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Northwestern Caswell Sub-basin

Release Area W16-3, North-western Caswell Sub-basin, Browse Basin, Western Australia

Highlights

Bids close 8 December 2016

  • Adjacent to Argus and Crown/Proteus gas accumulations
  • Close to Ichthys Gas Pipeline
  • Quality reservoirs and seals, multiple potential oil- and gas-prone source rocks and active petroleum systems
  • Further guidance available, please refer to 2016 Special Notices

Release Area W16-3 is located in the outer northwest margin of the Caswell Sub-basin, 300 km off the northwest coast and consists of 1 graticular block (85 km2) in water depths of 440–460 m (Figure 1). The Release Area is adjacent to the Argus and Crown/Proteus gas accumulations. It is also proximal to the Boreas 1, Poseidon 1, 2, Poseidon North 1 and Kronos 1 gas discoveries.

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 Caswell Sub-basin forms one of the major depocentres of the Browse Basin, containing up to 15 000 m of Paleozoic to Cenozoic sediments (Figure 2). The northern part of the Caswell Sub-basin is referred to as the Abalone Sub-basin by Lawrence et al (2014) based on Proterozoic and Paleozoic structural trends identified in gravity and seismic data.

Release Area W16-3 is located adjacent to, and on trend with, a number of outer Browse dry gas accumulations including from the southwest to the northeast; Calliance, Brecknock, Torosa, Crown/Proteus and Argus (Figure 1). These are reservoired in the Plover Formation, and sourced from a mixed terrestrial and marine organic matter, presumably within the Plover Formation.

The Lower–Middle Jurassic Plover Formation is the most laterally extensive reservoir across the Caswell Sub-basin and is characterised by massive high-energy, cross-bedded channel sandstones. At the Brecknock and Calliance accumulations, the upper Plover Formation is the main reservoir, while the lower Plover Formation acts as the main reservoir at Torosa. The older sandstones were deposited in a fluvial-dominated upper delta plain, with the younger sandstones reflecting deposition in a tidally influenced lower delta plain environment (Tovaglieri et al, 2013; Tovaglieri and George, 2014). Along the Scott Reef Trend, good reservoir properties are preserved, particularly within sandstones of the C. torosa spore/pollen zone, which may be evidence of either an early hydrocarbon charge or a minimum depth of burial (Blevin et al, 1998b).

Reservoirs are sealed by the regional Upper Jurassic–Lower Cretaceous upper Vulcan and Lower Cretaceous Echuca Shoals and Jamieson formations. The thickness of these seals ensures fault seal for the underlying reservoirs. Sections within the lower Vulcan Formation also form adequate seals for Plover Formation reservoirs. Potential intraformational seals occur within the Plover Formation (Blevin et al, 1998b), while marls and mudstones provide potential seals for Campanian–Maastrichtian turbidites and unconfined fan sandstones in the Puffin Formation (Benson et al, 2004). Proven combined structural and stratigraphic plays include Jurassic horsts/tilted fault blocks and associated drape anticlines, and Cretaceous submarine canyon fill and basin floor fans (Figure 3).

Figure 4 shows the existing data coverage in the region. Release area W16-3 has no well control but it is covered by the North Browse TQ and Poseidon 3D seismic. There is also 2D seismic coverage over this release area.






Figures

Figure 1

Structural elements of the Caswell Sub-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 in the Caswell Sub-basin and Brecknock-Scott Reef Trend 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 northwestern Caswell Sub-basin, Browse Basin


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Figure 4

Seismic and well data in the vicinity of the Release Area in the northwestern Caswell Sub-basin, Browse Basin


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