Peter W Haines
Geological Survey of Western Australia, 100 Plain Street, East Perth WA 6004, Australia
Bedded salt, including halite and anhydrite, may provide the most effective seal for long term accumulation and trapping of helium (He) and hydrogen (H2), particularly for gases generated by slow processes such as radioactive decay and associated radiolysis of water that may benefit from long accumulation times. While older salt deposits are known locally, the world’s oldest basin-wide bedded salt deposits occur in components of the Centralian Superbasin of Australia, most notably the Officer and Amadeus Basins. Salt deposition began in the Tonian (lower Neoproterozoic, c. 830 Ma) with sub-salt structural traps potentially formed during tectonic activity in the Neoproterozoic and Paleozoic, depending on locality. In the eastern Amadeus Basin, the few wells drilled beneath the Neoproterozoic salt seal encountered mixed gases including anomalous H2 and He. Notably, Jacko Bore 1 (formerly Mt Kitty 1) flowed gas from sub-salt fractured granitic basement with composition of 13% CH4, 4% C2H6, 11% H2, 9% He and 61% N2. Previous studies show that the He has a purely crustal source from radioactive decay of U and Th, while H2 could have multiple origins including radiolysis. No wells have been drilled beneath Tonian salt in the western Amadeus Basin, but outcrop around the basin margin suggests that sub-salt sandstone reservoirs are better developed in the west. Wells drilled to basement beneath salt in the western Officer Basin (GSWA Empress 1A, GSWA Lancer and Kanpa 1A) were not sited on sub-salt traps. However, recent fluid inclusion stratigraphy studies do show anomalous He and H2 from bulk analyses immediately below the salt in these wells. Two wells drilled to basement beneath salt in the South Australian Officer Basin did encounter minor sub-salt gas shows but these were not analysed for He and H2. The salt succession lies directly over basement in some areas, but the presence of sub-salt sandstone reservoir facies is also known, but the distribution poorly understood. Basement beneath the Officer Basin varies by location and can include granitic and mafic igneous rocks, and older sedimentary basins with abundant mafic intrusions of the Warakurna Magmatic Event (c. 1080 Ma). Such pre-Officer basins ultimately overly Neoarchean Yilgarn Craton containing granite and greenstone terranes and banded iron formation. The diversity of basement lithologies could provide opportunities for H2 generation via several mechanisms including redox reactions between water and iron-rich rocks, radiolysis, and over maturation of organic facies in older basins. Considering the age and large extent of the salt seal, the potential for large early sub-salt traps and sub-salt reservoirs, the Officer and Amadeus Basins offer interesting opportunities in the search for H2 and He rich gases. Challenges include remoteness and data sparsity.