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

The Pretty Hill Sandstone play exists in the onshore Otway Basin in South Australia.

Download play extent map of the Pretty Hill Sandstone play (JPG 6 MB)

The main producing reservoir in the South Australian sector of the onshore Otway Basin is the Pretty Hill Sandstone, which in the Katnook Field has porosities in excess of 25%, permeabilities in excess of 1000 mD. Average monthly production rates have been up to 938,000m³/33.1MMcfgd and 165bopd, both from Ladbroke Grove 3 (PEPS South Australia).

Reservoir permeability does not appear to decrease with increasing depth of burial, although only the upper part of the Pretty Hill Formation* is generally of good quality.
In total, 429,232 barrels of oil and 82.51BCF of gas were produced from the Pretty Hill Formation* in the onshore Otway Basin between January 1991 and October 2011. The following wells have produced oil and gas from the Pretty Hill (data from PEPS South Australia):

Boult et al (2004) interpreted the following for the Pretty Hill Sandstone* reservoir:

• The closures for wells Haselgrove 1, Haselgrove South 1 DW1, Katnook 2 and Redman 1 were gas-full to structural spill
• Limestone Ridge 1 had a paleo-gas column that was originally full to structural spill
• Zema 1 had a gas and oil palaeo-column that was full to spill
• Ladbroke Grove 1 had a very small palaeo-gas column
• Balnaves 1 had a possible partial palaeo-gas column, which originally was full to structural spill
• Pyrus 1 had a possible palaeo-gas column

* NB: This definition of the Pretty Hill Formation is from Morton et al (2002), which consisted of the Pretty Hill Sandstone, the Upper Sawpit Shale, the Sawpit Sandstone and the Lower Sawpit Shale which have been used in the 2023 DEM Petroleum Systems Study. For the study and this subsequent play analysis the reservoir interval at the top of the Pretty Hill Formation is called the Pretty Hill Sandstone Member.

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 Pretty Hill Sandstone, identifying several major braided and anastomosing river systems in the region 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 Pretty Hill Sandstone in the onshore Otway Basin (JPG 6 MB)

The seal to the Pretty Hill Sandstone over the southern portion of the area studied in the onshore Otway Basin in South Australia is the Laira Formation, while in the north where the Laira is absent seal is provided by shales of the Eumeralla Formation. However, in the north-west the Windermere Sandstone is present at the base of the Eumeralla, so it is likely that there may not be a competent seal for the Pretty Hill Sandstone if the Windermere directly overlies the Pretty Hill reservoir. In this instance the Pretty Hill Sandstone would have acted as a migration path for hydrocarbons into Windermere Sandstone traps, sealed by Eumeralla shales.

Similarly, in the Penola Trough Morton et al (2002) reported that “eventually the seal between the Katnook and Pretty Hill sands is lost. To the east (e.g. Haselgrove South 2) and NE (including the Victorian portion of the Penola Trough), the Laira Formation changes facies, sandstones of the upper Pretty Hill Formation* become less well developed, and the reservoir section is lost at the expense of better development of intra-Pretty Hill seals.”

* NB: The definition of the Pretty Hill Formation by Morton et al (2002) included the Upper Sawpit Shale Member, the Sawpit Sandstone Member and the Lower Sawpit Shale Member.

Extent of seals for Pretty Hill Sandstone reservoirs (JPG 6.4 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. However, the Laira Formation sits stratigraphically above the Pretty Hill Sandstone, and so it is unlikely that the latter has been charged by gas generated from the Laira Formation.

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 also 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 Pretty Hill Sandstone

The Upper Sawpit Shale is probably the main source rock interval that has charged the Pretty Hill Sandstone given it is directly below the reservoir section. However, wherever porous intervals may be juxtaposed through the section stratigraphically or via faulting the Pretty Hill Sandstone could have been charged with gas and/or oil expelled from the Lower Sawpit Shale and the Casterton Formation as well. It is unlikely that the Pretty Hill Sandstone has been charged by gas generated from the Laira Formation, as this sits stratigraphically above the reservoir section.

Extents of modelled gas and oil expelled from source rocks in the Upper Sawpit Shale, the Lower Sawpit Shale and the Casterton Formation that may have charged the Pretty Hill Sandstone.

Download potential charge for Pretty Hill Sandstone map (JPG 6.2 MB)

Play elements for the Pretty Hill Sandstone in the onshore Otway Basin, SA (JPG 7.3 MB)

Play map for the Pretty Hill Sandstone in the onshore Otway Basin, SA (JPG 6 MB)

Data from PEPS South Australia:

Proven production from the Pretty Hill Sst:	5 fields in South Australia have produced oil and gas from the Pretty Hill Formation
Number of producing wells at 31 March 2024:	None 9 wells in South Australia have produced oil and gas from the Pretty Hill Formation. The last wells to produce were Haselgrove 1, Haselgrove South 1 DW1 and Katnook 2, production ceasing in October 2021.
Total production at 31 March 2024:	429,232 barrels of oil and 82.51BCF of gas from Pretty Hill Formation between January 1991 and October 2011.
Hydrocarbon flow rates:	Highest average monthly production rates were 938,000m3/33.1MMcfd and 165bopd (both from Ladbroke Grove 3)

Undiscovered recoverable gas resources for the Crayfish Group, including the Pretty Hill Sandstone, 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.

Boult, PJ, Lyon, P, Camac, B, Edwards, D and McKirdy, 2004. Subsurface plumbing of the Crayfish Group in the Penola Trough: Otway Basin. PESA Eastern Australian Basins, Symposium II.

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.