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Transcript
REGIONAL GEOLOGY OF THE ROEBUCK BASIN
BASIN OUTLINE
The Roebuck Basin covers approximately 93,000 km2 on the North West
Shelf. It forms the central part of the Westralian Superbasin, which is a
northeast-trending passive margin of late Paleozoic and Mesozoic age. The
inboard part of the Roebuck Basin overlies a major northwest-trending
intracratonic basin (offshore Canning Basin) of Paleozoic age (Colwell and
Stagg, 1994). The temporal boundary between the intra-cratonic and passive
margin successions is marked by a major Pennsylvanian (late Carboniferous)
transpressional event (Meda Transpression) which probably represents the
peak of the Alice Springs Orogeny of Central Australia (Shaw et al, 1992).
Descriptions of the central North West Shelf basin elements are given by
Lipski (1993; 1994), Colwell and Stagg (1994), Smith (1999) and Smith et al
(1999), and the adjacent, onshore Canning Basin is described by Kennard et
al (1994).
The central North West Shelf has been divided into a number of major
structural elements. ‘Westralian’ passive margin sediments of the Bedout and
Rowley sub-basins comprise the Roebuck Basin. Sediments in these
depocentres disconformably overlie the Paleozoic intracratonic succession in
the Oobagooma Sub-basin (the offshore extension of the Fitzroy Trough) and
in the offshore Willara Sub-basin, which are separated by the west-northwesttrending structural high of the Broome Platform (Figure 1).
The Bedout Sub-basin consists of an east-northeast to west-southwesttrending Mesozoic depocentre (Figure 1) filled with approximately 2.5 km of
Paleozoic and 7 km of Mesozoic section (Smith et al, 1999). The Sub-basin is
separated from the Beagle Sub-basin to the west by the North Turtle Hinge
Zone, and partly bounded to the northwest by the Bedout High. The Mesozoic
succession has generally experienced only mild structuring, and thickens to
the west before pinching out against, and partly draping over, the Bedout
High. To the east and south the Mesozoic sediments thin and progressively
onlap the older Paleozoic succession.
The Bedout High locally separates the Bedout Sub-basin to the south from the
Rowley Sub-basin to the north (Figure 1). The high consists of uplifted and
eroded Permo-Carboniferous sedimentary rocks above an interpreted faulted
basement core, and is capped by Permian volcanics (Colwell and Stagg,
1994; Smith et al, 1999). It has a maximum relief of approximately 6 km above
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the surrounding major depocentres, and is associated with a Moho uplift of
about 7–9 km. Lower to Middle Triassic sediments onlap the feature from all
directions and approximately 3 km of Upper Triassic–Holocene sediments are
draped over the top. The upper surface of the Bedout High is a peneplain
approximately 30 km wide (Smith et al, 1999; Müller et al, 2005).
Three possible concepts for the origin of the Bedout High have been
proposed:
1. A pre-Triassic fault-bounded high, capped by extrusive volcanics,
which developed as a large volcanic island over a mantle hotspot
(Lipski, 1994);
2. A faulted core of partly thrust-controlled ‘basement’ rocks overlain by a
sequence of Carboniferous (or older) to Permian sediments (Colwell
and Stagg, 1994);
3. The uplifted core of a circular crater, formed by the impact of a large
extraterrestrial body near the end of the Permian (Gorter, 1996; Becker
et al, 2004).
Smith et al (1999) noted that the Bedout and Oobagooma highs comprise a
northeast–southwest structural lineament that forms a hinge-line between the
Rowley Sub-basin and the Bedout and Oobagooma sub-basins, and
concluded that these evolved during the same tectogenetic processes. This
lineament terminates older northwest–southeast-trending Paleozoic structures
(including the Broome Platform), and appears to have developed through
crustal thinning associated with a deep crustal detachment.
The Rowley Sub-basin is a major Mesozoic depocentre situated on the outer
continental shelf where it covers an area of approximately 66,000 km2
(Figure 1). It contains about 9 km of Permo-Carboniferous or older strata and
up to 6 km of Mesozoic–Holocene sediments (Figure 2); Smith et al, 1999).
Structurally, the sub-basin is separated from the Beagle Sub-basin of the
Carnarvon Basin to the southwest by the North Turtle Hinge Zone and Thouin
Graben (Figure 1 and Figure 3), whereas the separation from the Bedout and
Oobagooma sub-basins is mostly delineated by the Bedout High and
Oobagooma High, respectively. Paleozoic and lower Mesozoic strata in the
Rowley Sub-basin onlap the Bedout and Oobagooma highs and the Broome
Platform to the southeast (Figure 4). These sediments thicken seaward and
are terminated at the massive escarpment defining the present continent–
ocean boundary. A Lower Jurassic unconformity at the top of this sediment
wedge is overlain by prograding successions of Lower Jurassic–Holocene
sequences (Figure 4).
The Broome Platform is a west-northwest-trending largely unfaulted intrabasin basement high in the onshore and offshore Canning Basin (Colwell and
Stagg, 1994). It is capped by a thin package (1-2 km) of Ordovician, Devonian
and Permian rocks (Kennard et al, 1994). The platform dips gently to the
southeast and is flanked on its northern margin by fault-bounded terraces tens
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of kilometres wide that preserve thicker Paleozoic packages (2-4 km) of
mostly Ordovician and Devonian carbonates (Jones et al., 2007).
The Oobagooma Sub-basin is a northwest-southeast trending Paleozoic to
Holocene depocentre of the offshore Canning Basin. It lies between the
Broome Platform to the southwest, the Leveque Shelf of the Browse Basin to
the northeast and is separated from the Rowley Sub-basin by the Oobagooma
High (Figure 1). The 5.5 km thick Paleozoic succession of the Oobagooma
Sub-basin is the result of a depositional history and structural evolution similar
to that of the onshore Fitzroy Trough to the southeast. The 4.5 km thick
Mesozoic to Holocene succession is contiguous with the Rowley Sub-basin
succession to the west (Smith et al, 1999).
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BASIN EVOLUTION AND TECTONIC DEVELOPMENT
Seismic interpretation indicates the offshore Canning and Roebuck basins
first developed in the Ordovician as a result of intraplate extension. This was
followed by multiple phases of rifting with intervening periods of basin sag
during the Paleozoic and Mesozoic, and the formation of a passive margin
from the Late Cretaceous onward. Deposition within the offshore Canning and
Roebuck Basin was punctuated by rift-related uplift and compressional events
(Kennard et al, 1994; Smith et al, 1999). The major tectonic events and their
resultant unconformities divide the basin-fill into a series of sequences. The
post-Devonian stratigraphy of the Roebuck and offshore Canning basins is
summarised in Figure 2) (after Smith, 1999; Smith et al, 1999; and Nicoll et
al, 2009).
Ordovician–middle Carboniferous
Little is known about the nature of Paleozoic sedimentation offshore, as no
wells in the Roebuck Basin have penetrated this succession, and the
Carboniferous to Permian clastic succession in the Oobagooma Sub-basin,
encountered at the bottom of Wamac 1 and Lacepede 1a has been little
studied. A Paleozoic section is interpreted to onlap the Lambert Shelf,
Broome Platform and Bedout High and is presumed to be an extension of the
succession seen in the onshore Canning Basin (Passmore, 1991; Lipski,
1993; Colwell and Stagg, 1994; Smith, 1999). In the onshore Canning Basin,
Ordovician to middle Carboniferous sedimentary rocks primarily consist of
alternating sequences of marine clastic and carbonate rocks (Kennard et al,
1994).
The first extensional event recognised in the region was a northeast–
southwest extension in the Ordovician related to the separation of Chinese
blocks from the North West Shelf. This was followed by north–south
compression and uplift (Prices Creek Movement) in the Early Devonian. Three
northeast–southwest extensional events occurred in the Late Devonian to
Mississippian (early Carboniferous). The north-northwest–south-southeast
oblique-slip reactivation of pre-existing structures, termed the Meda
Transpression, terminated this phase of deposition (Kennard et al, 1994;
Smith et al, 1999).
Late Carboniferous–Permian
Initiation of the Westralian Superbasin in the Pennsylvanian (late
Carboniferous) was characterised by a change from northwest-oriented
structures associated with the Canning Basin to the predominantly northeastoriented structures of the Roebuck Basin. This was a period of transition from
a Paleozoic regime of northeast–southwest intracratonic extension to one of
northwest–southeast extension related to the separation of the Sibumasu
terrane from Gondwana (Metcalfe, 1988; Smith et al, 1999). In parts of the
Canning Basin, syn-rift sedimentation continued along reactivated pre-existing
intracratonic fractures formed during the northeast–southwest extension.
Upper Carboniferous (Pennsylvanian) fluvial deposits are overlain by a thick
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succession of Permian glacial deposits (Grant Group), in turn overlain by
Permian marine and fluvio-deltaic clastic rocks (Poole Sandstone,
Noonkanbah Formation, Liveringa Group). The complex nature of the basin fill
in the offshore Canning and Roebuck basins during this initial evolutionary
stage is poorly understood. Overlying the Paleozoic sequence is a prominent
regional unconformity possibly related to the formation of the Bedout High
(Colwell and Stagg, 1994).
Triassic–Early Jurassic
This period was dominated by thermal sag with transgressive marine and
fluvio-deltaic sedimentation (Locker Shale, Keraudren and Bedout
formations). Separating the lower and upper Keraudren Formation is the
Middle Triassic Cossigny Member, a widespread limestone unit seismically
expressed as a high-amplitude reflector. Triassic to Early Jurassic deposition
was punctuated by a series of northwest–southeast transpressional events
focused along the margins of the sub-basins (Fitzroy Movement). Smith et al
(1999) identified three phases of the Fitzroy Movement in the Roebuck Basin:
1. Fitzroy Movement I (Ladinian) responsible for large transpressional
‘flower structures’ along the North Turtle Hinge Zone;
2. Fitzroy Movement II (Norian) responsible for major en-echelon
anticlines in the Fitzroy Trough, and a subtle unconformity in the
Phoenix 1 and 2 wells; and
3. Fitzroy Movement III (Sinemurian) marking a major change in gross
stratal geometries within the Roebuck Basin from predominantly backstepping to prograding and aggrading.
Early Jurassic–Early Cretaceous
Following Early Jurassic uplift and erosion a broad prograding wedge of
fluvio-deltaic sediments (Depuch Formation) was deposited during thermal
subsidence across the shelf. Continental breakup of northwestern Australia
and Argo Land in the Callovian resulted in a second phase of prominent uplift
and erosion that marked the end of active rifting in areas adjacent to the
Roebuck Basin. Subsequent thermal subsidence drove rapid transgression
and accumulation of condensed marine mudstones (Baleine and Egret
formations) until the Early Cretaceous. An influx of siliciclastic material
(Broome Sandstone) occurred with further uplift of the sediment source in the
Valanginian, when Greater India moved away from the western margin of
Australia (Smith 1999, Smith et al, 1999).
Early Cretaceous–Holocene
Thermal relaxation of the crust soon after the Valanginian break-up led to the
development of a passive-margin succession of marine mudstones and marls.
Full oceanic circulation was established by the end of the Aptian. Reactivation
of some Paleozoic structural features, possibly related to the separation of
Antarctica and Australia and northward drift of the Australian Plate, resulted in
inversion and oblique slip movement, especially in the adjacent Oobagooma
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Sub-basin (Smith et al, 1999). A major progradational carbonate wedge
developed across the entire North West Shelf in the Cenozoic. Collision of the
Australian and Eurasian plates in the mid-Miocene led to transpressional
inversion of north-northwest-trending Paleozoic faults in the northeast
Oobagooma Sub-basin.
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REGIONAL HYDROCARBON POTENTIAL
Hydrocarbon Families and Source Rocks
Because of the perceived absence of a prolific source rock (Smith et al,
1999), the petroleum potential of the offshore Canning and Roebuck basins is
considered to be poor compared to other areas along the North West Shelf.
Interpreted mild structural deformation in the Late Jurassic and Early
Cretaceous suggests that restricted depocentres did not exist, thus claystone
source rocks typical of the adjacent Carnarvon and Browse basins could not
accumulate in the Roebuck Basin.
Ordovician, Upper Devonian, Carboniferous, Cisuralian (lower Permian),
Lower to Middle Triassic and Lower to Middle Jurassic successions have
been proposed as potential source rocks in the Roebuck and offshore
Canning basins (Goldstein, 1989; Taylor, 1992; Kennard et al, 1994; Smith,
1999; Geoscience Australia and Geomark Research, 2005). These source
rock intervals are known to be organic-rich from more inboard areas in the
Browse Basin and onshore Canning Basin, but high quality equivalent source
rock intervals are unproven in the Roebuck and offshore Canning Basin
(Goldstein, 1989; Kennard et al, 1994; Smith, 1999).
Lower Triassic transgressive marine shales within the Locker Shale are
frequently proposed as a potential source rocks within the Bedout, Rowley
and adjacent Beagle sub-basins (Blevin et al, 1993; Smith, 1999). The Locker
Shale remains an unproven source, but gas shows and discoveries within the
stratigraphically equivalent Keraudren Formation in Phoenix 1 are likely to be
intraformationally derived (Geoscience Australia and Geomark Research,
2005). The Lower–Middle Jurassic fluvio-deltaic sediments in the Roebuck
Basin contain organic facies identified from well cuttings, but these are poorly
constrained temporally and spatially. Sedimentary modelling of this
depositional system suggests the possible accumulation of high-quality
transgressive pro-delta marine shales (Smith et al, 1999; Geoscience
Australia and GeoMark Research, 2005).
Regional Petroleum Systems
Several petroleum systems potentially operate in the Roebuck Basin and
offshore Canning (Kennard et al, 1994; Smith 1999; Smith et al, 1999):
1. Ordovician–Larapintine 2
2. Devonian–Larapintine 3
3. Carboniferous–Larapintine 4
4. Pennsylvanian–Permian Gondwana 1
5. Early–Middle Triassic Gondwana 2
6. Early–Middle Jurassic Westralian 1
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Ordovician–Carboniferous Larapintine Petroleum Systems
In much of the Roebuck Basin, the Larapintine 2 system is not regarded as
highly prospective as it is likely to be poorly developed. However, within the
Ordovician succession of the onshore Canning Basin, organic-rich marine
shales are well documented (Taylor, 1992; Kennard et al, 1994). These
sources are characterised by the occurrence of the oil-prone alga
Gloeocapsomorpha prisca, and are particularly well developed on the terraces
along the northern flank of the Broome Platform, but generally have poor
source quality within the Willara Sub-basin (Kennard et al, 1994). However,
sub-salt algal coals on the northern margin of the Willara Sub-basin locally
have excellent source quality (Kennard et al, 1994; McCracken, 1994). These
potential Ordovician source facies are likely to be mature in the offshore
Willara Sub-basin/Samphire Embayment where the system is only shallowly
buried beneath the inboard Bedout Sub-basin. The Larapintine 3 and
Larapintine 4 systems are considered to be absent in the Roebuck Basin,
although within the offshore Canning Basin there is potential for both. The
marine shales at the base of the Devonian Pillara Sequence (Larapintine 3)
and the lower Laurel Formation (Larapintine 4) have fair to good generative
potential and are presumed to be present throughout the Oobagooma Subbasin (Kennard et al, 1994).
Early Permian–Permian Gondwana 1 Petroleum System
Cisuralian (lower Permian) transgressive marine shales of the Poole
Sandstone and Noonkanbah Formation are known to be organic-rich in the
Fitzroy Trough (Kennard et al, 1994). These Permian units form part of a
globally recognised Pennsylvanian (upper Carboniferous)–Permian source
interval (Warris, 1993) and may also be present, and likely mature, in the
Bedout Sub-basin, Rowley Sub-basin, Oobagooma Sub-basin and offshore
Willara Sub-basin. Marine shales of the underlying Grant Group are also
locally organic-rich, but generally have poor generative potential. Hydrocarbon
accumulations and shows within the Grant Group are believed to be sourced
from the underlying Laurel Formation within the Fairfield Group (Goldstein,
1989; Kennard et al, 1994).
Early–Middle Triassic Gondwana 2 Petroleum System
This petroleum system includes the source rock marine shale intervals Tr1
and Tr2 of Smith et al (1999), which are equivalent to the Gondwana 2
System of Bradshaw et al (1994), Kennard et al (1994) and Geoscience
Australia and Geomark Research (2005). Lower to Middle Triassic
transgressive marine shales (Locker Shale) are an unproven source, although
this section in Phoenix 1, Phoenix 2 and Keraudren 1 in the Bedout Sub-basin
contains potential gas-prone source rock facies (JNOC, 1987, Smith et al,
1999). The interpreted tight gas column in the lower Keraudren Formation
(Middle Triassic) at Phoenix 1 may have been charged from this Lower
Triassic source (Smith, 1999), which is early mature to marginally mature at
Phoenix 1 and Keraudren 1. Basin modelling indicates that postulated Lower
Triassic source rocks may be presently expelling liquids in the outer Rowley
Sub-basin and on the flanks of the Bedout High (O’Brien et al, 2003).
However, the lack of reported hydrocarbon shows in wells on the Bedout High
2011 Release of Australian Offshore Petroleum Exploration Areas
Regional Geology of the Roebuck Basin
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(Bedout 1, Lagrange 1) and within the Rowley Sub-basin (East Mermaid 1,
Whitetail 1, Huntsman 1) suggests this postulated source is not effective.
Westralian Petroleum Systems
As noted earlier, the petroleum potential of the Mesozoic section in the
Roebuck Basin is considered to be poor compared to other North West Shelf
basins due to the absence of proven effective source rocks. Nevertheless,
several potential source intervals in the Jurassic–Cretaceous section of the
Roebuck Basin have been identified, based on sequence stratigraphic
interpretations integrated with available geochemical data (Smith et al, 1999).
The fluvio-deltaic Lower–Middle Jurassic sediments of the Roebuck Basin
contain the source rock intervals of the Westralian 1 Petroleum System
(Geoscience Australia and Geomark Research, 2005). These are considered
mostly gas-prone, but do contain thin, oil prone, coaly and alga-rich layers.
The Nebo 1 oil discovery in the adjacent Beagle Sub-basin (Carnarvon Basin)
is presumed to be from an equivalent Early-Middle Jurassic deltaic coaly or
lacustrine mudstone source interval (Geoscience Australia and Geomark
Research, 2005).The lack of significant hydrocarbon shows in the Roebuck
Basin does not support the presence of modelled high-quality, transgressive,
pro-delta marine shales in this system (Smith et al, 1999).
In the Bonaparte and Northern Carnarvon basins, the Westralian 2 petroleum
system is characterised by Upper Jurassic, rift-related, restricted marine
source rocks (Geoscience Australia and GeoMark Research, 2005), but in the
Roebuck Basin, equivalent rift structures and facies were not developed.
Similarly, potential Lower Cretaceous source rocks documented in the Browse
Basin and parts of the Bonaparte Basin (Westralian 3 Petroleum System;
Geoscience Australia and GeoMark Research, 2005), are absent or immature
in the Roebuck Basin.
A detailed fluid inclusion investigation of potential reservoir horizons within the
offshore Canning and Roebuck basins suggested that widespread oil
migration has occurred at multiple Mesozoic and Paleozoic levels (Lisk et al,
2000). Samples from multiple horizons in key wells in the region (Bedout 1,
East Mermaid 1, Keraudren 1, Lagrange 1 and Phoenix 1) were tested and
grains with oil inclusions (GOITM) were discovered in each well, but GOI
values were below 0.6%, except in Phoenix 1 where it reached 3.3%. The
widespread distribution of oil inclusions led Lisk et al (2000) to propose that a
lack of valid traps, rather than a lack of oil charge, may be the principal reason
for the discouraging results experienced.
2011 Release of Australian Offshore Petroleum Exploration Areas
Regional Geology of the Roebuck Basin
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EXPLORATION HISTORY
The first significant phase of exploration drilling in the Roebuck and offshore
Canning Basin occurred between 1970 and 1974, with drilling in the Bedout
Sub-basin (Bedout 1, Keraudren 1 and Minilya 1), Rowley Sub-basin (East
Mermaid 1), and Oobagooma Sub-basin (Lacepede 1 and Wamac 1). The
remaining three wells in the Bedout Sub-basin (Phoenix 1 and 2 and
Lagrange 1) were drilled between 1981 and 1983. The remaining exploration
drilling in the Oobagooma Sub-basin was undertaken between 1980 and 1983
and comprised four shallow water inboard wells. Recent drilling activity in the
Roebuck Basin has been confined to the outboard Rowley Sub-basin, where
the deep-water potential has been tested unsuccessfully by Whitetail 1 (2003)
and Huntsman 1 (2006).
The initial phases of seismic exploration in the Roebuck Basin and offshore
Canning Basin were from the late 1960s to 1982. The approximately 15
surveys run during this time, focused on the Bedout Sub-basin and inboard
Rowley Sub-basin. During the period from 1986 to 1994, Geoscience
Australia and predecessor organisations gathered several regional seismic
lines across the Roebuck Basin, tying in existing wells. Over this same period,
six private seismic surveys were conducted, including tightly spaced seismic
grids in the Bedout and Rowley sub-basins, and more loosely spaced surveys
across the Oobagooma Sub-basin.
More recent activity includes several surveys conducted between 1998 and
2001. These included 2D surveys filling gaps in seismic coverage of the
western Rowley Sub-basin, and 3D grids delineated drilling prospects for
Huntsman 1 and Whitetail 1. Also noteworthy are the 2010 surveys of New
Dawn; crossing the Browse, Roebuck, and Northern Carnarvon Basin, and
the Golden Orb supplement within the offshore Canning Basin; both surveys
were conducted by Petroleum Geo-Services (PGS). Also, the Bedout Subbasin has been the focus of 2D and 3D surveys in 2009 and 2010 to delineate
prospects on recently released acreage.
Seismic exploration and drilling in the adjacent Beagle Sub-basin of the
Northern Carnarvon Basin has been more intensive, with the drilling of 25
exploration wells between 1971 and 2003. Oil was discovered in the Callovian
Calypso Formation sandstones at Nebo 1 (Osborne, 1994), but no discoveries
have been economic.
Two recent aeromagnetic surveys have been conducted in the Roebuck and
offshore Canning basins; a Geoscience Australia survey in 2007 focused over
the Oobagooma Sub-basin and a Carnarvon Petroleum and Finder
Exploration survey in 2010 over the Bedout Sub-basin. The two surveys cover
an area of more than 47,500 km2.
Geoscience Australia conducted a hydrocarbon seepage survey of the
Roebuck and offshore Canning basins in June 2006 (Survey SS06/06; Jones
et al, 2007). No definitive evidence of natural hydrocarbon seepage was
2011 Release of Australian Offshore Petroleum Exploration Areas
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Page 10 of 15
detected. Head space gas analyses of gravity core sub-samples, and
biomarker screening of sediments and carbonate concretions, revealed no
thermogenic hydrocarbons or biomarkers diagnostic of methane oxidation.
O’Brien et al (2003) previously interpreted vertical seismic features distributed
throughout the region, with a strong clustering over the Bedout High, as
hydrocarbon related diagenetic zones (HRDZs) or gas chimneys. Reevaluation suggests these features are more likely brittle faulting of strata
above the rigid basement core during Miocene structural reactivation (Logan
et al, 2010).
2011 Release of Australian Offshore Petroleum Exploration Areas
Regional Geology of the Roebuck Basin
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FIGURES
Figure 1:
Structural elements of the Roebuck Basin and adjacent basins
(after Smith et al, 1999).
Figure 2:
Generalised stratigraphy of the Roebuck Basin and the
adjacent Canning Basin regions, based on the Northern
Carnarvon Basin Biozonation and Stratigraphy Chart (Nicoll et
al, 2010), Smith et al (1999). Geologic Time Scale after
Gradstein et al (2004) and Ogg et al (2008).
Figure 3:
Regional seismic section along depositional strike of the
Rowley and Barcoo sub-basins showing AGSO seismic line
120/07 through Release Areas W11-4 and W11-5. Location of
the line is shown in Figure 1.
Figure 4:
Regional seismic section along depositional dip of the Rowley
Sub-basin and Broome Platform showing AGSO seismic line
120/03 through Release Areas W11-4, W11-5, and W11-6.
Location of the line is shown in Figure 1.
2011 Release of Australian Offshore Petroleum Exploration Areas
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REFERENCES
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2011 Release of Australian Offshore Petroleum Exploration Areas
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Front page image courtesy of Petroleum Geo-Services.
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