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Cape Wickham Sub-basin

Release Area T17-1, offshore Tasmania

Highlights

Bids close 19 October 2017

  • Cretaceous–Cenozoic intracratonic rift basin containing proven hydrocarbon reserves
  • Situated in shallow water (<83 m) close to southeast Australian gas markets and production infrastructure
  • Release area surrounding the Yolla Field production lease and adjacent to retention leases associated with the White Ibis, Bass, Trefoil and Rockhopper accumulations
  • Fault block, anticlinal closure and onlap plays occurring on the up-dip flanks of proven hydrocarbon kitchens
  • Release Area partly covered by recent 3D seismic surveys
  • Further guidance available, refer to 2017 Special Notices

Release Area T17-1 is located in the western Bass Basin, covering much of the central Cape Wickham Sub-basin (Figure 1). The Release Area surrounds the producing Yolla Field (BassGas Project), is traversed by the BassGas pipeline, and is adjacent to known accumulations at White Ibis, Bass, Trefoil and Rockhopper. The northwestern and eastern parts of the Release Area are partly covered by recently acquired 3D and 2D seismic surveys (Figure 2).

The Cretaceous–Cenozoic Bass Basin is a northwest-trending, intracratonic rift basin that underlies the Bass Strait between northern Tasmania and southern Victoria. The Bass Basin is separated from the Otway and Sorell basins to the west by the King Island High, and from the Gippsland Basin to the northeast by Flinders Island and the Bassian Rise. The Bass Basin is divided into the Cape Wickham Sub-basin in the west and the Durroon Sub-basin in the east (Blevin et al, 2005). The Cape Wickham Sub-basin is a proven hydrocarbon province, hosting several gas discoveries and a producing gas and condensate field (Yolla).

The Cape Wickham Sub-basin consists of a series of large northwest- to north-northwest-trending Cretaceous–Cenozoic half graben informally named the Cormorant, White Ibis, Yolla, Dondu and Pelican troughs (Lennon et al, 1999; Blevin et al, 2003, 2005; Cummings et al, 2004). Rift bounding faults generally dip to the southwest to west-southwest. Fault throws in the sub-basin are in the order of 3–5 km, with the total sedimentary succession (syn-rift and post-rift) reaching a thickness of 8–10 km in the main depocentres.

The evolution of the Cape Wickham Sub-basin was dominated by three distinct rift phases (Figure 3). The first, in the Barremian–earliest Cenomanian, was coeval with extension in the adjacent Otway Basin and resulted in deposition of the non-marine Otway Megasequence (Blevin, 2003b). A second phase of extension affected the basin during the Turonian–Campanian, driven by Tasman Basin rifting to the east. Half graben initiated in the previous rifting phase continued to develop and expand, forming a system of linked fluvio-lacustrine depocentres (Blevin et al, 2005). A third phase of extension affected the Cape Wickham Sub-basin in the Campanian–early Eocene (Bass rift phase) (Smit, 1988; Blevin, 2003b; Blevin and Cathro, 2008). This phase of extension focused particularly on structures within the Pelican, Yolla and Cormorant troughs, and resulted in episodic reactivation of existing Lower Cretaceous half-graben structures from the Campanian until the early Eocene. In the Cape Wickham Sub-basin, the oldest sediments penetrated by drilling are Campanian–Maastrichtian  (Bass Rift Phase; Figure 4, Figure 5).

Until the late Eocene, deposition of non-marine sediments in the Bass Basin was controlled by internal drainage systems that migrated from the uplifted flanks of the basin into the developing half-graben structures. Following the cessation of rift-related activity in the latest early Eocene, accommodation was controlled by normal post-rift subsidence and eustatic fluctuations. Widespread marine flooding of the basin occurred from west to east in the late middle Eocene, although indications of periodic marine incursions are recorded as early as the Paleocene.

Multiple periods of post-rift tectonic reactivation, including several episodes of Cenozoic inversion, affected the Bass Basin. The events formed large scale anticlines within the syn- and post-rift successions, with several of these structures targeted by exploration drilling (e.g., Cormorant 1). The Cape Wickham Sub-basin has been affected by multiple periods of volcanic activity during the syn- and post-rift phases of basin evolution resulting in the widespread emplacement of intrusive (sills and dykes) and extrusive rocks (volcanic mounds and flows; Figure 4).