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Yampi Shelf, Browse Basin, North-West Shelf, Australia: a test-bed for
constraining hydrocarbon migration and seepage rates using combinations
of 2D and 3D seismic data and multiple, independent
remote sensing technologies
G.W. O’Briena,*, G.M. Lawrenceb, A.K. Williamsc, K. Glennd, A.G. Barrettd, M. Lechd,
D.S. Edwardsd, R. Cowleye, C.J. Borehamd, R.E. Summonsf
aAustralian School of Petroleum, University of Adelaide, Adelaide, South Australia 5005, AustraliabTREICo Limited, Knebworth, Hertfordshire, UK
cNigel Press Associates, Crockham Park, Edenbridge, Kent TN8 6SR, UKdGeoscience Australia, GPO Box 378, Canberra, ACT 2601, Australia
eSignalworks Pty Ltd, 93 Hume Street, Greensborough, Victoria 3088, AustraliafDepartment of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
Received 9 March 2004; accepted 1 October 2004
Abstract
The Yampi Shelf on Australia’s North-West Shelf is highly prospective, with two discrete hydrocarbon sources producing dry gas and oil.
To reduce exploration uncertainty relating to gas flushing and poor top seal capacity, a study was undertaken to characterise hydrocarbon
migration in the area. It used a combination of seismic amplitude and structural data integrated with shipboard water column geochemical
sniffer (WaSi) data, satellite Synthetic Aperture Radar or SAR data and aircraft-acquired Airborne Laser Fluorosensor (ALF) data. Data were
acquired synchronously and in staged programs, to allow both direct comparison and time-series analysis of results. Massive natural dry gas
and oil seepage was detected, though the relative abilities of WaSi, SAR and ALF to detect and characterise this seepage were markedly
different. The spatial distribution, concentration, and relative composition of the detected seepage were controlled principally by the regional
seal’s thickness and capacity, rather than by the inherent composition and flux of the migrating hydrocarbons. WaSi preferentially identified
gas seepage, often in basin-ward locations, because the high relative permeability of gas favoured its early leakage, even through thick seals.
SAR preferentially identified oil seepage, which was episodic and largely restricted to the basin-margin at the regional zero-edge-of-seal,
reflecting the low relative permeability of oil, even through thin seals (it leaked ‘late’). ALF principally detected low-level oil seepage from
charged traps, and was hence most useful for trap ranking. The ability of these remote sensing tools, as well as that of seismic data itself, to
detect hydrocarbons appears critically dependant upon interplays between the relative sensitivity of the assorted tools to detect various
hydrocarbon phases and the capacity of the top seal itself. The study has demonstrated that the interactions between geology and hydrocarbon
charge are predictable, and that understanding these interactions is crucial for the reliable interpretation of remote sensing data.
q 2005 Elsevier Ltd. All rights reserved.
Keywords: North-West shelf; Hydrocarbon seepage; Top seal integrity; Remote sensing
1. Introduction
The Australian School of Petroleum and Geoscience
Australia, in collaboration with industry and research
0264-8172/$ - see front matter q 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.marpetgeo.2004.10.027
* Corresponding author. Tel.: C61 8 8303 3502; fax: C61 8 8303 4345.
E-mail address: gobrien@asp.adelaide.edu.au (G.W. O’Brien).
partners, have been carrying out an evaluation of the
relative sensitivities of different remote sensing techno-
logies at detecting hydrocarbon seepage. In particular, the
relative responses of these technologies to variations in both
the type (e.g. oil versus gas) and rate (e.g. high versus low)
of seepage has been investigated (O’Brien et al., 1996a,
1998a, 1998b, 2002a).
The overall goal of this research has been to develop a
suite of generic evaluation tools so that exploration
Marine and Petroleum Geology 22 (2005) 517–549
www.elsevier.com/locate/marpetgeo
Fig. 1. Location of study area; Yampi Shelf; offshore northwestern
Australia. MG, Malita Graben; SP, Sahul Platform; AP, Ashmore Platform;
VSB, Vulcan Sub-basin; LH, Londonderry High; LS, Leveque Shelf; BSB,
Bedout Sub-basin; FT, Fitzroy Trough.
G.W. O’Brien et al. / Marine and Petroleum Geology 22 (2005) 517–549518
uncertainty associated with hydrocarbon migration and
preservation, particularly in relation to top and fault seal
integrity, can be reduced.
In this paper, the results of an investigation of
hydrocarbon migration and seepage on the Yampi Shelf,
north-eastern Browse Basin, Australia (Fig. 1), are reported.
Data from four independent remote sensing technologies,
namely Synthetic Aperture Radar (SAR), water column
geochemical sniffer (WaSi), Mark III Airborne Laser
Fluorosensor (ALF,) and surface water analysis (using
multi-spectral fluorimetry) were acquired and interpreted,
and the relative responses compared. The results have been
integrated with interpretations derived from 2D and 3D
seismic data, as well as analytical results from exploration
wells. Emphasis has been placed upon understanding the
key inter-relationships between basement topography, top
seal capacity, and hydrocarbon seepage.
The main area investigated was in the vicinity of the
Cornea-1 and Londonderry-1 wells (Fig. 1), although some
information is presented regarding the Gwydion-1 well and
surrounding area, located approximately 100 km to the
southwest.
The 2D seismic and the water column sniffer data were
acquired and interpreted in 1995 and 1996, respectively,
following the drilling by BHP Petroleum of the Gwydion-1
oil and gas discovery in 1995 (Spry and Ward, 1997). An
assessment of the exploration implications of these seismic
and sniffer results was made by O’Brien et al. (1996a),
several months prior to the discovery by Shell in 1997 of the
Cornea oil and gas accumulation (Ingram et al., 2000). The
Cornea appraisal wells established the presence of a
minimum 25 m gas column and a minimum 18 m oil
column in the Albian reservoir sequence (Ingram et al.,
2000), with in-place reserves reported to be hundreds of
millions of barrels. The trap was interpreted to be filled to
structural spill-point. The ALF and SAR data used in this
paper were all acquired and interpreted in 1998. The ALF
data were reprocessed in 2000 in order to remove possible
‘false positives’ and to allow more quantitative interpret-
ation of the ALF data.
Key parts of the present study were undertaken while the
Yampi Shelf was still a largely undrilled, exploration
frontier, and hence a comparison between the pre-drill
predictions (based upon the seismic-sniffer-SAR-ALF
remote sensing study), and the post-drill results around the
Cornea field, as discussed by Ingram et al. (2000), is
possible.
2. Regional geology of study area
The Yampi Shelf is located in the north-eastern Browse
Basin, North-West Shelf, Australia (Figs. 1 and 2) and
comprises the inboard part of a Palaeozoic to Mesozoic
flexural ramp margin which dips to the northwest, away
from the flanking cratonic (Proterozoic) Kimberley Block.
The basement has a rugose topography, with some basement
blocks being elevated by as much as 500 m above the
surrounding basement. The basement topography itself
appears to be due to a combination of the basement grain,
differential erosion, and possibly small displacement
faulting.
Following continental break-up in the Callovian, the
interplay between early post-rift low- and high-stand
Cretaceous sand deposition around the basement highs,
and the progressive onlap of post-rift, Cretaceous sealing
shales, created a series of stratigraphic, combined structural-
stratigraphic and compactional-drape traps. Exploration
activity on the poorly explored Yampi Shelf was boosted
dramatically by the discovery of the Gwydion oil field on
the southern Yampi Shelf by BHP Petroleum in 1995 (Spry
and Ward, 1997). This discovery, whilst sub-commercial,
demonstrated long-range migration (50–80 km) of liquid
hydrocarbons onto the ramp margin, and entrapment around
a small, Proterozoic basement high. Similarly, the discovery
in 1997 of the Cornea field (Stein et al., 1998; Ingram et al.,
2000) on the northern Yampi Shelf (Fig. 1) by Shell
Development Australia and its partners, Chevron and Cultus
Petroleum, further boosted interest in the area. The oil in
both the Gwydion and Cornea fields appears to have
been generated within Early Cretaceous (Valanginian to
Barremian) source rocks (Spry and Ward, 1997; Blevin et
al., 1998; Ingram et al., 2000). Dry gas (!2% wet) is also
reservoired in these discoveries, suggesting that the source
rocks for the oil and the gas are probably quite different. The
reservoirs units at Cornea are of Albian age, with Albian to
Cenomanian shales providing the top seals.
The regional Cretaceous sealing units over the north-
eastern Yampi Shelf show characteristic distributions,
which are important to understanding both the prospectivity
Fig. 2. Bathymetry of the Timor Sea, showing the transition between the Timor and Browse Compartments. Location of key wells highlighted. Location of
Cornea, Londonderry and Gwydion wells indicated.
G.W. O’Brien et al. / Marine and Petroleum Geology 22 (2005) 517–549 519
of the area and the probable distribution of seepage. At the
first order, the regional seal progressively thins and becomes
sandier marginward. More locally, the sealing units thin
significantly and rapidly onto topographically prominent,
basement blocks. Inboard from the Cornea-1 and London-
derry-1 wells, several high-relief basement horsts are
partially to completely bald of seal, whereas at the basin
edge, the regional zero edge of seal appears to be controlled
by a prominent and extensive basement shelf onto which the
seal on-laps, but does not cover.
Structurally, the Yampi Shelf occurs at the transition
zone between two major margin-scale compartments which
are evident in the present day bathymetry (Fig. 2): the Timor
and the Browse compartments (O’Brien et al., 1996b, 1999).
The boundary zone between these two compartments is a
fundamental, north-west trending Proterozoic lineament or
fracture system, which has acted as a long-lived fault relay
zone. Major rift fault systems die out into, and gain
displacement away from, this fracture system and, as a
consequence of significant fault overlap across the linea-
ment, this zone has thus been a prominent structural high
through time. Fault displacements on the Yampi Shelf
decrease to the northeast into this transition zone and this,
combined with the fact that the Bonaparte-Browse Tran-
sition Zone is the boundary between wide and narrow
margin compartments, makes the south-eastern segment of
the transition zone a regional focus for present day
hydrocarbon migration from the more central Browse
Basin (Fig. 2).
3. Technical approaches
The area investigated during the present study extends
from the vicinity of the Gwydion-1 exploration well in the
southern Yampi Shelf to the northeast of the Londonderry-1
well (Figs. 1–3). The technical approaches employed to
evaluate the hydrocarbon migration and seepage character-
istics of the Yampi Shelf are described below.
3.1. Regional seal thickness
A general overview of the regional seal thickness in the
northern Browse Basin was produced by image processing
the derived thickness of the key sealing interval intersected
in all available exploration wells (wlate Aptian-Albian;
Ingram et al., 2000). The interval used was the 95–115 Ma
sequence.
3.2. 2D seismic
The Australian Geological Survey Organisation’s
(AGSO, now Geoscience Australia) Yampi Shelf Tie survey
(YST Survey 165) regional seismic grid, acquired in late
1995 on a grid size of between 5–10 km (Fig. 3) was
interpreted as part of the study. The grid comprised 2000 km
of data, acquired along 18 dip and 2 strike lines (O’Brien
et al., 1996a). The interpretative emphasis was placed on
developing an understanding of key factors in relation to
hydrocarbon migration and seepage, such as the regional
Fig. 3. Location map showing interpreted YST 165 seismic lines. Lines highlighted have sniffer data acquired directly over them and are used in the present
study. Positions of mapped HRDZs and gas chimneys, and shallow and deep seismic amplitude anomalies are indicated.
G.W. O’Brien et al. / Marine and Petroleum Geology 22 (2005) 517–549520
seal thicknesses and the distribution of seismic amplitude
anomalies.
Amplitude anomalies were mapped according to whether
they were deep (i.e. present at depths greater than 500 ms
(ms) two-way-time (TWT)) or shallow (less than 500 ms
TWT). All anomalies were in similar shallow water
(/100 m), so any potential effects that increasing water
depth might have on the mapping was insignificant.
Features similar to the Hydrocarbon-Related Diagenetic
Zones (HRDZs) of O’Brien and Woods (1995) were also
mapped; HRDZs are zones of high seismic velocity caused
by enhanced carbonate cementation related to hydrocarbon
seepage and oxidation. Since the mapped HRDZs were
typically associated with gas chimneys, and vice versa,
these features have essentially been grouped together for the
purposes of discussion, unless it is clear that the feature is a
simple gas chimney with no associated cementation.
3.3. 2D seismic
Shell Development Australia’s 1997 Cornea 3D marine
seismic survey was interpreted as part of the present study.
This survey was acquired in the vicinity of the Cornea field
(Fig. 3) on a NE–SW azimuth and covered an area of
2100 km2; line spacing was 12.5 m. These 3D data were
used to map in detail gas chimneys and HRDZs, though
basement, the top of reservoir, and the top of seal, inter alia,
were also mapped. Features such as seafloor pockmarks and
build-ups were noted where present.
3.4. Synthetic aperture radar (SAR)
Satellite-based Synthetic Aperture Radar data (Fig. 4) is
a low cost, regional tool that can provide an almost
instantaneous radar snapshot of an area 100–165 km2. It
can be used to map natural and anthropogenic oil slicks, and
to a lesser extent condensate slicks, via the dampening effect
that the liquid hydrocarbons have on wind-induced rippling
(i.e. capillary ripples) on the surface of the sea. This
dampening results in reduced radar return from the affected
area, so that oil slicks appear as relatively dark features on
the SAR scenes. The pixel size for SAR is about 25 m,
which means that individual slicks smaller than about 120 m
long cannot be mapped reliably. Heavier (high API) oils are
the easiest to detect because they have longer residence
times at the sea surface, whereas condensates and light oils
evaporate much more rapidly. Gas can only be mapped
rarely using SAR data, typically when it is associated with
condensate.
The SAR data used in this study were interpreted by NPA
Satellite Mapping/TREICo of the UK as part of a wider
study of the Bonaparte and Browse basins carried out by
AGSO/Geoscience Australia (O’Brien et al., 2001). That
study used 55 RadarSat Wide 1 Beam Mode SAR scenes,
Fig. 4. Schematic showing tools used for detecting and characterising hydrocarbon seeps. Schematics for acquisition of ship-based water column geochemical
sniffer (WaSi) data, aircraft-based Airborne Laser Fluorosensor (ALF) and satellite-based Synthetic Aperture Radar (SAR) data shown.
G.W. O’Brien et al. / Marine and Petroleum Geology 22 (2005) 517–549 521
which provided a minimum of double coverage over an area
of ocean exceeding 365,000 km2. The SAR coverage
extended from the coastline to abyssal water depths, and
covered a range of geological provinces and sub-basins.
Three additional scenes were acquired over the Yampi Shelf
throughout 1998, providing five-fold coverage of the area
investigated in the present study and allowing a time-series
analysis of oil seepage in the region.
The seepage slicks mapped in this study have been sub-
divided into two classes, according to the nomenclature
used by NPA Satellite Mapping/TREICo: Second Rank
slicks, which are relatively intense; and Third Rank slicks,
which are typically smaller, less intense seepage slicks.
Details of this classification scheme can be found in O’Brien
et al. (2001).
The oil in seepage slicks is typically considered to rise
rapidly to the surface and is often transported as thin skins on
the surface of gas bubbles (Mackintosh and Williams, 1990).
These bubbles have been shown to rise at speeds exceeding
those of ocean currents and hence seepage slicks are typically
developed initially on the sea’s surface no further away from
the seafloor seepage vent than a distance roughly equivalent
to the water depth (Mackintosh and Williams, 1990). Given
that most of the water depths through the study area are less
than 100 m, it is reasonable to expect that emission points for
any slicks will be situated within a lateral radius of
approximately 100 m from the source vent.
3.5. Water column geochemical sniffer
The geochemical sniffer data were acquired by the RV
Rig Seismic using AGSO’s purpose-built system.
This comprised a towed, 2.5 m long fish from which
bottom-water was pumped through a hollow nylon tube,
wrapped with a stainless steel braid, into the geochemical
laboratory on the ship. The towed ‘fish’ was typically
deployed within 10–15 m of the seafloor to minimise
dispersion from the potential sources of seepage (Fig. 4).
Light hydrocarbons were extracted from seawater in an
evacuated chamber and analysed by gas chromatographs
connected in parallel. Total hydrocarbon concentrations
were measured every 30 s (s), which, at a ship speed of 5
knots, represents a distance of about 30 m on the seafloor.
The light hydrocarbons (C1–C4) were measured every 2 min
(w240 m intervals on the seafloor), whereas the C5–C8
hydrocarbons were measured every 8 min (w1000 m
interval). Hydrocarbon anomalies were identified at sea by
comparing the measured light hydrocarbon concentrations
to the local background concentrations.
A variety of geochemical cross-plots can be used to
determine whether any detected anomalies are due to
hydrocarbon seepage, the hydrographic structure in the
water column, or in situ biogenic production. If the
anomaly is related to seepage, additional cross-plots can
be employed to determine the source of the seepage
(thermogenic gas versus gas-condensate versus oil-prone,
or biogenic gas). In the present study, carbon isotopic
measurements were undertaken (following completion of
the survey) on methane extracted from several of the more
intense seeps. This characterisation of the isotopic and
molecular composition of the seeps allowed direct
comparison with the composition of gases measured
within the reservoirs intersected during exploration
drilling in the area.
G.W. O’Brien et al. / Marine and Petroleum Geology 22 (2005) 517–549522
The water column geochemical sniffer data were
acquired in July–August 1996 to test concepts developed
during the interpretation of the 2D seismic data. These
sniffer data (total 730 km) were acquired in two areas, a
w340 km grid around the Gwydion-1 exploration well, and
w390 km of data around the general area of the Cornea
trend, which had not yet been drilled at the time of sniffer
acquisition. These data were almost exclusively acquired
directly along the existing, north-west trending, Yampi
Shelf Tie (YST) seismic dip lines, in order to allow direct
comparison between the underlying geology and the
distribution, composition and intensity of the hydrocarbon
seeps. YST lines overshot by WaSi data were: YST 165-03
(through the Gwydion-1 well); 165-07 (over the Cornea
trend), 165-08 (through the Londonderry-1 well); and lines
165-09 to -12. These lines are highlighted on Fig. 3.
3.6. Surface water fluorimetric analyses
During 1998, a total of 24 surface and near-surface
(within 2 m of the sea’s surface) seawater samples were
acquired during a water column geochemical survey over
the Yampi Shelf (Survey 207) by AGSO (Wilson, 1999).
These samples were taken to provide some ‘sea-truthing’ of
the seepage concepts which had arisen from the interpret-
ation of the existing SAR, sniffer, and seismic data-sets
described above.
The seawater samples were collected (during Survey
207) whenever a UV fluorescence anomaly was detected by
an onboard Safire fluorimeter, as documented in Wilson
(1999); Radlinski et al. (1998). A pre-cleaned, 1 l glass
bottle was rinsed with seawater immediately prior to
sampling; upon filling, sodium azide was added to the
sample to kill any microbes, and the bottled sealed. Each of
the 24 bottled samples was refrigerated for the duration of
the cruise.
The 24 seawater samples were analysed by ultra-violet
(UV) emission spectrometry (Edwards and Johns, 1999),
according to ASTM method D3650-90. The emission
spectrum was scanned at a fixed excitation wavelength of
266 nm (nm), rather than 254 nm as specified in the ASTM
method. The rationale for this was to enable comparison of
the seawater spectra with the spectral data obtained from the
266 nm Mark III ALF data from the same area (see below
for discussion). The fluorimetry (UVF) data were collected
using a Perkin Elmer LS 50B luminescence spectrometer.
The seawater sample was placed into a 4 mL quartz
cuvette for UVF analysis. Appropriate cleaning pro-
cedures were used to prevent cross-contamination of
samples and the spectrum of pure water (obtained using
Millipore filtres) was collected in between each sample
analysis. Prior to analysis, a stock solution of artificial
seawater (3.5% salinity) was prepared by dissolving
1.75 g ‘aquarium sea salts’ in 50 mL Millipore water,
and its UVF spectrum was acquired as a background
check (Edwards and Johns, 1999).
The Perkin Elmer LS 50B luminescence spectrometer
was operated in the single emission scan mode with the
excitation monochromator set at 266 nm. The seawater
sample was irradiated briefly with UV light and the
emission spectra were scanned from 270 to 720 nm at
0.5 nm intervals at a rate of 60 nm/min. The signal-to-noise
ratio for fluorescence in the Yampi Shelf region was 689:1.
The slit width was set at 2.5 nm for the excitation
monochromator and 5 nm for the emission monochromator.
The Raman scattering from the seawater was monitored as
an independent intensity marker.
3.7. Mark III airborne laser fluorosensor
The Airborne Laser Fluorosensor (ALF) was developed
by British Petroleum’s (BP) Research Centre during the
1980s as a means of identifying hydrocarbon seepage in
frontier basins around the world. The ALF technology was
subsequently sold to World Geoscience Corporation Ltd
(WGC) in 1990, as part of BP’s technology out-sourcing
program.
ALF technology used in the present study (Mark III)
comprised an aircraft-mounted laser, with an emitting
wavelength of 266 nm, which was pulsed rapidly and fired
vertically at the sea surface (Fig. 4). Each pulse illuminated
an area of approximately 20 cm2, with an average spacing
between samples (on the sea surface) of 1.5–2 m (Cowley,
2000a). Any aromatic hydrocarbons present at the sea surface
become excited by the laser and fluoresce; this fluorescence
was then measured on-board the aircraft using a solid state
diode array and presented as a digital spectral output.
ALF is an extremely sensitive tool and detects the
presence of thin (!1 micron) hydrocarbon films on the sea
surface. It can detect oil and condensate slicks equally well,
in contrast to SAR, though it cannot detect gas. Hydro-
carbon anomalies detected by ALF are called ‘fluors’.
The Yampi Shelf ALF survey was flown in several stages
in November 1998. The aircraft acquired 69 north-west
trending lines from an altitude of 80 m. The spacing
between lines was 700 m and line lengths ranged from 15
to 75 km, with a total of 3148 line km of data acquired.
2,149,037 ALF spectra were collected during the survey
(Cowley, 2000a).
The ALF anomalies in this paper are presented as a map
of the relative intensity of the anomalies (to background).
This data format, which was produced during reprocessing
of the original ALF survey using the ALF Explorere
software system (Cowley, 2000a), is constructed from all of
the ALF spectra acquired by determining the peak-area
ratios (of anomaly to background). This output can be scaled
so as to include only anomalies of high confidence (high
amplitude); this approach effectively eliminates most or all
of the potential ‘false-positive’ anomalies.
Very few studies discussing the results of ALF surveys
around the Australian margin are available in the public
literature. Exceptions are the work of Martin and Cawley
G.W. O’Brien et al. / Marine and Petroleum Geology 22 (2005) 517–549 523
(1991); Bishop and O’Brien (1998), (1998a) and (2002a),
who described the results of several ALF surveys on the
North West Shelf.
4. Results and discussion
4.1. Seismic interpretation
Two key factors need to be considered when using
seismic data to understand the related issues of hydrocarbon
migration and seepage (e.g. Cowley and O’Brien, 2000) on
the Yampi Shelf. These are:
†
Fig
are
the regional seal thickness; and
†
the distribution, both laterally and vertically, of seepageindicators, such as seismic amplitude anomalies, gas
chimneys/HRDZs.
Neogene fault and trap reactivation, which has resulted in
prolific palaeo- and present day seepage within the
Mesozoic section of the Bonaparte Basin to the north of
the study area, is minimal to absent across the Yampi Shelf,
and hence is not considered to be an important process in
localising seepage.
4.1.1. Regional seal thickness
Given the inboard location of the Yampi Shelf, and the
lack of Neogene fault reactivation, seal integrity issues are
more likely to be related principally to top seal capacity as
the seal thins and becomes sandier marginward.
. 5. The thickness of Early Cretaceous sealing units (derived from well data: f
as of thick seal (O200 m) are dark coloured.
The principal seals on the Yampi Shelf are Early
Cretaceous, principally late Aptian to Albian; outside the
study area, sealing units as young as Turonian can be locally
important. The seals are difficult to map seismically and
contain sandy intervals that could potentially act as ‘thief’
zones or cause the seal to fail totally.
An overview of the distribution of the regional seal in the
area, based upon well intersections, is shown on Fig. 5.
Overall, within the Browse Basin, the seal thins both to the
north and north-east and becomes much sandier within the
inboard parts of the Yampi Shelf. The north-west trending
Bonaparte-Browse Transition Zone is also an area of
characteristically thin seals. In the Browse Basin, more
basinward wells such as Asterias-1 and Echuca Shoals-1
have very thick sealing facies, whereas the sealing facies
over the Cornea field can be quite sandy (Ingram et al.,
2000). Similarly, within the nearby Londonderry-1 well
(Figs. 1 and 2), the sealing unit is thin and inter-bedded with
sands; further east, the effective seal pinches out altogether.
Clearly, the thin and sandy nature of the seals within the
northern part of the Yampi Shelf suggests that top seal
capacity may represent a key exploration risk in this region.
The Gwydion field, located to the southwest (Spry and
Ward, 1997), has a thicker sealing facies than that present
over the Cornea field, and would appear, therefore, to be less
likely to leak.
4.1.2. Distribution of seismic amplitude anomalies, gas
chimneys/HRDZs
The YST seismic grid was interpreted and the locations
of prominent seismic amplitude anomalies, gas chimneys
or interval w95–115 Ma). Areas of thin seals (!50 m) are light coloured,
G.W. O’Brien et al. / Marine and Petroleum Geology 22 (2005) 517–549524
and HRDZs mapped across the area. The results are
summarised in Fig. 3.
‘Deep’ amplitude anomalies (O600 ms TWT) are
present on many of the lines. Particularly prominent
anomalies are present within the Aptian and older sequences
over the Gwydion accumulation (Fig. 6a) on YST line
165-03 (Fig. 3), which subsequent amplitude-versus-offset
modelling (Spry and Ward, 1997) has shown to be due to
gas-saturated sands. Flat spots and gas-related, push-down
effects are also present over Gwydion. Numerous seismic
amplitude anomalies are distributed within the pre-
Callovian and Cretaceous sequences between the Gwydion
structure and YST Line 165-11, northeast of the London-
derry-1 well (Fig. 3). Prominent amplitude effects and gas
chimneys/HRDZs are present on lines YST 165-07 and -08,
along the Cornea trend. Further to the northeast, however
(YST Lines 165-12-20), these deeper amplitude effects are
rare to absent (Fig. 3), perhaps indicating that gas-saturated
sands are largely absent from this area.
Fig. 6. Amplitude anomalies on the Yampi Shelf. (a) Deep amplitude anomalies be
(YST Line 165-03). (b) Shallow seismic amplitude anomalies on the Yampi She
Shallow (!600 ms TWT) seismic amplitude anomalies
are absent in the southern part of the study area (YST Lines
165-03 to -05), but are very common to the north, between
lines YST 165-06 to -11. In some cases, the amplitude
anomalies are present both within the section and also at the
seafloor (Fig. 6b). These seafloor anomalies may indicate
the presence of active gas seepage and attendant hydro-
carbon oxidation-authigenic carbonate precipitation at the
sediment–water interface. Throughout this area, the distri-
bution and abundance of the anomalies are correlated
positively with those of the deeper anomalies—they are
present where the deeper anomalies are present-though the
reverse is not necessarily true. The shallower anomalies are
completely absent northeast of line YST 165-11 (Fig. 3).
High velocity zones, similar to the HRDZs described by
O’Brien and Woods (1995) in the Vulcan Sub-basin, are
present on four of the 2D lines. These HRDZs are grouped
around the Cornea-Londonderry trends, and are typically
associated with clusterings of shallow amplitude anomalies
tween w650–850 ms associated with the Gwydion oil and gas accumulation
lf (YST Line 165-06).
Fig. 7. Gas chimneys developed over landward-dipping basement blocks
near the Londonderry-1 well and the Cornea trend, Yampi Shelf.
Fig. 8. Distribution of mapped gas chimneys/HRDZs over the Cornea field
(outlined) on the Yampi Shelf, Browse Basin, posted over basement, which
shallows significantly from west to east. Group 1 represents gas chimneys
associated with Cornea field; Group 2 with leakage at edge of effective top
seal (for gas). Shell 3D seismic data used in mapping, with total area
covered of survey shown. No mapping was possible in north-east part of
survey area due to data corruption.
Fig. 9. Gas chimneys over the Cornea trend seen on 3D seismic data. The
chimney, which does not reach the seafloor, has significant velocity pull-up
associated with it (Shell 3D line XLN-3751).
G.W. O’Brien et al. / Marine and Petroleum Geology 22 (2005) 517–549 525
and occur above gas chimneys (Fig. 3). One HRDZ was
located directly over the Cornea trend on line YST 165-08,
up-dip from the Londonderry-1 well. Again, no HRDZs are
present within the northeast part of the area (lines YST
165-12 to -20).
Spectacular gas chimneys are seen on line YST 165-08
(Fig. 7). These chimneys are associated with both the
Londonderry and Cornea trends (O’Brien et al., 1998b) and,
in general, are closely associated with the HRDZs. The gas
chimneys appear to be developed where the regional seal
thins onto the highest parts of landward-dipping tilt or
basement blocks. At these locations, seal capacity may be
reduced by the combination of a thinner and progressively
more sand-prone seal, which favours capillary failure and
break-through of the gas. These chimneys also appear to
control the distribution of small, but prominent, seismic
amplitude anomalies within the shallow section.
Laboratory mercury-air capillary pressure data acquired
at the base of the Albian sealing shale in the Cornea South-1
well (Ingram et al., 2000) indicated that the seal there could
support a maximum 55 m column of gas, or a 157 m oil-
only column (228 API oil saturated with gas). The closure in
the Cornea field (which is filled to spill) is greater than the
calculated maximum supportable gas column height
(Ingram et al., 2000) and thus it is likely that the seal over
the field is failing continuously.
The gas chimneys/HRDZs were also mapped on the Shell
Cornea 3D data-set and their distribution is shown on Fig. 8;
basement structure is also shown. The hydrocarbon
chimneys/HRDZs range in size from 0.13 to 7 km2, with
the average size being approximately 1.3 km2. The mapped
gas chimneys are located in two distinct areas.
The first grouping (Group 1) quite accurately defines the
Cornea field itself and extends along strike for about 21 km.
Over the field, two of the gas chimneys are large and form
chimneys which extend for well over 4 km. Several other
chimneys are present; these are more typically in the range
1–2 km long or wide (Fig. 8). Some of the chimneys,
notably the largest chimney, which is located at the
southwestern end of the field (Fig. 8), lies right on
the seaward limit of the field, whereas a couple lie just
outside mapped closure further to the northwest. The
chimneys/HRDZs are typically located where the seal
thins onto a shallowing basement (Fig. 9), suggesting that
the seal may be becoming thinner and sandier near the apex
of the basement blocks. The influence of localised fracturing
around the apices of basement blocks may also contribute.
Significantly, the majority of the chimneys mapped over the
Cornea field do not reach the seafloor. Brightening seismic
amplitudes were often associated with the gas chimneys,
probably because of the gas-charging of shallow adjacent
sands. It is uncertain as to why the locations of the chimneys
are biased to the northwestern flank of the field, with
virtually none occurring on the southeastern flank. The fact
that some of the chimneys lie immediately outside closure
might suggest that the accumulation was previously slightly
larger and has since leaked; probably via seepage up
Fig. 10. Gas chimneys with associated seafloor build-up and possible
pockmark, on Shell 3D seismic line INL-1697.
G.W. O’Brien et al. / Marine and Petroleum Geology 22 (2005) 517–549526
the chimneys. Ingram et al. (2000) have reported, however,
that the Cornea field is now filled to spill, which seems at
odds with the distribution of the chimneys outside closure.
The second group (Group 2) of chimneys/HRDZs, which
appears to consist of two northeast trending sub-groups, is
located approximately 15–20 km east of the Cornea trend.
Both of the sub-groups extend for about 14 km along strike.
These Group 2 chimneys are somewhat more numerous than
those over the field itself (Fig. 8), perhaps suggesting more
pervasive seepage through a poorer top seal, and tend to
occur where the regional seal pinches out onto, or onlaps,
basement. These chimneys typically range between 500 and
2500 m in length and, in contrast to the grouping over the
field, often reach the seafloor.
There was only occasional evidence of pockmarks on the
Cornea 3D seismic data; seafloor (carbonate) build-ups were
also rare, but were somewhat more common than pock-
marks. Fig. 10 shows an example of where an interpreted
pockmark is present on the same line as a build-up, which
has formed directly over a chimney; another pockmark
appears to be present on seismic line YST 165-07 (Fig. 14b).
The absence of pockmarks may be due to the fact that the
water depths over the Cornea survey area are relatively
shallow (!100 m) and the sediments in the region are
coarse grained and high energy, which inhibits pockmark
formation (Hovland and Judd, 1988; Judd, 2001). The build-
ups may be similar to those described in the North Sea by
Hovland et al. (1994).
4.1.3. Summary of seismic observations
Overall, the regional 2D seismic data reveal that the
deeper amplitude anomalies are much more common in
the central and southern part of the study area, between the
Gwydion and Londonderry-Cornea areas (YST 165-03 to
-11), than in the northern part (YST 165-12 to -20).
Secondly, shallow amplitude anomalies are much more
common in the central part of the survey area, between the
Rob Roy-1, Londonderry-1 and Cornea-1 wells, than in the
south near Gwydion-1. Gas chimneys and HRDZs are only
present through areas where shallow amplitude anomalies
are also seen. These differences appear to be related, at a
first-order, to the thickness of the regional Cretaceous
sealing unit. Where the seal is relatively thick, as is the case
around Gwydion-1, or in more basinal areas, no shallow
seismic anomalies are present. Further north, the clustering
of shallow amplitude effects, gas chimneys, and HRDZs
(Fig. 3) present around the Londonderry and Cornea wells is
located where the regional seal thins rapidly onto the basin
margin (O’Brien et al., 1996a, 2000).
The area bulls-eyed by mapping the shallow and deep
anomalies on the 2D data relates specifically to the general
location of the Cornea oil and gas field, which lends
significant weight to the assertion that these seismic
anomalies are related to hydrocarbon migration and/or
seepage. It also demonstrates that regional chimney
mapping provides a robust framework with which to high-
grade areas for exploration. Detailed chimney mapping
using the 3D seismic data over and around the Cornea field
has shown that the distribution of gas chimneys/HRDZs
accurately defines the dimensions of the field (Group 1
chimneys)—and hence the field is actively leaking gas—as
well as defining another area, inboard from the field, where
the regional seal onlaps basement (the Group 2 chimneys).
An argument can be made that areas where chimneys are
absent over the Cornea field are areas with superior seal
capacity.
These observations strengthen the assertion that seismic
chimney mapping can allow the development of concepts
that can be tested by a range of geochemical remote sensing
tools. The data suggest that any remote sensing geochemical
programs would most likely be successful around the
Londonderry-Cornea area, for here it appears that relatively
thin seals are facilitating hydrocarbon seepage.
4.2. SAR results
The principal results of the SAR interpretation over the
Yampi Shelf are shown in Fig. 11a and b. These figures
show the interpreted seepage slicks posted on the regional
bathymetry (Fig. 11a) and regional seal thickness (Fig. 11b).
The principal concentration of slicks interpreted from
SAR data (Fig. 11a) on the Yampi Shelf is actually located
around the edge of the basin. A large, almost continuous,
clustering of slicks is located between 25 and 70 km inboard
from the Cornea field; in total, this group extends for
approximately 80–90 km on a broad, north-east to south-
west azimuth. The slicks in this cluster are typically linear to
cuspate in shape and 500–5000 m long. This clustering of
slicks is positioned over and along a significant bathymetric
break, which is located in water depths of between about 60
and 75 m. The location of this break appears to approximate
the position of the edge of effective regional seal (O’Brien
et al., 1998b, 2000), as is evident from Fig. 11b and also
seismic data (e.g., Fig. 14c). The distribution of slicks
within this broad clustering is interesting: slicks located
almost due east of the Cornea field are densely clustered and
are principally Rank 2, whereas those located southeast of
Cornea are much more sparse and are Rank 3.
Fig. 11. (a) Seepage slicks in the northern Browse Basin-Yampi Shelf region mapped from SAR. Footprints of RadarSat Wide Beam Mode scenes are
indicated. Rank 2 slicks are relatively intense and are of higher confidence and Rank 3 slicks are typically smaller, less intense, and of lower confidence. (b)
Seepage slicks in the Bonaparte Basin mapped from SAR, plotted on thickness of Early Cretaceous sealing units (derived from well data; w115–95 Ma). Areas
of thin seals (!50 m) are red, areas of thick seal (O200 m) are blue.
G.W. O’Brien et al. / Marine and Petroleum Geology 22 (2005) 517–549 527
Another broadly northeast trending cluster of Rank 2
slicks occurs 15–20 km east of the main cluster described
above, and is also located directly inboard from the Cornea
oil field. Unlike the first group, this cluster does not appear
to be associated directly with any resolvable seafloor
bathymetric features or sub-seafloor geology. Other smaller
clusters of Rank 2, and to a lesser extent Rank 3, slicks occur
east to east—southeast of the main cluster; some of these
slicks are associated with bathymetric features whereas
others are not.
No seepage slicks were detected over or immediately
around the Cornea oil and gas field using SAR, in spite of
the fact that Cornea contains a very significant amount of
hydrocarbons (Ingram et al., 2000), and both the seismic
chimney mapping and the measured low top seal capacity
suggest that leakage through the top seal over the field is
G.W. O’Brien et al. / Marine and Petroleum Geology 22 (2005) 517–549528
presently occurring. Moreover, the oil in Cornea would
actually be expected to induce a clear SAR response
because it has a low (18–228) API gravity (Ingram et al.,
2000) and would persist for extended periods at the sea’s
surface. One explanation for the lack of mapped slicks over
the field could be that the seepage of liquid hydrocarbons
from the Cornea field is occurring at rates, and in volumes,
too low to produce slicks which can be detected using SAR,
that is, any slicks present over the field are smaller than
about 120 m long.
The preferred interpretation of the SAR data over and
inboard from Cornea is that the apparent extensive seepage
along the inboard edge of the basin is due principally to oil
which is spilling from the Cornea field (driven by active gas
flushing) and is then migrating and leaking at the edge of the
effective regional top seal. Of the five scenes acquired and
interpreted, seepage slicks are present in significant
numbers along the edge of the basin on only two scenes.
This suggests that the seepage along the edge of seal is
episodic, with seepage being active for only about 40% of
the time. Fig. 12 combines two SAR scenes acquired several
months apart in 1998. Whilst many of the seeps repeat, there
are many more slicks on one scene (slicks coloured yellow)
than on the other (slicks coloured red). The slicks located
inboard from the Cornea oil and gas field are particularly
clear on the SAR data, which could be due to the fact that
the oil in the Cornea field, and probably from the region in
general, is heavy and hence the resulting slicks are relatively
thick and persistent.
The other seepage slicks mapped in the area are scattered
both inboard of, and outboard from, the edge of the basin.
Slicks are present around several of the carbonate shoals, for
example the Heywood Shoals, indicating the some of these
Fig. 12. Composite of seepage slicks derived from two SAR scenes (yellow
versus red) acquired on different dates in 1998. There is much more seepage
on one scene (slicks coloured yellow) than on the other (slicks coloured
red). Location of Cornea oil and gas field indicated.
may possibly have originally formed over hydrocarbon
seeps (O’Brien et al., 2002b). A number of slicks are also
present over the fault system along which the Heywood-1
well was drilled (Fig. 11a). This clustering may be related to
minor fault reactivation and seal failure along this major
fault system. Another clustering of slicks is located along
the Bonaparte-Browse Transition Zone, though the signifi-
cance of these slicks has been discussed elsewhere by
O’Brien et al. (2003).
The distribution of the thick Early Cretaceous depocentre
near the Brewster-1 well, outboard from the Cornea field, is
highlighted in Fig. 11b. Much of the oil and gas reservoired
within the Cornea field, and found within the wider area,
was probably generated from mature source rocks in this
depocentre and migrated up to the flanking regions.
A very limited side-scan sonar investigation (using RV
Franklin in 1999) of an area covering only part of the zone
of most intense SAR seepage slicks (i.e. at the zero edge of
the regional top seal inboard from Cornea) revealed the
presence of clusters of topographically negative, circular
features on the seafloor, at approximately latitude
13.80418S, longitude 124.95388E (Fig. 13). They were
clustered into groups that were approximately 50–150 m
across, with individual features being about 5–20 m across.
These features were previously described by O’Brien et al.
(2002b), who incorrectly identified them as topographically
positive features, possibly chemo-synthetic mounds or
build-ups which could have been living on the seeping
liquid hydrocarbons (Sassen et al., 1993). Whilst they have
not yet been sampled successfully, the mostly likely
explanation is that these features are actually pockmarks,
perhaps similar to those seen on the 3D seismic data.
Fig. 13. Side-scan sonar record of seafloor through region of oil seepage at
end of line 165-09 (see Fig. 14d). Features present appear to be pockmarks
at location 13.80418S, longitude 124.95288E.
G.W. O’Brien et al. / Marine and Petroleum Geology 22 (2005) 517–549 529
This part of the margin is essentially a ‘shaved shelf’ (James
et al., 1994), which is swept by strong tidal and other
currents, as well as being affected by intermittent tropical
cyclones. As a consequence, the seafloor sediments in this
immediate area are typically relatively coarse grained
(O’Brien, unpublished data), rather than being of the finer
grained type that favours pockmark preservation (Judd,
2001). The high energy conditions on the shelf suggest that
any pockmarks which formed in this area would be rapidly
filled in and/or destroyed—and hence their presence on the
seafloor should be quite transitory. Consequently, if the
features seen on Fig. 13 are indeed pockmarks, then it would
indicate that the fluid/gas escape processes which formed
them must be quite active and that they are being
continuously renewed.
The slicks mapped in the Yampi Shelf region have been
interpreted, based upon established criteria, to be most
likely due to the expression of natural hydrocarbon seepage
at the sea surface. Two features do, however, stand out
about the distribution of the mapped slicks in the area. The
first is that one set of slicks, the interpreted edge-of-seal
slicks east of Cornea, is preferentially associated with a
bathymetric feature, while a second set is localised around
the peripheries of some of the carbonate reefs and shoals.
Fig. 14. Water column geochemical sniffer profiles (methane) overlain on regiona
165-07. (c) Line YST 165-08. (d) Line YST 165-09. (e) Line YST 165-10. (f) Li
It might be possible, for example, that the first set of slicks
could have formed as a result of processes such as laminar
flow over seafloor topography, perhaps driven by tidal
forces. Similarly, coral spawning might be a process that
could produce slicks, though not hydrocarbon slicks, around
and more particularly over, carbonate reefs and shoals. Such
processes were considered during the present study, but
have been essentially discounted.
In the case of the first set of slicks, there are several
reasons why a natural seepage origin is favoured. Firstly, the
location of the slicks appears to be geologically controlled
by the combination of a major down-dip source of
hydrocarbons (the Cornea field and the source depocentre)
and the pinching out of the regional top seal. Secondly and
importantly, there are other clusters of slicks east and
southeast of Cornea that appears to be completely unrelated
to seafloor bathymetry. Thirdly, the bathymetric headland
east of Cornea is only one of several over which SAR data
were acquired during the wider study of the Yampi Shelf
and surrounds. Nevertheless, the region inboard from
Cornea was the only area in which apparently massive
seepage was observed, which is consistent with the slicks
having a petroleum geological, rather than oceanographic,
origin. Fourthly, the slicks are of a size and shape which is
l seismic lines from the Yampi Shelf: (a) Line YST 165-03. (b) Line YST
ne YST 165-11. (g) Line YST 165-12.
Fig. 14 (continued)
G.W. O’Brien et al. / Marine and Petroleum Geology 22 (2005) 517–549530
consistent with them having formed as a result of natural
seepage. Finally, thermogenic hydrocarbons were detected
at the sea surface through the area of these slicks (discussion
follows), as were apparent seafloor pockmarks on side-scan
sonar. Nevertheless, a non-seepage origin for these slicks
cannot be discounted, though it is not favoured: more work
to confirm the origin of the slicks seen on the SAR data
would be useful.
A natural seepage origin is also favoured for the second
set of slicks, which is developed preferentially around the
reefs and banks. Examination of records for coral
spawnings in the region (Dr Andrew Heyward, Australian
Institute of Marine Science, personal communication,
2004) has demonstrated that virtually none of the slicks
could have formed as a result of coral spawning processes.
Most of the relevant SAR scenes were acquired in April
2004 (one in October 2004), whereas the principal
spawning event takes place in February to early to mid-
March, with a much lesser event in November. In addition,
spawning is a regional phenomenon, which occurs at
the same time (for a few days) all along the North-West
Shelf and into Indonesia. The fact that slicks are present on
SAR scenes with varying dates, and that their distribution
is actually patchy over reefs and banks within even the
same SAR scene, would seem to indicate that spawning
cannot be a significant contributor to the slicks on these
scenes. Finally, coral spawning might be expected to
produce an abundance of slicks over, as well as around the
edges of, the banks, but this is not observed. Other
supporting factors for a seepage origin include the fact that
many of the banks/reefs that have slicks around them have
been drilled and they typically had strong shows/residual
columns at reservoir level, as well as abundant gas
chimneys on seismic data; some even had abundant
thermogenic hydrocarbons in the seafloor sediments
located directly under the slicks (O’Brien et al., 2003). If
these slicks are not seepage-related, then it would seem
much more likely that they are due to laminar flow or other
topography-related flow processes around the reef and
banks, rather than coral spawning.
Fig. 14 (continued)
G.W. O’Brien et al. / Marine and Petroleum Geology 22 (2005) 517–549 531
4.3. Water column geochemical sniffer (WaSi) program
A targeted water column geochemical sniffer program
was carried out over and around the trends tested by the
Londonderry and Cornea wells, along pre-existing seismic
lines, to test the migration/seepage model derived from the
seismic interpretation. A single line was also run over the
Gwydion-1 well location. The locations of these lines are
indicated on Fig. 3. The premise being tested was that
hydrocarbons would be present in anomalous concen-
trations in the water column in areas with thin seals and
abundant shallow amplitude anomalies.
The results of the program are summarised below. In
addition to the sniffer data, the approximate locations of any
(SAR) seepage slicks (see previous discussion) and/or
clusters of ALF fluors (see Section 4.3.1) are posted on the
seismic lines (Fig. 14a–g).
4.3.1. Gwydion area
No significant water column anomalies were detected in
the 340 km of acquisition over and around the Gwydion
field. Background methane values of approximately 4 ppm
methane and 0.016–0.018 ppm ethane were measured
throughout the region, suggesting that minimal amounts of
thermogenic hydrocarbons are migrating to the seafloor at
the present day.
An overlay of the sniffer profile on seismic line YST
165-03 is shown on Fig. 14a. The prominent (O600 ms)
amplitude anomaly associated with the Gwydion-1 well is
clearly seen, although no hydrocarbon anomalies were
present over this accumulation within the water column.
Significantly, no chimneys extend to, or near, the seafloor
along the line covered by the sniffer data.
4.3.2. Londonderry-Cornea area
The 390 km of sniffer acquisition through this region
revealed the presence of areally extensive gas seepage. This
seepage extended from around the Cornea trend on YST
165-08 (inboard from the Londonderry-1 well) for approxi-
mately 25 km to the east—southeast of Londonderry-1 and
Cornea-1 (Fig. 15). The most intense seepage is located
inboard of most of the mapped amplitude anomalies, gas
chimneys, and HRDZs (Fig. 15). An area of particularly
intense seepage extends over an area 5–6 km across, with
methane values peaking at 300 ppm (75–100 times back-
ground). Ethane peaked at over 2 ppm in the same area.
Fig. 14 (continued)
G.W. O’Brien et al. / Marine and Petroleum Geology 22 (2005) 517–549532
Overall, the area with greater than 5 times background gas
extends for approximately 750–800 square kilometres; the
region with greater than 20 times background covers over
200 square kilometres.
The composition of the seep gas is dry, averaging about
0.8% wet gas ð% wetnessZ ½fC2KC4g=fC1KC4g!100�Þ
over the full range of methane concentrations measured
(Fig. 16a). The carbon isotopic composition of the sniffer
gases was analysed at two locations in the area with the
highest gas concentrations. These analyses yielded d13C
ratios of K42.36 and K42.53. Given the molecular
composition of the seep gases (i.e. the high ethane/ethylene
ratios; Kvenvolden and Redden (1980)) and the d13C ratios
of the methane (average K42.45), the seep gases appear to
be almost entirely of thermogenic, rather than biogenic,
origin. The fact that the hydrocarbon wetness did not
increase with progressively increasing methane concen-
trations indicates that the seep gases is sourced by a gas-
prone, or perhaps overmature source, rather than an oil-
prone source. A plot of the concentration of ethane and
propane versus methane for the seep gases (Fig. 16b) shows
that there is a simple linear relationship between the
methane concentration and the concentration of ethane and
propane, which suggests that there is probably just one
source for the gas within the seeps on Australia’s Yampi
Shelf.
The composition of the seep gas (0.8% wet, d13CZK42.45) is remarkably similar to that of gas recovered from
the reservoir section within the Cornea-1 well (2.2% wet,
d13CZK40.60; Shell, personal communication, 1999). As
such, it is likely that the reservoir gas in the Cornea Field and
the seep gas have been generated from the same source rock.
This source rock is probably of older and perhaps more
thermally mature than the Valanginian source rocks that
generated the oil reservoired in Cornea-1 (Blevin et al.,
1998). It is also likely that both source rocks are continuing
to generate hydrocarbons at the present day (Spry and Ward,
1997; Blevin et al., 1998).
The sniffer data from the Londonderry-Cornea area are
overlain over the YST seismic lines in Fig. 14b–g. These
overlays allow a direct comparison between the underlying
geology and the position and intensity of the seeps within
the water column.
On line YST 165-07, the regional seal (approximately
the Late Aptian to Turonian interval) thins sharply onto
the basin margin near Cornea-1 (Fig. 14b). In spite of this
thin seal and the fact that Cornea-1 intersected a large
amount of hydrocarbons (Ingram et al., 2000), no
significant methane anomalies were detected in the bottom
water above or near the field. Overall, background levels
of methane were between 4 and almost 5 ppm along line
YST 165-07, compared to typical background concen-
trations of 3.5–4 ppm. Significant disturbances of the
seafloor topography are present directly over the Cornea
field (Fig. 14b) and also to its southeast. These
disturbances have the appearance of large pockmarks,
though the absence of water column hydrocarbon
anomalies associated with them indicates that they were
Fig. 14 (continued)
G.W. O’Brien et al. / Marine and Petroleum Geology 22 (2005) 517–549 533
not venting significant amounts of gas at the time of the
sniffer survey.
This highlights that both the gas seepage and the oil
seepage in this region can be quite episodic, spatially and
temporally. This episodic nature could be due to a vast array
of often poorly understood processes, which include earth
tides, changes in the hydrostatic head over the seepage vents
(associated with tidal cycles), buoyancy-driven top seal
failure (due to increasing hydrocarbon column heights at
depth), and the stress-state of the crust.
A distinct series of areally restricted bottom water seeps
with well-defined shapes were detected along line YST 165-
08, which traverses the Londonderry and Cornea trends
(Fig. 14c). These anomalies are present where the regional
seal thins onto the margin, with the most prominent located
directly above seismically prominent gas chimneys. The
sniffer data reveal that these chimneys are associated with
water column anomalies which are only 2–5 times back-
ground, up to a maximum of approximately 17 ppm
methane. Consequently, whilst these chimneys appear as
strong events on seismic data (Fig. 7), the total amount of
hydrocarbons passing through them to the seafloor may be
small, at least at the present day. It appears, therefore, that
the seismic response is not a good predictor of the total flux
of hydrocarbons through a given chimney leakage system.
There is no evidence within the sniffer data for the seepage
of wet gases, even directly over the Cornea oil accumu-
lation. Dry (!1% wet) gas is by far the dominant seep gas,
perhaps because of its high relative mobility (through the
top seal), particularly when compared to the heavy,
biodegraded oil (APIZ18–228) that is present within the
Cornea field (Ingram et al., 2000). The chimneys seen on 2D
seismic line YST 165-08, and which have active gas
seepage associated with them, actually correspond to some
Fig. 14 (continued)
G.W. O’Brien et al. / Marine and Petroleum Geology 22 (2005) 517–549534
of the Group 1 chimneys observed on the 3D seismic data
(Fig. 8).
By far the greatest amount of methane and wet gas
seepage, both geographically and in terms of concentration,
is present along seismic line YST 165-09 (Fig. 14d). There,
methane concentrations within the bottom waters increase
progressively from background levels of 3–4 ppm to a
maximum of 300 ppm methane near shot-point 2000. The
increase in the methane concentrations between shot-points
3000 and 2500 mirrors the thinning of the regional seal. A
rapid and massive increase in seepage was detected
associated with where the top seal pinches out against a
prominent basement high at approximately shot-point 2200.
Marginward of this bald basement high, the gas concen-
trations of the bottom waters show an exponential decrease
and fall rapidly towards background concentrations. This
region of rapidly decreasing methane in the bottom waters
corresponds broadly to the interpreted pinch-out edge of the
regional seal.
On seismic line YST 165-09, the area of focussed,
massive gas seepage above the basement high is expressed
principally as a zone of moderately poor reflection
coherency and attendant lack of continuity—a seismic
whiteout. Well-defined gas chimneys or amplitude effects
are only occasionally discernable on the 2D seismic data
(usually outboard from the most intense seepage), in spite of
the clear indications from the sniffer data that the area is one
of very strong hydrocarbon seepage through the sequences
above the bald high. The most reliable diagnostic signature
of the very intense seepage on the 2D seismic data appears
to be a very prominent seafloor amplitude anomaly
(Figs. 14(d) and 17). This anomaly coincides precisely
with the location of the most intense seepage and probably
relates to enhanced biological activity and carbonate
Fig. 14 (continued)
G.W. O’Brien et al. / Marine and Petroleum Geology 22 (2005) 517–549 535
cementation associated with oxidation of seep gases at or
near the seafloor, in a manner similar to that described by
Hovland et al. (1987) and Judd (2001). If the high seafloor
amplitudes seen along line YST 165-09 are due to the
presence of seep-related carbonate hard-grounds, then the
processes responsible for their formation may also be
analogous to those documented for HRDZ formation in the
sub-surface.
Organic geochemical analyses were carried out on
several surficial seafloor sediment samples that were
collected in 1999, using grab sampling, over the most
intense part of the gas seep on seismic line YST 165-09
(latitude 13.726008S, longitude 124.759558E, 85 m water
depth, 0.5% total organic carbon). The surficial, carbonate-
rich (91% CaCO3) sediments associated with the most
prolific gas seeps on the Yampi Shelf contained molecular
evidence for the presence of both aerobic and anaerobic
methane-oxidising microbial communities (Summons,
unpublished results). The aerobic processes were revealed
by the presence of diagnostic hopanoids, while anaerobic,
methane-oxidising consortia were revealed through their
gylcerol ether signature lipids, including archaeol and
glycerol monoethers similar to those found by Hinrichs
et al. (2001) in offshore California basins.
Over the last 300,000 years, this part of the shelf, which
is at water depths of 80–100 m, would have been sub-
aerially exposed as a result of eustatic sea-level variations
for about 30–50% of the time. Consequently, these gas seeps
have alternated regularly between submarine and sub-aerial
environments, with significant implications for the type of
diagenetic and bio-geochemical processes that may have
taken place within them.
Line YST 165-10 is an example of the significant control
that basement topography exerts on the distribution of
Fig. 15. Contour map of methane in bottom waters in the Londonderry-Cornea area. Massive gas seepage present through, and inboard from, the region
containing numerous shallow amplitude anomalies and HRDZs.
G.W. O’Brien et al. / Marine and Petroleum Geology 22 (2005) 517–549536
hydrocarbon seepage across the Yampi Shelf (Fig. 14e).
Four localised seeps, which vary from about 18 to 27 ppm
methane (5–7 times background) occur directly over
topographically prominent basement highs. The regional
seal either thins significantly, or is absent altogether, over
these highs. As such, these highs appear to act as
hydrocarbon catchments around which seepage is focussed.
Hydrocarbon concentrations within the bottom waters
drop rapidly between seismic lines YST 165-10 and -11
(Fig. 14f). Methane is consistently 1.5–2 times background
along the line and a very weak hydrocarbon anomaly
appears to be localised over a prominent basement high that
pierces through the regional seal.
Line YST 165-12 shows only minor gas seepage in the
bottom water data (Fig. 14g) and appears to mark the
northern limit of active seepage in this area. Significantly,
shallow seismic amplitude anomalies are also virtually
absent north of this line (Figs. 3 and 15).
The second clustering of gas chimneys which was
mapped on the 3D seismic data (Group 2 in Fig. 8) appears
to correlate very closely with the area of strong gas seepage
identified on the sniffer data inboard from Cornea, on
seismic lines YST 165-09 and -10. Clearly, chimney
mapping using the 3D seismic data can provide a very
reliable indication of where seafloor seepage is likely to be
active. Interestingly, this clustering of chimneys consists of
two sub-groups. The first of these sub-groups is located
approximately 2.5 km northwest of the highest methane
concentrations measured in the water column, whereas the
second sub-group is located about 7.5 km northwest of the
highest concentrations. Why the chimneys and water
column anomalies appear to be offset is unclear, though it
could be that the (probably) small-to-moderate bottom
water ‘kicks’ associated with these chimneys are simply
swamped by the very large seepage inputs coming from the
area of the bald basement high—that is, the signal from
these chimneys is overwhelmed.
4.3.3. Summary of sniffer observations
Water column geochemical sniffer data have identified
characteristic styles of hydrocarbon seepage into the bottom
waters of the Yampi Shelf.
Where the regional seal is thick, such as around the
Gwydion-1 well on the southern Yampi Shelf, or in
the more basinal areas, seep-related hydrocarbons within
the bottom waters tend to be either absent or occur at very
low levels.
In contrast, in the Londonderry-Cornea area, the gas
seepage signal ranges from weak to very strong (5–
300 ppm). Again, the principal control on the amount of
Fig. 16. Geochemical cross-plots from seeps near the Londonderry and
Cornea wells on the Yampi Shelf: (a) Methane concentration versus
hydrocarbon wetness. (b) Methane concentration versus ethane and propane
concentrations.
Fig. 17. Methane concentrations in bottom waters versus seismic amplitude
(32 ms window) at seafloor along 2D seismic line YST 165-09. Seep-
related cementation at seafloor produces hard grounds and attendant high
amplitudes.
G.W. O’Brien et al. / Marine and Petroleum Geology 22 (2005) 517–549 537
seepage of hydrocarbons within the bottom waters appears
to be the thickness and quality of the regional top seal. Gas
chimneys tend to be located at or near the apices of
topographically prominent tilt blocks, probably because seal
capillary failure at that point is facilitated by thinner and
perhaps sandier sealing facies, although a contribution from
enhanced, flexurally-induced fracturing at the apices cannot
be discounted. In spite of their obvious seismic character,
the gas chimneys appear to contribute only a small amount
of hydrocarbons to the bottom waters-they may represent a
relatively minor, point source of hydrocarbon input to the
bottom waters. The greatest amounts of seepage appear to
occur around topographically prominent basement highs,
where the seal thins markedly or is absent. These highs
appear to act as hydrocarbon catchments that focus seepage
around them. As such, they represent ideal locations over
which to capture a snapshot of the hydrocarbon charge
within a particular area.
It appears that apparently low rates of hydrocarbon
seepage, such as those seen to be associated with the
chimneys, can produce very prominent seismic effects in
the shallow section. In contrast, the areas of most intense
seepage are not so easily defined seismically using
regional 2D data with conventional display parameters,
although they produce prominent amplitude anomalies at
the seafloor. Such zones of seismic whiteout and clustered
chimneys are, however, readily mapped out using 3D
seismic data.
The seeps detected in the bottom waters were invariably
composed of dry, thermogenic gas, with wet gas contents of
less than 1% being typical. Based upon regional geological
considerations, this gas was probably sourced from a gas-
prone, possibly overmature, source rock, and probably
represents an older, more mature source than that which
sourced the oil in the Gwydion and Cornea fields. No
evidence was seen in the bottom water data of the heavy,
biodegraded oil that is reservoired in Cornea (Ingram et al.,
2000). This may in part relate to the fact that gas has a much
higher relative mobility (permeability) than heavy oil and
hence can leak through the marginal sealing facies much
more easily.
Gas flushing clearly represents a key exploration
uncertainty on Australia’s Yampi Shelf. However, fault
displacements decrease to the north-east of the London-
derry-1 and Cornea wells into the basement fracture
system/relay zone which separates the Timor and Browse
compartments. This means that a natural remigration
fairway for any displaced oil exists through this area,
which is located approximately 100 km north-east of
Cornea-1 (Fig. 3). If traps are present within this zone,
Fig. 18. Locations of sea surface samples analysed by fluorimetry shown in
by circles, with two anomalous samples are highlighted by pentagons.
Table 1
Locations of surface seawater samples analysed by multi-spectral
fluorometry on the Yampi Shelf, north-western Australia. Anomalous
samples are highlighted in bold
AGSO no. Survey
sample no.
Latitude (S) Longi-
tude
Sample details
19999314 207WS020 K13.9529 124.9644 Sea water
19999315 207WS021 K13.9529 124.9644 Sea waterCazide
19999316 207WS009 K13.4592 124.0000 Sea waterCazide
19999317 Sea waterCazide
19999318 207WS024 K13.6997 124.6807 Sea water
19999319 207WS010 K13.6619 124.7950 Sea waterCazide
19999321 207WS025 K13.8336 124.5404 Sea water
19999322 207WS026 K13.7916 124.5816 Sea water
19999324 K13.7970 124.7986 Sea water
19999325 207WS022 K13.7970 124.7986 Sea water
19999326 207WS023 K13.7970 124.7986 Sea waterCazide
19999327 207WS017 K13.6951 124.7294 Sea water
19999328 207WS015 K13.4532 124.4698 Sea waterCazide
19999329 207WS016 K13.6600 124.6914 Sea waterCazide
19999330 Sea waterCazide
19999331 207WS014 K13.5595 124.6263 Sea waterCazide
19999332 207WS013 K13.7592 124.8424 Sea waterCazide
19999333 207WS018 K13.9576 124.9780 Sea waterCazide
19999334 207WS019 K13.9576 124.9780 Sea water
19999335 207WS012 K13.9147 125.0180 Sea water
19999336 Sea water
19999337 207WS008 K13.4592 124.6356 Sea water
19999338 207WS011 K13.6619 124.7950 Sea water
19999339 207WS007 K13.9267 124.9984 Sea water
G.W. O’Brien et al. / Marine and Petroleum Geology 22 (2005) 517–549538
and they have adequate top seal capacity for oil, then they
may represent attractive exploration targets. This zone
appears to be currently receiving little gas charge, based
upon a general absence of amplitude anomalies and gas
chimneys within it (Fig. 3), which may suggest that the gas
has already bled out of the system.
Significantly, comparison with the SAR data shows that
there is a virtual absence of oil slicks within the region of
strong gas seepage identified by the sniffer. The prominent
group of slicks identified along the edge of seal are located
10–15 km inboard from the most intense gas seepage. This
observation supports the premise that the oil and gas in this
region have different sources and/or different migration
histories.
It is interesting that the possible pockmarks identified on
the side-scan sonar data (Fig. 13) were spatially associated
with the zone of clustered (SAR) slicks, approximately 10–
15 km inboard from the area of most intense gas seepage. It
might be expected that pockmarks would actually be much
more common though the area of strong gas seepage, though
this does not appear (at least from the seismic data) to be the
case. Sampling of the seafloor is unusually difficult through
the area with the strongest gas seepage (O’Brien, unpub-
lished data); gravity coring is impossible and grab samples
are only intermittently successful. It seems likely that this
part of the shelf is essentially an areally extensive (perhaps
O200 sq km) hard-ground, the formation of which is related
to diagenetic cementation processes associated with the
seepage. The overall indurated nature of the seafloor
through the zone of greatest gas seepage is the probable
explanation for the lack of common pockmarks through that
area.
Almost all of the hydrocarbon (especially the gas) seeps
on the Yampi Shelf occur in shallow (!120 m) water
depths. Consequently, most of the seeps would have been
sub-aerially exposed during the last glacial maximum
(LGM), approximately 18,000 years ago and would have
been exposed regularly throughout the Quaternary. An
important outcome of this is that the hydrocarbon input to
the atmosphere from these large seeps on the Yampi Shelf
during the LGM would certainly have been much greater
than that at present. This is the result of two processes.
Firstly, the seeps would have been venting hydrocarbons
directly into the atmosphere, rather than into the water
column at the seafloor. A large percentage of the hydro-
carbons, which seep from the seafloor into the water column
are dissolved, trapped beneath the thermocline, or con-
sumed by bacteria during their residence time within the
water. As such, a significantly lower percentage of
hydrocarbons reaches the atmosphere than is vented at the
seafloor. In contrast, when seepage is sub-aerial, almost all
of the seeping hydrocarbons reach the atmosphere.
Secondly, another factor favouring greater hydrocarbon
inputs to the atmosphere during the LGM would have been
the reduction (by approximately 130–190 psi) of the
hydrostatic head over the seeps, which resulted from
the ‘unloading’ of approximately 80–125 m of seawater
above them. This reduction in hydrostatic head, of itself,
could have been sufficient to increase the flux of
hydrocarbons emanating from these seeps.
It could be that the sub-aerial exposure during glacial
low-stands of hydrocarbon seeps such as those on the Yampi
Shelf, with the attendant increase in hydrocarbon-related
(especially methane) greenhouse gas input to the atmos-
phere, might have contributed marginally to the eventual
Fig. 19. Spectral responses (for an excitation wavelength of 266 nm) of
water column samples shown in Fig. 18 and Table 1. Two anomalous
samples were detected and are highlighted. Non-anomalous water samples
plot below the spectral signature of sample 19999329 and are depicted by
thin lines.
G.W. O’Brien et al. / Marine and Petroleum Geology 22 (2005) 517–549 539
flipping of the climate system back to a warmer, inter-
glacial status.
4.3.4. Sea surface fluorimetric analyses
The 24 surface and near-surface seawater samples were
analysed by ultra-violet (UV) emission spectrometry
(Edwards and Johns, 1999) across the Yampi Shelf
(Fig. 18, Table 1). Of these samples, two samples fluoresced.
Fig. 20. Location of ALF survey and identified ALF fluors on the Yampi Shelf. Size
key wells indicated by black dots. Background is bathymetry.
Sample 19999329 (Fig. 18) had a very strong emission
spectrum that was characteristic of crude oil, with a
maximum emission wavelength at 335 nm and a maximum
intensity of 181 fluorescence units, on a scale of 0–1000.
Sample 19999324 fluoresced weakly with maxima at 356 and
461 nm. The emission spectra for these samples, for an
excitation wavelength of 266 nm, are shown in Fig. 19.
The presence of these two anomalous, surface samples
confirms the presence of seepage indicated by both the SAR
data and the bottom water sniffer data. It should be noted,
however, that these samples were collected when the sea
was rough, which minimised the chances of actually
sampling a hydrocarbon slick. It would be expected, under
normal sea states, that a higher percentage of these samples
would be anomalous in terms of their hydrocarbon contents.
4.4. Airborne laser fluorosensor program
The location of the lines acquired during the 1998 Yampi
Shelf ALF survey is shown in Fig. 20. During the survey, a
total of 2,149,037 spectra were collected at an average
spacing of 1.38–2.13 m. Of these spectra, a total of 132
fluorescence spectra were interpreted as comprising ‘confi-
dent’ fluors, yielding an average fluor density of 61 fluors per
million spectra. The fluorescence area/Raman area ratio
ranged from 1.43 to 0.14 over the 132 picked fluors. The
spectra of interpreted medium and strong fluors are shown on
Fig. 21a and b, respectively. The emission wavelengths of
of fluors is directly proportional to strength (peak over Raman). Location of
Fig. 21. Representative ALF fluors from the Yampi Shelf survey. (a) Moderate intensity fluor. (b) High intensity fluor.
G.W. O’Brien et al. / Marine and Petroleum Geology 22 (2005) 517–549540
the fluors are fairly consistent across the survey and are
typically between approximately 330 and 340 nm.
The locations of the fluors interpreted from the Yampi
Shelf survey are presented on Fig. 20. The ALF anomalies
have been scaled according to the ‘Fluor’ to Raman ratio,
with larger dots representing stronger anomalies. According
to an analysis by Cowley (2000a,b), a value of about 0.3 or
greater in this region typically constitutes a ‘strong’ or high
confidence anomaly.
The high confidence ALF anomalies are focussed along
the oil-prone Cornea trend, especially along its north-
western flank (Fig. 20). A lesser number of fluors were also
scattered along the bathymetric break which equates to the
approximate edge-of-seal in this area. This was the same
area that showed a concentration of seepage slicks on the
SAR data. Very few ALF anomalies were present near the
Cortex-1 exploration well, where the sniffer detected a large
amount of methane seepage; this may be because ALF can
only detect liquid (oil and condensate) hydrocarbons.
Possible explanations for why the ALF did not respond
strongly to any edge-of-seal seepage, and also appears to
decrease somewhat across the field itself, are as follows.
Firstly, the degree of biodegradation (Ingram et al., 2000)—
and probably the extent of water washing—increases
Fig. 22. CCD line optical camera data acquired from aircraft during
hyperspectral survey over Cornea field. Small oil slicks (w5–30 m long)
are present. Flight height approximately 80 m; pixel size approximately
2 m.
G.W. O’Brien et al. / Marine and Petroleum Geology 22 (2005) 517–549 541
progressively to the south-east across the Cornea field. This
would result in the increased depletion of the aromatic
hydrocarbons within the oil columns—and it is these
aromatics which are responsible for the fluorescence
measured by ALF in the hydrocarbons that have filled,
and then been spilled, from traps across the field. Secondly,
the seepage in the region is episodic and it may be that there
was no seepage when the ALF survey was flown.
The explanation for why ALF so dramatically identified
the charged traps in this area, whereas SAR did not, may
come from Fig. 22. The image shown of the sea surface was
taken using a downward-looking CCD optical camera
during an aircraft-based hyperspectral survey that was
acquired (for Geoscience Australia) by Fugro Airborne
Surveys over the Cornea area in 2001 (Hausknecht, 2001).
This image has been interpreted to show a series of small oil
slicks, which were typically between 5 and 50 m long,
which were detected along the north-western flank of the
Cornea field, within a couple of kilometres of Hammer-1.
These slicks are located close to some of the chimneys
mapped using the 3D seismic data and were located through
the same area as the clusters of ALF anomalies discussed
above.
The combination of the ALF data and the optical
imagery suggests that oil slicks are present over the
Cornea oil and gas field, in spite of the fact that no
seepage slicks were detected over the field using SAR.
One possibility is that only very small amounts of oil (see
Rock-Eval discussion below)—along with, and probably
transported by, greater but still minor amounts of gas—are
leaking vertically within the gas chimneys over the
Cornea trend. This gas, with associated liquids, moves
up through the top seal via the chimneys until it reaches
the seafloor, where it rises rapidly to the sea surface as
bubbles. It may be that ALF, with its very high (w1–2 m)
sample rate, can detect the resulting small pancake slicks
and lenticular slicks (which are perhaps 5–50 m long) that
form on the surface of the sea as the bubbles, and clusters
of bubbles, burst. In contrast, SAR has a much larger
sample rate or sample size (w25–30 m pixel size) and
hence can only detect slicks significantly greater than
about 100–125 m in length.
These preliminary results suggest that high (1–2 m)
resolution tools, such as ALF, or optical and hyperspectral
imagery, have great potential for evaluating prospectivity at
both regional and prospect scales. In particular, it may be
that such tools could be particularly useful for ranking traps
at a prospect scale, especially in areas with minor seal
failure.
4.5. Rock-Eval analysis of hydrocarbons in the top seal
The presence of gas chimneys, sniffer anomalies, and
a clustering of ALF fluors over the Cornea trend, as well
as seal capillary measurements, all suggest that the top
seal is failing over the field. To test this further, a series
of Rock-Eval 6 analyses were made of the top seal above
the reservoir in a number of representative wells across
the field (Table 2). The purpose of these measurements
was to determine the amount of free (migrated) liquid
hydrocarbons present in the top seal, which provides an
indirect measurement of top seal capacity—with poorer
seals typically having higher contents of migrated
hydrocarbons. The total amount of free hydrocarbons
present in the seal (S1c) was calculated as follows. The
whole rock S1 and S2 measurements (S1W and S2W) were
determined (Table 2). S1W is principally low molecular
weight hydrocarbons (!C25). An extraction of all of the
free hydrocarbons was then made from the rock
reanalysed by Rock-Eval 6. The S2 measured in this
procedure (S2E) was assumed to be the immobile kerogen
inherent in the rock. The difference between S2W and S2E
is the amount of free, higher molecular weight, migrated
hydrocarbons within the pore spaces—and this was
contributing to a large component of the whole rock
S2W analysis. Consequently, the amount of free hydro-
carbons present in the top seal can be measured by
adding the whole rock S1W and S2W, and then subtracting
S2E. This yields S1c.
The results of this study are summarised in Fig. 23 and
Table 2. The highest concentrations of migrated hydro-
carbons detected in the top seals (S1c) were found in the
Hammer-1 and Cornea South wells (up to 17.84 mg gK1).
In contrast, the lowest concentrations were detected in the
top seal in the Tear-1 and Cornea-1B wells (0.06 mg gK1).
Table 2
Rock Eval 6 analyses of top seal facies in wells from the Cornea field
Well Depth GA No. Whole rock Solvent extracted rock Calculated
Top
(m)
Base
(m)
TMAXW
(8C)
S1W
(mg/g)
S2W
(mg/g)
TMAXE
(8C)
S1E
(mg/g)
S2E
(mg/g)
TOCE
(wt%)
HIE
(mg/
gTOC)
S1C
(mg/g)
S2C
(mg/g)
PIC
TEAR 1 726 20020002 410 0.06 0.35 419 0.04 0.39 1.99 20 0.06 0.35 0.15
TEAR 1 738 20020003 403 0.14 0.6 407 0.1 0.54 1.91 28 0.2 0.54 0.27
TEAR 1 753 20020004 402 0.08 0.45 405 0.08 0.39 1.49 26 0.14 0.39 0.26
TEAR 1 759 20020005 405 0.08 0.47 416 0.07 0.66 1.86 36 0.08 0.47 0.15
TEAR 1 771 20020006 407 0.07 0.31 412 0.07 0.32 2.61 12 0.07 0.31 0.18
MACULA 1 740 20020007 317 0.57 1.43 321 0.6 1.84 3.89 47 0.57 1.43 0.29
MACULA 1 748 20020008 417 0.1 0.88 418 0.1 0.8 2.54 32 0.18 0.8 0.18
MACULA 1 757 20020009 418 0.1 0.59 420 0.1 0.58 2.6 22 0.11 0.58 0.16
MACULA 1 766 20020010 420 0.14 0.69 419 0.14 0.58 3.39 17 0.25 0.58 0.30
MACULA 1 772 20020011 316 1.27 1.39 417 0.53 1.16 3.35 35 1.5 1.16 0.56
HAMMER 1 751 20020012 433 0.28 0.7 436 0.2 0.58 2.23 26 0.4 0.58 0.41
HAMMER 1 760 20020013 332 1.98 3.47 340 1.15 2.69 2.49 108 2.76 2.69 0.51
HAMMER 1 772 20020014 323 1.44 2.41 341 0.84 2.3 2.31 99 1.55 2.3 0.40
HAMMER 1 778 20020015 416 1.32 2.74 420 0.73 1.93 2.52 77 2.13 1.93 0.52
CORNEA 1B 735 20020016 409 0.06 0.28 420 0.02 0.2 1.13 18 0.14 0.2 0.41
CORNEA 1B 760 20020017 408 0.07 0.49 419 0.05 0.42 1.72 25 0.14 0.42 0.25
CORNEA 1B 775 20020018 417 0.05 0.42 423 0.04 0.4 1.79 22 0.07 0.4 0.15
CORNEA 1B 780 20020019 407 0.03 0.15 411 0.01 0.09 0.85 10 0.09 0.09 0.50
CORNEA
SOUTH 2ST
765 20020020 434 9.88 7.79 422 0.15 0.93 1.76 53 16.74 0.93 0.95
CORNEA
SOUTH 2ST
775 20020021 435 8.21 7.79 416 0.14 0.84 1.59 53 15.16 0.84 0.95
CORNEA
SOUTH 2ST
792 20020022 432 9.66 9.67 422 0.14 1.49 1.8 83 17.84 1.49 0.92
CORNEA
SOUTH 2ST
795 20020023 425 4.31 7.55 430 0.14 1.48 1.97 75 10.38 1.48 0.88
G.W. O’Brien et al. / Marine and Petroleum Geology 22 (2005) 517–549542
Both the Cornea South and the Hammer wells are located
near major mapped gas chimneys (Fig. 8), which suggests
that the top seal capacity in these wells is relatively low and
that liquids are present within the chimneys. In contrast, both
Fig. 23. Concentration of free hydrocarbons in top seal above reservoir un
the Cornea-1B and Tear-1 wells are located a significant
distance (w3000–5000 m) from any mapped gas chimneys,
perhaps indicating that the top seal capacity in these two
wells is relatively higher. This interpretation assumes that
it in the Cornea field, Yampi Shelf, Browse Basin (S1c in Table 2).
G.W. O’Brien et al. / Marine and Petroleum Geology 22 (2005) 517–549 543
the total hydrocarbon column and phase in these respective
wells is similar, which it appears to be (Ingram et al., 2000).
The observations show that liquid hydrocarbons are
present within the gas chimneys over the Cornea field.
Given that the sniffer data showed that the gas chimneys are
currently transporting gas to the seafloor, albeit often at low
rates, a mechanism presents itself for the transportation of
liquid hydrocarbons to the seafloor and ultimately the sea
surface. The liquids are transported to the seafloor in
association with the gas in the chimneys.
The observations that liquid hydrocarbons are present in
the top seal overlying the reservoir in the Cornea field
strongly suggest that an oil charge was emplaced in the
Cornea field prior to the gas charge.
5. Integration of observations
The observations presented regarding hydrocarbon see-
page on the Yampi Shelf are significant, for exploration both
within this area and elsewhere. The application of
the respective technologies has also revealed the key
processes in relation to hydrocarbon migration and seepage
in the region.
5.1. Relative response of different technologies
Hydrocarbon seepage on the Yampi Shelf has been
identified and independently confirmed via satellite-based
SAR, water column geochemical sniffer, surface water
fluorimetry, Airborne Laser Fluorosensor (ALF), high-
resolution optical, and 2D/3D seismic data. Evidence for
failure of the seal is also supported by the presence of free
hydrocarbons within the top seal. Each technology and
approach has, however, quite different relative responses
and sensitivities to different types and rates of hydrocarbon
seepage. The results of these assorted programmes are
shown together on Fig. 24a and b respectively.
2D seismic data, for example, appear to be well suited to
the rapid and ready identification of laterally restricted
features such as gas chimneys and discrete seismic
amplitude anomalies. These features, may, however, only
be responsible for relatively small contributions to the total
seep-related hydrocarbon inventory. In contrast, zones of
more distributed seepage, such as the massive and extensive
gas seepage which produces whiteout zones, can actually
initially appear less obvious than localised chimneys on 2D
data. In general, areas of strong gas seepage were
characterised by a general lack of coherency and continuity
of seismic events in the shallow section, and the presence of
prominent amplitude anomalies at the seafloor. 3D seismic
data were useful at mapping both localised chimneys and
especially the more diffusely defined areas that were
associated with high seepage rates.
SAR appears to have detected areally extensive oil
seepage along the south-eastern edge of the basin, near
the edge of the regional Cretaceous top seal (Figs. 11a,b and
24a,b). The strong SAR response is most readily explained if
these slicks are due to a relatively heavy oil (API of 18–228)
such as that found reservoired in the Cornea field (Ingram
et al., 2000). The most intense clustering of these seepage
slicks is located well to the east (marginward) of the Cornea
field, where Rank 2 seepage slicks dominate. Further south,
the slicks are less dense and tend to be Rank 3. These
observations perhaps suggest that most of the oil leaking at
the edge of the seal has been derived from the spilling
(tertiary migration) of oil displaced from the Cornea trend by
a later gas charge, with a lesser contribution coming via
secondary migration directly from the basin source system.
The oil seeps at the edge of the seal are located approximately
10–15 km inboard of the zone of maximum dry gas seepage
detected by the sniffer (near Cortex-1) and approximately
20–50 km inboard from the Cornea field. The prominent
seepage slicks at the edge of the basin were only seen on two
of the five SAR scenes acquired, suggesting that this edge-of-
seal seepage is quite localised and/or episodic and hence
could easily be missed. Seepage slicks are relatively sparse in
more basinward areas, such as along the Cornea trend, even
though well-developed chimneys are present there. This
shows that, in this case, SAR did not detect the accumulation
directly but was responding principally to tertiary migrated
oil from the Cornea accumulation.
Seepage slicks are virtually absent over the major zones
of gas seepage detected by the sniffer (Fig. 24a and b).
Slicks are, however, scattered along the reactivated fault
near which Heywood-1 was drilled. This is the major basin
margin fault system in the area and also controls the location
of several of the major carbonate banks (Fig. 11a and b).
Sniffer data accurately mapped dry, thermogenic gas
seeps. These are focussed in areas where the regional seal
thins, thereby leading to capillary failure—with respect to
gas—of the top seal. The greatest amounts of seepage are
associated with topographically prominent basement highs
over which the regional seal thins significantly or is absent
(near Cortex-1; Fig. 24a and b). Narrow gas chimneys,
although obvious on seismic data, often actually contribute
only minimally to the seepage-related hydrocarbon inven-
tory in the area. They are typically located at the apices of
tilt blocks, again where the seal is thinnest and probably
also sandiest. Whilst minor gas seepage occurred over the
Londonderry and Cornea trends, the majority of seepage
was found well inboard of these hydrocarbon accumu-
lations, in areas where the seal onlaps or is truncated
against basement highs. Here, gas concentrations within
the bottom waters reached 100 times background. It is
likely that methane within the bottom waters exceeds 20
times background over an area of at least 200–300 square
kilometres in this region. This gas has an almost identical
composition to that present within the reservoir in the
Cornea field, although whether the gas in the seeps is
Fig. 24. Cornea region, Yampi Shelf, showing results of chimney mapping and locations of seepage slicks (Rank 2 dark purple, Rank 3 light purple), water
column sniffer lines (lines colour-coded and correspond to methane concentration in bottom waters), and Airborne Laser Fluorosensor fluors. (a) On
bathymetry. (b) On mapped basement (red is shallower, blue is deeper).
G.W. O’Brien et al. / Marine and Petroleum Geology 22 (2005) 517–549544
G.W. O’Brien et al. / Marine and Petroleum Geology 22 (2005) 517–549 545
actually derived from gas spilling from the field, or from
gas migrating around the trap, is unknown. A combination
of both processes seems likely. The seepage is facilitating
the formation of a hard seafloor (producing a high seismic
amplitude), probably via authigenic carbonate cementation
and a consortia of aerobic and anaerobic methane oxidising
bacteria have been documented in this study within the
gas-seep field.
The Airborne Laser Fluorosensor (ALF) survey data
clearly identified the oil-charged Cornea trend. ALF was,
however, less efficient at detecting the oil seepage, mapped
using SAR, at the edge of seal, although this could have
been due to the episodic nature of this seepage. ALF
essentially did not respond at all to the area of intense gas
seepage between the Cornea trend and the edge of seal, near
the Cortex-1 well location.
ALF’s ability to identify charged traps probably relates to
its very high sampling rate compared to the other tools, which
clearly indicates that most of the key (i.e. exploration-
relevant at a prospect scale) slicks in this area are localised
and small. It appears that minor amounts of liquids are
present within the gas chimneys and that gas leaking to the
sea surface provides the transport mechanism. The same
appears to be true of the high-resolution optical imagery,
which also delineated small oil slicks over the Cornea trend.
Clearly, the mapping of the gas chimneys on the seismic
data, the observations made using Rock-Eval 6 data on the
top seals, and the distribution of the slicks detected by ALF
and the optical imagery all suggest that the Cornea-
Londonderry trends are actively leaking hydrocarbons.
These leaking hydrocarbons do have a genuine liquids
component, even though the volumes of liquids that are
leaking, and hence the attendant size of the oil slicks that are
produced, appear to be too small (/120 m long) to be
detected by commercial SAR data. It appears that these oil
slicks over the Cornea field are small and typically range
from less than 5 m to about 50 m in length. These small slicks
are, however, absolutely critical in identifying the location of
the most prospective structures within a province such as the
Yampi Shelf. It is likely that these small slicks have formed
via the leakage of small amounts of oil, probably either
dissolved in gas bubbles, or as rims on the outside of gas
bubbles, which break though the seal, and then burst when
they reach the surface of the sea, to form small slicks,
probably somewhat analogous to the pancakes observed in
the Gulf of Mexico.
In relation to defining or ranking potential oil prospects,
critical inter-relationships exist among: the location of gas
chimneys (mapped using 2D and 3D seismic data); the
unequivocal evidence for liquid—as opposed to dry gas—
hydrocarbon seepage (from both the ALF and optical data);
and the localisation of small oil slicks over a charged
structural horst, the Cornea trend. The examination of these
relationships allows the potential discrimination between
hydrocarbon leakage from an oil-charged and potentially
commercial trap, such as the Cornea field (which has
chimneys, focussed ALF anomalies, as well as small
slicks), as opposed to the leakage of voluminous dry gas at
the (non-prospective) edge of effective seal (where chimneys
are present but no ALF anomalies or slicks are present).
Similarly, SAR data were useful in reducing explora-
tion uncertainty about the presence or absence of a
working, liquids petroleum system on the Yampi Shelf;
the prominent, edge-of-seal slicks detected by SAR
showed that a working system was present. However,
these slicks were located well inboard from the charged
and prospective Cornea trend. Consequently, the SAR
data helped to get us into the right street—that is to the
right play fairway in the right part of the basin—but
lacked the spatial resolution to get us into the right
house—that to the trap itself.
From an exploration viewpoint, it would appear that a
hierarchical approach is the most appropriate. Firstly,
seafloor features such as pockmarks and any biological
build-ups should be identified, and shallow direct
hydrocarbon indicators (DHIs), gas chimneys, and
HRDZs should be mapped using available 2D and 3D
seismic data. These data should be combined with
regional charge history modelling (2D and 3D) and
structural mapping, and analysis of the distribution of
the regional sealing facies. These data should be
combined with regional SAR data to identify any areas
with liquids seepage, such as the inboard edge of the
Yampi Shelf. Combining the mapping of gas chimneys
with the SAR-plus the addition of limited additional data,
such as the Rock-Eval used in the present study, or for
example, fluid inclusion charge history analysis—can
further refine the exploration strategy.
The critical next step is to identify which structures are
most likely to contain oil, rather than gas. The key factor in
successfully achieving this is, at least in an environment
such as the Yampi Shelf, to acquire remote sensing data
that can reliably detect the small, i.e. the 5–50 m long, oil
slicks which are forming as a result of limited liquid
hydrocarbon seepage through relatively good (with respect
to oil) top seals, directly from accumulations. Some of the
newer aircraft and satellite platforms appear to offer the
potential to be able to map the small, subtle slicks which
the available commercial satellite SAR is currently unable
to resolve.
5.2. A model for hydrocarbon migration and seepage
The observations described in this paper have allowed
the construction of a simple model for present day
hydrocarbon migration and seepage across the Yampi
Shelf (Figs. 25 and 26).
In this model, mature, Early Cretaceous, oil-prone source
rocks and older (perhaps over-mature) gas-prone source
rocks have been, and still are, generating hydrocarbons 50—
80 km outboard of the Yampi Shelf (see Fig. 26). This oil
Fig. 25. Schematic model of hydrocarbon migration, leakage and seepage across the Yampi Shelf, with attendant responses of assorted remote sensing
responses. Size of arrows is proportional to amount of migrating or leaking hydrocarbons. Accumulations are developed over or around basement highs;
vertical exaggeration is extreme.
Fig. 26. Oblique, 3D bathymetric image of the boundary between the
Bonaparte and Browse basins, showing schematic hydrocarbon migration
pathways and zones of hydrocarbon seepage.
G.W. O’Brien et al. / Marine and Petroleum Geology 22 (2005) 517–549546
and gas is expelled and migrates out (secondary migration)
of the basin and onto the shelf, typically at low rates
(location ‘A’ in Fig. 25). Through this region, the seal
thickness and capacity is generally good, and hence little
seepage takes place. Hydrocarbons—probably oil initially-
accumulated in some of the structures (such as Cornea) on
the edge of the basin (location ‘B’ in Fig. 25). However, this
oil subsequently began to be partially displaced by gas,
resulting in the displacement of significant volumes of
reservoired oil, and eventually gas, marginward. It is likely
that this tertiary migration takes place at relatively high
rates and at high volumes compared to the secondary
migration from the basinal source rocks.
Where the regional seal thins significantly onto topo-
graphically prominent tilt blocks and basement highs, the
seal capacity is reduced sufficiently to allow migrating gas
(which has high relative mobility compared to oil) to break
through the seal, forming gas chimneys (location ‘C’ in
Fig. 25). These chimneys effectively represent point (and
G.W. O’Brien et al. / Marine and Petroleum Geology 22 (2005) 517–549 547
episodic) sources of seepage and, since the flux of
hydrocarbons through these chimneys is relatively small
(perhaps due to an effective high resistance to flow), only
weak seeps are produced at the seafloor. A minimal amount
of oil seeps to the surface in these locations, since the seal
capacity is essentially sufficient to contain the heavy,
biodegraded oil found in the area. In this region, no SAR
response is observed, although a weak sniffer response can
be recorded. In contrast, high sample rate tools such as ALF
(and related tools) respond strongly to the small oil slicks
that are present over and near the field.
Further inboard, the most topographically prominent
basement highs are often largely bald of sealing facies
(location ‘D’ in Fig. 25). These bald highs act as hydrocarbon
catchments for both secondary and tertiary migration and
localise massive gas seepage over a significant area, thereby
producing extensive and strong water column anomalies
(Fig. 26). Again, minimal leakage of liquid hydrocarbons
takes place in these locations and hence neither the SAR nor
the ALF has a significant response.
At the basin’s inboard edge (location ‘E’ in Figs. 25 and
26), a combination of migrating heavy oil (secondary
migration) and significant volumes of oil displaced by gas
from the more basinward traps such as Cornea (tertiary
migration), seep at, or close to, the effective regional zero
edge of seal (for oil). It is probable that the heavy oil that
characterises this region, such as that recovered from
Cornea-1 (Ingram et al., 2000) can migrate much further
inboard, through the zone of declining top seal quality, than
can the more mobile gas.
On the Yampi Shelf, progressive top seal capillary failure
towards the basin margin, as a result of an increasingly thin
and sandy sealing facies, appears to have produced a large-
scale, spatial compartmentalisation (capillary sieving) of the
seeping hydrocarbons over distances exceeding 100 km
(Figs. 25 and 26).
This phenomenon has important implications for the
exploration of this and similar provinces worldwide. For
example, if only limited data were available—just
geochemical sniffer or seismic data for example—such
seepage fractionation could easily lead to the erroneous
conclusion that large parts of the Yampi Shelf are
exclusively gas-prone, when in fact a significant liquid
petroleum system is present. The fact that the seeps
detected by the sniffer are dry gas would strengthen this
erroneous conclusion. This tendency to measure a more
gaseous/dry seep composition is related to the relative ease
with which gas can migrate to the surface, compared to oil.
Since gas is much more mobile, seeps are invariably biased
towards a drier gaseous composition, particularly in the
case of a ‘two component’ system such as exists on
the Yampi Shelf. In fact, some of the dry gas seep fields on
the Yampi Shelf are over 20 km across, which could
represent a significant part, or all, of an exploration permit
in some areas.
Conversely, if an explorer’s permit straddles the basin
edge, then the interpretation of SAR data in isolation
could also lead to the conclusion that the region is
exclusively oil-prone, when in fact the gas flushing of
pre-existing oil columns, particularly within low ampli-
tude traps with thin column heights, is a key exploration
risk.
6. Summary
Dry gas and oil seepage was detected over the Yampi
Shelf, though the respective abilities of SAR, WaSi and
ALF to detect and characterise this seepage were markedly
different.
The results of this study demonstrate the value that an
integrated, multi-disciplinary, multi-technology approach
has in obtaining a cost effective and accurate assessment
of the hydrocarbon migration and seepage in regions
such as the Yampi Shelf. Clearly, the primary determi-
nant of the location, volume and, composition of
hydrocarbon seeps on the Yampi Shelf—and probably
many other areas—is the combination of the geology and
the relative seal capacity, rather than simply the nature
and volume of the hydrocarbon charge in the sub-surface.
The study has demonstrated that the interactions between
geology and hydrocarbon charge are predictable and that
understanding these interactions is crucial for the reliable
interpretation of remote sensing data.
In particular, facies-controlled seal capillary failure
(i.e. capillary sieving) can potentially produce permit-
scale spatial compartmentalisation of the composition and
volume of seeping hydrocarbons. Nevertheless, these
observations suggest that prime areas in which to capture
a snap-shot of the present-day migration across a margin
are where the regional seal thins over inboard basement
highs, or particularly, where the seal itself pinches-out
regionally.
Acknowledgements
We thank RadarSat International, and especially Shawn
Burns, for their great support of this pilot project.
The authors wish to thank the operational crew of the
Australian Geological Survey Organisation (now
Geoscience Australia) vessel RV Rig Seismic, who
acquired the geochemical sniffer data used in this
study. The seafloor sediment samples analysed were
acquired during a survey of the National Facility vessel
RV Franklin and we thank its crew for their great efforts.
We especially thank Greg Blackburn and Jenny Baird for
their work on the YST Study, some of which has been
used in this paper. Maria de Farago Botella, formerly
G.W. O’Brien et al. / Marine and Petroleum Geology 22 (2005) 517–549548
OBS Operations Manager for Nigel Press and Associates,
performed all of the weather compliance research for this
paper. A.G. Barrett, M. Lech, D.S. Edwards, C.J.
Boreham and K. Glenn publish with the permission of
the CEO, Geoscience Australia.
This manuscript was reviewed by Dr Jean Whelan
(Woods Hole Oceanographic Institute) and an unknown
reviewer and their comments were extremely helpful during
the revision of the manuscript.
Geoff O’Brien wishes to thank Geoscience Australia,
where he was employed when some of this work was
undertaken. He also wishes to thank the Australian
Petroleum Cooperative Research Centre (APCRC) and
especially the APCRC Seals Consortium, and the Australian
School of Petroleum (University of Adelaide), whose
support allowed this study to be completed.
References
ASTM D3650-90. Standard Test Method for Comparison of Waterborne
Petroleum Oils By Fluorescence Analysis, 267–271.
Bishop, D.J., O’Brien, G.W., 1998. A multi-disciplinary approach to
definition and characterisation of carbonate shoals, shallow gas
accumulations and related complex, near-surface structures in the
Timor Sea. APPEA Journal 38 (1), 93–114.
Blevin, J.E., Boreham, C.J., Summons, R.E., Struckmeyer, H.I.M.,
Loutit, T.S., 1998. An effective Lower Cretaceous petroleum system
on the North West Shelf: Evidence from Browse Basin. In:
Purcell, P.G., Purcell, R.R. (Eds.), The Sedimentary Basins of Western
Australia, pp. 397–419.
Cowley, R., 2000a. 1998 Yampi Shelf, Browse Basin Airborne Laser
Fluorosensor Survey Interpretation Report [WGC Yampi Survey].
AGSO Record 2000;27.
Cowley, R., 2000b. Comparison of AGSO North-West Shelf Airborne
Laser Fluorosensor Survey Interpretations. AGSO Record 2000;27.
Cowley, R., O’Brien, G.W., 2000. Identification and interpretation of
leaking hydrocarbons using seismic data: a comparative montage of
examples from the major fields in Australia’s North West Shelf and
Gippsland Basin. APPEA Journal 40 (1), 121–150.
Edwards, D.S., Johns, N., 1999. UV Fluorescence Analysis of Seawater
Samples from AGSO Marine Survey 207, North-West Australia. AGSO
Record 1999;27.
Hausknecht, P., 2001. Fugro Airborne Services Limited ARGUS Offshore
test survey: Cornea April 2001. Preliminary report for the Australian
Geological Survey Organisation. Unpublished.
Hinrichs, K.-U., Summons, R.E., Orphan, V., Sylva, S.P., Hayes, J.M.,
2001. Molecular and isotopic analysis of anaerobic methane-oxidising
communities in marine sediments. In: Yalcin, M.N., Inan, S. (Eds.),
Advances in Organic Geochemistry 1999 Organic Geochemistry, 31,
pp. 1685–1701.
Hovland, M., Judd, A.G., 1988. Seabed Pockmarks And Seepage. Graham
and Trotman, London pp. 293.
Hovland, M., Talbot, M., Qvale, H., Olaussen, S., Aasberg, R., 1987.
Methane-related carbonate cements in pockmarks of the North Sea.
Journal of Sedimentary Petrology 57, 81–892.
Hovland, M., Croker, P.F., Martin, M., 1994. Fault associated seabed
mounds (carbonate knolls?) off western Ireland and north-west
Australia. Marine and Petroleum Geology 11 (2), 232–246.
Ingram, G.M., Eaton, S., Regtien, J.M.M., 2000. Cornea case study: lessons
for the future. APPEA Journal 40 (1), 25–34.
James, N.P., Boreen, T.D., Bone, Y., Feary, D.A., 1994. Holocene
carbonate sedimentation on the west Eucla Shelf, Great Australian
Bight: a shaved shelf. Sedimentary Geology 90, 161–177.
Judd, A.G., 2001. Pockmarks in the UK sector of the North Sea. Technical
Report TR-002. Technical report produced for Strategic Environmental
Assessment—SEA2.
Kvenvolden, K.A., Redden, G.D., 1980. Hydrocarbon gas in sediment from
the shelf, slope and basin of the Bering Sea. Geochimica Cosmochimica
Acta 44, 1145–1150.
Mackintosh, J.M., Williams, A.K., 1990. ALF survey of the Great
Australian Bight. Basic Data Report. Unpublished BP Report.
Martin, B.A., Cawley, S.J., 1991. Onshore and offshore petroleum
seepage: contrasting a conventional study in Papua New Guinea and
airborne laser fluorescing over the Arafura Sea. APEA Journal 31,
333–353.
O’Brien, G.W., Woods, E.P., 1995. Hydrocarbon-related diagenetic zones
(HRDZs) in the Vulcan Sub-basin, Timor Sea: recognition and
exploration implications. APEA Journal 35, 220–252.
O’Brien, G.W., Blackburn, G., Baird, J., 1996a. Yampi Shelf Tie (YST)
Basin Study and Interpretation Report: Yampi Shelf, Browse Basin,
Northwestern Australia. AGSO Record 1996; 60.
O’Brien, G.W., Higgins, R., Symonds, P., Quaife, P., Colwell, J., Blevin, J.,
1996b. Basement control on the development of extensional systems in
Australia’s Timor Sea: An example of hybrid hard linked/soft linked
faulting? APPEA Journal 36, 161–201.
O’Brien, G.W., Quaife, P., Cowley, R., Morse, M., Wilson, D., Fellows, M.,
Lisk, M., 1998a. Evaluating trap integrity in the Vulcan Sub-basin,
Timor Sea, Australia, using integrated remote sensing geochemical
technologies. In: Purcell, P.G., Purcell, R.R. (Eds.), Petroleum
Exploration Society of Australia (PESA) Western Australian Basins
Symposium, vol. 2, pp. 237–254.
O’Brien, G.W., Quaife, P., Burns, S., Morse, M., Lee, J., 1998b. An
evaluation of hydrocarbon seepage in Australia’s Timor Sea (Yampi
Shelf) using integrated remote sensing technologies, Proceedings of the
SEAPEX Exploration Conference, 2–3 December 1998, Singapore
1998 pp. 205–218.
O’Brien, G.W., Morse, M., Wilson, D., Quaife, P., Colwell, J.,
Higgins, R., Foster, C.B., 1999. Margin-scale, basement-involved
compartmentalisation of Australia’s North-West Shelf: a primary
control on basin-scale rift, depositional and reactivation histories.
APPEA Journal 39, 40–63.
O’Brien, G.W., Lawrence, G., Williams, A., Webster, M., Wilson, D.,
Burns, 2000. Using integrated remote sensing technologies to evaluate
and characterise hydrocarbon migration and charge characteristics on
the Yampi Shelf, north-western Australia: a methodological study.
APPEA Journal 40 (1), 230–255.
O’Brien, G.W., Lawrence, G., Williams, A., Webster, M., Cowley, R.,
Wilson, D., Burns, S., 2001. Hydrocarbon migration and seepage in the
Timor Sea and Northern Browse basin North-West Shelf, Australia: An
Integrated SAR, Geological and Geochemical Study. AGSO Report and
GIS. AGSO Record 2001;27.
O’Brien, G.W., Cowley, R., Quaife, P., Morse, M., 2002a. Characteris-
ing hydrocarbon migration and fault-seal integrity in Australia’s
Timor Sea via multiple, integrated remote sensing technologies. In:
Schumacher, D., LeSchack, L.A. (Eds.), Applications of geochem-
istry, magnetics, and remote sensing AAPG Studies in Geology
No. 48 and SEG Geophysical References Series No. 11, pp. 393–
413.
O’Brien, G.W., Glenn, K., Lawrence, G., Williams, A., Webster, M.,
Burns, S., Cowley, R., 2002b. Influence of hydrocarbon migration and
seepage on benthic communities in the Timor Sea, Australia. APPEA
Journal 42 (1), 225–240.
O’Brien, G.W., Cowley, R., Lawrence, G.H., Williams, A.K.,
Edwards, D.S., Burns, S., 2003. Margin to prospect scale controls on
fluid flow within the Mesozoic and Tertiary sequences, offshore
G.W. O’Brien et al. / Marine and Petroleum Geology 22 (2005) 517–549 549
Bonaparte and northern browse Basins, northwestern Australia. In:
Ellis, G.K., Ballie, P.W., Munson, T.J. (Eds.), Proceedings of the Timor
Sea Symposium, 2003. Timor Sea Petroleum Geoscience, pp. 99–124.
Radlinski, A.R., Edwards, D.S., Morse, M., Johns, N., 2000. Survey 207
direct Hydrocarbon Detection Offshore Canning Basin; Yampi shelf;
Southern Bonaparte Basin, Timor Sea, September/October 1998.
Unpublished AGSO Report.
Sassen, R., Brooks, J.M., MacDonald, I.R., Kennicut, M.C.,
Guinasso, N.L., Requejo, A.G., 1993. Association of oil seeps and
chemosynthetic communities with oil discoveries, upper continental
slope, Gulf of Mexico. Transactions of the Gulf Coast Association
Geological Societies 43, 349–355.
Spry, T.B., Ward, I., 1997. The Gwydion discovery: a new play fairway in
the Browse Basin. APPEA Journal 37, 87–104.
Stein, A., Myers, K., Lewis, C., Cruse, T., Winstanley, S., 1998. Basement
control and geoseismic definition of the Cornea discovery, Browse
Basin, Western Australia. In: Purcell, P.G., Purcell, R.R. (Eds.),
Petroleum Exploration Society of Australia (PESA) Western Australian
Basins Symposium, vol. 2, pp. 421–434.
Wilson, D.J., 1999. Survey 207 Direct Hydrocarbon Detection Offshore
Canning Basin; Yampi shelf; Southern Bonaparte Basin, Timor Sea,
September/October 1998. AGSO Record 1999; 51.