Stratigraphy of the onshore Otway Basin, with the Laira Sandstone highlighted in red and mature source rocks highlighted in purple. (PNG  2.2 MB)

The Laira Formation play exists in the onshore Otway Basin in South Australia:

Download play extent map of the Laira Formation play (JPG 5.2 MB)

There is no discussion of the Laira Formation being a potential reservoir in Morton et al (2002). Boult et al (2002) described the Laira Formation as “Lithology consists of medium to light grey to green siltstone and claystone, with minor fine-grained sandstone interbeds. Sandstones are quartzose with minor feldspar, mica, lithic fragments and carbonaceous material, and are cemented by diagenetic calcite, dolomite and minor silica.”

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

A review of 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 Laira Formation, identifying several braided and anastomosing river systems with sediment input primarily from the north-west, surrounded by extensive floodplains, which may contain smaller river systems and/or crevasse splays. In addition, there may have been alluvial fans formed in the hanging walls of some of the major faults.

Gross depositional environments of the Laira Formation (JPG 5.5 MB)

Any hydrocarbons that may have migrated into sandstone reservoirs in the Laira Formation are likely to be sealed by intra-Laira siltstones and shales, unless they are near the top of the formation; in this case Eumeralla Formation shales above would act as suitable seals, although where the Windermere Sandstone occurs at the base of the Eumeralla top seal is unlikely to occur.

Extent of potential seals for Laira Formation reservoirs (JPG  8.3 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.

Modelled gas expelled from the base of the Laira Formation (JPG  4.8 MB)

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.

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.

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

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 Laira Formation sandstone reservoirs

Any suitable reservoirs in the Laira Formation may have been charged with gas and/or oil expelled from one or more source rocks in the Laira Formation, the Upper Sawpit Shale, the Lower Sawpit Shale and the Casterton Formation.

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

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

Download potential charge for Laira Formation reservoirs map (JPG 5.7 MB)

Play elements for the Laira Formation in the onshore Otway Basin, SA (JPG 7 MB)

Play map for the Laira Formation in the onshore Otway Basin, SA (JPG 5.2 MB)

Data from PEPS South Australia:

Morton and Boult (2002) undiscovered recoverable gas resources for the Crayfish Group in the onshore Otway Basin. However, given that there is little reference in the Boult and Hibburt (2002) volume to the Laira Formation containing suitable reservoirs for hydrocarbons it is unlikely that this estimate included anything for the Laira Formation.

Therefore, no estimate has been made to date.

Boult, PJ and Hibburt, JE, eds, 2002. 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.

Boult, PJ, White, MR, Pollock, R, Morton, JGG, Alexander, EM and Hill, AJ, 2002. Lithostratigraphy and environments of deposition. Chapter 6, 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.

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.