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Bonaparte Basin
The Bonaparte Basin is a large, predominantly offshore sedimentary basin that covers approximately 270,000 square kilometres of Australia’s northwest continental margin.
The basin contains up to 15 kilometres of Phanerozoic, marine and fluvial, siliciclastic and carbonate sediments.
The basin has undergone two phases of Palaeozoic extension, a Late Triassic compressional event and further extension in the Mesozoic. Convergence of the Indo-Australian and Eurasian plates in the Miocene to Pliocene resulted in the formation of a major tectonic collision zone (Banda orogen), the 2000-3000m deep Timor Trough and widespread fault reactivation across the western Bonaparte Basin.
The Timor Trough marks a major tectonic boundary between the thickened, deformed and active crust of the Banda orogen to the north, and the relatively undeformed, stable Australian craton to the south.
Basin Setting
The Bonaparte Basin is structurally complex and comprises a number of Palaeozoic and Mesozoic sub-basins and platform areas.
The Basin adjoins the Browse Basin to the south along the southwest margins of the Ashmore Block and the Vulcan Sub-basin. In the northeast, beyond the limits of the Darwin Shelf, the Bonaparte Basin adjoins the Arafura and Money Shoal Basins. The northern margin of the basin is taken as the Timor Trough, where water depths can exceed 3,000 metres.
In the east, the northwest trending Petrel Sub-basin (referred to as the Bonaparte Basin by Gunn, 1988) underlies the Joseph Bonaparte Gulf (Figures 1).
Figure 1 Petrel Sub-basin - tectonic elements, exploration wells and bathymetry
The sub-basin developed during rifting in the Late Devonian to Early Carboniferous and contains a thick evaporitic sequence, which was mobilised in a subsequent episode of salt tectonism (Gunn, 1988; Lee and Gunn, 1988).
Offshore, the Petrel Sub-basin is orthogonally overprinted by a northeast and east-northeast trending, Mesozoic structural grain that resulted from rifting and the ultimate break-up of Gondwanaland in the Middle Jurassic (O’Brien et al., 1993). The Malita Graben, a major Triassic depocentre which lies between the Petrel Sub-basin and the Sahul Platform, developed at this time. The graben also contains a significant thickness of Cainozoic, Cretaceous and possibly, Late Jurassic sediments (Botten and Wulff, 1990; O’Brien et al., 1993)(Figure 2).
Figure 2 Structure and stratigraphy of the Malita Graben and adjacent terraces.
Basin Evolution
Key tectonic events in the evolution of the Bonaparte Basin include:
• A northwest-trending, Late Devonian to Early Carboniferous rift formed the
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 on the southern margins of the Petrel Sub-basin;
• In response to Mesozoic extension, the Vulcan Sub-basin, Malita Graben and Sahul Syncline became major, Jurassic depocentres;
• With the onset of thermal subsidence in the Valanginian, a thick wedge of fine grained, clastic and carbonate sediments prograded across the offshore Bonaparte Basin during the Cretaceous and Cainozoic;
• Regional compression and deformation associated with the collision of the Indo-Australian plate with the South East Asian microplates in the Miocene formed the Banda orogen, the Timor Trough and the strong flexural faulting and reactivation of the northern margin of the adjacent, Sahul Platform.
Stratigraphy
The Bonaparte Basin has undergone a complex tectonic history. Consequently, the stratigraphy varies considerably across the basin (Messent et al., 1994) – Palaeozoic sediments are largely restricted to the onshore and inboard portions of the Petrel Subbasin while Mesozoic and Cainozoic sequences are largely confined to the outboard portion of the Bonaparte Basin.
Sedimentation in the Petrel Sub-basin commenced in the Cambrian (Figure 3).
Figure 3 Petrel Sub-basin - stratigraphy, tectonics and petroleum discoveries
The pre-rift sequence comprises extensive evaporite deposits, but the precise age (Ordovician, Silurian or Devonian), lateral continuity and extent of these salt bodies is uncertain. Subsequent salt tectonics (flow, diapirism, and withdrawal) has controlled the development of numerous structural and stratigraphic traps within the sub-basin (Edgerley & Crist, 1974; Durrant et al., 1990; Miyazaki, 1997; Lemon & Barnes, 1997)/
Northeast-southwest rifting was initiated in the Late Devonian, when clastic and carbonate sediments were deposited in shallow marine and non-marine environments across the Petrel Sub-basin. This was followed by a thick, Carboniferous succession of marine, fluvio-deltaic and finally glacial sediments which were deposited in response to post-rift subsidence and salt withdrawal (Figure 4). Late Devonian to Late Carboniferous carbonate and clastic sequences are primary exploration objectives in the Petrel Sub-basin.
Figure 4 Geological cross-section of the Southern Petrel Sub-basin (modified after Miyazaki, 1997).
In the Late Carboniferous to Early Permian, the Late Devonian-Carboniferous rift-sag system was orthogonally overprinted by northeast-trending rifting. The proto-Malita Graben developed at this time (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. Several petroleum accumulations have been identified both within the Permian section in the Petrel Subbasin, and the Triassic, fluvio-deltaic and marginal marine sandstones in the south and east of the Vulcan Sub-basin.
Uplift in the Late Triassic caused widespread erosion on the Ashmore Platform,Londonderry High and on the southern margin of the Petrel Sub-basin. A thick succession of fluvial and fluvio-deltaic, Jurassic sediments (Plover Formation) were then deposited in the main depocentres within the basin (Vulcan Sub-basin, Sahul Syncline and Malita Graben) and across the Sahul Platform.
In the Northern Bonaparte Basin, a marine facies is developed at the top of the Plover Formation. This unit has been referred to as the Elang Formation, the Laminaria Formation or the ‘Montara beds’. Many of the petroleum accumulations identified in the Vulcan Sub-basin and on the Sahul Platform are structurally trapped in Plover and Elang/Laminaria Formation sandstones.
Plover Formation sediments are absent on the Ashmore Platform and on the crestal parts of the Londonderry High, but onlap the eastern flank of the Londonderry High from the Petrel Sub-basin. In areas south and east of the Malita Graben, however, the Plover Formation is not considered a primary exploration objective due to relatively shallow burial depths.
In the Late Jurassic, the rate of subsidence in the major grabens increased and fine grained sediments of the Flamingo Group were deposited over a basin-wide, Callovian unconformity. In the Vulcan Sub-basin, sediments of the Flamingo Group (Upper and Lower Vulcan Formations) have traditionally been considered good quality source rocks. A recent oil discovery in a Tithonian sandstone within the Upper Vulcan Formation (at Tenacious-1) indicates the unit also has reservoir potential. In the Sahul Syncline area, equivalent age sandstones within the Cleia Formation are also considered exploration targets. In the offshore Petrel Sub-basin, a sandstone of Late Jurassic age at the top of the Flamingo Group (Sandpiper sandstone) is considered a secondary exploration objective in the area.
Mesozoic extension ended in the Valanginian when a marine transgression flooded the Australian continental margin. With the onset of thermal subsidence, fine grained, clastics and carbonates of the Bathurst Island Group were deposited across the Bonaparte Basin. At the base of the group, the Echuca Shoals Formation provides a regional seal to the underlying Upper Vulcan Formation in the Vulcan Sub-basin and to the Cleia Formation in the Sahul Syncline area. The unit thins on the platform areas in the west of the Bonaparte Basin, and in the Petrel Sub-basin to the east, is equivalent in age to sediments within the lowermost Darwin Formation.
Late Cretaceous and Cainozoic sediments typically comprise thick, prograding, platform carbonates. Lowstands sands developed in the Maastrichtian (Puffin Formation) and Eocene (Grebe sandstone), however, are considered exploration targets in the Vulcan Sub-basin. Oil has been recovered on test from the channelled, fan sands within the Puffin Formation at Puffin-1.
Regional compression associated with the collision of the Australian plate with the South East Asian microplates reactivated Mesozoic faulting and breached many Middle to Late Jurassic, fault dependent structures on the Londonderry High, Sahul Platform and in the Vulcan Sub-basin - many exploration wells drilled in these areas have intersected residual oil columns within sands of the Plover and Laminaria/Elang Formations.
Petroleum Systems
The Bonaparte Basin is a proven petroleum province. The basin contains all the prerequisites for additional discoveries with good quality reservoirs, mature source rocks and traps overlapping over a wide area of the basin. At date of writing, 68 petroleum accumulations had been identified in the Bonaparte Basin.
The petroleum potential of the Bonaparte Basin has been summarised by numerous authors over the last decade including McConachie et al, (1996); Colwell and Kennard, (1996); and Kennard et al., (2002 and 2003). These authors discuss the use of petroleum systems as an integrated approach to basin analysis, recognising existing proven petroleum plays and presenting them as a tool for identification of further hydrocarbon opportunities.
Magoon and Dow, (1991 and 1994) define a petroleum system as a mature source rock and all its generated hydrocarbon accumulations. Individual petroleum systems that share source rocks of similar age and facies can be grouped together into petroleum supersystems (Bradshaw et al., 1994). These can provide a basis for prediction of hydrocarbon occurrences in less well explored areas of a basin or basins with similar age rocks.
The key features of a petroleum play (source, maturity, reservoir, seal, trapping mechanism) are used to define a petroleum system. Petroleum systems identified in the Bonaparte Basin at date of writing are summarized below.
Palaeozoic Petroleum Systems
Colwell and Kennard, (1996) recognized three active Palaeozoic petroleum in the Petrel Sub-basin (elsewhere in the Bonaparte Basin, Palaeozoic sediments are not considered prospective for petroleum).
• A Late Devonian (Ningbing Limestone / Bonaparte Formation) Petroleum System. The gas accumulations identified within these units at Garimala and Vienta in the onshore, Petrel Sub-basin form part of this system and may have been sourced from either the Milligans Formation (Laws, 1981), or from the Bonaparte Formation (Kennard et al., 2002). Residual oil observed in a core cut in the Ningbing Limestone at Ningbing-1, may have been sourced locally from algal material within the limestone. This system appears to be restricted to the onshore and near offshore areas in the south of the Petrel Sub-basin;
• An Early Carboniferous (Milligans / Tanmurra / Kuriyippi Formations) Petroleum System. Thermally mature, marine mudstones within the Milligans Formation, have probably provided an oil and gas charge for the petroleum accumulations identified at Turtle, Barnett, Weaber and Waggon Creek;
• A Permian (Hyland Bay / Keyling Formations) Petroleum System. Gas (and possibly oil) sourced from either the Hyland Bay Formation or the Keeling Formation / Treachery Shale has charged the accumulations identified at Blacktip, Petrel, Tern and Fishburn.
Mesozoic Petroleum Systems
Mesozoic petroleum systems in the Timor Sea area include:
• A Middle Triassic to Middle Jurassic Petroleum System - oil and gas accumulations within the Challis, Nome and Plover Formations charged by Plover Formation source rocks;
• A Late Jurassic to Neocomian Petroleum System - source rocks within the Lower Vulcan Formation provide a hydrocarbon charge for Plover and Upper Vulcan Formation reservoirs (includes Jabiru, Skua, Audacious, Oliver, Tenacious). This system may also provide a petroleum charge for Late Cretaceous and Cainozoic reservoirs (Puffin Formation, Grebe and Oliver sandstones) via migration up faults;
• A Late Cretaceous Petroleum System. Although the Echuca Shoals and Upper Vulcan Formations are thermally immature for petroleum generation over much of the Bonaparte Basin, in the major depocentres in the Northern Bonaparte Basin (Sahul Syncline, Malita Graben), these units may provide a hydrocarbon charge for Upper Vulcan, Elang / Laminaria and Puffin Formation traps.
Data source: Cadman, S.J. and Temple, P.R., 2004. Bonaparte Basin, NT, WA, AC & JPDA, Australian Petroleum Accumulations Report 5, 2nd Edition, Geoscience Australia, Canberra.
Следующий Бассейн: Canning