The fractured Basement play exists in the onshore Otway Basin in South Australia:

Download play extent map of the fractured Basement (JPG 9.9 MB)

Fractured Basement may provide a reservoir in the onshore Otway Basin although there has been no production to date. However, there have been recoveries of oil and gas:

• Sawpit 1:
  • 1.5 barrels of oil and 0.5 barrels of mud were recovered on test from open fractures in metavolcanics and low-grade metamorphosed argillite
  • The oil was 35⁰ API gravity at 65⁰ F and 32⁰ API gravity at 77⁰ F
• Kalangadoo 1:
  • Gas flowed at a maximum of 1.55MMcfgd from fractures in turbidites with minor tuff beds (Morton et al, 2002), although none of the tests were of long enough duration for adequate evaluation of the reservoir according to the Well Completion Report
  • The gas consisted of 96% CO2, 2.4% methane and 1.3% nitrogen

There were also weak gas shows with minor C3 and C4 hydrocarbons in metasediments at the top of the Basement in Tilbooroo 1 but attempts to test the interval were unsuccessful due to very bad hole conditions. Two successful DSTs were run deeper in the Basement but the zones were extremely tight and no hydrocarbons were indicated.

Morton et al (2002) reported that “bedding dips 40–80⁰ with no appreciable matrix porosity” in both Sawpit 1 and Kalangadoo 1; “consequently, the reserves for these two wells are probably very small. Open fractures are presumed to parallel mapped Cretaceous and Tertiary extensional fault trends (NW to SE), but other (presumed older) orientations may be closed. No orientated high deviation or horizontal drilling has been attempted for basement reservoirs in the Otway Basin. If matrix porosity were to be located in association with open fractures in the basement, then there is the potential for a significant accumulation, similar to the Palm Valley gasfield in the Northern Territory”.

Download top Basement depth map (JPG 8.6 MB)

The seal to potential fractured Basement reservoirs in the onshore Otway Basin in South Australia consists of shales and siltstones from the Casterton Formation through to the Eumeralla Formation, depending on what the Basement sub-crops. However, within each of these units there may not be a competent seal for the Basement where alluvial fans, river systems or turbidite complexes directly overlie the Basement, in which case these systems would probably have acted as migration paths for hydrocarbons into shallower traps.

Extent of seals for the fractured Basement (JPG 9.9 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. However, the Laira Formation sits stratigraphically above the Casterton Formation, 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 fractured Basement

Fractured Basement may have been charged with gas and/or oil expelled from source rocks in the Casterton Formation, the Lower Sawpit Shale, the Upper Sawpit Shale or the Laira Formation depending upon which formation directly overlies the Basement or is juxtaposed against it across significant faults.

Extents of modelled gas and oil expelled from source rocks that may have charged the fractured Basement:

Download potential charge for fractured Basement map (JPG 5.6 MB)

Play elements for the fractured Basement in the onshore Otway Basin, SA (JPG 9.9 MB)

Play map for the fractured Basement in the onshore Otway Basin, SA (JPG 5.5 MB)

Data from PEPS South Australia:

No estimate has been made to date.

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, 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.