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- Adjacent to proven oil-producing province; development planned for nearby Cash/Maple gas accumulation
- Shallow to deep water depths, 50–350 m
- Release Areas flank Jurassic depocentre with oil- and gas-prone source rocks
- Potential for structural and stratigraphic Triassic–Jurassic, Cretaceous and Paleogene plays
- Special Notices apply, refer to Guidance Notes
Release Areas AC15-1 and AC15-2 are located in the western Bonaparte Basin in the Timor Sea, approximately 300 km offshore and 600 km from Darwin. Release Area AC15-1 (4 graticular blocks) is within the Vulcan Sub-basin, while Release Area AC15-2 (4 full and 4 part graticular blocks) straddles the eastern margin of the Vulcan Sub-basin and the northwest margin of the Londonderry High (Figure 1). Release Areas AC15-1 and AC15-2 are located in water depths of 50–350 m and 50–100 m, respectively.
The Vulcan Sub-basin is a northeast-oriented Mesozoic extensional depocentre that contains Permian–Recent sediments (Figure 2). It is a proven Jurassic hydrocarbon province containing the producing Montara oil field and several depleted oil fields. The hydrocarbon province also includes the Cash/Maple gas and minor oil accumulation which is currently under development, and several sub-economic oil and gas accumulations. Upper Jurassic and Upper Cretaceous submarine fan sandstones are proven exploration targets within the sub-basin. Structural traps containing Triassic to Jurassic sandstone reservoirs are sealed by Lower Cretaceous claystones (Figure 3).
Release Area AC15-1 is on the flank of the Paqualin Graben, less than 5 km from the Oliver oil and gas accumulation to the north and the Pengana gas accumulation to the east. Hydrocarbons at Oliver and Pengana are reservoired in the Plover and Nome formations, respectively. Also nearby are the depleted Jabiru oil field, where oil was produced from the Jurassic Montara Formation, and the Cash/Maple accumulation, hosted in the Challis, Plover and Montara formations. In addition, numerous wells recording oil and gas shows in the Nome, Plover and upper Vulcan Formations (e.g. Pengana 1, Octavius 2 and Tenacious wells respectively; Figure 2) are found in the immediate vicinity of Release Area AC15-1.
Release Area AC15-2 presents both Vulcan Sub-basin (as for AC15-1) and Londonderry High play opportunities (Figure 3). The Londonderry High is characterised by highly faulted Paleozoic and Triassic rocks, which acted as a major source of sediment for adjacent depocentres during Late Jurassic rifting. The Triassic section is unconformably overlain by a relatively unfaulted Upper Jurassic and younger succession. Although most faulting terminates at the top of the Triassic succession, some faults show evidence of Miocene reactivation. Following uplift during the Late Triassic Fitzroy Movement the Triassic section was deeply eroded in places. Potential gas reservoirs in Release Area AC15-2 include the upper Vulcan Formation (Delamere accumulation) and carbonates of the Hyland Bay Subgroup (Osprey 1). Potential oil reservoirs include the Triassic Challis Formation (Challis accumulation), Puffin Formation (Challis 7, Puffin accumulation) and Paleocene Johnson Formation carbonates (Snowmass 1), although intraformational seals are required. The most recently drilled wells on the western flank of the Londonderry High are within 12 km of Release Area AC15-2 and were all drilled by PTTEP. They include Clairault 1 (2009), Ironstone 1 (2011) and Kingtree 1 (2011). Clairault 1 tested targets within the Triassic Pollard Formation and Berriasian upper Vulcan Formation.
Petroleum systems elements
Release Areas AC15-1 and AC15-2 are proximal to the Cash/Maple gas accumulation currently being planned for development by PTTEP. The nearest fixed infrastructure are the Montara platform 120 km to the southwest, and the Ichthys pipeline which is under construction approximately 100 km to the southeast. In the past, production of oil fields in the Vulcan Sub-basin has been through the use of floating, production, storage and offtake (FPSO) vessels. The port of Darwin is approximately 600 km from the Release Areas.
Structural complexity in the Vulcan Sub-basin has meant adequately imaging reservoir, trap and seals has historically proven challenging. Acquisition of 3D seismic data and the re-processing of existing seismic could assist with the re‑evaluation of Release Areas AC15-1 and AC15-2 and post-drill analysis of nearby discovery wells.
Repeated episodes of fault reactivation (particularly during the Neogene) and associated, remigration, water washing, and gas flushing have locally degraded oil accumulations in ‘leaky traps’ such as Jabiru. The risk of water washing degrading oil accumulations in AC15-1 is difficult to determine as water washing in the area is highly variable. For example – Jabiru and Tenacious oils to the south and west of the Release Area respectively show moderate water washing, Audacious 1 oil shows significant water washing, and Oliver 1 oils show no evidence of water washing. The Delamere 1 gas accumulation in Release Area AC15-2 is biodegraded. While remobilised Siluro–Ordovican evaporites as seen at Paqualin 1 are less extensive in the Vulcan Sub-basin than in other parts of the Bonaparte Basin, there is a risk that localised salt intrusions could impact on seal integrity. Elevated geothermal gradients during the Middle Jurassic as modelled in Jabiru 1 may affect the timing of hydrocarbon generation and expulsion in the Vulcan Sub-basin and adversely affect reservoir quality.
Recent drilling in and around Release Area AC15-2 has progressively improved knowledge of trap and seal mechanisms, and source and reservoir potential in this relatively under explored part of the Vulcan Sub-basin (Figure 4). Erosion on the Londonderry High has restricted the areal extent of reservoir and seal units and reduced the likelihood of widespread Jurassic reservoirs and source rocks in the eastern part of Release Area AC15-2. Migration pathways for hydrocarbons sourced in the Swan Graben up onto the eastern flank of the Vulcan Sub-basin and Londonderry High are at the proven limit of migration. Hence, a lack of a Jurassic charge is a risk for plays in Release Area AC15-2. However, it is possible that mature Permian source rocks could provide an alternative hydrocarbon charge for Triassic reservoirs on the Londonderry High.
Wells Data and Seismic Survey listings for the Vulcan Sub-basin are available in the Supporting Information section.
Geoscience Australia products
Regional geology and seismic
- 1996 Vulcan Sub-basin Airborne Laser Fluorosensor Survey Interpretation Report (WGC Vulcan Graben Survey Number 1113). Australian Geological Survey Organisation Record 2000/033 by Cowley, 2001
- Interpreted Horizons and Faults for Seismic Survey 98. Vulcan Basin, 2001. Data Package by Kennard et al, 2003
- Line Drawings of AGSO – Geoscience Australia’s Regional Seismic Profiles, Offshore Northern and Northwestern Australia, 2001. Australian Geological Survey Organisation Record 2001/036 by Colwell and Kennard, 2001
- Vulcan Sub-basin Composite Well Log, 1999. Petroleum Wells Dataset by Kennard, 1999
- Vulcan Sub-basin, Offshore WA, 1989 (P556). Airborne Geophysics Dataset by Richardson, 2002
- Vulcan Sub-basin, Timor Sea Survey 97: High Resolution Seismic Reflection Profiling and Direct Hydrocarbon Detection (Vulcan I): Post-Cruise Report: Project 121.19, 1991. Bureau of Mineral Resources Record 1991/068 by O’Brien and Marshall, 1992
- Vulcan Tertiary Tie (VTT) Basin Study, Vulcan Sub-basin, Timor Sea, Northwestern Australia, 1996. Australian Geological Survey Record 1996/061 by O’Brien, 1996
- Basement and Crustal Structure of the Bonaparte - Browse Basins, 2003. Conference Abstract by Goncharov et al, 2003
- Geoscience Australia’s Basin Biozonation and Stratigraphy Chart Series: Bonaparte Basin Biozonation and Biostratigraphy Chart 34, 2014. Geoscience Australia Stratigraphy Chart by Kelman et al, 2014
Petroleum systems and accumulations
- AGSO Marine Survey 176 Direct Hydrocarbon Detection North-West Australia: Yampi Shelf; Southern Vulcan Sub-Basin; Sahul Platform (July/September 1996) - Operational Report and Data Compendium. Australian Geological Survey Record 2000/042 by Wilson, 2000
- Ashmore-Cartier prospectivity package: an analysis of the petroleum prospectivity of vacant areas overlying the Ashmore Platform, Vulcan Sub-basin, Jabiru Terrace and northern Browse Basin, 1991. Bureau of Mineral Resources Record 1991/090 by Lavering, 1991
- Bonaparte Basin, Northern Territory (NT), Western Australia (WA), Territory of Ashmore and Cartier Islands Adjacent Area (AC), Joint Petroleum Development Area (JPDA), 2004. Geoscience Australia Report 5 by Cadman and Temple, 2004
- Characterisation of Natural Gases from West Australian Basins, 2000. Geoscience Australia Report by Boreham et al, 2000
- Light Hydrocarbon Geochemistry of the Vulcan Sub-basin, Timor Sea: Rig Seismic Survey 97: Project 121.19, 1992. Bureau of Mineral Resources Record 1992/062 by O’Brien, 1992
- Oils of Western Australia II, 2005. Geoscience Australia Report by Edwards and Zumberge, 2005
- Source Rock Distributions and Petroleum Fluid Bulk Compositional Predictions on the Vulcan Sub-basin, Offshore Western Australia, 2009. Geoscience Australia Report by Neumann et al, 2009
- Source Rock Time-slice Maps, Offshore Northwest Australia, 2004. Petroleum data by West et al, 2004
- The Petroleum Systems of the Bonaparte Basin, 2004. Dataset by Earl, 2004
- Vulcan Sub-basin – Hydrocarbon Shows Analyses, 2001. Data Package by Royal et al, 2001
- Hydrocarbon Migration and Seepage in the Timor Sea and Browse Basin, North West Shelf, Australia: An Integrated SAR, Geological and Geochemical Study, 2001. Dataset and Geoscience Australia Record 2001/11 by O’Brien et al, 2001
Contact Geoscience Australia’s Sales Centre for more information or to order these reports or products,
phone +61 (0)2 6249 9966, email email@example.com
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