Southern Australian Bight basin holds deepwater potential

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The Jurassic-Cretaceous Bight basin is a large, mainly offshore, basin that extends along the southern Australian margin, from the southern tip of Western Australia, across the Great Australian Bight to the western tip of Kangaroo Island. The basin underlies the continental shelf and slope, including two broad bathymetric terraces, in water depths ranging from less than 200 m to over 4,000 m.

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The Bight basin contains a Middle Jurassic to Late Cretaceous sedimentary succession that in the thickest depocenter, the Ceduna sub-basin, is in excess of 15 km thick. To the east of the main depocenter, a thin Bight basin succession overlies the western tip of the mostly Proterozoic Polda basin.

The Bight basin is overlain unconformably by the dominantly cool-water carbonates of the Cainozoic Eucla basin. To the south, the uppermost sequences of the Bight basin onlap highly extended continental crust and rocks of the continent-ocean transition on the abyssal plain between Australia and Antarctica.

In 1999-2003, Geoscience Australia undertook an integrated geological study of the Bight basin, which resulted in the development of a new chronostratigraphic framework and an improved understanding of the tectonic and depositional history of the basin. The sequence stratigraphic and structural interpretation of 8,600 line-km of newly acquired, high quality, regional 2D seismic reflection data was a key component of this study. These investigations revealed that the development of two large progradational delta systems in the Cenomanian and the Late Santonian-Maastrichtian dominated the Late Cretaceous evolution of the basin.

Exploration status

The Bight basin is a frontier basin. From the late 1960s to the early 1990s, three discrete cycles of petroleum exploration were undertaken in the region. During this time only nine wells were drilled in the entire offshore portion of the basin. All wells were drilled within relatively thin successions on the margins of the basin, and none encountered significant hydrocarbons.

The thickest depocenter, the Ceduna sub-basin, covers an area of more than 155,000 sq km, but prior to the current exploration round, only five petroleum exploration wells had been drilled within the sub-basin.

Since 1999, the Great Australian Bight has been the focus for the release of exploration areas by the Australian Commonwealth Government. In 2000, a consortium comprising Woodside Petroleum, EnCana Corp. (then PanCanadian), and Anadarko Petroleum were awarded three large exploration permits in the Ceduna sub-basin. Smaller permits have also been awarded in the Duntroon sub-basin to both the Woodside consortium and Australian explorer Santos Ltd.

The exploration programs associated with these permits include the acquisition of over 18,000 line-km of 2D seismic data, reprocessing of seismic data, and completion of geological and geophysical studies. Woodside and partners now are scheduled to drill the first well in the basin since 1993. Current exploration interest is focused largely on Late Cretaceous deltaic systems in the Ceduna and Duntroon sub-basins.

Tectonic setting

The Bight basin formed within a tectonic framework dominated by the break-up of eastern Gondwana and is one of a series of Mesozoic to Cainozoic depocenters that developed along Australia's southern margin. The basin was initiated during a period of Middle-Late Jurassic to Early Cretaceous upper crustal extension.

This extension represented the failed arm of a triple junction, with the other arms of the system along the incipient rifts between India and Antarctica, and India and western Australia. A northwest-southeast to north-south extension direction superimposed on east-west and northwest-southeast oriented basement structures resulted in oblique to strongly oblique extension and the formation of en echelon half grabens in the Eyre, inner Recherche, eastern Ceduna, and Duntroon sub-basins. The extent of early extensional structures beneath the thick Ceduna sub-basin cannot be determined at present; however, the anomalously thick nature of the sub-basin may indicate that Jurassic-Early Cretaceous rifts are present at depth.

Post-rift thermal subsidence was followed by a phase of accelerated subsidence, which began in the Late Albian and continued until continental break-up in the Late Santonian-Early Campanian. During this phase of enhanced subsidence, the dominant structural feature was a system of gravity-driven detached extensional and contractional structures, which formed in the Cenomanian as a result of deltaic progradation.

Evidence for upper crustal extension during this period is limited to Turonian-Santonian extensional faulting in the Ceduna sub-basin and the reactivation and propagation of Cenomanian growth faults. A period of thermal subsidence and the progradation of a large sandy deltaic system followed the commencement of seafloor spreading at about 83 Ma.

Sequence stratigraphy

In 2000 Geoscience Australia developed a new sequence stratigraphic scheme for the Bight basin. This scheme is based on the identification of four megasequences, each related to a different basin phase, and their component second-order supersequences.

• Mid-Late Jurassic – Sedimentation in the Bight basin began during a period of Jurassic intracontinental extension. Two rift-related depositional units have been identified–the Sea Lion and Minke supersequences. These units consist of fluvial–lacustrine sandstone, siltstone, and shale, with minor coal
• Early Cretaceous – A period of slow thermal subsidence throughout most of the Early Cretaceous followed the extensional phase. Deposition during this time was largely non-marine. The Berriasian Southern Right Supersequence consists mainly of fluvial–lacustrine sandstone and claystone, while the Valanginian to mid-Albian Bronze Whaler Supersequence is a thick dominantly fine-grained lacustrine succession. Some marine influence is evident within the upper Aptian-Albian part of the Bronze Whaler Supersequence
• Mid-Cretaceous – An abrupt increase in subsidence rate in the mid-Albian signalled the start of the third basin phase. This period of accelerated subsidence coincided with a period of rising global sea level. This combination of factors resulted in an increase in accommodation, the first major marine flooding event in the basin and the widespread deposition of marine silts and shales of the mid-Albian to early Cenomanian Blue Whale Supersequence. The present-day distribution of the supersequence indicates that the seaway at that time extended along the southern margin from the open sea in the west. Progradation of deltaic sediments into this seaway (White Pointer Supersequence) commenced in the Cenomanian, following uplift and erosion along the eastern margin of the continent. Deposition was rapid, resulting in the development of overpressure in the underlying marine shales, and a short-lived period of shale mobilisation and growth faulting throughout the northern half of the Ceduna sub-basin. The accumulation of the marine-marginal marine mixed clastic sediments of the Turonian-Santonian Tiger Supersequence followed deposition of the White Pointer Supersequence
• Late Cretaceous – A period of thermal subsidence and the establishment of the southern Australian passive margin followed continental break-up in the Late Santonian. During this phase a second large progradational delta developed, represented by the sand-rich fluvial, deltaic and marine sediments of the Hammerhead Supersequence. A dramatic reduction in sediment supply at the end of the Cretaceous saw the abandonment of deltaic deposition and the development of a cool-water carbonate margin during the Cainozoic.

Petroleum systems

Several factors point to the presence of active petroleum systems in the Bight basin. Many wells in the region had minor oil and/or gas indications, such as fluorescence or cut, and significant oil and gas shows were reported from Greenly 1 in the southern Ceduna sub-basin.

Fluid history analysis of samples from the base of Jerboa 1 in the Eyre sub-basin has revealed the presence of a paleo-oil accumulation. Analysis of the fluid-inclusion oil points to a lacustrine source. Wells in the Duntroon sub-basin show clear indications of migration at several stratigraphic levels.

Stranded coastal bitumens of natural origin have been recovered from Australia's southern shorelines for about 100 years. Many of these are asphaltites that originated from a Jurassic to Early Cretaceous marine source rock.

Recent geochemical and carbon isotopic studies have shown a correlation between the asphaltites, the Albian to Cenomanian Blue Whale supersequence of the Bight basin, and marine oil shales from the Albian Toolebuc Formation of the onshore Eromanga basin. These results provide further evidence for the presence of at least one marine source rock unit in the region, and strengthen the argument that the asphaltites originated in the Bight basin.

The presence of active hydrocarbon seepage also provides evidence for hydrocarbon generation in the Ceduna sub-basin. The majority of seepage slicks, interpreted from synthetic aperture radar data, occurs along the margins of the major depocenter, the Ceduna sub-basin, in areas where significant Late Tertiary to Recent faulting extends to the seafloor or at onlap points of potential reservoir/carrier beds or seals. Hydrocarbon seepage correlates with regional drainage foci and patterns of late stage reactivation faults, which have focused laterally migrating hydrocarbons to produce active, probably episodic seepage at specific locations in the basin. Where these features are absent, seepage may be passive and/or be governed by long distance migration to points of seal failure.

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The sequence stratigraphic framework developed by Geoscience Australia has allowed the identification of several potential hydrocarbon source intervals in the basin. These range from Late Jurassic syn-rift lacustrine shales to Late Cretaceous marine and deltaic facies. While the Jurassic-Early Cretaceous non-marine source intervals are important in the shallower, more proximal parts of the basin, the key to the petroleum prospectivity of the basin probably resides in the Late Cretaceous deltas and associated marine facies.

The Cenomanian and Campanian-Maastrichtian deltas built out into a narrow restricted seaway prior to and immediately after the commencement of slow seafloor spreading between Australia and Antarctica in the Late Santonian. This seaway would have provided an excellent environment for the development of organic-rich rocks.

The marine shales of the Albian-Cenomanian Blue Whale Supersequence, the mobile substrate to the Cenomanian growth faults, have proven source rock potential and, on geochemical evidence, have suggested asphaltites as the possible source.

The White Pointer Supersequence contains both marine condensed sections and coaly deposits and has good-to-excellent source potential for both oil and gas. The preservation potential for organic-rich rocks would have been enhanced in the high accommodation setting of the growth-fault controlled depocenters. In addition, seismic indications of thick shale-prone intervals within the dominantly marine Tiger Supersequence provide encouragement for the presence of Turonian age source rocks in the basin.

Geoseismic profile across the Ceduna sub-basin: Jurassic – Sea Lion and Minke; Cretaceous – Southern Right to Hammerhead; Cainozoic – Wobbegong and Dugong.

The dominantly progradational Hammerhead Supersequence has excellent reservoir potential, and there is a high likelihood of regionally extensive marine seal facies within the upper aggradational portion of the succession. The basinward thickening (up to 5,000 m) geometry of the Hammerhead is the key to the loading and maturation of successively younger source rocks. Modelling shows that generation and expulsion commenced from the Turonian onwards.

Finally, the Cenomanian growth faults set up a pattern of upward migration from mature source rocks into shallower reservoirs. These faults, which sole out in the Blue Whale shales, have been reactivated several times and formed the locus for the nucleation of new faults.

Conclusion

The Bight basin contains a thick, prospective Jurassic-Cretaceous sedimentary section. Recent work by both Geoscience Australia and the petroleum industry has increased our understanding of the structural and stratigraphic development, and the range of exploration opportunities available in this frontier basin. The presence of thick deltaic units and indications of active petroleum systems further enhance its prospectivity. Although the basin is being tested by new drilling, it remains one of the least explored passive margins in the world, and will require much more exploration to fully assess its potential.

Acknowledgments

The authors thank Fugro Multi Client Services for permission to use the seismic image. We publish with the permission of the chief executive officer, Geoscience Australia.

References

A full reference list is available from the author, email: [email protected].