Shipwreck and Sherbrook supersequence regional gross depositional environments, offshore Otway Basin

Introduction

The upper Cretaceous Shipwreck (Turonian–Santonian) and Sherbrook (Campanian–late Maastrichtian) supersequences (SSs) of the Otway Basin (Fig. 1a) formed during an extensional basin phase that commenced after basin-scale inversion (‘Otway Unconformity’) and ended with Australia–Antarctica separation. Both SSs consist of fluvial, coastal plain, deltaic, and marine shelf sediments. Fig. 1b shows the time-thickness variation of the upper Cretaceous succession. Across platform areas, thickness is generally less than 1.5 s TWT (two-way travel time). Beyond the outboard platform margin, the upper Cretaceous depocentre attains a thickness of over 3 s TWT. Accommodation for the upper Cretaceous depocentre was created by pronounced late Cretaceous crustal thinning and extension (Nicholson et al. 2024).

Fig. 1.

Regional geologic setting of the Otway Basin upper Cretaceous (a) Stratigraphic chart (modified from Kelman et al. (2015)) (b) Time-isochore and structural elements map. Revised basin and sub-basin boundaries are from Nicholson et al. (2024).

Regional gross depositional environment (RGDE) maps for the Shipwreck and Sherbrook SSs were compiled to constrain the distribution and composition of potential reservoir, seal, and source rock intervals of the upper Cretaceous Austral 3 petroleum system across the basin (Summons et al. 2002). The work presented here is part of an offshore Otway Basin pre-competitive program undertaken at Geoscience Australia. The program included acquisition of the Otway 2020 2D seismic survey over the deep-water region (Fig. 1b) and this dataset forms the basis of RGDE mapping in this under explored and poorly understood part of the basin.

RGDE characterisation

Observations from 35 wells (core and wireline logs), together with seismic facies analysis, were integrated to formulate basin scale RGDEs. The results of well-based GDE interpretation are presented in Cubitt et al. (2023) and associated data packages (Nguyen et al. 2022, 2024; Cubitt et al. 2024). Seismic facies classification integrated with well-based interpretations is presented by Abbott et al. (2023). Seismic sequence boundaries used to define RGDE intervals are included in Abbott et al. (2022). The characteristics of the main Shipwreck and Sherbrook SS RGDEs are summarised in the following, and reference core logs can be found in Cubitt et al. (2024).

Shelf RGDE

The Shelf RGDE includes marine mud-prone inner-, middle-, and outer-shelf depositional environments. High-frequency, continuous, variable amplitude reflections characterise this seismic facies. Reference core logs for this RGDE are from La Bella 1. In the Morum Sub-basin, the Shelf RGDE displays prominent divergent stratal geometries associated with growth-faulting (Fig. 2a). In contrast, in the Nelson Sub-basin, the Shelf RGDE is distinguished by shelf-scale clinoforms (Fig. 2b). The dominance of marine mud for the Shelf RGDE is consistent with the sigmoidal character of clinoforms within the progradational complex.

Fig. 2.

Interpreted seismic profiles showing RGDE distribution across the (a) Crayfish Platform and Morum Sub-basin, and (b) Shipwreck Trough and Nelson Sub-basin. Profile locations are shown in Fig. 3.

RGDE maps

For each mapped interval, the distribution of RGDE on seismic lines was constrained by well control and seismic facies (Fig. 2). Markers placed on RGDE boundaries on seismic profiles were used to guide the creation of RGDE polygons in plan view. Using this approach, three maps (Fig. 3) were prepared:

Fig. 3.

Regional gross depositional environment maps for the (a) Shipwreck Supersequence (LC1.1 sequence) (b) Shipwreck Supersequence (LC1.2 sequence) and (c) Sherbrook Supersequence. Well names and offshore well-based GDE data can be found in Nguyen et al. (2022, 2024).

Basin-wide map coverage was completed using published sources for the onshore basin. Transitions between RGDEs occur over distances of a few kilometres to several tens of kilometres.

Discussion and conclusions

The Shipwreck and Sherbrook SSs were deposited during a second phase of Otway Basin extension, and prior to the breakup of Australia and Antarctica. Plate tectonic reconstructions show that the Otway Basin was a NW–SE oriented embayment, flanked by the Australian continent to the north and east, Antarctica to the south, and faced the incipient Southern Ocean to the west. The RGDE maps in Fig. 3 reflect this broad palaeogeographic setting, as each is dominated by a marine Shelf RGDE across the deep-water region, a medial belt of Coastal-Delta Plain RGDE, and Fluvial Plain RGDE variably developed along the northeastern inboard basin margin.

The RGDE maps show fairly extensive development of the Coastal-Delta Plain and Fluvial Plain RGDEs over the Morum Sub-basin and adjacent platform area and depicts the development of a major fluvial-deltaic system that was active throughout the late Cretaceous. This system increases in extent across the three intervals depicted in the maps, progressively encroaches into the Morum Sub-basin, and coincides with the thickest accumulation of upper Cretaceous strata in the Morum depocentre (Fig. 1b). The upper Cretaceous deltaic complex in the northwest Otway Basin is analogous to the Hammerhead Delta (Krassay and Totterdell 2003) in the Bight Basin.

Each RGDE map shows a palaeogeographic embayment of Shelf RGDE over the central platform area which diminished in size as the fluvial-deltaic complex prograded into the basin. The embayment incorporates the Shipwreck Trough, a relatively subtle feature visible on the upper Cretaceous time-isochore map (Fig. 1b). Accommodation within the Shipwreck Trough has been influenced by the large Minerva Graben, several smaller half-graben and graben, and syn-depositional folding. Accommodation was enhanced along the eastern margin of the Shipwreck Trough by development of the Shipwreck Fault (Fig. 3), a N–S trending, basement involved, strike-slip fault zone.

Hydrocarbon accumulations primarily sourced and reservoired within the Shipwreck and Sherbrook supersequences (i.e. Austral 3 petroleum system) have yet to be discovered, probably because these intervals are immature across the explored inner Otway Basin (Totterdell et al. 2014). Gas accumulations reservoired in the Waarre and Flaxman formations (LC1.0 sequence, Fig. 1a) in the Shipwreck Trough are charged by Austral 2 (lower Cretaceous Eumeralla Supersequence) source rocks. These are sealed by thick Shelf RGDE (LC1.1 and LC1.2 sequences) and trapped within faulted anticlines and tilted fault blocks. This study confirms that accumulations in the Shipwreck Trough rely on the presence of the thick Shelf RGDE seal (i.e. ‘Belfast Mudstone’ regional seal of the Otway Basin literature) and now places this play configuration within a sequence stratigraphic and GDE context. The maps presented here indicate that distribution of this important seal play element and, therefore Shipwreck SS prospectivity, is greater in area than the published boundary of the Shipwreck Trough (Cliff et al. 2004, Fig. 3). Further, deltaic reservoir facies (Thylacine Sandstone and other seismically imaged bodies) are sealed within the Shelf to Coastal-Delta Plain RGDE transition of the LC1.2 sequence (Fig. 1a).

Petroleum systems modelling (Schenk et al. 2023) and geochemical analysis from outboard wells indicate that the Austral 3 petroleum system may have operated within the Sherbrook SS in the deep-water part of the basin where it is more deeply buried. The presence of thick intervals of marine mudstone (Shelf RGDE) interpreted across the deep-water basin indicate the potential for productive source rocks and effective cross-fault seal, corroborating the observations by Totterdell et al. (2014). Reservoir rocks in the deep-water basin are likely best developed in the Coastal-Delta Plain RGDE belt where it encroaches the Morum Sub-basin and resides in extensional growth wedges. The insights yielded by this study will inform area selection for detailed GDE mapping, formulation of new hydrocarbon and carbon dioxide (CO₂) storage plays, and provide new inputs for petroleum and CO₂ systems modelling.