Publication Name: New Petroleum Symposium 1993
Authors: Graham Bradley
Date Published: December 1993
Number of Pages: 39
Reference Type: Magazine Article
Abstract:The Permo-Triassic offshore Sydney Basin is dominated by three structural elements : The Offshore Syncline, Offshore Uplift, and Offshore Newcastle Syncline. The eastern edge of the Offshore Syncline is a complex fault zone formed by extensional overprinting of reverse/ thrust faulting.
The basin is a foreland basin containing in excess of 5 km of sediment in its depocentres. The clastic fill was dominated by the interplay between arc-derived acid-intermediate volcanic sands and craton-derived quartz-rich sands, with locally significant basic volcaniclastic sands derived from the Offshore Uplift.
The top of the Offshore Uplift is defined by high-amplitude reflectors inferred to be Mid Permian volcanics, which are onlapped progressively from east to west by Late Permian Coal Measures and Early Triassic Narrabeen Group. The uplift is the crest of a long-lived volcanic arch/ fault-propagation fold which was initiated in the latest Carboniferous, grew in structural relief and was being actively eroded until the mid-Triassic. The crest of the uplift was then buried by Garie and Newport Formations followed by Hawkesbury Sandstone and Wianamatta
The onshore and offshore Sydney Basin was buried during the Jurassic and Early Cretaceous by up to 3.5 km of sediment, which was eroded, following uplift during mid-Cretaceous breakup.
High-risk gas potential exists along the crest of the Offshore Uplift where:
1) Quartz-rich Newport Formation and Hawkesbury Sandstone reservoir quality could be significantly improved if reservoired hydrocarbons inhibited formation of quartz cement;
2) Excellent top-seal would be provided by Wianamatta Group and/ or Newport Formation shales. The volcanogenic substrate would form an excellent base-seal, and could provide fault-gouge dip seal;
3) The Late Permian Coal Measures could provide a viable, mature, gas/ condensate source along the length of the Offshore Uplift, with simple migration pathways updip to the crest of the uplift.