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AUSGEOnewsMARCH 2003 ISSUE No. 69

CO2down under

OFFSHORE ACREAGE RELEASED

Also: Use of acoustic surveys probed, potential new oily sub-basin, dinocyst zones realigned...

AusGEO news No.69 3/18/03 6:50 PM

A U S G E O N e w sMarch 2003Issue no. 69

Editor Julie Wissmann

Graphic Designer Karin Weiss

This publication is issued free ofcharge. It is published four times a yearby Geoscience Australia.

The views expressed in AusGeoNews are not necessarily those ofGeoscience Australia, or the editor, andshould not be quoted as such. Everycare is taken to reproduce articles asaccurately as possible, but GeoscienceAustralia accepts no responsibility forerrors, omissions or inaccuracies.

© Commonwealth of Australia 2003

ISSN 1035-9338

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On March 24 the Australian Government releases newacreage offshore for petroleum exploration. This year’sopportunities are in frontier areas and in producing basins,as well as in basins known to have petroleum systems (seepage 22). Geoscience Australia carries out a lot of researchto support this annual release of acreage to reduce the risksfor petroleum explorers. This issue of AusGeo News isdevoted to reporting some of that research.

Photo: ©Lowell Georgia/CORBIS

CONTENTS

Comment 3

Survey sounds fine in Antarctic waters 4

PETROLEUM SPECIAL…

Potential new oily sub-basin in Browse 8

Bass well data suggest a Tertiary problem 10

Big capacity to store greenhouse gas down under 11

Broad study looks at Otway’s petroleum 14

New resource estimates for Bonaparte on GA program 15

Crustal thickness turns heat up on sedimentary basins 16

Events calendar 18

Achieving new heights in Antarctica 18

Dinocyst zones realigned for Shelf explorers 19

Timor Sea activity significant 20

Product news 21


AusGEO News 69 March 2003 3

N E I L W I L L I A M SCEO Geoscience Australia

Comment

Last December, the CommonwealthGovernment announced thesuccessful bids under its 2002Cooperative Research Centres (CRC)program. Geoscience Australia willparticipate in four of thesesuccessful bids: the CRC forGreenhouse Gas Technologies, theCRC for Spatial Information, andthe extension bids for the CRC forthe Coastal Zone, Estuary andWaterway Management and theCRC for the Great Barrier ReefWorld Heritage Area.

Geoscience Australia willcontribute enormously to the CRCfor Greenhouse Gas Technologiesfollowing its success in the APCRC– Geological Sequestration ofCarbon Dioxide project (GEODISC).The main findings of this work arefeatured in this issue of AusGeoNews.

Oil and natural gas account formore than 50 per cent of Australia’senergy fuels. Australia has hugereserves in gas with several majordevelopments in the ‘pipeline’ andrecent discoveries on the NorthWest Shelf and in the Otway Basin.

Australia has also enjoyed a high level of self-sufficiency in liquidhydrocarbon, but its reliance on imported oil is likely to increase in thefuture.

Geoscience Australia forecasts that annual total liquid-hydrocarbonproduction in Australia will decline by 40 per cent in 10 years. This outlookis exacerbated by a marked decrease in exploration over the past year. Anew oil province needs to be found.

The Commonwealth Government is addressing the issues and problemsassociated with current levels of exploration in Australia. Minister forIndustry, Tourism and Resources, the Honourable Ian Macfarlane announceda House of Representatives Industry and Resources Committee Inquiry into‘Resources exploration impediments’ in May last year, which has alreadyattracted a considerable number of submissions. Last year, 28 new areas foroffshore petroleum exploration were also awarded with a total explorationcommitment valued at almost $600 million.

This month another 35 areas in basins around Australia are released forindustry bidding. Geoscience Australia, in conjunction with state authorities,provided the technical and geological information for the 2003 OffshoreAcreage Release.

The Government also conducted a major internal review of GeoscienceAustralia’s pre-commercial petroleum exploration and technical adviceprogram. This review is now being considered, and I would like to take thisopportunity to thank the petroleum exploration industry for theircontributions.

High-quality pre-commercial geoscientific information is crucial tomaintaining Australia’s competitive position in attracting petroleumexploration investment. Inside this issue of AusGeo News are some examplesof the work Geoscience Australia carries out to provide this information, inparticular in the Bass and Otway basins and off north-western Australia.

This issue also contains an article about a topical environmental issue,the impact of marine acoustic technology on whales, seals and other marinefauna. This work highlights how Geoscience Australia collaborates withscientists around the world to develop significant expertise in areas ofscience that are new to the organisation.


4 March 2003 AUSGEO News 69

The web has many reports of whale and dolphin strandings andhow some could be linked to acoustic technology used in marinesurveys and naval exercises. But is there hard evidence and, if so,

what practices and equipment are at fault?

Two years ago one Environmental Licensing Agencybanned its scientists from using acoustic equipment in Antarctic

waters. This stopped a large marine research program andprompted a review of Antarctic surveys and their potential effect on

marine animals by a team of scientists from various countriesinvolved in the Scientific Committee for Antarctic Research (SCAR).

Geoscience Australia’s Dr Phil O’Brien, convenor of SCAR’s adhoc working group on marine acoustic technology outlines the

group’s findings, a draft of which was released in February.

AntaAntaSURVEY SOUNDS FINE IN

TT he use of echosounders, acousticreleases and seismic

reflection equipment for marinesurveys has been questionedbecause these acoustic tools mayinterfere with the activities ofmarine animals that use sound,presumably for navigation,communication and acquiringfood. Overuse of these tools,particularly at close range,perhaps harms whales, seals andpenguins and the animals onwhich they feed.

Acoustic tools are crucial tomarine research. Without thesetools, very little would be knownabout the world’s oceans andcontinent formation.

Echo sounders aid navigation,map the sea-floor, and show thedistribution of fish and plankton.Seismic reflection equipmentimages the sediments beneath thesea-floor, which is important fordetermining resource potentialand climate history. Acousticreleases allow seabed moorings tobe placed and retrieved withoutthe long lines that can entangleanimals and icebergs.

Our task was to considerwhether acoustic technology hasan impact on major animal groupsin the Antarctic. The SCAR workinggroup comprised an internationalteam of geoscience and biologyexperts who know about Antarcticwildlife and the acoustic responseof wildlife, and the various surveyequipment used.

Our biggest problem wasfinding definitive data becauseresearch is at an early stage.Nevertheless, the working grouphas come up with a few guidelinesfor assessing survey risks and somemitigation strategies.



Sound pressureIn the ocean, sound travels as vibrations of water molecules thatexert push–pull pressure on objects in their paths. Sound is heard bypush–pull on an animal’s hearing mechanism.

Sound levels are usually expressed as decibels. Decibels areactually a ratio of the sound pressure against a standard referencepressure. The reference pressures for air and water are different, sosound levels in air and water cannot be compared directly. A soundunit in water is in decibels relative to one micropascal (dB re 1µpa).Typical Antarctic surveys have source levels of about 220 decibels,which diminish rapidly away from the source.

Another important characteristic of sound is its frequency or pitch.It is measured in cycles per second or hertz. Frequency governs whatcan be heard by a species. For example, baleen whales use soundwith frequencies as low as 20 hertz—a low rumble to humans.Dolphins use sound up to 24 000 hertz, which is inaudible to humans.

Most acoustic devices use pulsed sound (figure 1). Outputs varydepending on the equipment, filtering, the signal, and theenvironment. The figure given for the energy of the sound sourcedepends on how the energy is measured. A pulsed sound can bemeasured as the pressure difference from the zero to the first soundpeak or between the two major peaks. It can also be measured asthe total energy or as the energy in a range of frequencies. There arealso many ways of expressing the sound level, and each methodproduces different decibel numbers.

Sound energy is lost as it spreads. In the simplest case, soundenergy from a source spreads out much like light from a torch,becoming weaker with distance, halving every time the distancedoubles. In shallow water, the surface and sea-floor form boundariesthat channel the sound energy in a horizontal direction. In the deepocean, layering in the water can either increase or decrease theenergy reduction.

Sound is scattered when it strikes objects in the water. Theseobjects can be the sea-floor, the shore, the surface, bubbles, particlessuspended in the water, marine life and the thermal structure of theocean. Sound reflected from the sea-floor usually loses a lot ofintensity, particularly if the bottom is soft such as mud. Not as muchenergy is lost when the sea-floor is rock, and in some cases theenergy may refract and add to noise levels if the sound strikes hardshallow shelves.

Some acoustic energy will also be converted into heat, but howmuch depends on the temperature, concentration of magnesiumsulphate in the seawater, and the frequency of the sound wave. Highfrequency sound is absorbed more rapidly than low frequency sound.

Acoust ic equipmentThe dominant frequency determines the impact on animal hearing.Sound sources such as airguns produce more noise in the hearingrange of baleen whales than mid-frequency echo sounders, whichproduce more noise in the ranges used by seals and toothed whales.

rcticrctic

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Figure 1. Pulsed sound produced by asingle airgun showing the major features ofa pulsed sound wave form.

WATERS

Noise levelsIn the harsh Antarctic conditions,animals on pack ice and in the seatolerate a lot of noise and make soundsthat rise above the din. Sound fromwind-generated waves, sea ice andsediment movement can reach 180decibels. Whale songs, flipper slappingand breaching (some of which is notmeant to be friendly) can exceed 190decibels. Many whales can ‘sing’ forlong periods, with baleen whalesmaking sounds that last 16 seconds.

The thresholds of auditory damagein marine animals are difficult to assessbecause, for example, how do you testthe hearing of a great whale? There aredifferences across species and withinpopulations and, like humans at a rockconcert or a club on Saturday night,some individuals may be more tolerantof sound even when they experiencehearing loss. Like the human condition,safe hearing thresholds are probablylowered with repeated exposure.

The literature suggests that auditorydamage in marine animals is possiblefrom sound levels as low as 178decibels for sensitive species, to 224decibels for less sensitive species.

Initial impulse

Surface reflection (ghost)

Bubble pulses



from October to December anddepart by late February. Theyconcentrate along the retreating iceedge in spring and early summer.Later in the season they move to a 60nautical-mile wide krill belt seawardof the continental shelf edge.

Toothed whales Toothed whales vary in size, bodyshape and possibly hearingapparatus. They produce whistles,pulsed sounds and echo-locationclicks. Their general sounds reachmaxima of 180 decibels, and echo-location clicks reach 228 decibels.

Airguns produce enough high-frequency noise to be heard bytoothed whales, even though mostof the energy in airguns is infrequency ranges below theiroptimum hearing. Continuouspulsing from an echo sounderseems to produce less reaction thanshort sequences of sound pulsesfollowed by longer pauses.Antarctic killer whales willapproach and swim alongsidevessels operating echo sounders.

Reaction thresholds in dolphinsand porpoises can be as low as110–140 decibels, but responsesdiminish with time even for levelsas high as 170 decibels. Dolphinsexposed to sound pressure levels of192–201 decibels demonstratetemporary decreased sensitivity. Inhumans, similar but prolongedexposure would producepermanent hearing impairment.

Deep-diving sperm and beakedwhales may be more vulnerable,because their physiological needslimit their avoidance options andmake it hard for them to movelaterally to avoid an approachingsource. The well-documented casesof whale stranding involvingmilitary echo sounders and seismicequipment have mostly involvedbeaked whales.

In Antarctica, sperm and beakedwhales are found in deep water andalong the continental slope nearsquid, their main food source.

PenguinsNot much is known about penguinhearing, but they seem less likely tobe disturbed by marine acousticsurveys than whales. Theirsusceptibility is probablycomparable to humans.

Acoustic devices are designed with various beam shapes. Seismic airgunsthat are towed behind a ship produce a much broader, almost sphericalbeam compared with a hull-mounted, scientific echo sounder.

The maximum sound levels from most hull-mounted scientific echo soundersare directed vertically below the ship (figure 2). Amplitude levels emittedhorizontally are typically 20 to 40 decibels lower than those emitted vertically.

Baleen whales Baleen whales use sound extensively and in the dominant frequency ofseismic surveys. Their songs can degrade the quality of seismic survey databy drowning the return signals. The passing of survey vessels generally doesnot silence baleen whales. Some whales even change their calling patterns inresponse to echo sounders. They appear to avoid seismic sources and mid-frequency echo sounders, but do not react to high frequency pingers andacoustic tags.

Whale response to sound depends on their activity at the time. Studiesof humpback, bowhead and grey whales show they are less responsivewhen migrating or feeding than when suckling and resting. Individualreaction also varies. About 10 per cent of migrating grey whales avoidednoise at 163 decibels and about 50 per cent at 173 decibels, by divertingaround a sound source in their migratory path.

Studies of humpbacks along Australia’s North West Shelf (when they werenot migrating) show humpback cows with young calves move away at soundlevels of 126–129 decibels. Some whales however swim directly towards anairgun source up to a certain stand off distance, sometimes circle it, and thenswim off. Maybe the similarity of the sound level and frequency content of theairgun to a breaching event makes humpbacks inquisitive.

There are humpback whale populations on both sides of Australia.Those in the west have been exposed to intense industry seismic activity forseveral decades. The eastern population has not. The annual growth rate isabout the same for both populations, suggesting that the seismic activity offAustralia to date has not threatened humpback whales.

The closer an animal is to a loud sound source, the more likely it is to beinjured. In many jurisdictions, seismic surveys are required to gradually increasegun numbers and pressures so animals can swim away before sound reachesdangerous levels. There are no studies to verify that this works, though, and itassumes that the animals will avoid the noise and swim away.

Most baleen whales breed in temperate waters to the north of Antarcticaand migrate to the krill-rich summer feeding grounds in the Antarctic. Calves areolder and stronger by the time their pods reach Antarctic waters. They arrive

Figure 2. Beam pattern for a hull-mounted,multibeam echo sounder

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It is not known whether Antarctic penguins communicate under water.But on land, they use sound extensively for mate and chick recognition.Contact calls can be heard up to a kilometre from the originating bird.

Virtually the entire Antarctic continental margin is within foraging rangeof penguins, with large colonies in the northern Antarctic Peninsula andwestern Ross Sea. Breeding birds generally arrive later and depart earlier atsoutherly colonies than at northerly colonies. All penguins moult annuallyand this typically occurs at the end of breeding season.

SealsSeven species inhabit the Antarctic and sub-Antarctic. All are quite vocal,even under water, and their calling rates peak in spring. Weddell seals, forexample, produce a variety of calls with source levels of 153–193 decibels.

Australian fur seals have colonised oil production facilities in Bass Strait,showing that seals get used to sound. They can also be quite inquisitive.Airguns have been ineffective in scaring South African fur seals away fromfishing gear and, like Australian fur seals, they are often reported nearoperating seismic equipment and consistently damage it by biting.

From what is known about their hearing, seals are relatively insensitiveto sound below one kilohertz. Ringed and Weddell seals are apparentlyunaffected by acoustic tags fixed to them. But there is a report of harbourand grey seals avoiding small airguns at ranges of about two kilometres.These seals returned to the area soon after shooting ended, however. Theequipment with most potential risk for seals would be lower frequency echosounders used for sub-bottom profiling.

The seals’ sensitive periods are pupping, post-pupping and implantation,and when weaners leave breeding beaches to forage. Most Antarctic marinesurveys take place from late December to March and so avoid the breedingseasons of pack ice seals. As well, any risk to pack ice seals from seismicsurveys is reduced because seismic equipment trails behind the vessel andcannot operate easily in ice.

Food sourcesKrill and fish attract predators that could be adversely affected by acousticequipment, and there is also concern that survey tracks through swarms couldcause them to separate and coalesce, making them more vulnerable to predators.

Fish can be displaced by large airgun arrays. The injury radius for eggsand juvenile fish will be a few metres around large airguns, with lethaleffects at about 220 decibels. Krill will be affected in a similar way to fisheggs and juveniles. But with the low level of activity in Antarctica, the effectof acoustic surveys on krill and fish will be very small compared with fishingand predation.

Squid hearing is less sensitive than most fish, but they show alarm atabout 156–161 decibels and will eject ink at about 174 decibels. Squid spawnand hatch from April to early November, which is outside the optimumweather conditions for seismic surveys in Antarctic waters. Squid also wouldonly be killed within a few metres of individual, large airguns.

Crustaceans are thought to be insensitive to sound because they detect itthrough mechanoreceptors.

Survey impactDespite the unknowns about acoustic propagation and animal hearing andbehaviour, there is insufficient evidence to justify a ban on marine acoustictechnology in the Antarctic. But there is also insufficient evidence to say thatall equipment and surveys are safe.

Survey equipment with an output of less than 190 decibels producessound levels similar to natural sources. Once sound levels go beyond 210decibels, they can be above natural levels for a large area.

Mit igat ionSurveys should use the minimum source level possible to achieve therequired result. For powerful sources, increase the output slowly at the startof a line, because these ‘ramped’ starts theoretically allow animals to avoidequipment. There also should be shut-down zones around the source tominimise the exposure of animals that do not avoid the sound.


Continuous noise has morepotential to disrupt animalcommunications than pulsed orintermittent signals. It is also moredamaging to human hearing thanpulsed sounds, so a similar effect ispossible in animals.

Ship speed, line spacing, beamshape of the equipment, and surveyduration influence the degree ofimpact. This is multiplied whenthere are several ships producinghigh sound levels in a region,making it hard for animals to avoidexposure.

The sea-floor and any adjacentcoast can also restrict an animal’sability to avoid high source levels.There may be a choke point in themigration path, or the ship’sprogress may herd animals into abathymetric restriction or anembayment in fast ice. Provide anescape route for the animals,particularly for whales that cannothaul out to avoid sounds.

Close proximity to a colonyduring breeding season is likely toexpose more animals to a soundsource, as will a survey in the pathof migrating animals. Time surveysrelative to important stages inannual cycles. Seals should not bedisturbed when pupping andbonding, and when young begin toforage.

Even though the level ofsurveying in Antarctica is very lowcompared with many other parts ofthe world, the best way to avoidlong-term disturbance anddisplacement is to minimise repeatsurveying and ensure that areas arenot surveyed in consecutive years.This requires the full use of SCAR’sinternational coordination ofsurveys and data sharingarrangements.

Records of the locations,timing, frequency and nature ofhydroacoustic activities need to bemaintained to allow retrospectiveassessment of likely causes of anyfuture observed changes in thedistributions, abundance andproductivity of Antarctic species. Aswell, the effectiveness of mitigationstrategies needs to be investigated,and these strategies should beregularly reviewed.

Information on the surveyhistory of the Antarctic is availablefrom SCAR.

For more information phonePhil O’Brien on +61 2 6249 9409or e-mail [email protected] draft report is available atwww.geoscience.scar.org/geophysics/acoustics_1_2.pdf


Potential new oily sub-basin in BrowseA forecast of declining future Australian oil production has GeoscienceAustralia involved in four studies to help identify new potential oil-pronesub-basins off north-western Australia. The studies suggest potential in thedeepwater Browse Basin, which BHP Billiton is currently testing by drilling awell. It will be the first deepwater test in the outer Browse Basin.

Plate reconstruct ionsIn the first study, a series of plate tectonic reconstructions andpalaeogeographic maps were compiled for the entire north and north-westAustralian margin. The 19 maps show the structural and tectonic evolution ofthe region from the Early Permian (290 million years ago) to the present-day,including the distribution and type of sedimentary rocks deposited duringeach 10–20 million year interval. An accompanying report documents thetectono-stratigraphic evolution of each area, and discusses the stratigraphiccontrols on the region’s petroleum systems.


The palaeogeographic mapsshow the distribution of Jurassic riftbasins that have sourced the majoroil accumulations discovered in theregion (Exmouth, Barrow andDampier sub-basins in theCarnarvon Basin, and Vulcan Sub-basin and Sahul Syncline in theBonaparte Basin). The palaeogeo-graphic reconstruction for theOxfordian (155 million years ago)shows that the oil-prone rifts of theNW-trending Sahul Syncline and NE-trending Vulcan Sub-basin mostlikely step westward into the deep-water Browse Basin. They form agraben complex in the SeringapatamSub-basin, immediately outboard ofthe Brecknock–Scott Reef–BuffonHigh (figure 1).

Martin Norvick, University ofMelbourne, compiled the platereconstructions.

Transect restorat ionTo evaluate the structural andstratigraphic evolution of thisunexplored deepwater Browsegraben, Geoscience Australia’sregional seismic profile across thebasin has been back-stripped andstructurally restored. The workindicates that the graben was filledwith a thick syn-rift section ofinterpreted Early–Middle Jurassicand Late Jurassic–Berriasian agesediments (figure 2).

The sediments are likely tocontain organic-rich rocksdeposited within a restricted marineenvironment and are expected tobe oil prone, based on similarfacies within the other activelyexplored Jurassic rifts of the NorthWest Australian margin. The grabenalso probably received an influx ofcoarse clastic sediments depositedwithin sub-marine fans that weretransported across a fault-steepenedslope between the Brecknock–ScottReef–Buffon High and theSeringapatam Sub-basin. This mayhave allowed a favourablejuxtaposition of hydrocarbon sourceand reservoir rocks.

The depth-converted sectionalso shows a large Early Cretaceousinversion with several hundredmetres of relief that is onlappedand draped by deepwaterValanginian–Aptian sediments.These sediments are likely toprovide an effective regional sealfor any hydrocarbons generatedwithin the graben.

Kevin Hill and Nick Hoffman,University of Melbourne, carriedout the structural-stratigraphicrestoration work.

Major riverActive faulting and foldingnOceanic spreading ridge

Oceanic crust

Shallow marine and deltaicse

LevequePlatformq

Yampi

Shelf

Yamp

Prudho

e Terraceerracrra

LondonderryHigh

ScotttPlateau

NancarTroughNancarugu

Low erosional areae

deepwater clays, marls

clays

Figure 1. Mid Oxfordian(155 Ma) platereconstruction of theBrowse and southernBonaparte basins (modifiedafter Norvick, 2002)

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Fluid inclus ionA third study, in conjunction with CSIRO Petroleum, provides a basin-wideassessment of hydrocarbon charge history and oil migration pathways withinthe Browse Basin using the grains containing oil inclusion (GOI) technique.A focus of this work is looking for evidence of oil migration in wells on theBrecknock–Scott Reef–Buffon High that lies immediately inboard of theJurassic graben identified in the adjacent Seringapatam Sub-basin. Jurassicsandstone cuttings in these wells were analysed for oil-bearing inclusions.

Elevated GOI values (1.1–1.3%) provide evidence for early oil migrationprior to gas charge at the North Scott Reef-1, Scott Reef-2A and Brecknock-1wells. No evidence of oil migration was detected by the GOI technique inBuffon-1 to the north, or Barcoo-1 to the south. The source and migrationpathway of this inclusion oil has yet to be determined, but the early oilcharge may have migrated from the newly identified Jurassic graben in theadjacent Seringapatam Sub-basin.

Details of the Browse Basin fluid inclusion study will be presented at theTimor Sea Petroleum Geoscience Symposium to be held in Darwin in June(see www.dbird.nt.gov.au/ntgs).


Basement

Modelled site

Hydrocarbon modell ingA geohistory analysis has involvedassessing the geographic distribu-tion and timing of hydrocarbonexpulsion from identified sourceunits in the Browse Basin. Itincorporates a number of modelleddepocentre sites derived fromregional interpretations ofGeoscience Australia’s seismicsurveys in the basin, includingseveral sites within the Jurassicgraben of the deep-waterSeringapatam Sub-basin.

A maturity model for a site 35kilometres north-west of Brecknock-1indicates that the Upper Jurassicsection straddles the immature toearly mature oil zone, and that thevery thick Middle–Lower Jurassicsection spans the oil, wet-gas anddry-gas maturity zones (figure 3).Modelled oil expulsion from thedeeper portions of the Middle–EarlyJurassic section (Plover Formationequivalent) commenced at the endof the Cretaceous, and expulsionfrom successively shallowerportions occurred in the early tomid and late Tertiary, respectively.Expulsion models for the UpperJurassic section (Vulcan Formationequivalent) suggest no hydrocarbonexpulsion, unless these sedimentscontain exceptionally high-qualitysource units at this site.

The Browse Basin geohistoryanalysis was undertaken in conjunctionwith BuryTech. Full details will bepresented at the Timor Sea PetroleumGeoscience Symposium.

New deepwater wel lThe four studies provide supportfor a potential new oil-prone sub-basin in the deepwater BrowseBasin. This potential is currentlybeing tested by the Maginnis-1, 1Awell (BHP Billiton Permit WA-302-P), the first deepwater well in theouter Browse Basin. This well andthe recent deepwater Wigmore-1well in the outer Beagle Sub-basin(Kerr McGee, Permit WA-295-P)herald a significant new phase forexploration of the deepwater NorthWest Shelf.

For more information contact John Kennard on +61 2 6249 9204 or [email protected]

Figure 3. Modelled maturity plot for a depocentre site within the SeringapatamSub-basin, 35 kilometres north-west of Brecknock-1

Figure 2. Cross-section based onGeoscience Australia’s seismic lines 119/6and 128/1 across the outer Browse Basinshowing a thick Jurassic section in theSeringapatam Sub-basin. Note the largeinversion structure of the Middle–LowerJurassic section.

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Bass well data suggest a Tertiary problemPetroleum exploration in the BassBasin began in 1965 and although32 wells have been drilled to testthe basin’s potential, only the Yollafield is being considered forcommercialisation*.

As part of the WesternTasmanian Regional MineralsProgram, Geoscience Australia hastaken a closer look at data from thedrilled wells to find reasons forexplorers’ limited success.

An audit shows that about one-third of the wells in the basin wereinvalid tests. The drilling has beenoff-structure and poor data qualityhas led to geological misinterpre-tations.

Of the remaining wells, theprimary reasons for failure werelack of effective seal, timing ofhydrocarbon charge, trap validity,lack of access to mature sourcerocks or reservoir problems.

The Bass Basin is an intra-cratonic rift basin between northernTasmania and southern Victoria(figure 1). Hydrocarbon discoveriesand shows occur in Late Paleoceneto Early Eocene sandstones of theEastern View Group. Intraform-ational shales seal these accumu-lations. Oil and gas shows alsooccur higher in the stratigraphicsuccession below the regionalsealing facies of the Middle EoceneDemons Bluff shale.

In parts of the basin, theregional seal has undergone aperiod of structural inversion duringthe late Tertiary resulting in trapand seal breach (figure 2). Thisprocess particularly affectedanticlinal closures of Eocene age.Structures on fault-boundedbasement highs were less affected.

Recent fluid inclusion studiesby CSIRO and Geoscience Australiaidentified palaeo-oil zones at Yolla-1, Cormorant-1 and Bass-3 alongwith suspected zones at Aroo-1,King-1 and Pelican-5. Severalpotential palaeo-hydrocarbon zoneswere also identified at Yurongi-1,Chat-1, Tilana-1 and Squid-1.

The basin’s pattern of hydro-carbon distribution shows thathydrocarbons were generated inthe Cormorant and Pelican troughswith migration into structureswithin the depocentres and on theadjacent flanks. In the Yolla andWhite Ibis fields, access to maturesource rocks was provided bylarge-displacement faults that linkedthe upper Eastern View Groupreservoirs with deeper maturesource rocks.

Figure 2. Seismic section showing the Cormorant-1 well situated over an inversion-related anticlinal structure in the north-western Bass Basin

Figure 1. Location map of the Bass Basin showing wells, fields, current permitsand the 2002 and 2003 Release Areas

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In part, future exploration successes in the Bass Basin will meanexplorers need to find traps that were in place prior to the generation ofhydrocarbons, but did not undergo significant Tertiary inversion.

The Bass Basin study is a Commonwealth and Tasmanian governmentinitiative coordinated by Mineral Resources Tasmania. Some results of thestudy will be presented at the 2003 APPEA conference.

For more information phone Jane Blevin on +61 2 6249 9818 or e-mail [email protected]

* Origin Energy-operated BassGas project


igB


CAPACITYto store greenhousegas down under

Worldwide case Attempts to assess the potential for storage of CO2 in geological formationsare mostly at a regional scale or global level. The amount of detail andassessment methods vary substantially, from counting storage volume insedimentary basins to more simplistic approaches that try to estimate thepotential worldwide.

Worldwide volume assessments are often quoted as ‘very large’ withestimates ranging from 100s to 10 000s gigatonnes (Gt = 109 tonnes) of CO2.The huge volumes of CO2 that can be stored geologically compared withmany other sequestration methods have been relied upon to guide policydirections and decide future research proposals. However as knowledge hasincreased about the technical issues of geological storage of CO2, so has theuncertainty about details used in the assessment methods.

To decide future viable injection sites, especially where capital costsrange from less than $US50 million to more than $US1000 million, it isessential to use more sophisticated means to make storage potentialassessments.

In GEODISC, the initial deterministic approaches are being closelyintegrated with mapping emission sources and economic modelling. Thehuge numbers that can be generated for storage potential are being realignedto take account of the reality of economic and geological viability.

Austra l ian caseEstimates of storage potential and a deterministic risk assessment provide anidea of the enormous geological storage potential of CO2 in Australia. Butthey do not account for various factors in CO2 source to sink matching(emission location to storage site matching). A more realistic analysis can bederived if preferences due to CO2 source to sink matching are incorporated,and it is assumed that some economic imperative will apply to encouragegeological storage of CO2.

Emiss ion mapsThe National Greenhouse Gas Inventory calculated that Australia’s netgreenhouse gas emissions for 1999, not including emissions from landclearing, were 458.2 Mt of CO2 equivalents. Stationary energy emissions,primarily produced through electricity generation, represent 56.7 per cent ofnational emissions or 259.8 Mt. These stationary sources are the most likelyto be considered suitable for sequestration.

The Australian Petroleum Cooperative Research Centre’sGEODISC program has been examining the issue of geologicalstorage of CO2. Within GEODISC, Geoscience Australia and theUniversity of New South Wales recently completed an analysisof Australia’s potential for the geological storage of CO2. Morethan 100 sites were assessed, of which 65 are potentiallyEnvironmentally Sustainable Sites for CO2 Injection (ESSCIs).

Australia has the potentialto sequester a quarter of its

annual total net carbondioxide (CO2) emissions, or

about 100–115 milliontonnes (Mt) of CO2 a year,in underground reservoirs.

Photo: Australian Picture Library


GEODISC made maps of thelocation of all major, stationaryenergy emission sites, andestimated the likely supply rates ofCO2 for a 20-year period (figure 1).The emissions mapping shows thatthe top 35 point sources represent90 per cent of the emissions thatcan be potentially sequestered (thetop 50 point sources represent96%). It also shows that the majoremission sources are concentratedinto nodes. The presence of nodesmeans it will be possible to reducethe set-up costs of establishinginjection sites, provided there areviable injection sites in neighbouringregions.

Basin screeningGEODISC screened all sedimentarybasins in Australia to identify whereCO2 storage might be viable. Thisincluded assessing all sedimentarybasins that were adjacent to known,major emissions sources, or whichmight in the future require potentialinjection sites to store CO2 emissions.

About 300 known sedimentarybasins were screened, of which 48were considered viable based ontheir geological characteristics. Morethan 100 sites within these basinswere examined, resulting in theidentification of 65 potential ESSCIs.

Storage eff ic iencyTotal pore volume assessments (i.e.space in rock that can hold CO2)were calculated for each site basedon their reservoir conditions, aswell as their possible CO2 capacity.This required making a preliminaryassumption (although incorrect)that 100 per cent of the porevolume could be filled with CO2.

The total pore volume for the65 ESSCIs was 7 x 1012 cubicmetres, or a potential CO2 storagecapacity of 3.9 x 1012 tonnes. Thisestimate makes no adjustment forthe storage efficiency of CO2 in thepore space.

Storage efficiency varies witheach ESSCI because of differenttrap types (geological structure thatcan store CO2) and geologicalcharacteristics.

A detailed reservoir model isneeded to accurately assess a site’sstorage efficiency. Continuingresearch in the GEODISC programaims to model the most likely sitesfor future storage of CO2.


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Figure 3. Graph showing the ESSCI capacity (ESSCI chance multiplied by total porevolume) plotted against the number of ESSCIs for different ESSCI types.

Figure 1. Major CO2 point sources and percentage of total emissions estimated for a20-year period. Volume of CO2 for each node of point sources is shown in milliontonnes (Mt) and in trillion cubic feet (Tcf). Expected emission rates are in millioncubic feet per day (MMcfd).

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Figure 2. Graph showing the ESSCI chance (product of risk factors) plotted againstthe number of ESSCIs for different ESSCI types.

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