The intracratonic Polda Basin is an elongate easterly trending trough that extends for at least 350 km from the Eyre Peninsula to the centre of the Great Australian Bight. It ranges from 10 to 40 km in width and has a water depth that varies from 50 to 200 m at the western end of the basin. The Polda Basin contains sediments which range in age from possible Neoproterozoic to Jurassic, overlies the Paleo- to Mesoproterozoic Itiledoo Basin and is overlain by a thin veneer of Tertiary sediments.

See Polda Basin figures 1, 2 and 3

The Polda Basin has been explored for coal, uranium and petroleum. Petroleum exploration focussed on the offshore central Polda Basin following the discovery of the basin by the Shell aeromagnetic survey in 1966. 2D seismic data was acquired by Shell and Bridge Oil, who subsequently identified drilling targets. Gemini 1 was the first petroleum well drilled in 1975 by Outback Oil, while Mercury 1 and Columbia 1 were drilled by Australian Occidental in 1981–82. There has been no exploration carried out since 1982, except for five seismic lines (totalling 125 km within the offshore Polda Basin) obtained as part of a 1986 survey by the Bureau of Mineral Resources (BMR; now Geoscience Australia) and three seismic lines extending into the basin (104 km within the offshore Polda Basin) shot in 1998 by GHD Surveys Pty Ltd. One PEL application is current over the southern portion of the onshore basin and one immediately offshore, and two GSELs cover the whole of the onshore portion of the basin.

The Jurassic Polda Formation contains the Lock Coal Deposit. Coal exploration, centred on the Lock Sub-basin, has delineated 260 Mt of sub-bituminous coal in the measured-indicated category, with a proximate analysis of 26% moisture, 23% ash, 30% volatile matter, and 21% a fixed carbon. The average heat value of 14.6 MJ/kg is typical of South Australian coals.

entX Limited was granted gas storage permits GSEL 781 and 784 in 2023.

The Polda and Itiledoo basins are underlain by a suite of Archean and earliest Proterozoic crystalline basement rocks of the Gawler Craton known as the Sleaford Complex (~2560 – 2470 Ma), which together with the Mulgathing Complex in the northern Gawler Craton was probably part of an adjoining basin now dismembered by Paleoproterozoic tectonism (Gawler Craton | Energy & Mining). The Sleaford Complex contains a diverse range of metamorphosed and deformed rocks, of which granites and iron-rich mafic rocks may form potential natural hydrogen source rocks on Eyre Peninsula.

The Sleaford Orogeny was followed by approximately 400 Ma of tectonic quiescence in the eastern Polda Basin area. Late Paleoproterozoic extension led to widespread basin formation across the Gawler Craton, associated with sedimentation and bimodal magmatism. The Hutchison Supergroup (<2000 – 1770 Ma) comprises quartzite, dolomite, iron formation, schist and amphibolite, deposited in a passive margin setting. The Hutchison Supergroup was subsequently intruded by the Middle granites of the Peter Pan Supersuite (1745 – 1700 Ma) as part of the Kimban Orogeny, also likely to be good potential source rocks for hydrogen.

Juvenile felsic to mafic magmas of the St Peter Suite (1650 – 1610 Ma) occur over the northwest of the onshore Polda area, formed broadly in a continental magmatic arc setting.

Another period of extension occurred in the Polda area at the end of the Paleoproterozoic, leading to localised sedimentation and magmatism in the Itiledoo Basin (named by Flint and Rankin 1991) with a maximum age of approximately 1605 Ma inferred by radiometric dating of detrital zircons (Australian Stratigraphic Units Database). Deposition of sandstone, minor grit and pebble beds forming the Blue Range Beds (an equivalent of the Corunna Conglomerate shown in Figure 3) were reworked products of the Proterozoic and Archean basement so are likely to be a good source for natural hydrogen. The Blue Range Beds are exposed within the Polda Basin at Mount Wedge, approximately 13 km to the southeast of Mucka Cudla 1.

Soon after deposition of the Blue Range Beds the region was intruded by granites of the Hiltaba Suite, thought to be the source of the natural hydrogen encountered on the Yorke Peninsula. No Hiltaba Suite granite has been intersected in the Itiledoo Basin to date, although just to the north of the Polda Basin the granite encountered in Wudinna 1 was dated as 1519 ± 67 Ma, although Flint and Rankin (1991) expect that the true age is more likely to be 1600 – 1580 Ma. No zircons from the Hiltaba granite have been sampled from the Blue Range Beds, although zircons from the St Peter Suite do occur, indicating that the intrusion was post-deposition in the Itiledoo Basin.

The Polda Basin contains sediments which range in age from possibly Neoproterozoic to Jurassic, unconformably overlying the Itiledoo Basin with an age gap of at least 600 million years (Figures 2 and 3). The whole section was penetrated at the onshore drillhole Kilroo 1A or CRA83 KD1A and comprises from the base, the Kilroo Formation, a volcanic, evaporitic, red bed sequence, overlain unconformably by the Permian-Carboniferous glacial Coolardie Formation and again unconformably overlain by the Jurassic Polda Formation. An unconformity separates the Jurassic from unconsolidated siliciclastic Cainozoic above. Figure 3 summarises the basin stratigraphy.

The age of the Kilroo Formation is uncertain, but it is thought to have been deposited during the Neoproterozoic. A recent review of the conventional core from Kilroo 1A (Tiainen 2025) indicates the formation comprises variably red-brown-grey-white interbedded arkoses, siltstones, claystones and at least two basalt flows and minor anhydrite and rare salt. From prior dating (Flint et al. 1988) the lower basalt is dated as Neoproterozoic from three samples and the shallower basalt is dated as Neoproterozoic, Silurian and Triassic from four samples. The sedimentary features observed in the core including mud drapes, bi-directional bedding, syneresis cracks, and possible low diversity bioturbation have been interpreted as indicating deposition on in an inter to supra tidal coastal plain gross depositional environment.  Massive halite with minor interbeds of siltstone, claystone, dolomite and limestone extends for approximately 1700 m was penetrated offshore in Mercury 1 in 1981/2 (Well Completion Report (WCR) and Composite Log available in PEPS).

The Coolardie Formation unconformably overlies the Kilroo Formation and primarily consists of diamictite deposited in a terminoglacial environment. The thickness of the formation is variable, ranging offshore from 37 m at Gemini 1 to 87 m at Mercury 1. Based on palynology (Flint and Rankin 1981) the formation is correlative with deposition during the Gondwana Carboniferous-Permian glaciation.

The overlying Polda Formation is interpreted to have been deposited in an alluvial plain gross depositional environment (GDE) in response to the extensional tectonism that eventually led to the breakup of Gondwana, via reactivation of the deep basement bounding faults. In its type section in SADME Polda 1 (Flint and Rankin 1991) the Polda Formation comprises sandstone and carbonaceous and lignitic claystone around 86 m thick. In the Lock Sub-basin, the unit is divided into three intervals (Gatehouse 1995): a basal dark grey claystone with subangular ‘floating’ sand grains, a middle dark grey to black carbonaceous claystone with lignite, siltstone and fine-grained sandstone interbeds, and an upper dark brown and grey, medium to coarse-grained, angular to subangular and pyritic sandstone. Within the Lock Sub-basin, subbituminous coal seams up to 15 m thick are present. Similar coal seams and other lithotypes were intersected in drilling offshore and in the Kilroo Sub-basin. Coal seams pinch out abruptly towards basin margins to the north and south. In the offshore the formation is subdivided into two intervals, with the lower zone being sand-prone with regular interbeds of coal and siltstone, while the upper zone is dominated by claystone, siltstone and fining upward sandstone. The thickness of the formation ranges from 450–500 m in Columbia 1 and Mercury 1 but is highly variable across the basin.

Sedimentary structures in the Polda Formation include crossbedding, upward-fining sequences, and channelling. Poor sorting in coarser grained sandstone and conglomerate, with better sorting in finer grained intervals, and clay units beneath coal seams and within fine-grained sandstone beds, indicate a fluvial environment. The age of the Polda Formation is late Middle to Late Jurassic based on the presence of palynoflora from the Murospora florida Zone (Gatehouse 1995).

The depositional environment is interpreted as a broad valley with relatively low relief in which streams provided lateral mudflats, oxbow lakes and swamps in which coal accumulated. Lateral streams from the valley sides may have provided coarser grained sediments. The known distribution of the Polda Formation along the length of the graben (400 km) contrasts with a width of only 20–60 km. The recent review of Kilroo 1A confirms similar lithologies, and additionally micro conglomerates and variably pedogenically altered siltstones. Low angle planar cross beds dominate the micro conglomerate and arenite lithologies and support the fluvial plain interpretation. However, the lack of preserved vertical accretion deposits suggest an anastomosing channel habit rather than meandering, at least at this well. The finer fraction, siltstone and variably carbonaceous-coaly claystone represent vertical accretion and floodplain/mire deposits.

Interestingly no Cretaceous sediments comparable to those in the Bight and Duntroon Basins have been intersected in the Polda Basin, as the Polda Formation is overlain by a thin veneer of Cainozoic sediments, including the sequence of unconsolidated sands and claystones with a lignite base of the Eocene Poelpena Formation. The onshore portion of the Polda Basin is subdivided into the Lock Sub-basin and the Kilroo Sub-basin, containing up to 1500–2000 m of Neoproterozoic to Jurassic sediments in the eastern and western ends but only 500 m in the centre.

Reservoirs

Potential reservoirs include clastic sandstones and conglomerates in the Polda, Coolardie and Kilroo Formations. Reservoir potential has been demonstrated to be fair to excellent in several intervals (refer to WCRs in PEPS) and are summarised below.

• Reservoir quality is excellent, particularly in the poorly consolidated porous and permeable Jurassic Polda Formation.
  • The onshore Kilroo 1A drillhole shows oil stained, relatively unconsolidated and coarse-grained lithic to sub-lithic arenites in the Polda Formation, which based on sedimentary structures is interpreted as indicating deposition in an alluvial-delta plain gross depositional environment (Figure 4).
  • Sandstone intervals in Mercury 1 have vertical porosity distribution ranging for 0% to 25%, with a mean of 10.9%.
  • Those in Columbia 1 range from 0.7% to 29.5%, with a mean of 13.2%.
• Reservoirs of variable quality are present in the Permian Coolardie Formation.
• Neoproterozoic Kilroo Formation:
  • Log-derived porosities of up to 19% were recorded from the cleaner sandy intervals in Columbia 1, with thick quartzose sandstones in the basal sequence with porosities in the range 10–15% despite extensive silicification, although SP logs suggest that they might have limited permeability.
  • In Mercury 1 thin sandstone intervals in the top of the Proterozoic section had maximum log-derived porosities of 18%, whereas thick sandstone beds in the basal sequence showed porosities of around 13%.

There has been no petroleum or hydrogen exploration in the release area hence understanding of trapping mechanisms is limited. However, the deep-seated, east-west striking boundary faults of the Polda Basin have probably been active since the Archean, exerting a primary control on basin development during the Mesoproterozoic (Itiledoo Basin) and Neoproterozoic. Reactivation during Mesozoic rifting of Australia’s southern margin from Antarctica have had further impact on trap generation.

Potential plays onshore include unconformity traps at the top of the Kilroo Formation, intra-graben horsts, monoclinal drape features and clastic aprons adjacent to the steep southern boundary fault in particular. Salt-associated traps have been demonstrated in the offshore portion of the Polda Basin and may exist onshore although to a lesser extent due to a likely much thinner salt section.

Offshore, Mercury 1 tested an anticlinal closure above an interpreted halite pillow (Feitz et al. 2022; Figure 2) and may extend inboard to the onshore. Columbia 1 was designed to test a large horst structure in the western sector of the Polda Basin. Gemini 1 was terminated before reaching the target sequence. All three wells were considered to be valid hydrocarbon plays with significant vertical closures.

The deep-seated, east-west striking boundary faults of the Polda Basin have probably been active since the Archean, exerting a primary control on basin development during the Mesoproterozoic (Itiledoo Basin) and Neoproterozoic. They were then reactivated during Mesozoic extension of Australia’s southern margin and have had an impact on generating potential traps. They may have acted as long-lived conduits for circulation of fluids and gases and hence as pathways for any generated hydrocarbons and/or hydrogen to migrate towards potential traps.

Figure 6 shows a prospectivity map for natural hydrogen in South Australia, generated from analysis of potential hydrogen source rocks, potential migration pathways (faults, dykes and permeable lithologies) and known trapping mechanisms such as evaporites, clay, dolerite, and basin structure (Rumi Daruso 2023, shown in Margiono et al. 2024). This suggests that there is potential for natural hydrogen accumulation within portions of the onshore Polda Basin.

See Polda Basin figures 2 and 6

There is no estimate of undiscovered resources.

Figure 7 shows the licence status at the time of publication.

Information on holders of petroleum tenements in South Australia (PDF)

See Polda Basin figure 7

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