The Petrel Sub-basin is located in northwestern Australia, with most of the sub-basin occurring offshore in the Joseph Bonaparte Gulf, where water depths are typically less than 100 m. The southernmost part of the sub-basin extends onshore in the area lying between the Ord and Victoria rivers (Figure 2 [PDF, 110KB]). The Northern Territory/Western Australia boundary trends northwest along the axis of the sub-basin.
Local Tectonic Setting
The tectonic and stratigraphic development of the Petrel Sub-basin has been discussed in detail by Gunn (1988), Lee and Gunn (1988), Mory (1988, 1991), Gunn and Ly (1989), Petroconsultants Australasia Pty Ltd (1990), BRS (1994), McConachie et al (1996) and Colwell and Kennard (1996). It is most recently summarised by Kennard et al (2002) and Cadman and Temple (2004). Details of the onshore part of the sub-basin are discussed by Mory and Beere (1988).
The Petrel Sub-basin is an asymmetric, northwest-trending Paleozoic rift that contains a succession of thick Paleozoic and thinner Mesozoic sediments (Figure 3 [PDF, 148KB] and Figure 4 [PDF, 152KB]). 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 are overlain by a thin cover of Phanerozoic sediments and are developed 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 extends eastward into the Cambridge and Turtle-Barnett highs, where it is flanked by the Lacrosse Terrace (Figure 2 [PDF, 110KB]).
Strata within the Petrel Sub-basin dip regionally to the northwest with 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.
Interpreted horizons for industry seismic lines through and nearby the 2009 Special Release Areas in the inboard Petrel Sub-basin are shown in Figure 5 [PDF, 64KB].
Structural and stratigraphic evolution
Late Givetian–Frasnian to 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 Visean basin 'sag' known as the Petrel Deep (Figure 2 [PDF, 110KB]), indicating a possible partitioning between the mechanisms that controlled upper-crustal extension and the subsequent sag-dominated phase of the basin's tectonic evolution.
These rift-related extensional structures are bounded by major normal faults (and/or fault systems) and include planated basement platforms (eg, Berkley Platform and Moyle Platform), horst blocks (eg, Cambridge High and Barnett-Turtle High), rotated fault-blocks (eg, Lacrosse Terrace and Kulshill Terrace), and graben (eg, Cambridge Trough and Keep Inlet Sub-basin). Many of these features lie within, or are adjacent to, the 2009 Special Release Areas (Figure 2 [PDF, 110KB]), and hence are considered in more detail later in this section.
The basin continued to receive sediment during the post-rift subsidence that occurred throughout the Carboniferous, Permian and Triassic. The Fitzroy Movement was a compressional event during the Late Triassic to Early Jurassic that is associated with salt mobilisation and resulted in the inversion of many earlier extensional faults. It is responsible for creating many traps within the sub-basin, including the anticlinal structures that host the Petrel and Tern gas accumulations. During this event, the onshore portion of the sub-basin was uplifted and eroded, resulting in the rapid thickening of the sediments from the south to the north.
The Berkley Platform is an area of planated shallow basement (tholeiitic dolerite was intersected in Berkley 1) that essentially forms an offshore extension of the Proterozoic Kimberley Basin. 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, given its linear nature, may be fault controlled. The platform is overlain by about 2500 m of Pennsylvanian 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 extends northward into the Darwin Shelf. The basement is overlain by ?Pennsylvanian–Cisuralian sediments, as intersected by the Moyle 1 well.
The Cambridge High is an eastward-dipping, narrow basement horst-block that extends from the Berkley Platform 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 both the south (Cambridge Trough) and north (Lacrosse Terrace and 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 northwest-trending high-standing basement block, which juxtaposes the Cambridge High and Lacrosse Terrace. 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–Early Mississippian (i.e. during syn-rift deposition) the Turtle-Barnett High was a high-standing feature that probably shed sediments 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 the Lesueur 1 well, and comprises a rotated basement block that is overlain by syn-rift and younger sediments. The Lacrosse Terrace dies out as it merges into a series of deeper fault-blocks northwest of Lesueur 1.
The Kulshill Terrace has been variously applied to features in the southeast of the basin, but its usage herein is restricted to the onshore part of the basin to the west of the Moyle Fault. Two wells have been drilled in this area, Kulshill 1 and 2; both intersected thick Paleozoic sections.
The Cambridge Trough is a graben that acted as a major Late Devonian–Mississippian depocentre lying to the south of the Cambridge High. Its geology is probably largely contiguous with that of the onshore Carlton Sub-basin, which lies to the south of a reactivated, east–west (wrench?) fault zone extending through the Pelican Island area. Sediments in the trough onlap the Berkley Platform to the west, and are bounded to the east by the Turtle-Barnett High. By the late Visean, the Cambridge Trough had filled and ceased to exist as a discrete structural entity.
Keep Inlet Sub-basin
The term Keep Inlet Sub-basin has been applied inconsistently over the years. It is used herein for the poorly developed depocentre lying east and southeast of the Turtle-Barnett High. It extends onshore in the south to the northeast of Keep River 1 and possibly to the east as part of the Kulshill Terrace.
The stratigraphy of the Petrel Sub-basin has been compiled from Beere and Mory (1986), Mory and Beere (1988), Mory (1991), Gorter (1998) and Gorter et al (1998). The stratigraphy shown in Figure 3 [PDF, 148KB] has been updated to the Geologic Time Scale 2004 after Gradstein et al (2004), and revised to incorporate the most recent stratigraphic definitions by Gorter et al (2004, 2005, 2008 and 2009). Due to the complexity of the revised Petrel Sub-basin stratigraphy, Figure 4 [PDF, 152KB] shows the relationships of the Devonian to Triassic subgroups and formations.
Sedimentation in the Petrel Sub-basin commenced in the Cambrian and continued into the Early Ordovician, with the deposition of shallow marine clastics and carbonates. This was followed, most probably in the Late Ordovician, by extensive evaporite deposits of unknown lateral continuity. These evaporites appear to be of similar age as those of the Carribuddy Group in the Canning Basin. A detailed account of salt diapirs and salt-related tectonics are given by Edgerley and Crist (1974), Woodside Australian Energy (2002a) and Leonard et al (2004).
Rifting was initiated in the Late Devonian and siliciclastic sediments and carbonates were deposited in terrestrial and shallow marine environments. In the southern onshore portion of the basin, the Frasnian sediments of the Cockatoo Group are dominated by coarse clastics and conglomerates, some of which may be non-marine (Mory and Beere, 1988). Northward, the coarse-grained Cockatoo Group facies are gradually replaced by siltstones and shales with interbedded sandstones and sandy limestones, known as the Bonaparte Formation (Mory and Beere, 1988).
In the Famennian, the clastics of the Cockatoo Group are replaced by the reefal carbonates of the Ningbing Group around the margins of the basin, while in the deeper central onshore and offshore portions of the basin, deposition of the Bonaparte Formation continued essentially unchanged (Mory and Beere, 1988).
By the Mississippian, rifting had produced a northwest-trending basin, in which marine, fluvio-deltaic and glacial sediments accumulated as a result of post-rift subsidence and salt withdrawal during the Carboniferous and Permian. These Permo-Carboniferous sediments represent the bulk of the basin-fill in the Petrel Sub-basin.
In the Tournaisian the reefal facies was replaced by mixed carbonates, and fine-grained clastics of the Langfield Group (Mory and Beere, 1988; Gorter, 2006a) on the basin margins, with deposition of the Bonaparte Formation continuing in the deeper portion of the basin. The type section for the Bonaparte Formation is defined between 2280 and 3210 mKB total depth (TD) in Bonaparte 1 (Beer and Mory, 1986; Mory, 1991) and comprises a sequence of shale, siltstone, sandstone and minor sandy limestone. Towards the end of the Tournaisian, an unconformity separates the Langfield Group and offshore Bonaparte Formation from the overlying Weaber Group, represented at its base by the Milligans Formation.
The Mississippian Weaber Group, as developed in the southern part of the Petrel Sub-basin, is a complex package of clastic and carbonate sediments separated by several unconformities. The original definition of the Weaber Group as described by Mory and Beere (1988) and Mory (1991) has been revised by Gorter et al (2005), so that it now comprises the Milligans Formation (including the Waggon Creek facies), Yow Creek Formation, Utting Calcarenite, Kingfisher Shale/Burvill Formation, Tanmurra Formation, Sandbar Sandstone and Sunbird Formation.
The Tournaisian–Visean Milligans Formation was originally defined by Mory (1991) as consisting of fossiliferous shales and siltstones with the type section occurring over the depth range 44–155 m in Milligans No.1 Bore (Veevers and Roberts, 1968), Thick Milligans Formation is penetrated in the onshore wells Keep River 1 and Bonaparte 1 and 2, with the thickest offshore section occurring within Kingfisher 1 (Gorter et al, 2005). The 'Milligans Beds' is a term used to describe the shales overlying the Langfield Group in some of the earliest wells drilled in the Petrel Sub-basin, but may not correspond to the Milligans Formation as currently defined. The Milligans Formation extends throughout the Cambridge Trough, Keep Inlet Sub-basin and the onshore parts of the sub-basin. The age of this formation has been redefined and is regarded as being of latest Tournaisian–early Visean in age (Gorter et al, 2004, 2005). The basin margin equivalent of the Milligans Formation is known as the Waggon Creek facies that comprises predominately pebbly sandstones and conglomerates overlain by sandstones and shales, as intersected at Keep River 1 and Waggon Creek 1 (Beere, 1984; Gorter et al, 2005).
The Visean Yow Creek Formation is a basinal shale that commonly contains ironstone, which is bounded by unconformities at the base and top (Gorter et al, 2005). The type section is between 980–1160 mKB in Bonaparte 1. The overlying Utting Calcarenite is named from the type section in Utting Gap and comprises fossiliferous sandy limestone and calcareous sandstone (Veevers and Roberts, 1968; Mory and Beere, 1988). This may be the low stand section of the Yow Creek Formation (Gorter et al, 2005). The abrupt lithological change from the Utting Calcarenite to the carbonaceous claystone of the Kingfisher Shale is taken to represent a rapid deepening event (Gorter et al, 2005). The type section for the Kingfisher Shale is between 1950–2091 mRT in Kingfisher 1. The Utting Calcarenite and Kingfisher Shale are lateral equivalents of the coarse clastic-dominated Burvill Formation developed near the basin margin.
The Visean Tanmurra Formation unconformably overlies the Kingfisher Shale and comprises a thick succession of calcareous and dolomitic sandstones and sandy carbonates deposited throughout the Carlton Sub-basin, Cambridge Trough and Keep Inlet Sub-basin. Carbonates of this age are known as the 'Medusa Beds' in Lacrosse 1 (Arco Limited, 1969). Gorter et al (2005) redefined the type section for the Tanmurra Formation from that of Mory (1991) to being between 220–497 mKB in Bonaparte 1. Shales within the sandstones contain high organic contents as intersected at NBF-1002, Keep River 1, and possibly within a poorly sampled section of Kingfisher 1.
The uppermost units of the Weaber Group are the Visean Sandbar Sandstone and overlying Visean–Serpukhovian Sunbird Formation (Gorter et al, 2005). The Sandbar Sandstone consists of mixed lithologies with the lower part dominantly carbonate and the middle and upper parts comprise carbonates and interbedded quartzose sandstones. The type section is between 1634–1695 mRT in Sandbar 1. The Sandbar Sandstone occurs within the Cambridge Trough, but it is not present on the Barnett-Turtle High and in Matilda 1. Seismic profiles show that the Sandbar Sandstone either pinches out or is truncated by the base of the Sunbird Formation. The Sunbird Formation is present widely throughout the southern Petrel Sub-basin and comprises massive limestone with minor quartzose sandstone, with the type section being between 2236.5–2598.5 mRT in Sunbird 1 (Gorter et al, 2005).
The Late Mississippian–Early Pennsylvanian Wadeye Group is represented on the basin margins by the Point Spring Sandstone, consisting of sandstones, pebbly sandstones and minor siltstones, and in the deeper parts of the basin by the finer grained clastics of the Arco and Aquitaine formations (Gorter et al, 2005). Note that the Wadeye Subgroup of Gorter et al (2008) has not been adopted herein. The base of the Wadeye Group is characterised by canyon incision as a result of a major fall in sea level.
The Wadeye Group is overlain unconformably by the Early Pennsylvanian–Cisuralian Kulshill Group. The Kulshill Group, as redefined by Gorter (2006b) and Gorter et al (2008) comprises the Kuriyippi, Treachery, Quoin, Ditji and Keyling formations. The Kulshill Group was deposited in an overall transgressive cycle, overprinted by the onset of glaciation in the Kuriyippi Formation (Mory, 1991). The Bashkirian–Asselian Kuriyippi Formation, and its western basin-margin equivalent Border Creek Formation and eastern onshore sub-basin equivalent the Keep Inlet Formation, are overlain by the regional Treachery Formation. The Kuriyippi Formation, as defined by Mory (1991), is a thick succession of upward fining cycles of sandstones, siltstones, shales and minor coals, overlain by glacial sandstones and conglomerates. The complex incised channel network at the top of the Kuriyippi Formation suggests that the area lay under an ice sheet at this time (Gorter et al, 2008). The capacity of this formation to entrap oil is demonstrated at Barnett and Turtle on the Turtle-Barnett High and gas at Blacktip. The type section is named after the Kuriyippi Hills and is defined in Lesueur 1 between 1784–2801 mKB, which is the thickest section penetrated in the Petrel Sub-basin (Mory, 1991).
The Sakmarian Treachery Formation extends throughout the southern Petrel Sub-basin where it unconformably overlies the Kuriyippi Formation (Gorter 2006b; Gorter et al, 2008). It comprises tillites, diamictites, carbonaceous shales, varved siltstones, sandstones and minor limestones and coals, and provides top seal to accumulations in the underlying Kuriyippi Formation. The type section is between 1094–1227 mRT in Kulshill 1 where it was named the Treachery Shale (Mory, 1991). However, Gorter et al (2008) renamed the unit the Treachery Formation, and includes the informally named Blacktip member, a reservoir at Blacktip 1. These authors also propose a reference section between 2827.3–3072.8 mRT in Blacktip North 1. In the Keep Inlet Sub-basin, the formation is over 300 m thick in Kingfisher 1, Sunbird 1 and Kulshill 1, with the formation thinning towards the basins margins.
The Sakmarian Quoin and Ditji formations as defined by Gorter et al (2008) were originally included within the overlying Keyling Formation of Mory (1991). The type section of the Quoin Formation is between 1145–1350 mKB in Barnett 1, with a reference section established in Blacktip 1 (Gorter et al, 2008). The Quoin Formation is a sharp-based blocky sandstone that fines upwards into thinner sandstones, siltstones and shales. These sediments were deposited in a fluvial environment after de-glaciation when melt water from the ice sheet carried vast quantities of sediments into the basin. The formation is thickest (˜750 m) in the vicinity of Kulshill 1 and 2. The overlying Ditji Formation is interpreted as a transgressive sequence deposited in response to the end of glaciation (Gorter et al, 2008). The type section of the Ditji Formation is between 1721–1795 mKB in Kinmore 1 with a reference section of 1079–1132 mKB in Barnett 1 where a characteristic ash bed is present (Gorter et al, 2008). The formation comprises hard calcareous sandstone grading into sandy limestone, with minor interbedded coals. The marine transgression was terminated by the prograding coarse-grained sediments of the Keyling Formation.
The Sakmarian Keyling Formation was originally defined as the type section between 254–1094 mRT in Kulshill 1 (Mory, 1991). Gorter et al (2008) redefined the formation and suggested a reference section within Blacktip 1 (2152.5–2601.5 mRT). The formation probably unconformably overlies the Ditji Formation, and is unconformably to conformably (in the north) overlain by the Fossil Head Formation. The formation comprises delta-plain and marginal marine sandstones, siltstones, shales and minor coals and limestones. The coals are intersected in the eastern Petrel Sub-basin and on the Darwin Shelf by Kinmore 1 and Flat Top 1, respectively. The coals and marginal marine shales have moderate to very good oil and gas potential. The Keyling Formation is present throughout the southeastern Petrel Sub-basin being 525 m thick at Polkadot 1 and 450 m thick at Blacktip 1. The Keyling Formation is about 300 m thick in Kulshill 1 and 2. It generally thins towards the southern basin margin, where it is truncated below the base of the Fossil Head Formation. The Keyling Formation was deposited in a marginal marine environment. The Keyling Formation is the primary reservoir below the Fossil Head Formation regional seal, and is a gas-bearing reservoir at Blacktip 1 (Leonard et al, 2004) and Tern 1 and contains oil at Turtle 1.
The Cisuralian to Middle Triassic Kinmore Group of Mory (1991) has been redefined by Gorter et al (1998) and Gorter et al (2009) so that it now comprises the Fossil Head Formation, Hyland Bay Subgroup and Mount Goodwin Subgroup (Figure 3 [PDF, 148KB] and Figure 4 [PDF, 152KB]). The Sakmarian–Roadian Fossil Head Formation comprises carbonaceous siltstones and mudstones with sandstones and minor limestones. The type section is between 2993–3569 mKB in Tern 1 (Mory, 1991). It occurs throughout the southern Petrel Sub-basin, south of Petrel 1 and was deposited under marine shelfal conditions. This transgressive sequence forms the regional seal in the Petrel Sub-basin.
The Hyland Bay Formation of Mory (1991) was revised to the Hyland Bay Subgroup by Gorter (1998), with the terminology used interchangeably in the well completion report extracts herein depending on the vintage of the report being quoted. The Hyland Bay Subgroup consists of pro-delta, deltaic and shoreface mudstones, siltstones and sandstones, as well as open shelf carbonates that are particularly thick (up to around 2300 m) in the central and outer parts of the Petrel Sub-basin. Gorter (1998) and Gorter et al (1998) divided the subgroup into five formations; Pearce, Cape Hay, Dombey, Tern and Penguin formations. However, the basal Torrens Member, as defined between 1208–1230 mRT in Torrens 1 (Gorter, 1998) has since been given formation status (Figure 3 [PDF, 148KB] and Figure 4 [PDF, 152KB]), and the Penguin Formation is now classed as the base of the Mount Goodwin Subgroup (Gorter et al, 2009). Robinson and McInerney (2004) published palaeogeographic reconstructions of the most important reservoir units within the Hyland Bay Subgroup. The Torrens Formation hosts gas at Penguin 1, Petrel 2 and Polkadot 1. The Pearce Formation is represented by a shelf and platform carbonates. The Cape Hay Formation (Gorter, 1988) is the equivalent to the Hay Member of Bhatia et al (1984). It is the reservoir unit for the gas accumulations at Ascalon 1A, Petrel, Tern 4 and oil shows at Turtle 2. It was deposited as part of a widespread, river-dominated delta system with restricted shoreface conditions (Robinson and McInerney, 2004). The Dombey Formation carbonates provide the top-seal to the Cape Hay Formation. The Tern Formation is the reservoir for the Tern gas accumulation and gas shows at Ascalon 1A. The formation is interpreted to represent shoreface and shoal environments (Robinson and McInerney, 2004). The Tern Formation comprises open-marine sediments and forms a broad, prograding shoreface system. The Hyland Bay Subgroup is conformably overlain by the thick transgressive claystones of the latest Permian–Early Triassic (Changhsingian–Olenekian) Penguin and Mairmull formations, which provides both vertical and lateral seal across the Petrel Sub-basin. Where the Penguin Formation is absent, the Mairmull Formation unconformably overlies the Hyland Bay Subgroup.
Collectively the Penguin, Mairmull, Ascalon and Fishburn formations comprise the Mount Goodwin Subgroup (Gorter et al, 2009). The type section for the Penguin Formation is between 2400–2449 mKB in Tern 3 (Gorter, 1998) and is believed to have been deposited in a lacustrine setting. Although the formation is predominantly claystones, it hosts gas at Fishburn 1. The Mairmull Formation consists of claystones and siltstones with the type section being between 2121–2320 mRT in Fishburn 1 (Gorter et al, 2009), where it was deposited in a shallow water marine environment. The Olenekian Ascalon Formation is a prominent, widespread sandstone and siltstone unit deposited throughout the southern Bonaparte Basin. The type section is between 4072.5–4105 mRT in Ascalon 1A (Gorter et al, 2009). The sandstones were deposited in a marginal marine setting and probably represent a lowstand package. Gas is reservoired within the Ascalon Formation at Blacktip 1 and gas shows occur at Ascalon 1A. The Olenekian–Anisian Fishburn Formation consists of claystones with minor siltstone and sandstone, probably deposited in a near-shore environment. The type section is between 1904.5–2084 mRT in Fishburn 1 (Gorter et al, 2009).
The Kinmore Group is unconformably overlain by the Troughton Group in the eastern Bonaparte Basin, and the partially time equivalent Sahul Group in the western Bonaparte Basin (Mory, 1991; Gorter et al, 2009). The Middle Triassic to Middle Jurassic Troughton Group comprises the Cape Londonderry, Malita and Plover formations (Mory, 1991). It is a thick clastic sequence of marginal-marine to marine sandstones, siltstones and dolomitic shales. The Sahul Group is also marine to marginal marine, but contains more carbonates and shales than the Troughton Group and is defined from well penetrations on the Ashmore Platform, Vulcan Sub-basin and Londonderry High.
The Anisian Osprey Formation is recognised in wells in the Vulcan Sub-basin and on the Ashmore Platform and Londonderry High. In the eastern Bonaparte Basin, the Osprey Formation is equivalent to the basal part of the Cape Londonderry Formation (Mory, 1991). The formation has been mapped to extend within wells in the central Petrel Sub-basin as a package of interbedded sandstones and shales, with some minor carbonates underlying the sandstone-dominated Cape Londonderry Formation (Gorter et al, 2009).
The regressive Cape Londonderry Formation (Anisian–Norian) consists of sandstones and minor amounts of siltstones and shales, and was deposited during a relatively stable sag phase in a fluvial to braided stream environment. The type section is between 2471–2887 mKB in Petrel 1 (Helby, 1974; Mory, 1991). In the Middle Triassic, uplift and northeast–southwest rifting was initiated, which resulted in the widespread erosion of the Cape Londonderry Formation and parts of the Mount Goodwin Subgroup. Depositional environments changed from marine to terrestrial, culminating in red-bed deposition of the Late Triassic (Norian–Rhaetian) Malita Formation. The type section is between 2229–2471 mKB in Petrel 1 (Helby, 1974; Mory, 1991). Late Triassic compressional inversion related to the Fitzroy Movement involved extensive uplift and erosion along the southern margin, and created structural traps within the sub-basin.
The Early–Middle Jurassic Plover Formation ('Petrel C' of Arco Australia Limited (1971a) contains sandstones that may have excellent reservoir qualities, as well as shales with some source potential. These sediments were deposited as thick sequences in deltaic to near-shore environments within the central and northern Petrel Sub-basin.
The Middle Jurassic–Early Cretaceous Flamingo Group represents a time of minor extension and subsidence and herein comprises the Elang Formation, Lower Frigate Shale/Cleia Formation, Frigate Shale and Sandpiper Sandstone, as modified from Mory (1991), Pattillo and Nicholls (1990) and Whittam et al (1996). The Flamingo Group includes the packages of sediments referred to as 'Petrel A' and 'Petrel B' by Arco Australia Limited (1971a). The Callovian–Oxfordian Elang Formation is an excellent reservoir elsewhere in the Bonaparte Basin, whereas the basal marine shale, the Oxfordian–Tithonian Cleia Formation/Lower Frigate Shale and Frigate Shale, can form an excellent top seal and cross-fault seal to the Plover Formation. The overlying Berriasian–Valanginian Sandpiper Sandstone is an excellent reservoir unit, although no hydrocarbon shows have been recorded in this formation in the Petrel Sub-basin.
The intra-Valanginian unconformity separates the Flamingo Group from the overlying Cretaceous Bathurst Island Group that comprises the Echuca Shoals Formation, Darwin Radiolarite and Wangarlu Formation. During this time, the Bonaparte Basin was submerged during a post-rift sag phase, and widespread, thick, shale-dominated marine sediments were deposited across the basin, with the thickest sections occurring in the Petrel Sub-basin, Malita Graben and Calder Graben. The basal glauconitic claystones of the Valanginian–Aptian Bathurst Island Group were originally defined as the Darwin Formation by Mory (1988, 1991). However, many subsequent workers have referred to this section as the Echuca Shoals Formation, and applied the term 'Darwin Formation' to the overlying Aptian to Albian section of radiolarian-bearing calcareous claystones and calcilutites (referred to herein as the 'Darwin Radiolarite').
The mid-Valanginian–early Aptian Echuca Shoals Formation comprises a condensed section of glauconitic, marine claystones and siltstones. These sediments were deposited widely across the Bonaparte Basin as a result of the foundering of the Australian margin following continental break up, sea-floor spreading and subsidence. The dark-grey to black claystones within this formation contain good quality, oil-prone kerogen that provides a potential liquid source within the northern Bonaparte Basin. The peak of the transgression is represented by a condensed sequence of radiolarian cherts, claystones and calcilutites (Whittam et al, 1996; the 'Darwin Radiolarite').
The overlying Wangarlu Formation is an Albian to mid-Campanian progradational sequence that was deposited in a marine shelf to slope environment. The basal section predominantly comprises massive claystones with subordinate siltstones and minor sandstones. These lithologies grade into claystones, calcilutites and marls until the Santonian where sandstones are locally developed in the upper part of the formation, in the Vulcan Sub-basin and northern Bonaparte Basin.
A regional erosional event occurred between the Late Cretaceous (Santonian) and Miocene leading to the accumulation of the Woodbine Group on a progradational shelf of the passive margin. The collision of the Australian Plate with the Banda Arc resulted in north–south compression and minor inversion of the generally east–west normal faults and possible strike-slip along older northeast–southwest-trending Triassic-aged faults.