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Durroon Sub-basin

Release Area T17-2, offshore Tasmania

Highlights

Bids close on Thursday 19 October 2017

  • Under-explored rift basin containing a Cretaceous–Cenozoic succession up to 7000 m thick
  • Two wells drilled in the Release Area
  • Numerous untested structural and stratigraphic plays
  • Potential source rocks in Eumeralla Formation (Otway Basin) equivalent coaly rocks
  • Situated in shallow water (<82 m) close to infrastructure, including the BassGas production facility at the Yolla Field, and southeastern Australian gas markets
  • This release area is considered to be lightly explored, refer to supporting information
  • Further guidance available, refer to 2017 Special Notices.

Release Area T17-2 is located in the eastern Bass Basin, covering much of the frontier Durroon Sub-basin and extending into the easternmost Cape Wickham Sub-basin (Figure 1). The Release Area is located approximately 60 km east of the producing Yolla Field (BassGas Project) in the Cape Wickham Sub-basin, and is traversed by the Gippsland–Tasmania gas pipeline. The data coverage for the Release Area is shown in Figure 2. Only two wells—Chat 1 and Durroon 1—have been drilled in the Durroon Sub-basin, and the region has a moderately good coverage of mostly 2D seismic data.

The Cretaceous–Cenozoic Bass Basin is a northwest-trending, intracratonic rift basin that underlies 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). While the Cape Wickham Sub-basin is a proven hydrocarbon province, hosting several gas discoveries and a producing gas and condensate field (Yolla), the Durroon Sub-basin is underexplored, and no discoveries have been made in this portion of the Bass Basin.

The Durroon Sub-basin is characterised by a series of northwest- to north-northwest-trending half graben bounded by northeast-dipping faults, and these half graben are generally narrower and more intensely faulted than those in the western part of the basin (Blevin, 2003b). The sub-basin contains a Cretaceous–Cenozoic rift and post-rift succession up to 7000 m thick (Baillie and Pickering, 1991).

Two Early Cretaceous rift phases can be identified across the Durroon Sub-basin (Figure 3, Figure 4). 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). Where intersected in Durroon 1, it comprises stacked fluvial channel sands interbedded with thin, coaly overbank–floodplain shales (Figure 5). A second phase of extension (Durroon Rift Phase) 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 depocentres (Blevin et al, 2005). Thick shales within the megasequence are interpreted to have been deposited in deep, freshwater lacustrine environments (Blevin et al, 2005; Partridge, 2003).

Extension had generally ceased by the mid-Campanian and until the late early Eocene, deposition took place within an internal drainage basin, with fluvial systems feeding into the slowly subsiding depocentres from the uplifted basin flanks (DITR, 2004). Due to erosion, the upper Paleocene–lower Eocene coaly succession that is known to have sourced hydrocarbon accumulations in the Cape Wickham Sub-basin (Aroo Sequence; Boreham et al, 2003) is either poorly developed or absent in the Durroon Sub-basin (Blevin et al, 2005).

The overlying middle Eocene–lower Oligocene Flinders Sequence is an overall transgressive–regressive succession deposited in shallow bay to open marine environments (Blevin et al, 2005). Massive sandstone units near the top of the sequence are assigned to the Boonah Formation, which is generally considered the top reservoir unit across most of the Bass Basin. The uppermost unit of the Flinders Sequence, the “Demons Bluff Shale”, is regarded as the regional seal; in the Durroon Sub-basin it is generally thin and relatively coarse-grained (Blevin et al, 2005; Figure 5). The Flinders Sequence is overlain unconformably by marine strata of the upper Oligocene–Holocene Torquay Sequence. The Durroon Sub-basin was only mildly affected by the Oligocene and younger magmatic events that characterise the Cape Wickham Sub-basin (Blevin et al, 2005).