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

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

Download play extent map of the Sawpit Sandstone (JPG 5.9 MB)

There has been production from the Sawpit Sandstone in only one well in the South Australian sector of the onshore Otway Basin, Haselgrove 3 ST1, which produced 2,061.3 barrels of oil and 0.16Bcf of gas between February 2018 and September 2022 (data from PEPS South Australia):

The highest average monthly production rates were 224,000m³pd/7.9MMcfd and 25.45bopd.

There have been a number of other hydrocarbon tests in the Sawpit Sandstone in the basin:

• Jacaranda Ridge 1: Flowed 408bopd and 0.7MMcfgd. The oil was 44.3⁰ API gravity.
  • Boult et al (2004) interpreted this to be a partial palaeo-oil column
• Jacaranda Ridge 2: Flowed 235bopd and 2.4MMcfgd.
• Killanoola 1 DW1: Recovered 11 barrels of gas-cut oil of 34.8⁰ API gravity
• Killanoola Southeast 1: recovered oil-cut mud and water
• Wynn 1: Flowed up to 107.2bcpd, 4.1MMcfgd and 511bwpd from a number of DSTs and cased hole tests, with recovered oil of 46⁰ API gravity.
  • NB: The Well Completion Report (February 1995) notes that Wynn 1 was the first well in the Otway Basin to flow oil to surface from a Crayfish Group reservoir.
  • Boult et al (2004) interpreted this to be a partial palaeo-condensate column

Boult et al (2004) also interpreted a possible paleo-oil column in Penley 1.

Morton et al (2002) reported that “Only the upper sands [of the Pretty Hill Formation] have so far been exploited as petroleum reservoirs. A lower unit, which is informally referred to as the ‘Sawpit sandstone member’, has been intersected in Sawpit 1, Wynn 1, Pyrus 1, Jacaranda Ridge 1, Viewbank 1, Killanoola 1 and Penley 1.” It is this lower unit which is termed the Sawpit Sandstone in the 2023 DEM Petroleum Systems Study.

Morton et al (2002) also reported that pore volume reduction with depth is not considered a major risk for the Pretty Hill Formation* due to the suppression of quartz overgrowth formation, and that porosity may be found even in the deepest parts of the Penola Trough. This may well be the case for the Sawpit Sandstone.

* 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, 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 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 Sawpit Sandstone, identifying a major canyon in the north depositing a large alluvial fan system which in turn fed sediment into several major braided and anastomosing rivers. Smaller river systems were present in the north-east, providing sediment into the Penola Trough, where a delta plain fed into a lake. In addition, there may have been alluvial fans formed in the hanging walls of some of the major faults.

Gross depositional environments of the Sawpit Sandstone in the onshore Otway Basin (JPG 6 MB)

The seal to the Sawpit Sandstone reservoirs over most of the onshore Otway Basin in South Australia is the floodplain facies of the Upper Sawpit Shale immediately above, with floodplain facies of the Sawpit Sandstone providing lateral seal and potentially acting as a top seal as well.

However, where the Upper Sawpit Shale river systems and alluvial fans are present there may not be a competent seal for Sawpit Sandstone reservoirs as the former may have acted as migration paths for hydrocarbons into reservoirs higher in the section.

Extent of seals for Sawpit Sandstone reservoirs (JPG 6.6 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 Sawpit 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. However, the Upper Sawpit Shale sits stratigraphically above the Sawpit Sandstone, 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.

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 Sawpit Sandstone

The Lower Sawpit Shale directly below is probably the main source rock interval that has charged the Sawpit Sandstone with hydrocarbons, given it is directly below the reservoir section. However, gas and/or oil expelled from the Casterton Formation may have been able to migrate into the Sawpit Sandstone given suitable stratigraphic juxtaposition or via faulting. It is unlikely that the Sawpit Sandstone has been charged by the stratigraphically younger Laira Formation or Upper Sawpit Shale source rocks.

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 Upper Sawpit Shale reservoirs:

Download potential charge for Sawpit Sandstone map (JPG 6 MB)

Play elements for the Sawpit Sandstone in the onshore Otway Basin, SA (JPG 7.1 MB)

Play map for the Sawpit Sandstone in the onshore Otway Basin, SA (JPG 5.9 MB)

Data from PEPS South Australia:

Proven production from the Upper Sawpit Shale:	1 field in South Australia has produced oil and gas from the Sawpit Sandstone
Number of producing wells at 31 March 2024:	None Only 1 well in South Australia has produced oil and gas from the Sawpit Sandstone.
Total production at 31 March 2024:	2,061.3 barrels of oil and 0.16Bcf of gas from Sawpit Sandstone in Haselgrove 3 ST1 between February 2018 and September 2022
Hydrocarbon flow rates:	Highest average monthly production rates were 224,000m3pd/7.9MMcfd of gas and 25.45bopd from Haselgrove 3 ST1 Jacaranda Ridge 1 flowed 408bopd

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.

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.