The role of salt basins in the race to net zero: a focus on Australian basins and key research topics

Energy storage in salt caverns

Salt caverns provide various benefits for storing resources and managing energy supply. Manufactured salt caverns are created by an engineering process known as solution mining, in which fresh or semi-fresh water is pumped into a salt formation to generate partial dissolution. The resulting brine is pumped to the surface to be evaporated and analysed for critical minerals, creating caverns within the subsurface. Salt caverns have unique sealing properties (Ratigan 1995) that are ideal for storing various substances, including crude oil, natural gas, hydrocarbons, chemicals, hydrogen, compressed air, biofuels, and nuclear waste.

Salt deposits are often interlayered with other sedimentary deposits, sometimes deformed into structurally complex folds and faults (Rowan et al. 2019). Because the internal sedimentological and structural characteristics are understudied within salt sequences, their properties and influence on salt caverns are largely unknown. Furthermore, the unique physical and chemical properties of salt that make it excellent for storage also make it difficult to image through geophysical methods. Thus, characterising heterogeneities within the salt succession is challenging (Kernen et al. 2021). Salt heterogeneities are a significant issue in Australian basins, further complicated by poor historical 2D seismic data density and quality. New outcrop and subsurface research within salt sequences and diapirs characterising internal architecture and heterogeneities must be conducted to ensure optimal salt cavern placement (Fig. 1, Table 1).

Fig. 1.

Schematic cross-section through a salt basin depicting undeformed and deformed subsurface strata. Potential cavern locations for energy storage are illustrated in halite (white).

CO₂ storage

Carbon capture and storage (CCS) will be necessary to decarbonise hard-to-abate industrial sectors. Carbon dioxide (CO₂) can be injected and permanently stored within porous sedimentary deposits in the subsurface. The injected CO₂ is initially buoyant and will flow upward and away from the injection point until a trap is reached. The trap containing the CO₂ must be sealed by an overlying impermeable unit, such as shale or salt, so that the CO₂ is contained within the trap, dissolves in the pore water and eventually becomes mineralised. A long migration path from the injection point to the final trap can increase the overall storage capacity by increasing residual trapping efficiency and avoiding pressure build-up.

Salt has a unique rheology in sedimentary basins, deforming as a viscous fluid on geological timescales and generating complex 3D geometries during halokinesis. The associated structural deformation of the sedimentary deposits overlying the salt layer creates a variety of potential trap structures, and salt basins, therefore, host large potential capacities for CO₂ storage (Duffy et al. 2023). Salt deformation also influences the deposition of overlying sediments by creating basinal highs and lows around which depositional systems are redirected, and local erosion of those deposits can occur. This process leads to complex sedimentological heterogeneity near the salt body and therefore affects the flow of CO₂. Additionally, the salt layer itself can make an excellent seal for CO₂ if traps in the subsalt strata are targeted. Further research into the impact of salt tectonics on subsurface fluid flow and seal integrity is required to unlock the great potential for CO₂ storage in salt basins, as highlighted by observations in key Flinders and Willouran Ranges outcrop analogues and subsequent modelling.

Geothermal energy

Geothermal energy can provide an excellent low-carbon fuel or local heating system if exploited cost-effectively. The thermal conductivity of halite is two to four times higher than that of non-evaporitic sediments (Daniilidis and Herber 2017). This means that salt bodies provide paths of low thermal resistance for heat conduction from depth to the surface. As a result, heat is focussed through the salt body rather than surrounding sediments. The focussed heat will re-enter the sediments near the apex of the salt structure so that sediments around the salt apex are warmer than the sediments at the same depth, away from the salt structure. These shallow geothermal anomalies above the crests of salt structures provide accessible heat sources. In 2009, temperatures of 98.4°C were observed at a depth of 1807 m in Treebeard 1A, Flinders Ranges (NS Energy 2009). Treebeard 1A encountered high temperatures much shallower than expected due to the nearby salt bodies. Further research modelling heat flow associated with salt bodies is needed to understand their potential for geothermal energy production in salt basins.

Critical mineral deposits

The relationship between salt diapirs, fluid flow, and base metal mineralisation is widely recognised across the US Gulf Coast (Caesar et al. 2019), Southern Europe (Leach et al. 2006), North Africa (Rddad et al. 2019) and South Australia (Coats and Blissett 1971; Kernen et al. 2020). In the Willouran, Gammon, and Flinders Ranges, diapir-related Mississippi Valley-Type lead-zinc deposits, copper, and gold are distributed around regional-trending faults linking diapirs at or near the salt–sediment interface or within the diapiric matrix occurring as stratabound veins, stockworks, and cavity-filling orebodies. These outcrops provide an opportunity for excellent research case studies for characterising the style and orientation of mineralisation trends (Fig. 2).

Fig. 2.

Copper (green), gold (yellow), lead/zinc (orange), manganese (red), and uranium (white) occurrences (from SARIG). Neoproterozoic–Cambrian stratigraphy (green, blue, purple), sedimentary cover (beige), layered evaporite sequence (light grey), diapiric matrix (dark grey).

Natural hydrogen

Natural hydrogen accumulation in salt basins occurs because halite is a highly effective seal (Hand 2023). There are several different origins for naturally occurring hydrogen in salt basins: (1) early biodegradation of interbedded organic shales or coal where trapped hydrogen is within the crystalline halite or potash fluid inclusions; (2) production by radiolysis associated with sylvite and carnallite (Smetannikov 2011); and (3) migration from an underlying basement source (Hand 2023). Reports of economic hydrogen accumulations are often associated with fossil fuels such as methane (Smetannikov 2011); therefore, we must utilise existing knowledge from the petroleum industry to research natural hydrogen systems. To better understand the economic potential of these scenarios, characterising the outcropping layered evaporite sequences containing sylvite and carnallite in the Willouran and Gammon Ranges will be essential in understanding where those sequences are for any regional-trending faults linking diapirs at or near the salt–sediment interface or within the diapiric matrix.