Seismic Sequence Stratigraphy of the Late Cretaceous Ceduna Sub-Basin, Great Australian Bight
Honours Degree, 2001
University of Adelaide
The development of a seismic sequence stratigraphic framework in frontier basins is crucial for hydrocarbon exploration. Sequence stratigraphy facilitates the prediction of reservoir, source, and seal facies, including insights into petroleum generation, migration, and trapping. In the Ceduna Sub-basin of the southern Australian margin, there is only one well intersection (Potoroo-1), hence the interpretation of play elements is heavily dependent upon seismic facies analysis.
This study integrated 2D seismic reflection data, well data, biostratigraphy, gravity data, and aeromagnetic data to develop a seismic sequence stratigraphic framework, seismic facies maps, and context-sensitive depositional models that aid in the regional understanding and prediction of hydrocarbon systems. The Late Cretaceous sediments were deposited in a wave-influenced, shelf-margin/deltaic depositional environment in palaeo-latitudes equivalent to sub-arctic conditions of today. The onset of marine incursions began with the Cenomanian break-up of Australia and Antarctica, resulting in the development of a narrow seaway across the southern margin. Sediment accumulation was concentrated in the southeast due to a combination of increased subsidence in that region and wave reworking of sediments fed from the north.
Three large progradational shelf-margin/deltaic systems (Supersequences 1-3 (95-65 Ma) consisting of Seismic Packages 1-7) were identified within the Late Cretaceous post-rift sediments of the Ceduna Sub-basin based on stratal geometries and the influence of shale tectonics. The supersequences were controlled by low tectonic subsidence rates and high sedimentation rates, with the Campanian to Maastrichtian siliciclastic sediments of basal Supersequence 3 displaying seismically resolvable, higher frequency depositional packages. Accommodation for the high sediment supply rates in the Late Cretaceous was controlled by total subsidence (tectonic and load-induced) and eustasy. The aggradational nature of Seismic Package 7 (Supersequence 3) was produced by high sedimentation rates resulting in load-induced subsidence and relative sea level rise during a global sea level fall.
A change from Early Cretaceous volcaniclastic sediment to terrigenous clastic sediments during the Late Cretaceous signified the onset of erosion of the uplifted Eastern Highlands and establishment of an internal drainage pathway to the Ceduna depocentre. The continental drainage system passed through the uplifted Eromanga Basin, around the northern edge of the Gawler Craton, through the Ackaringa and Officer Basins, and finally into the Ceduna Sub-basin from the north. This study also highlighted a change in the vector of shelf-margin/deltaic progradation within Supersequence 1, formed by either a delta lobe switch or a change in provenance to the northeast.
Two plays displaying similar characteristics to the Tertiary Niger Delta petroleum system were identified within Supersequence 1. Both plays possess good reservoir and seal potential, with the reservoirs comprising non-marine, nearshore, and deepwater siliciclastics, sealed by marine shale. The main trapping mechanisms are rollovers associated with growth faulting, which were enhanced by Tertiary inversion. Hydrocarbon charge is from syn-rift and post-rift candidate source rocks, with faults acting as conduits for hydrocarbon migration. The key risks of the identified plays are presence of hydrocarbon charge and fault breach.