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

Ashmore Platform, Bonaparte Basin

Bonaparte Basin Geology

Petroleum Potential

Hydrocarbon families and source rocks

Hydrocarbon families and their postulated source rocks have been extensively documented within the Bonaparte Basin. Recent papers that published the detailed geochemistry of oils and source rocks from the Petrel Sub-basin are Edwards et al (1997, 2000), Gorter et al (2004, 2005), and Gorter (2006). Geochemical studies of Vulcan Sub-basin oils include those by Carroll and Syme (1994), George et al (1997, 1998, 2004a), van Aarssen et al (1998a, b), Edwards et al (2004) and Dawson et al (2006a, b). In the northern Bonaparte Basin appraisal of the hydrocarbon potential of the Jurassic–Early Cretaceous source rocks has been undertaken by Brooks et al (1996a, b), and Preston and Edwards (2000). Gas studies were undertaken by AGSO and Geotech (2000). Oil–oil and oil–source rock correlations in the northern Bonaparte Basin have been made by Gorter and Hartung-Kagi (1998), and Preston and Edwards (2000), while George et al (2002a, b, 2004a, b and c) carried out oil–fluid inclusion oil correlations.

Oil-oil and gas-condensate/oil comparisons have been made throughout the Bonaparte Basin by Edwards and Zumberge (2005) and Edwards et al (2006), respectively, from which much of the following text is taken. Figure 3 shows the hydrocarbon families of the Bonaparte and Browse basins and their interpreted origin after Edwards et al (2004). Two Palaeozoic and five Mesozoic families were recognised in the Bonaparte Basin, and two separate Mesozoic families were identified in the Browse Basin.

In the Petrel Sub-basin, an oil family comprising the Barnett, Turtle and Waggon Creek oils was recognised (Figure 3), of which the offshore oils at Barnett and Turtle have undergone biodegradation. This oil family was generated from anoxic marine mudstones. Such source rocks have been located at 208 m depth in the NBF-1002 mineral hole by McKirdy (1987), Edwards and Summons (1996) and Edwards et al (1997), and were placed within the Early Carboniferous Milligans Formation. However, recent reappraisal of the Petrel Sub-basin stratigraphy by Gorter et al (2004, 2005) and Gorter (2006) assigned these sediments to the earliest Early Carboniferous (early–middle Tournaisian) Langfield Group.

Most of the gas discoveries reservoired in the Late Permian Hyland Bay Formation in the outboard Petrel Sub-basin and on the Londonderry High are attributed to Permian source rocks within the Hyland Bay Formation and/or Keyling Formation (Edwards et al, 1997, 2000; Edwards and Zumberge, 2005). This hydrocarbon family is represented in Figure 3 by condensate recovered from the Petrel gas accumulation. The stable carbon isotopic signatures of the gases recovered from the Petrel, Tern and Blacktip accumulations indicate that at least two source units generated these gases (Edwards et al, 2006). The biomarker signature of the recovered condensates from Petrel and Tern are consistent with derivation from land-plant material. As yet, gas-source rock correlations have not been undertaken to determine the exact sources of the gases, but both the Hyland Bay and Keyling formations are rich in land-plant remains and were deposited in prodelta marine and deltaic to coastal plain environments, respectively.

In the Vulcan Sub-basin two oil families are recognised; a marine oil family comprising the oils from Birch, Challis, Jabiru, Puffin, Skua and Talbot, and waxy terrestrial oils from Bilyara, Maret and Montara (Figure 3). The majority of the oil accumulations (including all produced oils) throughout the sub-basin are sourced from the Late Jurassic lower Vulcan Formation. Their source rocks comprise marine mudstones that contain variable amounts of terrigenous organic matter (Carroll and Syme, 1994; Edwards et al, 2004; Dawson et al, 2006a). The most likely source of the waxy oil family is from fluvio–deltaic to marginal marine mudstone facies, possibly within the Plover Formation, which contains a greater terrestrial component than the lower Vulcan Formation (Edwards et al, 2004). The oils from Oliver 1 and Puffin 3 are mixtures, hence plotting separately from the other Vulcan Sub-basin families in Figure 3.

In the central northern Bonaparte Basin (Laminaria and Flamingo highs), oils reservoired within the Middle–Late Jurassic Plover and Elang formations, which includes all the commercial accumulations, have been divided into two end-member families by Preston and Edwards (2000), as shown in Figure 3. The first family includes the relatively land-plant-influenced oils in the northwestern part of the area (eg, Buffalo, Corallina, Jahal and Laminaria accumulations), and the second family includes the relatively marine-influenced oils/condensates to the southeast (eg, Bayu/Undan accumulation). Oils of intermediate composition occur between these accumulations.

While none of the oils can be uniquely correlated with a single source unit, Preston and Edwards (2000) concluded that all of the accumulations in this area are sourced predominantly from the Middle Jurassic Plover Formation and Late Jurassic Elang Formation, with additional contributions from the overlying sealing units: the land-plant-rich, Late Jurassic Frigate Formation in the northwest, and the marine-dominated, Late Jurassic–Early Cretaceous Flamingo Group in the southeast.

In the central northern Bonaparte Basin, a separate oil family is found comprising the non-commercial oils from Elang West 1, Layang 1 and Kakatua North 1, reservoired in the Early Cretaceous Darwin Formation (Preston and Edwards, 2000). These oils are believed to originate from the Sahul Syncline that contains post-rift, organic-rich marine sediments in the Early Cretaceous Echuca Shoals Formation. The oil from Elang West 1 has a similar composition to the Early Cretaceous-sourced oils (eg, Caswell 2) in the Browse Basin (Figure 3).


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Reservoirs

Throughout the Bonaparte Basin, most of the commercial oil and gas accumulations are reservoired in the Middle Jurassic Plover Formation and Late Jurassic Montara/Elang formations. In the Vulcan Sub-basin, commercial accumulations also occur in Late Triassic (Challis and Nome formations) and Late Cretaceous (Puffin Formation) sands. In the Petrel Sub-basin and on the Londonderry High, gas accumulations occur in the Late Permian Hyland Bay Formation. Oil accumulations occur within Carboniferous (Milligans, Tanmurra and Kuriyippi formations) to Permian (Treachery and Keyling formations) reservoirs in the inshore Petrel Sub-basin.

Petroleum Systems

Numerous petroleum systems of various ages have been documented within the Bonaparte Basin (Bradshaw et al, 1994, 1997; Colwell and Kennard, 1996; McConachie et al, 1996; Kennard et al, 1999, 2000, 2002; Barrett et al, 2004; Edwards and Zumberge, 2005), and are summarised by Cadman and Temple (2004) and produced in montage format by Earl (2004);

At least four petroleum systems are relevant to the 2007 release areas in the Bonaparte Basin;

Larapintine 4 Petroleum System

Figure 4 shows the extent of the Carboniferous Milligans-Kuriyippi/Milligans(.) petroleum system in the Petrel Sub-basin after Barrett et al (2004). This diagram was constructed on the premise that the NBF-1002 source rocks belonged to the Milligans Formation, and not the slightly older Langfield Group, as since determined by Gorter et al (2004, 2005). However, plays based on oil generation from the Milligans Formation, in addition to those from the Langfield Group, have been proposed by Taylor (2006). Hence, a Langfield/Milligans-Kuriyippi/Milligans(.) petroleum system may still be valid, albeit with modifications. Further work is required to determine whether the oil shows at Lesueur 1, Kinmore 1, Kulshill 1 and 2 and Sunbird 1, have the same or different chemistries to the oils at Barnett and Turtle to provide a better understanding of the Early Carboniferous petroleum system.

In summary, the Larapintine 4 Petroleum System comprises hydrocarbons generated from Early Carboniferous marine mudstones in the Petrel Sub-basin. The oils at Barnett and Turtle are reservoired in sandstones of the Milligans Formation, Tanmurra Formation, Kuriyippi Formation, Treachery Shale, and Keyling Formation. The Early Permian Treachery Shale regional seal is partially fault-breached across the Turtle-Barnett High (Durrant et al, 1990; Colwell and Kennard, 1996). Intraformational seals include; marine shales within the Milligans Formation, carbonates and marine mudstones of the Tanmurra Formation, and mudstones within the Point Spring and Kuriyippi formations.


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Gondwanan 1 Petroleum System

In the Petrel Sub-basin, the updated Permian Gondwanan 1 Petroleum System of Bradshaw et al (1994, 1997) is based on the gas accumulations at Blacktip, Penguin 1, Polkadot 1, Petrel, Tern and Fishburn 1 (Colwell and Kennard, 1996; Edwards et al, 1997, 2000, 2006; Kennard et al, 2002). These accumulations are thought to have been sourced from either prodelta marine mudstones of the Late Permian Hyland Bay Formation, or marine–deltaic and coastal plain shales and coaly shales of the Early Permian Keyling Formation, and are reservoired within deltaic and shoreface sandstones of the Hyland Bay Formation. This petroleum system has accordingly been designated the ‘Hyland Bay/Keyling-Hyland Bay(.) Petroleum System’ (Kennard et al, 2002; Barrett et al, 2004). A map showing the extent of this petroleum system is presented in Figure 5.

Regional seal is provided by transgressive marine shales of the Mount Goodwin Formation. The main trap types within this petroleum system are faulted anticlines, large-scale inversion anticlines, stratigraphic traps and pinchouts, and drape/pinchout associated with diapiric salt. A schematic diagram of the Gondwanan 1 Petroleum System is shown in Figure 6 after Colwell and Kennard (1998).

The gas accumulation at Kelp Deep 1 on the Sahul Platform is reservoired in, and presumably sourced by, Permian rocks. Likewise, the gas accumulation at Prometheus/Rubicon on the Londonderry High is also attributed to probable Permian source rocks. These accumulations are reservoired in the Hyland Bay Formation and form extensions or outliers of the Permian Gondwanan 1 Petroleum System.

Oil shows in Torrens 1 also provide encouragement for Permian-sourced oil on the Londonderry High (Edwards et al, 2000, Kennard et al, 2000; Ruble et al, 2000). This outlying Permian petroleum system has been designated the ‘Permian-Hyland Bay (?)’ Petroleum System by Barrett et al (2004). A map showing the extent of this petroleum system is shown in Figure 7.

Westralian Petroleum Supersystem

The Westralian Petroleum Supersystem is named after the Westralian Superbasin (Bradshaw et al, 1988) that includes all of the northeast–southwest-trending rift basins of the western Australian margin. In the Bonaparte Basin, this petroleum supersystem is represented in the Vulcan Sub-basin, Sahul Syncline and surrounding highs, and in the Malita Graben and Calder Graben.

Westralian 1 Petroleum System

The Westralian 1 Petroleum System comprises the gas accumulations on the Sahul Platform (Greater Sunrise), Troubadour Terrace (Evans Shoal), Malita Graben and Calder Graben (Lynedoch, Barossa, Caldita, and Abadi in Indonesian waters), reservoired within the Middle Jurassic Plover Formation. Recent geochemical studies indicate that the Greater Sunrise gas field was sourced locally from the Plover Formation on the Malita Graben and Troubadour Terrace (Longley et al, 2002). The presumed sources of the gas accumulations east of Sunrise, from Evans Shoal to Abadi, are numerous source units within the Plover Formation (Edwards et al, 2006). The regional seal for the Plover reservoirs is the thick claystone unit of the lower Bathurst Island Group. Trap types are large faulted anticlinal structures at the base regional seal level (Longley et al, 2002).

Westralian 2 Petroleum System

The majority of oil accumulations in the Vulcan Sub-basin belong to the Westralian 2 Petroleum System and are sourced from the marine shales of the Late Jurassic lower Vulcan Formation. Figure 8 shows the extent of the Jurassic Vulcan-Plover(!) petroleum system in the Vulcan Sub-basin after Barrett et al, (2004) with the pods of effective source rocks being modelled by Kennard et al (1999). Depending on the structural juxtaposition of the horsts blocks, and the migration pathways from the source kitchens in the Swan and Paqualin graben, the hydrocarbon accumulations may be found at several stratigraphic horizons, as presented in the schematic diagram in Figure 9. Many oil and gas accumulations (including the discovery at Audacious 1) are reservoired within the Plover Formation and sealed by the thick claystones of the Echuca Shoals Formation. The accumulations at Cassini, Challis and Talbot are reservoired within Late Triassic sandstones of the Challis and Nome formations and sealed by the Echuca Shoals Formation. In addition, the oil accumulation at Puffin, as well as the gas at Swan and East Swan, are reservoired in the Late Cretaceous Puffin Formation sands. The accumulations discovered in the recently drilled wells at Tenacious and Katandra are reservoired within the upper Vulcan Formation.

Trap types range from complex horst structures as at Jabiru, Challis and Skua, to smaller horst blocks in the southern part of the sub-basin, in the vicinity of Montara.

Westralian 1/Westralian 2 Hybrid Petroleum System

In the Sahul Syncline, the commercial oil accumulations located on and adjacent to the Laminaria High and the commercial gas accumulations at Bayu/Undan are believed to be sourced predominantly from the Middle Jurassic Plover Formation and Late Jurassic Elang Formation, with additional contributions from the overlying sealing units (Preston and Edwards, 2000). Hence, these central northern Bonaparte accumulations represent a hybrid Westralian 1/Westralian 2 petroleum system. Barrett et al (2004) referred to this petroleum system as the ‘Elang-Elang (!) petroleum system’. A map showing the extent of this petroleum system is shown in Figure 10. Similarly, in the Vulcan Sub-basin, gas is modelled to have been sourced by both the Middle Jurassic Plover Formation and the Late Jurassic Vulcan Formation (Kennard et al, 1999), resulting in the variable gas compositions recorded for this sub-basin (Edwards et al, 2006). The oil at Oliver 1 also appears to be of mixed composition.


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