Distribution of Triassic Reefs in the Northern Exmouth Plateau and Offshore Canning Basin : (Project 121.23) BMR Offshore Sedimentary Basins Program

Petroleum exploration on the Exmouth Plateau has ceased, althoughthe giant Scarborough Gas Field has been retained by Esso-BHP.Research cruises by BMR's vessel "Rig Seismic" and the associated assessment of other data, indicate that the plateau still has petroleumpotential, but the combination of deep water and low oil pricesmilitates against further exploration at present. However, the Ocean Drilling Program (ODP) core drilling on theWombat Plateau, in the far north of the Exmouth Plateau area, showedthat Rhaetian (Latest Triassic) reefs are present there, one beingdrilled at Site 764 (Fig. 1). This discovery was the first of aTriassic reef in Australia, and indicates that suitable conditions forreefal development are likely to be widespread on the Northwest Shelf.The existence of Late Triassic reef complexes suggests a new petroleumexploration play, with the reefs forming traps, and lagoonal faciesperhaps providing oil source potential. BMR has recently suggestedthat there is a 50% chance of Triassic reefs hosting 60 million barrelsof undiscovered oil on the Northwest Shelf. BMR's Marine Division intends to assess the potential of suchreefs and other known Triassic and Jurassic shelf carbonates in twophases. The first is to map the known reef complexes on the WombatPlateau, and to see how far they can be extended along the northernmargin of the Exmouth Plateau and into the Canning Basin. The secondis to take our knowledge of their seismic character, and the bestseismic parameters for mapping them, into the outer Bonaparte Basinwhere Triassic carbonate sequences up to 1000 m thick have beendrilled, but no reefs have yet been identified. The present project makes up the first phase of work, on thenorthern Exmouth Plateau. It is proposed that a 34 day cruise of "RigSeismic" be carried out in early 1990. This cruise should result inthe acquisition of 2500 km of multichannel seismic profiles on thenorthern Exmouth Plateau, about 15 dredges aimed at Triassic andJurassic carbonates, and up to 5 cores. The cruise and its post-cruisefollow-up work aim to (i)Map the extent of and facies changes within the Triassic/Jurassic carbonates in and south of Wombat Plateau, andin the Rowley Sub-basin of the southwestern Canning Basin, usingseismic profiling and dredging techniques. (ii)Define the extent, character and petroleum potential of any Triassic or Jurassic reefs found. (iii)Use the cruise results, along with other data, to help assessthe petroleum prospects of Mesozoic reef plays elsewhere on theNorthwest Shelf. (iv)Better define the geological history of the northern Exmouth Plateau and the adjacent southwest Canning Basin.