PETROLEUM GEOLOGICAL SUMMARY
RELEASE AREA W11-1
CASWELL SUB-BASIN, BROWSE BASIN,
WESTERN AUSTRALIA
Bids Close – 13 October 2011

Under-explored area adjacent to Gwydion, Prelude and Concerto
discoveries and undeveloped fields including the giant Ichthys gas field
and Cornea oil field.

Proven structural and stratigraphic plays nearby, including Upper
Jurassic to Lower Cretaceous drape on erosional basement highs on
the Yampi Shelf, Jurassic horsts/tilted fault blocks and associated
drape anticlines and Cretaceous submarine canyon fill, basin floor fans
and ‘ponded’ turbidite stratigraphic traps.

Organic-rich rocks with fair to moderate oil potential within the Plover
Formation, Lower and Upper Vulcan formations, Echuca Shoals
Formation as well as Lower and Upper Jamieson formations.

Gas-prone, high-quality coals and pro-delta shales in the Lower-Middle
Jurassic Plover Formation.

Excellent reservoir and seal facies demonstrated.

Planned development of a pipeline through Release Area to a new
onshore LNG processing plant.

Water depths 90-220 m.
2011 Release of Australian Offshore Petroleum Exploration Areas
Release Area W11-1, Caswell Sub-basin, Browse Basin, WA
Release Area Geology
Page 1 of 20
LOCATION
Release Area W11-1 is located in the Caswell Sub-basin, Prudhoe Terrace
and Yampi Shelf of the Browse Basin, some 86 to 120 km off the northwest
coast of Western Australia (Link to Browse Basin CASWELL Sub-basin
Figure 1). Water depths vary from approximately 90 m in the inboard part to
approximately 220 m in the outboard part. The Release Area is located
adjacent to current offshore petroleum exploration permits and is situated
some 40 km east of the Ichthys gas field (Caswell Sub-basin), 35 km east of
the Prelude 1A discovery and 40 km southwest of the Cornea oil field (Yampi
Shelf). No wells have been drilled in the Release Area, however, exploration
wells Braveheart 1, Buccaneer 1 and Prudhoe 1 ST1 are less than 2 km from
the Release Area.
Release Area W11-1 comprises 40 full graticular blocks with a total area of
approximately 3,320 km2 (Link to Browse Basin CASWELL Sub-basin
Figure 2).
Gas production facilities are currently being proposed for the Ichthys gas field
and Prelude accumulation. Inpex have proposed the construction of a gas
export pipeline from the Ichthys gas field to onshore facilities at Blaydin Point
near Darwin in the Northern Territory. This pipeline will pass through the W111 Release Area (Inpex, 2008a). Proposed development of the Prelude gas
accumulation by Shell includes what could be the world’s first operational
floating LNG facility (Gas Today, 2010).
2011 Release of Australian Offshore Petroleum Exploration Areas
Release Area W11-1, Caswell Sub-basin, Browse Basin, WA
Release Area Geology
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RELEASE AREA GEOLOGY
Local Tectonic Setting
The Caswell Sub-basin is the northernmost major depocentre of the Browse
Basin and contains up to 15 km of Paleozoic to Cenozoic sediments
(Struckmeyer et al, 1998; Link to Browse Basin CASWELL Sub-basin
Figure 3). The southeastern margin of the sub-basin is underlain by shallow
basement, which is typically highly eroded with a distinct, rugose
palaeotopographic relief and is onlapped by Permian to Mesozoic sediments
(Struckmeyer et al, 1998). This area is termed the Yampi Shelf in the central
and northern parts of the basin (Link to Browse Basin CASWELL Sub-basin
Figure 3) and the Leveque Shelf to the south (Hocking et al, 1994). The
basinward boundary of the Leveque and Yampi shelves is defined by a ‘hinge’
where the dip of the basement changes from relatively flat lying to gently
basinward-dipping, beyond which lies the Prudhoe Terrace, a fault-bounded
terrace at intermediate depth (Link to Browse Basin CASWELL Sub-basin
Figure 3);Struckmeyer et al, 1998). The Caswell Sub-basin is bounded to the
west by a major north to north-northeast trending structural zone, the BuffonScott Reef-Brecknock Anticlinal Trend (Link to Browse Basin CASWELL Subbasin Figure 3). This outer high trend passes westward into the deep water of
the Scott Plateau, the inboard (eastern) portion of which is underlain by the
Seringapatam Sub-basin (Hocking et al, 1994).
Structural Evolution and Depositional History of the Sub-basin
The lithostratigraphic scheme used herein is based on the Browse Basin
Biozonation and Stratigraphy Chart (Nicoll et al, 2009) and is tied to the
Geologic Time Scale 2004 (Gradstein et al, 2004).
The compartmentalisation of the Browse Basin into the distinct sub-basins
resulted from the Mississippian to Cisuralian (Late Carboniferous to Early
Permian) extensional phase (Symonds et al, 1994; Struckmeyer et al, 1998).
In the Caswell Sub-basin, structures resulting from this extensional phase
controlled the distribution and nature of the sedimentary fill and the location of
subsequent reactivation events (Struckmeyer et al, 1998).
The Carboniferous section in the Caswell Sub-basin is dominated by fluviodeltaic sediments, while the Cisuralian sediments (mainly limestones and
shales) were deposited in a marine environment. The higher Permian section
consists of sandstones grading into shales and limestones (Link to Browse
Basin CASWELL Sub-basin Figure 4). The oldest Triassic rocks intersected
in the Caswell Sub-basin are marine claystones, siltstones and volcaniclastic
sediments that were deposited during a regional Early Triassic marine
transgression (Link to Browse Basin CASWELL Sub-basin Figure 4). Permian
and Lower Triassic rocks were intersected near the Release Area at Echuca
Shoals 1. Overlying Triassic rocks include fluvial and marginal to shallowmarine sandstones, limestones and shales (Link to Browse Basin CASWELL
Sub-basin Figure 4).
2011 Release of Australian Offshore Petroleum Exploration Areas
Release Area W11-1, Caswell Sub-basin, Browse Basin, WA
Release Area Geology
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The Permo-Triassic thermal subsidence (sag) phase was terminated by
compressional reactivation in the Late Triassic to Early Jurassic, resulting in
partial inversion of Paleozoic half-graben and the formation of large scale
anticlinal and synclinal features within their hanging walls (Link to Browse
Basin CASWELL Sub-basin Figure 3). This event is marked by a regional
unconformity through the Release Area (Trmid seismic horizon; Link to
Browse Basin CASWELL Sub-basin Figure 4 and Link to Browse Basin
CASWELL Sub-basin Figure 5) that is correlated with the Fitzroy Movement
in the Canning and Bonaparte basins (Etheridge and O’Brien, 1994).
The Early to Middle Jurassic extensional phase resulted in widespread smallscale faulting and the collapse of the Triassic anticlines. Extensional faulting
was concentrated in the northeastern part of the Caswell Sub-basin and along
the adjacent outer margin of the Prudhoe Terrace (Link to Browse Basin
CASWELL Sub-basin Figure 3); Struckmeyer et al, 1998). The Lower–Middle
Jurassic syn-rift sediments (Plover Formation) comprise sandstones,
mudstones and coals (Link to Browse Basin CASWELL Sub-basin Figure 4)
that accumulated in deltaic and coastal-plain settings. Widespread erosion
and peneplanation in the Callovian coincided with continental breakup and the
initiation of sea-floor spreading in the Argo Abyssal Plain.
From the Late Jurassic to the Cenozoic, accommodation space was
controlled by the interplay of thermal subsidence, minor reactivation events
and eustasy. Upper Jurassic interbedded sandstones and shales (Link to
Browse Basin CASWELL Sub-basin Figure 4) onlap and drape the preCallovian structures, providing a thin, regional seal across much of the subbasin (Link to Browse Basin CASWELL Sub-basin Figure 4 and Link to
Browse Basin CASWELL Sub-basin Figure 5). An overall transgressive cycle
began in the Early Cretaceous and peaked in the mid-Turonian, with open
marine conditions established throughout the sub-basin by the Aptian (Link to
Browse Basin CASWELL Sub-basin Figure 4). Thick marine claystones
deposited during this interval (Echuca Shoals and Jamieson formations)
provide a regional seal in the Release Area (Link to Browse Basin CASWELL
Sub-basin Figure 4 and Link to Browse Basin CASWELL Sub-basin Figure 5)
and contain potential source rocks, with particularly high total organic carbon
(TOC) values recorded at the maximum flooding surfaces of several Early
Cretaceous transgressive cycles (Blevin et al, 1998a).
The Turonian–Cenozoic section represents a major progradational
(regressive) cycle in which the shelf edge migrated northwestwards to the
outer limits of the Buffon–Scott Reef–Brecknock Anticlinal Trend. The
development of submarine canyons on the Yampi Shelf and deposition of
turbidite mounds within the central Caswell Sub-basin occurred during the
middle to late Campanian (Benson et al, 2004). Inversion in the Late Miocene
occurred as a result of the convergence of the Australia-India and Eurasia
plates (Shuster et al, 1998).
2011 Release of Australian Offshore Petroleum Exploration Areas
Release Area W11-1, Caswell Sub-basin, Browse Basin, WA
Release Area Geology
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EXPLORATION HISTORY
The Caswell Sub-basin is well explored and hosts significant discoveries of
gas, condensate and to a lesser extent, oil. Five gas fields have been
discovered in the Caswell Sub-basin, but remain undeveloped: Torosa (Scott
Reef), Brecknock, Calliance (Brecknock South), Ichthys (Brewster) and Crux.
There are also several other gas discoveries in the Caswell Sub-basin
(Abalone 1 ST1, Adele 1, Argus 1, Columba 1A ST1, Concerto 1, Echuca
Shoals 1, Kaleidoscope 1, Marabou 1 ST1 and Prelude 1A) as well as both oil
and gas discoveries in Caswell 2 ST2. Gas shows and/or oil shows have been
found in several other wells in the Caswell Sub-basin. The Yampi Shelf hosts
a significant discovery of oil and gas in the undeveloped Cornea field. There
are additional oil and gas discoveries on the Yampi Shelf at Gwydion 1 and
Focus 1. An oil discovery was made at Sparkle 1 (near Cornea oil field) and a
gas discovery was made at Caspar 1A (near Gwydion 1), which also has an
oil show. The Release Area has no exploration wells and is only lightly
explored. However, exploration wells Braveheart 1, Buccaneer 1 and
Prudhoe 1 ST1 are less than 2 km from the Release Area.
Exploration wells were drilled near the Release Area during the 1970s: Rob
Roy 1 (1972), Yampi 1 (1973) and Londonderry 1 (1973), along the basin
margin faults of the Yampi Shelf; and Heywood (1974) and Prudhoe 1 ST1
(1974) in the central Caswell Sub-basin (Link to Browse Basin CASWELL
Sub-basin Figure 1). Oil shows were found in Yampi 1 (1973) within the
Callovian sequence (Lower Vulcan Formation; Link to Browse Basin
CASWELL Sub-basin Figure 4). Yampi 2 was drilled 860 m northeast of
Yampi 1 in 1994 and reported oil shows in the Lower Vulcan Formation
(reported as the Montara Formation equivalent) as well as the Plover
Formation. Drilling of Heywood 1 (1974) resulted in high gas readings and
traces of live oil encountered in the Lower Cretaceous to Lower Jurassic
sequence (Echuca Shoals and Upper Vulcan formations).
During the 1980s, a significant discovery was made near the Release Area in
the central Caswell Sub-basin at Echuca Shoals 1 (1983; Link to Browse
Basin CASWELL Sub-basin Figure 1). Log interpretation of Echuca Shoals 1
indicated gas in separate reservoirs within the Upper Jurassic (Tithonian)
Upper Vulcan Formation and in the Lower Cretaceous (Berriasian) Brewster
Member (Willis, 1988).
In the late 1990s, the testing of stacked Lower Cretaceous seismic amplitudes
near the Release Area on the Yampi Shelf lead to the Gwydion 1 (1995) oil
and gas discovery, the Cornea oil and gas discovery (1996-1997) and the
Caspar 1A (1998) gas discovery (Link to Browse Basin CASWELL Sub-basin
Figure 1). Gwydion 1 (1995) discovered oil/gas-bearing zones in Barremian
to Albian shallow marine sandstones (Echuca Shoals and Upper Jamison
formations) draped over a prominent basement high (Spry and Ward, 1997).
The Cornea 1, 1B and 2 wells (1996-1997) encountered a 25 m gas column
overlying an 18 m oil column in the base Albian reservoir sequence (Ingram et
2011 Release of Australian Offshore Petroleum Exploration Areas
Release Area W11-1, Caswell Sub-basin, Browse Basin, WA
Release Area Geology
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al, 2000; Upper Heywood Formation; Link to Browse Basin CASWELL Subbasin Figure 4). Caspar 1A discovered gas accumulations and oil shows in
the Lower Cretaceous sandstones (Echuca Shoals Formation; Link to Browse
Basin CASWELL Sub-basin Figure 4). The reservoir facies for these wells
were encountered in less than 1,000 mRT.
Significant gas discoveries were made in the central Caswell Sub-basin in
2000 with the drilling of Titanichthys 1, Dinichthys 1 and Gorgonichthys 1 on
the Brewster structure (Link to Browse Basin CASWELL Sub-basin Figure 1).
Appraisal drilling in the Ichthys gas field was completed in 2003–2004
(Ichthys 1A, Ichthys 2A ST2 and Ichthys Deep 1; Link to Browse Basin
CASWELL Sub-basin Figure 1). Here, gas is primarily reservoired within the
Upper Vulcan Formation (Brewster Member) and the Plover Formation. Gas
was also encountered within Callovian sandstones (reported as Ichthys
Formation, renamed Montara Formation; (Link to Browse Basin CASWELL
Sub-basin Figure 1) and in basal Oxfordian sandstones of the Lower Vulcan
Formation (Ban and Pitt, 2006; Link to Browse Basin CASWELL Sub-basin
Figure 4). Drilling continued in this region in 2007–2008 with Dinichthys
North 1 and Ichthys West 1 (DoIR, 2008a, b). During this period, Mimia 1
(2008) recorded a 72 m gas column, confirmed as a gas and condensate
discovery by subsequent production testing (Inpex, 2008b).
Shell Development (Australia) Pty Ltd drilled twelve exploration wells
(Prelude 1, Prelude 1A, Gigue 1, Bouree 1, Bouree 1A, Rigaudon 1,
Tocatta 1, Rondo 1, Trio 1, Fortissimo 1, Intermezzo 1, Minuet 1, Concerto 1
and Crescendo 1) in this region during 2007–2009 (Link to Browse Basin
CASWELL Sub-basin Figure 1) .Shell reported that Prelude 1A, Fortissimo 1,
Concerto 1, Intermezzo 1 and Minuet 1 were discoveries (DoIR, 2007, 2008a,
2009).
Nexus Energy Ltd (2007) reported that the Fossetmaker 1 appraisal well (Link
to Browse Basin CASWELL Sub-basin Figure 1), drilled on the Echuca
Shoals structure 7 km east-northeast of Echuca Shoals 1 and 6 km northwest
of the Release Area reached a total depth of 3,822 m in September 2007.
Wireline logs indicate the well encountered a 10 m tight gas interval with a low
net to gross at a depth just below the 70 m main gas column seen in the
Echuca Shoals 1 well. Nexus Energy Ltd (2007) reported that further studies
will be required to assess the commercial implications of this result for the
Echuca Shoals gas accumulation.
In December 2009–January 2010, Braveheart 1 was drilled 1.3 km east of the
Release Area and approximately 34 km southwest of and along trend from the
Cornea field (Link to Browse Basin CASWELL Sub-basin Figure 1). The well
encountered the targeted Barremian submarine fans (Echuca Shoals
Formation) over a gross interval of 30 m, of which 22.7 m of the net porous
sands had an average total porosity of 28.7%. Wireline formation testing
showed residual hydrocarbons in the top ~4 m of the targeted sandstone
reservoir (Exoil Limited FPO, 2010a).
2011 Release of Australian Offshore Petroleum Exploration Areas
Release Area W11-1, Caswell Sub-basin, Browse Basin, WA
Release Area Geology
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Well Control
There are currently no wells drilled in the Release Area. However, a number
of wells have been drilled in adjacent petroleum permits, including
Braveheart 1, Buccaneer 1 and Prudhoe 1 ST1 which are less than 2 km from
the Release Area (Link to Browse Basin CASWELL Sub-basin Figure 1). The
Release Area is also located some 40 km east of the Ichthys gas field
(Caswell Sub-basin), 35 km east of the Prelude 1A discovery and 40 km
southwest of the Cornea oil field (Yampi Shelf; Link to Browse Basin
CASWELL Sub-basin Figure 1).
Rob Roy 1 (1972)
Rob Roy 1 targeted a possible carbonate buildup on an anomalous dip zone.
The well intersected Cenozoic-Lower Jurassic and Lower Permian sediments
and bottomed in Proterozoic quartzite basement. No carbonate buildup was
penetrated and the anomalous dip zone was found to be the pre-Jurassic
erosional surface. No significant hydrocarbon shows were recorded in this
well.
Yampi 1 (1973)
Yampi 1 was drilled on a well defined easterly trending, Permo-Triassic fault
controlled anticline. The well penetrated a sedimentary and volcanic
succession ranging in age from Holocene to Early Permian. Numerous minor
hydrocarbon shows were recorded during drilling but Formation Interval Tests
(FITs) carried out over the intervals with the best shows produced only water
and mud. Traces of residual oil were extracted during core analysis of a core
cut in the Montara Sands (Lower Vulcan Formation; Link to Browse Basin
CASWELL Sub-basin Figure 4). The well was plugged and abandoned at a
total depth of 4,176 mRT in Lower Permian sediments.
Heywood 1 (1974)
Heywood 1 was drilled to evaluate the hydrocarbon potential of a
Permo/Triassic-Middle Jurassic horst block. The well penetrated a
sedimentary section ranging in age from Holocene to Early Jurassic. High gas
readings and traces of live oil were encountered in the Lower Cretaceous to
Lower Jurassic succession. Two wireline formation tests (FITs) were run to
test Kimmeridgian-Oxfordian and Hauterivian-Berriasian sandstones, but no
hydrocarbons were recovered. Good oil shows in the Lower-Middle Jurassic
section could not be tested due to mechanical problems.
Prudhoe 1 ST1 (1974)
Prudhoe 1 ST1 was drilled to test a seismically defined fault block on the inner
margin of the basin. The well penetrated a sedimentary succession ranging in
age from Pliocene-Middle Miocene to Lower Permian. No significant
hydrocarbons were encountered and wireline logs indicated that the
sandstones are 100% water saturated.
Echuca Shoals 1 (1984)
Echuca Shoals 1 was designed to test multiple objectives. Shallower targets
were Barremian-Tithonian sandstones (Echuca Shoals and Upper Vulcan
formations) and Middle Jurassic sandstones (Plover Formation) above a horst
2011 Release of Australian Offshore Petroleum Exploration Areas
Release Area W11-1, Caswell Sub-basin, Browse Basin, WA
Release Area Geology
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block. Deeper objectives were sandstones within a Permo-Triassic horst
block. The well encountered gas in two separate reservoirs within the
Tithonian? (and Berriasian?) section. RFT data implied a gas column of
17.5 m net pay averaging 12% porosity for the upper sand and 23.7 m net pay
averaging 15.1% for the lower sand. No detailed testing was conducted and
the well was plugged and abandoned as a gas discovery.
Browse Island 1(1986)
Browse Island 1 was drilled to determine the thickness and interval velocity of
the surface reef associated with Browse Island. The well was drilled to assist
identification of the pull-up effect of a deeper, seismically defined structure
beneath the island. The well was plugged and abandoned at a total depth of
405.5 mRT in a Late Pliocene fossiliferous limestone grading to wackestone.
Buccaneer 1 (1990)
Buccaneer 1 was drilled to test Middle-Upper Jurassic Plover and Lower
Vulcan Formation fluvio-deltaic sandstones in a fault controlled roll-over
closure on the downthrown side of the basin margin fault. The primary Middle
Jurassic objective was absent immediately below the main unconformity.
However, fair hydrocarbon shows were encountered in the Upper Vulcan
Formation (Tithonian-Berriasian) and the Middle Triassic (Pollard Formation),
consistent with the presence of residual oil.
Yampi 2 (1994)
Yampi 2 was drilled on the Yampi structure, 860 m south southeast of
Yampi 1 well, to follow-up hydrocarbon shows reported in the Yampi 1 well.
Significant hydrocarbon indications were observed during drilling the Montara
Sands (Lower Vulcan Formation; Link to Browse Basin CASWELL Sub-basin
Figure 4) and a core was cut. No significant direct fluorescence was noted
and only very slow crush cut fluorescence was observed in the core. Minor
hydrocarbon shows were noted in the Plover Formation and an RFT only
recovered mud and water. Log interpretation estimates high water saturation
throughout both zones indicating the hydrocarbons are immobile. The well
was plugged and abandoned at a total depth of 3,321 mRT terminating in
volcanics within the Lower Plover Formation.
Gwydion 1 (1995)
Gwydion 1 tested a series of stacked Lower Cretaceous seismic amplitude
anomalies draped over a prominent basement high. Three separate gas
reservoirs, ranging in thickness from 4-6 m, were intersected in Albian to
Aptian glauconitic sandstones. An RFT program consisting of pretests and
samples confirmed the presence of gas in each of these reservoirs. A deeper
gas-oil reservoir was also encountered. This lowermost hydrocarbon zone is
Hauterivian–Barremian in age and consists of 9.5 m net oil-filled sandstone
6 m of net gas-filled glauconitic sand (Spry and
Ward, 1997). The reservoirs were encountered in shallow depths between
675-809 mRT. The well has validated a new Lower Cretaceous drape (over
basement) play on the Yampi Shelf that is dependent on long distance oil
migration from the Browse Basin depocentre (this play was subsequently
extended by the Cornea discovery wells). Oil recovered at both Gwydion 1
2011 Release of Australian Offshore Petroleum Exploration Areas
Release Area W11-1, Caswell Sub-basin, Browse Basin, WA
Release Area Geology
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and the Cornea Field indicate that the late gas charge in the Browse Basin
has not excluded oil from all traps.
The Cornea field is located some 40 km northeast of the Release Area.
Exploration on the Yampi Shelf resulted in the discovery of oil and gas
accumulations at Cornea 1 and 1B in 1997 by Shell. These wells tested a
direct hydrocarbon indication (DHI) supported closure within Albian
sandstones, within a large, northeast-trending, elongate drape anticline over a
highly eroded rugose basement (Ingram et al, 2000). Small quantities of oil
and gas were recovered from the Albian section during wireline formation
tests (FITs) in Cornea 1 and oil only was recovered from Cornea 1B, but no
production tests were attempted. Cornea 1 and 1B established the presence
of a minimum 25 m gas column and a minimum 18 m oil column in the Albian
sandstone reservoir (Ingram et al, 2000). Subsequent drilling and re-appraisal
of the shallow Cornea field (<900 mRT) suggested that fundamental
development problems existed due to: the thin, laterally discontinuous oil rim;
the low API gravity and relatively high viscosity of the oil; the low effective
porosity and permeability of the reservoir; the likely rapid gas breakthrough
during production; and partial degradation of the oil (Ingram et al, 2000).
Cornea 3 was drilled in 2009 and penetrated exploration targets in the upper
and lower Jamison Formation (Upper and Lower Heywood formations; Link to
Browse Basin CASWELL Sub-basin Figure 4). An oil column of 20.4 m was
intersected in the top Middle Albian reservoir sand (Lower Vulcan Formation)
between 788-808.4 mRT. Pressure testing results have established oil and
water gradients and the free water level. High quality log data implies an
average effective free fluid porosity of 16.4% and an average inferred
permeability of 100 mD for the reservoir (Exoil Limited FPO, 2010b).
Caspar 1A (1998)
Caspar 1A was drilled to test a small drape closure within sandstones of the
Echuca Shoals Formation overlying basement. Logs and MDT sampling
indicate a 5 m gas column between 978-983 mRT in low porosity sandstones
underlain by traces of residual biodegraded oil, believed to represent an
earlier hydrocarbon charge. No hydrocarbons were encountered in other
potential reservoir facies of the Echuca Shoals Formation in the well,
indicating that migration across the Yampi Shelf is potentially complex and
possibly restricted to specific carrier beds.
The giant Ichthys gas field is located some 40 km west of the Release Area
(Inpex, 2008b). The exploratory wells Brewster 1A ST1 (1980),
Dinichthys 1 (2000) and Gorgonichthys 1 (2000) resulted in gas
discoveries. Gas is primarily reservoired within the Brewster Member of the
Upper Vulcan Formation and within the Plover Formation. Gas was
encountered within Callovian sandstones (Montara Formation; Link to Browse
Basin CASWELL Sub-basin Figure 4) and in basal Oxfordian sandstones of
the Lower Vulcan Formation (Ban and Pitt, 2006).
Fossetmaker 1 (2007)
Fossetmaker 1 was drilled on the Echuca Shoals structure, 7 km eastnortheast of Echuca Shoals 1. Wireline logs indicated a 10 m tight gas interval
2011 Release of Australian Offshore Petroleum Exploration Areas
Release Area W11-1, Caswell Sub-basin, Browse Basin, WA
Release Area Geology
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with a low net to gross at a depth just below the 70 m main gas column seen
in the Echuca Shoals 1 well (Nexus Energy Ltd, 2007). The well was plugged
and abandoned at a total depth of 3,822 mRT. Detailed results of the well
have not been released.
Prelude 1A (2007)
Prelude 1A was drilled to evaluate the hydrocarbon potential of a dip-closed
structure east of the Ichthys field. The well penetrated a sedimentary
succession and encountered hydrocarbons. Drill stem testing (DST) flowed
26.5 MMscfd of gas and 1,080 stbd of condensate at depth interval 4,145.54,233 mRT. The well was plugged and abandoned at a total depth of
4,361 mRT in a claystone. Detailed results of the well have not been released.
Braveheart 1 (2010)
Braveheart 1 targeted Barremian submarine fans within the Echuca Shoals
Formation. The well encountered the targeted sandstone reservoir over a
gross interval of 30 m, of which 22.7 m of net porous sands had an average
total porosity of 28.7%. Wireline formation testing showed residual
hydrocarbons in the top ~4 m of the reservoir interval. (Exoil Limited FPO,
2010a). The well validated the depositional model relied upon to support the
presence of a high quality reservoir interval. Detailed results of the well have
not been released.
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_caswell_AR11.xls
2011 Release of Australian Offshore Petroleum Exploration Areas
Release Area W11-1, Caswell Sub-basin, Browse Basin, WA
Release Area Geology
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Data Coverage
The Release Area has been covered by a broad grid of reconnaissance
seismic lines. To view image of seismic coverage follow this link:
http://www.ga.gov.au/energy/projects/acreage-release-andpromotion/2011.html#data-packages
The data is of varying vintage and quality – the 1970s data is of poor quality
but the later data, acquired in the 1990s, is generally of fair to good quality.
The Braveheart 2D M.S.S. (2005) consisted of 1,353 line km of 2D seismic
data over 39 lines across the Release Area and two permits WA-333-P and
WA-342-P. Line spacing was 1–15 km (line density is greatest to the NE in
the Release Area) and the survey included well tie lines to Prudhoe 1,
Buccaneer 1, Rob Roy 1 and the Cornea field wells. The Braveheart Infill 2D
M.S.S. (2008) acquired a further 770 line km of new infill 2D seismic across
the Release Area and part of WA-333-P. This seismic survey will become
open file in November 2011.
WesternGeco acquired the Adele Trend speculative 3D seismic survey
(Phase I and Phase II) in 2001. These surveys were acquired across several
blocks including the northern edge of the Release Area (WesternGeco, 2010).
The Release Area also falls within the boundaries of the North West Shelf
Digital Atlas (North West Shelf Digital Atlas, 2010).
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Release Area W11-1, Caswell Sub-basin, Browse Basin, WA
Release Area Geology
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PETROLEUM SYSTEMS AND HYDROCARBON POTENTIAL
Table 1: Petroleum Systems Elements Summary
Lower–Middle Jurassic Plover Formation (oil and gas prone)
Callovian–Valanginian Lower and Upper Vulcan formations (oil
prone)
Sources
Valanginian–Aptian Echuca Shoals Formation (oil prone)
Aptian–Cenomanian Lower and Upper Jamieson formations
(oil prone)
Multiple stratigraphic levels within:
Jurassic – Plover Formation (fluvio-deltaic)
Reservoirs
Cretaceous – Upper Vulcan Formation, Echuca Shoals
Formation, Lower and Upper Jamieson formations (shallow
marine sandstones, submarine fans and ’ponded’ turbidite
mounds)
Upper Jurassic-Lower Cretaceous Vulcan Formation
claystones (regional seal)
Seals
Lower Cretaceous Echuca Shoals Formation claystones
(regional seal)
Intra-formational Lower–Middle Jurassic Plover Formation
shales
Upper Jurassic to Lower Cretaceous drape of erosional
basement highs on the Yampi Shelf
Play Types
Jurassic horsts/tilted fault blocks and associated drapes
Cretaceous submarine canyon fill, basin floor fans and
‘ponded’ turbidite stratigraphic traps
Play Types and Migration
The potential play types in the Release Area W11-1, include Upper Jurassic
to Lower Cretaceous drape on erosional basement highs on the Yampi Shelf,
Jurassic horsts/tilted fault blocks and associated drape anticlines and
Cretaceous submarine canyon fill, basin floor fans and ‘ponded’ turbidite
stratigraphic traps (Link to Browse Basin CASWELL Sub-basin Figure 5).
Hydrocarbon accumulations have been encountered near the Release Area
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W11-1 in both structural and stratigraphic plays in the Caswell Sub-basin and
Yampi Shelf. These play types include Upper Triassic faulted anticlines (e.g.
Crux 1), Jurassic horsts/tilted fault blocks and associated drape anticlines
(e.g. Yampi 1 and 2) , Lower Cretaceous drape on erosional basement highs
on the Yampi Shelf (e.g. Gwydion 1, Caspar 1A, Cornea field) and
Cretaceous submarine canyon fill, basin floor fans and ‘ponded’ turbidite
stratigraphic traps (e.g. Braveheart 1).
The Release Area W11-1 has the potential to be on the migration pathway of
known petroleum systems. Significant quantities of oil and gas have migrated
within the Caswell Sub-basin and from the Caswell Sub-basin to the Yampi
Shelf, as evident by the Ichthys gas field, Cornea oil field and other
hydrocarbon discoveries and major shows. Gwydion 1 validated a Lower
Cretaceous drape (over basement) play on the Yampi Shelf that is dependent
on long distance oil migration from Caswell Sub-basin depocentres. This play
was subsequently extended by the Cornea discovery wells. Oil recovered at
both Gwydion 1 and the Cornea Field indicate that the late gas charge in the
Browse Basin has not excluded oil from all traps. Multiple indications of
hydrocarbon seepage on the Yampi Shelf (including synthetic aperture radar,
airborne laser fluorescence, water-column sniffer and seismic amplitude
effects) confirm the oil and gas expulsion and migration (O’Brien et al, 2000).
However, migration across the Yampi Shelf is potentially complex and
possibly restricted to specific carrier beds as demonstrated in Caspar 1A
where hydrocarbons were only encountered in one of multiple reservoir facies
of the Echuca Shoals Formation.
Source Rocks
Potential source rocks near and in the Release Area W11-1 exist in multiple
stratigraphic levels within the Plover Formation (Lower-Middle Jurassic), the
Lower and Upper Vulcan formations (Middle-Upper Jurassic), the Echuca
Shoals Formation (Early Cretaceous) and the Lower and Upper Jamieson
formations (Early Cretaceous; Blevin et al, 1998a, 1998b, Kennard et al,
2004). A comprehensive assessment of the source rock potential of the
Browse Basin was undertaken by Boreham et al (1997) and the results
summarised by Blevin et al (1998a, 1998b), which include generalised
distribution maps of the Upper Jurassic-Lower Cretaceous source units.
These studies recognised organic-rich rocks with fair to moderate oil potential
at multiple stratigraphic levels within the Permian–Lower Cretaceous section
and that some local, thin, high-quality coals and pro-delta shales occur within
the Lower–Middle Jurassic, fluvio-deltaic Plover Formation. However,
hydrocarbons generated from the fluvio-deltaic Plover Formation are likely to
be dominated by gas rather than oil (Blevin et al, 1998b).
The Lower-Middle Jurassic Plover Formation is very thick near the Release
Area W11-1 (Link to Browse Basin CASWELL Sub-basin Figure 5) and is
intersected near the Release Area in Rob Roy 1 (Link to Browse Basin
CASWELL Sub-basin Figure 3). The Plover Formation is dominated by fluviodeltaic facies, including pro-delta shales and coastal plain shaly coals that
have significant source potential (Blevin et al, 1998b). Kerogen kinetics data
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shows that the Plover Formation source sediments within the Prudhoe
Terrace are mature for oil and wet-gas generation (Kennard et al, 2004).
The Callovian-Tithonian Lower Vulcan Formation (Middle-Upper Jurassic)
experiences localised thickening of the potential source units near and in the
Release Area W11-1 (Blevin et al, 1998a; Link to Browse Basin CASWELL
Sub-basin Figure 5). On the Prudhoe Terrace, the Lower Vulcan Formation
consists of stacked sequences of fluvial to marine shelf sediments.
Thick sections of Lower Cretaceous potential source sediments (Echuca
Shoals to Upper Jamieson formations) occur within the Release Area (Blevin
et al, 1998a; Link to Browse Basin CASWELL Sub-basin Figure 5) and
contain mixed marine and terrestrial organic matter with moderate to good
source potential (Blevin et al, 1998a). Pyrolysis data also suggests that these
potential source sediments have liquids potential (HI=150–350 mgH/gTOC;
Kennard et al, 2004, table 1). Oil-source correlations have also been
established between the Cornea-Gwydion oils/stains and marine organic
matter in the Lower Cretaceous Echuca Shoals Formation (Valanginian S.
areolate and Barremian lower M. australis dinoflagellate zones; Boreham et
al, 1997; Blevin et al, 1998a).
Reservoirs and Seals
Reservoir facies are best developed within the fluvio-deltaic Lower–Middle
Jurassic Plover Formation as well as shallow marine sandstones, submarine
fans and ’ponded’ turbidite mounds of Tithonian, Berriasian, Barremian,
Aptian and Albian ages (Upper Vulcan to Upper Jamieson formations). The
reservoirs are of potential high quality, with Gwydion 1 encountering a shallow
marine Hauterivian–Barremian sandstone reservoir that consisted of 24-27%
porosity (Spry and Ward, 1997). The separate reservoirs encountered in
Gwydion 1, Caspar 1A and the Cornea field are all between 650-1,000 mRT.
Upper Jurassic-Early Cretaceous claystones (Vulcan Formation) and Lower
Cretaceous claystones (Echuca Shoals Formation) provide regional seals
throughout much of the basin (Link to Browse Basin CASWELL Sub-basin
Figure 5). Potential intraformational sealing shales occur within the Lower–
Middle Jurassic Plover Formation (Blevin et al, 1998b).
Critical Risks
Proven hydrocarbon systems and migration have already been established
near the Release Area W11-1 in the Caswell Sub-basin and Yampi Shelf (Link
to Browse Basin CASWELL Sub-basin Figure 3). Therefore, continued
success relies on the identification of additional valid traps with access to
charge from source rocks. Potential traps in the Release Area W11-1 include
Upper Jurassic to Lower Cretaceous drape on erosional basement highs on
the Yampi Shelf, Jurassic horsts/tilted fault blocks and associated drape
anticlines and Cretaceous submarine canyon fill, basin floor fans and ‘ponded’
turbidite stratigraphic traps. In the inboard part of the Release Area, another
risk can arise from the shallow nature and quality of the potential reservoirs
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that can have fundamental development problems, as identified in the Cornea
field (<900 mRT) by Ingram et al (2000).
Key exploration tools that are likely to contribute to continued structural and
stratigraphic trap identification in the Release Area are 3D seismic and AVO
technology (Longley et al, 2002; Williamson and Kroh, 2007). Detailed highresolution (system tract level) sequence stratigraphic control would also assist
in stratigraphic trap identification as well as enable the accurate modelling of
the generation and expulsion history of the discontinuous lenses of organicrich intervals within the Lower Cretaceous section (Kennard et al, 2004).
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FIGURES
Figure 1:
Location map of Release Area W11-1 in the Caswell Sub-basin
and Yampi Shelf, Browse Basin. Exploration wells relevant to
Release Area W11-1 are also shown.
Figure 2:
Graticular block map and graticular block listings for Release
Area W11-1 in the Caswell Sub-basin and Yampi Shelf, Browse
Basin.
Figure 3:
Structural elements of the Caswell Sub-basin, Prudhoe Terrace
and Yampi Shelf showing the 2011 Release Area and the
exploration wells relevant to Release Area W11-1. The location
of the seismic section in (Link to Browse Basin CASWELL Subbasin Figure 5).is also shown
Figure 4:
Generalised stratigraphy of the Caswell Sub-basin, Prudhoe
Terrace and Yampi Shelf. The lithostratigraphic scheme is
based on the Browse Basin Biozonation and Stratigraphy Chart
(Nicoll et al, 2009). Geologic Time Scale 2004 after Gradstein
et al (2004) and Ogg et al (2008).
Figure 5:
AGSO Seismic line 119/04 across Release Area W11-1 in the
Caswell Sub-basin, Prudhoe Terrace and Yampi Shelf
demonstrating some structural and stratigraphic play types.
Location of the seismic line is shown in (Link to Browse Basin
CASWELL Sub-basin Figure 3. Regional seismic horizons are
shown in (Link to Browse Basin CASWELL Sub-basin Figure
4)
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