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 (2006a). 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 (2007). 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).
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 (2006a) 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, Cassini, Challis, Jabiru, Puffin, Skua, Talbot and Tenacious, 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, 2007. The most likely source of the waxy oil family is from fluvio-deltaic to marginal marine mudstones, 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 and hence plot 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 (Bluff, Buffalo, Corallina, Jahal, Krill and Laminaria accumulations), and the second family includes the relatively marine-influenced oils/condensates to the southeast (Bayu, Elang, Hingkip, Kakatua, Kakatua North, Trulek and Undan accumulations). 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 reservoired in the younger Early Cretaceous Darwin Formation from Elang West 1, Layang 1 and Kakatua North 1 (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 oils sourced from the Early Cretaceous (eg, Caswell 2) in the Browse Basin (Figure 3).
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; Edwards and Zumberge, 2005);
Barrett et al (2004), following the nomenclature proposed by Magoon and Dow (1994), defined seven petroleum systems in the offshore Bonaparte Basin, consisting of three Jurassic, three Permian and one Permo-Carboniferous systems;
The distribution of these petroleum systems are shown in Figure 4, and are presented in montage format by Earl (2004). As noted earlier, the source of the Permo-Carboniferous system in the southern Petrel Sub-basin is now believed to be the Langfield Group (Gorter et al, 2004, 2005; Gorter, 2006a), rather than the Milligans Formation, so this system requires redefinition and re-mapping.