Release area NT02-1 comprises 56 whole and part graticular blocks and covers an area of approximately 4200 square kilometres. NT02-1 is located in the eastern Timor Sea, 400 kilometres northwest of Darwin (Figure 1). Water depths range from around 60 metres in the south to 200 metres in the central north of the area. Along the northern boundary of NT02-1, water depth increases rapidly to around 600 metres. Water depths at Sunset Shoal, Troubadour Shoals (central portion of NT02-1) and Loxton Shoal (northeastern part of NT02-1) are less than 20 metres.
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Area NT02-1 is located near the eastern margin of the Sahul Platform (southern flank of the Troubadour High) in the northern Bonaparte Basin (Figure 2). The greater Sahul Platform comprises a series of horsts and grabens, bounded by the Sahul Syncline to the southwest, the Timor Trough to the northeast and the Calder and Malita Grabens to the east and southeast. The Flamingo Syncline separates the Flamingo High at the southwest margin of the platform.
The present day structure of the Bonaparte Basin has resulted from the intersection of two major rift systems. The older, northwest-trending Petrel rift system is of Late Devonian to Early Carboniferous age and extends from the Petrel Sub-basin out into the Timor Sea (Malita Graben). The younger, northeast-trending rift system is the result of Early Permian and Late Jurassic-Early Cretaceous extension, with post-rift subsidence in the Malita Graben and Sahul Syncline continuing throughout the Cretaceous and Early Cainozoic. A period of significant structuring occurred in the Late Miocene, with the progressive, oblique collision of the Australian plate with the South East Asian microplates forming the Timor Trough and establishing the present configuration of the Sahul Platform and its strongly faulted northern margin. Later fault rejuvenation during the Pleistocene to the Holocene created the Timor Trough and probably represents a trap-forming episode on the Sahul Platform. The stratigraphy of the region is summarised in Figure 3.
Troubadour-1, drilled on the northern margin of the Sahul Platform in 1974, encountered recrystallised, Late Permian carbonates of the Hyland Bay Formation overlying granitic basement (Figure 4). These units are overlain in the Sahul Platform area by Late Permian to Early Triassic marine siltstones and shales of the Mount Goodwin Formation. This was followed by deposition of the Triassic Sahul Group, a mixed clastic-carbonate succession deposited in shallow marine to fluvio-deltaic environments. A Late Triassic marine regression, induced in part by regional uplift associated with the Fitzroy compressional movement, culminated in the deposition of fluvio-deltaic redbeds (Nome and Malita Formations) across the region.
Subsequent transgression in the Early to Mid Jurassic resulted in the deposition of thick fluvio-deltaic to marine sediments of the Plover Formation, including the important reservoir sandstones of the Sunrise and Troubadour fields (Figure 3). Marine influence within the Plover Formation increases from the southwest to the northwest across the Sahul Platform.
Over the greater Sunrise area, the Plover Formation is para-conformably overlain by marine sandstone/shale sequences of late Callovian to early Oxfordian age (Laminaria Formation). This unit constitutes both a reservoir and source rock interval in the Sunrise-Troubadour field. In other areas of the Bonaparte Basin, a period of block faulting and uplift during the Callovian resulted in an unconformity between the Plover Formation and the Laminaria Formation/Frigate Shale. Across the Sahul Platform and greater Sunrise area, however, the main episode of faulting occurred during the latest Jurassic. This led to the development of east-west trending horsts and grabens and a major Late Jurassic unconformity.
Deposition of marine claystones and siltstones of the overlying Flamingo Group (Tithonian to Berriasian in age) is widespread in the Malita Graben. This unit onlaps the Sahul Platform. The areal distribution of the Flamingo Group and its potential as an exploration target in area NT02-1 remains uncertain.
Following the onset of sea-floor spreading in the mid-Valanginian, subsidence of the Australian continental margin resulted in the widespread deposition of a condensed section of glauconitic, marine claystone (Valanginian to Early Aptian, Echuca Shoals Formation). The peak of this trasgression is represented by a condensed, radiolarian claystone and calcilutite (Darwin Formation, sensu Whittam et al., 1996). A thick Aptian to Maastrichtian progradational section of claystone, calcilutite and marl (Jamieson and Wangarlu Formations) overlies this condensed section and fills the accommodation space created by the rapid subsidence of the Australian margin after break-up. This section is in turn overlain by a thick succession of Cainozoic marine shelf carbonates.
Exploration in the offshore Bonaparte Basin commenced in the early 1960's with gravity and aeromagnetic surveys. These were followed by regional and semi-detailed 2D seismic surveys. The first exploration wells in the Bonaparte Basin were drilled in the early 1970's.
Area NT02-1 was previously awarded as exploration permit NT/P56 for a six year term (commencing in January 1999) to Roma Petroleum Company Pty Ltd and Guinness Peat plc. In early 2001, at the conclusion of Year 2 of the primary exploration program, exploration permit NT/P56 was cancelled due to non-fulfilment of commitments in the minimum guaranteed work program by the permittees.
The area is covered by regional, 2D seismic surveys acquired in 1965 and 1966. In 1974, more detailed seismic coverage led to the drilling of the first exploration well in the vicinity of NT02-1 (Troubadour-1). To date, no wells have been drilled within release area NT02-1.Wells
Troubadour-1 (1974) was drilled northwest of NT02-1 to test a northeast-trending faulted anticline located on the crest of the Sahul Platform. Gas and condensate were recovered from the Jurassic Plover and Laminaria Formations. A drill stem test taken over the interval 2228-2244 metres within the Plover Formation flowed gas at 279 000 cubic metres (9 850 000 cubic feet) per day and condensate at 39 cubic metres (245 barrels) per day.
Sunrise-1 (1974) was drilled 17 kilometres north of Troubadour-1 on a large northeast-trending anticline on the northern margin of the Sahul Platform. Gas and condensate were recovered on RFT from two sandstone intervals in the Jurassic Plover Formation.
An appraisal well (Sunrise-2) was drilled in 1998. This well was located 15 kilometres north-northwest of Sunrise-1 and programmed to delineate the northern extension of the gas/condensate accumulation intersected by Sunrise-1. Sunrise-2 intersected a 151 metre gross gas column. A drill stem test taken over the interval 2112-2124 metres KB, flowed gas at a maximum rate of 849 500 cubic metres per day through a two inch choke.
Loxton Shoals-1 (1995) was also drilled on a northeast-trending anticline on the northern margin of the Sahul Platform. Located 12 kilometres northeast of Sunrise-1, gas and condensate were recovered from the Plover Formation with an MDT tool. The Loxton Shoals gas/condensate discovery is thought to be a separate accumulation trapped in a fault-block, adjacent and to the north of the Sunrise accumulation.
Sunset-1 (1997) was drilled on the same horst block and to the west of Sunrise-1, in the then Timor Sea Zone of Cooperation (ZOCA 95-19). A drill stem test taken in the Plover Formation flowed gas at a rate of 906 100 cubic metres (32 million cubic feet) per day and condensate at 4.45 cubic metres (28 barrels) per day through a 28.5 millimetre choke.
Sunset West-1 (1998), drilled approximately 8 kilometres to the west of Sunset-1, flowed gas on drill stem test from the Plover Formation at a rate of 555 000 cubic metres (19.6 million cubic feet) per day.
The Sunrise and Troubadour fields comprise a complex of large, east-west elongated fault blocks with approximately 180 metres of vertical relief. The complex is bounded to the northwest by a large fault that also forms a spill-point for the structure. A central, east-trending fault separates the Sunrise and Troubadour accumulations (Seggie et al., 2000). The main phase of structuring at Sunrise/Troubadour occurred during the Pleistocene when the Sunrise-Troubadour High was reactivated as a consequence of rapid subsidence of the Timor Trench associated with the emplacement of the Timor accretionary prism.
The P50 estimate for gas-in-place and condensate-in-place at Sunrise / Sunset / Troubadour / Loxton Shoal is 473 billion cubic metres (16.7 trillion cubic feet) of gas and 8.2 gigalitres (325 million barrels) of condensate, respectively. The average condensate-gas ratio has been calculated at 247 cubic metres of condensate per million cubic metres (44 barrels of condensate per million cubic feet) of gas (Seggie et al, 2000). Development proposals for Sunrise/Troubadour are currently being evaluated with possible production to begin as early as 2005 (Seggie et al., 2000).
The successful delineation of a major gas condensate resource in the greater Sunrise area has enhanced the perceived prospectivity of the region. Release area NT02-1 lies between the Sunrise / Sunset / Troubadour / Loxton Shoal gas/condensate accumulation(s) and the main source "kitchen" area, the Malita Graben. Source rocks in the area are largely gas-prone but liquids content should be significant given the condensate reserves established in the greater Sunrise/Troubadour area. Isotopic and biomarker analysis of condensates recovered from the Sunrise area suggest these accumulations represent a separate hydrocarbon family to other oils and condensates recovered from the Bonaparte Basin. A definitive condensate-source correlation has yet to be established for Sunrise/Troubadour (Edwards et al., 2000).
East-northeast trending horst blocks provide the main traps in the area and the primary exploration targets are the Plover and Laminaria Formations. Latest Jurassic, Flamingo Group sandstones comprise a secondary target. The source, reservoir and seal relationships operating within NT02-1 should be analogous to those existing in the greater Sunrise area, as described by Seggie et al (2000).
Source rocks intervals occur in the Plover Formation, Laminaria Formation, Frigate Shale and Flamingo Group (Preston & Edwards, 2000). Little is known of these source units in the Malita Graben but they are expected to be mature for gas generation in this location. It is likely that local source pods on the northern flank of the Malita Graben are oil-mature and could charge traps that occur within migration shadows from the main gas charge from the deeper portions of the Malita Graben. There may also be some potential for oil charge from the Sikitan Syncline to the southwest. At Troubadour-1, the Plover and Laminaria Formations and the Flamingo Group are at optimum thermal maturity for oil generation, whilst the Permian Hyland Bay Formation is gas mature.
Full information on wells, seismic surveys and other open-file data relevant to this area is available from the Northern Territory Department of Mines and Energy (NTDME), Darwin, as well as from Geoscience Australia (GA), Canberra. Digital wireline logs from wells drilled in adjacent acreage are available from Wiltshire Geological Services, Adelaide.
A full listing of the relevant open-file data held in Geoscience Australia's Repositories is available. Geoscience Australia's geological databases provide detailed biostratigraphic (STRATDAT), geochemical (ORGCHEM) and reservoir, hydrocarbon show and interpreted depositional environment (RESFACS) information from exploration wells.
Exploration wells and seismic surveys that impinge on or lie within both NT02-1 and adjacent acreage are tabulated below:
|Well||Year||NTDME Ref No.|
|Loxton Shoals 1||1995|
PR95/27(basic data only)
|Sunset 1||1997||Available from GA|
|Sunrise 2||1998||PR99/125 basic data only|
|Sunset West 1||1998||Available from GA|
|Bard 1||1998||Available from GA|
|Seismic Survey||Year||NTDME Ref No.|
Edwards, D.S., Kennard, J.M., Preston, J.C., Summons, R.E., Boreham, C.J. & Zumberge, J., 2000 - Bonaparte Basin: Geochemical characteristics of hydrocarbon families and petroleum systems. AGSO Research Newsletter 33, 14-19.
Northern Territory Geological Survey, 1990 - Petroleum Basin Study Bonaparte Basin. Prepared by Petroconsultants Australasia Pty Ltd. Northern Territory Government Printer.
Preston, J.C. & Edwards, D.S., 2000 - The petroleum geochemistry of oils and source rocks from the northern Bonaparte Basin, offshore Northern Australia. The APPEA Journal 40(1), 257-281.
Seggie R.J., Ainsworth R.B., Johnson D.A., Koninx J.P.M., Spaargaren B. & Stephenson P.M., 2000 - Awakening of a Sleeping Giant: Sunrise-Troubadour gas-condensate field. The APPEA Journal 40(1), 417-435.