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AUSGEOnewsMarch 2004 ISSUE No. 73

Also: Also: Oi l province hunt , source rock studies , new map for rai l buffs

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A U S G E O N e w sMarch 2004 Issue no. 73

Editor Julie Wissmann

Graphic Designer Katharine Hagan

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 2004

ISSN 1035-9338

Printed in Canberra by National Capital Printing

Geoscience AustraliaGPO Box 378Canberra ACT 2601 Australia

cnr Jerrabomberra Ave & Hindmarsh DrSymonston ACT 2609 Australia

Internet: www.ga.gov.au

Chief Executive OfficerDr Neil Williams

SubscriptionsPhone +61 2 6249 9249Fax +61 2 6249 9926www.ga.gov.au/about/corporate/ausgeo_news.jsp

Sales CentrePhone +61 2 6249 9519Fax +61 2 6249 9982E-mail sales@ga.gov.au

GPO Box 378Canberra ACT 2601 Australia

Editorial enquiriesJulie WissmannPhone +61 2 6249 9249Fax +61 2 6249 9926E-mail julie.wissmann@ga.gov.au

AusGeo News is available on the web at www.ga.gov.au/about/corporate/ausgeo_news.jsp

Australia is naturally gas rich, but our oil production isexpected to fall by 40 per cent over the next decade. We need some major new oil discoveries.

Geoscience Australia contributes by providing vitalgeological and seismic data to help reduce exploration risk.

Photo: Natural gas plant © Australian Picture Library/ RF

CONTENTS

Comment 3

Frontiers in oil province hunt 4

More gas, less oil in forecast update 6

Events calendar 7

Submerged slice of Australia target of sea voyage 8

Otway source rocks probed for liquids potential 9

New tool splits petroleum populations 10

In brief… 12

Marine remnants add to Darling Basin history 14

Mineral exploration expenditure up but Australia’s share down 16

New maps to aid North West Shelf assessments 18

Product news 19

The Petroleum Open Day on April 1 is immediately afterAPPEA 2004 in Canberra. It will be held at GeoscienceAustralia’s headquarters. See the proposed program at www.ga.gov.au

AusGEO News 73 March 2004 3

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

This month Canberra is hosting the annual conference of the AustralianPetroleum Production and Exploration Association. The APPEA conference isan opportunity for petroleum industry professionals to get to know moreabout what Geoscience Australia and other parts of the Australiangovernment can and are doing for the industry.

The theme of this year’s conference, ‘Contributing to our nation’underscores the importance of the petroleum industry to Australia,particularly in economic and energy security terms. This importance hasbeen recognised by government and demonstrated in the 2003 budgetdecision which allots $61 million over four years to Geoscience Australia’spetroleum program.

The funding is to provide the pre-competitive information to supportindustry’s search for a new oil province. The funds will maintain GeoscienceAustralia’s core petroleum program and initiate a vital new phase of dataacquisition and the preservation of the data archive.

I am happy to announce that this new initiative is now well under way.The first marine surveys have just returned from sea—one from dredging thefrontier basins off the south-west margin, and the other has been criticallyassessing techniques for identifying active petroleum systems in the TimorSea. As well, the remastering of deteriorating seismic tapes onto new, stablemedia has begun. The articles in this issue of AusGeo News describe thesenew programs in more detail and outline our plans for the future.

The outcome of the new data acquisition program is the eventualoffering of new prospective areas in the annual release of offshorepetroleum acreage. The 2004 release will be announced by the Minister atthe APPEA conference. The areas available for bidding are outlined in this issue.

In future years we hope to offer new and further opportunities forinvestment in frontier and other basins underpinned by critical geoscienceinformation that reduces risk and demonstrates petroleum prospectivity.

Geoscience Australia has a booth at the APPEA conference, and anumber of our researchers are presenting papers in the technical program.Our Petroleum Open Day immediately follows the conference. The OpenDay will provide an informal atmosphere in which to discuss industry issueswith people from Geoscience Australia, the Department of Industry, Tourismand Resources, and the Department of Environment and Heritage.

There will also be anopportunity to review the optionsfor the 2004–2007 petroleumprogram, and to see the results ofcurrent projects. Industry input isessential for our planning of thenew data acquisition campaign, so Iencourage your contributions to thediscussions now and in the future.The dredge samples fresh off theboat from the Bremer and Denmarksub-basins will be on display, aswill be our 3D theatre and world-class petroleum data and corerepository.

I would like to take thisopportunity to welcome APPEAdelegates to Canberra and Iencourage you to visit GeoscienceAustralia while in town.

Comment

4 March 2004 AUSGEO News 73

Australia’s oil production is expected to fall by 40 per cent over the next 10years and this will seriously affect our energy self-sufficiency unless there aremajor new discoveries.

Australia is handling this situation in a number of ways. Existing oilprovinces that may have further potential are being re-examined, and newtechniques to locate oil using satellite technology are being tested. But thesolution is thought to lie in under-explored areas offshore.

Australia has comparatively few petroleum exploration wells—less thaneight thousand wells in an area of more than 12 million square kilometres—and it has some un-drilled frontier basins that could contain oil (figure 1).

Companies considering oil exploration in Australian territory look forgeological and seismic data to reduce exploration risk. Geoscience Australiaprovides this vital data.

The Australian Government has allocated $61 million to GeoscienceAustralia over four years to provide basic geological information thatencourages petroleum exploration investment.

$36 million is for the core petroleum program, and $25 million is fornew data acquisition and data preservation and archive.

Stakeholder inputSince the Australian Governmentallocation, Geoscience Australia hasconsulted the exploration industryin Australia and overseas abouthow best to achieve a new oilprovince.

An integrated program ofseismic acquisition, geologicalsampling and oil-seep detectionwas proposed, along with aportfolio of potential projects(figure 2).

One of the preferred deep-water frontiers was the Bremer Sub-basin and there was interest in theshallow-water Arafura Basin. TheLord Howe Rise was viewed as alonger term prospect. It is too largeto ignore and new data for the areacould radically change perceptionsof its petroleum potential.

Even though frontier basins areAustralia’s best chance of findingsubstantial new oil provinces,stakeholders stressed the need tocontinue efforts in existingexploration areas.

Part of Geoscience Australia’sprogram therefore involves regionaloverviews and analysis ofpetroleum systems in provenprovinces, and providing access tothe data.

Figure 1. Australia has less than eight thousand petroleum exploration wells in an areaof more than 12 million square kilometres, and a number of un-drilled frontier basins.

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Figure 2. Potential project areas forGeoscience Australia in the search fora new oil province

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Firs t phaseIn February, Geoscience Australiabegan acquiring data in the Bremerand Denmark sub-basins offWestern Australia between Albanyand Esperance (figure 3). Thegeophysical and sampling survey isin water depths of 100 to 4500metres.

A large number of dredgesamples and new high-speedseismic data will provide importantinformation about the petroleumgeology of the area, the evolutionof the sub-basins, and riftingbetween Australia and Antarcticaabout 100 million years ago.

At this stage, the petroleumpotential can only be inferred. Thesub-basins have not been drilled,and until this survey no rocks hadbeen dredged from the seabed.

Both sub-basins appear to bethick enough to have generated oil,providing there is the rightcombination of petroleum sourceand reservoir rocks. If theseconditions prove true, based onseismic-section interpretations, theBremer Sub-basin has largegeological features that could havetrapped the hydrocarbons.

A dozen deep canyons, calledthe Albany Canyons, provide theideal dredge sites. These slice 1000metres deep into the sediment pileand occur where the sub-basinscross the continental slope.

If successful, the first dredgesamples will be on display atGeoscience Australia’s PetroleumOpen Day on April 1 (following the2004 APPEA conference).

Seep detect ionAs Earth’s crustal plates bump andgrind, cracks or faults formallowing hydrocarbons to seep tothe surface. Around Australia, oceangenerally covers the seeps andwater pressure restricts their flow.But some surfaces as gas or an oilslick which can quite quicklyevaporate.

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Figure 3. Possible dredge sites inthe Bremer and Demark sub-basins

AusGEO News 73 March 2004 5

Geoscience Australia has begun locating natural hydrocarbonseepage around Australia. A combination of approaches is beingused including remote sensing, conventional geological interpretation,and marine surveys to acquire water column and seafloor samples forgeochemical analysis in Geoscience Australia’s laboratories.

The first marine survey, around the Ashmore Platform and Yampi Shelfregion is in March. Current knowledge of the region, its shallow-watersetting and petroleum potential, make it an attractive location to survey.

Previous work around the Cornea area of the Yampi Shelf providedevidence of hydrocarbon seepage. A range of seep detection and samplingtools will be tested in this area and then the survey will focus on theAshmore Platform using the understanding gained on the Yampi Shelf.

Potential seepage areas have been identified from remote sensing data—a combination of Synthetic Aperture Radar and Landsat satellite imagery, aswell as Airborne Laser Fluorescence—and from interpreting features inseismic lines and sub-bottom profiles. The features that suggest seepageinclude seabed faults, pockmarks, mounds, and changes to sediment since itwas deposited (hydrocarbon-related diagenetic zones or HRDZs).

Sub-bottom profiles, side-scan sonar, swath-mapping and fluorometerdata will be collected during the survey to better target sampling sites.

Informat ion shar ingDifferentiating natural oil slicks from other phenomena requires carefulassessment, particularly when using remote sensing tools and satelliteimagery to map differences in the sea surface. A number of things can causeslicks or change the surface tension of the ocean, including passing ships orcoral spawning.

Geoscience Australia recently hosted a science–industry workshop on theremote sensing of natural oil seeps. Australian as well as internationalconsultants and researchers presented and discussed different remote sensingtechniques and their geological integration. The workshop has beenextremely useful for evaluating potential survey sites around Australia.

Another valuable workshop delivered by Michael Abrams, leader of alarge, multi-client study on surface geochemistry calibration for the Energyand Geoscience Institute, University of Utah has resulted in a newpartnership for Geoscience Australia.

The two agencies will be collaborating on methods of seep analysis anddetection.

Forward programGeoscience Australia has identified study areas in the south-west region,North West Shelf and the Lord Howe Rise, and scoped a preliminaryprogram of data acquisition up to 2007. These will be modified with surveyresults and stakeholder input. Further industry–government workshops willbe held on an as-needs basis.

The surveys will help determine whether a suitable geological historyhas occurred to form large oil accumulations and whether oil is still present.The targeted areas range from shelf depths to deep water.

Data acquisition will be linked to the Australian Government’s annualrelease of petroleum exploration acreage. Explorers can study the results andjudge the prospectivity of acreage for themselves as the data will be free viathe web or at the cost of transfer.

For more information phone Marita Bradshaw on +61 2 6249 9452 ore-mail marita.bradshaw@ga.gov.au

The medium-term forecast for thediscovery of hydrocarbons in theBrowse Basin was recently updatedby Geoscience Australia.

In the next 10–15 years,Geoscience Australia expects thatseven gigalitres (46 million barrels)of oil, 119 billion cubic metres (4.2trillion cubic feet) of gas and 18gigalitres (115 million barrels) ofcondensate could be discovered inthe Browse Basin (table 1).

This discovery forecast wascalculated using GeoscienceAustralia’s resource assessmentprogram, AUSTPLAY.

It is not an estimate of thebasin’s ultimate potential.

The previous Browse Basinassessment released in 1998forecast that 62 gigalitres (391million barrels) of oil and 93 billioncubic metres (3.3 trillion cubic feet)of gas could be discovered.

It reflected the optimismassociated with finding oil in theCornea Field in 1996–1997, which

Mean P10

Oil GL 7.3 23.9

mmb 46 150

Gas bcm 118.6 338.5

tcf 4.2 12.0

Condensate GL 18.3 51

mmb 115 321

Table 1. Summary of GeoscienceAustralia’s medium-term assessment for theBrowse Basin (GL gigalitres, mmb millionbarrels, bcm billion cubic metres, tcftrillion cubic feet)

has dimmed with subsequent exploration activity. On the other hand, theprevious gas assessment proved to be pessimistic, with the subsequentdiscovery of gas at Brecknock South, Crux and Argus alone exceeding theforecast.

Forecast uncerta int iesThe drilling forecast for the next 10–15 years strongly influenced theassessment, even though predicting future drilling activity is complex.

Current permit drilling commitments were used as a starting point. Theproblem is permit drilling commitments extend a maximum of six years fromthe date the exploration permit is granted. As well, drilling commitments inthe later stage of a permit life generally have a lower expectation of actuallybeing drilled.

The current assessment (table 1) is based on a forecast drilling model of16 wildcat wells expected to be drilled in the next 10–15 years.

An alternative model based on the number of wells to be drilled to meetcurrent permit commitments (21 wells) produces an outcome of 215 billioncubic metres (7.6 trillion cubic feet) of gas and 29 gigalitres (180 millionbarrels) of condensate. The oil outcome does not change because themodifications occur in the pure gas assessments.

This alternative drilling model was considered too optimistic based onpermit drilling commitments for outer areas, and so the number of modelledwells was reduced for the current assessment.

Assessment uni ts Three assessment units were defined for the Browse Basin: an outer dry-gasarea, an inner ‘wetter’ gas area, and a Cretaceous oil assessment unit (figures1 & 2).

The outer dry-gas area comprises accumulations where condensate–gasratios are generally less than 20 barrels of condensate per million cubic feetof gas. These include Scott Reef, Brecknock, Brecknock South and Argus gasaccumulations, and the deeper Middle Jurassic Plover Formation gas reservoirof the Brewster accumulation.

In the inner ‘wetter’ gas area, condensate–gas ratios are greater than 20but less than 70 barrels of condensate per million cubic feet of gas. Itincludes the Crux accumulation and the shallower Late Jurassic/EarlyCretaceous Vulcan Formation gas reservoir of the Brewster accumulation.

The Cretaceous oil assessment unit comprises parts of the Caswell Sub-basin and the Yampi Shelf. These two distinct structural regions requireddifferent sets of input parameters to be properly assessed.

MORE GAS,LESS OILin forecast update

6 March 2004 AUSGEO News 73

AusGEO News 73 March 2004 7

Figure 2. Assessment units for the Browse Basin

Figure 1. Location of the Browse Basin

USGS assessmentThe United States Geological Survey included the Browse Basin in its worldpetroleum assessment in 2000.

Its discovery forecast for the Browse Basin was 168 gigalitres (1055million barrels) of oil, 569 billion cubic metres (20.1 trillion cubic feet) of gasand 148 gigalitres (934 million barrels) of condensate.

The current Geoscience Australia and USGS assessments clearly differ. The main difference is the timeframe for the forecasts; Geoscience

Australia’s assessment is for a 10–15 year period, while the USGS assessmentis for a 30-year period.

Geoscience Australia has adopted the USGS gas–condensate assessmentas the ultimate potential for the Browse Basin, but it believes the oil forecastis overly optimistic and is probably influenced by the Cornea oil discovery.

Other updatesGeoscience Australia is currently assessing the potential of the Dampier Sub-basin. Updated assessments for the remainder of the Carnarvon Basin andthe Gippsland Basin will occur in future years.

The Bonaparte Basin assessment and the AUSTPLAY program werepresented and discussed at the Timor Sea Geoscience Symposium in Darwinin June last year.

For more information about Geoscience Australia’s resourceassessment program phone Andrew Barrett on +61 2 6249 9502 or e-mail andrew.barrett@ga.gov.au

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EVENTS calendarcompiled by Steve Ross

AAPG 2004 American Association of PetroleumGeologists 18 to 21 April Dallas, Texas Contact: AAPG ConventionDepartment, PO Box 979, TulsaOklahoma 74101-0979 USAphone +1 918 560 2679fax +1 918 560 2684e-mail convene@aapg.or

Coast to Coast ’04 Coastal CRC 19 to 23 April Grand Chancellor Hotel, HobartContact: Conference Design, POBox 342, Sandy Bay, Tasmania 7006phone +61 3 6224 3773fax +61 3 6224 3774e-mail mail@cdesign.com.au

Spatial Data Forums Geoscience AustraliaCanberra 21 May, Adelaide 25 May,Perth 27 May, & Brisbane 11 June Contact: Andrew Beer, GeoscienceAustralia, GPO Box 378, CanberraACT 2601 phone +61 2 6249 9034fax +61 2 6249 9937e-mail andrew.beer@ga.gov.au

17th Geophysical Conference &Exhibition Australian Society of ExplorationGeophysicists & PetroleumExploration Society of Australia(NSW Branch) 15 to 19 August Sydney Convention CentreContact: Conference Action, POBox 576, Crows Nest NSW 1585phone +61 2 9437 9333fax +61 2 9901 4586

PESA 2nd Eastern AustralasianBasin SymposiumPetroleum Exploration Society ofAustralia 19 to 22 SeptemberAdelaideContact: Rob Bulfield, SAPROConference Management, PO Box187, Torrensville SA 5031phone +61 8 8352 7099fax +61 6 8352 7088e-mail scm@sapro.com.au

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8 March 2004 AUSGEO News 73

An important piece of the tectonicjigsaw of the south-west Pacific willbe probed by Geoscience Australiaduring a sea expedition in May thisyear.

The Kenn Plateau expedition isthe first stage of an investigationinto remote and poorly understoodplateaus that lie off Australia’snorth-east margin.

The Kenn Plateau is a large,submerged continental block thatrifted 63–52 million years ago fromwhat is now southern Queensland.

It is a submarine plateauseparated from the Marion Plateauby the 3000-metre-deep CatoTrough. The Bellona Plateau lies tothe north-east and the MiddletonBasin to the south-east (figure 1).

The Australian part of the Kenn Plateau is about 100 000 squarekilometres in area, of which about 40 000 square kilometres (half the area ofTasmania) is in water shallower than 2000 metres. The plateau is shallowestalong its north-western margin and generally deepens to the south-east.

It is a rare area that lies at the junction of the Coral Sea and Tasman Sea,between seafloor-spreading terrains to the south, and ridge propagation andother terrains to the north.

The Kenn Plateau is being targeted initially because of its setting, andbecause of a buried sedimentary succession more than two kilometres thickthat should answer some questions about the geology of the greater LordHowe Rise region.

Seismic profiles of the region are generally of low quality, and there arevery few sediment or rock samples from the plateau. It is presumed tocontain rift basins younger than 65 million years old.

The Southern Surveyor will be used for the expedition. A program ofshallow, high-speed seismic, bathymetric and magnetic profiling will establishthe plateau’s framework and setting. The research program will also includedredging and coring to obtain rocks and sediments.

The program addresses several themes in Australia’s Ocean Policy andMarine Science and Technology Plan. Scientific questions being probed bythe expedition are outlined in table 1.

The expedition should greatly improve understanding of the geologicevolution and geographic history of the Kenn Plateau, as well as its long-term resource potential, which is currently believed to be small.

For more information phone Tony Stephenson on +61 2 6249 9472 ore-mail tony.stephenson@ga.gov.au

Submerged

Australia targetof sea voyage

Figure 1. The submarine Kenn Plateau lies at the junction of the Coral Sea and TasmanSea. Its geology is the focus of an upcoming expedition on the Southern Surveyor.

Table 1. Scientific questions for theKenn Plateau expedition

• The nature and distribution ofacoustic basement(continental and/or volcanic?);

• The nature and tectonic stylesof the plateau’s margins tohelp constrain plate tectonichypotheses;

• The geometry and age of therift basins, and the nature oftheir sediments;

• The controls on the initiationand development of eastAustralian carbonate platformssince the early Miocene;

• The nature and ideally the ageof several Cainozoic volcanicpedestals, assumed to be partsof the Tasmantid volcanicchain;

• The nature of modernsediments on this remotemarginal plateau and theirpotential influence onbenthic habitats.

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AusGEO News 73 March 2004 9

Figure 1. Geochemical log of source and maturity parameters based on Rock Eval and vitrinitereflectance data, which can be generated on-line from Geoscience Australia’s web site.

Organic geochemistrystudyKey offshore and onshore wells aswell as a regional grid of conven-tional and deep-seismic data arecurrently being interpreted. Thiswork will provide a time-series forsource-rock development.

Well logs, geochemical plotsand web-based geochemicalprofiles have been reviewed toselect new and existing samples ofpotential source rocks. A total of169 sediments from 32 onshore and15 offshore wells were chosen formore detailed study.

New vitrinite reflectance andfluorescence analyses based on theNewman technique were alsocarried out on samples from fourkey offshore wells (Mussell-1,Troas-1, Trumpet-1 and Voluta-1).Kerogen-specific VRF analyses arecritical for accurate burial historymodelling and determiningmaturation.

On-l ine data The primary resource for organicgeochemical data for the OtwayBasin is Geoscience Australia’sORGCHEM database. Users canaccess open-file information in thedatabase via the web.

They can build geochemicalprofiles of Rock Eval (S2, HI, PIand Tmax) and vitrinite reflectance(VR) data for most offshoreexploration wells (figure 1). Aftersubmitting an initial well query, adownhole profile of geochemistryresults can be generated by clickingthe ‘Graph’ option under the‘OrgChem’ header.

Sample ages are automaticallyestimated using GeoscienceAustralia’s STRATDAT database(figure 1).

The Otway Basin in south-east Australia is becoming a major commercial gasprovince. Sub-commercial oil occurrences suggest the basin also has liquidspotential.

The basin’s oil families are well understood through detailed carbonisotope and biomarker studies, and potential source rocks have beenidentified in a range of Late Jurassic to Cretaceous (150–65 million year old)lithostratigraphic units.

But the effective source rocks for oil and gas are yet to be identified andthis requires integrating source-rock characteristics with the geochemistry.

Geoscience Australia is studying the basin’s oil-source correlations tolearn more about the spatial variations in petroleum potential,condensate–gas ratios, gas–oil ratios, and maturation.

Otway source rocksprobed for liquids potential

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10 March 2004 AUSGEO News 73

Oil and gas samples from the Otway Basin reveal a lot about their heritagewhen analysed with Geoscience Australia’s new tool.

The GC-IRMS, or gas chromatography-stable isotope ratio massspectrometer, analyses carbon isotopes (13C/12C), and last year a newcapability for analysing hydrogen (D/H) was added.

This addition has returned some interesting findings about the Otwaygases and oils and the value of hydrogen isotope analysis in tracingpetroleum family history.

Petroleum systemsPrevious geochemical studies of theOtway Basin focused mainly on theoils and oil stains. These studiessuggest, with varying levels ofconfidence, that the Otway hostsone of the largest number ofpetroleum systems of any basin inAustralia. Up to five different oilpopulations have been identified.

The Otway oils are thought tobe sourced from a range ofdepositional environments: fromfresh to saline lacustrine, to fluvio-lacustrine and peat swamp, tomarine. Potential source rocks areof Late Jurassic to Late Cretaceousage (150–65 million years).

Changes in depositional settingand source-rock character areclosely linked to the basin’sdevelopment (from initial rifting tothermal subsidence).

Figure 3. Maturity profile of vitrinitereflectance versus depth for offshorewells in the Otway Basin

Figure 2. Age profile of total organiccarbon for offshore wells in the OtwayBasin

Figure 1. The analysed gas and oil were sampled from these locations in the Otway Basin.

Prel iminary resul tsThe study will not be completeduntil June, but there are somepreliminary results: • Potential source rock intervals

through time can bedetermined using the estimatedages (Ma) at the top and baseof the samples. The onset ofcoal deposition at around 115million years is clearly markedby increases in total organiccarbon (TOC) concentrations(figure 2).

• Most Early Cretaceous (135–97Ma) potential source rocksoffshore are predominantly gasprone (HI<300 mg hydro-carbons/g TOC) with onlyminor liquids potential.Potential source rockscommonly enter the maturitywindow (VR > 0.65%) forpetroleum generation between2200 and 3400 metres depth inoffshore wells (figure 3).

• Vitrinite reflectance (VR)datasets vary greatly in qualityand consistency (figure 1). VRFanalyses help constrain existingVR data and are important forburial history modelling.

NEW TOOL splits petroleum populations

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Final results of the study will be presented at the Eastern Australian BasinSymposium II in Adelaide in September 2004.

For more details phone Chris Boreham on +61 2 6249 9488 or e-mailchris.boreham@ga.gov.au

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AusGEO News 73 March 2004 11

Gas chromatography-stableisotope mass spectrometer used inhydrogen isotope analysis

Figure 2. Gas components fromthe Otway Basin showing (a)hydrogen isotope values and (b)carbon isotope values, and gasand oil individual n-alkanecomponents for which hydrogenisotope values (c), and carbonisotope values (d) are shown.The open and solid symbolsrepresent the eastern andwestern Otway Basin,respectively.

Gas i sotopesThe analysed natural gases and oils (including condensates) show the Otwaypetroleum systems are strongly compartmentalised. Most hydrocarbons arerecovered from the Early Cretaceous Pretty Hill Formation in the westernOtway Basin or from the Late Cretaceous Waarre Sandstone in the easternOtway Basin (figure 1).

The hydrogen isotopic composition of individual C1–C5 hydrocarbon gascomponents is plotted in figure 2a. Two populations of natural gases areclearly seen with large hydrogen isotopic separations of 90‰ or morebetween populations.

The wide isotopic separation probably reflects the different climatic andhydrological conditions which had an impact on the hydrogen isotopiccomposition of water during the Early Cretaceous.

Carbon isotopes also differentiate the two gas populations, although atmuch lower isotopic resolution (figure 2b). The isotopic compositionaldifferences are also evident in the individual n-alkanes from oils andcondensates.

Gas–oi l corre lat ion The extended (C2+) n-alkane D-isotopic profile for representative gas–oilpairs indicate two significant populations in the Otway (figure 2c). Thewestern Otway gas–oil population is isotopically light (depleted in D)compared with the eastern population.

Carbon isotopes do not offer the same clear distinction of oils (figure2d). Nevertheless, the natural gases show a strong isotopic association withtheir respective oils, especially for D (figure 2c). This suggests that they weregenerated together from the same source.

The gases, oils and their effective source rocks also have a strongstratigraphic and geographic relationship, inferring mainly short- to medium-migration distances from source to trap.

Next s tep Hydrogen isotopes provide a more concise picture than carbon isotopes ofthe genetic relationships between gases and oils, and clearly distinguish twomain populations in the Otway.

Further testing is required to see if these D-isotope results are influencedby the higher susceptibility of hydrogen to isotopic exchange in thegeological environment. This aspect is being clarified through conventionalbulk carbon isotopic and biomarker studies, and an analysis of the hydrogenisotopic composition of the source organic matter.

The results of the Otway studies will be presented at the EasternAustralian Basin Symposium in Adelaide in September.

For more information phone Chris Boreham on +61 2 6249 9488 or e-mail chris.boreham@ga.gov.au

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12 March 2004 AUSGEO News 73

I N B R I E F

ElectoralboundariesreviewLocating electoral boundariesrequires mapping expertise, whichhas Geoscience Australia involvedin a review of ATSIC electoralboundaries.

Voting in the Aboriginal andTorres Strait Islander Commission(ATSIC) elections is based onzones, regions and wards—theboundaries of which crisscross thecountry. There are 16 zones dividedinto 35 regional councils, whichmay be sub-divided into wards.

The Aboriginal and TorresStrait Islander Commission Act(1989) requires elections to be heldevery three years. Voting is notcompulsory, but more than 53 000votes were recorded in the electionheld in 2002.

Under the Act a panel isconvened after each election toseek and review comments aboutthe election. The panel considerssubmissions and, if necessary, visitscommunities to discuss issues. Theprocess takes three to four months.

The Act requires GeoscienceAustralia to nominate a member tothe five-person panel because of itsmapping expertise. Its represen-tative is Bill Hirst (with assistancefrom Peter Richardson).

The other panel members arethree indigenous people appointedby the Minister and a representativeof the Australian ElectoralCommission.

The panel recently completedtwo reports to be presented to bothhouses of parliament. One reportoutlines suggested changes to theelectoral system and the otherrecommends changes to theelectoral boundaries. Theboundaries report will be availablefor public comment, so there maybe new submissions.

An augmented panel with aGeoscience Australia representativewill be appointed in March/April toconsider the report’s comments. Afinal boundaries report will then beprepared. This will complete theprocess required by the ACT—thatis until the after next ATSICelection.

Dog Fence mappedThe entire, dog-legged wire fence that winds 5400 kilometres across SouthAustralia and into southern Queensland is finally being surveyed, and willfeature on Geoscience Australia’s 1:250 000 topographic maps.

Geoscience Australia geographer Brian Polden says that prominentgeographic features can be sparse in remote areas.

‘A conspicuous fence is an important landmark for mapping’, he says. The Dog Fence was built in the late 1800s by graziers and governments

to keep dingoes away from sheep, although some sections along the NewSouth Wales border were originally rabbit proof fences.

The fence begins near Ceduna, then meanders north and east, and endsnear the Queensland border town of Goondiwindi.

It is almost two metres high and thought to be one of the longest builtstructures in the world. China’s Great Wall is 6700 kilometres long.

The Dog Fence is known by various names including Dingo Fence, WildDog Fence, Vermin Proof Fence, and Buffer Fence. It is sometimes confusedwith Western Australia’s famous Rabbit Proof Fence (or State Barrier Fence).

In New South Wales it is generally called the Border Fence because itfollows the border with South Australia and Queensland.

Vermin proof fences are vital in the management of dingoes and feralpests in rural areas. Ironically, on the ‘protected’ side of the fence, thenumbers of kangaroos, emus and some feral animals have increased withoutdingoes.

The whole Dog Fence will be mapped using the latest global positioningsystems. But it will take time to record and check the coordinates for everycorner and bend in the fence.

Satellite images will help to fix the position of the Dog Fence becausethe imagery can easily detect strips of cleared vegetation around the largefence.

Field verification and other information from the states with the DogFence will be used in conjunction with Geoscience Australia satellite imageryand map data to ensure that the fence is mapped as accurately as possible.

For more information phone Brian Polden on +61 2 6249 9466 or e-mail brian.polden@ga.gov.au

AusGEO News 73 March 2004 13

I N B R I E F

Improved nationalDEMThe nine-second Digital Elevation Model is the highestresolution, nationally consistent DEM of Australia. Sometimesthis resolution does not provide enough detail about terrainvariations, particularly for studies into soil salinity, landscapeevolution, and catchment management.

Geoscience Australia plans to improve the national DEM by using higherquality satellite data.

A potential source is the stereo imagery from the ASTER (AdvancedSpaceborne Thermal Emission and Reflectance Radiometer) instrument onboard the Terra spacecraft launched into Earth’s orbit in 1999.

ASTER acquires near-simultaneous stereo pairs of imagery (a stereo paircovers a 60-kilometre square of the Earth’s surface). This is an importantbenefit compared with stereo pairs collected on different dates, whereground cover conditions may have changed between acquisitions.

The spatial resolution (pixel size) of ASTER stereo data is 15 metres, butan ASTER DEM is normally at 30–50 metre resolution to reduce noise.

Geoscience Australia has been using a range of commercial software toassess the quality of ASTER DEMs. The results show that ASTER data couldprovide DEMs with a Root Mean Squared Error (RMSE) of about 15 metres inundulating terrain and about 20 metres in rugged terrain.

The generation of DEMs from the more advanced applications takes oneto two hours on high-performance PCs. The most time-consuming element isthe selection of Ground Control Points (GCPs) in the image pairs.

The GCPs are required to improve the planimetric accuracy of the DEMsto about 10 metres (RMSE), and to translate the DEM to a local height datum.

The quality of the DEMs generated by the different software wasgenerally comparable. The main differences were the use of GCPs in DEMgeneration and geolocation, and options in grid cell size.

Geoscience Australia is also testing the semi-automated use of GCPs inASTER DEM generation. The results will help determine the best datasets forthe improved national DEM.

Federal Industry Minister IanMacfarlane opened the vault to oneof the world’s largest and mostcomprehensive collections ofseismic data in GeoscienceAustralia’s headquarters inFebruary.

The old data tapes werecollected by mineral and petroleumexplorers over 40 years up to 1990.

The data are stored on 25thousand one-inch 21-track tapesand 300 thousand half-inch nine-track tapes, some of which arestarting to deteriorate.

Access to this data is critical tothe resources industry so the dataare being remastered onto moderncartridges. It will take up to threeyears and is part of a $25 milliongovernment funding allocation toacquire and preserve vitalinformation for petroleum andmineral exploration.

The data transfer will reducestorage space from a massive 6000shelf-metres to just 30 and willmake it easier and cheaper to sharethe data across the continent.

The $10 million remasteringexercise is being done by threePerth companies. SpectrumData andGuardian Data are copying thetapes, while GeoCom Services willprovide quality control.

New satellite data pendingSatellites are crucial to Geoscience Australia’s mapping program. It acquiresand interprets data from a number of satellites including the Landsat seriesand NASA’s Terra satellite.

Another satellite that interests Geoscience Australia is the Advanced LandObserving Satellite (ALOS) being developed by the Japan AerospaceExploration Agency (JAXA).

ALOS is a major global geospatial project and is scheduled to belaunched from the Tanegashima Space Centre, Japan in the northern summerof 2004. It will be one of the world’s largest earth-observation satellites.

ALOS is designed for high-resolution and precise land observation andwill contribute to disaster monitoring, resource surveying and mapping via itsthree main sensors: • the Panchromatic Remote-sensing

Instrument for Stereo Mapping (PRISM)composed of three sets of opticalsystems to measure precise landelevation;

• the Advanced Visible and NearInfrared Radiometer type 2 (AVNIR-2) to observe coverage of land surfaces; and

• the Phased Array type L-band Synthetic ApertureRadar (PALSAR) that enables day-and-night and all-weather landobservation.

Geoscience Australia (ACRES) will acquire and archive raw data fromJAXA, and process it for users in the Oceania region.

Vault openinglets data copying begin

Digital Elevation Modelfrom ASTER data

Nine-second DigitalElevation Model

14 March 2004 AUSGEO News 73

Interest in the Darling Basin in New South Wales was recently rekindled bythe Kewell East-1 stratigraphic well in the Neckarboo Trough. It reached1224 metres and penetrated a section containing marine microfossils.

Until this find, the few petroleum exploration wells drilled in the basinintersected organically lean continental sediments about 400–350 millionyears old, suggesting the basin has an unfavourable history for hydrocarbons.

There is now no doubt that 400 million years ago, some of thissprawling 100 000 square kilometre basin was covered by sea, and thatsediments deposited during this time could provide petroleum source rocks.

The Moomba–Sydney gas pipeline crosses the Darling Basin (figure 1),adding to its attractiveness for exploration.

Other studies suggest some structures in the ancient rocks that formedbefore the basin itself, may be worth a closer look for their mineral potential.

Recent s tudiesGeoscience Australia and the New South Wales Department of MineralResources have been interested in the Darling Basin for some time. Theyhave been carrying out seismic studies to better understand the basin’shistory and some of its poorly defined structures.

The work includes a seismic sequence analysis of the Pondie RangeTrough using 1980’s vintage BHP profiles, and a 160 kilometre-long deep-seismic reflection transect (figure 1).

The transect extended in a north-easterly direction across the BancanniaTrough (a remnant of the Darling Basin) and the ancient rocks of theKoonenberry Zone (about 1500–500 million years old). It was to investigatebasement structures that controlled basin partitioning, and highlight anyimplications for petroleum or minerals.

Pondie Range TroughThe 420–350 million-year-old Pondie Range Trough comprises a 12 000-metresedimentary section underlain by a deformed basement (figure 2).

The Early Devonian sediments (about 400 Ma) appear to include marinerocks that are a potential hydrocarbon source. Younger deposits (after ~384Ma) are less promising.

At that time the sea retreated and continental folding and uplift carriedriver and lake sediments into the basin. It became dominated by red-beds—layers of sediment deposited on the continent, which are red due to acoating of ferric oxide or hematite.

The traditional targets forexploration such as the PondieRange structure formed during thisperiod (384–370 Ma) when north-east to south-west compressivestress created various thrusts,anticlines and transpressionalstructures.

Other, possibly lower-riskexploration options are now morehighly rated. These includeanticlines unbreached by faulting,and compaction drape anticlinesover the flank of the Pondie RangeTrough at about 1300 metres depth,and possibly biohermal growths (amound or reef-like mass of rockbuilt up around the remains ofcorals, molluscs and other seacreatures).

Bancannia TroughtransectThe crustal structure north-east ofBroken Hill probably comprisesdiscrete south-west-dipping thrustzones superimposed on an older,deformed north-east-dippingbasement. From the Broken HillBlock, a Proterozoic shear(2500–545 Ma) seems to dip underthe Bancannia Trough and probablydetaches at Moho depths.

Most south-west-dipping thrustzones seem to relate to faultsystems mapped at the surface, butthey were thought to dip the otherway—that is, to the east or north-east, away from Broken Hill.

Marine remnants add to Darling Basin history

AusGEO News 73 March 2004 15

The distribution of blocksand shear zones north-east ofBroken Hill, and probablythroughout the Darling Basin, islargely due to events that tookplace 384 to 335 million yearsago. The shear zones extenddeep into the crust and mayhave been conduits for mineral-rich brines. These shears,especially around theKoonenberry Zone, shouldinterest mineral explorers.

The largely undeformedBancannia Trough is probablycomposed of Darling Basinsediments overlying older(~500–430 Ma) sediments orvolcanics. Much younger strataand alluvium veneer the area.

The Bancannia Troughunderwent north-east–south-westcompression (~384–370 Ma) andboth flanks appear to have beenoverthrust. The anticlines thatdeveloped near the margins wereeventually inverted and eroded.

By about 350 million years,the climate cooled and the searetreated even further from thecontinent. Glacial ice coveredareas bordering the basin by 285million years.

An in-depth look at thestructural evolution and resourcepotential of the Darling Basin hasbeen published as GeoscienceAustralia record 2003/05.

For more information phoneBarry Willcox on +61 2 6249 9273 or e-mailbarry.willcox@ga.gov.au

Figure 1. Darling Basin location map showing Pondie Range Trough, BancanniaTrough, the Moomba–Sydney gas pipeline, and the seismic transect

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