AUSTRALIA AND EASTERN INDONESIA.PDF

SEAPEX Exploration Conference 2001Orchard Hotel, Singapore

4th – 6th April 2001

Copyright SouthEast Asia Petroleum Exploration Society

AUSTRALIA AND EASTERN INDONESIA AT THE CROSS-ROADS OF GONDWANA AND TETHYS - THE IMPLICATIONSFOR PETROLEUM RESOURCES

By Marita Bradshaw,AGSO, GPO Box 378, Canberra 2601 [email protected]

Since Wallace drew his line, the Australasian region has been recognised asa pivotal place on the earth's surface, where across a sharp gradient we stepfrom one world to another. In the biological realm, Asian elephants, tigers andweaver birds are juxtaposed to Australian cockatoos, giant goannas andmarsupials across a narrow gap of deep water running between Borneo andSulawesi and continuing between Bali and Lombok (van Oosterzee, 1997).These faunal changes reflect a geological history along the border zonebetween the great southern continent of Gondwana, the ancient Tethyanocean and an evolving Asian landmass.

The legacy of this history for petroleum exploration is a complex and diverseregion where a multitude of petroleum systems, ranging in age from theCambrian to the Neogene, can be exploited (Bradshaw et al., 1997).Hydrocarbons have been generated from marine, lacustrine, deltaic and coalswamp facies; and both carbonate and siliciclastic source rocks occur. Themost prolific petroleum systems are the product of organic rich rocksaccumulated in tropical environments, not unlike the present day regime.

There are important synergies to be gained in considering the petroleumgeology of the Australasian region as a whole, and in particular by recognisingthe shared petroleum systems between northern Australia, New Guinea,Timor and other parts of eastern Indonesia. The complex geology of theregion is a natural consequence of its location at the plate boundary today andin the past. This history is being progressively unravelled and a number ofdetailed plate model reconstructions (Hall, 1999; Metcalfe, 1997) are nowavailable for the region. Just as macro-fossils such as Glossopteris helped inthe recognition of the various fragments of Gondwana, so molecular fossilssuch as biomarkers in oils (Peters and Moldowan, 1993) may also be used toidentify relationships between areas later disconnected by plate movements.

The comparisons and linkages between the petroleum systems and oilgeochemistry of the region has been previously discussed by ten Harven andSchiefelbein (1995); Bradshaw et al., (1997), Howes (1997), Schiefelbein andCameron (1997), Todd et al., (1997) and others. This abstract aims to providean up date for the Australian region to the information compiled in Fraser etal., (1997) and Howes and Noble (1997). In the intervening years, newpetroleum systems have been identified and have come into production. Thegulf in age between the Indonesian systems, dominated by oil sourced from




Tertiary rocks, and the Jurassic and older Australian petroleum systems hasnarrowed. Discoveries attributed to Permian source rocks have been madeboth sides of the border - giant gas fields Irian Jaya, and more modestaccumulations of gas, condensate and oil on Australia’s North West Shelf.The Cretaceous is now recognised as a proven source rock interval in theBonaparte and Browse Basins and in onshore Irian Jaya.

Early PalaeozoicIn the Cambrian Australia was locked into the jig-saw puzzle of Gondwanawith various continental blocks now located in Asia arrayed along the currentNorth West Shelf margin (Fig.1). As today, the region straddled the equatorand shallow seas stretched through central and northern Australia, across thewhat is now the Arafura Sea and the southern half of New Guinea; and on intothe continental blocks that later amalgamated to form China and south-eastAsia (Metcalfe, 1997). There is oil production from Ordovician reservoirs at theopposite ends of this now dismembered shallow seaway - in central Australia(Mereenie field, Amadeus Basin) and in the Tarim Basin of western China(Fig. 1).

Carbon-rich oil prone source rocks were deposited in these warm shallowwaters of the palaeo-tropics. In terms of organic matter content, the Early




Palaeozoic marine shales deposited in northern and central Australia are therichest potential source rocks on the continent. They have been linkedtogether as part of the Larapintine Petroleum Supersystem (Bradshaw et al.,1994) and their distinctive oil geochemistry has been described by AGSO andGeoMark (1996), Edwards et al., (1997) and Summons et al., (1998). Richsource rock intervals are recognised in the early to middle Cambrian, theEarly Ordovician and Late Devonian.

The prospective zone for the Larapintine Petroleum Supersystem extendsacross the stable platform from northern Australia into eastern Indonesia(Moore et al., 1996; Bradshaw et al., 1997) as further confirmed by the recentinterpretation of re-processed seismic data reported by Moss (2001).Significant oil shows have been reported from the Palaeozoic sediments ofthe Arafura Basin in both Australian and Indonesian waters. Oils correlated toCambrian source rocks (Edwards et al., 1997; Summons et al., 1998) havebeen recovered from Ordovician carbonates and Devonian sandstones in theAustralian sector in Arafura -1 (Bradshaw et al., 1990); and oil shows arereported from Siluro-Devonian carbonates in Kola-1 (Moss, 2001) locatedoffshore Irian Jaya. Advances in the understanding of the thermal history ofthe region (Moore et al., 1996) and improved seismic coverage will enable




explorers to better target the Palaeozoic potential of the Arafura Basin thanthe previous limited drilling.

Late PalaeozoicIn the late Devonian, the Palaeo-Tethys ocean opened separating the SouthChina, Tarim, Indochina and North China blocks from Gondwana (Metcalfe,1997). The Caledonia Orogeny in China and the Alice Springs Orogeny inAustralia signal these events; and from this time onwards the geologicalhistories of Australia and China diverged dramatically.

Australia as part of Gondwana, shifted southwards into polar regions, and bythe Late Carboniferous-earliest Permian was partially covered by an ice sheet.Tropical conditions continued on the Chinese continental fragments, with thecharacteristic Cathaysian faunas being in sharp contrast to the glacialdeposits and high latitude coals typical of the Gondwanan continents.

Onshore Permian coal basins are important suppliers of domestic gas inAustralia (Bowen and Cooper Basins) and undeveloped Permian gasaccumulations also occur offshore in the Petrel Sub-basin, of the BonaparteBasin (Fig. 2). Kennard et al., (2000) and Edwards et al., (in press) detail theevidence for a Permian petroleum system that has sourced oil, as well as gasand condensate in the Bonaparte Basin. There is a close similarity of the LatePermian sedimentary sequences from northern Australia and Irian Jaya. Thegeological and geochemical evidence (Perkins and Livsey, 1993; Bradshaw etal., 1997) indicates that the coals and associated shales of the Permian AinimFormation are the likely source of the giant gas fields (Wiriagar, Vorwata) inthe Birds Head (Fig. 2). In this context, one is left to speculate whetherPermian sediments are also the source for the recent Abadi-1 gas/condensatediscovery in the Indonesian Masela PSC Block (Oil and Gas Australia, 2001)in the eastern Timor Sea.

MesozoicBy the Triassic, the Meso-Tethys had opened as the Cimmerian continentrifted off from the Gondwanan core (Metcalfe, 1997). The Mesozoic carbonatebelt running along the northern shores of Tethys is well known from theAustrian Alps to the Middle East (Baudin, 1995), and evidence hasaccumulated in recent years of a similar zone mirrored on the southern shoresof Tethys (Fig. 3). ODP drilling unexpectedly recovered Rhaetian reefalcarbonates from the northern margin of Australia's Exmouth Plateau (Exon etal., 1991). There is also some geochemical evidence to suggest that the oilssourced from Mesozoic carbonate facies seen in Seram, Buton, Buru andTimor may also extend to the outer Bonaparte and Browse Basins (Edwardset al., in press) and to the Carnarvon Basin (George, et al., 1998). A picture isemerging of another petroleum system developed out beyond the typical LateJurassic Westralian system (Bradshaw et al., 1994) of the North West Shelf.This “Sahul” system (Summons et al., 1998) is characterised by hydrocarbonssourced from deep water marls rather than marine shales with a significanthigher plant input as found in the inboard failed rifts (Bradshaw et al., 1994).




By the Late Jurassic, the last of the continental fragments had rifted fromAustralia's north-west margin, and the Indian Ocean began to open as themajor blocks of Gondwana separated. The Late Jurassic WestralianPetroleum Supersystem extends along the western and northern Australianmargin from the Carnarvon Basin to the Papuan Basin (Bradshaw et al.,1994); and there is now evidence of viable Cretaceous oil source rocks over asimilar wide geographic range. Oil accumulations sourced from EarlyCretaceous marine shales are now proven in the Browse and BonaparteBasins (Blevin et al., 1998; Preston and Edwards, 2000); and McConachie etal., (2000) report an Early Cretaceous petroleum system from Warim in thefoldbelt of Irian Jaya. However, in the global context, the Australasian regionis very under-represented in oil reserves derived from mid Cretaceous(Aptian-Turonian) source rocks (Klemme and Ulmishek, 1991). This gap in thedistribution may be filled with further finds on the North West Shelf and intoeastern Indonesia, as well as along Australia's southern margin (Totterdell etal., 2000).

CainozoicThe region has had a dramatic Cainozoic tectonic history as the Australian,Pacific and Asian plates have collided, subducted, buckled, fragmented andsheared (Hall, 1999) producing a host of young petroliferous basins.




Cainozoic petroleum systems are dominant in Indonesia, ranging fromEocene lake basins in Sumatra, deltaic complexes in Kalimantan, to Miocenecarbonates in the Salawati Basin of the Birds Head (ten Haven andSchiefelbein, 1995). In contrast, on the old Gondwanan core of Australia mostknown hydrocarbons are derived from Mesozoic and Palaeozoic sources. Theone significant exception being the Gippsland Basin in south-eastern Australiawhere the Late Cretaceous to Eocene lower delta plain deposits of theLatrobe Group are the accepted source rock. However, Eocene oil shalesoccur onshore in Queensland in small lake basins close to the currentcoastline where a pilot plant has been established to retort oil from thecrushed shale. Perhaps these organic-rich facies may also occur offshore inbasins where they may be mature for oil generation (Longley et al., 2001).

ACKNOWLEDGMENTSBrian Hack is thanked for drafting, and John Laurie, Roger Summons andClinton Foster are thanked for reviewing the text. The author publishes withthe permission of the Chief Executive Officer, Australian Geological SurveyOrganisation.

FiguresFigure 1. Reconstruction of eastern Gondwana for the Cambro-Ordovicianfrom Metcalfe (1997). Land shown in orange, shallow seas in light blue, deepsea in dark blue and basins with Cambro-Ordovician petroleum systemsshown in green. NC = North China, SC = South China, T = Tarim, I =Indochina, QI = Qiangtang, L = Lhasa, S = Sibumasu, WB = West Burma, GI= Greater India. Land and sea distribution compiled from Golonka et al.,(1994) and Struckmeyer & Totterdell (1990).

Figure 2. Reconstruction of the Tethyan region for the Late Permian fromMetcalfe (1997). Land shown in orange, shallow seas in light blue, deep seain dark blue. Gas fields in the Bonaparte Basin, Australia and Irian Jayaconsidered to be sourced from Late Permian source rocks indicated by redstars. NC = North China, SC = South China, I = Indochina, WC = WesternCimmerian Continent, QI = Qiangtang, L = Lhasa, S = Sibumasu. Land andsea distribution compiled from Golonka et al., (1994) and Struckmeyer &Totterdell (1990).

Figure 3. Reconstruction of the Tethyan region for the Late Triassic fromMetcalfe (1997). Land shown in orange, shallow seas in light blue, deep seain dark blue. Areas of carbonate facies shown in green from Baudin (1995).NC = North China, SC = South China, I = Indochina, SG = Songpan Ganziaccretionary complex, WC = Western Cimmerian Continent, QI = Qiangtang,S = Sibumasu, L = Lhasa, WB = West Burma. Land and sea distributioncompiled from Golonka et al., (1994) and Struckmeyer & Totterdell (1990).




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