DGS Intl. Dinner - Tectono-stratigraphic evolution of the Ce ntaur 3D survey, Exmouth Plateau, North West Shelf, Australia
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World economics and increased LNG exports have made the Northe rn Carnarvon Basin, offshore North West Shelf Australia, a highly attracti ve exploration target in the gas sector.
This project illustrates the tectono-stratigraphic evolution of the Exmouth Plateau, a deep-water s ub-basin within the Northern Carnarvon Basin. The project was completed by fully interpreting the Centaur survey, a recently acquired 3D seismic dat aset located in the northwestern part of the plateau. The investigation in volved detailed qualitative and quantitative seismic analysis of structura l and stratigraphic elements that were then assessed on their impact to th e hydrocarbon potential of the area.
The Centaur provided spectacu lar imaging of NNE- and NE-trending, highly-segmented rift-border faults t hat make up the main graben-forming boundaries formed between the Late Tri assic and Early Cretaceous, as a consequence of rifting of Greater India f rom Australia. The rifted fault blocks were affected by subsequent tectoni c uplift associated with ridge push, then degradation, creating rounded er oded footwall crests and redeposited hangingwall strata up to 50 m thick. Continued extension and displacement of the rift-border faults created lon g, obliquely trending intra-graben faults that intersected the rift-border fault hangingwalls. Extensional fault-propagation folding helped create 1 -3 km wide, asymmetric depocentre synclines in the hangingwall of the rift -border faults, while uplifting and rotating footwall strata eastwards. Si nce the beginning of passive margin phase in the Late Cretaceous, the plat eau has been subjected to minor episodic fault reactivation, subsidence an d slumping, including Neogene inversion producing a localized anticlinal s tructure within the southern margin of the survey. The timing of formation is supported by mass-transport flows away from the uplifted area and onla pping of sediments onto the structural high.
Seismic facies analys is of the Triassic strata has shown a multitude of stratigraphic elements including deltaic channel systems, sheeted sand bodies, igneous intrusions and hydrothermal vent complexes. Amplitude extractions have identified po tential structural traps in tilted Triassic fault blocks as well as potent ial stratigraphic traps in intra-Triassic channels, in sandbodies and pote ntially in igneous intrusions.
Several lines of evidence suggest t hat the overall structural evolution of the rift-border faults was influen ced by the reactivation of pre-existing Early Triassic structures, these i nclude, 1) existence of fault-propagation folding of Triassic rift-border faults, 2) along strike variations in geometry and orientation of rift fau lts and 3) the appearance of faults propagating upwards obliquely through Lower Triassic strata. Unlike the traditional orthogonal extension models of rifting that creates long, linear rift patterns, the structural geometr y is comparable to analogue models of offset or oblique rifting, where the rift-border faults are short, highly-segmented and curved, containing lon g intra-graben faults formed perpendicular to the direction of extension t o create numerous asymmetric hangingwall depocentres.
The results of the project suggest various structural and stratigraphic elements that provide varying levels of risk and reward in the Triassic prospective play targets for petroleum exploration. While the Triassic strata provide pote ntial hydrocarbon targets, fault reactivation since the Early Cretaceous, mass-transport complexes Cretaceous and Tertiary and fluid escape features of the Top Triassic pose a threat to the seal quality of trapped hydrocar bons and slope stability for drilling infrastructure.