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Sahul Syncline

Bids close 29 October 2015

  • Adjacent to a producing gas and oil province
  • Shallow to deep water depths, <50–300 m
  • Proximal to the Bayu/Undan gas pipeline
  • Release Area flanks Jurassic depocentre with oil- and gas-prone source rocks
  • Proven plays in Mesozoic tilted fault blocks, potential for structural and stratigraphic plays in Triassic–Jurassic and Cretaceous reservoirs
  • Special Notices apply, refer to Guidance Notes

Release Area W15-2 is located in the Sahul Syncline, immediately adjacent to the Bayu/Undan gas field and the associated pipeline in the Joint Petroleum Development Area (JPDA), as well as the Laminaria/Corallina, Kitan, Elang (JPDA), and Buffalo oil fields, and the Kakatua, Kakatua North, Jahal, and Kuda Tasi oil accumulations (all JPDA; Figure 1). This large Release Area (42 full and 16 part graticular blocks) is 470 km from the northwest coast of Australia, and contains twenty three exploration and development/appraisal wells. More than twenty other wells are found within 10 km of Release Area W15-2.

The Sahul Syncline, and its western extension, the Nancar Trough, are prominent Paleozoic to Mesozoic northwest‑trending troughs located between the Londonderry and Flamingo highs in the northern Bonaparte Basin. These depocentres are the source of the petroleum accumulations on the adjacent Laminaria and Flamingo highs. Proven source rocks in the region are the Plover, Elang and Frigate formations. Recent research suggests that the Echuca Shoals Formation may also have some hydrocarbon potential in the region.

Release Area W15-2 contains the depleted Buffalo oil field that was reservoired in the Elang Formation within an east‑trending, fault-bounded Tithonian horst block. Other effective reservoir targets within Release Area W15-2 are demonstrated by oil shows in the Plover Formation and the Flamingo Group as seen in Buller 1, Bluff 1, Bogong 1 and Iris 1 (Figure 2). Reservoirs may also be present within the Bathurst Island Group (including the Woolaston and Darwin formations) if the conditions found in Coleraine 1 in the Flamingo Syncline are replicated in the Sahul Syncline. Oil shows at Elang West 1 and Kakatua North 1A, and gas shows at Bluff 1 provide some evidence of this. In the north of the Release Area, the Cenozoic Oliver and Grebe Sandstone Members, and the argillaceous sandstones of the Johnson Formation may also constitute exploration targets as indicated at Buller 1; though top seal for these younger reservoirs is problematic. The reservoir potential of the southwest flank of the Sahul Syncline is less well-known, being penetrated only by Franklin 1 and Rambler 1. Rambler 1 tested a Mesozoic fault block with a probable overlying fault drape play. Reservoir quality in the Plover Formation at this location is poor with low porosity and permeability, and the Flamingo Group facies were shalier than prognosed, but had some fracture permeability. In spite of the poor reservoir quality, the well recorded oil shows in the Sandpiper Sandstone and significant gas shows in the Plover Formation. Franklin 1 targeted the Upper Cretaceous marginal marine Turnstone Formation and failed to encounter reservoir facies, raising questions over lithofacies distribution in the region.

Extensive Jurassic and Cretaceous shales and claystones provide competent top seals in the region. Lateral seal capacity is also potentially good with significant onlap relationships. However, seals reliant on stable fault geometries are, as with the rest of the northern Bonaparte Basin, a risk due to fault reactivation, particularly in the Neogene.

The primary trap styles identified within the Release Area are tilted fault blocks (e.g. Rambler 1, Bogong 1 and Buffalo field) and associated stratigraphic drapes (e.g. Firebird 1, Figure 3), or some combination of both, as seen at Tanjil 1.

Petroleum systems elements

Sahul Syncline

Sources
  • Lower Cretaceous transgressive marine shales of the Echuca Shoals Formation
  • Upper Jurassic–Lower Cretaceous transgressive marine shales of the Frigate and Elang formations
  • Lower–Middle Jurassic fluvio-deltaic Plover Formation
  • Permo-Triassic marine-deltaic shales of the Kinmore Group including the Fossil Head and Ascalon formations
Reservoirs
  • Cretaceous carbonates of the Bathurst Island Group including the Darwin and Woolaston formations
  • Upper Jurassic–Lower Cretaceous submarine fans of the Flamingo Group
  • Lower–Middle Jurassic fluvio-deltaic Plover Formation
Seals Regional Seals
  • Upper Jurassic–Lower Cretaceous marine Flamingo Group shales
  • Cretaceous Bathurst Island Group shales and claystones including the Echuca Shoals and Wangarlu formations

Intra-formational Seals

  • Lower–Middle Jurassic fluvio-deltaic Plover Formation
  • Paleogene–Neogene Woodbine Group carbonates
Traps
  • Jurassic horsts/tilted fault blocks and associated drapes
  • Stratigraphic traps at the Flamingo Group level

Infrastructure

Release Area W15-2 is close to the Bayu/Undan gas field and the associated Bayu/Undan pipeline in the JPDA. The Bayu/Undan and Kitan (JPDA), and Laminaria/Corallina fields are currently being produced using FPSO facilities. Development of the Kitan and Bayu/Undan fields is ongoing with several new development wells being completed during 2014. The Release Area is also close to the Kakatua, Kakatua North, Jahal, and Kuda Tasi oil accumulations, and the depleted Buffalo and Elang oil fields. The port of Darwin is approximately 500 km from the Release Area.

Critical risks

Although proven petroleum systems and traps have been identified in the region, a detailed understanding of reservoir distribution and fault-associated migration pathways in the relatively underexplored Sahul Syncline is needed. The acquisition of additional 3D seismic data over the southern part of Release Area W15-2 would augment the current data suite (Figure 4).

Elevated geothermal gradients may preclude the generation and preservation of liquid hydrocarbons in deeper parts of the Sahul Syncline. Heterogeneity of source rocks, and particularly within the Plover Formation, is a risk that may affect charge potential within the Release Area. Reservoir quality, hydrocarbon migration pathways and potential hydrocarbon recovery factors have been adversely impacted by sandstone diagenesis in the deeper parts of the Sahul Syncline. While diagenetic effects are pervasive throughout the region, significant local variation can occur. At Buller 1, for example, better than prognosed reservoir quality was observed in the primary objective despite poor reservoir quality being encountered in other wells within the Release Area (e.g. Iris 1, Cleia 1, Rambler 1 and Wowo Wiwi 1). Diagenetic and structural overprinting together reduce the probability of sufficient charge reaching traps above the centre of the syncline. However, lateral migration towards the flanks of the syncline is more successful as evidenced by the number of discoveries on the margins (Figure 3). Lateral variation in reservoir lithofacies as seen at Bogong 1, Buller 1, Iris 1 and Heifer 1 have impacted on net reservoir thickness. Post-tectonism thinning has also locally impacted net reservoir thickness as seen at Iris 1 where Berriasian reservoirs thin over the Tithonian horst block. The relatively poorly understood nature of reservoirs on the western flank of the Sahul Syncline pose an additional exploration risk within the Release Area.

Hydrocarbon preservation risks in the Sahul Syncline include water washing and evaporative fractionation caused by a combination of Neogene fault breaching (as seen at Bogong 1) and gas flushing. The pervasive effects of this post‑expulsion alteration is seen in fields on the Laminaria and Flamingo highs and at the Tanjil 1 well location.

Data sets

Wells Data and Seismic Survey listings for the Sahul Syncline are available in the Supporting Information section.

Geoscience Australia products

Regional geology and seismic

Stratigraphy

Petroleum systems and accumulations

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ABBASSI, S., DI PRIMIO, R., HORSFIELD, B., EDWARDS, D.S., VOLK, H., ANKA, Z. AND GEORGE, S.C., 2015—On the filling and leakage of petroleum from traps in the Laminaria High region of the northern Bonaparte Basin, Australia. Marine and Petroleum Geology, 59, 91–113.

ABBASSI, S., GEORGE, S.C., EDWARDS, D.S., DI PRIMIO, R., HORSFIELD, B. AND VOLK, H., 2014—Generation characteristics of Mesozoic syn- and post-rift source rocks, Bonaparte Basin, Australia: new insights from compositional kinetic modelling. Marine and Petroleum Geology, 50, 148–165.

AMBROSE, G.J., 2004—Jurassic sedimentation in the Bonaparte and northern Browse Basin: new models for reservoir-source rock development, hydrocarbon charge and entrapment. In: G.K. Ellis, P.W. Baillie, T.J. Munson (Eds.), Timor Sea Petroleum Geoscience, Proceedings of the Timor Sea Symposium, Darwin, Northern Territory (2004), 125–142

BARRETT, A.G., HINDE, A.L. AND KENNARD, J.M., 2004—Undiscovered resource assessment methodologies and application to the Bonaparte Basin. In: Ellis, G.K., Baillie, P.W. and Munson, T.J. (eds), Timor Sea Petroleum Geoscience: Proceedings of the Timor Sea Symposium, Darwin, 19-20 June 2003. Northern Territory Geological Survey, Special Publication 1, 353–372.

BICKFORD, G.P., BISHOP, J.H., O'BRIEN, G.W. AND HEGGIE, D.T., 1992—Light Hydrocarbon Geochemistry of the Bonaparte Basin : Including the Sahul Syncline, Malita Graben and Northern Petrel Sub-basin : Rig Seismic Survey 99 : Project 121.24. Record 1992/050. Bureau of Mineral Resources, Geology and Geophysics, Canberra.

BOTTEN, P.R. AND WULFF, K., 1990—Exploration potential of the Timor Gap Zone of Cooperation. The APEA Journal, 30(1), 53–68.

CADMAN, S.J. AND TEMPLE, P.R., 2004—Bonaparte Basin, NT, WA, AC and JPDA, Australian Petroleum Accumulations Report 5, 2nd Edition, Geoscience Australia, Canberra. https://www.ga.gov.au/products/servlet/controller?event=GEOCAT_DETAILS&catno=60865 (last accessed 20 December 2014).

DE RUIG, M.J., TRUPP, M., BISHOP, D.J., KUEK, D. AND CASTILLO, D.A., 2000—Fault architecture and the mechanics of fault reactivation in the Nancar Trough/Laminaria area of the Timor Sea, northern Australia. The APPEA Journal, 40(1), 174–193.

EDWARDS, D.S. AND ZUMBERGE, J.E., 2005—The Oils of Western Australia II. Regional Petroleum Geochemistry and Correlation of Crude Oils and Condensates from Western Australia and Papua New Guinea. Geoscience Australia, Canberra and GeoMark Research Ltd, Houston. https://www.ga.gov.au/products/servlet/controller?event=GEOCAT_DETAILS&catno=37512 (last accessed 20 December 2014).

GEORGE, S.C., LISK, M. AND EADINGTON, P.J., 2004b—Fluid inclusion evidence for an early, marine-sourced oil charge prior to gas-condensate migration, Bayu-1, Timor Sea, Australia. Marine and Petroleum Geology, 21(9), 1107–1128.

GRADSTEIN, F.M., OGG, J.G. SCHMITZ, M.D. AND OGG, G.M. (EDITORS), 2012—The Geologic Time Scale 2012; Volumes 1 and 2. Elsevier BV, 1144pp.

KELMAN, A.P., EDWARDS, D.S., KENNARD, J.M., LAURIE, J.R, LEPOIDEVIN, S., LEWIS, B., MANTLE, D.J. AND NICOLL, R.S., 2014—[Web page] Bonaparte Basin Biozonation and Stratigraphy, Chart 34, Geoscience Australia. https://www.ga.gov.au/products/servlet/controller?event=GEOCAT_DETAILS&catno=76687 (last accessed 20 December 2014).

LANGHI, L. AND BOREL, G.D., 2008—Reverse structures in accommodation zone and early compartmentalisation of extensional system, Laminaria High (NW shelf, Australia). Marine and Petroleum Geology, 25, 791–803.

LONGLEY, I.M., BUESSENSCHUETT, C., CLYDSDALE, L., CUBITT, C.J., DAVIS, R.C., JOHNSON, M.K., MARSHALL, N.M., MURRAY, A.P., SOMERVILLE, R., SPRY, T.B. AND THOMPSON, N.B., 2002—The North West Shelf of Australia – a Woodside perspective. In: Keep, M. and Moss, S.J. (eds), 2002, The Sedimentary Basins of Western Australia 3: Proceedings of the Petroleum Exploration Society of Australia Symposium, Perth, WA, 2002, 27–88.

NEWELL, N.A., 1999—Water washing in the northern Bonaparte Basin. The APPEA Journal, 39(1), 227–247.

O’BRIEN, G.W., LISK, M., DUDDY, I.R., HAMILTON, J., WOODS, P. AND COWLEY, R., 1999—Plate convergence, foreland development and fault reactivation: primary controls on brine migration, thermal histories and trap breach in the Timor Sea, Australia. Marine and Petroleum Geology, 16, 533–560.

ROBINSON, P.H., STEAD, H.S., O’REILLY, J.B. AND GUPPY, N.K., 1994—Meanders to fans: a sequence stratigraphic approach to Upper Jurassic – Early Cretaceous sedimentation in the Sahul Syncline, north Bonaparte Basin. In: Purcell, P.G. and Purcell, R.R. (eds), The Sedimentary Basins of Western Australia: Proceedings of the Petroleum Exploration Society of Australia Symposium, Perth, 1994, 223–242.

WHITTAM, D.B., NORVICK, M.S. AND MCINTYRE, C.L., 1996—Mesozoic and Cainozoic tectonostratigraphy of western ZOCA and adjacent areas. The APPEA Journal, 36(1), 209–231.

Figures

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Figure 1

 Bonaparte Basin-Sahul Syncline-figure-1 map image
Structural elements of the Sahul Syncline and surrounds showing petroleum fields and discoveries, the location of the 2015 Release Area and seismic line (Figure 3)
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Figure 2

 Bonaparte Basin-Sahul Syncline-figure-2 map image
Stratigraphy and hydrocarbon discoveries in the Sahul and Flamingo synclines, and the Laminaria and Flamingo highs, based on the Bonaparte Basin Biozonation and Stratigraphy Chart 34 (Kelman et al, 2014). Geologic Time Scale after Gradstein et al (2012)
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Figure 3

 Bonaparte Basin-Sahul Syncline-figure-3 map image
Seismic line n116/09 across the Sahul Syncline and the Londonderry High, Flamingo High and Flamingo Syncline. Interpretation after AGSO (2001). Location of the seismic line is shown in Figure 1
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Figure 4

 Bonaparte Basin-Sahul Syncline-figure-4 map image
Seismic and well data in the vicinity of Release Area W15-2 in the Sahul Syncline, Bonaparte Basin
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