Geoscience Australia has conducted a feasibility analysis on underground gas/hydrogen storage (UGS/UHS) through creating salt caverns in the offshore Polda Basin, South Australia. The Mercury structure in the offshore part of the Polda Basin has massive halite deposits in the Neoproterozoic Kilroo Formation at depth intervals of 1376.8–2383.8 mSS (mSS: depth (m) below mean sea level) (salt pillows), 2383.8–2538.8 mSS and 2807.8–3083.8 mSS (salt interbeds). The halite is distributed over 217 km2 approximately 20 km N-S and approximately 20 km E-W, forming an anticlinal structure near Mercury 1 well. Well data (Mercury 1) suggests the total net thickness of halite is up to 1000 m over the lower Kilroo Formation and 468 m above 2000 mSS, which is within the depth range considered suitable for UHS. The potential storage site analysed is located approximately 50-70 km offshore, west of the Eyre Peninsula and approximately 200 km from Port Lincoln. The lower thermal gradient (cold basin) observed, and overburden and formation fracture gradients, are favourable for salt cavern design. The 1376.8–1539.8 mSS and 1575.6–2367.6 mSS halite intervals in Mercury 1 were identified as potential candidates for cavern creation in the future UHS program. A conceptual design of a halite cavern is presented for the depth range of 1650–2000 mSS. The cylindrical halite cavern is evaluated for two diameters (60 m and 100 m) with the calculated hydrogen storage capacity of approximately 240 GWh and 665 GWh (equivalent to approximately 7200 and 20000 tonnes), respectively. Potentially multiple halite caverns could be built within the thick halite deposits of the Mercury structure. Compared to one of Australia’s non-hydrocarbon energy storage resources currently under construction (Snowy 2.0 hydropower) at 350 GWh, UHS within a halite cavern in the offshore Polda Basin provides an alternative for large-scale energy storage that is potentially quicker to build, has less environmental footprints and is not impacted by drought.

The integrated use of seismic and gravity data can help to assess the potential for underground hydrogen storage in salt caverns in the offshore Polda Basin, South Australia. Geophysical integration software was trialled to perform simultaneous modelling of seismic amplitudes and traveltime information, gravity, and gravity gradients within a 2.5D cross-section. The models were calibrated to existing gravity data, seismic and well logs improving mapping of the salt thickness and depth away from well control. Models included known salt deposits in the offshore parts of the basin and assessed the feasibility for detection of potential salt deposits in the onshore basin, where there is limited well and seismic coverage. The modelling confirms that candidate salt cavern storage sites with salt thicknesses greater than 400-500 m should be detectable on low altitude airborne gravity surveys. Identification of lower cost onshore storage sites will require careful calibration of gravity models against measured data, rather than relying on the observation of rounded anomalies associated with salt diapirism. Ranking of the most prospective storage sites could be optimized after the acquisition of more detailed gravity and gradiometry data, preferably accompanied by seismic reprocessing or new seismic data acquisition.