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Sahul Syncline

Release Area W16-2, Sahul Syncline, Bonaparte Basin, Western Australia

Highlights

Bids close 23 March 2017

  • Shallow water depths 60–100 m
  • Bayu/Undan gas pipeline intersects Release Area
  • Proven plays in Mesozoic tilted fault blocks, potential for structural and stratigraphic plays in Triassic–Jurassic and Cretaceous reservoirs
  • Release Area contains the Sahul Syncline and Malita Graben source kitchens
  • Special Notices apply, refer to Guidance Notes

Release Area W16-2 is located in the northern Bonaparte Basin, encompassing the Sahul Syncline and parts of the Malita Graben (to the northeast) and Londonderry High (to the southwest) in water depths up to 100 m (Figure 1). It is intersected by the pipeline from the Bayu/Undan gas field in the Joint Petroleum Development Area (JPDA). This Release Area contains 60 full graticular blocks (5 025 km2) and is 200 km from the northwest coast of Australia. The Release Area is located adjacent to an oil show at Shilling 1 and an oil and gas discovery at Rambler 1 within the Sahul Syncline. On the Londonderry High, the Release Area is close to a gas discovery at Saratoga 1 and oil and gas shows at Backpacker 1 and Ascalon 1A, respectively. Two wells were drilled in the Malita Graben in 2010, of which Laperouse 1 is within the Release Area, while Durville 1 is located to the north of Laperouse 1 and is just outside of the Release Area.

The present day configuration of the northern Bonaparte Basin is the result of three cycles of rifting: an initial northwest-trending Late Devonian rift extending outboard from the Petrel Sub-basin; a northeast-trending Carboniferous–Permian rift; and lastly, similarly oriented Jurassic rifts in the Malita and Calder graben and the Vulcan Sub-basin (Figure 2). The northern Bonaparte Basin contains two major depocentres; the northeast-trending Malita and Calder graben, and the northwest-trending Sahul Syncline and its southwestern extension, the Nancar Trough. These northern Bonaparte depocentres are flanked to the north by the Sahul Platform and to the south by the Londonderry High (Figure 1). The Sahul Syncline, and its western extension, the Nancar Trough, are prominent Paleozoic to Mesozoic northwest-trending troughs located between the Londonderry and Flamingo highs in the northern Bonaparte Basin (Figure 1). These depocentres are the source of the petroleum accumulations on the adjacent Laminaria and Flamingo highs. Proven source rocks in the region include the Plover, Elang and Frigate formations. Recent research suggests that the Echuca Shoals Formation may also have some hydrocarbon potential in the region. The Malita Graben forms a major northeast-trending rift system located between the Petrel Sub-basin and the Sahul Platform (Figure 1). The graben contains a thick (up to 10 000 m) succession of Mesozoic and Cenozoic sediments that are underlain by a thick section of Pennsylvanian–Permian sediments (Figure 2). The graben is bounded by northeast to east-northeast–trending faults that have large displacement. The Londonderry High is characterised by a highly faulted succession of Paleozoic and Triassic rocks overlain by a relatively unfaulted, Upper Jurassic and younger faulted succession (Whibley and Jacobsen, 1990; de Ruig et al, 2000).

The reservoir potential of the southwest flank of the Sahul Syncline is not well known, being penetrated only by Franklin 1, Helvetius 1 and Rambler 1. Franklin 1 targeted the Upper Cretaceous marginal marine Turnstone Formation and failed to encounter reservoir facies, raising questions over lithofacies distribution in the region. Helvetius 1 targeted the Sandpiper Sandstone horst bock with good quality, but water saturated, reservoir sands in the Sandpiper Sandstone. Rambler 1 tested a Mesozoic fault block with a probable overlying fault drape play. Reservoir quality in the Plover Formation at this location is poor with low porosity and permeability, with the Flamingo Group facies shalier than predicted, but with some fracture permeability. In spite of the poor reservoir quality, oil shows were recorded in the Sandpiper Sandstone and significant gas shows in the Plover Formation. Schilling 1 was drilled in the Malita Graben targeting the Sandpiper Sandstone in a four way dip closure associated with the Malita fault. The well failed due a combination of poor reservoir and lack of hydrocarbon charge. In the Londonderry High part of the Release Area, there is potential in older Permian plays hosted in the Hyland Bay Formation (e.g. Ascalon 1). Backpacker 1 encountered oil shows in the Sandpiper Sandstone within a tilted fault block.

Extensive Jurassic and Cretaceous shales and claystones provide competent top seals in the region. Lateral seal capacity is also good with significant onlap relationships. However, as with the rest of the northern Bonaparte Basin, seals reliant on stable fault geometries are a risk due to fault reactivation, particularly in the Neogene.

The potential primary trap styles identified within the Release Area include tilted fault blocks, horsts, four way dip closure and associated stratigraphic drapes, or combinations of these (Figure 3).

Figure 4 shows the data coverage in the region. The Release Area contains the Laperouse 1 well and is covered by dense 2D seismic. The Dunnart 3D grid extends into the western margin of the Release Area.







Figures

Figure 1
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Structural elements of the Sahul Syncline and surrounds, showing petroleum fields, key wells, and the 2015 and 2016 Release Areas


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Figure 2

Stratigraphy, basin phases and hydrocarbon shows in the Flamingo and Sahul synclines, and the Londonderry high and Malita Graben, based on the Bonaparte 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 over the Londonderry High, Sahul Syncline and Malita Graben, Bonaparte Basin


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Figure 4
Due to the complexity of this document and the niche scientific target audience, no alternative description has been provided. Please email Geoscience Australia at clientservices@ga.gov.au for an alternative description

Seismic and well data in the vicinity of the Release Area over the Londonderry High, Sahul Syncline and Malita Graben, Bonaparte Basin


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