Stratigraphy of the onshore Otway Basin, with the Lower Sawpit Shale reservoirs highlighted in red and mature source rocks highlighted in purple. (JPG 2.2 MB)

The Lower Sawpit Shale play exists in the onshore Otway Basin in South Australia:

Download play extent map of the Lower Sawpit Shale (JPG 5.5 MB)

There have been few hydrocarbon shows within the Lower Sawpit Shale in the onshore Otway Basin but no production. The shows there have been were generally weak, both oil and gas, but may indicate possible migration paths in the area.

A review the stratigraphy in the basin, description of core data and seismic mapping conducted by DEM in 2022/23 reviewed the gross depositional environments of the Lower Sawpit Shale, identifying turbidite fans deposited in a deep lake extending from the Robe Trough in the west through to the Penola Trough in the east (Tiainen et al, 2024). The turbidites have been termed the McEachern Sandstone by Cubitt et al (2024). In addition, there may have been alluvial fans formed in the hanging walls of some of the major faults.

Gross depositional environments of the Lower Sawpit Shale in the onshore Otway Basin (JPG 5.8 MB)

The lacustrine upper section of the Lower Sawpit Shale acts as a seal for the turbidite and alluvial fan reservoirs below.

Extent of seals for Lower Sawpit Shale reservoirs (JPG 5.9 MB)

There are several intervals in the onshore Otway Basin that contain suitable material to have acted as potential source rocks for charge of hydrocarbons into the Pretty Hill Sandstone reservoir:

1. the Early Cretaceous Eumeralla Formation
2. the Early Cretaceous Laira Formation of the Crayfish Subgroup
3. the Early Cretaceous Upper Sawpit Shale of the Crayfish Subgroup
4. the Early Cretaceous Lower Sawpit Shale of the Crayfish Subgroup, and
5. the Late Jurassic Casterton Formation.

Basin modelling in the onshore Otway Basin conducted by DEM (2023 DEM Petroleum Systems Study) suggests that both gas and oil have been expelled from major source rocks in the onshore Otway Basin.

Download total gas expelled from Otway Basin source rocks map (JPG 5.9 MB)
Download total oil expelled from Otway Basin source rocks map (JPG 5.5 MB)

1. Eumeralla Formation

Basin modelling in the onshore Otway Basin suggests that the Eumeralla Formation has not been mature for generation and expulsion of hydrocarbons in the onshore Otway Basin.

2. Laira Formation

The Laira Formation has been recognised as a potential source interval for hydrocarbons in the onshore Otway Basin since the 1990s, containing algal-rich zones with high TOC deposited in lacustrine conditions (Hill and Boult, 2002). Hydrocarbon Index (HI) values indicate that the Laira Formation is predominantly Type IV grading to at best Type III kerogen so is mainly gas-prone.

Basin modelling in the onshore Otway Basin conducted by DEM in 2023 suggests that the Laira Formation has only ever been mature for generation and expulsion of gas in the deepest parts of the Penola Trough and Tantanoola Trough, where it is over 1000m thick; maturity of this source rock interval has not previously been identified in the Tantanoola Trough.

See the Laira Formation play for a map of the modelled gas expelled from the base of the Laira Formation.

3. Upper Sawpit Shale

The Upper Sawpit Shale is the interval referred to as the “Intra-Pretty Hill Formation (Sawpit) shales” by Hill and Boult (2002), with the source rock interval representing lacustrine deposits over 200m in thickness in the Penola Trough and over 100m in the Robe Trough. Source richness of this interval is fairly uniform and constitutes a good source rock (mean TOC = 1.22%). Source quality is marginal to moderate, with little variation in kerogen type and source potential between the unnamed basal shale and intra-Pretty Hill shale.

The DEM basin modelling indicates that significant amounts of gas may have been generated from the Upper Sawpit Shale in the deepest portions of the Penola Trough close to the border between South Australia and Victoria, although the interval appears to have been mature for gas generation in the rest of the Penola Trough and in the St Clair Trough, the Rivoli Trough and the Robe Trough. However, the Upper Sawpit Shale sits stratigraphically above the Lower Sawpit Shale, and so it is unlikely that the latter has been charged by gas generated from the Upper Sawpit Shale.

See the Upper Sawpit Shale play for a map of the modelled gas expelled from the Upper Sawpit Shale.

4. Lower Sawpit Shale

The Lower Sawpit Shale interval is of significant thickness in the onshore Otway Basin, being over 3000m thick in the deepest parts of the Penola Trough, and over 1500m thick in the Robe and St Clair Troughs, according to seismic mapping. It consists of two separate intervals deposited in deep lacustrine deposits, some of it interpreted to have been deposited in algal lakes. The basal shale interval is equivalent to the McEachern Sandstone turbidites, and is referred to by Hill and Boult (2002) as the “unnamed basal shale”, the major source rock of the Pretty Hill Formation.

The basin modelling conducted by DEM indicates that the most mature area for generation of hydrocarbons from the Lower Sawpit Shale is the deepest portions of the Penola Trough, with significant amounts of both gas and oil generated. The interval also appears to have been mature for gas generation in most of the rest of where the interval occurs, with potentially large amounts of gas and oil having been generated in the deeper areas of the Robe Trough, the St Clair Trough and the Rivoli Trough.

Modelled gas expelled from the Lower Sawpit Shale in the onshore Otway Basin (JPG 5.3 MB)
Modelled oil expelled from the Lower Sawpit Shale in the onshore Otway Basin (JPG 5.7 MB)

5. Casterton Formation

The Casterton Formation represents the richest source rock of the Otway Supergroup, although it has only been sparsely intersected. Seismic mapping suggests that it may be over 1000m in places. It comprises pre-rift to early syn-rift interbedded shales, siltstones and sandstones and volcanic lithologies, such of which were reworked from the Basement, deposited under lacustrine conditions, very deep in some places. The richest source rocks contain algal material.

The results of the DEM basin modelling show that the Casterton Formation is mature for generation of gas and oil over just about the whole of the areas where it is present, the exception being in the very north of the Otway Basin.

See the Casterton Formation play for a map of the modelled gas expelled from the Casterton Formation.

Charge for the Lower Sawpit Shale

Gas and oil generated and expelled from mature Lower Sawpit Shale source rocks are likely to have been the main charge for sandstone reservoirs within the interval given their juxtaposition. However, gas and/or oil expelled from the older Casterton Formation below may have been able to charge Lower Sawpit Shale reservoirs given suitable stratigraphic migration paths or via faulting as well. It is unlikely that the Lower Sawpit Shale has been charged by gas generated from the Laira Formation or Upper Sawpit Shale above.

Extents of modelled gas and oil expelled from source rocks in the Lower Sawpit Shale and the Casterton Formation that may have charged Lower Sawpit Shale reservoirs:

Download potential charge for Lower Sawpit Shale map (JPG 5.5 MB)

Play elements for the Lower Sawpit Shale in the onshore Otway Basin, SA (JPG 6.5 MB)

Play map for the Lower Sawpit Shale in the onshore Otway Basin, SA (JPG 5.5 MB)

Data from PEPS South Australia:

Undiscovered recoverable gas resources for the Crayfish Group, including the Upper Sawpit Shale, in the onshore Otway Basin have been estimated as follows (PIRSA estimate August 2002, Morton and Boult, 2002).

Download Crayfish Group prospectivity map (JPG 1.2 MB). Only wells penetrating Crayfish Group are shown.

Cubitt, C, Kirk, R, Rady, T and Tiainen, S. 2024. Gross depositional environment mapping in the onshore South Australian Otway Basin: an example of data integration. The AEP Journal 64

Hill, A and Boult, PJ, 2002. Source rock distribution. Chapter 8, Boult, PJ and Hibburt, JE, eds: The Petroleum Geology of South Australia. Volume 1, Second Edition: Otway Basin, South Australia. Department of Primary Industries and Resources. Petroleum Geology of South Australia Series.

Morton, JGG and Boult, PJ, 2002. Undiscovered petroleum resources. Chapter 13, Boult, PJ and Hibburt, JE, eds: The Petroleum Geology of South Australia. Volume 1, Second Edition: Otway Basin, South Australia. Department of Primary Industries and Resources. Petroleum Geology of South Australia Series.

Morton, JGG, Sansome, A and Boult, PJ, 2002. Reservoirs and seals. Chapter 10, Boult, PJ and Hibburt, JE, eds: The Petroleum Geology of South Australia. Volume 1, Second Edition: Otway Basin, South Australia. Department of Primary Industries and Resources. Petroleum Geology of South Australia Series.

Tiainen, S, Kirk, R, Cubitt, C and Rady, T, 2024. Revisiting the rocks discovers turbidites and a new exploration play in the Penola Trough, Otway Basin, South Australia. The AEP Journal 64