Late Permian Evaporite Facies Variation in the Forth Approaches Basin, North Sea: Implications for Hydrogen Storage

Abstract

Hydrogen is expected to play a key role in decarbonizing industry and storing energy from intermittent sources such as wind energy. Underground salt caverns are an attractive target for storage due to their large volumes and effective sealing capacity. Despite ambitious goals to become a world leader in hydrogen, there are no onshore salt basins in Scotland. Therefore, the offshore Forth Approaches Basin (FAB), currently undergoing development of the Seagreen Offshore Wind Farm, could provide a critical storage site. Re-evaluation of petrophysical data from five legacy hydrocarbon wells allowed an updated assessment of the composition and variability of the Late Permian Zechstein evaporite sequence. Well analysis is combined with seismic interpretation to understand the salt bodies and their suitability for solution mining. Three halite formations are identified: (1) the Stassfurt Halite Formation, which has insufficient thickness for solution mining; (2) the Leine Halite Formation, which comprises three subunits with a KCl-dominated unit separating two halite-dominated units; and (3) the Aller Halite Formation, which is only identified in the centre of the FAB. Where halokinesis has occurred, the Leine Halite Formation reaches sufficient thicknesses (>300 m) and purity for salt cavern placement; however, heterogeneities are challenging to predict. Layered evaporites only reach sufficient thickness where the Aller Halite Formation is present and could be developed with the underlying Leine Halite Formation. Heterogeneities can be correlated across wells within the layered sequences, aiding prediction. A strong understanding of evaporite facies distribution is required to ensure that halite bodies are suitable for safe and economic solution mining in the FAB and other salt basins globally.