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Abstract

The Ceduna subbasin forms part of the under-explored but highly prospective frontier Bight Basin located on the southern margin of South Australia. Structural mapping of the Ceduna subbasin reveals two separate delta lobes/systems deposited in the late Albian-Santonian and late Santonian-Maastrichtian. Each system is comprised of an updip delta top linked via a shale detachment to a down-dip delta toe or deep-water fold-thrust belt. These delta lobes are separated by a transgressive sequence of Turonian-Santonian age that deposited a thick marine mud, up to 2000 m in places. Like the Niger Delta, this marine mud forms the detachment for the overlying Santonian-Maastrichtian delta—deep-water fold-thrust belt system and is also a proposed source rock. An Albian marine mud forms the detachment for the older Cenomanian system and is also thought to exhibit source rock potential.

We examine the differences in structural style between the western and eastern parts of the basin, the west being dominated by the Cenomanian lobe and the east by the Santonian-Maastrichtian lobe. Recent work on the sedimentary provenance of the basin suggests two different mechanisms were responsible for deposition of the delta lobes in the west and east, as well as significant changes in regional tectonics dominating the basin fill history. This has resulted in the deposition of two delta—deep-water fold-thrust belt systems rather than a continuous system as is commonly observed elsewhere in Cenozoic analogues such as the Niger Delta. Evidence from the drilling of Gnarlyknots-1A on the delta-top suggests excellent reservoir quality in the Santonian-Maastrichtian system; the potential for seal and source development increases farther offshore toward the deep-water fold-thrust belts. The abundant availability of deep-water contractional targets, combined with modeled increase in source and sealing potential farther offshore, results in a highly prospective system in both the Cenomanian and Santonian-Maastrichtian deep-water fold-thrust belts.

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