Bremer Sub-Basin, Australia: Evaluation of Petroleum Potential in a Frontier Basin Through Analyses of Sub-Sea Dredge Samples

Boreham, Chris J.¹, Andrew Barrett¹, Jane Blevin¹, Irina Borissova¹, Barry Bradshaw¹, Marita Bradshaw¹, Neville Exon¹, Richard Howe¹, Cameron Mitchell¹, Mike MacPhail², Eric Monteil¹, Chris Nicholson¹, Robin O’Leary¹, Damien Ryan¹, Jennifer Totterdell¹ (1) Geoscience Australia, Canberra, Australia (2) Australian National University, Canberra, Australia

The Bremer Sub-basin on the rifted southwestern continental margin of Australia is a frontier basin in which no wells have been drilled. The petroleum potential of such frontier basins is generally limited to theoretical assessments from seismic data and analogues. However, a series of submarine canyons have incised the Bremer Sub-basin, allowing geo­logical sampling of the upper 2.5 km of the basin succession. Geochemical, petrographic and palaeontological analyses of 136 rock samples recovered from 30 dredge sites, integrat­ed with interpretation from a regional seismic grid, indicate that the Bremer Sub-basin con­tains a succession of up to 7km of Jurassic to Tertiary age sediments containing the essen­tial petroleum system elements (source, reservoir and seal) to generate and trap hydrocar­bons. Source rock analyses indicate Early Cretaceous coaly and lacustrine organic facies have the best oil potential with hydrogen indices (HI) up to 370 mg hydrocarbons/g TOC. Similar fluvio-lacustrine organic facies are recognised sources for oil in the adjacent Perth and eastern Bight basins. Furthermore, the identification of late Early Cretaceous marine anoxic organic facies in the Bremer Sub-basin supports the concept of a local southern Australian margin origin for widespread coastal bitumens termed asphaltites. Berriasian to Hauterivian age strata within the Bremer Sub-basin have the greatest potential to reservoir hydrocarbons, where lacustrine mudstones overlie fluvial sandstones in anticlines and fault block traps. The largest anticline may be capable of trapping up to 500 million barrels of oil in-place (P50 estimate; 900 million barrels P10 estimate).