The Vulcan Sub-basin is a proven oil producing province. It contains 26 hydrocarbon discoveries from 121 exploration wells, giving an overall success rate of about 21%. The latest published estimated reserves are 1.3 Tcf of gas, 31 MMbbl of condensate and 357 MMbbl of oil (Longley et al, 2002); however, these reserves do not include the results of more recent appraisal/development drilling at Puffin, or new discoveries in the period 2002-present (Cash, Katandra, Vesta, Swift North, Swallow). Commercial production has occurred from Late Triassic (Challis and Cassini fields), Early-Middle Jurassic (Jabiru and Skua fields), Late Jurassic (Jabiru field) and most recently Late Cretaceous (Puffin field) reservoirs. Development of the Montara Project (encompassing the Montara, Skua, Swift North and Swallow fields) is scheduled to commence in late 2008, and production from the Talbot Field is also scheduled to start in late 2008.
Barrett et al (2004) grouped all of the accumulations and significant hydrocarbon shows in the southern Vulcan Sub-basin and adjacent flanks of the Ashmore Platform and Londonderry High into the Vulcan-Plover(!) Petroleum System (Figure 9). This system is predominantly underpinned by Late Jurassic source rocks of the lower Vulcan Formation, and although reservoirs occur at several stratigraphic levels, the volumetrically dominant accumulations occur within the Plover Formation. A schematic time-space diagram of this system is shown in Figure 10.
The majority of the recovered oils and condensates from the Vulcan Sub-basin have a mixed marine and terrestrial geochemical signature, and are divided into two groups (Edwards et al, 2004). Group A with the stronger marine source affinity is found at Audacious, Challis, Puffin, Jabiru, Skua, Talbot and Tenacious, which are sourced from the Late Jurassic Lower Vulcan Formation. The condensate from Swan 3 is categorised as a subset of this marine group. Group B with the stronger terrestrial source affinity is found at Bilyara, Montara and Padthaway, which are probably sourced from the Early-Middle Jurassic Plover Formation. The Puffin oils are assigned to Group A, but may have an additional input from the Middle Jurassic Plover Formation, or possibly older (Permo-Triassic?) sediments (Edwards et al, 2004). The oil at Oliver is a mixture of both groups.
Late Jurassic oil-prone marine source rocks have been intersected by several wells in the Vulcan Sub-basin. Kennard et al (1999) and Edwards et al (2004) identify the Oxfordian-Kimmeridgian lower Vulcan Formation and underlying Montara Formation as a fair to very good source throughout the sub-basin. The organic matter is predominantly assigned to Type II/III kerogen that has the potential to generate both oil and gas. These sediments presently lie within the main oil to wet-gas generative windows in the Swan and Paqualin grabens, and the early oil window in the Cartier Trough. They are largely immature for hydrocarbon generation where intersected on the Montara Terrace.
The Early-Middle Jurassic Plover Formation is characterised by coaly, fluvio-deltaic and shallow marine sediments. On the Montara Terrace and on the flanks of the Cartier Trough, the formation contains organic-rich mudstones with fair to very good source potential (Edwards et al, 2004). Source quality varies from gas-prone (Type III) kerogen to oil- and gas-prone (Type II/III) kerogen. Some of these mudstones are within the early oil generation window in both localities. Coals and carbonaceous mudstones within the Plover Formation have only been intersected on the Montara Terrace, but are generally immature for hydrocarbon generation. If these facies extend into the Swan and Paqualin grabens the increased depth of burial would allow generation and expulsion of hydrocarbons.
The upper Vulcan Formation has fair to moderate potential yields in the Cartier Trough, Paqualin Graben and the Montara Terrace, but its low organic richness (TOC <1-2%) and marginal maturity may inhibit expulsion of oil and gas (Edwards et al, 2004).
The Early Cretaceous Echuca Shoals Formation, which acts as a regional seal, has good source rock potential in the Cartier Trough, and fair to moderate source potential elsewhere. However, these sediments are thermally immature to marginally mature for oil generation within much of the sub-basin (Edwards et al, 2004).
Various models have been proposed to predict hydrocarbon expulsion and migration in the Vulcan Sub-basin. The 1D modelling undertaken by Kennard et al (1999) predicted relatively restricted areas of oil and gas expulsion from the lower Vulcan Formation in the Swan and Paqualin grabens. Subsequent 2D and 3D modelling undertaken by Chen et al (2002) and Fujii et al (2004) showed more widespread expulsion extending northward into the Cartier Trough, and migration paths within the underlying Plover Formation towards the west onto the flank of the Ashmore Platform and eastward to the Skua, Cassini-Challis and Jabiru accumulations. Significantly, Chen et al (2002) modelled hydrocarbon migration westward to Puffin and towards Rainbow 1 and Warb 1A on the Ashmore Platform (Figure 11). Indeed, the residual oil analysed from the Miocene carbonates of the Oliver Formation at Warb 1A suggests that it was sourced from the Late Jurassic succession of the Vulcan Sub-basin. This is the most westerly known extent of oil migration from that depocentre, and migration does not appear to extend as far west as Lucas 1, Pascal 1, Rainbow 1 or Sahul Shoals 1.
None of the models generated by Kennard et al (1999), Chen et al (2002) or Fujii et al (2004) show charge from the lower Vulcan Formation into the Talbot accumulation on the eastern flank of the Montara Terrace. However, this accumulation is believed to have been sourced from the lower Vulcan Formation in the Swan Graben since;
The Talbot oil also has the lowest maturity of oils in the Group A family (Edwards et al, 2004), thus suggesting that it is an early generated and expelled, long-distance migration oil.
Fujii et al (2004) also modelled generation and migration from jointly the lower Vulcan and Plover formations, which resulted in far more pervasive expulsion, migration and accumulation in the eastern portion of the Vulcan Sub-basin, especially on the Montara Terrace adjacent to Release areas AC08-5 and AC08-6 (Figure 12). This model suggests mid Tertiary expulsion and migration from the Plover Formation in the Kimberley Graben (the southwestern extension of the Swan Graben) into the Montara-Tahbilk structures, which is consistent with the geochemical interpretation of Edwards et al (2004) that these oils are sourced from the Plover Formation. An active latest Miocene oil charge from the Plover Formation on the Montara Terrace is also predicted by their model (Figure 12), which has important implications for charge of potential structures in Release areas AC08-5 and AC08-6.
The source-rich claystones of the Late Jurassic Lower and Upper Vulcan Formation also act as regional seals across the Vulcan Sub-basin, and although they have variable seal potential, they are unlikely to fail as a result of capillary failure (Kivior et al, 2002). These claystones generally form competent seals above the Callovian, Oxfordian and Kimmeridgian unconformities.
Following continental margin collapse in the Valanginian, regional marine flooding led to the deposition of the Echuca Shoals Formation which forms a widespread and competent seal above the Valanginian unconformity. This unit has good sealing capacity, extent and integrity, but it has inconsistent sealing thickness (Kivior et al, 2002). However, if this seal is thin or absent, claystones and marls of the overlying Jamieson Formation usually provide a reliable top seal.
The reservoirs of the Puffin Formation are sealed by carbonates of the Paleocene Johnson Formation, and the Eocene Grebe Formation sandstones are sealed by carbonates of the Hibernia Formation. For the deeper Permian plays, seal is provided by Early Triassic shales of the Mount Goodwin Formation.
Many Jurassic/Triassic reservoirs within tilted fault bocks in the Vulcan Sub-basin rely on competent, intra-formational and cross-fault seals for trap integrity. In structurally complex areas of the sub-basin, such as along the major northeast-trending horsts along its western and eastern flanks, faulting is often difficult to image on seismic data. Recent advances in seismic acquisition and processing are leading to more accurate predictions of trap geometries and fault seal integrity (Maxwell et al, 2004; Peresson et al, 2004).
Exploration of the western margin of the Vulcan Sub-basin and adjacent flank of the Ashmore Platform has focussed on the Late Triassic (Nome and Challis formations, Middle Jurassic Plover Formation and Tithonian sandstone fans (upper Vulcan Formation) that are developed below Late Jurassic and Early Cretaceous regional seals. Late Cretaceous sandstone fans (Puffin Formation) draped over Triassic-Jurassic horsts have been additional targets. The Cretaceous and Tithonian sandstones generally have excellent to good reservoir qualities, respectively, whereas quality is good to locally poor within the intersected Middle Jurassic (eg Cash 1) and Triassic sections (eg Maple 1). All of these plays rely on Late Jurassic source charge from the Swan-Paqualin-Cartier depocentres.
Triassic tilt blocks have been tested at Pollard 1, Warb 1A, Rainbow 1, Langhorne 1, Yarra 1, Maple 1 and Cash 1. Tithonian and Oxfordian lowstand fan deltas and submarine fans have been interpreted and mapped along the western margin of the Paqualin Graben (Figure 13; O'Brien, Sturrock and Barber, 1996), and Tithonian fans are a proven play type at Tenacious (Woods and Maxwell, 2004). Thin reservoir quality Tithonian submarine fan sandstones were intersected in Paqualin 1, and were subsequently targeted but absent in Rothbury 1, Caversham 1 and Warb 1A. High quality 3D seismic data and newly available 3D megasurveys (Edwards et al, 2005) will assist better definition of these and other subtle stratigraphic traps in the region.
The Puffin Formation represents a channel-fed submarine fan system that contains six main depositional lobes formed in a deep-marine setting (de Boer, 2004). These lobes extend across the junction of the Bonaparte and Browse basins, and across the southeastern portion of the Ashmore Platform and northward into the southern most portion of Release Area AC08-4 (eg Yarra 1 and Great Eastern 1).
Secondary potential targets also include Eocene and Miocene carbonates and lowstand sandstones (Grebe and Oliver formations) in which residual oil shows were encountered in Puffin 1 and Warb 1A, respectively.
Drilling to date on the eastern margin of the Montara Terrace has primarily focussed on Late Triassic horst blocks (oil-bearing Challis Formation at Talbot) and transgressive Oxfordian shoreface/barrier bar sandstones of the Montara Formation (oil and gas-bearing at Montara, Bilyara and Tahbilk), sometimes with a secondary objective of Maastrichtian sandstones of the Puffin Formation. The Montara Formation sandstones are interpreted to have been deposited within transgressive fan deltas along the margin of the Montara Terrace/Londonderry High (Figure 14; O'Brien, Sturrock and Barber, 1996). Wells on the adjacent flanks of the Londonderry High have targeted deeper Permian plays within the Hyland Bay Formation (Osprey 1, Anderdon and Sleeper 1), but these older sediments generally have poor reservoir quality.
Tithonian and Berriasian glauconitic sandstones within the Upper Vulcan Formation at Delamere 1 and Halcyon 1, respectively, are gas-bearing and have good porosity, and a small gas accumulation has been identified in Santonian sandstones of the Gibson Formation at Tahbilk 1.
Traditional challenges for explorers in the Vulcan Sub-basin have been the seismic definition of potential traps, and as indicated by the identification of numerous palaeo-oil columns (Lisk et al, 1998) and hydrocarbon related diagenetic zones (HRDZs; O'Brien and Woods, 1995), the retention of hydrocarbons in tectonically reactivated structures (Woods, 2004). The seismic definition of these traps can now be better imaged with 3D datasets, including the PGS 3D Vulcan Sub-basin MegaSurvey (Edwards et al, 2005). Trap integrity and fault seal issues have been variously addressed by studies of, for example, trap breach by Neogene fault reactivation (Shuster et al, 1998; O'Brien, Morse et al, 1999), in situ stress regimes (Castillo et al, 2000), fault-tip propagation (Cooper et al, 1998), capillary seal potential (Kivior et al, 2002), intersecting fault trends (Gartrell et al, 2002), integrated fluid inclusion and buoyancy pressure studies (Liu et al, 2004) and quantification of fault-seal trapping mechanisms (Mildren et al, 2004). It is likely that these multiple techniques will continue to play an important role in identifying exploration targets in the more structurally complex areas of the Vulcan Sub-basin.
Critical risk factors for the western Vulcan Sub-basin Release Area AC08-4 are seismic definition of complex structural and subtle stratigraphic traps, trap integrity due to fault reactivation, and suitable migration pathways onto the adjacent flank of the Ashmore Platform.
Critical risk factors for the eastern Vulcan Sub-basin Release areas are the existence of suitable long distance migration pathways from proven Late Jurassic source rocks within the Swan Graben, or effective generation and charge form potential Plover Formation source rocks on the Montara Terrace. Trap integrity is also a potential risk, due to Miocene fault reactivation, and cross-fault seal due to juxtaposition of foot wall and hanging wall porous sandstone sections. Seismic definition of potential plays is also a challenge in this area.