Towards a regional understanding of Sherbrook Supersequence Gross Depositional Environments, offshore Otway Basin

The Sherbrook Supersequence (Sherbrook SS, Campanian-Maastrichtian) is the youngest of four Cretaceous supersequences in the Otway Basin and was deposited during a phase of crustal extension that culminated in breakup. This study presents a basin-scale gross depositional environment (GDE) map for the Sherbrook SS, and the significance of the map for the Austral 3 (upper Cretaceous, lower Cenozoic) petroleum system is discussed. Supersequence thickness is typically less than 1,000 ms TWT across the inboard platform. Beyond the platform edge up to 2,800 ms TWT of Sherbrook SS sediments were deposited in the deep-water Morum and Nelson sub-basins. Analysis of wireline logs and cores from wells yielded fluvial, deltaic, coastal, and shelf gross depositional environments (GDEs). As the number of regionally mappable seismic facies is much less than the number of well-derived GDEs, the integration of well-based environmental interpretations with seismic facies resulted in three main regional GDEs (RGDEs); Fluvial Plain, Coastal/Deltaic Plain, and Shelf. The Fluvial Plain and Coastal/Deltaic RGDEs are almost entirely restricted to the inboard platform areas of the basin. The mud-prone Shelf RGDE is widespread across the deep-water part of the basin where it forms the depocentres of the Morum and Nelson sub-basins. The Shelf RGDE is well imaged on the Otway 2020 2D seismic data that was acquired over the deep-water Otway Basin. In the Morum Sub-basin, the Shelf RGDE is strongly influenced by growth on extensional faults. In contrast, the Shelf RGDE in the Nelson Sub-basin is a relatively unstructured progradational complex. The presence of mass-transport and incision complexes are consistent with tectonism during Sherbrook SS deposition. Reservoir rocks in the deep-water basin are expected where the Coastal/Deltaic RGDE encroaches into the Morum Sub-basin. In this setting, the Austral 3 petroleum system was potentially active within the Sherbrook SS.