A new perspective on regional structural architecture across the offshore Otway Basin

Introduction

The Otway Basin (Fig. 1) formed during the rifting of Australia from Antarctica and contains an Upper Jurassic to Cenozoic sedimentary succession that consists of unconformity-bound seismic supersequences (Krassay et al. 2004). Geological understanding of the deep-water region was previously limited by sparse seismic data coverage but has been improved by recent acquisition and reprocessing. The 2020 Otway Basin 2D seismic program included acquisition of over 7000 line-km of 2D seismic, gravity and magnetic data, together with reprocessing of over 10,000 line-km of legacy 2D data. The 2023 Otway 3D post-stack Mega Merge seismic dataset (Gunning et al. 2024) has increased the confidence of mapping between the deep-water and inner Otway Basin. Revisions to the stratigraphic framework and structural architecture for the Upper Cretaceous Shipwreck and Sherbrook supersequences have recently been published (Nicholson et al. 2022). New work presented here includes interpretations of the Lower Cretaceous Crayfish (latest Jurassic-Barremian) and Eumeralla (Aptian-Albian) supersequences, basement, basement-involved faults, reflection MOHO (Mohorovičić discontinuity), and the top of lower laminated continental crust (TLLCC). Results highlight the distribution of Lower Cretaceous depocentres, the basin and basement structural architecture, and crustal thinning trends across the inner and deep-water Otway Basin extending to the outer margin highs (Fig. 2).

Fig. 1.

(a) Structural elements map of the Otway Basin showing the 2020 new and reprocessed seismic data, legacy 2D seismic data, petroleum wells and fields, the locations of Fig. 3 seismic transects and updated and previous basin boundaries. (b) Updated basin boundaries, basement-involved faults, fold axes and outer margin high outlines overlying the Cretaceous (latest Jurassic-base Cenozoic) depocentre isochore map.

Fig. 2.

Seismic horizon structure and isochore maps of the offshore Otway Basin Lower Cretaceous interval and deep crustal reflectors. Faults intersecting structure surfaces are marked in black. (a) Crayfish Supersequence isochore map (EC2-Basement). (b) Eumeralla Supersequence isochore map (LC1-EC2). (c) Basement structure surface (d). EC2 (base Aptian-Albian Eumeralla Supersequence) structure surface. (e) Basement to Mohorovičić discontinuity (MOHO) isochore map. (f) top of lower laminated continental crust (TLLCC) to MOHO seismic isochore map.

Cretaceous depocentre

The Cretaceous depocentre (Fig. 1b) is based on interpretation of the base Wangerrip Supersequence (base Cenozoic) breakup unconformity (T1) and the top basement surface (base Crayfish Supersequence) (Fig. 3) across the offshore Otway Basin. The depocentre attains a maximum thickness of ~5800 ms two-way time (TWT) in the Morum Sub-basin and the northwestern part of the Nelson Sub-basin. On the outboard margin of the basin, the depocentre thins as Upper Cretaceous sediments onlap a series of outer margin highs (Figs 1b, 3b). The southeast margin of the Cretaceous depocentre is marked by a series of half graben that formed over shallow basement across the Sorell Fault Zone. In the northwest, a series of small half graben step up from the Morum Sub-basin onto the Crayfish Platform and abut the Trumpet Fault and basement high (Fig. 1). The Upper Cretaceous Shipwreck and Sherbrook supersequences have been documented elsewhere (Nicholson et al. 2022). The Lower Cretaceous sequences are summarised in the following.

Fig. 3.

Seismic stratigraphy and structure interpretation of the Cretaceous and deep crustal reflectors on representative seismic profiles across the offshore Otway Basin. Proximal, Necking, Distal and Outer structural domains illustrate progressive crustal thinning outboard, corresponding with thickening of Cretaceous sediments. See Fig. 1 for locations. (a) Dip line from the Crayfish Platform to the Morum Sub-basin. (b) Dip line from the Shipwreck Trough to the Nelson Sub-basin. (c) Dip line from the Prawn Platform to the Nelson Sub-basin.

Crustal architecture

Basement architecture is broadly aligned with structural geometries observed in the Cretaceous depocentre. In the southeast, a dominant NNW–SSE basement grain controls faulting, basement features, and basin architecture (Gibson et al. 2011). An associated series of NNW-SSE oriented basement-involved faults from the Sorell Fault Zone defines the Otway and northeastern Sorell basin boundaries (Figs 1a, b, 2c). Further outboard along the Mussel Hinge Zone, a series of en échelon, NNW–SSE to NW–SE oriented, crustal-scale detachment faults, step down from the platform edge and offset basement (Figs 1b, 2c, 3b, c). Across the Prawn Platform, Shipwreck Trough and Mussel Platform, basement faults predominantly trend between NE–SW and N–S and are commonly aligned with Cretaceous fold axes (Fig. 1a, b), indicating that deformation was basement-controlled. In the northwest, the ENE–WSW oriented Trumpet Fault (Fig. 1a) marks the basin boundary. It is accompanied by a series of E–W oriented faults that offset basement to the south (Figs 2c, 3a). The EC2 unconformity (Fig. 2d) broadly follows the structural architecture of the underlying basement surface (Fig. 2c). However, it also inherits the largely NW–SE oriented fault architecture of an overlying Late Cretaceous synrift succession from the Shipwreck Supersequence (Nicholson et al. 2022).

The top of lower laminated continental crust (TLLCC) and Mohorovičić discontinuity (MOHO) seismic picks were constrained by a package with distinctly layered seismic facies (Fig. 3). This reveals a zone of crustal thinning that is clearly imaged by the Basement-MOHO and TLLCC-MOHO isochores (Fig. 2e, f). The laminated lower continental crust and combined upper and lower crust are thickest beneath the inner Otway Basin (Figs 2e, f, 3) and thin outboard across the deep-water Otway Basin.

Discussion

Conclusions

Work presented here clarifies the distribution of the Cretaceous depocentre, and component supersequence depocentres of the offshore Otway Basin. The distribution of Lower Cretaceous depocentres and structural elements along the inner Otway Basin and outboard trends in crustal thinning and rifting during the Late Cretaceous have important implications for the distribution and potential maturity of petroleum systems. This work also highlights the need to review and update structural elements. For example, the boundary between the Otway and Sorell basins is now geologically constrained. The separation of the Otway and Sorell depocentres as distinct tectonostratigraphic entities may have important implications for their petroleum systems characteristics.