TECTONIC SETTING, STRATIGRAPHY AND HYDROCARBON POTENTIAL OF THE BEDOUT SUB-BASIN, NORTH WEST SHELF

Abstract

The tectonic and depositional histories of the Bedout Sub-basin are closely related to more widely explored areas of the southern North West Shelf, i.e. the Barrow and Dampier Sub-basins. The Mesozoic Bedout Sub-basin onlaps and overlies the Palaeozoic offshore Canning Basin sequence. Four distinct tectonic regimes characterised the Triassic, Early to Late Jurassic, Late Jurassic to Late Cretaceous, and Tertiary to present:

During the Triassic, the Bedout Sub-basin was part of a broad intracratonic downwarp that also encompassed the Barrow, Dampier and Beagle Sub-basins. A thick sequence of Locker Shale and Upper and Lower Keraudren Formations (Mungaroo Formation equivalent) was deposited.

During the Jurassic rifting phase, the Bedout Sub-basin was a subsiding rim basin, landward of the uplifted rift margin. Sedimentation was dominated by a thick sequence of fluviodeltaic to marginal marine deposits.

In the post-break-up phase from the Callovian to latest Cretaceous, a transgressive regime resulted in deep open marine conditions with widespread claystone and minor carbonate deposition over the southern North West Shelf.

Through the Tertiary to the present, shallow shelf conditions prevailed and sedimentation was dominated by a thick prograding carbonate wedge.

Hydrocarbon source is provided by a thick sequence of Triassic Locker Shale and Lower Keraudren Formation. The Locker Shale is presently mature for hydrocarbon generation over most of the Bedout Sub-basin and has the potential to generate both oil and gas. The Lower Keraudren Formation is a mature source mainly for gas/condensate in deeper sections of the sub-basin. Jurassic marine claystones, which represent a prolific source in the Barrow and Dampier Sub-basins, are not present in the Bedout Sub-basin.

Reservoir rocks exist in the Triassic and Jurassic sections. However, gentle Jurassic rim basin tectonic activity has resulted in minor faulting compared to the adjacent rift. This has limited migration pathways from Triassic source to Jurassic reservoirs. The primary reservoir objectives are sandstones of the Triassic Upper and Lower Keraudren Formations.

Although large structural traps are uncommon, there is considerable potential to host large hydrocarbon accumulations in stratigraphic traps. A giant prospect involving the onlap of the Triassic sequence has been identified in the eastern Bedout Sub-basin. Pursuit of this play should accelerate exploration in this sparsely drilled area.