|Assoc Prof Simon C. Lang,
Dr. Brian McGowran (Department of Geology & Geophysics, University of Adelaide),
Dr. Qianyu Li (Department of Geology & Geophysics, University of Adelaide)
Project support: Origin Energy; Primary Industries and Resources, South Australia
Scholarship support: Origin Energy
The Gambier Sub-basin forms part of the Australian Southern Rift System, which is located on the southern margin of Australia. The Australian Southern Rift System is a divergent, passive continental margin that has experienced Jurassic-Palaeogene rifting and spreading, resulting in the separation of Australia and Antarctica. A regional study of seismic and available well data, including new biostratigraphic data, has led to a revision of the understanding of the sequence stratigraphy and depositional environments of the southern margin.
The Cainozoic Wangerrip Group in the Gambier Sub-basin was deposited during the Late Paleocene and Early Eocene over 10 M.y. and comprises marginal marine and deltaic siliciclastic sediments up to 530 m thick in the South Australian Voluta Trough. The Wangerrip Group is divided into four supersequences based on seismic stratigraphy. The boundaries of the supersequences fall on seismically resolvable, regional, erosional surfaces, although the duration of these hiatuses are beyond palynological resolution (less than 2 M.y.). Each supersequence comprises up to three sequences with individual systems tracts resolvable locally. This clastic succession contains several potential reservoir targets (Pebble Point and Dilwyn Formations) as well as local, immature source rocks and seals (Pember Mudstone, Dilwyn Formation).
Deposition of the clastic Wangerrip Group terminated during the early Middle Eocene and the next event was a major marine transgression in the late Middle Eocene. The transgression resulted from an increase in the spreading rate between Australia and Antarctica from 43 Ma, which led to thermal subsidence and a rise in relative sea level as well as the establishment of the episodic warm-water Leeuwin Current. This led to the starvation of clastic sediments, but the generation of bryozoan carbonates was encouraged and they prograded across the shelf. These carbonates are fine-grained and may represent a regional seal, but there are significant risks associated with faulting. The carbonates in the Gambier Sub-basin are dominated by three major submarine canyon complexes on the outer shelf, below the present day shelf break. There is no expression of the canyons on the seafloor. The initial canyon-cutting event occurred at approximately the end of the Early Oligocene, followed by up to ten major events in succession that can be mapped on a regional seismic grid. Mechanisms of canyon formation might include sub-aerial incision during lowstands and submarine mass slope failure, followed by headward erosion.
These canyons are 0.15 to 0.54 sec TWT deep (approx 170-615 m) and 2.3-5.8 km wide. On seismic patterns, the canyon cuts are represented by high-amplitude reflectors that bound low amplitude canyon-fill packages displaying prograding to divergent internal geometries. Highstand shedding during the Late Oligocene and Miocene has resulted in a thick (up to 700 m) wedge on the upper slope. This wedge is dominated by highstand systems tracts and is interpreted as reworked carbonate sediment from the shelf. Seismic reflection character suggests that these carbonates are fine-grained, hence they may represent a regional seal but there are significant risks associated with faulting. The canyon systems in the Gambier Sub-basin are too shallow to be part of a potential hydrocarbon system, however they may serve as a useful analogue for comparable settings on the southern margin where viable hydrocarbon systems may be present.