The Bonaparte Basin has a complex structural history. The Phanerozoic evolution of the region has been described by Gunn (1988), Gunn and Ly (1989), Veevers (1988), Pattillo and Nicholls (1990), O'Brien et al (1993, 1996a), AGSO NW Shelf Study Group (1994), Baillie et al (1994), Whittam et al (1996), Kennard et al (2002) and Peresson et al (2004). Neogene tectonism and its implications for petroleum exploration in the Bonaparte Basin are 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:
- Widespread volcanism and subsidence initiated deposition in the onshore portion of the Petrel Sub-basin in the Cambrian.
- Late Devonian to Early Carboniferous extension formed the northwest-trending Petrel Sub-basin.
- Extension in the Late Carboniferous to Early Permian overprinted the older trend with a northeast-oriented structural grain. The proto-Vulcan Sub-basin and Malita Graben developed at this time.
- A compressional event in the Late Triassic caused uplift and erosion on the Londonderry High, the Ashmore and Sahul platforms, and the southern margins of the Petrel Sub-basin.
- In response to Mesozoic extension, the Vulcan Sub-basin, Sahul Syncline, Malita Graben and Calder Graben became major Jurassic depocentres. This structuring coincided with the commencement of sea-floor spreading in the Argo Abyssal Plain to the west of the Browse Basin.
- With the onset of thermal subsidence in the Early Cretaceous (Valanginian), a thick wedge of fine-grained, clastic and subsequently carbonate sediments prograded across the offshore Bonaparte Basin throughout the Cretaceous and Cenozoic.
- Regional compression associated with the collision of the Australia-India plate and Southeast Asian microplates in the Miocene formed the Timor Trough and the strongly faulted northern margin of the adjacent Sahul Platform.
The stratigraphy of the basin is summarised in Figure 2 [PDF, 530KB] after Cadman and Temple (2004). The stratigraphy of the Bonaparte Basin has been defined by Beere and Mory (1986) and Mory (1988, 1991), with many localised revisions since, such as those by Gorter (1998, 2006a, b); Gorter et al (2004, 2005); Whittam et al (1996) and Labutis et al (1998). Paleozoic 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. Paleogeographic reconstructions of the Northwest Shelf region, including the Bonaparte Basin are provided by Bradshaw et al (1988) and Norvick (2001).
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-trending 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 (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). Sandstones within this succession form the reservoir facies for the gas discoveries in the Petrel Sub-basin and on the Londonderry High.
Compression in the Late Triassic resulted in reactivation and inversion of the previous Paleozoic 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 the 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 sandstones (Elang 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 mudstones (Vulcan Formation and Frigate Formation) within the Vulcan Sub-basin, Sahul Syncline, Malita Graben and Calder Graben. These marine sediments may contain good quality oil-prone source rocks; however, they are gas-prone 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 and northern Bonaparte 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 sandstones accumulated in the Maastrichtian (Puffin Formation) and Eocene (Grebe Sandstone Member).
Regional compression, associated with the collision of the Australia-India Plate and 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).
Petrel Sub-basin
The Petrel Sub-basin is an asymmetric, northwest-trending Paleozoic rift that contains a succession of thick Paleozoic 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.
Sediments within the Petrel Sub-basin dip regionally to the northwest about a northwest-plunging synclinal axis, resulting in exposure of Early Paleozoic sediments in the southern onshore area, and in the progressive subcropping of Late Paleozoic, Mesozoic and Cenozoic sediments offshore. The Late Paleozoic-Mesozoic section exceeds 15000 m 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 (Figure 1 [PDF, 602KB]). 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. 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, while the Jabiru Terrace borders 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, 1996b, 1999). Thermal sag phase sedimentation continued until the Neogene, resulting in over 10000 m of sediment-fill in 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 breakup of the Argo margin to the south led to tilted fault-block development prior to widespread peneplanation, subsidence and burial in the Cretaceous-Cenozoic. The Ashmore Platform has been subjected to fault reactivation due to the Miocene-Pliocene convergence of the Australia-India Plate and the Southeast Asian microplates.
Londonderry High
The Londonderry High is characterised by a highly faulted sequence of Paleozoic and Triassic rocks that acted as a major sediment source 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 defined 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; the northeast-trending Malita and Calder graben, and the northwest-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 (Figure 1 [PDF, 602KB]).
The stratigraphy and geological history of the northern Bonaparte Basin has been described by Mory (1988), Mory and Beere (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 Carboniferous-Permian rifting, and Jurassic rifts in the Malita and Calder graben and Vulcan Sub-basin. The pre-existing Paleozoic 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 trend of both the Sahul and Flamingo synclines (Whittam et al, 1996).
This northwest-trending 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, and to east-west in the area of the Flamingo and Laminaria highs. Woods (1992) attributes 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:
- The strong influence of the Permo-Carboniferous rifting event in the distribution and thickness of the Triassic succession.
- The tectonic event at the Triassic-Jurassic boundary, which marks the onset of extension during the Mesozoic.
- The relative unimportance of the Callovian phase of tectonism that initiated subsidence in the Vulcan Sub-basin.
- The Tithonian extensional event resulted in the development of east-trending horsts and graben that characterise the structure of the Sahul Syncline and Flamingo Syncline region, which have proven to be the most prospective structural traps in the area.
- The identification of the base-Aptian disconformity as a regional seismic marker that is the principal structural mapping horizon in the region and the most reliable indicator of regional structure at the top of the Callovian reservoir section.
These differences have important implications for petroleum exploration in the region. 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 sandstones would be widespread on the Laminaria and Flamingo highs and Sahul Platform if the major Callovian extension that affected 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.
The Permian to Cenozoic 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. Most of the Sahul Platform lies within the JPDA between Australia and Timor Leste, with the northern-most part located in Australian and Indonesian waters (Figure 1 [PDF, 602KB]). The Sahul Platform is an area of relatively shallow basement. It is divided into the Troubadour High in the east, where basement is approximately 3000 m deep, 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). Sediment thicknesses vary from 3000 m on the Troubadour High to more than 5000 m on the Kelp High. The Troubadour Terrace is an area of relatively shallow basement that is arbitrarily separated from the Sahul Platform. The southern boundary of the Sahul Platform is marked by northeast-trending Mesozoic normal faults showing displacement down into the Malita and Calder graben creating a series of prominent blocks and terraces. The Heron Terrace is a perched, down-faulted block covering an extensive area adjacent to the Troubadour Terrace.
The Sahul Platform was originally part of a broad, northeast-trending Late Paleozoic 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 breakup in the Callovian produced a series of narrow, confined depocentres (Malita Graben and Sahul Syncline) to the south and west of the elevated Sahul Platform. Late Jurassic and Early Cretaceous sediments are mainly confined to these depocentres, and both consist of thin, condensed marine mudstones across the Sahul Platform and Troubadour Terrace or are absent. Late Miocene to Pliocene convergence of the Australia-India Plate and the Southeast Asian microplates resulted in flexural down-warp of the Timor Trough to the north, and generation of the Kelp High and Troubadour High faulted anticline structures. The Late Cretaceous to Cenozoic sediments consist predominantly of marine carbonates.
Hydrocarbon discoveries on the Sahul Platform include the Greater Sunrise and Evans Shoal gas fields and the gas accumulation at Chuditch 1. Recent drilling has encounter gas accumulations at Heron 2 and Blackwood 1. The Indonesian Abadi gas field is located on what is thought to be the northeastern extension of the Sahul Platform.
Middle Jurassic Plover Formation sediments contain the main reservoir and source rock units. There is also additional, but limited, reservoir potential in Permian to Triassic sediments; gas flowed on test from the Hyland Bay Subgroup at Kelp Deep 1. A regional seal is provided by Late Jurassic and Cretaceous mudstones. The main exploration targets are complex, faulted anticlines with hydrocarbons trapped at the apex of large, regional structural closures.
The Sahul Syncline (and its western extension, the Nancar Trough) is a prominent Paleozoic to Mesozoic northwest-trending trough located between the Londonderry and Flamingo highs in the northern Bonaparte Basin. 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 suggested that the latest Carboniferous to earliest 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, as a result of minor Callovian and then more pronounced Tithonian extension, controlled the deposition of the Late Jurassic to Early Cretaceous clastic sequences (Elang Formation, Frigate Formation and Sandpiper Sandstone).
In axial areas of the syncline, the sandstones of the Plover and Elang formations lie too deep to constitute valid exploration objectives, but these units form good quality reservoirs on the Laminaria and Flamingo highs and Sahul Platform. Following continental breakup in the Valanginian, a thick Cretaceous-Cenozoic thermal sag section accumulated across the Sahul Syncline.
The Malita and Calder graben form a major, northeast-trending rift system that contains a significant thickness of Late Paleozoic, Triassic, Jurassic and Early Cretaceous sediments. These graben are bounded by northeast to east-northeast-trending faults that show large displacement. Mesozoic and Cenozoic sediments are probably up to 10000 m thick in the graben and are underlain by a considerable section of Late Carboniferous-Permian sediments. Key features of the stratigraphic succession deposited in these areas are:
- Early-Middle Jurassic Plover Formation sediments thicken markedly into the graben, and may include good quality source rocks.
- Mudstones of the late Middle Jurassic-Early Cretaceous Flamingo Group may have some source potential in the area.
- Tithonian turbiditic sandstones (which were intersected in Heron 1) may provide valid exploration targets in the graben.
- The Early Cretaceous Echuca Shoals Formation may provide additional source potential in the graben.
- The Cretaceous-Cenozoic section exceeds 4000 m in thickness in the central Malita Graben.
Exploration in the Malita and Calder graben has resulted in the discovery of the Barossa (Lynedoch) and Caldita gas accumulations.