Areas NT07-3, NT07-4, NT07-5 and W07-4

Petrel Sub-basin, Bonaparte Basin

Petroleum Potential

The petroleum prospectivity is largely restricted to the Palaeozoic succession in the Petrel Sub-basin per se (Figures 6, 7, 8 and 9) Only in the northern-most part of the sub-basin, where it merges with the flanks of the Malita Graben, do Mesozoic sediments become potential exploration targets.

Hydrocarbon families and source rocks

Hydrocarbon families and their postulated source rocks have been extensively documented within the Bonaparte Basin. Papers that published the detailed geochemistry of oils and source rocks from the Petrel Sub-basin are McKirdy (1987), Jefferies (1988), Edwards et al (1997, 2000), Gorter et al (2004, 2005), and Gorter (2006).

The stable carbon isotopic composition for C7+ n-alkanes of the condensates recovered from the Petrel and Tern gas accumulations clearly distinguish them from the oils recovered from Turtle, Barnett and the onshore well Waggon Creek 1 (Figure 10). The wet gases from Tern show a similarly 13C-enriched isotopic composition as the condensate, indicating that the gas and condensate were generated from the same source. The gases from Blacktip 1 are somewhat depleted in 13C and exhibit a different shaped profile in comparison to the Tern 5 gas. The most likely source of the Blacktip gas is the Permian Keyling Formation, but until gas-source rock studies are undertaken, a Carboniferous source may also be possible (Edwards et al, 2006).

Petroleum systems

At least two active petroleum systems have been identified and defined in the offshore portion of the Petrel Sub-basin (Bradshaw et al, 1994; Edwards et al, 1997; Kennard et al, 2002; Barrett et al, 2004; Earl, 2004; Gorter et al, 2004, 2005):

Timing of generation and expulsion

Hydrocarbon expulsion models have been generated by Kennard et al (2002) for three basin-wide source rock units: Early Carboniferous Milligans Formation, Early Permian Keyling Formation and Late Permian Hyland Bay Formation.

Only expulsion models for the Keyling and Hyland Bay formations are reproduced here since these source units are prospective for gas throughout a large portion of the sub-basin, including the Northern Territory 2007 release areas. The modelling for the Early Carboniferous Milligans Formation needs to be modified since it has been proven that the oils at Turtle and Barnett are generated from the slightly older Langfield Group (Gorter et al, 2004, 2005; Gorter, 2006). Further work is required to determine whether any of the oil shows in the sub-basin are indeed sourced from the Milligans Formation.

Keyling source unit
Modelled gas and oil expulsion from shales and coaly shales of the Early Permian Keyling Formation is restricted to the central and outer portions of the Petrel Deep (to the north and northwest of Penguin 1 and Bougainville 1, respectively, beneath Area NT07-3). Modelled expulsion from the outer Petrel Deep occurred in the Late Permian–Early Triassic. Expulsion from the central Petrel Deep (eg, below TD of the Petrel wells) commenced and peaked in the Early Triassic and continued to the mid-Cretaceous. To date, no oil accumulations have been discovered that are likely to have been sourced from the Keyling Formation or underlying Treachery Shale and Kuriyippi Formation.

Structural traps at the Petrel, Tern and Penguin gas accumulations (Figures 6, 7 and 8) predominantly formed in the Late Triassic. These structures post-date the main phase of modelled oil expulsion from any oil-prone source units within the Keyling Formation, Treachery Shale and Kuriyippi Formation. They are unlikely to have received the main phase of oil charge from these sources. Having said this, the occurrence of interpreted SAR oil slicks east and southeast of the Petrel field may indicate oil migration pathways from local oil-prone coaly facies within these units. These SAR anomalies occur within or adjacent to the 2007 offshore release areas.

Hyland Bay source unit
Modelled gas expulsion from the Late Permian Hyland Bay Formation is limited to the outboard portion of the Petrel Sub-basin adjacent to the Malita Graben. It occurred throughout the Jurassic and Cretaceous, with peak expulsion in the mid- to Late Cretaceous. This unit is considered too lean to expel significant quantities of oil. The Petrel, Tern, Penguin, Polkadot and possibly Fishburn gas accumulations are most probably sourced from the Hyland Bay or Keyling formations, although a Carboniferous input to the Fishburn and Blacktip gas accumulations has not been discounted.


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Reservoirs

The most prospective reservoirs within the 2007 release areas are the Milligans, Kuriyippi and Hyland Bay formations.

The oil accumulations at Barnett and Turtle occur within the Carboniferous (Milligans, Tanmurra and Kuriyippi formations) to Permian (Treachery and Keyling formations) reservoirs in the inshore Petrel Sub-basin. Onshore, the oil and gas accumulation at Waggon Creek 1 is hosted in turbiditic sandstones within the Milligans Formation.

The Late Permian Hyland Bay Formation constitutes the main reservoir for the gas accumulations in the central part of the sub-basin including; Penguin 1, Polkadot 1, Tern, Petrel and Fishburn 1 (Robinson and McInerney, 2004). Gas at Blacktip is reservoired in three units; the Keyling Formation (primary objective) that contained 339 m cumulative gross gas and sealed by the Fossil Head Formation (Leonard et al, 2004). These authors report that the Mount Goodwin Formation was the secondary objective and contained 20 m gross gas, and two further gas columns were encountered in the Treachery Shale. However, in the southern inshore part of the sub-basin the Hyland Bay Formation is eroded (Lee and Gunn, 1988).

Seals

The Permian Fossil Head Formation and Treachery Shale provide regional seals for the respective underlying Keyling and Kuriyippi formations in the inboard Petrel Sub-basin. However, the regional seal is partially fault-breached across the Turtle-Barnett High (Durrant et al, 1990; Colwell and Kennard, 1996). Shaly members of the Point Spring Sandstone, Keyling Formation, Milligans Formation and Langfield Group may form effective local seals in a variety of trap geometries. Salt diapirs are also likely to provide effective seals in a variety of settings.

In the central and outboard areas of the Petrel Sub-basin, the transgressive, thick marine shales of the Mount Goodwin form the regional seal to the reservoirs of the Hyland Bay Formation. Also within the Hyland Bay Formation there are the intraformational marine shales of the Cape Hay Member, and the biomicritic limestones of the Dombey and Pearce members (Colwell and Kennard, 1996; McConachie et al 1996).

Play types

Structural and stratigraphic traps that contain both sandstone and carbonate reservoirs, have been identified at numerous stratigraphic levels in the southern Petrel Sub-basin, as shown in the schematic diagram in Figure 11 after Colwell and Kennard (1996) and Miyazaki (1997).

The main play types within the Early Carboniferous petroleum system of the inboard Petrel Sub-basin are Milligans Formation pinch-out plays against the Turtle-Barnett High and lowstand basin-floor fans. Sandbar 1 targeted such a basin floor fan, but did not encounter hydrocarbons. Other play types include Tanmurra Formation reefal plays, which appear to have formed on salt-induced seafloor mounds and Langfield Group combination plays. There are also Kulshill Group rollover anticlines on fault blocks down-thrown against the Lacrosse Terrace.

The Fitzroy Movement is responsible for creating large-scale inversion anticlines, such as those drilled at Petrel and Tern (Figures 6 and 7), as well as anticlines associated with faulting, for example those at Blacktip 1 (Leonard et al, 2004), Lacrosse 1 and Lesueur 1 (Figures 8 and 9)

Salt tectonics (flow, diapirism and withdrawal) has created numerous structural and stratigraphic potential petroleum traps. These features have either been identified or are thought to be present across most of the sub-basin (Edgerley and Crist, 1974; Durrant et al, 1990). Salt movement may have triggered petroleum migration and influenced migration pathways throughout the development of the Petrel Sub-basin.

Salt-related petroleum plays in the Petrel Sub-basin range from salt-core plays to salt-withdrawal basin plays. The timing of salt movements in the sub-basin varies widely, although many such salt-related traps may have formed too late with respect to hydrocarbon generation and migration. There is abundant evidence on seismic data for the presence of turbidites, basin-floor sands, slope-fan sands and coastal onlap of sand bodies within local depocentres over laterally migrating, slug-like, salt bodies (Lemon and Barnes, 1997; Miyazaki, 1997). These sandstones now constitute primary exploration objectives when found in favourable trap geometries.


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