Deep-Water Otway Basin: A New Assessment of the Tectonics and Hydrocarbon Prospectivity

Abstract

The northwest-trending Otway Basin in southeast Australia formed during the separation of Australia and Antarctica between the latest Jurassic and the Early Cainozoic. A new, deep-seismic data set shows that the basin comprises two temporally and spatially overlapping rift components:the mainly Late Jurassic to mid-Cretaceous, east-west trending, inner Otway Basin—comprising the onshore basin and most of the continental shelf basin; andthe northwest–southeast to north–south trending depocentres beneath the outer shelf and continental slope, extending from eastern South Australia to the west coast of Tasmania, and a relatively minor and ill-defined sub-basin underlying the continental rise in water depths greater than about 4,500 m. This rift system was most active from the mid-Cretaceous to Palaeogene, and was strongly affected by sinistral strike-slip motion as Australia and Antarctica separated.The continental slope elements contain the bulk of the sediment volume in the basin. From northwest to southeast, these elements comprise the Beachport and Morum Sub-basins, the north-south trending Discovery Bay High, and the Nelson Sub-basin which appears to be structurally and stratigraphically continuous with the Sorell Basin off west Tasmania.The reflection character of the crust and upper mantle varies widely across the basin, and there is a strong correlation between that character and the basin configuration. It appears that accommodation space beneath the slope basin was created largely by extension and removal of most of the laminated deep continental crust.There is encouragement for hydrocarbon exploration in the deep-water basin. Firstly, there are indications of diagenesis related to fluid flow in and above the strongly faulted Cretaceous section in the Morum Sub-basin. As an Early Cretaceous petroleum system is already proven beneath the continental shelf, this suggests that the same system is also active in deep-water. Secondly, existing sample data suggest that a second, Late Cretaceous petroleum system could be active where any source rocks are sufficiently deeply buried; this condition would probably be met in the Nelson Sub-basin.

Overpressures in the central Otway Basin: the result of rapid Pliocene–Recent sedimentation?

“…Structural closures south of this fault zone are associated mainly with titled fault blocks; large structures that have been targeted by wells include Bridgewater Bay-1, Voluta-1, Discovery Bay-1 and Normanby-1. A major, NE-SW trending structural culmination, variously named the Bridgewater High (Lavin and Mein, 1995), the Bridgewater Arch (Krassay et al, 2004) or the Discovery Bay High (Moore et al, 2000) effectively separates the Morum and Nelson sub-basins and is thought to have developed during the late-Maastrichtian (Lavin and Mein, 1995). Comparatively little structural activity occurred during the Cenozoic within this area, with relatively few normal faults observed; there is, however, evidence for localised inversion along the Tartwaup-Mussel fault zone (Geary and Reid, 1998).…”

“…This paper focuses on the Voluta Trough (which has been subdivided further into the Morum and Nelson sub-basins ( Fig. 1; Moore et al, 2000)), where overpressured units occur within upper Cretaceous Sherbrook Group shales and sand units.…”

Analysis of gravity-driven normal faults using a 3D seismic reflection dataset from the present-day shelf-edge break of the Otway Basin, Australia

“…We aim to temporally and spatially constrain the growth of a gravity-driven normal fault assemblage, imaged by 3D seismic data and located within Upper Cretaceous sedimentary rock at the present-day shelfedge break of the Otway Basin Australia (Figure 1). The Otway Basin is a Late Jurassic to Cenozoic age, rift-to-passive margin basin, which has undergone two stages of crustal extension during the Tithonian-Barremian and Turonian-Maastrichtian, after which it developed into a passive margin basin following the breakup of Australia and Antarctica (Moore et al 2000;Krassay et al 2004;Stacey et al 2013;Holford et al 2014). The Otway Basin extends from SE South Australia to NW offshore Tasmanian and contains a latest Jurassic to Maastrichtian siliciclastic sedimentary succession, followed by Cenozoic mixed carbonate and siliciclastic rocks (Moore et al 2000;Krassay et al 2004).…”

“…The Otway Basin is a Late Jurassic to Cenozoic age, rift-to-passive margin basin, which has undergone two stages of crustal extension during the Tithonian-Barremian and Turonian-Maastrichtian, after which it developed into a passive margin basin following the breakup of Australia and Antarctica (Moore et al 2000;Krassay et al 2004;Stacey et al 2013;Holford et al 2014). The Otway Basin extends from SE South Australia to NW offshore Tasmanian and contains a latest Jurassic to Maastrichtian siliciclastic sedimentary succession, followed by Cenozoic mixed carbonate and siliciclastic rocks (Moore et al 2000;Krassay et al 2004). Five major depocentres comprise the Otway Basin and these include the Eastern Torquay Sub-Basin, the mostly onshore Inner Otway Basin and the deepwater Hunter, Nelson and Morum Sub-Basins ( Figure 1a, Moore et al 2000).…”

“…The Otway Basin extends from SE South Australia to NW offshore Tasmanian and contains a latest Jurassic to Maastrichtian siliciclastic sedimentary succession, followed by Cenozoic mixed carbonate and siliciclastic rocks (Moore et al 2000;Krassay et al 2004). Five major depocentres comprise the Otway Basin and these include the Eastern Torquay Sub-Basin, the mostly onshore Inner Otway Basin and the deepwater Hunter, Nelson and Morum Sub-Basins ( Figure 1a, Moore et al 2000). Separating the Inner Otway Basin from the deepwater and frontier Nelson Sub-Basin is the present-day shelf-edge break, where this study is located (Amrit 3D, Figure 1).…”