Areas AC07-1, AC07-2, AC07-3, AC07-4 and AC07-5

Ashmore Platform, Bonaparte Basin

Bonaparte Basin Geology

Regional Geology

The Bonaparte Basin is a large sedimentary basin, located predominantly offshore (Figure 1). It covers an area of approximately 270,000 km2 of Australia’s northwest continental margin. The basin contains up to 15 km of Phanerozoic, marine and fluvial siliciclastics, as well as marine carbonates. The regional geology, structural evolution and petroleum potential have been described by Laws and Kraus (1974), Gunn (1988), Lee and Gunn (1988), Gunn and Ly (1989), MacDaniel (1988), Mory (1988, 1991), Botten and Wulff (1990), Hocking et al, 1994, and Woods (1994), and summarised by Cadman and Temple (2004). Recent papers on the petroleum geology of the region are presented in the Proceedings of the Timor Sea Symposium, Darwin, June 2003 (Ellis et al, 2004).

The Bonaparte Basin is bounded to the northwest by the Timor Trough, where water depths exceed 3000 m. In the northeast, beyond the limits of the Darwin Shelf, the basin adjoins the Arafura and Money Shoal basins. To the southwest, the basin is contiguous with the Browse Basin.

Structurally, the Bonaparte Basin is complex and comprises a number of Palaeozoic and Mesozoic sub-basins and platform areas (Figure 1). The basin developed during two phases of Palaeozoic extension, followed by Late Triassic compression, and then further extension in the Mesozoic that culminated in the break up of Gondwana in the Middle Jurassic (O’Brien et al, 1993). Convergence of the Australia-India and Eurasia plates in the Miocene to Pliocene resulted in flexural downwarp of the Timor Trough and widespread fault reactivation across the western Bonaparte Basin.

The Petrel Sub-basin is a northwest-trending Palaeozoic rift that occurs in the eastern portion of the Bonaparte Basin and extends onshore. The sub-basin contains a thick section of mostly Palaeozoic and thinner Mesozoic sediments, and is underlain by Proterozoic crystalline basement (dolerite in well sections) and sediments of the Proterozoic Kimberley Basin (Colwell and Kennard, 1996). The eastern and southwestern margins of the sub-basin are flanked by platforms of relatively shallow basement and thin sediment cover. Sedimentation in the sub-basin commenced in the Cambrian, and northeast–southwest rifting was initiated in the Late Devonian to Early Carboniferous. Offshore, the Petrel Sub-basin is orthogonally overprinted by a northeast-trending structural grain that resulted from Late Palaeozoic and Mesozoic rifting.

The Malita and Calder graben form a major, northeast-trending, rift system that lies between the Petrel Sub-basin and the Sahul Platform. The graben contain a significant thickness of Late Palaeozoic, Triassic, Jurassic and Early Cretaceous sediments.

The Sahul Platform, which underlies most of the Joint Petroleum Development Area (JPDA), is an area of relatively shallow basement. The Permo–Triassic succession in this area was uplifted to form a structural high during Jurassic extension of the adjacent Malita and Calder graben.

The Vulcan Sub-basin is a major northeast-trending, Late Jurassic rift depocentre in the western part of the Bonaparte Basin. It is flanked to the southeast and northwest by Permo–Triassic platforms; the Londonderry High and the Ashmore Platform, respectively.

The Sahul and Flamingo synclines are northwest-trending depocentres that link and offset the northeast-trending Malita and Calder graben and Vulcan Sub-basin rift systems. These synclines are separated by the Laminaria and Flamingo highs.

Basin evolution and tectonic development

The Bonaparte Basin has undergone a complex structural history. The Phanerozoic evolution of the region has been described by Gunn (1988), Veevers (1988), Pattillo and Nicholls (1990), O’Brien et al (1993), AGSO NW Shelf Study Group (1994), Baillie et al (1994), Whittam et al (1996) and Kennard et al (2002). Neogene tectonism (and its implications for petroleum exploration in the Bonaparte Basin) is described by McCaffrey (1988), Shuster et al (1998), Keep et al (1998, 2002) and Longley et al (2002). Key events in the evolution of the Bonaparte Basin include:

The stratigraphy varies widely across the basin – Palaeozoic sediments are largely restricted to the onshore and inboard portions of the Petrel Sub-basin, while Mesozoic and Cenozoic sequences are largely confined to the outboard portion of the Bonaparte Basin. The stratigraphy of the basin is summarised in Figure 2.

Volcanic and clastic sedimentation commenced in the onshore Petrel Sub-basin in the Cambrian. This pre-rift sequence contains extensive evaporite deposits, but the precise age (Ordovician, Silurian or Devonian), lateral continuity and extent of these salt bodies is poorly known.

Subsequent salt tectonics (flow, diapirism and withdrawal) has controlled the development of numerous structural and stratigraphic traps within the sub-basin (Edgerley and Crist, 1974; Gunn, 1988; Durrant et al, 1990; Lemon and Barnes, 1997).

Northeast–southwest rifting was initiated in the Late Devonian, and clastic and carbonate sediments were deposited in shallow marine and non-marine environments within the Petrel Sub-basin. During the Carboniferous, a thick succession of marine and fluvio–deltaic (Bonaparte Formation to Point Spring Sandstone) and, finally, glacial sediments (Kuriyippi Formation and Treachery Shale) were deposited in response to post-rift subsidence and salt withdrawal.

The initial northwest-trending Late Devonian–Early Carboniferous rift-sag system (ie, Petrel Sub-basin in the eastern Bonaparte Basin) was orthogonally overprinted in the Late Carboniferous to Early Permian by northeast-trending rifts to form the proto-Malita Graben and probably a proto-depocentre in the Vulcan Sub-basin (O’Brien, 1993; Baxter, 1996). A succession of northwest-thickening, shallow marine to fluvio–deltaic, Permian and Triassic sediments was then deposited across the Bonaparte Basin (Keyling to Cape Londonderry formations). These form the reservoir facies for the gas discoveries in the Petrel Sub-basin and on the Londonderry.

Compression in the Late Triassic resulted in reactivation and inversion of the previous Palaeozoic fault systems (O’Brien et al, 1993) and caused widespread uplift and erosion on the Ashmore Platform, Londonderry High and in the southern portion of the Petrel Sub-basin. Late Triassic–Early Jurassic fluvial sedimentation (Malita Formation) was followed by a thick widespread succession of Early–Middle Jurassic fluvial and coastal plain deposits (Plover Formation) throughout most areas of the Bonaparte Basin except for the Ashmore Platform and crest of the Londonderry High. The Plover Formation forms a major source and reservoir unit over much of the northern Bonaparte Basin.

The onset of rifting in the mid-Callovian resulted in a widespread marine transgression and the deposition of retrogradational deltaic sands (Elang, Laminaria and Montara formations), which form reservoir units in many of the commercial petroleum accumulations in the northern Bonaparte Basin. Continued rifting and rapid subsidence resulted in the deposition of a thick succession of marine mudstone (Vulcan Formation and Frigate Shale) within the Vulcan Sub-basin, Sahul Syncline, Malita Graben and Calder Graben. These marine sediments contain good quality oil-prone source rocks, but source quality decreases within the Malita and Calder graben.

Mesozoic extension ceased with the onset of sea-floor spreading in the Valanginian and was followed by widespread thermal subsidence and flooding of the western Australian continental margin. Fine grained clastics and carbonates of the Bathurst Island Group were deposited across the Bonaparte Basin during this phase. At the base of the Bathurst Island Group, claystones of the Echuca Shoals Formation provide a regional seal for the hydrocarbon accumulations in the Vulcan Sub-basin. This unit thins onto the platform areas in the west (Ashmore and Sahul platforms) and in the Petrel Sub-basin to the east. The Late Cretaceous and Cenozoic sections typically comprise thick, prograding platform carbonates. Lowstand sands developed in the Maastrichtian (Puffin Formation) and Eocene (Grebe Formation).

Regional compression associated with the collision of the Australia-India Plate with the Southeast Asian Microplates reactivated Mesozoic faulting and breached many fault-dependent structures in the Vulcan Sub-basin and adjacent areas. This regional tectonism resulted in the loss of hydrocarbons from previous accumulations (O’Brien and Woods, 1995; O’Brien et al, 1999; Longley et al, 2002) and leakage to the sea floor that appears to have controlled the development and distribution of present-day biohermal mounds in the region (Bishop and O’Brien, 1998; O’Brien et al, 2002).


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Petrel Sub-basin

The Petrel Sub-basin is an asymmetric, northwest–southeast-trending Palaeozoic rift that contains a succession of thick Palaeozoic and thinner Mesozoic sediments. The eastern and western faulted margins of the sub-basin converge onshore to form a southern termination. To the south and east of the Petrel Sub-basin, extensions of the Halls Creek–Fitzmaurice Mobile Zone separate this sub-basin from the Precambrian Victoria River Basin and Pine Creek Geosyncline. Extensive basement shelves overlain by a thin cover of Phanerozoic sediments lie on the eastern, western and southern margins of the Petrel Sub-basin. To the east, the Kulshill Terrace and Moyle Platform extend to the north-northeast into the Darwin Shelf. In the southwest, the Berkley Platform has been sub-divided into several, smaller southeast-trending horst (Lacrosse Terrace and Turtle-Barnett High) and graben (Cambridge Trough) structures.

Strata within the Petrel Sub-basin dip regionally to the northwest about a northwest-plunging synclinal axis, resulting in exposure of Early Palaeozoic sediments in the southern onshore area, and in the progressive subcropping of Late Palaeozoic, Mesozoic and Cenozoic sediments offshore. The Late Palaeozoic–Mesozoic section exceeds 15 km in thickness in the central and northern Petrel Sub-basin.

Vulcan Sub-basin

The Vulcan Sub-basin is a northeast-trending, Mesozoic, extensional depocentre in the western Bonaparte Basin. The sub-basin comprises a complex series of horsts, graben and marginal terraces, and abuts the Londonderry High to the east-southeast and the Ashmore Platform to the west-northwest (Figure 1). The structurally significant and proven hydrocarbon source provinces of the Swan Graben and Paqualin Graben die out to the northeast beneath the younger (Neogene) Cartier Trough. The Montara Terrace flanks the Swan Graben to the east, and the Jabiru Terrace boarders the eastern margin of the Cartier Trough. The southern boundary of the Vulcan Sub-basin with the northern Browse Basin is somewhat arbitrary. O’Brien et al (1999) considered that the boundary is marked by a fault relay zone that overlies a major northwest-trending Proterozoic fracture system.

The Vulcan Sub-basin developed as part of an upper plate rift margin (O’Brien, 1993). The rift margin developed as a linked array of northwest-trending accommodation zones orthogonal to northeast-trending normal faults (Etheridge and O’Brien, 1994; O’Brien et al, 1996, 1999). Thermal sag phase sedimentation continued until the late Tertiary, resulting in over 10 km of sediment infilling the deeper graben (Baxter et al, 1997).

Ashmore Platform

The Ashmore Platform is an extensive, elevated and highly structured block. It borders the Vulcan Sub-basin to the east, the northern Browse Basin to the south and deepens into the Timor Trough to the west. On the platform, up to 1500 m of flat-lying Cretaceous and Cenozoic strata overlie up to 4500 m of heavily faulted and folded Permo–Triassic sediments. Rifting through to the Late Jurassic break up of the Argo margin to the south led to tilted fault-block development prior to widespread peneplanation, subsidence and burial in the Cretaceous–Cenozoic. It has been subjected to fault reactivation due to the Miocene–Pliocene convergence of the Australian Plate and the Southeast Asian Microplates.


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Londonderry High

The Londonderry High is characterised by a highly faulted sequence of Palaeozoic and Triassic rocks that acted as a major source of sediment for adjacent depocentres during the Late Jurassic rifting (Whibley and Jacobsen, 1990; de Ruig et al, 2000), overlain unconformably by a relatively unfaulted, Late Jurassic and younger succession. Although most faulting terminates at the top of the Triassic sequence, some faults show evidence of Miocene reactivation. On higher parts of the Londonderry High the Triassic section is deeply eroded. Uplift and erosion are less pronounced on the eastern and northern flanks where the unconformity is underlain by progressively younger sediments.

Northern Bonaparte Basin

The northern Bonaparte Basin, as redefined by Whittam et al (1996), encompasses the area to the northwest of the Petrel Sub-basin that contains a thick Mesozoic and Cenozoic succession. Two major depocentres of Late Jurassic to Early Cretaceous age are recognised in the northern Bonaparte Basin (Figure 1); the northeast–southwest-trending Malita and Calder graben, and the northwest–southeast-trending Sahul Syncline, including its western extension, the Nancar Trough. These depocentres are flanked to the north by the Sahul Platform and to the south by the Londonderry High.

The stratigraphy and geological history of the northern Bonaparte Basin has been described by Mory (1988), Gunn (1988), MacDaniel (1988), Veevers (1988), Pattillo and Nicholls (1990), O’Brien et al (1993), Whittam et al (1996), Labutis et al (1998) and Shuster et al (1998), and is summarised by Cadman and Temple (2004).

The present day configuration of the northern Bonaparte Basin results from the intersection and superimposition of three cycles of rifting: an initial northwest–trending Late Devonian rift extending outboard from the Petrel Sub-basin, northeast-trending Late Carboniferous–Permian rifting, and Jurassic rifts in the Malita and Calder graben and Vulcan Sub-basin. The pre-existing Palaeozoic structural grain had considerable influence on the distribution and thickness of the Mesozoic and Cenozoic succession on the western part of the Sahul Platform (particularly during the Triassic), and is expressed in the northwest–southeast-trend of both the Sahul and Flamingo synclines (Whittam et al, 1996).

This structural grain is cross-cut by a series of Jurassic faults, the strike of which varies from northeast–southwest in the area adjacent to the Londonderry High, through north-northeast to south-southwest at the western-end of the Malita Graben, to east–west in the area of the Flamingo and Laminaria highs. Woods (1992) attribute this latter east–west-trend to Tithonian tectonism.

Whittam et al (1996) concluded that the geological history in the northern Bonaparte Basin and Vulcan Sub-basin are broadly similar, but there are significant differences recognised in the northern Bonaparte Basin:

These differences have important implications for petroleum exploration in the area. Variations in the subsidence history and timing of tectonic events between the two regions influenced the distribution and preservation of potential reservoir and source rocks (Whittam et al, 1996). For example, it is considered unlikely that deposition of the Elang/Laminaria Formation reservoir sands would be widespread on the Laminaria and Flamingo highs and Sahul Platform if the major Callovian extension that occurred in the Vulcan Sub-basin had occurred on the western part of the Sahul Platform. Similarly, differences in subsidence history and in the thickness of the mid-Cretaceous to Cenozoic succession had a major impact on the timing of hydrocarbon generation, and on the extent to which later episodes of faulting affected the integrity of Jurassic traps.


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Sahul Platform and Troubadour Terrace

The Permian to Cainozoic Sahul Platform is a structural element of the Bonaparte Basin located offshore on Australia's northwestern margin in water depths of 50 to 1500 m. Much of the Sahul Platform falls within the Joint Petroleum Development Area between Australia and East Timor (Figure 1). The Sahul Platform is an area of relatively shallow basement. It is further divided into the Troubadour High in the east, where basement is at approximately 3 km sub-sea, and the Kelp High in the west, where basement is interpreted to be significantly deeper (Whittam et al, 1996). The Troubadour High is also referred to as the Sunrise High (Longley et al, 2002). The Troubadour Terrace forms part of the same structural feature as the Sahul Platform, and is only arbitrarily separated from it. Sediment thicknesses vary from 3 km on the Troubadour High to more than 5 km on the Kelp High.

The Sahul Platform was originally part of a broad, northeast to southwest-trending Late Palaeozoic sag basin. Following Early Jurassic rifting, the platform became a depocentre for non-marine and marginal to shallow marine clastics in the Early to Middle Jurassic. Subsequent break up in the Callovian produced a series of narrow, confined depocentres, the Malita Graben and Sahul Syncline, to the south and west of the elevated Sahul Platform. Late Jurassic and Early Cretaceous strata are mainly confined to these depocentres, and either consist of thin condensed marine mudstones across the Sahul Platform and Troubadour Terrace or are absent. Late Miocene to Pliocene convergence of the Australian and Eurasian plates resulted in flexural down-warp of the Timor Trough to the north, and generation of the Kelp High and Troubadour High faulted anticline structures. Late Cretaceous to Tertiary strata consists predominantly of marine carbonates.

Hydrocarbon discoveries on the Sahul Platform include the Greater Sunrise gas field, and a gas accumulation at Chuditch. Middle Jurassic strata are of greatest economic significance containing the main reservoir and source rock units. There is also limited reservoir potential in Permian to Triassic strata. Gas was recovered from the Hyland Bay Formation at Kelp Deep 1 (Figure 1). Late Jurassic and Cretaceous mudstones form the regional seal. The main exploration targets are complex faulted anticlines with hydrocarbons trapped at the apex of large regional structural closure. To date, the Troubadour High has proven to be the most viable exploration area on the Sahul Platform.

Exploration on the Troubadour Terrace is sub-mature with four wells drilled by 2000, resulting in the discovery of the Evans Shoal gas accumulation. Since this time, drilling has been concentrated to the east (ie, Barossa, Caldita and Evans Shoal South). The Abadi gas accumulation is located on what is thought to be the northeastern extension of the Sahul Platform and Troubadour Terrace. The Middle Jurassic Plover Formation is of greatest economic significance containing the main reservoir and source rock units. Late Jurassic and Cretaceous mudstones form the regional seal, with the main exploration targets being complex faulted anticlines.

Sahul Syncline

The Sahul Syncline (and its western extension, the Nancar Trough) is a prominent Palaeozoic to Mesozoic northwest-trending trough located between the Londonderry and Flamingo highs in the northern Bonaparte Basin (Figure 1). It is the primary source kitchen for petroleum accumulations discovered on the adjacent Laminaria and Flamingo highs.

Botten and Wulff (1990) considered that the Sahul Syncline formed in the Late Triassic to Middle Jurassic, whereas Durrant et al (1990) believe it formed as part of the Late Devonian rift system in the Petrel Sub-basin. O’Brien et al (1993) and Robinson et al (1994) described the Sahul Syncline as a ‘sag’ feature, and considered that Late Carboniferous to Early Permian extension reactivated pre-existing, northwest-trending fault zones (such as the Sahul Syncline) as transfer faults.

Subsidence in the Permian and Triassic led to the deposition of a thick sedimentary succession in the region between the Londonderry High and Sahul Platform (including the present day Sahul Syncline, Flamingo High and Flamingo Syncline). Tectonic compression in the Late Triassic resulted in uplift and erosion of the Flamingo High, but deposition continued within the Sahul Syncline where a thick section of the Plover Formation was deposited.

Further subsidence resulting from minor Callovian and then more pronounced Tithonian extension controlled the deposition of the Late Jurassic to Early Cretaceous clastic sequences (Elang/Laminaria Formation and Flamingo Group).
In axial areas of the syncline, the Plover and Elang/Laminaria sands lie too deep to constitute valid exploration objectives, but these units form good quality reservoirs on the Laminaria and Flamingo highs. Following continental break up in the Valanginian, a thick Cretaceous–Cenozoic thermal sag section was deposited across the Sahul Syncline.

Malita and Calder graben

The Malita and Calder graben form a major, northeast-trending rift system that contains a significant thickness of Late Palaeozoic, Triassic, Jurassic and Early Cretaceous sediments. The graben are bounded by large displacement, northeast to east-northeast-trending faults. While no exploration well has penetrated the entire Mesozoic section in the central part of the graben, Mesozoic and Cainozoic sediments are probably up to 10 km thick, and are underlain by a considerable section of Late Carboniferous–Permian sediments. Key features of the stratigraphic succession deposited in these graben are:


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