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

Abstract

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.