Hydrocarbon Families and Source Rocks
Hydrocarbon families and their postulated source rocks have been extensively documented within the Bonaparte Basin. Recent papers on the detailed geochemistry of oils and source rocks from the Petrel Sub-basin are by 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 [PDF, 21KB] 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 [PDF, 21KB]), 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 onshore NBF-1002 mineral hole (McKirdy, 1987; Edwards and Summons 1996; Edwards et al 1997), and were postulated as being within the Carboniferous (late Tournaisian-Visean) Milligans Formation. However, reappraisal of the Petrel Sub-basin stratigraphy by Gorter et al (2004, 2005) and Gorter (2006a) assigned these sediments to the early-middle Tournaisian Langfield Group.
Most of the gas discoveries reservoired in the Late Permian Hyland Bay Subgroup in the outboard Petrel Sub-basin and on the Londonderry High are attributed to Permian source rocks within the Hyland Bay Subgroup and/or Keyling Formation (Edwards et al, 1997, 2000; Edwards and Zumberge, 2005); however there are no proven gas-source correlations in the literature. This hydrocarbon family is represented in Figure 3 [PDF, 21KB] 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 the Petrel and Tern accumulations are consistent with derivation from land-plant material.
In the Vulcan Sub-basin, two oil families are recognised; a marine oil family comprising oils from the Birch, Cassini, Challis, Jabiru, Puffin, Skua, Talbot and Tenacious accumulations, and waxy terrestrial oils from the Bilyara, Maret and Montara accumulations (Figure 3 [PDF, 21KB]). The majority of the oil accumulations (including all produced oils) throughout the sub-basin are sourced from the Late Jurassic lower Vulcan Formation (Edwards and Zumberge, 2005). 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 of the two sources and hence plot separately from the other Vulcan Sub-basin families (Figure 3 [PDF, 21KB]).
In the central northern Bonaparte Basin (Laminaria and Flamingo highs), oils reservoired within the Jurassic Plover and Elang formations, which include all the commercial accumulations, have been divided into two end-member families by Preston and Edwards (2000). As shown in Figure 3 [PDF, 21KB], one family includes the strongly land plant-influenced marine oils in the northwestern part of the area (Bluff, Buffalo, Corallina, Jahal, Krill and Laminaria accumulations), and the other family includes the marine oils/condensates to the southeast (Elang, Hingkip, Kakatua, Kakatua North, Trulek, Bayu and Undan 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 Plover Formation, with additional contributions from the Elang Formation and overlying sealing units: the land-plant-rich, Frigate Formation in the northwest, and the marine-dominated, 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 wells (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 (e.g., Caswell 2) in the Browse Basin (Figure 3 [PDF, 21KB]).
Recent geochemical studies of the gases from accumulations on the northern Sahul Platform, and in the Malita and Calder graben indicate that they are sourced from the Plover Formation in the main depocentres and on the Heron and Troubadour terraces (Longley et al, 2002; Edwards et al, 2006;)
Regional 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; Edwards and Zumberge, 2005);
- A Late Devonian-sourced petroleum system (Larapintine 3),
- An Early Carboniferous-sourced petroleum system (Larapintine 4)
- A Permian-sourced petroleum system (Gondwanan 1),
- An Early-Middle Jurassic-sourced petroleum system (Westralian 1),
- A Late Jurassic-sourced petroleum system (Westralian 2), and
- An Early Cretaceous-sourced petroleum system (Westralian 3).
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;
- Elang-Elang(!) Petroleum System (Sahul Syncline and Flamingo High)
- Plover-Plover(.) Petroleum System (Malita Graben and Sahul Platform)
- Vulcan-Plover(!) Petroleum System (Vulcan Sub-basin)
- Hyland Bay-Hyland Bay(?) Petroleum System (Kelp High)
- Hyland Bay/Keyling-Hyland Bay(.) Petroleum System (central Petrel Sub-basin)
- Permian-Hyland Bay(?) Petroleum System (Londonderry High)
- Milligans-Kuriyippi/Milligans(!) Petroleum System (southern Petrel Sub-basin)
The distribution of these petroleum systems are shown in Figure 4 [PDF, 208KB], 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.