Richard Kempton

Building the regional understanding of the deep-water geology and benthic ecology of the Great Australian Bight

The Great Australian Bight (GAB) represents one of Australia’s most prospective frontier hydrocarbon exploration regions. Its largest subregion – the Ceduna sub-basin – is a deep (slope to abyss) area of 126 300 km2 with a 15-km deep sedimentary sequence that remains effectively untested. Knowledge of the Ceduna sub-basin’s geology is rapidly evolving following recent collection of 3D seismic datasets, but many questions remain about its geological evolution. The composition of the seabed biota and its ecology in the deep GAB was virtually unknown. To address a range of geological and biological questions, the multidisciplinary Great Australian Bight Deepwater Marine Program aims to build a more comprehensive regional understanding of the geology of the deep (~700–5437 m) GAB, with a focus on rocky outcrops, segif and volcanic seamounts, and to document aspects of the biota and benthic ecology for the first time. A field campaign of 63 days in 2015 aboard the RV Investigator and a second support vessel for an Autonomous Underwater Vehicle completed a detailed mapping of 10 225 km2 of seabed. In addition, physical geological and biological sampling collected 1.3 tonnes of volcanic and sedimentary rocks and over 25 553 biological specimens. A surprisingly complex deep-water sedimentary environment was revealed, including several previously unmapped deep-water canyons and 10 previously unmapped volcanic seamounts. A total of 430 species were collected, of which nearly half appeared to be unknown to science. This paper uses results from this survey to provide insights into the geological processes that have shaped the GAB, and briefly describes the makeup of biological assemblages present on the seabed.

Insights into the Great Australian Bight gained through marine geology and benthic ecology studies

While the Great Australian Bight (GAB) represents one of the most prospective deep water basins in Australia, its vast geographic extent and deep sedimentary sequences remain poorly characterised. Recently, multidisciplinary research has been conducted to better characterise the continental and abyssal slope of the Ceduna Sub-basin. The Great Australian Bight Deepwater Marine Program (GABDMP) aimed to build a regional understanding of the deep water GAB marine geology and benthic ecology. This three-year research program encompassed four research voyages that aimed to sample and characterise deep water outcropping facies, volcanic seamounts, potential seeps and their associated biological communities. These voyages used a variety of equipment to achieve the research goals and included the deployment of autonomous underwater and remotely operated vehicles and a seafloor coring system. Numerous sites across the Ceduna Sub-basin from 700 to 5501 m water depth were studied. Sampling operations collected over 2.8 tons of rocks, 148 m of core, 55 698 biological specimens and 48 097 km2 of mapping data. Nearly 4000 geological samples have been analysed to date. This paper will summarise the key findings from the GABDMP and the geological and biological insights that have been revealed through this multidisciplinary research program.

Petroleum migration in the Bight Basin: a fluid inclusion approach to constraining source, composition and timing

The Bight Basin in southern Australia is a vast under-explored offshore area with promise of, but as of yet, limited proof for hydrocarbons. Fluid inclusions (FIs) offer a unique method to test for petroleum migration, composition and timing, which would otherwise remain hidden in the rocks, and more direct evidence to calibrate basin models. A reconnaissance-scale FI study, using CSIRO’s Grain with Oil Inclusion (GOI™) technique, was undertaken to detect liquid hydrocarbons in Jurassic to Cretaceous sandstones. Oil-bearing, and in some cases gas-rich, inclusions were detected at low abundance, and their presence provides proof of oil generation and migration in the Ceduna Sub-basin. Geochemical fingerprinting of FI oil was undertaken using the Molecular Composition of oil Inclusions (MCI) technique on an intra-Coniacian interval in Gnarlyknots-1A and a Cenomanian interval in Greenly-1. The results show differences in the type of organic matter input, with algal co-sourcing significant for the central Ceduna Sub-basin. The timing of oil migration from pressure-temperature (PT) reconstructions was interpreted in Gnarlyknots-1A, Greenly-1, Duntroon-1 and Potoroo-1. The results indicate oil charge during the Late Cretaceous in the basin depocentres, explained by sediment loading of the Upper Cretaceous succession by the Hammerhead Supersequence and oil, gas-condensate and gas charge to the depocentres and basin margins during the Miocene. The Great Australian Bight Research Program is a collaboration between BP, CSIRO, the South Australian Research and Development Institute (SARDI), the University of Adelaide and Flinders University. The Program aims to provide a whole-of-system understanding of the environmental, economic and social values of the region, providing an information source for all to use.