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 Permian Zechstein Basin (Fig. 1) is among the largest ancient evaporitic basins in the world as well as being one of the most studied for both industrial and academic purposes (e.g., Allsop et al., 2022; Barnett et al., 2023; Brackenridge et al., 2023; Glennie et al., 2003; Peryt et al., 2010; Stewart and Clark, 1999; Tucker, 1991; Warren, 2016). The Zechstein Basin has been geographically subdivided into two domains separated by the Mid North Sea and the Ringkøbing-Fyn structural highs: the Northern Permian Basin extending between the central UK and south Norway and the Southern Permian Basin extending from the south UK to Poland (Rhys, 1974; Ziegler, 1990) (Fig. 1). Eustatic variations coupled with arid conditions resulted in the development of five large-scale evaporitic cycles in the Zechstein Group (Tucker, 1991). Each cycle includes four main types of lithologies: carbonate, sulfate, halite, and K-Mg salts (polyhalite, carnallite, sylvite, kieserite, or bischofite) (Colter and Reed, 1980; Raith et al., 2017; Tucker, 1991; Warren, 2016). Carbonate and sulfate deposits formed marginal platforms, whereas halite and K-Mg salts accumulated in depocenters during evaporative basin drawdown maxima (Tucker, 1991). Although Zechstein Basin marginal platform deposits have been extensively studied (e.g., Becker and Bechstädt, 2006; Dyjaczyński and Peryt, 2014; Grant et al., 2019; Mawson and Tucker, 2009; Mulholland et al., 2019; Reijers, 2012), the spatial distribution of K-Mg salts relative to halite has yet to be accurately determined. Typically, published stratigraphic charts of the Zechstein Group do not differentiate K-Mg salts and halite (e.g., Glennie et al., 2003; Jackson et al., 2019). Paleogeographic reconstructions that consider the occurrence of K-Mg salts have been produced for the Dutch sector of the Southern Permian Basin (Pichat, 2022) but not for the Northern Permian Basin.

Halite begins to precipitate when the concentration in a brine is 10–12 times that in seawater (Usiglio, 1849). In contrast, K-Mg salts form from hyper-saline brines at concentrations that are more than 70–90 times that in the original seawater, with bischofite as the ultimate salt to precipitate before complete evaporation of seawater (when considering a modern chemical composition of seawater) (Usiglio, 1849). In salt basins, K-Mg salts commonly form in depressions hydrologically isolated from deeper depocenters or in the main depocenters of salt basins during periods of maximum basin desiccation (e.g., García-Veigas et al., 2009; Katz and Starinsky, 2009; Krumgalz et al., 2002). Accordingly, the spatial distribution of K-Mg salts can highlight the former hydrology and paleogeography of the salt basin (Blanc-Valleron and Schuler, 1997; Célini et al., 2024; Garrett, 1996; Pichat et al., 2024; Warren, 2016). K-Mg salts also tend to be less viscous than halite and can therefore influence halokinetic deformation (Luangthip et al., 2017; Raith et al., 2016; Słotwiński et al., 2020). Evaporitic sections rich in K-Mg salts should deform faster and more easily than halite-pure formations (Jackson and Hudec, 2017; Słotwiński et al., 2020). This difference in deformation character is of particular interest for understanding post-salt deformation and minibasin development (e.g., timing of diapir initiation or wavelength of minibasin structures). In addition, understanding the distribution of K-Mg salts is important for drilling operations because the fast-creep properties of K-Mg salts can be problematic for well completion and long-term integrity (Dusseault et al., 2004; Firme et al., 2019; Poiate et al., 2006). Finally, predicting the proportions of K-Mg salts in bedded or diapiric salt is crucial when considering the development of salt caverns for H₂ or natural gas storage (Duffy et al., 2023). Indeed, K-Mg salts can compromise the shape of caverns during leaching operations because they may induce washout structures inside the halite body (Xue et al., 2020). The fast-creep properties of K-Mg salt may also be a risk for the long-term stability of the salt cavern, which would progressively collapse during storage operations (Duffy et al., 2023; Warren, 2016; Xue et al., 2020).

The Northern Permian Basin contains important post-salt hydrocarbon resources and is targeted as a possible area for the development of offshore salt caverns (Royal Society of London, 2023). Numerous exploration wells and seismic surveys have been conducted in the basin, providing ample data to constrain the broad-scale stratal architecture of the Zechstein Group (Brzozowska et al., 2003; Eriksen et al., 2003). This information has been made publicly available and/or published in the UK and Norway (e.g., Brackenridge et al., 2023; Jackson et al., 2019; Marín et al., 2023; Patruno et al., 2015; Taylor, 1998). In this paper, we compile bibliographical data and updated interpretations of intra-salt well logs and seismic sections of the Northern Permian Basin to refine the stratigraphic architecture of the Zechstein Group, with a focus on K-Mg salts. Our work highlights the spatial distribution of K-Mg salts over the Northern Permian Basin and gives insight into the depositional settings and paleogeography of the Zechstein Group. The results also enable a discussion of the interplays between the Northern Permian Basin and the Southern Permian Basin during the different stages of salt deposition. Finally, the maps produced have implications for targeting the best areas for salt cavern development.

The Northern Permian Basin initiated with the collapse of the Variscan orogenic belt, which was controlled by thermal subsidence during the Permo-Carboniferous (Coward et al., 2003) and possible lower crustal metamorphic processes (post-orogenic rock density increase and subsequent volume reduction; Brink, 2005). Orogenic collapse was followed by continental extension with the onset of the break-up of Pangea during the Triassic Period (Glennie and Underhill, 1998; Sørensen et al., 1992; Van Wees et al., 2000). This extension resulted in the reactivation of Variscan fault zones, which controlled the formation of structural lows and highs and which had a significant impact on the thickness and facies variations of Permian and Mesozoic deposits (Coward et al., 2003; Zanella et al., 2003).

From the early to late Permian, the Rotliegend Group was deposited under lacustrine to fluvial and arid continental environments (Glennie et al., 2003). In the late Permian, a global glacio-eustatic sea-level rise and active rifting in the North Sea led to the formation of a seaway between the Permian Basin and the Boreal Sea (Paul, 1995). For 2.8–7 m.y. (Menning et al., 2006; Soto et al., 2017), restricted phreatic to surface marine influxes and arid conditions promoted the precipitation of salt deposits of the Zechstein Group above the Rotliegend Group. Published restored isopach maps of the Zechstein Group in the Northern Permian Basin (Glennie et al., 2003; Taylor, 1998; Smith et al., 1993) highlight the probable occurrence of two main depocenters, one on each side of the Central Graben, where as much as 1.5 km of salt may have initially been deposited (Fig. 2A).

After salt deposition, Triassic extensional events triggered halokinesis of the Zechstein evaporites, initiating development of diapiric structures and related salt minibasins, especially over the marginal part of the Northern Permian Basin and around structural highs in the Central Graben (Høiland et al., 1993; Stewart and Clark, 1999) (Figs. 3A and 3B; Fig. 4, steps 1–3). The thickness of the minibasins was directly controlled by the initial halite thickness (Høiland et al., 1993; Jackson et al., 2018). Thin minibasins associated with initially thin salt deposits have been reported above structural highs in the Central Graben (Figs. 3A and 3B) (Gowers et al., 1993; Smith et al., 1993; Stewart, 1993). An erosional phase occurred at the end of the Triassic that resulted in a significant loss of the initial salt volume (as much as 30% according to Bishop [1996]) (Fig. 4, step 3). During the Middle Jurassic, thick-skinned extension renewed halokinesis where the Zechstein evaporites were thick (Høiland et al., 1993; Stewart and Clark, 1999; Ziegler, 1981) (Fig. 4, step 4). In addition, rifting accentuated the relief of structural highs in the Central Graben and sustained the dissolution of salt due to continued subaerial exposure (Høiland et al., 1993; Goldsmith et al., 1995). Cretaceous and Tertiary marine carbonate and siliciclastic strata were deposited during several shortening events that led to diachronous basement inversion over the basin (Pegrum, 1984; Stewart and Clark, 1999) (Fig. 4, step 5). Cenozoic deposits have sealed most of the faults and diapirs in the basin, but Tertiary contractional events locally rejuvenated deeply rooted diapirs in the Central Graben area, resulting in diapirism that occurred until the mid-Miocene (Davison et al., 2000; Stewart and Clark, 1999).

Eustatic variations influenced the deposition of the Zechstein Group, dividing it into four main evaporite cycles named Z1 to Z4 and a minor one named Z5 (Fig. 5) (Cameron, 1993; Glennie et al., 2003; Stemmerik, 2000; Taylor, 1998). Each cycle may include shale, carbonate, sulfate, halite, and K-Mg salts. At the base of Z1, Z3, and Z4, three shale horizons (T1, T3, and T4, respectively), deposited in deep marine anoxic conditions, mark maximum flooding surfaces (Tucker, 1991). In Z1, Z2, and Z3, carbonate strata formed marginal platforms (Ca1, Ca2, and Ca3, respectively) deposited as high-stand systems tracts in open marine conditions (Fig. 5) (Tucker, 1991). During marine regressions (low-stand systems tracts), increased basin restriction and salinity enabled the precipitation of sulfate platforms (A1, A2, A3, and A4), which prograded over the carbonate platforms in Z1, Z2, and Z3 and over basal shale in Z4, respectively (Taylor, 1998). Basinward, sulfate deposits were restricted in terms of thickness and preservation due to lower precipitation rate and bacterial sulfate reduction processes in an anoxic deepwater environment (Garland et al., 2023; van de Sande et al., 1996). With the exception of Z1, sulfates in each cycle were capped by halite units followed by K-Mg salts, which developed in depocenters under maximal drawdown conditions likely linked to the lowest eustatic levels (Na2, Na3, and Na4) (Tucker, 1991) (Fig. 5). The Zechstein Group ends with the Z5, which records anhydrite and localized occurrences of halite interlayered with lacustrine to fluvial shale and sandstones (Brackenridge et al., 2023; Smith, 1980). This Z5 sequence records the final infill of the salt basin and gradual transition to a continental environment (Glennie et al., 2003).

To characterize the spatial composition of the Z ed along the echstein Group over the Northern Permian Basin, Clark et al. (1998) subdivided the basin into four depositional zones (DZ1 to DZ4), which were later updated by Jackson et al. (2019) in the Norwegian Basin (Fig. 5). DZ1 includes less than 10% halite plus K-Mg salts and characterizes basin margins (Clark et al., 1998). DZ2 and DZ3 respectively include 10%–50% and 50%–80% halite plus K-Mg salts and characterize the basinward-dipping ramp (Clark et al., 1998). DZ4 includes more than 80% halite plus K-Mg salts and characterizes the center of the basin (Clark et al., 1998) (Fig. 5).

Intra-salt minibasins have been describ West Central Shelf (Clark et al., 1998; Joffe et al., 2023; Stewart and Clark, 1999). These minibasins developed above the Z2 halite (Na2) due to differential subsidence caused by lateral lithological variations and/or a regional slope at the base of the salt (Fig. 2B, 3C, and 3E) (Clark et al., 1998; Joffe et al., 2023; Stewart and Clark, 1999). During the deposition of Z3, an early folding also affected evaporite deposition in the Ling Depression in Norway (Marín et al., 2023) and the Forth Approaches Basin in the UK (Cartwright et al., 2001).

Our analysis and data synthesis are based on w s, seismic data, structural m ireline logs, well report aps, and isopach maps. We examined 55 wells drilled in the UK and 24 wells drilled in Norway using the Facies Map Browser (FMB4.0) database provided by the TGS company (Asker, Norway, https://www.tgs.com/seismic/multi-client/subsurface-interpretation/facies-map-browser). Sixty-eight of these intersect the entire Zechstein Group, whereas 11 intersect more than 200 m of salt in diapiric structures (Fig. 2B). The FMB4.0 database provides first-order interpretations of the Zechstein lithologies, which we refined or modified using wireline logs including gamma ray (GR), caliper (CALI), density (RHOB), sonic (DT), and neutron porosity (NPHI) logs. Our lithologic interpretations also considered the descriptions of cuttings produced during well drilling operations and reported in well reports. We carried out wireline log data compilation and intra-salt lithological interpretation using Petrel software (https://www.slb.com/products-and-services/delivering-digital-at-scale/software/petrel-subsurface-software/petrel). We also used a series of two-dimensional (2-D) seismic lines (from TGS and Western Geco Company [Crawley, United Kingdom] surveys) and a three-dimensional (3-D) seismic block (NNS CNS MegaSurvey from TGS Company) covering most of the offshore domain to identify wells located in bedded evaporites and those in diapiric structures or intra-salt minibasins (Figs. 2B and 3).

After an initial interpretation of all borehole intra-salt lithologies, we carried out well-by-well stratal correlations. Identification of the base of the Z2 and Z3 cycles was achieved with relatively high confidence using regionally correlative carbonate and/or anhydrite markers (e.g., Barnett et al., 2023; Brackenridge et al., 2023; Geluk, 2007). Identification of the base of the Z4 and Z5 cycles was less certain and relies on regionally correlative shale-rich intervals coupled with the presence of thin anhydrite beds highlighting the main flooding events (Brackenridge et al., 2023; Houghton et al., 2024; Tucker, 1991).

Given the intense halokinetic deformation that has affected the Zechstein evaporites, we could not consider the Zechstein intervals with thick halite deposits to be strictly undeformed. However, we did identify wells where internal Zechstein stratigraphy was estimated to be well preserved (i.e., with the expected succession of shale, carbonate, anhydrite, and halite units composing the Z2 to Z4 evaporitic sequences), either across the entire Zechstein Group or in specific intervals. During this identification, we considered: (1) the reference stratigraphic sequence established by the bibliographic review in the UK and Norway and extended from well to well, (2) anomalous symmetric stratigraphic intervals suggesting folded strata (Hirlemann, 1993), and (3) seismic data allowing the characterization of halokinetic deformation.

By calculating the proportions of the different salts reported in each well studied, we updated and completed the Zechstein depositional zones of Jackson et al. (2019). In light of preceding research on the Zechstein Group, we did not consider polyhalite as a primary mineral but rather as a diagenetic salt byproduct of anhydrite that had undergone transformation due to the flushing of K-Mg-rich brines (Colter and Reed, 1980; Peryt et al., 1998, 2005). Accordingly, we assigned polyhalite to anhydrite when the proportion of the various salt types identified by wireline interpretations was calculated. Where well data were absent, we approximately assigned the depositional zone of the evaporites using 2-D and 3-D seismic surveys based on: (1) the thickness of the Zechstein Group, which is assumed to increase from DZ1 to DZ4; (2) intra-salt seismic reflections, given that carbonate or anhydrite embedded in halite is commonly tied to high-amplitude reflectors under DZ1 to DZ3; and (3) the presence of diapirism and associated minibasins filled with post-salt deposits, given that halokinetic structures consequently developed in thick halite units within DZ3 and DZ4. Post-salt dissolution above structural highs in the Central Graben, such as the Forties-Montrose High (Fig. 2) (e.g., Høiland et al., 1993), resulted in the disappearance of most of the salts that were initially present. However, seismic profiles still show evidence of Triassic minibasins and collapse structures, indicating the former presence of salt and allowing for an assessment of its initial amount (Fig. 3B). Accordingly, we used seismic interpretation and the thickness of insoluble carbonate-anhydrite platform deposits found in some wells above structural highs to assess the depositional zone of the Zechstein prior to salt dissolution. Salt dissolution was also previously demonstrated along a western channel that crosscuts the Mid North Sea High (Jenyon's Channel; Jenyon et al., 1984). There, the former spatial extension of halite was estimated using the restoration approximation proposed by Jenyon et al. (1984). Finally, around the Mid North Sea High, the studies of Mulholland et al. (2019), Patruno et al. (2018), and Browning-Stamp et al. (2023) were considered to help in interpreting the well data and mapping the spatial distribution of the carbonate-anhydrite platform and thick salt units.

From the well data, we identified 11 main lithologies in the Zechstein Group: shale, marl, dolostone, sandstone to siltstone, anhydrite, halite, polyhalite, carnallite, bischofite, and sylvite, the latter four being the identified K-Mg salts. In the UK, the stratigraphy of the Zechstein sequences in halite-rich units can be correlated in several representative wells, including those initially crossing anticlinal-shaped diapirs (Figs. 6–9). In Norway, only three wells are reported in which the halite-rich units could be considered as relevant for stratigraphic correlation within moderately deformed evaporites (Figs. 8 and 9). We hereafter summarize the lithologic architecture of Zechstein Group units Na2 to Na4.

Where Z2 could be identified, Na2 has a thickness ranging from 80 to 320 m. The maximum thickness of Na2 was reported north of the Ling Depression (Norway; Fig. 2) in well-bedded evaporites and far from the main depocenters of the Zechstein Group. In the UK, Na2 commonly begins with a basal unit that is 10–60 m thick (lying atop the A2 unit), composed of thin alternating layers of halite, anhydrite, and polyhalite (Figs. 7 and 8). Up section, well interpretations suggest that the halite may be interstratified with two main intervals rich in K-Mg salts. The first one, observed locally in the UK only, is mixed with shale in the middle part of Na2 (Figs. 6 and 7). The second, observable in the UK and locally in Norway, is thicker and marks the top of Na2 (Figs. 7–9).

We have interpreted a bischofite-dominated interval to the south of the West Central Shelf (well UK 29-27-1; Fig. 6), which was previously interpreted as tachyhydrite dominated by Taylor (1993). Our reinterpretation is based on density log readings of ~1.58 g/cm³ (bischofite density = 1.56 g/cm³, tachyhydrite density = 1.67 g/cm³). Bischofite is the final salt to precipitate from the evaporation of SO₄-rich seawater such as that found in the Permian ocean (Weldeghebriel et al., 2022). Tachyhydrite may be mixed with bischofite (e.g., Pichat et al., 2024), which may explain RHOB values higher than 1.56 g/cm³. However, tachyhydrite can only be of hydrothermal origin in the Zechstein Basin, given that it requires a large excess of Ca-rich brines to form (Warren, 2016). Tachyhydrite precipitation might have been during or after bischofite deposition (Houghton et al., 2024; Keith et al., 2018; Taylor, 1993). The presence of bischofite is also strongly inferred in the Forth Approaches Basin at the top of Z2. This is supported by the caliper records of a major washout interval that is laterally correlative in at least three wells (UK 20-19-1, UK 23-12-1, and UK 26-14-1; Figs. 7–9) and by the presence of Mg-rich fluids in some of these washout intervals as reported in well reports. Bischofite is never found in cuttings due to its very high solubility, and its presence is classically inferred from significant washouts, such as in the Congo and Gabon Basins (Gindre-Chanu et al., 2022; Hirlemann and Jaillard, 1993; Pichat et al., 2024). Accordingly, we assigned bischofite layers, likely also containing carnallite, to the top of Z2 in the Forth Approaches Basin.

Na3 is the thickest halite accumulation in the Central North Sea basin, reaching more than 750 m in thickness in some representative wells of the UK and Norway (Figs. 6 and 8). South of the West Central Shelf and in Jenyon's Channel and the few representative wells of the Norwegian Basin, Na3 is composed almost entirely of halite (Fig. 6). Elsewhere in the UK, it exhibits a wide range of lithologies, including polyhalite (which is found mainly just above the basal anhydrite platform), anhydrite, and several intervals rich in K-Mg salts, with significant lithological and thickness variations between wells (Figs. 7 and 8). The thickest and most correlative interval rich in K-Mg salts in the UK is located at the top of Na3. In the Forth Approaches Basin and to the north of the West Central Shelf, lateral lithology variations are significant, and large correlative washouts are once again interpreted as characterizing bischofite-rich intervals, more or less mixed or interlayered with carnallite or halite according to RHOB and GR signatures (Fig. 8). There, shale and sandstone deposits interlayered with halite are also commonly observed (Figs. 7–9).

In moderately deformed areas, the Na4 layer is 20–200 m thick and widespread over the Forth Approaches Basin. The halite tends to have a high content of clastic deposits and anhydrite beds, with thin and sparse K-Mg salt layers. An exception arises for well NO 17-4-1 (Norway), which displays a relatively high content of K-Mg salt in the Na4 layer.

Our updated map of the Zechstein depositional zones (Fig. 10) complements the previous version of the map (Jackson et al., 2019) in the Central Graben area on structural highs and along the southern part of the basin. Above the Forties-Montrose, Jæren, and Mandal structural highs, evidence of Triassic minibasin development was reported (Figs. 3B and 3D) (Stewart and Clark, 1999). There, several wells intersecting the Zechstein Group display shale, mudstone, and anhydrite layers (Fig. 9; UK 22-22a-7Z and UK 22-23b-4RE). As per previous publications (Goldsmith et al., 1995; Høiland et al., 1993; Stewart and Clark, 1999), this is interpreted as resulting from salt dissolution during the Jurassic and Cretaceous emersion phases of the highs. Based on the Triassic halokinetic structures above these highs and the relative thickness of insoluble residue reported in the wells, we propose that the depositional zones range between DZ2 and DZ3, as previously suggested by Stewart and Clark (1999) and Jackson et al. (2019) (Fig. 10). Above the Jæren High, the DZ3 assignment is supported by well NO 6-3-2, which penetrates a halite-rich salt wall (Fig. 9). Southward, the map (Fig. 10) highlights the western salt-filled Jenyon's Channel, which connects the Northern Permian Basin with the Southern Permian Basin (Day et al., 1981; Jenyon et al., 1984; Ziegler, 1975). The absence of halite deposits from the Grensen Nose to the rest of the Mid North Sea High suggests a depositional setting that was dominated by marginal carbonate and anhydrite in DZ1 (Fig. 10), as previously demonstrated (Browning-Stamp et al., 2023; Mulholland et al., 2019; Patruno et al., 2018). Although it is likely that previous halite deposits were dissolved during Jurassic to Cretaceous uplift events, the absence of halokinetic deformation suggests that the depositional zone has remained relatively free of mobile salts (see seismic lines in Patruno et al. [2018]).

The lithological pie charts on the map (Fig. 10) s how the proportion of K-Mg salts in each studied well, allowing for an initial assessment of their spatial distribution relative to halite. The charts indicate that K-Mg salts are relatively scarce in the southwestern domains of the Northern Permian Basin (UK; Fig. 10), with the exception of well UK 29-27-1, which reached a bischofite depocenter in Z2. In contrast, the northern part of the West Central Shelf and the Forth Approaches Basin have a relatively high proportion of K-Mg salts, reaching as much as 26% (Fig. 10). Elsewhere (particularly in Norway), well data are less representative due to incomplete drilling in diapiric structures, and K-Mg salt proportions range between 0% and 20% without any distinguishable spatial distribution pattern.

None of the representative wells studied in the Northern Permian Basin display K-Mg salt intervals with well-defined regional correlations. This suggests that K-Mg salt deposition occurred in both the Na2 and Na3 units in localized depocenters (tectonic or depositional) of the salt basin, likely under shallow depositional settings during periods of basin drawdown. In the main regional depocenters (West Central Graben and Norwegian Basin), well data are lacking, but it is reasonable to expect that wider and thicker K-Mg salt depocenters could have developed there during periods of maximum desiccation.

In Na3, the absence of K-Mg salts along the West Central Shelf and Jenyon's Channel (Fig. 10) suggests that the hydrographic conditions were not conducive to retaining hyper-saline brines and related K-Mg salt saturation during maximum drawdown conditions, most likely because the area formed a topographic high away from the main Northern Permian Basin depocenters (Fig. 2A). In Na2 and Na3, the proportion of K-Mg salt is particularly significant around the West Central Graben depocenter and in the Forth Approaches Basin (Fig. 10). Hyper-saline conditions in these areas were likely promoted by the structural architecture of the Central North Sea. Indeed, during drawdown conditions, marine-water influxes reaching the Northern Permian Basin through the Viking Graben probably had to bypass the Forties-Montrose High before reaching the Forth Approaches Basin (Fig. 10). This flow pattern likely restricted seawater influxes in the area and favored the pre-concentration of the inflowing brines through evaporation. However, it should be noted that this area was also affected by early halokinetic movements during Z3 deposition (Cartwright et al., 2001; Clark et al., 1998). The syn–salt deposition early buckling and flowing that occurred during Z3 likely created confined shallow-water hydrological conditions in minibasins or synclines, resulting in higher salinity ranges and K-Mg salt precipitation, such as proposed in the Santos Basin (Brazil) (Célini et al., 2024). Extreme salinity conditions in the area may have been intensified by salt recycling processes, which involve the dissolution of diapiric salt and reprecipitation in minibasin depocenters (Pichat et al., 2018; Warren, 2008). In the area south of Devil's Hole High (Fig. 10), the intra-Zechstein minibasins were interpreted to be anhydrite rich based on strong seismic impedance contrasts in the minibasins (Joffe et al., 2023), which is supported further south by well UK 20-19-1, which intersects an anhydrite-rich minibasin (see Clark et al., 1998; Figs. 3C and 8). However, well UK 20-15-1 (Figs. 3E and 8) demonstrates that intra-salt minibasins may also contain significant amounts of K-Mg salts. These salts can induce strong seismic impedance contrast when interbedded with halite and anhydrite (Teixeira and Lupinacci, 2019), and the presence of minibasins rich in K-Mg salt over the West Central Shelf is thus also likely.

Finally, the early halokinetic movements that prevailed in the Forth Approaches Basin and in the West Central Shelf (Fig. 2B) must have promoted the development of separated sub-basins with independent flooding and desiccation events (Brackenridge et al., 2023). This segmented paleogeography likely explains the high lithological variations in the area, with high siliciclastic and shale content in the halite of sub-basins recording fluvial flooding events against high K-Mg salt content in hydrographically more isolated sub-basins (Brackenridge et al., 2023). The siliciclastic deposits were likely sourced by the same sedimentary systems as those of the Bosies Bank Formation, a continental formation coeval with the Zechstein Group that has been identified in the nearby Moray Firth Basin (Fig. 2) (Cameron, 1993; Glennie et al., 2003).

Four sub-zones (SZ1 to SZ4) can be defined based on well data interpretations and previous discussion, representing areas with varying proportions of K-Mg salts in the Zechstein Group (Fig. 11). In the Norwegian Basin, considering the limited number of wells penetrating undeformed evaporite series, the spatial distribution of the sub-zones remains more speculative than in the UK side. Accordingly, the proposed K-Mg salt distribution in Norway relies mainly on the depocenter configuration of the salt basin to date, as published in Glennie et al. (2003), considering that in a drawdown scenario for each evaporite cycle, the highest proportions of K-Mg salts should be at first order preferentially preserved in the main basin depocenters.

In SZ1, K-Mg salt layers are either absent or very minor (<5% of the Zechstein Group). This sub-zone is present mostly in the southern domain of the West Central Shelf and in the western salt-filled Jenyon's Channel (Fig. 11). SZ1 likely represents areas that preferentially emerged during maximum drawdown periods and thus did not specifically allow K-Mg salt accumulations. In SZ2, the K-Mg salts account for ~5%–10% of the Zechstein Group. This sub-zone is interpreted as characterizing large areas where the K-Mg salts precipitated in a flat depositional setting without well-defined depocenters and thus without significant accumulations. SZ2 typically surrounds SZ3, where the occurrence of K-Mg salts should be systematic and in proportion potentially as high as 20%. SZ3 is interpreted to represent the main depocenters of the Central North Sea, i.e., the West Central Graben, as supported by a few well data in diapiric structures (Fig. 10), and the central part of the Norwegian Basin where well data are unfortunately lacking (Fig. 11). Finally, SZ4 defines a restricted area in the northern part of the West Central Shelf and the Forth Approaches Basin (Fig. 11). Well data suggest that the proportion of K-Mg salts may exceed 20% in this area because of the hydrographic isolation induced by geographic position and early halokinetic deformation, as previously discussed above (in the section Depositional and Hydrological Conditions of K-Mg Salts in the Northern Permian Basin).

Previous studies have highlighted that the Northern and Southern Permian Basins exhibit different stratigraphic architectures in terms of salt thickness in the Z2 and Z3 evaporite cycles (Taylor, 1993). Recently, Houghton et al. (2024) proposed a model illustrating the joint evolution of the Northern Permian Basin and the Southern Permian Basin during Z2 and Z3. However, this model does not consider the distribution of K-Mg salts in both basins, nor does it differentiate between polyhalite deposits and other K-Mg salts (carnallite, kieserite, or bischofite). Indeed, polyhalite is commonly described as a secondary mineral after anhydrite replacement (Colter and Reed, 1980; Peryt et al., 1998, 2005) but can also highlight primary deposits developed during freshening events (Biehl et al., 2014; Colter and Reed, 1980). In contrast, other K-Mg salts are usually tied to primary precipitates in higher salinity ranges during basin drawdown phases. Accordingly, based on our review in the Northern Permian Basin and previous studies in the Southern Permian Basin (Cameron et al., 1992; Colter and Reed, 1980; Geluk, 2007; Pichat, 2022; Taylor, 1998), we propose an update of the evolution model of the Zechstein Basin published by Houghton et al. (2024), considering a cross section extending from the UK platform in the Northern Permian Basin to the Dutch Platform in the Southern Permian Basin (Fig. 12).

In the Southern Permian Basin, more than 600 m of salt deposits accumulated during Na2. There, Na2 records two main sub-cycles that end with K-Mg salts (Pichat, 2022). During the first sub-cycle, halite precipitation began in a deepwater setting with the formation of a halite platform along continental shores due to a brine salinity gradient from north to south (Fig. 12, step 2). Polyhalite layers reported at the base of this halite unit were most likely deposited during the transition stage between sulfate and halite saturation phases and may record freshening events. Polyhalite wedge shapes locally reported along the southern side of the Mid North Sea High platform (Garland et al., 2023) suggest that primary anhydrite to polyhalite deposits kept prograding over the margin platform. Pichat (2022) also proposed that part of these polyhalite layers in the basin center formed from anhydrite turbidites having reworked margin sulfate platforms (platform A2). After the halite saturation phase, the first K-Mg salts precipitated within the Southern Permian Basin main depocenter as the basin experienced a basin drawdown (Pichat, 2022) (Fig. 12, step 3). In the second sub-cycle of Na2, halite precipitation filled and flattened the Southern Permian Basin, and K-Mg salts developed over a widespread area, under shallow-water to subaerial conditions (Fig. 12, step 4) (Geluk, 2007; Peryt et al., 2010).

In the Northern Permian Basin, well data suggest that Na2 was at least half as thin as in the Southern Permian Basin (Fig. 12, steps 1 and 2). The two main K-Mg salt units found in Na2 of the Northern Permian Basin may be time equivalent to these found in Na2 of the Southern Permian Basin. However, unlike in the Southern Permian Basin, the first K-Mg salt unit is very thin and appears to be limited to the marginal areas of the Northern Permian Basin given that it was not possible to highlight stratigraphically equivalent K-Mg salt in the deeper part of the Northern Permian Basin. This suggests that the Northern Permian Basin did not experience the same degree of salinity or basin drawdown as the Southern Permian Basin during the first Na2 sub-cycle. According to this interpretation, a relatively deep subaqueous setting might have prevailed in the basin center of the Northern Permian Basin during the first Na2 sub-cycle with salinities maintained under the halite-saturation phase. K-Mg salt precipitation was favored only along the marginal part of the basin, where isolated shallow-water depocenters could develop under an evaporitic salina shelf (Fig. 12, step 3). This interpretation also suggests a difference in the brine level between the Northern Permian Basin and the Southern Permian Basin and, therefore, a highly reduced hydrological connection between the two basins at this stage of Zechstein deposition, such as documented by Houghton et al. (2024) south of Jenyon's Channel.

For the second Na2 sub-cycle, it is not possible to determine if the Northern Permian Basin was entirely filled and flattened by halite in its center, as was the case in the Southern Permian Basin. The basin center might have remained underfilled and within deepwater conditions, as suggested by Houghton et al. (2024). However, K-Mg salt deposits atop Z2 in several wells from the West Central Shelf to the depocenter of the West Central Graben (e.g., well UK 21-24-1, Fig. 9) may favor the hypothesis of a significant drawdown phase at the end of Z2 on a flattened basin (Fig. 12, step 4).

Finally, the overall lower thickness of Na2 in the Northern Permian Basin compared to the Southern Permian Basin may have resulted from two factors: lower accommodation space initially available in the Northern Permian Basin and/or lower salinity conditions having induced lower salt precipitation rates. Indeed, as highlighted by Houghton et al. (2024), given the linkage between the Northern Permian Basin and the Viking Graben, it is likely that direct seawater inflows promoted lower salinity conditions in the Northern Permian Basin relative to the Southern Permian Basin. This, in turn, would have resulted in a reduced rate of salt precipitation in the Northern Permian Basin, which would have concomitantly contributed to limiting the subsidence rate of the Northern Permian Basin in comparison to that of the Southern Permian Basin (Houghton et al., 2024; van den Belt and de Boer, 2007).

In the Southern Permian Basin, Na3 is ~400 m thick and composed of two sub-cycles that end with K-Mg salts. In the first sub-cycle, a basal halite unit flattened the basin topography, and K-Mg salts formed successively in restricted depocenters, mostly in a marginal domain isolated from marine or other freshwater influxes (Fig. 12, step 6) (Pichat, 2022). During the second sub-cycle, halite and K-Mg salt were deposited in a wide and flat shallow-water environment, with possible local influence of early halokinetic movements (Fig. 12, step 7) (Barabasch et al., 2019; Pichat, 2022; Raith et al., 2016).

In the Northern Permian Basin, well data indicate that Na3 is ~200 m thicker than in the basin center of the Southern Permian Basin (Fig. 12, step 5). This cannot be explained by a higher isostatic compensation after the Na2 accumulation, given that Na2 was thicker in the Southern Permian Basin compared to the Northern Permian Basin (van den Belt and de Boer, 2014). In the early stages of Z3, a higher basin bathymetry in the Northern Permian Basin is suggested by the thickness differences of the Ca3-A3 unit in the basin centers, which range from 30 to 70 m in the Southern Permian Basin compared to ~10–40 m in the Northern Permian Basin. Indeed, less-developed carbonate-anhydrite deposits in the Northern Permian Basin may have resulted from a deeper-water setting that prevented both the development of carbonates and the preservation of sulfate precipitates (due to bacterial sulfate reduction processes in deep anoxic conditions) (Fig. 12, step 5). The higher bathymetry of the Northern Permian Basin during the early stages of Z3 may have been due to the lower accumulation of salt during Z2, in comparison to the Southern Permian Basin, which possibly left the Northern Permian Basin underfilled at the end of Z2. Alternatively, active extension occurred in the Central Graben during the early phases of Z3, linked to normal fault movements and a regional tilt of the margins (Fig. 12, step 5). Such basement movements were inferred along the east of the Devil's Horst High to explain the down-dip salt evacuation of Z2 and related intra-salt minibasin development reported in Z3 by Joffe et al. (2023) and Clark et al. (1998). With a deeper depositional setting in the Northern Permian Basin, it is likely that the filling and flattening of the basin by halite during the early phase of Na3 lasted longer in the Northern Permian Basin than in the Southern Permian Basin. Accordingly, it is inferred that during the first sub-cycle of Na3, K-Mg salts could form on a flattened basin in the Southern Permian Basin, while in the Northern Permian Basin they could only form in marginal domains of the basin in isolated ponds probably influenced by early halokinetic movements, as previously discussed (in the section Depositional and Hydrological Conditions of K-Mg Salts in the Northern Permian Basin) regarding the Forth Approaches Basin (Fig. 12, step 6). The Northern Permian Basin could experience regional accumulation of K-Mg salts after the basin was flattened, likely contemporaneous with the second sub-cycle in the Southern Permian Basin (Fig. 12, step 7).

In the Southern Permian Basin, Na4 is ~70 m thick cludes multiple layers of K-Mg salts. The basin w and in as flat, and a shallow depositional setting favored significant lateral lithology variations (Pichat, 2022) (Fig. 12, step 7). In the Northern Permian Basin, Na4 appears to be locally ~100 m thicker than in the Southern Permian Basin, indicating that basin subsidence was still more active in the Northern Permian Basin during Z4. The depositional setting may thus have been deeper than this in the Southern Permian Basin, during the early stage of the Z4 infill at least. A higher bathymetry would have delayed the occurrence of extreme salinities, which may explain why Na4 is overall less carnallite rich in the Northern Permian Basin compared to what was shown in the Southern Permian Basin (Pichat, 2022) (Fig. 12). The change in the composition of the brine on its way from the Northern to the Southern Permian Basin likely also explains the higher potash content in the Southern Permian Basin. Finally, the minibasins that developed in the marginal domains of the Northern Permian Basin during Na3 may have still been active during Na4. This is suggested by the relatively thick Z4 intervals reported in the core of minibasins (Fig. 12, step 7).

Figure 13 displays a map with the depositional zones and sub-zones of the Northern Permian Basin superposed by areas of bedded evaporites, salt diapirs, and salt pillows that have a top of the Zechstein Group that does not exceed a depth of 1700 m and a salt thickness greater than 200 m, these two requirements being important for developing salt caverns (Caglayan et al., 2020; Warren, 2016). The map highlights that the West Central Shelf and the Forth Approaches Basin have a large area of bedded salt along with several diapiric and pillow structures. In contrast, diapirs of interest are more scattered elsewhere.

To ensure the development of a salt cavern with an ideal stable capsule shape during leaching operations, it is also important to have a pure halite salt composition (e.g., Wang et al., 2013; Xue et al., 2020). Accordingly, the depositional zone and sub-zone areas respectively enable one to assess the risk of encountering insoluble carbonate-anhydrite deposits and hyper-soluble K-Mg salts in eligible salt structures. At first order, the map highlights that the optimal location for salt cavern development is in the area extending south of the West Central Shelf, north of the Grensen Nose and in Jenyon's Channel, where diapiric structures or bedded evaporites are present in both DZ4 and SZ1 (Fig. 13). Conversely, the Forth Approaches Basin poses a higher risk due to the presence of SZ4, which predicts a high content of K-Mg salts, the hyper-solubility of which should promote the development of undesirable enlargements in the salt cavern (e.g., Wilke et al., 2001; Warren, 2016). However, it could be argued that some of the main diapiric structures border intra-salt minibasins, which may contain the main concentrations of insoluble and hyper-soluble materials. Therefore, it is possible that some of the syn-salt diapiric structures have a purer halite composition than the bordering intra-Zechstein minibasins.

As previously discussed by several authors (e.g., Brackenridge et al., 2023; Duffy et al., 2023; Marín et al., 2023), the main challenge in diapiric structures is predicting intra-salt deformation where the halite is not pure. Indeed, modern geophysical tools are not able to well image the very complex intra-salt architectures in diapirs (e.g., Barnett et al., 2023; Marín et al., 2023). South of the West Central Shelf, well data highlight that three anticlinal-shaped diapirs are dominantly composed of Z3, whereas Z2 remains relatively thin in the basal part of the diapir (Fig. 14). This observation indicates that the Z2 halite may be too thin to contribute to the central inflation of the diapirs, which is entirely accommodated by Z3. Based on this pattern, it is likely that the anticlinal-shaped diapirs in the Central North Sea are mainly composed of Z3. However, it is possible that this pattern differs in the deepest parts of the basin, where Z2 salts may be locally thicker and thus were more involved in vertical deformation (Richter-Bernburg, 1987; Bornemann et al., 2008; Strozyk et al., 2014). This is suggested for instance by wells NO 7-3-1 and NO 8-10-3, which cross diapiric structures near the Central Graben and in which Z2 could not be identified at the basal part of the salt (Fig. 9; see Marín et al., 2023, their figures 7c and 7d, for seismic sections). Finally, it should be noted that compared to the Southern Permian Basin, the lower thickness of the carbonate-anhydrite stringer in Z3 and its position in the lower section of the Zechstein Group should reduce the risk of encountering stringer fragments in diapiric structures of the Northern Permian Basin.

Using previously published seismic and well data, we reviewed the stratigraphy and depositional setting of the evaporitic Zechstein Group in the Northern Permian Basin, with emphasis on the halite and K-Mg salt deposits. The results have enabled an updated depositional map, highlighting the spatial distribution of the insoluble lithologies and hyper-soluble K-Mg salts across the basin and showing that K-Mg salt deposits developed preferentially to the west of the Central Graben and in the Forth Approaches Basin. This is likely due to confined hydrological conditions, which were favored by (1) the area's geographic position, isolated by the Forties-Montrose High from marine-water influxes coming from the Viking Graben and (2) early halokinetic movements during the Z3 and the Z4 evaporite cycles having promoted localized isolated depocenters. The proposed spatial distribution of the insoluble deposits and K-Mg salts in the Zechstein Group of the Northern Permian Basin suggests that the most halite-pure area extends south of the West Central Shelf, north of the Grensen Nose and in Jenyon's Channel. The shallow buried salt pillows and salt diapirs present in this area are of particular interest for salt cavern development given that non-halite lithologies may compromise the shape and the geomechanical stability of the salt caverns.

The results of this study allow us to propose a refined tectonostratigraphic evolution of the Zechstein Group in the Northern and Southern Permian Basins. In this model, the Southern Permian Basin remained more confined from marine-water influxes because of the influence of the Mid North Sea High. This likely resulted in higher salinity ranges in the Southern Permian Basin and thus higher K-Mg salt accumulations in the basin center. Moreover, although K-Mg salt units were likely synchronous in both basins, the marginal versus basinal distribution of these units differed between the Northern Permian Basin and Southern Permian Basin as a result of different bathymetry evolutions and hydrological conditions. In particular, a deeper depositional setting in the Northern Permian Basin might have been favored because of underfilled condition during Z2 and a high subsidence rate (possibly tectonically induced) during Z3 and Z4.

This study emphasizes the importance of constraining the distribution of K-Mg salts in salt giants to identify areas recording higher accommodation and/or higher salinities resulting from greater hydrological isolation. Constraining the distribution of K-Mg salts allows for a more comprehensive understanding of the large-scale stratigraphic and hydrological evolution of the basin and can elucidate the presence of sub-basins with different salinities, hydrological evolutions, or tectonic to halokinetic processes.

d out as part of the This study was carrie multidisciplinary Deep in Salt project, led by TotalEnergies (France) and the Laboratoire des Fluides Complexes et leurs Réservoirs laboratory of the Université de Pau et des Pays de l'Adour (France); this project includes the development of analogue models, jointly with seismic imaging and numerical simulation works. TotalEnergies is thanked for its financial support and the supply of the working software. Sabine Delahaye (TotalEnergies) and Sidonie Revillon (SEDISOR [France]) are warmly thanked for their technical support during the study. We are also grateful to Mar Moragas, Dan Larsen, and Todd LaMaskin (Geosphere associate editor) for their relevant corrections and suggestions that improved the manuscript. Samuel Brooke-Barnett is warmly thanked for his reading and correction of the English throughout the whole manuscript. We used the artificial intelligence (AI) tool DeepL Write (https://www.deepl.com/fr/write) to improve and correct English usage through rephrasing and sentence modifications; there are no solely AI-generated sentences in this paper.