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Roebuck Basin

Release Areas W17-4, Offshore Western Australia


Bids close on Thursday 19 October 2017

  • Recent major oil discovery at Phoenix South 1
  • Liquids-rich gas discoveries at Phoenix South 2, Roc 1 and Roc 2
  • Adjacent to the proven Northern Carnarvon Basin and Browse Basin hydrocarbon provinces
  • Shallow to deep water, 150−400 m
  • Further guidance available, refer to 2017 Special Notices

Release Area W17-4 straddles the boundary between the Rowley and Bedout sub-basins of the Roebuck Basin, offshore Western Australia (Figure 1). The Release Area lies 215 km from the coast in water depths of 150-400 m. The area comprises 78 graticular blocks. Only 16 wells have been drilled in the Roebuck Basin, including discoveries of oil at Phoenix South 1 and liquids-rich gas at Phoenix South 2, Roc 1 and Roc 2.

The Roebuck Basin comprises the Bedout and Rowley sub-basins (Figure 1). The Bedout Sub-basin is filled with approximately 2.5 km of Paleozoic and 7 km of Mesozoic sediments. The Rowley Sub-basin contains about 9 km of Permo-Carboniferous or older strata and up to 6 km of Mesozoic–Holocene sediments. The Rowley and Bedout sub-basins are separated by a structural trend that includes the Bedout High. The anticlinal Bedout High is linked to a late Permian inversion event referred to as the Bedout Movement. The stratigraphy of the Roebuck Basin is summarised in Figure 2.

The only well within the Release Area is East Mermaid 1, located near the northeastern boundary. This well reached TD within strata of Jurassic age. Otherwise, well control is provided by Lagrange 1 (Figure 3) and Bedout 1, both located on the Bedout High to the south of W17-4. These wells intersected strata as old as Triassic (Bedout and upper Keraudren formations) before reaching TD within the Permian Bedout Volcanics. Figure 4 shows the wells and seismic data coverage in the region.

Release Area W17-4 lies over the structural trend that separates the Rowley sub-basin from the Bedout Sub-basin, and includes the northeastern part of the Bedout High (Figure 1). Figure 5 is an interpreted seismic section that depicts the geology of the central part of the Release Area. Structure within the central part of the Release Area is characterised by a series of normal faults that step-down towards the Rowley Sub-basin depocentre. Syn-rift growth is evident in the Permo-Carboniferous succession. In the vicinity of East Mermaid 1, Totterdell et al (2014) have interpreted the presence of large Permo-Carboniferous syn-rift wedges. A pre-Westralian, Carboniferous and older Paleozoic succession is inferred to underlie the Release Area. The conspicuous unconformity at the top-Permian is linked to the Bedout Movement. The Permo-Carboniferous and Triassic intervals both decrease in thickness over the faulted zone. Jurassic and Cretaceous intervals are of essentially uniform thickness across the region, while the Cenozoic succession thickens towards the modern shelf edge located over the inner Rowley Sub-basin.

An active petroleum system in the Bedout Sub-basin was demonstrated by the discoveries at Roc and Phoenix South. The source rocks have been shown to be pre-Jurassic and probably Lower Triassic Locker Shale and Lower to Middle Triassic lower Keraudren Formation (Molyneux et al, 2016). The presence of oil in Phoenix South 1 has challenged the interpretation of a predominantly gas-prone petroleum system in the Roebuck Basin. Petroleum systems modelling indicates that Lower Triassic source rocks are also mature in the Rowley Sub-basin at East Mermaid 1, and between 1500 m and 2500 m water depth in the outer Rowley Sub-basin (Smith et al, 1999; O'Brien et al, 2003).

Other potential source rocks include the fluvio-deltaic Depuch Formation (Jurassic) which includes coaly and algal-rich mudstone and pro-delta marine shales, and Lower Cretaceous marine mudstone. However, these intervals may only be mature where they lie beneath the Cenozoic carbonate wedge in the inner Rowley Sub-basin (Smith et al, 1999). In the absence of well intersections, the presence of Palaeozoic source rocks in the Roebuck Basin, such as those known from the onshore Canning Basin (e.g., Poole Sandstone, Noonkanbah Formation, and marine shales of the Grant Group), is speculative.

Numerous play types have been proposed in the Roebuck Basin where Triassic and Jurassic strata thin towards basin highs. Such a geological setting characterises the greater Phoenix area, as well as the southern margin of the Bedout Sub-basin where it onlaps the Broome Platform of the offshore Canning Basin (Figure 6). Release Area W17-4 occupies a similar geological setting in which Mesozoic strata onlap the structural high between the Bedout and Rowley sub-basins. Upper and lower Keraudren Formation stratigraphic, combination stratigraphic-structural, and stratigraphic pinch-out plays are potentially present. The basal Triassic Locker Shale source rock may be overstepped by younger strata towards the structural high.

The Depuch Formation in East Mermaid 1 is dominated by sandstone and numerous coal intervals that may provide source potential. The potential for plays based on Jurassic reservoirs within the Release Area is limited by light structuring, although the presence of the Cenozoic wedge in the Release Area indicates that the source rocks of this age may be buried deeply enough to be mature. The stratigraphy in East Mermaid 1 indicates that the entire Cretaceous interval in the Release Area is likely to be fine-grained.

Plays based on Permian and older reservoirs are poorly constrained because of the absence of Paleozoic well intersections in the Roebuck Basin. Nonetheless, plays may be postulated based on features evident in the Release Area such as tilted fault blocks, syn-rift wedges, and a potential Lower Triassic regional seal over the top-Permian unconformity.