Zechstein Research Articles

The role of stratigraphic juxtaposition for seal integrity in proven CO 2 fault-bound traps of the Southern North Sea

ABSTRACT Exploration well 50/26b-6 in the UK Southern North Sea discovered a trap containing a gas-bearing Rotliegend Group (Leman Sandstone Formation) reservoir which was a major surprise at the time of drilling in that its gas composition was approximately 50% CO 2 (with 9% N 2 and the remainder methane). Christened the ‘Fizzy Discovery’, the accumulation was appraised by well 50/26b-8. Subsequently, another CO 2 -rich discovery (Oak) was made along-strike in nearby block 54/1b. Column heights at the well locations are of the order of a few tens of metres, but at the Fizzy Discovery the column height at the trap crest is estimated to be over 200 m. Interpretation of a high fidelity PSTM 3D seismic dataset has been constrained by 33 exploration wells allowing fault geometries and stratigraphic offsets to be determined with confidence. Despite late-stage (Late Cretaceous) structural inversion, the net boundary-fault offset is sufficient in both the Fizzy and Oak discoveries to almost breach the Zechstein Group evaporite super-seal, and the CO 2 -bearing Rotliegend Group in the footwall is now juxtaposed against hanging wall sediments of the uppermost Zechstein Group. Hence, these Zechstein Group units evidently act as a robust long-term side-seal for the carbon dioxide column. The Fizzy and Oak accumulations are noteworthy in providing a natural demonstration of top seal and fault side-seal integrity for carbon dioxide in a subsurface reservoir, that has remained intact over a geological timescale in what is otherwise a prolific methane-rich reservoir play fairway.

Cyclicity and duration of the Zechstein Group

Cyclicity and duration of the Zechstein Group

The Permian–Triassic transition and the onset of Mesozoic sedimentation at the northwestern peri-Tethyan domain scale: Palaeogeographic maps and geodynamic implications

The main aim of this paper is to review Middle Permian through Middle Triassic continental successions in European. Secondly, areas of Middle–Late Permian sedimentation, the Permian–Triassic Boundary (PTB) and the onset of Triassic sedimentation at the scale of the westernmost peri-Tethyan domain are defined in order to construct palaeogeographic maps of the area and to discuss the impact of tectonics, climate and sediment supply on the preservation of continental sediment.At the scale of the western European peri-Tethyan basins, the Upper Permian is characterised by a general progradational pattern from playa-lake or floodplain to fluvial environments. In the northern Variscan Belt domain, areas of sedimentation were either isolated or connected to the large basin, which was occupied by the Zechstein Sea. In the southern Variscan Belt, during the Late Permian, either isolated endoreic basins occurred, with palaeocurrent directions indicating local sources, or basins underwent erosion and/or there was no deposition. These basins were not connected with the Tethys Ocean, which could be explained by a high border formed by Corsica–Sardinia palaeorelief and even parts of the Kabilia microplate. The palaeoflora and sedimentary environments suggest warm and semi-arid climatic conditions.At the scale of the whole study area, an unconformity (more or less angular) is observed almost everywhere between deposits of the Upper Permian and Triassic, except in the central part of the Germanic Basin. The sedimentation gap is more developed in the southern area where, in some basins, Upper Permian sediment does not occur. The large sedimentary supply, erosion and/or lack of deposition during the Late Permian, as well as the variable palaeocurrent direction pattern between the Middle–Late Permian and the Early Triassic indicate a period of relief rejuvenation during the Late Permian. During the Induan, all the intra-belt basins were under erosion and sediment was only preserved in the extra-belt domains (the northern and extreme southern domains). In the northern domain (the central part of the Germanic Basin), sediment was preserved under the same climatic conditions as during the latest Permian, whereas in the extreme southern domain, it was probably preserved in the Tethys Ocean, implying a large amount of detrital components entering the marine waters. Mesozoic sedimentation began in the early Olenekian; the ephemeral fluvial systems indicate arid climatic conditions during this period. Three distinct areas of sedimentation occur: a northern and southern domain, separated by an intra-belt domain. The latter accumulated sediments during the Early–Middle Permian and experienced erosion and/or no-deposition conditions between the Middle–Late Permian and the beginning of Mesozoic sedimentation, dated as Anisian to Hettangian. At the top of the Lower Triassic, another tectonically induced, more or less angular unconformity is observed: the Hardegsen unconformity, which is dated as intra-Spathian and is especially found in the North European basins. This tectonic activity created new source areas and a new fluvial style, with marine influences at the distal part of the systems. During the Anisian and Ladinian, continental sedimentation was characterised by a retrogradational trend. In other words, the fluvial system evolved into fluvio-marine environments, attesting to a direct influence of the Tethys Ocean in the southern and northern domains. Both at the end of the Olenekian (Spathian) and during the Anisian, the presence of palaeosols, micro- and macrofloras indicate less arid conditions throughout this domain.

Geological controls on Upper Permian Plattendolomit Formation reservoir prospectivity, Wissey Field, UK Southern North Sea

ABSTRACT Interpretation of a large, well-calibrated 3D seismic volume, electrical well-log data and core samples from the Quadrant 53 area of the Southern North Sea provides a new-found basis for understanding the controls on gas production in the Wissey Field, a successful test of the Upper Permian (Zechstein Group; Z3 Cycle) Plattendolomit Formation carbonate play in the Southern Permian Basin (SPB). The petrophysical assessment of wells combined with facies analysis demonstrates that the Plattendolomit Formation represents the northward progradation of an important upward-shoaling carbonate platform. The results show that the highest primary porosity values, lie within the uppermost reservoir sub-units of the Plattendolomit Formation, which consist of brecciated packstones and overlying oolitic grainstones developed at the shelf edge of the Z3 carbonate ramp and sealed by back-barrier lagoonal anhydritic mudstones (ascribed to the Scolt Head Formation). It can now be shown that the play fairway is enhanced where two (WSW–ESE and NNW–SSE-striking) fault trends intersect and cause fracturing of the reservoir interval, providing the additional permeability critical for reservoir production. Seismic stratigraphic studies demonstrate that early (pre-Cretaceous) normal fault sets were formed on both trends and were locally affected by contractional deformation (structural inversion) during the Late Cretaceous. As well as explaining the key geological factors that contribute to production success in the Wissey Field itself, the results provide a foundation for evaluating the prospective potential that the Plattendolomit Formation has elsewhere in the SPB.

From the intra‐desert ridges to the marine carbonate island chain: middle to late Permian (Upper Rotliegend–Lower Zechstein) of the Wolsztyn–Pogorzela high, west Poland

AbstractThe tectonic Wolsztyn–Pogorzela palaeo‐High (WPH) is the south‐eastern termination of the Brandenburg–Wolsztyn High (western Poland), which during Late Permian times was an intra‐basin ridge surrounded by Upper Rotliegend sedimentary basins within the Southern Permian Basin. The geological history and structural framework of the WPH are complex. The High belongs to the Variscan Externides, consisting at present of strongly folded, faulted and eroded Viséan to Namurian flysch deposits capped by a thick cover of Upper Carboniferous–Lower Permian volcanic rocks. This sedimentary‐volcanic complex was strongly fragmented and vertically differentiated by tectonic movements and subsequently eroded, resulting in the deposition of coarse clastics surrounding uplifted tectonic blocks. During late Rotliegend time, arid climatic conditions significantly influenced occurrences of specific facies assemblages: alluvial, fluvial, aeolian and playa. Sedimentological study helped to recognize the interplay of tectonic and palaeoclimatic factors and to understand the phenomenon of aeolian sandstones interbedded with coarse deposits of alluvial cones close to fault scarps.Subsequent tectonic and possible thermal subsidence of the studied area was synchronous with inundation by the Zechstein Sea. The rapid inundation process allowed for the preservation of an almost perfectly protected Uppermost Rotliegend landscape. Based on 3D seismic data from the base Zechstein reflector, a reconstruction of Rotliegend palaeogeomorphology was carried out, which shows examples of tectonic rejuvenation of particular tectonic blocks within the WPH area before inundation by the Zechstein Sea.The inundation led to the deposition of the marine Kupferschiefer Shale followed by the Zechstein Limestone. In the deeper parts of the basin the latter is developed in thin basinal facies: in shallow parts (e.g. uplifted tectonic blocks forming in some cases islands), carbonate buildups were formed. The remarkable thickness of those buildups (bryozoan reefs) is interpreted as due to stable tectonic subsidence together with a rise of sea level. A detailed study of carbonate buildups has showed that their internal structure reflects changes in shallow marine environments and even emersion events, caused by sea‐level oscillations and tectonic movements of the reef substrate. Copyright © 2010 John Wiley & Sons, Ltd.

A coelacanthid-rich site at Hasbergen (NW Germany): taphonomy and palaeoenvironment of a first systematic excavation in the Kupferschiefer (Upper Permian, Lopingian)

Plants, invertebrates and fish vertebrates were studied in an excavation campaign of the 30-cm-thick Upper Permian Kupferschiefer Member (Werra Formation: Lower Zechstein, Lopingian) in the Huggel (Osnabrucker Bergland, NW Germany). Taphonomy was mapped at six different planal levels on a 12 m2 area. Plant remains belong mainly to terrestrial conifers Ullmannia frumentaria (Schlotheim) and Pseudovoltzia liebeana (Geinitz), and rare Quadrocladus solmsii (Gothan and Nagelhard). Marine invertebrates consist mainly of the common nautilid Peripetoceras freieslebeni (Geinitz). Of importance is a rich fish fauna with rare skeletons of elasmobranchs Wodnika striatula Munster and Janassa bituminosa (Schlotheim) and numerous actinopterygians, dominated (83%) by Palaeoniscum freieslebeni Blainville as juvenile to adult individuals in different stages of disarticulation. Other, rarer, ray-finned species, including Pygopterus humboldti Agassiz, Platysomus striatus Agassiz, Eurysomus macrurus (Agassiz), Dorypterus hoffmanni Germar and Acentrophorus glaphyurus (Agassiz) are typical for the diverse European Late Permian Zechstein Sea fauna. Most important are concentrations of Coelacanthus granulatus Agassiz (3% of total), also represented by juvenile to adults. Fossil density variation within the Kupferschiefer reflects paleoenvironment in a shallowing-up sequence and bathymetrical change from deeper water to zones close to coastal patch reefs. The shallow-water submarine swell zone at Hasbergen can be related to uplift of the regional Huggel Horst. Charcoalified wood indicates forest fires, and palaeocurrent orientation of fossils correlates with the paleoclimatic of SW Passat wind-direction models based formerly on palaeodune analyses.

Temporal and spatial development of a gravity-driven normal fault array: Middle–Upper Jurassic, South Viking Graben, northern North Sea

Three-dimensional seismic and well data from the South Viking Graben, northern North Sea Basin, is used to investigate the temporal and spatial development of a gravity-driven normal fault array above an evaporite-rich detachment. Two moderate throw (500–900 m), Middle to Upper Jurassic normal faults (the Gudrun and Brynhild Faults) are developed within the study area. Both faults die-out laterally and tip-out upwards at different structural levels within the syn-rift succession. Both faults terminate downwards into Late Permian evaporites (Zechstein Group) and do not offset pre-evaporite basement units. This thin-skinned fault array developed in response to westwards tilting of the hangingwall of the South Viking Graben during Late Jurassic rifting, and consequent westward gliding and extensional break-up of units above the mechanically-weak evaporite horizon. Isochron mapping and well-based correlation of Middle to Upper Jurassic syn-rift units allow constraints to be placed on the temporal evolution of the fault array. Several stages of structural development are observed which document; (i) a period of relatively minor, early (i.e. pre-rift) halokinesis; (ii) variable spatial activity on individual faults within the array; and (iii) the progressive upslope migration of active faulting within the array as a whole. The progressive upslope migration of fault activity is interpreted to reflect progressive “unbuttressing” and extensional faulting of upslope, post-evaporite units. The overall structural style and kinematic evolution identified here shares many characteristics with both ‘ rift–raft tectonics’ documented in other rifts developed above an evaporitic sub-stratum and ‘ raft tectonics’ described from passive margin basins containing thick mobile salt or shale intervals. This style of fault array evolution differs from that observed in rifts lacking mobile layers at-depth and highlights the importance of these units in the structural development of rifts.

A comprehensive seismic velocity model for the Netherlands based on lithostratigraphic layers

AbstractA seismic velocity model is necessary to map depth and thickness of subsurface layers interpreted from seismic reflection images. We have built a seismic velocity model (VELMOD-1) for the entire Netherlands area, both onshore and offshore, using non-confidential data (sonic logs, time-depth pairs, lithostratigraphic marker depths and downhole position data) of 720 boreholes in DINO - National Geoscientific Portal, and a preliminary isochore map (in seismic traveltime representation) of the layer of the Zechstein Group. The model is based on the Vint-zmidmethod applied to the following lithostratigraphic layers: Lower, Middle and Upper North Sea groups; Chalk Group; Rijnland Group; Schieland, Scruff and Niedersachsen groups; Altena Group; Lower and Upper Germanic Trias groups; Upper Rotliegend Group; and Limburg Group. Per layer, the linear least squares approximation, applied to Vintas a function of zmid, provides parameters V0and K for a linear velocity function V(z) = V0+ K · z. In VELMOD-1, K is constant, at least at the scale of structural elements, whereas V0varies with location. At borehole locations, V0is calibrated such that traveltime through the layer according to the linear velocity model equals the traveltime according to the borehole data. A kriging procedure is applied to the calibrated V0(x, y)-values resulting in an estimated V0-value at any other location. The model V0-values were determined on an areal grid with cells of 1 km × 1 km. On the same grid, kriged interval velocities constitute the model for the Zechstein Group. These interval velocities stem directly from interval velocities at borehole locations; at other positions they are also dependent on the thickness (in terms of seismic traveltime isochores) of the layer of the Zechstein Group. Maps are presented of the distributions of both V0and its standard deviation for two layers: that of the Chalk Group and that of the Lower and Upper Germanic Trias groups.

δ18O and δ13C of Permian brachiopods: A record of seawater evolution and continental glaciation

Two hundred sixty-four δ 18O and δ 13C values of Permian articulate brachiopod shells were analyzed and 81 of these values were characterized as well preserved and biostratigraphically well defined. These were then utilized for construction of baseline oxygen and carbon isotope curves for the Permian interval. In addition, 21 δ 13C whole rock values are reported for the Wordian and Capitanian. The early Permian, Asselian to Artinskian, times are characterized by ∼ 2.5‰ decrease in oxygen isotope values, from ∼ − 0.7‰ to − 3.3‰ (V-PDB). This is attributed to a ∼ 4–7 °C increase in temperature in the Southern Urals, concomitant with the retreat of the Permo-Carboniferous ice sheets and return of the 18O-depleted melt water into the oceans. The Late Permian samples from Iran (Jolfa at Kuh-e-Ali-Bashi) and China (Meishan) yield δ 18O values, and presumably temperatures, similar to those that followed the termination of the large-scale glaciation in the Lower Permian. In between, the upper Kungurian to Capitanian samples from the Delaware Basin (Guadalupe Mountains) are enriched in 18O, at − 1.5‰ to − 3‰. We have no definitive explanation for these data, but tentatively suggest that the “anomaly” can potentially be a result of evaporative enrichment of seawater in 18O, due to intracratonic arid setting of the basin. The 18O-enriched nature of the Zechstein samples (− 1.2‰ to + 2.5‰), on the other hand, is in all probability a reflection of the high evaporation rates in the Zechstein sea. The Permian interval is characterized by a relatively constant δ 13C, at about 4‰. The exceptions are again the brachiopods from the Delaware Basin (Guadalupe Mountains), which show ∼ 1.6‰ increase in the Guadalupian, to values of up to 5.9‰ in the Wordian. A tentative explanation, as in the case of oxygen, is based on the proposition that the semi-enclosed Delaware Basin was likely stratified, with sequestration of the 13C-depleted carbon to the deeper water layers and a complementary 13C enrichment in the upper oxygenated layer. The coeval open ocean water DIC may have been similar to that of the remainder of the Permian interval, at ∼ 4‰, as indicated by whole rock carbonate samples from Oman, Sicily, and Iran. In the latest Permian, the trend mimics the well-known δ 13C drop at the Permian/Triassic boundary.

Role of Cenozoic fault reactivation in controlling pre-rift plays, and the recognition of Zechstein Group evaporite-carbonate lateral facies transitions in the East Orkney and Dutch Bank basins, East Shetland Platform, UK North Sea

The East Orkney and Dutch Bank basins are located to the north of the Moray Firth rift arm in the UK North Sea. The East Orkney Basin measures approximately 60 by 30 km and is bounded to the west, north and east by large normal faults. The West Fladen High consists of a series of horsts and grabens, and separates the East Orkney Basin from the Dutch Bank Basin, which is situated immediately to the north of the Witch Ground Graben and to the west of the Fladen Ground Spur. Although exploration well results for the limited penetrations of structural highs on the margins of the East Orkney Basin have been disappointing, geophysical and geochemical analyses all appear to indicate the presence of a mature source in the region. Integration of wireline data, core samples, and the interpretation of 2D seismic data within the area allow mapping of the main structures in the basins and correlation to the adjacent well penetrations, thus not only enabling a tectonic and stratigraphic history, but also a framework for prospectivity to be established. A pillow-shaped sedimentary package at approximately 3 seconds two-way traveltime within the centre of the East Orkney Basin represents the downdip equivalent of stratigraphy on the West Fladen High, and is interpreted to be a Zechstein Group salt body, underlain by a thick sequence of Lower Permian (Rotliegend Group) and Devonian strata. Cored intervals from well 14/2-1 on the West Fladen High contain carbonates, evaporite dissolution breccias and anhydrites, implying a lateral change in sedimentary facies between structural highs and lows. Rifting took place through the Triassic and Jurassic, followed by relative tectonic quiescence in the Cretaceous, but as in adjacent areas, the Cenozoic witnessed structural inversion related to plume-generated uplift in the North Atlantic and the activation/reactivation of underlying faults coupled with deformation of the basin fill adjacent to these structures. Regional tilting from the Paleocene to early Eocene led to an influx of siliciclastics into the Dutch Bank Basin. The main play type is thought to consist of Rotliegend Group and Devonian sandstone reservoirs on the West Fladen High, sealed by Zechstein Group anhydrites or Cretaceous/Tertiary mudstones and charged by a deeply buried Devonian (Orcadian) lacustrine source. Although hydrocarbons are interpreted to have utilized normal faults as migration pathways, comparison with adjacent areas and observations of numerous oil seeps on the sea surface suggest that reservoir breach through reactivation of these faults to the seabed poses the main exploration risk. This fault reactivation has resulted from differential uplift combined with synchronous extensional faulting that affected adjacent areas, including the Inner Moray Firth.

Discussion on the North Sea Silverpit Crater: impact structure or pull-apart basin?

Ken Thomson writes: The suggestion that the Silverpit Crater is a pull-apart basin (Smith 2004) and not an impact structure (Stewart & Allen 2002) demonstrates that its origin requires further investigation. As noted by Underhill (2004) the coincidence of the structure with a salt withdrawal basin suggests that halokinesis may provide an alternative mechanism. Although the model provided by Smith involves salt movement it is overly complex as strike-slip detachment tectonics are not required. Instead gravitational collapse of the post-salt cover provides a simpler mechanism. The Southern North Sea is dominated by the development of large linear salt swells (Jenyon 1988; Stewart & Coward 1995). Elevation and tilting of the swell flanks results in gravitational instability and a range of deformation styles can develop (Jenyon 1988; Stewart & Coward 1995; Thomson 2004). The instability commonly leads to listric faulting of the post-salt cover down slope, towards the adjacent salt low (Jenyon 1988; Stewart & Coward 1995; Thomson 2004). Detachment levels for the listric faults occur at a number of stratigraphic levels. The top of the Upper Permian Zechstein Group salt, which forms the swells, provides a detachment at the base of the post-salt cover whilst detachment within the post-salt cover in both the Triassic Rot Halite and within the Upper Cretaceous chalk have also been recognized (Jenyon 1988; Stewart & Coward 1995; Thomson 2004). The majority of the flank collapse faults dip in the same direction as the cover rocks they displace (i.e. towards the salt withdrawal basin and away from the swell), but occasionally listric faults can be found which dip away from the salt withdrawal basin, towards the swell (Jenyon 1988). Associated with the listric faulting rollover anticlines can develop in the hanging walls (Stewart & Coward 1995) whilst slump folds …

Reconstruction of palaeohydrological conditions in a lagoon during the 2nd Zechstein cycle through simultaneous use of ?D values of individual n-alkanes and ?18O and ?13C values of carbonates

For the first time δ18O and δ13C values from carbonates and δD values of individual n-alkanes were used to reconstruct palaeohydrological conditions in a lagoon at the southern margin of the Central European Zechstein Basin (CEZB). A 12-m core covering the complete Ca2 interval and adjacent anhydrites (A1 and A2) was analyzed for δ18O and δ13C values of dolomitized carbonates and δD values of individual n-alkanes. δ18Ocarb values (+2‰ to +5‰ vs. VPDB) were strongly influenced by evaporation and temporal freshwater input into the lagoon. The δ13Ccarb values (−1‰ to +4‰ vs. VPDB) were controlled mainly by changes in primary production. Both isotopic ratios show an inverse relationship throughout most of the core, contradicting diagenetic alteration, since δ13Ccarb values are not altered significantly during dolomitization. Assuming a temperature range of 35–40 °C in the lagoon, δ18Ocarb values of +2.5‰ to +8‰ (vs. VSMOW) for the lagoonal water can be reconstructed. The lagoon may have desiccated twice during the Ca2 interval, as indicated by very high δ18Ocarb and low δ13Ccarb values, coinciding with increasing amount of anhydrite in the analyzed sample. These events seem to reflect not just local but a regional intra-Ca2 cyclicity. Measured δD values of the short-chain n-alkanes, namely n-C16 and n-C18 which are widely used as indicators for photosynthetic bacterial and algal input, reflect the isotopic composition of the lagoonal water. Assuming constant fractionation during incorporation of hydrogen into lipids of −160‰, an average δD value of +70‰ (vs. VSMOW) can be reconstructed for the lagoonal water, accounting for very arid conditions. The long-chain n-alkanes n-C27, n-C28, n-C29 and n-C30, thought to be derived from the leaf waxes of terrestrial higher plants, were shown to be depleted in D relative to the short-chain alkanes, therefore indicating a different hydrogen source. Terrestrial plants in arid areas mainly use water supplied by precipitation. By using a smaller fractionation of −120‰ due to evaporation processes in the leaves, reconstructed values vary between −74‰ and −9‰ (vs. VSMOW). These values are not indicating extremely arid conditions, implying that the long-chain n-alkanes were transported trough wind and/or rivers into the lagoon at the Zechstein Sea coast. δDwater values, reconstructed using the n-C16 alkane and δ18O water values, independently reconstructed on the same sample using the temperature dependant fractionation for dolomites are good agreement and suggest high amounts of evaporation affecting the coastal lagoon. Altogether, our results indicate that hydrogen isotopic ratios of n-alkanes give information on their origin and are a useful proxy for palaeoclimatic reconstruction.

The Clipper Field, Blocks 48/19a, 48/19c, UK North Sea

Abstract The Clipper Gas Field is a moderate-sized faulted anticlinal trap located in Blocks 48/19a, 48/19c and 48/20a within the Sole Pit area of the southern North Sea Gas Basin. The reservoir is formed by the Lower Permian Leman Sandstone Formation, lying between truncated Westphalian Coal Measures and the Upper Permian evaporitic Zechstein Group which form source and seal respectively. Reservoir permeability is very low, mainly as a result of compaction and diagenesis which accompanied deep burial of the Sole Pit Trough, a sub basin within the main gas basin. The Leman Sandstone Formation is on average about 715 ft thick, laterally heterogeneous and zoned vertically with the best reservoir properties located in the middle of the formation. Porosity is fair with a field average of 11.1%. Matrix permeability, however, is less than one millidarcy on average. Well productivity depends on intersecting open natural fractures or permeable streaks within aeolian dune slipface sandstones. Field development started in 1988. 24 development wells have been drilled to date. Expected recoverable reserves are 753 BCF.

Predicting distribution of remobilized aeolian facies using sub-surface data: the Weissliegend of the UK Southern North Sea

Aeolian sandstones of the Permian Rotliegend Group comprise the principal gas reservoirs of the Southern North Sea Basin. The upper portion of the reservoir interval comprises a unit informally called the ‘Weissliegend’. This unit comprises mass-flow and in-situ soft-sediment deformed deposits formed when the Zechstein Sea catastrophically flooded the Rotliegend sand sea. The Weissliegend typically exhibits poorer reservoir properties than the underlying Rotliegend reservoirs and its thickness and distribution are notoriously difficult to map. Hanging vertical sections on a regionally extensive, intra-Rotliegend super-surface has led to the recognition of a preserved topography at the top of the Rotliegend. The thickness differences within the unit between the super-surface and the top Rotliegend (termed here the Upper Aeolian Unit, UAU) represent the original dune topography modified by flood-related processes, and provide an insight into the nature and scale of the aeolian bedforms that existed within the basin prior flooding. The relationship between known Weissliegend distribution and this topography provides a predictive tool for understanding Weissliegend distribution. Mapping the preserved topography reveals preserved bedforms up to 85 m high, with a bedform spacing of 8 to 10 km. The dunes deposited ‘transverse type’ strata but the scale and spacing of the bedforms are too big compared with modern transverse dunse. Instead a more complex model is proposed. The Weissliegend has highly variable thickness (0–26 m) and is commonly thicker within the interdune hollows. The model for a preserved topography at the top of the Rotliegend is further supported by thickness trends in the overlying Zechstein units which increase into the proposed interdune lows.

Molecular evidence for basin-scale photic zone euxinia in the Permian Zechstein Sea

Previous studies of maleimides (1- H-pyrrole-2,5-diones), oxidative degradation products of chlorophylls and bacteriochlorophylls, and aromatic compounds derived from certain carotenoids (i.e., isorenieratene) indicate that photic zone euxinia (PZE) occurred at least intermittently during deposition of the German Kupferschiefer (Late Permian). However, that work was restricted to a stratigraphic profile through the Kupferschiefer of the Lower Rhine Basin, a shallow lagoon setting, and it was not established if photic zone euxinia prevailed in the larger and deeper basins of the Permian Zechstein Sea. Here, we present maleimide and porphyrin distributions in sediments from the Northern and Southern Permian basins. In samples from both basins, methyl, iso-butyl (Me, i-Bu) maleimide was recovered from the free extractable fraction and upon oxidation of vanadyl and nickel alkyl porphyrins. On structural grounds, Me, i-Bu maleimide derives ultimately from bacteriochlorophyll (bchl) c, d, or e and is thus indicative of the presence of green sulfur bacteria in the ancient Zechstein Sea. Consistent with this, C 34 and C 35 porphyrins apparently derived from bacteriochlorophyll d were also identified in the Northern Basin samples. Because green sulfur bacteria require both light and free H 2S, these results indicate that photic zone euxinia occurred at least periodically during the early history of the Zechstein Sea.

Novel haloarchaeal 16S rRNA gene sequences from Alpine Permo-Triassic rock salt.

Prokaryotic diversity in Alpine salt sediments was investigated by polymerase chain reaction (PCR) amplification of 16S rRNA genes, sequencing of cloned products, and comparisons with culturable strains. DNA was extracted from the residue following filtration of dissolved Permo-Triassic rock salt. Fifty-four haloarchaeal sequences were obtained, which could be grouped into at least five distinct clusters. Similarity values of three clusters to known 16S rRNA genes were less than 90%-95%, suggesting the presence of uncultured novel taxa; two clusters were 98% and 99% similar to isolates from Permo-Triassic or Miocene salt from England and Poland, and to Halobacterium salinarum, respectively. Some rock salt samples, including drilling cores, yielded no amplifiable DNA and no cells or only a few culturable cells. This result suggested a variable distribution of haloarchaea within different strata, probably consistent with the known geologic heterogeneity of Alpine salt deposits. We recently reported identical culturable Halococcus salifodinae strains in Permo-Triassic salt sediments from England, Germany, and Austria; together with the data presented here, those results suggest one plausible scenario to be an ancient continuous hypersaline ocean (Zechstein sea) populated by haloarchaea, whose descendants are found today in the salt sediments. The novelty of the sequences also suggested avoidance of haloarchaeal contaminants during our isolation of strains, preparation of DNA, and PCR reactions.

Bryozoen der oberpermischen Zechstein-Formation von Deutschland

The investigated bryozoan fauna of the Upper Permian Zechstein Formation includes 17 species of 9 genera, belonging mainly to the fenestellids and trepostomids and one species belonging to the tubuliporids,Corynotrypa voigtiana (King 1850). Three new species are described (Dyscritella microstoma sp. nov.,Ulrichotrypella incrustata sp. nov., andKingopora baderi sp. nov.). The Zechstein species of the generaAcanthocladia, Synocladia, Penniretepora andKalvariella display close resemblance in their internal morphology, and their derivation from a common ancestor is suggested. The distribution of the bryozoans in the Zechstein Sea was apparently controlled by factors such as lateral sedimentation, distribution of microbial mats and quality of substrates.

REEFS IN THE BASINAL FACIES OF THE ZECHSTEIN LIMESTONE (UPPER PERMIAN) OF WESTERN POLAND: A NEW GAS PLAY

Zechstein (Upper Permian) reefs have long been known to occur along the margins of the Polish Zechstein Basin where they host important gas reserves. During the last decade, reefal build‐ups have also been found in the centre of the basin on pre‐Zechstein highs such as the Wolsztyn Ridge in Western Poland. Here, early 2D seismic and subsequent more detailed 3D seismic studies and integrated geophysical and geological interpretations have led to a number of major gas discoveries. The reef complexes in the Koscian– Wielichowo region are thought to have recoverable gas reserves of more than 32B cu m.The basin‐centre reefs have been identified on the basis of palaeogeographic analysis and from the seismo‐acoustic properties of the Zechstein Limestone of which they are composed. 3D seismic profiles show that the reefs have irregular shapes in map view. Drilling results indicate that their thickness may exceed 80m, but characteristically their thickness decreases sharply at their margins corresponding to a facies change.In the Wolsztyn Ridge area, the Zechstein Limestone can be divided into five lithological units which are capped by the Werra Anhydrite. Four of these units reflect deposition in subaqueous environments whereas the uppermost unit originated in a very shallow‐water or temporarily subaerial setting. Typical “ecological” bryozoan‐stromatolitic reefs occur in the middle and upper (but not the uppermost) parts of the Zechstein Limestone, while the lower parts are dominated by bioclastic packstones and grains tones. Average reservoir porosity is 10–20%. The permeability varies significantly (0–400 mD) but values of a few tens of mD are common.The occurrence of intraclastic breccia in many boreholes suggests that the reefs may have developed on local palaeoslopes which are generally of tectonic origin. Blocks of contemporaneously lithifiedZechstein Limestone as well as steeply‐dipping laminar sheets are common in the reefs of the Wolsztyn Ridge and have also previously been recorded in the English Zechstein. Palaeoslopes elsewhere in the Wolsztyn Ridge and possibly also in other parts of the Variscan externides may host similar reefal build‐ups which could be of exploration potential.

Constraints on the genesis of the mineralization of the Alston Block, Northern Pennine Orefield, northern England

SUMMARY New evidence from fluid inclusion compositions and simple physical modelling provide important constraints on the genesis of the ores of the Alston Block in northern England. Evidence from fluid inclusions shows that probably at least four, hydrothermal fluids were involved, two of which are formed by evaporation of seawater, in some cases well beyond halite saturation. Some of the high-temperature fluid dissolved evaporites during its evolution. Physical modelling shows that neither compactional dewatering of the adjacent Carboniferous sedimentary basins nor compressional tilting and throughflow within the Northumberland Trough could have contributed significantly to the hydrothermal fluids of the orefield. Most of the high temperature hydrothermal flow must have been generated by penetrative circulation of surface brines to 8 to 10 km depth in the basement, with the upflow focused and accelerated by the high heat production of the Weardale Granite that unconformably underlies the Carboniferous rocks of the Alston Block. The orefield is considered to have formed during the Upper Permian by the following sequence of events. Initially, fluid movement in the basement was prevented by sealing of pre-existing cracks, and temperatures rose at depth. An episode of extension allowed saline water, probably from the Upper Permian Zechstein Sea, to penetrate deep into the basement, reacting with the rocks there, and becoming heated to as much as 200°C. Penetration to 10 km took a few tens of thousands of years. The granite, at 300°C at that depth, up to 100°C hotter than the surrounding basement, acted to focus the flow, and to generate concentric zones of increasing temperature fluid towards the centre. Once the basement had become cooled, the flow slowed, cracks became sealed, and the episode of hydrothermal circulation and ore formation was complete.

Palaeoceanography of the early Zechstein Sea during Kupferschiefer deposition in the Lower Rhine Basin (Germany): A reappraisal from stable isotope and organic geochemical investigations

Abstract In the study area of the Lower Rhine Basin the Permian Kupferschiefer was deposited in a shallow water environment. Water exchange from this marginal basin with the Zechstein Sea was restricted by palaeohighs. The burial depth of the shale did not exceed 1000 m during the depositional history of most parts of the basin. Maturation of the organic matter was governed only by the geothermal gradient not exceeding 68°C/km during the Late Carboniferous. Due to minor thermal alteration of organic matter, diagenetic effects on the biomarker composition and the light stable isotope ratios (C, H, O, N) of organic matter and carbonates are of minor importance. In the present study, isotopic and organic geochemical data are interpreted with respect to palaeoceanographic aspects. The data provide information about salinity variations, the nature of organisms living in the water column and the importance of bacterial activity in the sediment during deposition. From the degree of methylation of 2-methyl-2-trimethyl-tridecylchromans (MTTC) an euhaline to mesohaline (30–40‰) sea water salinity during Kupferschiefer sedimentation can be inferred. The high abundance of biomarkers derived from green/purple sulphur bacteria suggests H2S saturation of the bottom waters and maximum water depths below 100 m in the basin because these organisms live near the boundary between the photic zone and the anoxic (euxinic) bottom water at a depth of 10–30 m. Primary production in the upper water column was dominated by photosynthetic cyanobacteria or green algae. In the sediment, sulphate reduction occurred due to the availability of abundant sulphate and organic detritus from the overlying water column. Furthermore, methanogenesis was active mostly during early Kupferschiefer deposition. This is reflected by the light carbon isotopic composition of organic matter originating from recycling of CO2 generated by methane-oxidizing bacteria in the water column. Saccate pollen are the only morphologically preserved organic matter in the sediment.