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Caswell Sub-basin

Release Areas AC17-5 and W17-3, offshore Western Australia

Highlights

Bids close on Thursday 22 March 2017

  • Situated between the Ichthys and Crux gas accumulations, and outboard of the Cornea-Focus oil and gas accumulations
  • Close to the recently completed Ichthys gas pipeline
  • Quality reservoirs and seals, multiple potential oil- and gas-prone source rocks, and active petroleum systems
  • Further guidance available, refer to 2017 Special Notices

The contiguous Release Areas W17-3 and AC17-5 are located in the northeastern Caswell Sub-basin (Browse Basin), 220 km off the northwest coast of Western Australia, in water depths of 30–350 m (Figure 1). Release Area W17-3 consists of 39 graticular blocks (2715 km2). Release Area AC17-5 comprises 43 graticular blocks (3510 km2). 

The Browse Basin comprises six major tectonostratigraphic phases; 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 middle Miocene thermal subsidence, and middle Miocene to Holocene inversion (Figure 2). The Caswell Sub-basin is a Browse Basin depocentre that contains up to 15 000 m of Paleozoic to Cenozoic sediments (Figure 2).

Contiguous Release Areas W17-3 and AC17-5, referred to hereafter simply as the Release Areas, are situated over the Heywood Graben in the northeastern Caswell Sub-basin, between the Ichthys and Crux gas accumulations, and outboard of the Cornea-Focus oil and gas accumulations (Figure 1). Figure 3 is an interpreted seismic path over the southern part of the Release Areas. Buccaneer 1 (Figure 4) is one of the few wells in the region to intersect the Triassic, and provides an indication of the stratigraphy of the Release Areas.

Figure 5 depicts some important play types in the Caswell Sub-basin, and their lithostratigraphic and sequence stratigraphic context. The most common exploration play within the Release Areas is extensional fault blocks and associated roll-over anticlines. The main reservoir units associated with this play are the Lower-Middle Jurassic Plover Formation, and the lowermost Cretaceous Brewster Member, but also include the Jurassic Montara Formation and the Triassic Nome and Pollard formations. The Plover Formation is an important reservoir in the Ichthys and Crux gas fields, while the Brewster Member is the main reservoir in the Ichthys gas field.

Lower Cretaceous M. australis sands (informal, Echuca Shoals Formation) and D. davidii sands (informal, lower Jamieson Formation), both of which are shallow marine deposits, have received attention from explorers within the Release Areas, in plays that typically involve ‘drape folding’ over structural highs. Other Lower Cretaceous sandstone intervals are regarded as submarine fan deposits (e.g. Asterias Member at Kalyptea 1 ST1). The Upper Cretaceous Puffin Formation, which includes submarine fan deposits, has also been targeted.

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, while marls and mudstones provide potential seals for submarine fan sandstones in the Puffin Formation.

Figure 6 shows the well and seismic data coverage in the region. Three wells lie within Release Area AC17-5 and 5 wells lie within Release Area W17-3. Numerous wells lie in the near vicinity of both Release Areas. Partial 3D seismic coverage of Release Area AC17-5 is provided by the North Browse TQ, Arlo, Canis, Coppernicus and Octanis surveys. Almost complete 3D seismic coverage of Release Area W17-3 is provided by the Adele Trend TQ and Canis surveys. Extensive 2D seismic coverage exists over both Release Areas.