O'Brien at Al. 2005 Yampi Shelf, Browse Basin, NW Shelf Australia

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: [email protected] (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

http://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.


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.


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.


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


(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 seepage
indicators, 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,


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).


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.


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.


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


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.


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)


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)


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)


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)


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)


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)


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.


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.


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


19999317 Sea waterCazide

19999318 207WS024 K13.6997 124.6807 Sea water


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


19999327 207WS017 K13.6951 124.7294 Sea water



19999330 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


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.


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.


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.


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


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).


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).



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.


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


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


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


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.

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