HYDROCARBON POTENTIAL OF THE OFFSHORE OTWAY BASIN

Abstract

The Otway Basin is one of three sedimentary basins in the Bass Strait region and is situated west of the Bass and Gippsland Basins. It trends NW-SE, straddling the southern Australian coastline for 500 km between the Mornington Peninsula in Victoria and Cape Jaffa in South Australia. It has an average width of 200 km and an average offshore width, in water depths of less than 200 m, of 50 km.

The offshore basin consists of three main tectonic units: the Mussel Platform in the east, the Voluta Trough, which occurs in the centre of the basin, and the Crayfish Platform in the west. Structures are formed predominantly by Cretaceous normal faults, downthrown to the continent-ocean boundary, and displacing landward-dipping Cretaceous strata. The sedimentary sequence can reach 10 km in thickness and consists of terrestrial Early Cretaceous sediments of the Otway Group, Late Cretaceous transgressive-regressive terrigenous sediments of the Sherbrook Group, Paleocene-Eocene transgressive-regressive, terrigenous and carbonate sediments of the Wangerrip and Nirranda Groups, and Oligocene-Miocene shelf carbonates of the Heytesbury Group.

Since the early 1960s, about 50 exploration wells have been drilled onshore and 17 offshore. Shows of oil, gas and condensate have been widespread in both onshore and offshore wells, though only two small economic fields have so far been discovered; both are onshore. Exploration, particularly during the 1960s and 1970s, was hampered by poor seismic data quality, due primarily to the presence of shallow carbonates.