SEDIMENTATION AND PETROLEUM POTENTIAL OF THE JURASSIC SEQUENCE IN THE SOUTHWESTERN GREAT ARTESIAN BASIN

Abstract

Along the western margins of the southwestern Great Artesian basin the Jurassic sequence is an almost continuous sandstone section. In the eastern part of the same area, this sandstone sequence is broken by two shale-siltstone intervals, the Birkhead and Westbourne Formations. Towards the western margins of the basin these facies change into sandstone. The Murta Member of the Mooga Formation develops in the area north of Lake Frome through a facies change in the upper part of this formation from sandstone to siltstone and shale. It is postulated that the depositional conditions in the southwestern Great Artesian basin were dominantly fluviatile during most of the Jurassic and that the fine grained sediments of the Birkhead and Westbourne Formations and the Murta Member of the Mooga Formation were deposited under low energy lacustrine conditions. Abundant good quality potential petroleum reservoir rock exists throughout the entire Jurassic sequence. The lack of hydrocarbon filled traps found to date and the change of the main siltstone-shale intervals into sandstone facies in the west and southwest, imply that the Jurassic in the southwestern Great Artesian basin has been effectively flushed. However, the complex facies relationship of the sandstone and shale beds indicates that stratigraphically controlled traps may exist. The most prospective part of the Jurassic for commercial hydrocarbons appears to be in the lower part of the Hutton Sandstone.