K-Mg salt distribution in the Zechstein Group of the Northern Permian Basin (UK and Norway)—Interplays with the Southern Permian Basin and implications for salt cavern development

The Zechstein Group in the Northern Permian Basin (UK and south Norway sectors of the North Sea) is subdivided into four halite-rich evaporitic sequences. These sequences contain K-Mg salts, the amount and distribution of which are still poorly constrained. Understanding the lithological variations of the evaporites is important for understanding the syn- to post-salt basin evolution and for predicting the development of salt caverns. We compiled well data to perform intra-salt correlations and to constrain the stratal architecture of the halite-rich units. Our results enable refinement of depositional zones of the Zechstein Group in the Northern Permian Basin with emphasis on the spatial distribution of the K-Mg salt deposits. Our analysis suggests that K-Mg salts were preferentially precipitated in the Forth Approaches Basin and north of the West Central Shelf. This was likely the result of geographic position restricting the direct influx of marine water and early halokinetic movements associated with salt relief that promoted the development of isolated intra-salt minibasins. We then use the revised stratigraphy of the Zechstein Group to propose an evolutionary scenario of the Zechstein Group that considers both the Northern Permian Basin and the Southern Permian Basin and that highlights discrepancies in the bathymetric conditions of halite deposition and the spatial repartition of the K-Mg salts. Finally, our results allow an assessment of the potential risk of finding insoluble deposits or K-Mg salts in bedded salt, salt pillows, or salt diapirs that are otherwise suitable in terms of depth and thickness for the development of salt caverns in the Northern Permian Basin.