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Map of the study area in the southern margin of the Mid North Sea High showing the general depositional settings of the Hauptdolomit Formation (platform, slope and basin). Platform margins and toe-of-slopes are mappable on 3D seismic and gross depositional environments were verified by cuttings analyses. Wells discussed in this paper are located by red dots. Selected seismic lines: A = Fig. 8c; B = Fig. 8c; C = Fig. 9c; D = Fig. 11c; E = Fig. 11c; F = Fig. 12; G = Fig. 13b; H = Fig. 13b.
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This paper provides an updated understanding of the reservoir stratigraphy, sedimentology, palaeogeography and diagenesis of the Upper Permian Hauptdolomit Formation of the Zechstein Supergroup (“Hauptdolomit”) in a study area on the southern margin of the Mid North Sea High. The paper is based on the examination and description of core and cutting...
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... to provide a regional evaluation of the palaeogeography, sedimentology and diagenesis of the Z2 Hauptdolomit Formation of the Upper Permian Zechstein Supergroup ("Hauptdolomit") in the southern margin of the Mid North Sea High (MNSH). The study area extends from UK Quadrants 35 and 41 in the west to the A and E Blocks in the offshore Netherlands (Fig. ...
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... Z2 Hauptdolomit interval is characterised by diverse depositional settings in the study area, but overall by the development of shallow-water carbonate platforms with complex embayments and irregular margins (Fig. 1). In addition, isolated platforms and pinnacles also developed here locally (Fig. ...
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... Z2 Hauptdolomit interval is characterised by diverse depositional settings in the study area, but overall by the development of shallow-water carbonate platforms with complex embayments and irregular margins (Fig. 1). In addition, isolated platforms and pinnacles also developed here locally (Fig. ...
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... in shallow-water environments resulted in relatively rapid nucleation and growth of gypsum crystals, and precipitation and accumulation rates were high (van der Baan, 1990). Indeed, where precipitation rates of gypsum exceeded rates of subsidence and sea-level rise, gypsum accumulations rapidly built up to sea level (Van de Sande et al., 1996; also see figure 17 in Grant et al., 2019). However in deeper-water areas, gypsum saturations were lower and thus precipitation rates were reduced. ...
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... wells were studied as part of a larger-scale investigation, of which eight wells were selected to demonstrate the key facies variations and diagenetic processes affecting the Hauptdolomit in the study area (Table 1) Fig. 1). Two of these eight wells (38/29-1 and 44/07-1) had short cores through the Hauptdolomit interval. The cores were logged at a 1:50 scale using the slabbed cut held at the BGS store at Keyworth, Nottingham. Selected core samples were chosen from core for petrographic ...
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... study area for this paper covers a large part of the Mid North Sea High totalling around 20,000 km 2 (Fig. 1). The integration of microfacies analysis from 25 wells with the generation of synthetic seismic data for well-to-seismic ties, together with seismic interpretation of key events, has allowed a mapping methodology to be established for the areas of 3D coverage centred on the areas where Spirit Energy held exploration licences (Fig. 1). ...
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... 20,000 km 2 (Fig. 1). The integration of microfacies analysis from 25 wells with the generation of synthetic seismic data for well-to-seismic ties, together with seismic interpretation of key events, has allowed a mapping methodology to be established for the areas of 3D coverage centred on the areas where Spirit Energy held exploration licences (Fig. 1). Learnings from areas covered only by 2D seismic were also incorporated, as were regional analogues and publications from the wider Southern Permian Basin including areas in the Netherlands, Germany and ...
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... acoustic impedance is represented by a trough (hard event), coloured red. The synthetic seismic displays (Fig. 6) were generated from sonic and density logs using a 25Hz Ricker wavelet for consistency across the wells. Seismic mapping has allowed the broad-scale palaeogeography of the Hauptdolomit platforms in the study area to be established ( Fig. 1) (see Browning- Stamp et al. (2023 this issue) for a slightly modified interpretation). Isolated platforms and attached platforms/peninsulas can be distinguished based on the seismically-defined platform ...
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... wells have been chosen to represent the variability of facies in the Hauptdolomit platform interior: 43/05-1 and 42/04-1 ( Fig. 7; locations in Fig. 1). The interior setting occupies a large proportion of the Hauptdolomit platform in the study area ( Fig. 1), and there are therefore local variations in depofacies. The Hauptdolomit in the platform interior is generally expressed on seismic by a peak, although the amplitude is commonly weaker than the response along the platform margin ...
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... wells have been chosen to represent the variability of facies in the Hauptdolomit platform interior: 43/05-1 and 42/04-1 ( Fig. 7; locations in Fig. 1). The interior setting occupies a large proportion of the Hauptdolomit platform in the study area ( Fig. 1), and there are therefore local variations in depofacies. The Hauptdolomit in the platform interior is generally expressed on seismic by a peak, although the amplitude is commonly weaker than the response along the platform margin due to the lower average porosities (Fig. 8c). The platform area around well 42/04-1 has a low level of ...
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... of the Hauptdolomit platform margin in the study area ( Fig. 7; locations in Fig. 1). These wells are important for reservoir characterisation purposes as the Hauptdolomit facies in this setting has the best reservoir ...
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... thrombolitic build-up is present at the base of the cored interval (Fig. 10) and, although not fully cored, has a height of around 20 cm. The thrombolitic textures are dominated by mesoclots with irregular shapes and contain oncoidal and coated grains. The thrombolites are closely associated with stromatolitic textures which appear to coat and overlie the build-ups. The thrombolitic build-ups are typical of ...
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... the western part of the Mid North Sea High, two wells (43/02-1 and 36/26-1: Fig. 1) penetrate Hauptdolomit slope facies. The Hauptdolomit is thin (15-20 m) in both locations, and overlies a slightly thickened Werraanhydrit and is overlain by a thin Basalanhydrit (Fig. 6). Together the amalgamated Werraanhydrit, Hauptdolomit and Basalanhydrit form a bright trough on seismic profiles that is characteristic of the slope ...
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... a slightly thickened Werraanhydrit and is overlain by a thin Basalanhydrit (Fig. 6). Together the amalgamated Werraanhydrit, Hauptdolomit and Basalanhydrit form a bright trough on seismic profiles that is characteristic of the slope facies location and is often mappable on 3D and 2D seismic data, allowing platform areas to be identified ( Fig. 11c; Fig. ...
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... 2018). The strong trough seismic response of the shelf polyhalites in slope locations connects with the strong trough seismic response of the basinal polyhalites in the basin, creating a characteristic wing-like geometry which is observed throughout the platform margin where polyhalites are recognized in both basinal wells and slope wells (Fig. 12). Further to this, the slope wells 43/02-1 and 36/26-1 have a halite section below the shelfal polyhalites which forms a strong peak within the Stassfurt Halite. This seismic peak event may appear to be a continuation of the Hauptdolomit peak within the platform, but the two slope wells demonstrate that the peak is related to the ...
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... facies variation of the Hauptdolomit in wells 36/26-1 and 43/02-1 is presented in Fig. 11. In 36/26-1, there is a relatively high proportion of low-energy depositional facies such as dolomicrites, laminated dolomicrites and fine crystalline dolomites (Fig. 11a, photomicrographs 1, 5 and 6), all of which indicate low-energy deposition well below wave base. Indeed, the base of the Hauptdolomit appears to contain dark, silty ...
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... facies variation of the Hauptdolomit in wells 36/26-1 and 43/02-1 is presented in Fig. 11. In 36/26-1, there is a relatively high proportion of low-energy depositional facies such as dolomicrites, laminated dolomicrites and fine crystalline dolomites (Fig. 11a, photomicrographs 1, 5 and 6), all of which indicate low-energy deposition well below wave base. Indeed, the base of the Hauptdolomit appears to contain dark, silty mudstones (claystones) which may represent the basinal, organic-rich Stinkschiefer facies. However, there is also a significant proportion of microfacies which would ...
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... silty mudstones (claystones) which may represent the basinal, organic-rich Stinkschiefer facies. However, there is also a significant proportion of microfacies which would normally be associated with a shallowplatform setting -in particular dolomitised grainstones, dolomitised ooid grainstones and microbial bindstone (stromatolitic) textures (Fig. 11a, photomicrographs 2, 3, 4). The presence of these shallow-water microfacies can be interpreted in two ways: firstly, it is possible that they represent shallow-platform material which has been reworked into a slope setting in the form of gravity flows, storm deposits or turbidites; this appears to be likely given the admixture of both grainy and mud-supported ...
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... 43/02-1 is important in terms of the Hauptdolomit's seismic response, which clearly demonstrate the characteristic slope morphologies discussed above (Fig. 11c). However, only two cuttings samples were recovered from the Hauptdolomit interval for sedimentological analysis (Fig. 11b). Both samples were characterised by a mixture of fine crystalline dolomites and anhydrite, although there are ghosts of microbial grains within the anhydrite (?microbial lumps: Fig. ...
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... 43/02-1 is important in terms of the Hauptdolomit's seismic response, which clearly demonstrate the characteristic slope morphologies discussed above (Fig. 11c). However, only two cuttings samples were recovered from the Hauptdolomit interval for sedimentological analysis (Fig. 11b). Both samples were characterised by a mixture of fine crystalline dolomites and anhydrite, although there are ghosts of microbial grains within the anhydrite (?microbial lumps: Fig. ...
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... slope morphologies discussed above (Fig. 11c). However, only two cuttings samples were recovered from the Hauptdolomit interval for sedimentological analysis (Fig. 11b). Both samples were characterised by a mixture of fine crystalline dolomites and anhydrite, although there are ghosts of microbial grains within the anhydrite (?microbial lumps: Fig. ...
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... most of the basinal wells that were studied, the Hauptdolomit has been subjected to a degree of dedolomitisation and calcitisation (as shown in the map in Fig. 13). Dedolomitisation (discussed in more detail below) has important implications for the seismic response which is variable. In the western part of the Mid North Sea High, the Hauptdolomit can be mapped on seismic as a peak above a Werraanhydrit/ Zechsteinkalk section that is dominated by carbonates, rather than anhydrites, and below a ...
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... that can generate a Hauptdolomit peak (average porosity of 7% and 5%) above a Werraanhydrit section that is dominated by anhydrites, and below a thin Basalanhydrit and a thick Stassfurt Halite (with thick polyhalites at the base). However, in the south (well 41/15-1) and east (wells 44/07-1 and 38/18-1) of the Mid North Sea High study area (Fig. 1), the Hauptdolomit peak becomes less easy to distinguish from the underlying Werraanhydrit trough, and becomes an intermittent peak or a trough Fig. ...
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... thin Basalanhydrit and a thick Stassfurt Halite (with thick polyhalites at the base). However, in the south (well 41/15-1) and east (wells 44/07-1 and 38/18-1) of the Mid North Sea High study area (Fig. 1), the Hauptdolomit peak becomes less easy to distinguish from the underlying Werraanhydrit trough, and becomes an intermittent peak or a trough Fig. ...
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... (Smith et al. 2014;Kemp, 2018). This follows descriptions from the onshore Boulby Mine (located near Middlesborough in NE England) where polyhalites are mined from the Fordon Evaporite Formation, the onshore equivalent of the Stassfurt Halite. The basinal polyhalites influence the seismic signature as illustrated by wells 44/07-1 and 38/18-1 ( Fig. 14b), and produce a trough above the Hauptdolomit/Basalanhydrit (e.g. at wells 36/23-1, 41/10-1, 42/09-1, 43/06-1, 44/06-1 and 44/07-1). In the UK onshore, basinal polyhalites are recorded in the Great Hatfield 1 well (location shown in Knott and Cross, 1992), 4 km north of West Newton oil field (Quirk and Archer, 2022). The seismic ...
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... without basinal polyhalites are present in a few small areas of the Mid North Sea High, for example at wells 37/23-1, 38/18-1 ( Fig. 14b) and 38/24-1 (Fig. 12). In these areas, no wing-like geometries are developed in seismic profiles at the slope adjacent to the platform margins, supporting the interpretation that the geometries are created by the presence of polyhalites with the Stassfurt Halite and are not due to prograding Hauptdolomit. Browning-Stamp et al. (2023, ...
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... without basinal polyhalites are present in a few small areas of the Mid North Sea High, for example at wells 37/23-1, 38/18-1 ( Fig. 14b) and 38/24-1 (Fig. 12). In these areas, no wing-like geometries are developed in seismic profiles at the slope adjacent to the platform margins, supporting the interpretation that the geometries are created by the presence of polyhalites with the Stassfurt Halite and are not due to prograding Hauptdolomit. Browning-Stamp et al. (2023, this issue) interpret ...
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... well, 44/07-1, has been cored in the Hauptdolomit basinal facies and is considered to be representative of this setting. A 10 m core was taken from the upper part of the Hauptdolomit and reveals a succession of fine, often laminated, carbonates and evaporites which have been intensely calcitised, brecciated and fractured (Fig. 14a). The lower parts of the core are characterized by repeated, thinly-bedded, fining-upwards intervals that are interpreted as distal turbidites (Fig. 14a). Minor scour surfaces are present, and the background sedimentation is characterised by bedded, fine crystalline dolomites or microspar which contains dark, organic-rich partings, ...
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... core was taken from the upper part of the Hauptdolomit and reveals a succession of fine, often laminated, carbonates and evaporites which have been intensely calcitised, brecciated and fractured (Fig. 14a). The lower parts of the core are characterized by repeated, thinly-bedded, fining-upwards intervals that are interpreted as distal turbidites (Fig. 14a). Minor scour surfaces are present, and the background sedimentation is characterised by bedded, fine crystalline dolomites or microspar which contains dark, organic-rich partings, sometimes as thick as 1 cm (Fig. 14a, core photo 6). There is evidence of small syn-sedimentary faults and minor slumping (Fig. 14a, core photos 5 and 4). ...
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... parts of the core are characterized by repeated, thinly-bedded, fining-upwards intervals that are interpreted as distal turbidites (Fig. 14a). Minor scour surfaces are present, and the background sedimentation is characterised by bedded, fine crystalline dolomites or microspar which contains dark, organic-rich partings, sometimes as thick as 1 cm (Fig. 14a, core photo 6). There is evidence of small syn-sedimentary faults and minor slumping (Fig. 14a, core photos 5 and 4). The facies contain a very restricted faunal assemblage consisting only of rare ostracods and small benthic ...
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... interpreted as distal turbidites (Fig. 14a). Minor scour surfaces are present, and the background sedimentation is characterised by bedded, fine crystalline dolomites or microspar which contains dark, organic-rich partings, sometimes as thick as 1 cm (Fig. 14a, core photo 6). There is evidence of small syn-sedimentary faults and minor slumping (Fig. 14a, core photos 5 and 4). The facies contain a very restricted faunal assemblage consisting only of rare ostracods and small benthic ...
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... upper sections of the cored interval are characterised by an increased proportion of evaporites. Core observations suggest that the initial sediment was finely laminated (Fig. 14a, core photos 1, 2, 3, 4). The minerology of the original sediments is difficult to determine as they have undergone extreme dedolomtisation, but it is probable they consisted of carbonate mudstones with fine anhydrite interlaminations. Frequent layers of crystalline calcite ...
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... open marine occur in the upper core intervals and are considered to most likely represent former coarse bottom-growth selenitic gypsum crystals (with preserved twins) which have now been calcitised (Fig. 14a, core photos 2, 3, 4). Anhydrite nodules are present throughout the ...
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... area in order to determine the diagenetic history (paragenesis) and to investigate conditions of dolomitisation, cement precipitation, porosity creation, fracturing and hydrocarbon migration. Three of the cored wells are located on the platform (38/22-1, 44/02-1 and 38/29-1), and the fourth well is in a basinal setting (44/07-1) (locations in Fig. 1). Whilst the Hauptdolomit in the platform wells has a dolomitic mineralogy, in the basinal well it has been significantly modified by later calcitisation/dedolomitisation (Fig. ...
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... migration. Three of the cored wells are located on the platform (38/22-1, 44/02-1 and 38/29-1), and the fourth well is in a basinal setting (44/07-1) (locations in Fig. 1). Whilst the Hauptdolomit in the platform wells has a dolomitic mineralogy, in the basinal well it has been significantly modified by later calcitisation/dedolomitisation (Fig. ...
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... total of 19 diagenetic events have been recognised in the Hauptdolomit sediments analysed, the most important of which are discussed below (Fig. ...
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... common feature of ooid and coated-grain grainstones is the presence of a very early circumgranular cement (i.e. Fig. 16a; Fig. 8b photomicrograph 3; Fig. 9a photomicrograph 3). Cement crystals are approximately 60 µm in length and most likely had an original aragonitic mineralogy which was later replaced by dolomite. The cements are interpreted to have been precipitated under marine conditions and provided some limited mechanical stability against later ...
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... the intertidal and supratidal facies (i.e. laminated stromatolites), very early formed gypsum is common and takes the form of small, felted nodules or small, lath-like or acicular crystals dispersed within the sediment (Fig. 16b). Early formed gypsum also occurs locally as interlaminations in basinal sediments (Fig. ...
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... the intertidal and supratidal facies (i.e. laminated stromatolites), very early formed gypsum is common and takes the form of small, felted nodules or small, lath-like or acicular crystals dispersed within the sediment (Fig. 16b). Early formed gypsum also occurs locally as interlaminations in basinal sediments (Fig. ...
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... Very fine crystalline (aphanocrystalline) replacive dolomite (Fig. 9a, photomicrographs 1, 2 & 4; Fig. 8b photomicrograph 1) which replaces ooid laminae, the cortices of ooids and grains such as peloids, and locally replaces interparticle sediment. This dolomite type is in general non planar-a to planar-s with poorly developed crystal ...
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... Very fine crystalline (aphanocrystalline) replacive dolomite (Fig. 9a, photomicrographs 1, 2 & 4; Fig. 8b photomicrograph 1) which replaces ooid laminae, the cortices of ooids and grains such as peloids, and locally replaces interparticle sediment. This dolomite type is in general non planar-a to planar-s with poorly developed crystal ...
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... Fine to medium crystalline replacive dolomite, whose crystals have a non planar-a to planar-s morphology ( Fig. 9a photomicrograph 6; (Fig. 16a). Where peloids have been replaced, the dolomite crystals have a characteristic "equant" shape ( Fig. 16c, ...
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... Fine to medium crystalline replacive dolomite, whose crystals have a non planar-a to planar-s morphology ( Fig. 9a photomicrograph 6; (Fig. 16a). Where peloids have been replaced, the dolomite crystals have a characteristic "equant" shape ( Fig. 16c, ...
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... Replacement dolomite with an acicular/needlelike character ( Fig. 16e) with crystals up to 200 µm in length. This form of dolomite is especially common in the laminated microbial bindstone facies and locally within ooids, and is considered to represent replacement of very early formed gypsum. Under cathodoluminescence illumination (CL), the replacement dolomites have a dull red/orange colour (Fig. 16d) ...
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... character ( Fig. 16e) with crystals up to 200 µm in length. This form of dolomite is especially common in the laminated microbial bindstone facies and locally within ooids, and is considered to represent replacement of very early formed gypsum. Under cathodoluminescence illumination (CL), the replacement dolomites have a dull red/orange colour (Fig. 16d) or locally a cloudy, mottled dull yellowgreen-brown colour. Whilst the dull red/orange colour is typical of replacement dolomites, the less usual yellow-green-brown colour may indicate a different chemistry or crystallography. It is likely that these early dolomites are calcium-rich and are far from ...
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... overgrowth cements are present in some microbial facies and represent a continuum of the dolomitisation process. Cement crystals have subhedral to euhedral faces and form around clotted grains and peloids (Fig. 16f, g, h); the cement is generally 15-20 µm thick but can be up to 60 µm. Where peloids have been replaced by dolomite, there is often a dolomite cement overgrowth around the whole grain approximately 10-20 µm thick (Fig. 16f, g, h). Under CL, the cements is characterised by a fine red-orange zonation (Fig. ...
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... of the dolomitisation process. Cement crystals have subhedral to euhedral faces and form around clotted grains and peloids (Fig. 16f, g, h); the cement is generally 15-20 µm thick but can be up to 60 µm. Where peloids have been replaced by dolomite, there is often a dolomite cement overgrowth around the whole grain approximately 10-20 µm thick (Fig. 16f, g, h). Under CL, the cements is characterised by a fine red-orange zonation (Fig. ...
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... form around clotted grains and peloids (Fig. 16f, g, h); the cement is generally 15-20 µm thick but can be up to 60 µm. Where peloids have been replaced by dolomite, there is often a dolomite cement overgrowth around the whole grain approximately 10-20 µm thick (Fig. 16f, g, h). Under CL, the cements is characterised by a fine red-orange zonation (Fig. ...
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... the micritic/organic-rich rims of thrombolitic clotted grains, resulting in a microporous or dissolved/ mouldic rim (Fig. 16i, j); • peloids, which are commonly surrounded by dolomite cement but have frequently been dissolved, resulting in a mouldic pore surrounded by a "sheath" of residual remnants of dolomitised grain and calcite cement ~10-20 µm in thickness (Fig. 16d, f, g, h); • the cores of ooid grains, which are commonly dissolved, as are selected ooid ...
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... rims of thrombolitic clotted grains, resulting in a microporous or dissolved/ mouldic rim (Fig. 16i, j); • peloids, which are commonly surrounded by dolomite cement but have frequently been dissolved, resulting in a mouldic pore surrounded by a "sheath" of residual remnants of dolomitised grain and calcite cement ~10-20 µm in thickness (Fig. 16d, f, g, h); • the cores of ooid grains, which are commonly dissolved, as are selected ooid laminae (Fig. 9a, photomicrographs 3 & 4; Fig. 8a, photomicrographs 3 & 4; Fig. 8b, photomicrograph 3; Fig. 16k, l) and bioclasts (e.g. bivalves); dissolution has probably preferentially affected parts of an ooid that had an original aragonitic mineralogy ...
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... resulting in a mouldic pore surrounded by a "sheath" of residual remnants of dolomitised grain and calcite cement ~10-20 µm in thickness (Fig. 16d, f, g, h); • the cores of ooid grains, which are commonly dissolved, as are selected ooid laminae (Fig. 9a, photomicrographs 3 & 4; Fig. 8a, photomicrographs 3 & 4; Fig. 8b, photomicrograph 3; Fig. 16k, l) and bioclasts (e.g. bivalves); dissolution has probably preferentially affected parts of an ooid that had an original aragonitic mineralogy or which were microbial and therefore less stable. Some dolomites have a "chalkified" or "leached" appearance which may be the result of dissolution. Finally, dissolution may have resulted in ...
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... observations indicate that the circumgranular dolomite cements pre-dated dissolution (Fig. 16d, f, g, h). The processes responsible for dissolution are discussed in more detail ...
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... Hauptdolomit in many of the basinal wells (and in some slope wells) in the study area has been affected by dedolomitisation/ calcitisation (Fig. 13), and the resulting textures range from microspar to coarsely crystalline and spherulitic fabrics. The widespread dolomitisation of the Hauptdolomit precedes dedolomitisation, even in basinal settings; thus, in well 44/07-1, it is clear that dolomites are engulfed by, and subsumed within, the replacement calcite (Fig. ...
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... calcitisation (Fig. 13), and the resulting textures range from microspar to coarsely crystalline and spherulitic fabrics. The widespread dolomitisation of the Hauptdolomit precedes dedolomitisation, even in basinal settings; thus, in well 44/07-1, it is clear that dolomites are engulfed by, and subsumed within, the replacement calcite (Fig. ...
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... is more intense in the upper parts of the Hauptdolomit compared to the base. Whilst macrotextures are generally preserved, microscopic textures are often obliterated. Replacement calcite crystals are 50-12000 µm in size (Fig. 17b, c), and larger crystals commonly have a pronounced fanshaped sweeping extinction under crossed-polarised light. The replacement calcite cross-cuts all former sedimentological and diagenetic boundaries and locally has an almost spherulitic or concretionary appearance. Early sulphate precipitates are also calcitised (Fig. 14). Under CL, ...
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... 50-12000 µm in size (Fig. 17b, c), and larger crystals commonly have a pronounced fanshaped sweeping extinction under crossed-polarised light. The replacement calcite cross-cuts all former sedimentological and diagenetic boundaries and locally has an almost spherulitic or concretionary appearance. Early sulphate precipitates are also calcitised (Fig. 14). Under CL, neomorphic calcite has a non-to dullbrown colour. Calcite microspar is a dedolomitisation texture in which calcite crystals are typically only 10-25 µm in size (Fig. 17d). Microspar is often associated with fine crystalline ...
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... former sedimentological and diagenetic boundaries and locally has an almost spherulitic or concretionary appearance. Early sulphate precipitates are also calcitised (Fig. 14). Under CL, neomorphic calcite has a non-to dullbrown colour. Calcite microspar is a dedolomitisation texture in which calcite crystals are typically only 10-25 µm in size (Fig. 17d). Microspar is often associated with fine crystalline ...
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... A significant phase of fracturing/brecciation and dissolution post-dated dedolomitization in the basinal Hauptdolomit sediments. This resulted in a vuggy and fractured texture, commonly with relict porosity within larger fractures/vugs (Fig. 14a, core photographs 1 and 6; Fig. 17e). The vugs may result from the dissolution of former anhydrite ...
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... A significant phase of fracturing/brecciation and dissolution post-dated dedolomitization in the basinal Hauptdolomit sediments. This resulted in a vuggy and fractured texture, commonly with relict porosity within larger fractures/vugs (Fig. 14a, core photographs 1 and 6; Fig. 17e). The vugs may result from the dissolution of former anhydrite ...
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... and fractures are commonly cemented by a complex zoned suite of ferroan and non-ferroan calcite cements (Fig. 17e). Crystals are up 3000 µm in size, and have an initial ferroan followed by a non-ferroan zonation. The stratigraphy of the calcite cements can be reconstructed under CL. The initial pore-lining cement (calcite A) has an orange CL colour is inclusion-rich and massive (Fig. 17h, i). This is followed by a coarse euhedral cement which has a ...
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... by a complex zoned suite of ferroan and non-ferroan calcite cements (Fig. 17e). Crystals are up 3000 µm in size, and have an initial ferroan followed by a non-ferroan zonation. The stratigraphy of the calcite cements can be reconstructed under CL. The initial pore-lining cement (calcite A) has an orange CL colour is inclusion-rich and massive (Fig. 17h, i). This is followed by a coarse euhedral cement which has a dark brown, dull CL colour (calcite B; Fig. 17f, g, h, i), and which is postdated by a zoned cement with a bright orange-brown CL colour (calcite C; Fig. 17f, g, h, i). A cement with a uniform brown CL colour is the final phase and is present next to open pore space (calcite D; ...
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... size, and have an initial ferroan followed by a non-ferroan zonation. The stratigraphy of the calcite cements can be reconstructed under CL. The initial pore-lining cement (calcite A) has an orange CL colour is inclusion-rich and massive (Fig. 17h, i). This is followed by a coarse euhedral cement which has a dark brown, dull CL colour (calcite B; Fig. 17f, g, h, i), and which is postdated by a zoned cement with a bright orange-brown CL colour (calcite C; Fig. 17f, g, h, i). A cement with a uniform brown CL colour is the final phase and is present next to open pore space (calcite D; Fig. 17h, ...
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... can be reconstructed under CL. The initial pore-lining cement (calcite A) has an orange CL colour is inclusion-rich and massive (Fig. 17h, i). This is followed by a coarse euhedral cement which has a dark brown, dull CL colour (calcite B; Fig. 17f, g, h, i), and which is postdated by a zoned cement with a bright orange-brown CL colour (calcite C; Fig. 17f, g, h, i). A cement with a uniform brown CL colour is the final phase and is present next to open pore space (calcite D; Fig. 17h, ...
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... 17h, i). This is followed by a coarse euhedral cement which has a dark brown, dull CL colour (calcite B; Fig. 17f, g, h, i), and which is postdated by a zoned cement with a bright orange-brown CL colour (calcite C; Fig. 17f, g, h, i). A cement with a uniform brown CL colour is the final phase and is present next to open pore space (calcite D; Fig. 17h, ...
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... calcite crystal silt A bituminous calcite crystal silt is present is the remaining pore space. This sediment contains brokenup calcite crystals which sometimes have a ferroan core and non-ferroan rim as well as dolomite rhombs (Fig. 17e and j). The surrounding fill locally appears to be clay-rich and appears to have been created through mechanical grinding/breakage of the calcite cement phases, associated with a phase of fracturing/ brecciation/bitumen emplacement. Sulphur (or possibly sphalerite) is also associated with the vuggy bituminous fill (Fig. 17e and j). In ...
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... as dolomite rhombs (Fig. 17e and j). The surrounding fill locally appears to be clay-rich and appears to have been created through mechanical grinding/breakage of the calcite cement phases, associated with a phase of fracturing/ brecciation/bitumen emplacement. Sulphur (or possibly sphalerite) is also associated with the vuggy bituminous fill (Fig. 17e and j). In addition, pyrite is ...
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... final cement phase which affects all Hauptdolomit sediments to variable degrees is a late anhydrite which occludes pores in both the microbial and the oolitic facies ( Fig. 9a photomicrograph 2; Fig. 9b photomicrograph 2; Fig. 8a photomicrograph 4; Fig. 11a, photomicrograph 3; Fig. 11b, photomicrograph 2; Fig. 16a and i), and is also the final cement phase in vugs in the basinal dedolomites (Fig. 17k). Anhydrite cementation has therefore reduced overall reservoir quality significantly. The cement is coarse and blocky or bladed, with crystals generally 100-500 µm in size. The distribution is patchy and not all ...
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... final cement phase which affects all Hauptdolomit sediments to variable degrees is a late anhydrite which occludes pores in both the microbial and the oolitic facies ( Fig. 9a photomicrograph 2; Fig. 9b photomicrograph 2; Fig. 8a photomicrograph 4; Fig. 11a, photomicrograph 3; Fig. 11b, photomicrograph 2; Fig. 16a and i), and is also the final cement phase in vugs in the basinal dedolomites (Fig. 17k). Anhydrite cementation has therefore reduced overall reservoir quality significantly. The cement is coarse and blocky or bladed, with crystals generally 100-500 µm in size. The distribution is patchy and not all interparticle pores are ...
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... sediments to variable degrees is a late anhydrite which occludes pores in both the microbial and the oolitic facies ( Fig. 9a photomicrograph 2; Fig. 9b photomicrograph 2; Fig. 8a photomicrograph 4; Fig. 11a, photomicrograph 3; Fig. 11b, photomicrograph 2; Fig. 16a and i), and is also the final cement phase in vugs in the basinal dedolomites (Fig. 17k). Anhydrite cementation has therefore reduced overall reservoir quality significantly. The cement is coarse and blocky or bladed, with crystals generally 100-500 µm in size. The distribution is patchy and not all interparticle pores are ...
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... the paragenesis for the samples of the Hauptdolomit dolomites and limestones analysed can be established based on petrographic observations (Fig. 15), further analysis is needed to investigate the conditions (temperature/ salinity) under which diagenetic changes occurred. Oxygen and carbon stable isotope data is plotted in Fig. 18, and shows that Hauptdolomit dolomites from the wells studied plot within the known parameters of Permian marine carbonates ( Leary and Vogt, 1986). The ...
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... the paragenesis for the samples of the Hauptdolomit dolomites and limestones analysed can be established based on petrographic observations (Fig. 15), further analysis is needed to investigate the conditions (temperature/ salinity) under which diagenetic changes occurred. Oxygen and carbon stable isotope data is plotted in Fig. 18, and shows that Hauptdolomit dolomites from the wells studied plot within the known parameters of Permian marine carbonates ( Leary and Vogt, 1986). The stable isotope data is comparable to that of other Zechstein dolomites from Germany ( Schoenherr et al., 2018) and the Netherlands (Reijers, 2012), and suggests that the dolomites were ...
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... in near-surface/early burial conditions. Fluid inclusion analysis was performed on one Hauptdolomit sample from well 38/22-1. Homogenisation temperatures for fluid inclusions in the dolomite cement surrounding peloid grains suggest pore fluid temperatures ranging from 65 to 95°C, and that the fluids were highly saline with 22-24 wt.% NaCl (Fig. 19). Using a geothermal gradient of 32°C/ km (based on bottom-hole temperatures from well 42/04-1 and a surface temperature of ~25-30°C during the Permian: Wygrala, 1989), this suggests that the dolomite cements were precipitated between depths of ~1 and 2 ...
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... may result in well-developed mouldic porosity. The distinctive moulds associated with the microbial and peloidal facies must have post-dated, or been coeval with, cement formation (Fig 16f, g, h). Since cement formation occurs at temperatures of 65-95°C, early meteoric diagenesis can be ruled out. ...
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... mouldic porosity. The distinctive moulds associated with the microbial and peloidal facies must have post-dated, or been coeval with, cement formation (Fig 16f, g, h). Since cement formation occurs at temperatures of 65-95°C, early meteoric diagenesis can be ruled out. Four dedolomite samples were collected for O and C stable isotope analyses (Fig. 18). Dedolomitisation of Zechstein carbonates is well-documented in the Netherlands (Reijers, 2012;Clark, 1986;Clark, 1980, Van der Baan, 1990), Germany ( Schoenherr et al., 2018), andPoland (Peryt andScholle, 1996), and the petrographic and stable isotope characteristics of dedolomitised limestones in these studies are remarkably similar ...
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... and fractures are locally cemented by a suite of calcite cements, and fluid inclusion analyses allowed the conditions of precipitation to be constrained (Fig. 19). The evolution of the calcite cements (B to C to D) in samples from well 44/07-1 shows an increasing temperature profile: thus early cements precipitated at temperatures ranging from 77 to 103°C, and later cements precipitated at ~107 to 112°C. Fluid inclusion temperatures for the calcite cements (77-112°C, showing an increasing ...
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... quality in the Hauptdolomit is a function of both depositional facies and later diagenesis and burial trends. By comparing porosity logs for the key wells in the different gross depositional environments ( Fig. 8; Fig. 9; Fig. 11; Fig. 14), it is clear that the best reservoir porosities are associated with high energy, grain-supported platform margin settings. Mouldic and interparticle porosities are most common in oolitic grainstones (Fig. 9a, photomicrographs 3, 4, 5; Fig. 9b, photomicrographs 2, 3), whilst framework and mouldic porosities are locally preserved in ...
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... are associated with high energy, grain-supported platform margin settings. Mouldic and interparticle porosities are most common in oolitic grainstones (Fig. 9a, photomicrographs 3, 4, 5; Fig. 9b, photomicrographs 2, 3), whilst framework and mouldic porosities are locally preserved in microbial build-ups (i.e. cored interval in well 38/22-1; Fig. 21). Anhydrite cementation is the diagenetic process most detrimental to reservoir quality (Fig. 9a, photomicrograph 2 Fig. 8a photomicrograph 4; Fig. 11a photomicrograph 1; Fig. 21), but despite detailed evaluation the distribution and intensity of late anhydrite cement is difficult to ...
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... (Fig. 9a, photomicrographs 3, 4, 5; Fig. 9b, photomicrographs 2, 3), whilst framework and mouldic porosities are locally preserved in microbial build-ups (i.e. cored interval in well 38/22-1; Fig. 21). Anhydrite cementation is the diagenetic process most detrimental to reservoir quality (Fig. 9a, photomicrograph 2 Fig. 8a photomicrograph 4; Fig. 11a photomicrograph 1; Fig. 21), but despite detailed evaluation the distribution and intensity of late anhydrite cement is difficult to ...
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... limited core data from wells located on interpreted Hauptdolomit platforms in the study area is available for a thorough examination of porositypermeability relationships (Fig. 21). In a short core from well 44/02-1, ooid and coated grain grainstones are intensely cemented by anhydrite, resulting in generally poor porosities and permeabilities; the porosity, where present, is typically mouldic and partly interparticle. In well 38/22-1, it is clear that there is a facies control on reservoir quality (Fig. 21). ...
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... relationships (Fig. 21). In a short core from well 44/02-1, ooid and coated grain grainstones are intensely cemented by anhydrite, resulting in generally poor porosities and permeabilities; the porosity, where present, is typically mouldic and partly interparticle. In well 38/22-1, it is clear that there is a facies control on reservoir quality (Fig. 21). Thrombolitic boundstones offer some of the best reservoir qualities; framework porosity is only patchily cemented by anhydrite, and mouldic porosity is also developed (Fig. 21). By contrast, laminated microbial bindstones (stromatolites) are strongly cemented by anhydrite and have poorer reservoir ...
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... the porosity, where present, is typically mouldic and partly interparticle. In well 38/22-1, it is clear that there is a facies control on reservoir quality (Fig. 21). Thrombolitic boundstones offer some of the best reservoir qualities; framework porosity is only patchily cemented by anhydrite, and mouldic porosity is also developed (Fig. 21). By contrast, laminated microbial bindstones (stromatolites) are strongly cemented by anhydrite and have poorer reservoir ...
Citations
... 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). ...
... 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). ...
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.
... Recent studies and subsequent drilling campaigns have revealed a play fairway within the Upper Permian Zechstein Group across Quadrants 41-43 on the MNSH (Patruno et al., 2018;Browning-Stamp et al., 2023). Recent success involving the Ossian-Darach, Crosgan and Pensacola discoveries have demonstrated hydrocarbon prospectivity in the Zechstein Z2 Hauptdolomit Fm. 3D seismic and sedimentological facies mapping (Garland et al., 2023;Browning-Stamp et al., 2023) unveiled the Orchard Platform: A Z2 Hauptdolomit Fm. carbonate platform spanning Quadrants 36-38 and 42-44 (Fig. 2). Whilst our understanding of the characteristics of the Orchard Platform has improved significantly, the greatest uncertainty now resides with the overlying Zechstein Group formations which must be analysed to de-risk future exploration of the Zechstein system on the MNSH. ...
... Shallow marine Z2 Hauptdolomit Fm. progradation on top of Z1 Werraanhydrit Fm. clinoforms helped to create a tabular carbonate system known as the Orchard Platform ( Fig. 3; Patruno et al., 2018;Garland et al., 2023) at the southern entrance to Jenyon's Channel (Fig. 2a). As straits and seaways control the exchange of nutrients between basins (Bahr et al., 2022), it is envisaged that nutrient-rich waters arriving via Jenyon's Channel could have helped stimulate the growth of the Z2 Orchard Platform at its intersection with the Anglo-Polish Basin. ...
... Map of the Orchard Platform study area including the subset of the 3D seismic volume (rectangle) and the locations of the local well penetrations that were tied to guide seismic interpretation. The shape of the carbonate platform is after Browning-Stamp et al. (2023) and Garland et al. (2023). ...
The influence of the Mid North Sea High and Seaway on the distribution of salts in the Northern and Southern North Seas during the Zechstein.
The Mid North Sea High (MNSH) region represents one of the least explored areas for the Late Permian Zechstein Hauptdolomit play in the Southern Permian Basin although some of the first offshore wells drilled in the UK were located here. In other parts of the basin such as onshore Poland, the Hauptdolomit Formation (“Hauptdolomit”) is an active and attractive exploration target, with oil and gas production from commercial‐sized fields. In the UK, the play has been overshadowed by drilling campaigns in areas to the south of the MNSH which tested plays in the underlying Rotliegend and Carboniferous successions. However, with these areas now in decline, there is increased exploration interest in the Hauptdolomit in the MNSH region, particularly since 2019 when 3D seismic data were acquired and the first hydrocarbon discovery was made at Ossian (well 42/04‐01/1Z). Geochemical data from the latter discovery have pointed to the presence of a prolific petroleum system with the potential for Hauptdolomit reservoirs to be charged both by Zechstein‐generated oils and Carboniferous condensate/gas. With regard to hydrocarbon migration and preservation in the southern MNSH, a detailed evaluation of the effects of the Mid Miocene Unconformity has allowed for a greater understanding of the main factors controlling hydrocarbon preservation and remigration. Reservoir characterization of the Hauptdolomit play has been achieved by integrating petrographic microfacies analyses, core data and petrophysical interpretations. The most important factors controlling reservoir quality are the presence and extent of anhydrite cementation and the presence of high energy shoal facies. Thicker and coarser grained shoal facies are expected to occur along the yet‐to‐be explored Orchard platform margin where numerous prospects have been mapped and refined using recently acquired 3D seismic data.
A multidisciplinary approach combining geological mapping based on seismic and well data with petrographic analyses of core and cuttings samples was used to gain a better understanding of the distribution of Upper Permian (Zechstein, Z2) Hauptdolomit platforms and their depositional facies around the Elbow Spit High in the northern Dutch offshore. A detailed understanding of the Hauptdolomit's lateral facies variability is of great importance for assessing its reservoir potential, since both the thickness and reservoir properties of these carbonate platforms greatly depend on local accommodation within different palaeo‐depositional environments. The platforms generally contain the thickest Hauptdolomit sequences and are largely characterised by a mix of oolitic and coated grainstones, as well as by some dolomicrites. Porosities of around 15% are reached at well E02‐02 within the grainstone intervals, and interconnectivity between the pores is generally present. Seismic mapping has indicated a rim of isolated Hauptdolomit platforms, which are up to 10 km wide, around the southern and NW margins of the Elbow Spit High. No Hauptdolomit platforms are present on the NE margin of the High, likely because the palaeo‐ basin margin was too steep and hence lacked accommodation for carbonate growth. Discoveries made in recent years in the UK sector of the southern North Sea have highlighted the importance of the Hauptdolomit hydrocarbon play, and the results of the current study provide a solid base for assessing the reservoir potential of this play in the relatively underexplored northern part of the Dutch offshore.
