
indicates approximately 4,000 m
of vertical fault displacement of the Vulcan sub-basin adjacent to the
Londonderry High (Pattillo and Nicholls, 1990). Subsidence occurred in
two adjacent half grabens in the southern Vulcan sub-basin from Middle
Jurassic (Callovian) time to middle Late Jurassic (Kimmeridgian) time.
Slower subsidence with widespread deposition continued to Early Cretaceous
(Valanginian) time. Pattillo and Nicholls (1990) suggest that submarine
fans and fan deltas were deposited along the graben flanks during Late
Jurassic subsidence. Post-subsidence deposition prograded northwest from
the Londonderry High across to and thinning over the Ashmore Platform.
Stratigraphic nomenclature is not well established and
varies among areas and authors (Symonds and others, 1994; Maung and others,
1994). Most authors refer to rock units in time without using stratigraphic
names. Nomenclature from the adjacent Bonaparte Gulf area and the Vulcan
sub-basin is sometimes applied (Fig. 3).
The presence of the Browse sedimentary basin was recognized
from an aeromagnetic survey in 1963. Seismic surveys followed in 1964 and
the first well was drilled in 1967 on the Ashmore Platform (Butcher, 1989).
Between 1971 and 1997, thirty wells were drilled in the Browse Basin. Water
depths of greater than 200 m to 2000 m has impeded exploration (Maung and
others, 1994). Seismic data imaging problems beneath the Cretaceous and
Tertiary carbonate shelf have been encountered in the Browse Basin. There
are reported drilling difficulties due to clay sensitivity, excess pore
pressure, and lost circulation in the Tertiary carbonate section can be
severe (Willis, 1988).
The Scott Reef giant gas discovery made in 1971, probably
the largest gas field in Australia (IKODA Pty Ltd, 1997b), was only the
third well drilled in the Browse Basin. Three more gas discoveries, one
interpreted gas discovery, three oil discoveries, and one interpreted oil
discovery have been made with only one oil discovery considered potentially
commercial, Cornea (IKODA Pty Ltd, 1997a) (Table 1).
|
