Structural Characterization of the Late Cretaceous Ceduna Sub-Basin Delta Systems, Bight Basin, Australia: Implications for Potential Hydrocarbon Trap Geometries

MacDonald, Justin ¹; King, Rosalind ²; Backe, Guillaume ¹; Hillis, Richard ³
(1)Australian School of Petroleum, The University of Adelaide, Adelaide, SA, Australia. (2) School of Earth and Environmental Sciences, The University of Adelaide, Adelaide, SA, Australia. (3) Deep Exploration Technologies Cooperative Research Centre, Adelaide, SA, Australia.

The Ceduna Sub-basin is an under-explored prospective hydrocarbon province located within the Bight Basin, on Australia’s passive southern margin. Recently completed structural analysis using two-dimensional (2D) and three-dimensional (3D) seismic data demonstrates two Late Cretaceous delta—deepwater fold-thrust belts (DDWFTBs), which are overlain by Cenozoic sediments.

The Cenomanian White Pointer and Campanian-Maastrichtian Hammerhead DDWFTBs both display a well-documented link between extension on the delta top and down-dip compression in the delta toe. The two systems are structurally linked by stacking of the delta tops due to the overlap of the delta lobes. However, the down-dip deepwater fold-thrust belts remain independent of each other, with little evidence for structural linkage between the two systems toward the basin. Two pulses of inversion are identified in the Ceduna Sub-basin and correlate with onset of rifting and fault reactivation in the Santonian.

The constrained geometry and restorations of the two systems demonstrate that the amount of observed extension is balanced against the amount of observed shortening. Two different restoration techniques were applied to the Hammerhead DDWFTB, a Finite Element Method-based two-dimensional geomechanical restoration (using Dynel 2D) and a geometric-based two-dimensional kinematic restoration (using 2D MOVE). Both modelling techniques indicate that the Hammerhead DDWFTB system is near-balanced, demonstrating approximately equal amounts of up-dip extension and down-dip compression during formation.

A present-day normal fault stress regime and margin parallel maximum horizontal stress (SHmax) have been identified in the Bight Basin. Thus, Andersonain faulting theory suggests NW-SE striking delta top normal faults are optimally oriented for reactivation. Faults interpreted from 3D seismic data were modelled using the Poly3D geomechanical code and results indicate delta-top normal faults that dip 40^○ to 70^○ are at moderate to high risk of reactivation, while acute variations in fault plane roughness can increase the risk of reactivation.

Primary structural controls such as delta tectonics control fault geometries and syn-depositional structures. Post-depositional inversion and fault reactivation have significant impacts on potential trap geometries and the presence of breached hydrocarbon traps, such as those encountered in the Jerboa 1 and Potoroo-1 wells of the Eyre and Ceduna Sub-basins.

AAPG Search and Discovery Article #90135©2011 AAPG International Conference and Exhibition, Milan, Italy, 23-26 October 2011.