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Southern Browse Basin

Release Area W16-4, Southern Browse Basin, Western Australia

Highlights

Bids close 23 March 2017

  • Proven Mesozoic oil and gas province
  • Water depths less than 250 m
  • Large variety of structural and stratigraphic plays including Jurassic horsts/tilted fault blocks and associated drape anticlines, Cretaceous drape over erosional, fractured basement highs and Cretaceous submarine canyon fill and basin floor fans
  • Further guidance available, please refer to 2016 Special Notices

Release Area W16-4 is located mostly on the Yampi and Leveque shelves of the Browse Basin, a mature exploration province 100 km off the northwest coast of Western Australia. The Release Area also extends into the Caswell Sub-basin (Figure 1). Release area W16-4 contains 312 graticular blocks (approximately 25 655 km2) and water depths of 60-250 m. To the north, the permit is adjacent to the Burnside and Prelude gas fields. The Caspar 1/1A (gas) and Gwydion 1 (gas and oil) discoveries are located to the northeast of the release area. The release area contains the Psepotus 1 gas discovery, as well as gas shows at Leveque 1. The Ichthys Pipeline intersects the northern part of the release area.

The Browse Basin is one of a series of extensional basins that form the Westralian Superbasin and has undergone six major tectonic phases during its development. This includes middle Carboniferous–early Permian extension; early Permian to Late Triassic thermal subsidence; Late Triassic to Early Jurassic inversion; Early to Middle Jurassic extension; Late Jurassic to Cenozoic thermal subsidence; and Middle Miocene to Holocene inversion (Figure 2).

The Yampi and Leveque shelves marks the southeastern edge of the Browse Basin (Figure 1), and is characterised by shallow, gently basinward-dipping to relatively flat-lying basement. This basement is typically highly eroded with a distinct, rugose palaeotopographic relief onlapped by Permian to Mesozoic and Cenozoic sediments that progressively thin across the terrace and shelf from about 5 km to less than 500 m thick (Figure 3). The Leveque Shelf forms the offshore continuation of the King Leopold Mobile Zone, and the northern margin of the mobile zone marks the boundary between the Yampi and Leveque shelves. The edges of these shelves are characterised by faulting in places that are of varying ages and not necessarily linked. The basinward boundary of the Leveque and Yampi shelves is defined by a ‘hinge’ where the dip of the basement changes from relatively flat lying to gently basinward-dipping. Beyond this hinge the Prudhoe Terrace forms an intermediate depth fault-bounded terrace along the southeastern flank of the Caswell and Barcoo sub-basins (Hocking et al, 1994; Struckmeyer et al, 1998). The Caswell Sub-basin (Hocking et al, 1994) is one of the major depocentres of the Browse Basin and contains over 15 000 m of Paleozoic to Cenozoic sediments (Figure 1).

Numerous petroleum systems have been proven within the Caswell Sub-basin and the Cornea accumulation on the Yampi Shelf hosts oil and gas in the Cretaceous Heywood Formation sandstone reservoir. The Release Area is adjacent to three proven Mesozoic petroleum systems in the Browse Basin, which could have sourced hydrocarbons that may be present with the release area. The oils at Gwydion 1 on the Yampi Shelf, gas and oil are encountered in Echuca Shoals Formation sandstones. The gas is dry as a result of biodegradation (AGSO and Geotech, 2000). Likewise, although the oil is slightly biodegraded, it has a molecular composition similar to those of the Cornea and Caswell 2 oils which have been correlated to marine source rocks in the Echuca Shoals Formation (Boreham et al, 1997; Blevin et al, 1998a; Edwards and Zumberge, 2005). An additional input to the accumulation from the underlying Jurassic section has also been proposed by Spry and Ward (1997). The wet gas discovered in the Brewster Member of the upper Vulcan Formation in the Ichthys accumulation is believed to be sourced from the Vulcan Formation (Grosjean et al, 2015) with the adjacent Burnside gas accumulation a possible extension of this petroleum system. The basinwide, dry gas-prone petroleum system believed to be sourced from the Lower–Middle Jurassic Plover Formation may also have generated gas that has migrated to the margins of the basin, as proven by the isotopically heavy 13C signature of neo-pentane at Cornea South 2 and Macula 1 (Boreham and Edwards, 2008; Grosjean et al, 2015).

The Caswell Sub-basin contains proven combined structural and stratigraphic plays including Jurassic horsts/tilted fault blocks and associated drape anticlines, and Cretaceous submarine canyon fill and basin floor fans (Figure 3). On the Prudhoe Terrace and the Yampi and Leveque shelves, stratigraphic traps include Cretaceous drape over erosional, fractured basement highs; for example, the gas accumulation at Psepotus 1, and the Cornea and Gwydion oil accumulations (Brown, 2015; Rollet et al, 2016) (Figure 3).

Figure 4 shows the existing data coverage of the region. The release area contains the Carbine 1, Eupheme 1, Leveque 1 and Psepotus 1 wells. There is variable 2D seismic coverage, with the largest line spacing around 5 km. The Adele Trend 3D seismic covers the northern edge of the release area.






Figures

Figure 1

Structural elements of the southern Browse Basin in the vicinity of the 2016 Release Area, showing petroleum fields, key wells, and the 2015 and 2016 Release Areas


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Figure 2

Stratigraphy and hydrocarbon shows on the Leveque and Yampi shelves, based on the Browse 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 Area in the southern Browse Basin


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Figure 4

Seismic and well data in the vicinity of the Release Area in the southern Browse Basin


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