Stratigraphic framework and structural architecture of the Upper Cretaceous in the deep-water Otway Basin – implications for frontier hydrocarbon prospectivity

Keywords: deep‐water, hydrocarbon prospectivity, Otway Basin, seismic interpretation, stratigraphic architecture, stratigraphic framework, structural architecture, structural elements.

The Otway Basin is a passive margin basin located in southeastern Australia (Fig. 1a). Unconformity-bound seismic supersequences are associated with rifting and thermal subsidence basin phases (Krassay et al. 2004). Totterdell et al. (2014) recognised that the sparse coverage and poor imaging in legacy seismic data was a limiting factor in the geological understanding of the deep-water Otway Basin. This limitation was addressed by the 2020 Otway Basin 2D seismic program (Karvelas et al. 2021). The seismic data collected (Fig. 1a) provides new seismic coverage across the deep-water region that is tied, via reprocessed lines, to inboard wells. Geoscience Australia has undertaken a regional study to map the structural and stratigraphic architecture across the entire offshore basin using the new and reprocessed, as well as legacy 2D seismic data. This work updates interpretations across the ‘inner’ offshore basin, refines interpretations across the ‘inner’ and ‘deep-water’ basin boundary, and thereby provides a basin-wide perspective on structural and stratigraphic architecture across the offshore Otway Basin.

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, and the locations of Fig. 3 seismic transects. Basin and sub-basin outlines are from Totterdell et al. (2014) and Moore et al. (2000). (b) New structural observations overlying the Late Cretaceous (Turonian–Maastrichtian) isochron. Faults on the SE inboard platform areas are sourced from Romine et al. (2020).

The objective of this study is to extend Cretaceous supersequence and fault mapping across the offshore Otway Basin, building on the regional sequence framework of Krassay et al. (2004), Romine et al. (2020), Schenk et al. (2021) and Karvelas et al. (2021). With emphasis on the Upper Cretaceous Shipwreck and Sherbrook supersequences, mapping was refined across the 2020 Otway Basin 2D grid and across an additional ~40 000 line-km of legacy 2D seismic data. The supersequences were tied to 18 wells with reference to updated well biozonations (MGPalaeo 2020). Supersequence and fault interpretations were used to generate structure surfaces and isochron maps (Fig. 2). Established structural elements are labelled in Fig. 1a and new structural observations are depicted in Fig. 1b.

Fig. 2.  Seismic horizon structure and isochron maps of the offshore Otway Basin Upper Cretaceous interval. Faults intersecting structure surfaces are marked in black. Faults on the SE inboard platform areas are sourced from Romine et al. (2020). (a) T1 (base of Maastrichtian–Lutetian Wangerrip Supersequence) structure surface. (b) LC2 (base of Campanian–Maastrichtian Sherbrook Supersequence) structure surface. (c) LC1 (base of Turonian–Santonian Shipwreck Supersequence) structure surface. (d) Upper Cretaceous (LC1–T1, Turonian–Maastrichtian) isochron. (e) Sherbrook Supersequence isochron (T1–LC2). (f) Shipwreck Supersequence isochron (LC2–LC1).

The Shipwreck Supersequence (Figs 2f, 3, Turonian–Santonian) is a largely fluvial-deltaic succession, and is bounded at its base by the Lower Cretaceous 1 unconformity (LC1, base Phyllocladidites mawsonii spore-pollen zone). Supersequence thickness is generally less than 1000 ms two-way time (TWT) over the platforms and, for the Mussel and Prawn platforms, varies over Early Cretaceous folds. Thickness increases outboard of the platform edges, and is accommodated to varying degrees by growth-wedges (Fig. 3b, c). Shipwreck Supersequence depocentres occur in the Morum and Nelson Sub-basins (Fig. 2f) with up to 2000 and 3000 ms TWT sediment fill respectively. In addition to the growth at platform edges, generally smaller growth-wedges are observed in seismic profiles of the Shipwreck Supersequence throughout the southeast part of the offshore basin. The supersequence marks the commencement of a second extension phase and rift-related subsidence in the Otway Basin.

Fig. 3.  Seismic stratigraphy and structure interpretation of the Cretaceous on representative SW–NE seismic profiles across the offshore Otway Basin. (a) Dip line from the Crayfish Platform to the Morum Sub-basin showing progressive outboard listric faulting detaching in the Eumeralla Supersequence and prominent outer margin high. (b) Dip line from the Mussel Platform to Nelson Sub-basin showing folding on the Normanby Terrace and Portland Trough, Shipwreck and Sherbrook supersequence sediments thickening into a growth-wedge outboard of the platform edge, and a zone of mass transport complexes within the Sherbrook Supersequence. (c) Dip line from the Prawn Platform to the Nelson Sub-basin showing uplifted hanging-wall anticline geometries above crustal-scale faults that offset the base-Mesozoic reflector, and large-scale clinoforms within the Sherbrook Supersequence in the outboard deep-water region.

The Sherbrook Supersequence (Figs 2e, 3, Campanian–Late Maastrichtian) is a largely fluvial-deltaic succession, and is bound at its base by the Lower Cretaceous 2 unconformity (LC2, base Nelsoniella aceras dinocyst zone). Across the platform areas, Sherbrook Supersequence thickness is typically less than 1000 ms TWT. Thickness increases across the platform edges are variably accommodated by extensional growth, and large growth-wedges are associated with the Sherbrook depocentre (up to 2800 ms TWT) in the Morum Sub-basin (Fig. 3a). A thinner Sherbrook depocentre (<2000 ms TWT) extends across the southern Nelson Sub-basin (Fig. 3b) where this supersequence thickens as it oversteps the Shipwreck Supersequence to the south. Large-scale clinoforms are a characteristic feature of the Sherbrook Supersequence across the Nelson Sub-basin (Fig. 3c). Small-scale clinoforms are also evident locally within the intensely faulted Sherbrook succession in the Morum Sub-basin. Over the central part of the deep-water region, clinoform geometry is disrupted by mass-transport complexes (MTCs). The Sherbrook Supersequence is bounded at its top by the Tertiary 1 unconformity (T1, intra Upper Manumiella druggii). Together, the Shipwreck and Sherbrook Supersequences form a contiguous Upper Cretaceous depocentre in the Deep-water Otway Basin (Figs 1b, 2d) that attains a maximum thickness of 3900 ms TWT.

Across the platform areas, faults trend NW–SE, are closely spaced and have relatively small strike-lengths and offsets (Figs 1b, 2b, c). In contrast, in the deep-water Otway Basin, they are more openly spaced, tend to be longer in strike-length and have larger offsets (Figs 1b, 3a–c). In the northwest part of the deep-water area, elongate NW–SE oriented faults dominate, while the outboard southeastern area is characterised by shorter, en-echelon WNW–ESE to NW–SE oriented faults (Figs 1b, 2a–c). The far southeast of the basin is characterised by NNW–SSE oriented basin-bounding faults within the Sorell Fault Zone (Figs 1, 3c). Faulting across the basin is mostly confined to the Cretaceous interval and is commonly truncated at the top of the Sherbrook Supersequence (T1, Fig. 3a–c). Across the deep-water region, fault growth is predominantly accommodated in the Sherbrook Supersequence in the west and the Shipwreck Supersequence in the east (Figs 2e, f, 3a–c), showing a westerly shift in the focus of synrift deposition.

The far northwest is characterised by listric faults that detach within a highly reflective Eumeralla Supersequence (Lower Cretaceous) with hanging-wall fault blocks increasingly rotated outboard (Fig. 3a). Towards the southeast, extensional fault growth is dominated by crustal-scale faults stepping down from the outboard platform edges, contributing to Shipwreck and Sherbrook thickening, and offset of the base-Mesozoic reflector (Fig. 3b, c).

Southerly plunging NE–SW oriented folds have been mapped across Mussel and Prawn platforms. Similarly oriented folds occur to the south in the deep-water region (Fig. 1b). In the far southeast of the basin, folds are associated with major faults within the Sorrell Fault Zone where they exhibit more northerly trends and uplifted hanging-wall anticline geometries that indicate inversion (Fig. 3c). Some fold axes in the far southeast of the study area are visible in the Shipwreck and Sherbrook supersequence isochrons as N–S to NE–SW areas of comparatively thin sediment (Fig. 2e, f). Over the Normanby Terrace and Portland Trough, a large-scale anticline–syncline pair trends NW–SE parallel to the edge of the Mussel Platform (Figs 1b, 3b).

The mapping presented here has implications for several established structural elements across the offshore Otway Basin. For example, Moore et al. (2000) documented the Discovery Bay High as a complex north–northeast–south–southwest oriented structural high. This feature has continued to be depicted on structural elements maps (e.g. Totterdell et al. 2014). Our maps show instead that it is a zone of relatively thin sedimentation of no particular orientation separating the Shipwreck depocentres of the Morum and Nelson Sub-basins (Figs 1b, 2e, f). Similarly, the Mussel and Tartwaup hinge zones have been previously depicted as major fault zones that separate the inner from deep-water Otway Basin. This study reveals instead that these features are more appropriately described as structural and subsidence hinges over which strata thickens, in varying degrees due to Late Cretaceous growth faulting. The outboard edge of the Normanby Terrace, in combination with parts of the Tartwaup and Mussel hinges, marks the transition from the platform regime to the more highly extended deep-water region (Fig. 1a). These examples highlight the need for future investigations and mapping of structural elements and basin boundaries across the offshore Otway Basin as these influence the perceptions of basin evolution, and therefore the understanding of hydrocarbon prospectivity.

The Shipwreck and Sherbrook Supersequences are both largely fluvio-deltaic, progradational stratigraphic intervals. Seismic facies and geometry indicate that the Shipwreck Supersequence in the deep-water region is mud-dominated while well intersections indicate that sand-rich sediments are prevalent across platform areas. For the Sherbrook Supersequence, sand-dominated sediments are similarly distributed across the platforms, and clinoform geometries indicate that the sandy sediments were distributed across the deep-water basin. Deposition of Upper Cretaceous supersequences was strongly influenced by growth against the hinge zones that define platform edges. Furthermore, Shipwreck depocentres are bounded to the north and east by the Tartwaup and Mussel hinge zones (Fig. 2f), while the Sherbrook depocentre in the Morum Sub-basin is bounded to the northeast by the Normanby Terrace, emphasising the shift in extensional regime between these supersequences. This regional characterisation of Upper Cretaceous supersequences provides the basis for ongoing investigations of the basin’s structural evolution, and the distribution of gross depositional environments.

Petroleum systems modelling by Schenk et al. (2021) indicates that the Austral 3 (i.e. Upper Cretaceous) system has the potential to generate hydrocarbon accumulations in the deep-water part of the basin. The new observations presented here will have implications for the modelling of this system and is likely to improve the understanding of the hydrocarbon prospectivity in the underexplored outboard part of the Otway Basin.

The data that support this study are available in the National Offshore Petroleum Information Management System (NOPIMS) at http://www.ga.gov.au/nopims.

All authors confirm there are no conflicts of interest.

No funding from external organisations was received for this research.