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Ashmore Platform

Release Areas AC16-3 and AC16-4, Ashmore Platform, Bonaparte Basin, Territory of Ashmore and Cartier Islands

Highlights

Bids close 23 March 2017

  • In the vicinity of the producing Montara Oil Project and the planned development at Crux
  • Good quality Triassic and Upper Cretaceous reservoirs
  • Effective source rocks in adjacent Vulcan Sub-basin and the northern Browse Basin
  • Untested source potential in Late Jurassic tilted fault blocks
  • Play types include Triassic horst blocks and Miocene reefs, as well as fan sandstones on the eastern margin

Release Areas AC16-3 and AC16-4 are located about 750 km west of Darwin in the Timor Sea, within the offshore area of the Territory of Ashmore and Cartier Islands that adjoins the border with Indonesia. Water depths range from about 90 m to 600 m (Figure 1). Release Area AC16-3 consists of 18 graticular blocks and lies within the northern part of the Browse Basin, extending onto the Ashmore Platform. Release Area AC16-4 comprises 50 graticular blocks and also lies within the northern Browse Basin, but extends into the western Vulcan Sub-basin and onto the Ashmore Platform. This Release Area is located adjacent to the Puffin oil field and the Skua oil field (part of the Montara Project), and also contains two gas discoveries; Keeling 1 and Great Auk 1. The nearest production facilities are located 20 km southeast in the adjacent Vulcan Sub-basin associated with the Montara Project.

The Ashmore Platform is an extensive and structurally complex feature. Tilted fault blocks developed prior and in association with Late Jurassic rifting in the Vulcan Sub-basin to the east, and during the breakup of the Argo margin to the southwest. This resulted in up to 4 500 m of shallow marine to fluvio-deltaic Permo-Triassic sediments (Figure 2) being heavily faulted and folded. This was followed by peneplanation, subsidence and burial in the Cretaceous–Cenozoic, with the deposition of up to 2 000 m of flat-lying shallow marine to deep marine Cretaceous and Cenozoic strata. The stratigraphic succession is similar to the neighbouring Vulcan Sub-basin but Jurassic sediments are either thin or absent. This includes the Plover and Vulcan formations, major gas sources and reservoir units within the Vulcan Sub-basin (Figure 2). The platform is divided into two structural provinces; a western terrain with mainly west-dipping faults, and an eastern terrain with both east- and west-dipping faults. These terrains are divided by a westerly concave fault zone that formed in response to pre-breakup rifting (towards the west of the Ashmore Platform) and rifting in the Vulcan Sub-basin (towards the east). The Release Areas are both located in the eastern terrain. Miocene-Pliocene convergence of the Australian and southeast Asian microplates resulted in extensive fault reactivation on the Ashmore Platform.

The Ashmore Platform has good quality Triassic Sahul Group and Upper Cretaceous Puffin Formation sandstone reservoirs sealed by the regional Bathurst Island Group, although this unit thins onto the platform. The source potential of the Late Jurassic fault blocks is untested but there are likely migration pathways from the neighbouring source kitchens of the Vulcan Sub-basin and the northern Browse Basin, at least onto the margins of the platform. Play types include numerous Triassic horst blocks, tilted fault blocks, anticlines and Miocene reefs as well as Puffin Formation fan sandstones on the eastern margin (Figure 3). The oil in the Puffin field is hosted in Upper Cretaceous fan sandstones of the Puffin Formation in the Puffin horst and is sourced from the Upper Jurassic lower Vulcan Formation in the adjacent Swan Graben. Brown Gannet 1 tested a large faulted anticline and encountered gas shows within the Paleogene–Neogene Cartier Formation.

Release Area AC16-3 extends into the Vulcan Sub-basin and contains the Keeling 1 and Great Auk 1 gas discoveries that are hosted within anticlinal closures in Triassic and Cretaceous reservoirs, respectively. The neighbouring Skua field is located on the Skua High and had produced more than 20 MMbbl from the Plover Formation before being abandoned (Mining Oil Gas Pty Ltd., 2015).

Release Area AC16-4 is located largely on the Ashmore Platform but extends into the neighbouring northern Caswell Sub-basin of the Browse Basin where Gryphaea 1 and Discorbis 1 wells produced oil shows. Gryphaea 1 targeted Upper Triassic sandstones and an Aptian calcarenite in a fault-dependent closure on an east-west rotated fault block. The reservoir quality at this location is poor. Discorbis 1 targeted the Puffin and Echuca Shoals formations in a large fault-dependent crest (Le Poidevin et al, 2015). Here, the Puffin Formation has good porosity but sandstones of the Echuca Shoals Formation were absent and there was uncertainty concerning structural closure.

Figure 4 shows the existing data coverage for the region. The wells Champagny 1/ST1, Delta 1, Great Auk 1, Grebe 1, Keeling 1, Lucas 1, Prion 1, Snipe 1/ST1 and Woodbine 1 all lie within Release Area AC16-3, while Release Area AC16-4 contains the wells Arlo 1, Discorbis 1/ST1, Gryphaea 1/ST1, Mount Ashmore 1, 1A, 1B/ST2 and Turbo 1. Both Release Areas are partially covered by 3D seismic. Release Area AC16-3 is partially covered by the Onnia2 3D cube. Release Area AC16-4 is partially covered by the North Browse 3D cube. The Ashmore Platform region is covered by regional data of various vintages, with good quality regional 2D speculative seismic survey grids acquired across the platform between 1996 and 1998 (Figure 4).






Figures

Figure 1

Structural elements of the Ashmore Platform in the vicinity of the 2016 Release Areas, showing petroleum fields, key wells, and the 2016 Release Areas


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Figure 2

Stratigraphy, basin phases and hydrocarbon shows in the Ashmore Platform and Vulcan Sub-basin of the bonaparte Basin, and the Caswell Sub-basin of the Browse Basin, based on the Bonaparte Basin Biozonation and Stratigraphy Chart (Kelman et al, 2014). Geologic Time Scale after Gradstein et al (2012)


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Figure 3

 Play types in the vicinity of the Release Areas on the Ashmore Platform and Vulcan Sub-basin, Bonaparte Basin


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Figure 4

Seismic and well data in the vicinity of the Release Areas on the Ashmore Platform and Vulcan Sub-basin, Bonaparte Basin


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