Play analysis in the onshore Otway Basin, South Australia

Introduction

The Otway Basin is one of the best-known Mesozoic rift basins that span the southern coastline of Australia forming as a result of rifting between the Antarctic and Australian plates. The onshore portion of the basin covers nearly 10,000 km² in South Australia (SA) (Fig. 1a), from which nearly half a million barrels of condensate/oil and 86 BCF of gas have been produced.

Fig. 1.

(a) Central Otway Basin (SA) well and seismic line location map. (b) Onshore Otway Basin stratigraphy (modified from Kirk 2024), with plays reviewed in red and source rocks in purple.

In 2023, the Department for Energy and Mining conducted an integrated study to prepare a petroleum systems model of the onshore Otway Basin in SA, with the model showing significant hydrocarbon generation in the Penola Trough as expected. For the first time it appears that significant hydrocarbons may have been generated and expelled in the lightly explored Robe and Tantanoola Troughs (Edwards et al. 2024).

Play analysis of key intervals in the basin was conducted by the Geological Survey of South Australia (GSSA) following completion of this model as an important part of highlighting the petroleum prospectivity of this region.

Play analysis

What is a petroleum play? A ‘play’ is defined by Rose (2017) as a trend or family of geologically related leads, prospects and possibly producing oil and gas fields, with primary elements of similar geological history of hydrocarbon charge, reservoir/seal pairs and trap controls on their occurrence. They are typically defined by stratigraphic age, with plays generally separated by a regionally significant master seal.

Play analysis, or play-based exploration, is a technique to build and leverage an understanding of geological basins and petroleum systems. The methodology integrates many tools such as seismic and well log data, structural analysis, and geochemistry to define a series of play maps that can be used to focus exploration strategy and improve prospect chance assessment. See Strong (2024) for further discussion of the methodology.

In theory, there is no chance of the play being found outside of its approximate extent, but as with all models the limits are only as good as the data used to construct them. Therefore, this process should not be static, requiring iteration of interpretation following new information from drilling, studies, etc., to update the knowledge base and keep it as evergreen as possible.

Iteration of interpretation

The Casterton Formation represents the first sedimentation on top of the economic Basement in the onshore Otway Basin during the early rifting episode separating the Antarctic and Australian plates in the Jurassic (Fig. 1b). The interval primarily consists of medium to dark brown and dark grey, silty carbonaceous mudstone deposited in a lacustrine environment (Boult et al. 2002).

Fig. 2a shows sketches of the gross depositional environment (GDE) for the Early, Middle and Late Casterton Formation defined from interpretations of seismic and core data (Cubitt et al. 2024), with the drafted summary GDE map displayed in Fig. 2b, also from Cubitt et al. (2024).

Fig. 2.

(a) GDE sketches showing the evolution of the Casterton Formation (from Cubitt et al. 2024). (b) First pass Casterton Formation GDE map (Cubitt et al. 2024). (c) Final Casterton Formation GDE map from this study.

Further review of well data during play mapping of the Casterton Formation resulted in iteration of the GDE map. The Basement section in several wells was reinterpreted to have been reworked during the latest Jurassic as turbidite sands within the Casterton, deposited in the deep lake that filled the Penola Trough. This highlights potential for further prospectivity in the region, with gas shows in the formation in two wells. The final GDE map is shown in Fig. 2c.

GDE mapping

GDE mapping enables an understanding of the distribution of a potential reservoir section. Cubitt et al. (2024) described such mapping of six intervals within the onshore Otway Basin in SA, and Tiainen et al. (2024) discussed the reinterpretation of reservoir intervals, focussing on the Lower Sawpit Shale interpreted as being lacustrine turbidites in the Penola Trough. The final GDE map of the Lower Sawpit Shale is shown in Fig. 3a.

Fig. 3.

(a) GDE map of the Lower Sawpit Shale. (b) Seal map for Lower Sawpit Shale reservoirs. (c) Distribution of oil and gas expelled from the Lower Sawpit Shale and Casterton Formation. (d) Lower Sawpit Shale play map, showing sealed reservoir with likely charge in green.

Conclusion

Conducting play analysis is one method of stimulating exploration for hydrocarbons in a region, by understanding the distribution of reservoirs, seals, mature source rocks and hydrocarbon migration pathways. Play analysis is simply a technique to build and leverage an understanding of geological basins and petroleum systems, following a methodology that incorporates the integration of many tools. As we have shown in this study, this process should not be static, but iterative as new information from for example drilling, seismic data and studies to update the knowledge base and keep it evergreen; an example of this in the onshore Otway Basin in SA is the Casterton Formation.

The GSSA has recently conducted play analysis in the onshore Otway Basin, with eight intervals reviewed for reservoir distribution, presence of a sealing interval above, and likely charge from potential source rocks that may have generated hydrocarbons. The results of this analysis for the Lower Sawpit Shale have been presented in this paper.