GEOLOGICAL INFORMATION
RELEASE AREAS W11-16 AND W11-17, SOUTHERN
CARNARVON BASIN
WESTERN AUSTRALIA
Bids Close – Second Round – 12 April 2012

Poorly explored area immediately to the south of a producing
hydrocarbon province.

Northeastern part of Release Area W11-16 lies on the same structural
trend as Rough Range and Parrot Hill oil discoveries.

Proximal to existing pipeline at Carnarvon.

Vast majority of the Release Areas lie in water depths from less than
100 m to 1000 m.

Structural and stratigraphic plays at multiple stratigraphic levels.

Cretaceous regional seal overlies source rocks and quality reservoirs
of Paleozoic and Mesozoic age.
2011 Release of Australian Offshore Petroleum Exploration Areas
Release Areas W11-16 and W11-17, Southern Carnarvon Basin Western
Australia Release Area Geology
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LOCATION
Release Areas W11-16 and W11-17 lie on the continental shelf and slope of
the Cuvier margin (Figure 1 and Figure 2) in water depths ranging from
100 m to over 2500 m. They lie to the south of the Mesozoic Northern
Carnarvon Basin, the major Australian oil and gas producing basin. The
northern part of Release Area W11-16 lies on the same structural trend as the
Rough Range anticline, on which an early commercial oil discovery was made
in 1953. The Release Areas are located about 100 km to the west and
northwest of the township of Carnarvon (Figure 1), which provides access to
Western Australian infrastructure.
Release Areas W11-16 and W11-17 lie partly within the Paleozoic Bernier
Platform and Gascoyne Sub-basin of the Southern Carnarvon Basin and
partly on the southernmost Mesozoic Exmouth Sub-basin of the Northern
Carnarvon Basin. This is a frontier area with only one well (Pendock 1A,
1969) drilled in the Release Area W11-16 and one (Herdsman 1) about 5 km
to the north of it. Pendock 1A, which is located on the Bernier Platform had
minor shows in the Paleozoic succession, while Herdsman 1, which was
located in the Exmouth Sub-basin, was dry. The area has very limited seismic
coverage (>25 km line spacing), except for the northeastern part of Release
Area W11-16, where the data coverage is much denser.
Release Area W11-16 comprises 264 graticular blocks with a total area of
approximately 20,735 km2 and Release Area W11-17 comprises 230
graticular blocks with a total area of approximately 17,925 km2 (Figure 2).
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Australia Release Area Geology
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RELEASE AREA GEOLOGY
Local Tectonic Setting and Structure
The Release Areas include three different structural elements: the Gascoyne
Sub-basin, Bernier Platform and Exmouth Sub-basin (Figure 3). The northern
Release Area (W11-16) overlaps all three elements, whereas the southern
Release Area (W11-17) lies almost entirely on the Bernier Platform. Structural
architecture of the area is documented only at reconnaissance level. Faults
shown on Figure 3 include major faults mapped from seismic data at the
Valanginian level combined with some onshore and offshore faults from
published structure maps (Hocking et al, 1987, Lockwood and D’Ercole, 2004,
Woodside Energy Ltd., 2003b, c). The obvious lack of detail in the southern
part of the area is due to sparse seismic data.
The Gascoyne Sub-basin and Bernier Platform are also described in the
literature as the Gascoyne Platform (Gascoyne Sub-basin and eastern part of
the Bernier Platform), the Bernier Ridge and the Bernier Terrace (Figure 3).
The Bernier Ridge is an uplifted northerly trending part of the Paleozoic
platform separated from the Gascoyne Platform by a series of en echelon
faults (Lockwood and D’Ercole, 2004). It coincides with a large scale positive
gravity anomaly (Figure 3). Gravity modelling undertaken by Lockwood and
D’Ercole (2004) suggests that the ridge corresponds to a faulted crystalline
basement high. The area lying between the Bernier Ridge and Mesozoic
depocentres of the southern Exmouth and northern Houtman sub-basins
(Figure 3) is known as the Bernier Terrace. From the limited seismic data
available and regional geological knowledge (Iasky et al, 2003, Mory et al,
2003, Lockwood and D’Ercole, 2004) it appears that it has a similar structure
and depositional history to that of the Gascoyne Platform. The Bernier
Terrace is separated from the Mesozoic depocentres by a series of large
basement-involved faults, which strike northwest-southeast along the
boundary with the southern Exmouth Sub-basin and north-northwest-southsoutheast along the boundary with northern Houtman Sub-basin (Figure 3).
The Exmouth Sub-basin is the most southerly of the northeast-trending
Mesozoic Sub-basins which form part of the Exmouth-Barrow-Dampier intracratonic rift system of the Northern Carnarvon Basin. The southern Exmouth
Sub-basin is described as a ramp basin formed by a series of shallowly
dipping detachment faults (Partington et al, 2003). It is separated from the
Exmouth Plateau by the Kangaroo Syncline and bounded on the southeast by
the Rough Range Fault (Figure 3). This fault is a large arcuate normal fault
with a throw, locally, of more than 4,000 m (Partington et al., 2003),
separating the Gascoyne Sub-basin from the Exmouth Sub-basin. The Rough
Range Fault was active throughout the Jurassic, resulting in the deposition of
several kilometres of Jurassic sediments in the northern part of Exmouth Subbasin.
The architecture of the southernmost part of the Exmouth Sub-basin
extending into the Release Areas is poorly known. Geoscience Australia
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Australia Release Area Geology
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seismic data collected during 2008-09 under the Offshore Energy Security
program defines several en-echelon Mesozoic depocentres with complex fault
geometries (Figure 4). Steeply dipping normal faults on the boundary
between the Paleozoic platform and Mesozoic depocentres are shown in
Figure 5. A strike line across one of these depocentres (Figure 6) shows
significant faulting and folding within the syn-rift strata as well as a large
number of sills and dykes. Maximum thickness of sediments within these
depocentres exceeds 3.5 s TWT, which is about 8, 000 m according to
Partington et al. (2003). This includes about 2.5 s TWT of syn-rift and 1 s
TWT of post-rift strata. Based on interpretation of the regional 2D and
Coverack 3D seismic surveys (Figure 7), Partington et al. (2003) suggested
that the northeast-southwest trending faults bounding the Mesozoic
depocentres, may have originated as Early Paleozoic listric growth faults.
These en echelon faults are offset by a series of northwest-southeast trending
relay zones, which were interpreted to have formed in the Permian. These
relay zones are intersected by a series of compressional/transpressional
northwest-southeast oriented faults inverted during the Santonian and
Miocene compressional events (Partington et al, 2003).
Santonian inversion structures are widespread both in the Mesozoic
depocentres and on the Bernier Platform (Figure 4 and Figure 5). This
inversion is coincident with and may have been caused by the major plate reorganisation in the Indian Ocean, when Greater India changed its path from
northwest to north (Gibbons et al., 2010).
Miocene inversion is even more evident on the seismic (Figure 5). It also
affected the whole region and is related to the collision of Australia and
Eurasia in the middle Miocene (Partington et al, 2003). A large number of
major faults have been inverted and form well defined local anticlines in the
post-breakup strata.
In the Exmouth Sub-basin, igneous rocks (predominantly sills and dykes) are
widespread within the Mesozoic and possibly within the pre-rift upper
Paleozoic succession (Figure 6). Most of the igneous activity is associated
with the Early Cretaceous breakup on the western Australian margin (Muller
et al, 2002).The volume of volcanic rocks noticeably increases to the south,
towards the Wallaby Plateau. This part of the margin is described as a
volcanic rifted margin characterised by excessive volcanism overprinting all
pre-exiting structures (Direen et al, 2008; Symonds et al, 1998) The Wallaby
Saddle lies about 80 km to the west of W11-17 and is dominated by seaward
dipping reflector sequences (SDRS) interpreted as interbedded lava flows and
volcaniclastics, and large intrusive bodies. In the Release Areas, the
increased volume of volcanic rock leads to significant difficulties in seismic
imaging and consequent difficulties in seismic interpretation.
Structural Evolution and Depositional History of the Sub-basin
The Paleozoic basinal succession in the region is underlain by the
northernmost extension of the Pinjarra Orogen (Lockwood and D’Ercole,
2004). The Gascoyne Sub-basin and Bernier Platform contain an Ordovician
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Australia Release Area Geology
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to Carboniferous sedimentary succession deposited in the northward-opening
intracratonic basin (Mory et al, 2003; Lockwood and D’Ercole, 2004). The
Permian to Lower Cretaceous section in this area is largely absent due to
erosion. Permian (Lyons Group) sediments are present in the onshore
northern part of the Gascoyne Sub-basin (Iasky et al, 2003) and have been
interpreted to extend offshore across the northern part of the Bernier Platform,
as well as beneath the Mesozoic Exmouth Sub-basin. For a detailed summary
of the stratigraphy of the onshore Southern Carnarvon Basin see Regional
Geology of the Southern Carnarvon Basin. The stratigraphy of the offshore
Southern Carnarvon and the southernmost part of the Exmouth Sub-basin is
poorly documented due to lack of wells. Below is an attempt to reconcile
current understanding of the main tectonic events in the region with the
depositional phases identified from seismic interpretation.
Major supersequences in the region have been mapped using existing
industry 2D coverage and new seismic data collected by Geoscience
Australia in 2008-2009 (survey 310). These supersequences were then
correlated to regional tectonic events to define major depositional basin
phases (Figure 8).
Two Paleozoic supersequences were indentified in the pre-rift succession:
Ordovician-Silurian and Devonian-Pennsylvanian:
Ordovician-Silurian supersequence (pre-rift 1)
Initial formation of the intracratonic basin has been linked to the breakup of
Rodinia, which caused uplift of the Pilbara and Northampton Blocks and
subsidence of the basement to the west (Lockwood and D’Ercole, 2004). The
basin may have stated to form as early as the Late Cambrian with deposition
of fluvial and marine sediments. Deposition within this intracratonic basin
continued until the Pennsylvanian.
The base of the Paleozoic succession is not well imaged on seismic. The
Ordovician-Silurian succession is generally characterised by low amplitude,
low frequency, continuous to discontinuous reflections (Figure 5). This
supersequence has been mapped only within Paleozoic platform areas and it
is correlated to coarse grained red beds of the Tumblagooda Sandstone and
restricted marine mudstone of the Dirk Hartog Group (Figure 8).
Devonian - Pennsylvanian supersequence (pre-rift 2)
As a result of Early Devonian tectonism, depocentres shifted to the north with
accumulation of mostly shallow marine sediments (Kopke and Sweeney Mia
Formations). The formation of an extensive carbonate shelf in the Late
Devonian (Frasnian) led to growth of stromatoporoid reefs over parts of the
basin. Shallow marine deposition continued into the Mississippian.
Supersequence Pre-rift 2 has been mapped on the Paleozoic platform and on
the flanks of the Mesozoic depocentres (Figure 5). Lack of seismic resolution
in deeper parts of the Mesozoic depocentres (Figure 6) makes it uncertain
whether this sequence is present beneath the syn-rift succession. The
sequence is characterised by low amplitude, medium to low frequency
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Australia Release Area Geology
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continuous reflections which are correlated to shallow marine to restricted
marine sandstones and carbonate units of the Southern Carnarvon Basin
(Figure 8).
In the mid Carboniferous the Devonian-Mississippian sequences were folded
and faulted along north-trending axes (Mobil, 1994), possibly as a result of the
collision of Gondwana with Laurasia.
Pennsylvanian/Cisuralian – Base Mesozoic (syn-rift 1/ erosion 1)
The southern margin of Tethys evolved by successive shedding of
microcontinents, which subsequently drifted and accreted to Southeast Asia.
Uplift of central Australia in the middle Carboniferous was followed by
initiation of the Westralian Superbasin during Pennsylvanian-Cisuralian
extension. This first major rifting event in the region resulted in the formation
of north-northeast-south-southwest trending graben on the North West Shelf
(Norvick, 2002) and north-south trending graben of the Southern Carnarvon
and Perth basins (Mory et al, 2003; Partington et al, 2003). The same event
led to the uplift of the Gascoyne Sub-basin and Bernier Platform (Iasky et al,
2003).
The Pennsylvanian-Cisuralian succession, intersected in a number of onshore
wells (Airey Hill 1, Chargoo 1, Gnaraloo 1 and Warroora 1) is restricted to the
northernmost part of the Gascoyne Sub-basin. It consists predominantly of
shale and sandstone deposited in a glacio-marine environment (Lyons Group;
Hocking et al, 1987; Figure 8). The rest of the Gascoyne Sub-basin and
Bernier Platform remained relative structural highs with no deposition until the
earliest Cretaceous. It is estimated that up to 6 km of section may have been
removed by erosion between the Permian and Early Cretaceous (Mory et al.,
2003). Only a thin cover of Cretaceous and Cenozoic strata overlies the
Paleozoic succession in these areas.
The Top Permian has been mapped in the southern Exmouth Sub-basin on
Coverack 3D survey by Partington et al. (2003). Current assessment has
used only 2D data. Multiple faulting and folding of the basin fill makes it
impossible to reliably identify the early syn-rift package. The presence of the
syn-rift 1 unit is assumed beneath the Mesozoic succession (Figure 4) and is
tentatively interpreted as Cisuralian. This unit is characterised by high
amplitude, low frequency reflections of high to medium continuity.
Base Mesozoic – Lower Jurassic (erosion 2/subsidence 2)
From at least the Early Triassic, deposition continued within the ExmouthBarrow-Dampier intra-cratonic rift system (Figure 8). The Gascoyne Platform,
which remained an elevated horst complex, diverted sediments to the north
and the south. The Exmouth Sub-basin became the major depocentre for
Triassic and Jurassic sediments. The Locker Shale was deposited in shallow
shelf environment during a widespread Early Triassic marine transgression
which is recognised along the whole western Australian margin from the
Bonaparte Basin to the Perth Basin (Figure 8). It is generally a uniform shale
sequence, with local sandy horizons and the carbonate Cunaloo Member near
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Australia Release Area Geology
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the base (Hocking et al, 1987). The Locker Shale is overlain by a thick
succession of mainly fluvio-deltaic to marginal marine sediments of the
Mungaroo Formation. The reservoir units of the Mungaroo Formation host
giant gas accumulations on the Rankin Platform in the Northern Carnarvon
Basin. The sand percentage in this formation is variable, which significantly
affects the economic viability of hydrocarbon accumulations (Hocking et al,
1987). Shaly intervals of the Mungaroo Formation are considered as potential
source rocks.
The interpreted Base Mesozoic to Upper Jurassic succession (Figure 4 and
Figure 6) is characterised by medium to high amplitude, medium frequency,
variable continuity reflections. Seismic data shows a large number of igneous
features, mostly sills and dykes, especially in the lower part of this
succession. In the central parts of the depocentres the thickness of this unit
exceeds 2 s TWT (two way time).
Lower Jurassic - Lower Cretaceous (syn-rift 2)
Two units mapped within the Jurassic succession correspond to two different
extensional episodes (Figure 8): the Early Jurassic and the Middle to Late
Jurassic. The first extensional phase is associated with breakup of Argoland
(Norvick, 2002). The second extensional phase starting in the Middle Jurassic
preceded the breakup between Australia and Greater India. The Jurassic
supersequence within the Exmouth Sub-basin includes the Brigadier
Formation and Dingo Claystone, as well as the Learmonth Formation, the
deposition of which was restricted to the basin margin (Partington et al, 2003).
The Brigadier Formation is a paralic to shallow-marine deposit characterised
by siltstone with claystone, shale and fine sandstone. The overlying Dingo
Claystone is a thick unit with a very consistent lithology dominated by grey
argillaceous siltstone (Hocking et al, 1987). The Dingo Claystone is
considered to be the source rock for most of the hydrocarbon discoveries in
the Exmouth Sub-Basin (Hocking et al, 1987). The distribution of the
Learmonth Formation was controlled by major faults which were active at the
time of the deposition. It is an alluvial to shallow-marine fan complex formed
at the foot of elevated fault blocks (Hocking et al, 1987).
The Berriassian uplift of the Gascoyne Sub-basin and Bernier Platform prior to
the breakup provided the sediment source for the Barrow Delta which
prograded northward over the Exmouth Sub-basin, however there is no
evidence that the Barrow Group is present in the southernmost part of the
Exmouth Sub-basin (Partington et al, 2003, Figure 8).
The Lower Jurassic to Lower Cretaceous succession (Figure 4 and Figure 6)
is characterised by medium to high amplitude, high frequency and high
continuity seismic facies. The supersequence is highly faulted and includes a
number of unconformities of limited spatial extent.
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Australia Release Area Geology
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Valanginian to Barremian (Post-rift 1)
Valanginian breakup was accompanied by a major structural inversion which
resulted in the uplift of the Ningaloo Arch and erosion of the Barrow Group
and older Jurassic sediments across much of the Exmouth Sub-basin (Tindale
et al, 1998). The delta sediments were reworked and re-deposited as the
Birdrong Sandstone, which overlies the breakup unconformity (Figure 8). The
Birdrong Sandstone is primarily a coastal to near-shore deposit which formed
at the onset of the post-breakup subsidence. It is the reservoir for the Rough
Range oil accumulation and it has been the primary target for exploration in
the Southern Carnarvon Basin.
The regional marine transgression during the Hauterivian resulted in the
deposition of the Muderong Shale (Figure 8), which is the main regional seal
in the Exmouth Sub-basin. Although named the Muderong Shale, the unit is
dominantly an argillaceous siltstone with thin lenses of siltstone and fine
sandstone (Hocking et al, 1987).
This supersequence is fairly thin (less than 100 ms). It fills lows in the
Valanginian surface and is often absent over the highs (Figure 4 and
Figure 5 and Figure 6). It is a highly reflective seismic unit with high
amplitude, medium frequency and high continuity reflections.
Aptian to Turonian (Post-rift 2)
In the Aptian, shallow marine conditions spread to current onshore areas of
the Gascoyne Sub-basin. At the same time in the offshore areas an open
marine environment resulted in the onset of widespread carbonate
sedimentation. The onshore Aptian-Albian Windalia Radiolarite is a uniform
white radiolarian siltstone. It is overlain by argillaceous Gearle Siltstone
deposited in a low energy, restricted marine environment (Hocking et al,
1987), possibly inner shelf for the Gascoyne Sub-basin and outer shelf for the
Exmouth Sub-basin.
There is a prominent unconformity within this supersequence. It is tentatively
correlated with the middle Albian and is more pronounced in the deeper water
parts of the area. Overall the supersequence is characterised by medium
amplitude, medium to high frequency, high continuity reflections
Turonian to Base Cenozoic (Post-rift 3)
In the early Santonian regional uplift and fault reactivation largely overprinted
previously formed structures in the Northern Carnarvon Basin (Tindale et al,
1998). In the Release Areas, Santonian tectonism caused inversion on a
number of major faults and formation of anticlinal structures in the pre-existing
post-rift succession (Figure 4 and Figure 5). Following a brief depositional
hiatus, carbonate deposition recommenced with the onset of full ocean
circulation in the Turonian and deposition of the Toolonga Calcilutite. The
Toolonga Calcilutite disconformably overlies the Winning Group (Figure 8)
and consists of fossiliferous calcilutite and calcisiltite deposited in a lowenergy, middle-shelf marine environment. In the northern Gascoyne Platform
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Australia Release Area Geology
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the Toolonga Calcilutite grades up into the overlying Korojon Calcarenite,
which consists of silty calcarenite and calcisiltite deposited in a moderateenergy marine environment.
In the Release Areas, the post-rift 3 supersequence is highly variable in
thickness. It is very thin in shallow water areas and increases up to 500 ms in
deeper areas of the slope, where the basal part of the unit develops bright
chaotic reflections possibly suggesting turbidite origin. The rest of the unit is
characterised by medium to low amplitude, high frequency and generally
continuous reflections.
Base Cenozoic to Miocene (Post-rift 4)
The Cenozoic succession in the Southern Carnarvon Basin is predominantly
flat lying and consists of shallow-marine carbonates (Cardabia and Giralia
Calcarenites and Trealla Limestone). Offshore, thick prograding carbonates
were deposited throughout most of the Cenozoic (Hocking et al., 1987).
Progradational succession is clearly imaged on the seismic (Figure 4) in the
inboard part of the continental slope. It is bounded by Base Cen and Olig
seismic horizons (Figure 8). In deeper parts of the area this sequence is
highly disrupted by buried channels and .slump deposits. It is characterised by
medium to high amplitude, high frequency, high continuity reflections, which
become less continuous in its upper part.
Miocene to Holocene (Post-rift 5)
The Miocene collision between Australia and Eurasia reactivated major preexisting faults and led to the formation of widespread compressional
structures (Iasky et al, 2003). Miocene inversion anticlines are widespread
both over Paleozoic and Mesozoic parts of the basin (Figure 4 and Figure 5).
This sequence is generally thin (less than 150 ms) and is characterised by low
to high amplitude, high frequency, high continuity reflections.
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Release Areas W11-16 and W11-17, Southern Carnarvon Basin Western
Australia Release Area Geology
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EXPLORATION HISTORY
Release Areas W11-16 and W11-17 lie within frontier under-explored part of
the Carnarvon Basin. Over most of the area, seismic coverage has more than
25 km line spacing (Figure 7) with only one well (Pendock 1A) drilled in the
shallow water eastern part of Release Area W11-16. Northern and eastern
parts of the Release Areas have the best seismic coverage with 1-5 km line
spacing and a 3D grid in the northernmost part of W11-16.
In the last 20 years, permits have existed only over parts of the Release Area
W11-16. Between 1991 and 1995 Mobil Oil Australia Ltd held a permit on the
southern inboard part of W11-16, and between 2000 and 2003 Shell
Development (Australia) Pty Ltd had two permits covering the northern part of
W11-16 and extending further north into the Exmouth Sub-basin. The work
program within these permits was focused on seismic acquisition with only
one well (Herdsman 1) drilled in the southern Exmouth Sub-basin. Current
permits WA-284P and WA-385P, held by Shell, lie immediately to the north of
the Release Area W11-16 (Figure 1). Lack of exploration activity in the area
could be partly attributed to focus on hugely successful exploration in the
adjacent Northern Carnarvon Basin.
To view image of seismic coverage follow this link:
http://www.ga.gov.au/energy/projects/acreage-release-andpromotion/2011.html#data-packages
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Australia Release Area Geology
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Well Control
Only one well was drilled within the Release Areas, Pendock 1A (1969). A
more recent well (Herdsman 1, 2003) was drilled just to the north of W11-16.
Both wells were dry, however minor shows are noted in the Silurian and
Devonian section in Pendock 1A. The lack of success in both wells is
attributed to poor understanding of source rock distribution and their maturity.
Herdsman 1 (2003)
Herdsman 1 was drilled in 2003 by Woodside Energy Ltd within the Exmouth
Sub-basin in the Northern Carnarvon Basin, in a water depth of 556.7 m
(Figure 1), approximately 227 km north of Carnarvon and 46 km north of
Pendock 1. Herdsman 1 is the southernmost well in the Exmouth Sub-basin
(Woodside Energy Ltd, 2003a). The well tested a structural closure over a
tilted fault block with the primary targets in the Lower Cretaceous Birdrong
Sandstone and Middle Jurassic Learmonth Formation.
The well reached a total depth of 2010 mRT intersecting Cenozoic,
Cretaceous and Jurassic sections. The deepest units intersected were the
Jurassic Athol (1397.7-1462.7 mRT) and Learmonth (1462.7 mRT to TD)
formations (Woodside Energy Ltd, 2003a). The Jurassic section was thinner
and sandier than anticipated. The Learmonth Formation penetrated in this
well comprises a lower non-marine medium-grained sandstone package
(>80 m) and a locally carbonaceous claystone interval (115 m) that grades
upwards into a non-marine medium-grained sandstone package (352 m). The
overlying Athol Formation comprises 65 m of silty claystone grading into
argillaceous siltstone deposited in a non-marine to marginal marine
environment. The Cretaceous breakup unconformity separates the Athol
Formation from the overlying Birdrong Sandstone (1367.7-1397.7 mRT), a
non-marine to near-shore sandstone containing minor silt. The Birdrong
Sandstone is in turn overlain by the 57.5 m of Muderong Shale, 51.5 m of
Windalia Radiolarite, 285 m of Gearle Siltstone followed by undifferentiated
Upper Cretaceous and Cenozoic carbonates.
No indications of hydrocarbons were seen in the Birdrong Sandstone and it
was evaluated as water-bearing. Reservoir quality was good with an average
porosity of 28% and a net-to-gross ratio of 87%. The secondary objective, the
Learmonth Formation sandstones, were penetrated about 390 metres
shallower than expected, due to more extensive erosion at the breakup
unconformity. The Learmonth Formation is also evaluated as water bearing
with good reservoir properties (average porosity of 26% and a net-to-gross
ratio of 86%).
The Herdsman trap was interpreted to be valid. The lack of hydrocarbon
charge was thought to be due to the absence of Jurassic source rocks at this
location and extensive erosion during the Valanginian. It has been suggested
that if Jurassic source rocks are present, they are small in volume and are
insufficiently buried to have matured (Woodside Energy Ltd, 2003a). Low
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Australia Release Area Geology
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reservoir temperatures (40 to 60°C) were also confirmed. However, spore
colouration indicated the section is marginally mature for oil at the base of the
Cretaceous interval and is mature for both oil and gas generation in the
Jurassic interval. This is confirmed by the vitrinite reflectance analysis which
found the lower part of the section to be quite mature (Woodside Energy Ltd,
2003a).
Geochemical analyses were performed by Geotech on four sidewall core
samples from 1,428 to 1,920 mRT and cuttings from 1,950 to 1,955 mRT
(Woodside Energy Ltd, 2003a). The sidewall cores yielded 0.64 to 3.10%
Total Organic Carbon (TOC), the maximum occurring at 1,895 mRT. The
Hydrocarbon Indices (HI) are very poor ranging from 47 to 157, with the
lowest at 1,895 mRT, suggesting gas source potential only.
Fluid Inclusion Stratigraphy (FIS) was performed on cuttings from 1090 to
2010 mRT by Fluid Inclusion Technologies Inc (Woodside Energy Ltd, 2003a,
Appendix 3). Results of this work suggest that very poor Lower Jurassic or
Upper Triassic source rocks were mature some time before the Valanginian
breakup. The migrated compounds were limited in volume and consisted of
wet gas and possibly other liquids. Very high net-to-gross ratio in the
Learmonth Formation strata provides effective pathways for migration but is
unfavourable for trap formation. Any traps that might have existed were
disrupted by the Valanginian tectonism. The remaining traces of hydrocarbons
have been biologically altered due to temperatures less than 80°C (Woodside
Energy Ltd, 2003a).
Pendock 1A (1969)
Pendock 1 and Pendock 1A were drilled in 1969 by the Canadian Superior Oil
(Aust.) Pty Ltd. Both wells were located approximately 181 km north of the
town of Carnarvon within the Gascoyne Sub-Basin of the Southern Carnarvon
Basin. Pendock 1 was drilled in a water depth of 132.6 m (10.4 mKB) and
reached a TD of 242.6 mKB. Indications of excessive deviation and drill string
hang-ups led to abandoning the well and spudding of Pendock 1A
approximate 12 m away from the original location (Canadian Superior Oil
(Aust.) Pty Ltd, 1970). Pendock 1A was drilled in 131.1 m water depth and
reached a TD of 2,501 mKB. The well tested a large anticlinal structure
beneath the breakup unconformity interpreted from seismic data. The primary
target was the Lower Cretaceous Birdrong Sandstone and the secondary
targets were possible Triassic to Jurassic sandstone intervals.
The lower 651 m of Pendock 1A penetrated the Upper Ordovician to Silurian
Dirk Hartog Formation. Above the Dirk Hartog Group the well intersected
184.7 m of the Lower Devonian Nannyarra Formation., 558.1 m of Devonian
Gneudna Formation, and 77.1 m of the Mississippian Moogooree Limestone.
Within the Devonian Gneudna Formation the well encountered a dense
dolomite unit containing abundant stromatoporoids and corals, which was
interpreted to represent a reef or a carbonate bank deposit.
The breakup unconformity at the top of the Moogooree Limestone suggests
erosion or non-deposition from Cisuralian to lowest Cretaceous. The presence
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of Mesozoic strata has been incorrectly inferred from the aeromagnetic data,
which indicated over 5,000 m of sediments at this location. Above the breakup
unconformity, the well intersected 202 m of the Aptian to Cenomanian
Winning Group, including only 7.3 m of the Birdrong Sandstone. The upper
part of the well intersected 685 m of Upper Cretaceous to Holocene
carbonates.
Minor shows of oil were found in the Devonian and Silurian section (GENOA
Oil N.L., 1970: Canadian Superior Oil (Aust.) Pty Ltd, 1970). Oil stains were
detected in the Nannyarra (1718 mKB) Formation and the overlying Point
Maud Member of the Gneudna Formation (1,409 and 1,413 mKB). Oil staining
and fluorescence were also detected within the Coburn Formation of the Dirk
Hartog Group (e.g. 2,201 mKB). Methane gas shows occurred throughout the
Paleozoic with the maximum readings in the interval 2,121-2,124 mKB.
Hydrocarbon shows in the Paleozoic imply this interval is prospective
elsewhere in the basin. Rock-Eval of a limited number of samples from the
Silurian/Devonian source interval indicated that these rocks are thermally
immature. More recently work has shown that the Gneudna Formation is
currently mature at Pendock 1A (Ghori et al, 2005).
Although Pendock 1A failed to intersect anticipated Triassic and Jurassic
strata, minor shows encountered in the Devonian and Silurian succession
provide important information about a possible Paleozoic petroleum system in
this region.
For further details regarding wells and available data follow this link:
http://www.ret.gov.au/Documents/par/data/documents/Data%20list/data%20li
st_sthcarnvarvon_AR11.xls
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Data Coverage
Seismic Data
Between 1966 and 1973, nine surveys acquired seismic data over this region.
Most of these surveys acquired only a few widely spaced lines of limited
quality in the Release Areas. The exceptions are the Carnarvon Basin West
(1966) and Gnaraloo West (1967) surveys, both of which provide a tight grid
with a line spacing of about 5 km. This data imaged a large pre-breakup
anticline tested by Pendock 1A (1969).
From 1979 to 1982, four surveys acquired seismic data over the Release
Areas. These were small local grids on the inboard edge of Release Area
W11-16 (From 1992 to 1994, four surveys collected seismic data in the
region: AGSO surveys 135 (1994) and 136 (1994), the Carnarvon Terrace
Detail (1992) and the Cuvier (1992) surveys. These surveys acquired seismic
in the southeast corner and northeast corner of W11-16.
The more recent seismic data include the tight grid of the 1998 Western Geco
Carnarvon Terrace 2D Speculative Survey and the 2002 2D and 3D Coverack
surveys (Woodside Energy Ltd, 2003b, c). Shell Development (Australia) also
collected aeromagnetic data over the same region which partially covers
Release Area W11-16. Analysis of these data resulted in identification of a
number of prospects, one of which was tested by Herdsman 1.
In 2008-09 under the Offshore Energy Security Program, Geoscience
Australia acquired regional seismic data in frontier areas of the western
Australian margin (survey GA-310), including parts of the Southern Carnarvon
Basin (Figure 7). Seismic lines targeted potential depocentres delineated by
gravity lows and cover mostly north-western deep-water parts of the Release
Areas. These datasets, together with open file industry data were used in the
current hydrocarbon prospectivity assessment.
In November 2009, Searcher Sesimic (2010) shot the non-exclusive 2D
Acheron seismic survey with line spacing of 25 km across the both Release
Areas (Figure 7). These data provide a regional grid over the Paleozoic
Bernier Platform and have not been interpreted.
To view image of seismic coverage follow this link:
http://www.ga.gov.au/energy/projects/acreage-release-andpromotion/2011.html#data-packages
Swath bathymetry, gravity, magnetic data and dredge samples
In 2008-09, Geoscience Australia conducted a marine reconnaissance survey
over vast areas of the continental slope on the Western Australian margin.
This survey acquired high resolution bathymetry, gravity and magnetic data,
as well as a number of dredge samples in the region (Figure 7). Dredge
samples were collected from the walls of the deeply incised canyons, the aim
being to sample the pre-breakup succession. All sampling sites are located in
deep water to the west of the Release Areas. Unfortunately, many samples
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lacked microfossils and their age could not be determined. The oldest dated
samples are of Berriasian age. Detailed sample descriptions are given by
Daniell et al (2010).
Satellite data
Satellite hydrocarbon slick detection in the region was undertaken by Shell in
2000. Datasets collected included ALF, SAR, Landsat 1, Spot, IRS, IKONOS
and Earth Resources satellite scenes. Analysis of these data resulted in
identifying two medium confidence slicks; one within the Release Area W1116 and one further to the east, close to the coastline (see Figure 3 in
Partington et al, 2003).
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PETROLEUM SYSTEMS AND HYDROCARBON POTENTIAL
Petroleum systems
The region has potentially two active petroleum systems: Paleozoic and
Mesozoic. So far no commercial discoveries from Paleozoic petroleum
systems have been found in the Southern Carnarvon Basin, however oil and
gas shows were detected in a number of wells. Within the Paleozoic
Gascoyne Sub-basin and Bernier Platform, source rocks are present in the
Silurian and Upper Devonian. Effective reservoirs and seals are present both
in the Paleozoic and the post-breakup Cretaceous succession. The Mesozoic
petroleum system of the southern Exmouth Sub-basin of the Northern
Carnarvon Basin includes potential source rocks in the Triassic Mungaroo
Formation and Jurassic Dingo Formation with multiple reservoir and seal units
in the Triassic, Jurassic and Cretaceous. The potential of the Mesozoic
petroleum system in the southernmost part of the Exmouth Sub-basin has not
been proven. A test of a valid structure at Herdsman 1 suggests that the
thickness of the Mesozoic succession is a critical factor for hydrocarbon
generation in this part of the basin.
Source Rocks
The best Paleozoic source rocks are within the Silurian Coburn Formation,
and Devonian Gneudna Formation (Ghori et al, 2005; Figure 9). Silurian–
Devonian source rocks have been shown to have good potential for both oil
and gas generation. Silurian source beds have organic richness of over
7% TOC, potential yield (S1 + S2) of up to 38 mg/g, and hydrogen index of up
to 505 mgHC/gTOC. The main source rocks in the Gascoyne Sub-basin are
organic rich and oil-prone laminated mudstones within carbonate facies of the
Devonian Gneudna Formation. Devonian source beds have organic richness
of up to 13.5% TOC, potential yield of up to 40 mg/g, and hydrogen index of
up to 267 mgHC/gTOC. However, all of these source rocks are thin and
probably of limited extent. Maturity and petroleum generation modelling of the
Paleozoic succession (Ghori et al., 2005) showed that the maturity of these
units progressively increases from immature in the south-southeast to mature
in the north-northwest, commensurate with increasing depth of burial.
The presence of Triassic and Jurassic source rocks is inferred from the
northern part of the Exmouth Sub-basin, where they charge a number of
commercial accumulations. These may include organic-rich units of the Lower
Triassic Locker Shale and deltaic Upper Triassic Mungaroo Formation. The
Upper Jurassic Dingo Claystone is the principal source for oil in the Exmouth
Sub-basin (Tindale et al, 1998), however it is not clear how far south it
extends and whether it is present within the Release Areas, as this unit is
missing at Herdsman 1.
Reservoirs and seals
The Paleozoic succession contains a number of potential reservoir and seal
units (Figure 9). The Ordovician red beds (Tumblagooda Sandstone) has
variable porosity and permeability, although even at depths greater than
1,000 m porosity is typically good, with an average of 13%. The Gneudna
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Formation provides the prime reservoir potential, with the reefal Point Maud
Member demonstrating favourable reservoir properties. Seal is inferred to be
provided by thick marine shales and marls of the upper Gneudna Formation
overlying the Point Maud Member.
The Triassic Mungaroo Formation and Lower Cretaceous Birdrong Sandstone
are the main reservoir units in the Mesozoic succession (Figure 9). The
Birdrong Sandstone overlies the breakup unconformity, has excellent
reservoir characteristics and hosts a number of oil and gas accumulations
both onshore and offshore in the Northern Carnarvon Basin. The Birdrong
sandstone has been the prime exploration target in the area. The Dingo
Claystone, Muderong Shale, Windalia Radiolarite and Gearle Siltstone are
possible seals in the Mesozoic succession (Figure 9). The Muderong Shale
and Gearle Siltstone are proven effective seals in the northern Exmouth Subbasin (Iasky et al., 2003).
Generation, expulsion and migration
Burial, thermal and erosional histories are complex and poorly constrained in
the Gascoyne Sub-basin (Iasky et al., 2003). Geohistory modelling by Ghori et
al. (2005) indicated that petroleum generation and migration from Silurian and
Devonian source rocks peaked during the Permian, whereas generation from
Permian source rocks peaked during the Triassic. Therefore, in the Gascoyne
Sub-basin, mid-Carboniferous to Cisuralian structures are the most
prospective, although there is the risk of breaching during later tectonic
events.
On the Bernier Platform there is some controversy as to how much of the
Triassic and Jurassic section was eroded during the uplift preceding the
Valanginian breakup (Lockwood and D’Ercole, 2004). Maturity of the source
rocks would depend on the thickness of the eroded section. In the model with
major erosion during the Permian, the maximum rate of hydrocarbon
generation from Silurian and Devonian source rocks occurs at the end of the
Permian (Iasky et al, 2003). For the model with major erosion during the Early
Cretaceous the peak of hydrocarbon generation for these units extends from
the Permian to Middle Jurassic. If no deposition occurred between the
Permian and Cretaceous, peak hydrocarbon expulsion may have occurred
more recently which would result in filling of Miocene structures only.
In the Exmouth Sub-basin, the thickness of the Mesozoic synrift succession
and the presence of good source rocks in the area are the main risk.
Maximum thickness of the Jurassic rocks is interpreted to be up to 5-6 km in
the northeastern part of Release Area W11-16. These rocks are buried deep
enough to have generated oil before the breakup (Partington et al, 2003).
However, across the basin, thickness of the interpreted Jurassic succession is
highly variable, which suggests that some of the Jurassic source rocks would
have generated much later. The inferred Triassic succession is buried up to
8 km below the breakup unconformity and is also likely to have generated
hydrocarbons (Partington et al, 2003), whereas the underlying Paleozoic
section is likely to be mostly overmature.
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Plays
The Paleozoic Gascoyne Sub-basin hosts a number of structural and
stratigraphic plays. Structural plays include fault block and Miocene
reactivation anticline plays (Figure 10). Paleozoic traps include Ordovician,
Silurian and Devonian reservoir rocks in rotated fault blocks created by midto Lopingian and Late Jurassic rifting. Such traps may be effective if sealed by
intraformational shales. Low dips and small fault displacements imply that
such traps may be quite large (Mory et al, 2003). In the northern part of the
sub-basin, east of Pendock 1A, there are a few untested faulted anticlines, in
which reefal carbonate facies of the Point Maud Member of the Gneudna
Formation may be sealed intraformationally (Iasky et al., 2003). In the
southwestern offshore Bernier Platform, there are fault block plays in which
the Kockatea Shale seals the Tumblagooda Sandstone. These traps could
have been charged by hydrocarbons that migrated updip from more deeply
buried sections of the Kockatea Shale in the Abrolhos Sub-basin (Mory et al,
2003).
The main objective for petroleum exploration in the Gascoyne Sub-basin has
been the Birdrong Sandstone, sealed by the Muderong Shale in Miocene
anticlinal structures formed as a result of normal fault inversion. A few
untested Miocene structures have been identified by Iasky et al (2003) in the
northern part of the area surrounding Pendock 1A. A likely problem with the
Birdrong Sandstone is that a strong artesian flow could flush hydrocarbons
from all but the most robust traps (Mory et al, 2003).
Possible stratigraphic plays in the Gascoyne Sub-basin include incised
channels filled by Birdrong Sandstone and sealed by the Muderong Shale. In
the northern part of the sub-basin there may be additional traps in which the
Birdrong Sandstone is missing, thereby allowing the Muderong Shale to seal
dipping Paleozoic reservoir rocks (Iasky et al., 2003). Mobil (1994) suggested
some of the stratigraphic traps may be charged by long-distance migration
from the Mesozoic source rocks in the adjacent Exmouth Sub-basin.
The prime risks for Paleozoic plays are the volume of available source rock,
trap integrity because of the long period of preservation required, and relative
timing of generation versus trap formation (Ghori et al, 2005). Some of the
trapped hydrocarbons from Silurian-Lower Devonian sources are likely to
have been lost during Permian and Valanginian rifting and Miocene inversion
(Partington et al., 2003)
In the Exmouth Sub-basin several types of structural and stratigraphic plays
are present. Within the synrift succession, the main structural plays are fault
blocks and faulted anticlines (Figure 10). They could include Upper Triassic
fluvio-deltaic sandstones sealed by Lower to Middle Jurassic claystone, or
Upper Jurassic transgressive sandstone (Oxfordian to Tithonian) sealed by
Muderong Shale (Partington et al., 2003). In the post-rift succession,
structural traps include Santonian and Miocene anticlines (Figure 10) formed
as a result of inversion on normal faults during major tectonic reactivation
events. These traps may be present at the Birdrong Sandstone or Windalia
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Sand Member level and charged from underlying Jurassic source rocks or
through secondary migration along the reactivated faults from deeper
stratigraphic levels (e.g. Triassic source rocks). The main risk for these plays
is breach of trap integrity in the Pliocene to Holocene. Seismic data suggest
that some of the faults extend to the sea floor.
Stratigraphic plays are associated with pinchouts of the Birdrong Sandstone
onto the Valanginian unconformity highs, or drapes over these highs
(Figure 10). These plays may be charged by Jurassic or Triassic source
rocks and sealed by the Muderong Shale.
Risks
Overall, the main risks for plays in the Release Areas are:

The presence of mature source rocks in the syn-rift succession
(Exmouth Sub-basin)

The volume of available source rock (Paleozoic section)

breached seals

artesian flow (Birdrong Sandstone)

biodegradation
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FIGURES
Figure 1:
Location of Release Areas W11-16 and W11-17, showing
wells, permits and bathymetry.
Figure 2:
Graticular block map and graticular block listings for Release
Area W11-16 and W11-17 Southern Carnarvon Basin, Western
Australia.
Figure 3:
Tectonic elements of the Southern Carnarvon Basin based on
GA interpretation and compilation from Hocking et al (1987),
Lockwood and D’Ercole (2004) and Woodside Energy Ltd
(2003b, c). Also shown is the location of the seismic sections in
(Figure 4 and Figure 5 and Figure 6).
Figure 4:
Interpretation of GA seismic line 310-42 intersecting three enechelon Mesozoic depocentres separated by relay zones. See
location of the line of the line in Figure 3.
Figure 5:
Dip line (gpctr-93-0405) across the northern part of the
Gascoyne Platform, intersecting Pendock 1A. See location of
the line in Figure 3.
Figure 6:
Strike line (wg98ct-4) intersecting northern part of the Bernier
Platform and the Mesozoic depocentre of the southern
Exmouth Sub-basin. See location of line in Figure 3.
Figure 7:
Recently acquired seismic data, extent of GA Marine
Reconnaissance Survey 2476 and geological sampling sites.
Figure 8:
Generalised stratigraphy of the Gascoyne and Exmouth subbasins, showing regional tectonic events, basin phases and
seismic horizons, based on the Northern Carnarvon Basin
Biozonation and Stratigraphy Chart (Nicoll et al, 2010).
Geologic Time Scale after Gradstein et al (2004) and Ogg et al
(2008).
Figure 9:
Petroleum systems elements and possible play types for the
Release Areas. Geologic Time Scale from Gradstein et al,
2004 and Ogg et al, 2008.
Figure 10:
Conceptual play diagram for the Release Areas. Source rock
intervals: SR1 – Coburn Formation; SR2 – Gneudna
Formation; SR3 – Locker Shale; SR4 – lacustrine shale of the
Mungaroo Formation; SR5 – Dingo Formation.
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REFERENCES
CANADIAN SUPERIOR OIL (AUST) PTY LTD, 1970—Well completion report,
Pendock 1A, Unpublished report, 111p.
DANIELL, J., JORGENSEN, D.C., ANDERSON, T., BORISSOVA, I., BURQ,
S., HEAP, A.D., HUGHES, M., MANTLE, D., NELSON, G., NICHOL, S.,
NICHOLSON, C., PAYNE, D., PRZESLAWSKI, R., RADKE, L., SIWABESSY,
J., SMITH, C. AND SHIPBOARD PARTY, 2010—Frontier Basins of the West
Australian Continental Margin: Post‐ survey Report of Marine
Reconnaissance and Geological Sampling Survey GA2476. Geoscience
Australia, Record 2009/38, 229pp
DIREEN, N.G., STAGG, H.M.J., SYMONDS, P.A. AND COLWELL, J.B.,
2008— Architecture of volcanic rifted margins: new insights from the Exmouth
- Gascoyne margin, Western Australia. Australian Journal of Earth Sciences,
55 (3). pp. 341-363.
GENOA OIL N.L., 1970—Final report on Pendock N 1 well, Western Australia.
Unpublished report, 151p.
GHORI, K.A.R., MORY, A.J. AND IASKY, R.P., 2005—Modeling petroleum
generation in the Paleozoic of the Carnarvon Basin, Western Australia:
Implications for prospectivity. AAPG Bulletin, v. 89, no. 1 (January 2005), pp.
27–40.
GIBBONS A., WHITTAKER J. , MÜLLER M.R., 2010—Revised plate tectonic
history of the west Australian margin reveals how the Gascoyne Terrane
docked at West Burma, ASEG Extended Abstracts 2010, 1–4.
GRADSTEIN, F., OGG, J. AND SMITH, A. (EDITORS), 2004—A Geologic
Time Scale 2004. Cambridge: Cambridge University Press, 589p.
HOCKING R.M., MOORS H.T. AND VAN DE GRAAFF . J.E., 1987—
Carnarvon Basin: diagrammatic cross-sections and palaeogeographic
reconstructions, GSWA Bulletin 133 p3.
IASKY, R.P., D’ERCOLE, C., GHORI, K.A.R., MORY, A.J. AND
LOCKWOOD, A.M., 2003—Structure and petroleum prospectivity of the
Gascoyne Platform, Western Australia: Western Australia Geological Survey,
Report 87, 56p.
LOCKWOOD, A.M. AND D’ERCOLE, C., 2004—The evolution of the Bernier
Ridge, southern Carnarvon Basin, Western Australia: implications for
petroleum prospectivity. The APPEA Journal, 44 (1), 241–67.
MOBIL, 1994—WA-229-P Cuvier seismic survey interpretation report,
Unpublished report, 25p.
MORY, A.J., IASKY, R.P. AND GHORI, K.A.R., 2003—A summary of the
geological evolution and petroleum potential of the Southern Carnarvon
Basin, Western Australia: Western Australia Geological Survey, Report 86,
26p.
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Australia Release Area Geology
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MÜLLER, R.D., MIHUT, D., HEINE, C., O'NEILL, C. AND RUSSELL, I.,
2002—Tectonic and volcanic history of the Carnarvon Terrace: Constraints
from seismic interpretation and geodynamic modelling, In: The Sedimentary
Basins of Western Australia 3, ed: Gorter, J., Petroleum Exploration Society of
Australia, Perth, 719-740.
NORVICK, M.S., 2002—Palaeogeographic Maps of the Northern Margins of
the Australian Plate: Final Report. Unpublished report for Geoscience
Australia.
OGG, J.G., OGG, G. AND GRADSTEIN, F.M., 2008—The Concise Geologic
Time Scale. Cambridge: Cambridge University Press, 177p.
PARTINGTON, P.A., AURISCH, K., CLARK, W., NEWLANDS, I., PHELPS,
S., SENYCIA, P., SIFFLEET, P. AND WALKER, T., 2003—The hydrocarbon
potential of exploration permits WA-299-P and WA-300-P, Carnarvon Basin: a
case study. The APPEA Journal, 43 (1), 339–61.
SEARCHER SEISMIC, 2010—[Web page] Acheron 2D Non-Exclusive
Seismic Survey, Carnarvon Basin, Australia,
http://www.searcherseismic.com/projects/australia/carnarvon_basin/Acheron.
aspx (last accessed 26 November 2010)
SYMONDS P.A., PLANKE, S., FREY, Ø. AND SKOGSEID, J., 1998—
Volcanic development of the Western Australian continental margin and its
implications for basin development. The Sedimentary Basins of Western
Australia 2: Proceedings of Petroleum Exploration Society of Australia
Symposium, edited by P.G. and R.R. Purcell, Perth, 33-54.
TINDALE, K., NEWELL, N., KEALL, J. AND SMITH, N., 1998—Structural
evolution and charge history of the Exmouth Sub-basin, Northern Carnarvon
Basin, Western Australia. In: Purcell, P.G. and Purcell, R.R. (eds), The
Sedimentary Basins of Western Australia 2: Proceedings of the Petroleum
Exploration Society of Australia Symposium, Perth, 1998, 447–472.
WOODSIDE ENERGY LTD., 2003a—WELL COMPLETION REPORT
Herdsman-1 Interpretive Data (WA-299-P, Carnarvon Basin)
WOODSIDE ENERGY LTD., 2003b— Coverack 2001/2 3D, Seismic
Interpretation Report, WA-299-P, Exmouth Sub-Basin, Unpublished report,
20p.
WOODSIDE ENERGY LTD., 2003c— Coverack 2001/2 3D, Seismic
Interpretation Report, WA-300-P, Exmouth Sub-Basin, Unpublished report,
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Australia Release Area Geology
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