Publication Name: Eastern Australian Basins Symposium 2001
Authors: D. Palmowski, K.C. Hill, N. Hoffman and T. Bernecker
Date Published: November 2001
Number of Pages: 30
Reference Type: Magazine Article
Abstract:
The Otway Basin is part of the Southern Ocean Rift System between Australia and Antarctica. Balancing and restoring deep seismic profiles, this paper presents a new look at the tectonic development of the basin and its petroleum implications.The development of a Lower Cretaceous half graben under the modern shelf along the entire Otway coast ended with Aptian-Albian regional post-rift subsidence. The onset of a second rift-phase in the Late Cretaceous developed a hinge zone south of the first rift-system and involved widespread, pervasive extensional faulting and subsidence, allowing regional deposition of reservoir rocks in shallow marine or coastal facies. The widespread faulting and subsidence were probably due to development of a low-angle, north-dipping detachment zone
between Australian and Antarctic crust and the 'pulling out' of Antarctic crust from underneath. In the Campanian, associated with break-up in the Great Australian Bight, extension was restricted to a few very large growth faults allowing deposition of a 5 km thick Campanian-Santonian sequence above a 50 km wide northeast-dipping rollover anticline. The hydrocarbons generated at this time would all have migrated southwest and now reside in traps that are currently in deep water more than 100 km from shore. With continental break-up during the Mid-Eocene, the entire margin of the basin foundered and deepwater conditions dominated, until the modern shelf built out over the hinge zone.
Restricted marine source rocks in Late Cretaceous Sherbrook Group equivalents south of the hinge zone probably entered the hydrocarbon generation window in the Paleogene, and may have charged reservoirs in late Sherbrook Group equivalents onlapping onto the hinge zone and also overlying Paleocene to Late Eocene shelf progrades. Deepwater conditions after break-up allowed the deposition of Eocene Narrawaturk Marl equivalents, acting as a regional seal.