Areas NT07-3, NT07-4, NT07-5 and W07-4
Petrel Sub-basin, Bonaparte Basin
Release Area Geology
The Petrel Sub-basin lies off northwest Australia where it mostly underlies the Joseph Bonaparte Gulf, extending onshore in the south. It straddles the Northern Territory/Western Australia boarder. Over most of the sub-basin, water depths are less than 100 m.
The stratigraphic and tectonic development of the Petrel Sub-basin has been discussed in detail by Mory (1988, 1991), Gunn (1988), Lee and Gunn (1988), Gunn and Ly (1989), McConachie et al (1996) and Colwell and Kennard (1996). It is most recently summarised by Kennard et al (2002) and Cadman and Temple (2004).
The Petrel Sub-basin is an asymmetric, northwest–southeast-trending Palaeozoic rift that contains a succession of thick Palaeozoic and thinner Mesozoic sediments (Figures 2, 3 and 4). 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.
Rift-related structures
Late Devonian (?late Givetian/Frasnian) to earliest Carboniferous (Tournaisian) upper-crustal extension produced a series of rift-related structures, particularly in the south and southwest of the basin (Gunn, 1988; O’Brien et al, 1993; Colwell and Kennard, 1996). These structures lie to the southwest of the axis of the main post-late Tournaisian basin ‘sag’ known as the Petrel Deep (Figure 2), indicating a possible partitioning between the mechanisms controlling upper-crustal extension and the subsequent sag-dominated phase of the basin’s development.
These rift-related extensional structures are bounded by major normal faults (and/or fault systems) and include planated basement platforms (eg, Berkley and Moyle platforms), horst blocks (eg, Cambridge High), rotated fault blocks (eg, Lacrosse Terrace), and graben (eg, Cambridge Trough). These features lie within or are adjacent to the 2007 release areas (Figure 2), and hence are considered in more detail.
The Berkley Platform is an area of planated basement that essentially forms an offshore extension of the Proterozoic Kimberley Basin and comprises tholeiitic dolerite. It dips to the northeast and is bounded on its northeastern margin by a major down-to-basin fault. Its landward extent approximates to the Kimberley coastline which, from its linear nature, may be fault controlled. The platform is overlain by about 2.5 km of Late Carboniferous and younger sediments.
The Moyle Platform forms the eastern (largely-onshore) flank of the Petrel Sub-basin where it consists of shallow crystalline basement, probably equivalent to those of the Pine Creek Geosyncline and Victoria River Basin. It is bounded on its eastern side by major faults of the Fitzmaurice Mobile Zone, and on its western side by the Moyle Fault. It passes northward into the Darwin Shelf.
The Cambridge High is an eastward-dipping, narrow basement horst block extending from the Berkley Terrace in the west to the Turtle-Barnett High in the east. It is bounded by reactivated normal fault systems and flanked by major depocentres to the south (Cambridge Trough) and to the north (Lacrosse Terrace, Petrel Deep). Initially, much of the syn-rift sediment in the southern Petrel Sub-basin appears to have been trapped south of the Cambridge High and adjacent Turtle-Barnett High. As the available accommodation space was filled, syn-rift sediments spread out as a series of alluvial fans across the highs and onto the developing Lacrosse Terrace to the north, and beyond. Movement on the faults bounding the Cambridge High during the late Tournaisian at the end of the ‘syn-rift’ phase led to widespread erosion of syn-rift sediments across the high.
The Turtle-Barnett High is a fault-bounded, approximately north–south-trending high-standing basement block, which juxtaposes the Cambridge High and Lacrosse Terrace. Area W07-4 is located in this structural setting (Figure 2). The position and trend of the high suggest that it may be related to reactivation of faults along the western edge of the Halls Creek Mobile Zone. Fault movements along its northwestern flank appear to post-date the formation of the main down-to-basin faults that form the northern margins of the Cambridge High and Lacrosse Terrace. However, during much of the Late Devonian–earliest Carboniferous (ie during syn-rift deposition), the Turtle-Barnett High was a high-standing feature probably shedding sediment into the adjacent developing depocentres of the Cambridge Trough and Keep Inlet Sub-basin. Like the adjacent Cambridge High, the feature was covered by sediments of the Bonaparte Formation and it was probably uplifted and eroded during the late Tournaisian at the end of the syn-rift phase.
The Lacrosse Terrace is largely restricted to the area between the Turtle-Barnett High and Lesueur 1, and comprises a rotated basement block overlain by syn-rift and younger sediments.
Stratigraphy
The generalised stratigraphy of the Petrel Sub-basin is shown in Figure 4. Detailed revisions to the Carboniferous stratigraphy of the sub-basin have been published by Gorter et al (2004, 2005), as shown in Figure 5.
Sedimentation in the Petrel Sub-basin commenced in the Cambrian. The pre-rift sequence contains extensive evaporite deposits. However, the precise age (Ordovician, Silurian or Devonian) and lateral continuity and extent of these salt bodies remain unknown.
Rifting was initiated in the Devonian, when clastic rocks and limestones were deposited in shallow marine and non-marine environments. The ‘Bonaparte Formation’, as defined by Beere and Mory (1986) and Mory and Beere (1988), comprises a thick succession of shale, siltstone, sandstone and sandy limestone, and unconformably overlies Proterozoic or Cambrian rocks. The onshore equivalent of the Bonaparte Formation is the Cockatoo Formation, Ningbing Group and Langfield Group. The complexity of this part of the stratigraphic column, as shown in Figure 5, is beyond the scope of this document. The reader is referred to the detailed description of the Late Devonian to Early Carboniferous stratigraphy as published by Gorter et al (2004, 2005).
By the Early Carboniferous, rifting had produced a northwest-trending basin, into which marine, fluvio-deltaic and glacial sediments were deposited in the Carboniferous and Permian, as a result of post-rift subsidence and salt withdrawal. These Carboniferous and Permian sediments represent a great proportion of the gross rock volume deposited in the Petrel Sub-basin, and specifically where the 2007 release areas are located.
The Early Carboniferous Milligans Formation, as defined by Mory (1991), was intersected in Bonaparte 1 between 497–2280 m, and extends throughout the inboard (Cambridge Trough and Keep Inlet Sub-basin) and onshore parts of the sub-basin. This formation has been redefined and is regarded as being of latest Tournaisian to late Visean in age (Gorter et al, 2004, 2005). The Tanmurra Formation comprises a thick succession of sandstones and sandy carbonates deposited throughout the Carlton Sub-basin, Cambridge Trough and Keep Inlet Sub-basin. The Point Spring Sandstone consists of sandstone, pebbly sandstone and minor siltstone. The Late Carboniferous Kuriyippi Formation, as defined by Mory (1991), is a thick succession of sandstone, shale and minor coal, overlain by glacial sandstone and conglomerate.
The latest Carboniferous to Early Permian Treachery Shale comprises tillite, carbonaceous shale, siltstone, sandstone and minor limestone and coal. The Keyling Formation is defined at the type section in Kulshill 1 (Mory, 1991) and comprises organic-rich delta-plain mudstones and coals, and marginal marine shales that have moderate to very good oil and gas potential in parts of the sub-basin. The Fossil Head and Hyland Bay formations comprise a thick (up to around 2300 m) succession in the central and outer parts of the Petrel Sub-basin (Figures 6 and 7). The Hyland Bay Formation consists of pro-delta marine mudstones and carbonates.
During the Triassic, depositional environments changed from marine to non-marine, ending with the deposition of red-beds. Late Triassic compressional inversion related to the Fitzroy Movement involved extensive uplift and erosion along the southern margin, and created structural trapping geometries within the sub-basin.
Jurassic, Cretaceous and Cainozoic sediments were deposited in a northwest-plunging
synclinal sub-basin. These strata are absent in the southern part of the
sub-basin, but thicken to the northwest toward the adjoining Malita Graben.
This sedimentation cycle is more closely affiliated with the Mesozoic rifting
of the Malita Graben and Sahul Syncline than the original Palaeozoic rifting
of the Petrel Sub-basin itself.