Zechstein Evaporites Research Articles

Mesozoic and Cenozoic evolution of salt structures within the Polish basin: An overview

Abstract The Permian–Cretaceous Polish Basin belonged to the system of epicontinental depositional basins of Western and central Europe and was filled with several kilometres of siliciclastics, carbonates, and also thick Zechstein (approximately Upper Permian) evaporites. Its axial part (the so-called Mid-Polish Trough) characterized by the thickest Permo-Mesozoic sedimentary cover, developed above the Teisseyre–Tornquist Zone, lithospheric-scale boundary separating the East European Craton and the Palaeozoic Platform. The Polish Basin was inverted in Late Cretaceous–Paleocene times. A synthesis of studies based on seismic reflection data allowed some general rules regarding salt tectonics of the Polish Basin to be formulated. Two general classes of structures genetically related to the presence of the Zechstein evaporites have been described: peripheral structures located within NE and SW flanks of the Polish Basin, outside its axial part and structures located within its axial part. The first class of structures includes grabens bounded by listric faults detached above salt or salt pillows that developed where Zechstein evaporites were of relatively smaller thickness and where sub-Zechstein fault tectonics played a relatively smaller role. The second class of structures includes more mature salt structures such as salt pillows and salt diapirs and is related to the more axial part of the basin, characterized by relatively thicker Zechstein evaporites and by more intense basement tectonics. First salt movements (salt pillowing) took place in the Early Triassic that in certain cases was followed by the Late Triassic salt diapirism and extrusion. In Jurassic–Early Cretaceous times, no significant growth of salt structures took place. Most of the salt diapirs have been finally shaped by the Late Cretaceous inversion tectonics. Some salt diapirs also underwent Cenozoic reactivation, associated with localized Oligocene or Miocene subsidence that in some cases was followed by younger (Pliocene–Quaternary) inversion and uplift.

Lithostratigraphy of Zechstein evaporites of the central and north-western parts of the Mogilno Salt Diapir, based on boreholes Z-9 and Z-17

This paper presents the results of comprehensive mineralogical and petrographic studies conducted on evaporite rocks of the Zechstein (Upper Permian) period, extracted from boreholes drilled in the Mogilno diapir. Based on the research results, the occurrence of rock salts, potash-magnesium salts (kieseritic sylvinites), clayey salts, zubers, and anhydrites was identified. Those formations were as-signed to cyclothems PZ-3 and PZ-2. No presence of evaporites belonging to cyclothems PZ-1 and PZ-4 were discovered. The recognized rocks were mainly composed of halite, sylvine, kieserite and anhydrite. Smaller quantities of polyhalite, kainite, carbonates and clayey minerals were also found. A number of accessory minerals were identified, with their salt rock contents from several to about several tens of ppm.

Zechstein saline brines in Poland, evidence of overturned anoxic ocean during the Late Permian mass extinction event

Bromine concentrations in halite, sulfate isotopes (δ34S and δ18O), and major ion concentrations in primary fluid inclusions from three boreholes in the Late Permian Zechstein evaporites have revealed sharp variations in marine derived brines within the Polish sector of the European Southern Permian Basin. The base of the Older Halite (Na2), during the latest Permian, registers a change from sulfate-rich brines, similar in composition to modern evaporated seawater, to sulfate-depleted brines (calcium-rich). This change coincides with a drop in δ34S to values close to +9‰, not observed in δ18O counterparts. Opposite isotope (δ34S–δ18O) trends through the Na2 unit cannot be explained by changes in restriction conditions. We propose that the change to sulfate-depleted (calcium-rich) brines during halite deposition of the PZ2 (Stassfurt) cycle is related to the overturn of anoxic sulfidic deep-waters from the Panthalassa stratified superocean coinciding in time with the Permian–Triassic mass extinction event.The reconstruction of chemical changes in brines reveals two major evaporite sequences of increasing concentration that do not match the classic lithostratigraphic cycles. The first evaporite sequence (PZES-1) contain the evaporite units of the PZ1 (Werra) cycle, the PZ2 (Stassfurt) cycle, the Main Anhydrite (A3), and the base of the Younger Halite (Na3) of the PZ3 (Leine) cycle. The second evaporite sequence (PZES-2) is represented by almost the entire Na3 unit and the PZ4 (Aller) cycle.

Chemistry and isotopic composition of Rotliegend and Upper Carboniferous formation waters from the North German Basin

Element concentrations and isotopic composition ( δD, δ 18O, δ 15N of ammonium, 87Sr/ 86Sr) have been determined in highly-saline formation waters from the Rotliegend Altmark gasfield and the Upper Carboniferous Husum–Schneeren gasfield in the North German Basin. Both reservoirs have been covered by huge Zechstein evaporite caps since the Late Permian. Formation waters from the Altmark gasfield are characterized by high total dissolved solids and are enriched in δ 18O relative to meteoric waters and either show positive or negative δD values which typically follow the proposed δD vs. δ 18O paths for seawater evaporation. Formation waters stored in Upper Carboniferous sandstones show much more variable element concentrations and δ 18O and δD values than the Rotliegend Formation waters. The Br vs. Cl concentrations of the brines suggest seawater evaporation being the principal source of salinity at both sites. The ammonium concentrations in the analyzed brines are highly variable and the δ 15N isotopic composition of dissolved ammonium is controlled either by the isotopic composition of the gas in the reservoir (Rotliegend) or that of the host rocks (Upper Carboniferous). At both sites, early formation waters interacted with the sedimentary host rocks after deposition. 87Sr/ 86Sr ratios suggest that re-equilibration between fluids and their host rocks occurred in pre-Triassic times. There is no chemical or isotopic evidence that original formation waters which occur in sandstone below the overlying Zechstein evaporites were flushed out or mixed by later meteoric water as has been documented in deep-seated Mesozoic and other Paleozoic aquifers of the North German Basin.

GEOCHEMICAL AUREOLES AROUND OIL AND GAS ACCUMULATIONS IN THE ZECHSTEIN (UPPER PERMIAN) OF POLAND: ANALYSIS OF FLUID INCLUSIONS IN HALITE AND BITUMENS IN ROCK SALT

Fluid inclusions in halite and bitumens in rock salt in Upper Permian Zechstein evaporites in West Poland were studied in locations where the evaporites lie above oil and gas reservoir rocks. The samples were taken from halite intercalated within the Basal Anhydrite; this unit lies above the Main Dolomite which serves as both source rock and reservoir. Samples came from a depth of 2.3–3.2 km. A characteristic feature of the fluid (gas‐brine) inclusions was their high methane content together with the occasional presence of bitumen globules of near‐spherical form. Geochemical analyses of the bitumen in bulk samples of rock salt (including content and distribution of n‐alkanes and isoprenoids, and carbon isotope ratios) suggest an algal origin, similar to that of the oil in the underlying source rocks. For comparison, we studied fluid inclusions in halite from Zechstein evaporites in northern Poland, where hydrocarbon accumulations do not occur in underlying strata and where mostly single‐phase (brine) inclusions with a low methane content have been recorded. However, published data indicate that similar inclusions to those occurring in the Zechstein of West Poland (comprising brine with a high methane content, bitumen films and/or oil droplets) are present in other salt‐bearing sequences, where their origin is related to the thermal degradation of organic material dispersed within the salt itself. Accordingly, such fluid inclusions in an evaporite succession can only be considered to form a geochemical aureole where the bitumens in the rock salt (including those in the fluid inclusions in halite) can be compositionally linked to those in the associated oil accumulation.

Geological factors controlling Early Carboniferous carbonate platform development in the Netherlands

AbstractA deep pre‐Silesian carbonate play has recently come into focus in the Netherlands and poses some real opportunities and challenges to the explorationist. Underneath the very thick cover of Westphalian and Namurian clastic sediments, which cover almost the entire Netherlands, a yet undiscovered petroleum system may be present in Dinantian carbonates. However, lack of well control and the significant depth involved has caused pre‐Silesian formations to be under‐explored. Furthermore, seismic definition below the widespread Permian Zechstein evaporites is often very poor. Nevertheless, recent discoveries in the Caspian Sea area have shown that during the Lower Carboniferous very thick carbonate bioherms can develop. These giant reservoirs related to large build‐ups can provide the required size for deep pre‐Silesian prospects in the southern North Sea area and Dutch onshore. From the sparse well control and extrapolation of limited geological data in the Netherlands, the United Kingdom, Belgium and Germany, it is suggested that thick Dinantian platform carbonates may be present at the northern fringe of the London‐Brabant Massif. For realistic reservoir models of these platforms, areas outside the Netherlands have to be studied, for example the UK Midlands, and south of the London‐Brabant Massif, in eastern and southern Belgium. Although, the latter area has been affected by the Variscan Orogeny, the Dinantian‐aged rocks at outcrop provide a good model for the understanding of the frequency of the major depositional cycles, solution collapse, erosion and dolomitization processes.Dolomitization may be one of the most important processes that can provide good reservoir quality, and primary porosity related to reef growth, the second important process. Reservoir studies show, that significant reef growth, with associated grainstone facies on the windward side, is an important factor, to understand Dinantian‐aged reservoirs in the Caspian Sea area. Similarly, a dominant east to west wind direction may explain, the development of build‐ups and grainstone facies as an eastern fringe to the Dutch Dinantian platforms, but not on the leeward‐side of the London‐Brabant Massif in southern Belgium. Copyright © 2007 John Wiley & Sons, Ltd.

Effective sub-Zechstein salt imaging using low-frequency seismics — Results of the GRUNDY 2003 experiment across the Variscan front in the Polish Basin

The extent of the Variscan deformation front is one of the key problems of the regional geology of the Central European Permian Basin system, particularly in its Polish part. Conventional reflection seismics usually fails to produce a satisfactory image of the pre-Permian strata due to the shielding effect of Zechstein (Upper Permian) evaporites. Thus we used a novel seismic acquisition technique to study the base of the Permian complex and its Variscan basement. In the GRUNDY 2003 experiment we combined wide-angle reflection–refraction measurements with the near-vertical reflection seismics by the use of the constant geophone array with dense (100 m) receiver spacing occupying 50-km long profile. 3D design of the experiment, covering 50 × 10 km area, helped in eliminating the effect of out-of-plane propagations and local inhomogeneities. An effective integration of traveltime tomography, CDP reflection processing and prestack depth migration of wide-angle reflections applied to our data, allowed us to present the model in which we deduced the contact zone of the Variscan overthrust structure (Variscan front) with its molasse-filled foredeep. The latter might be a gas-generation zone, which is of a great importance for hydrocarbons prospecting in this area.

Basin evolution of the northern part of the Northeast German Basin — Insights from a 3D structural model

A 3D structural model for the entire southwestern Baltic Sea and the adjacent onshore areas was created with the purpose to analyse the structural framework and the sediment distribution in the area. The model was compiled with information from several geological time-isochore maps and digital depth maps from the area and consists of six post-Rotliegend successions: The Upper Permian Zechstein; Lower Triassic; Middle Triassic; Upper Triassic–Jurassic; Cretaceous and Cenozoic. This structural model was the basis for a 3D backstripping approach, considering salt flow as a consequence of spatially changing overburden load distribution, isostatic rebound and sedimentary compaction for each backstripping step in order to reconstruct the subsidence history in the region. This method allows determination of the amount of tectonic subsidence or uplifting as a consequence of the regional stress field acting on the basin and was followed by a correlation with periods of active salt movement. In general, the successions above the highly deformed Zechstein evaporites reveal a thickening trend towards the Glückstadt Graben, which also experienced the highest amount of tectonic subsidence during the Mesozoic and Cenozoic. Two periods of accelerating salt movement in the area has been correlated with the E–W directed extension during the Late Triassic–Early Jurassic and later by the Late Cretaceous–Early Cenozoic inversion, suggesting that the regional stress field plays a key role in halokinesis. The final part of this work dealt with a neotectonic forward modelling in an attempt to predict the future topography when the system is in a tectonic equilibrium. The result reveals that many of the salt structures in the region are still active and that future coastline will run with a WNW–ESE trend, arguing that the compressional stresses related to the Alpine collision are the prime factor for the present-day landscape evolution.

Different modes of the Late Cretaceous–Early Tertiary inversion in the North German and Polish basins

Several selected seismic lines are used to show and compare the modes of Late-Cretaceous–Early Tertiary inversion within the North German and Polish basins. These seismic data illustrate an important difference in the allocation of major zones of basement (thick-skinned) deformation and maximum uplift within both basins. The most important inversion-related uplift of the Polish Basin was localised in its axial part, the Mid-Polish Trough, whereas the basement in the axial part of the North German Basin remained virtually flat. The latter was uplifted along the SW and to a smaller degree the NE margins of the North German Basin, presently defined by the Elbe Fault System and the Grimmen High, respectively. The different location of the basement inversion and uplift within the North German and Polish basins is interpreted to reflect the position of major zones of crustal weakness represented by the WNW-ESE trending Elbe Fault System and by the NW-SE striking Teisseyre-Tornquist Zone, the latter underlying the Mid-Polish Trough. Therefore, the inversion of the Polish and North German basins demonstrates the significance of an inherited basement structure regardless of its relationship to the position of the basin axis. The inversion of the Mid-Polish Trough was connected with the reactivation of normal basement fault zones responsible for its Permo-Mesozoic subsidence. These faults zones, inverted as reverse faults, facilitated the uplift of the Mid-Polish Trough in the order of 1–3 km. In contrast, inversion of the North German Basin rarely re-used structures active during its subsidence. Basement inversion and uplift, in the range of 3–4 km, was focused at the Elbe Fault System which has remained quiescent in the Triassic and Jurassic but reproduced the direction of an earlier Variscan structural grain. In contrast, N-S oriented Mesozoic grabens and troughs in the central part of the North German Basin avoided significant inversion as they were oriented parallel to the direction of the inferred Late Cretaceous–Early Tertiary compression. The comparison of the North German and Polish basins shows that inversion structures can follow an earlier subsidence pattern only under a favourable orientation of the stress field. A thick Zechstein salt layer in the central parts of the North German Basin and the Mid-Polish Trough caused mechanical decoupling between the sub-salt basement and the supra-salt sedimentary cover. Resultant thin-skinned inversion was manifested by the formation of various structures developed entirely in the supra-salt Mesozoic–Cenozoic succession. The Zechstein salt provided a mechanical buffer accommodating compressional stress and responding to the inversion through salt mobilisation and redistribution. Only in parts of the NGB and MPT characterised by either thin or missing Zechstein evaporites, thick-skinned inversion directly controlled inversion-related deformations of the sedimentary cover. Inversion of the Permo-Mesozoic fill within the Mid-Polish Trough was achieved by a regional elevation above uplifted basement blocks. Conversely, in the North German Basin, horizontal stress must have been transferred into the salt cover across the basin from its SW margin towards the basins centre. This must be the case since compressional deformations are concentrated mostly above the salt and no significant inversion-related basement faults are seismically detected apart from the basin margins. This strain decoupling in the interior of the North German Basin was enhanced by the presence of the Elbe Fault System which allowed strain localization in the basin floor due to its orientation perpendicular to the inferred Late Cretaceous–Early Tertiary far-field compression.

The Mesozoic–Cenozoic structural framework of the Bay of Kiel area, western Baltic Sea

A dense grid of multichannel high-resolution seismic sections from the Bay of Kiel in the western Baltic Sea has been interpreted in order to reveal the Mesozoic and Cenozoic geological evolution of the northern part of the North German Basin. The overall geological evolution of the study area can be separated into four distinct periods. During the Triassic and the Early Jurassic, E–W extension and the deposition of clastic sediments initiated the movement of the underlying Zechstein evaporites. The deposition ceased during the Middle Jurassic, when the entire area was uplifted as a result of the Mid North Sea Doming. The uplift resulted in a pronounced erosion of Upper Triassic and Lower Jurassic strata. This event is marked by a clear angular unconformity on all the seismic sections. The region remained an area of non-deposition until the end of the Early Cretaceous, when the sedimentation resumed in the area. Throughout the Late Cretaceous the sedimentation took place under tectonic quiescence. Reactivated salt movement is observed at the Cretaceous Cenozoic transition as a result of the change from an extensional to compressional regional stress field. The vertical salt movement influenced the Cenozoic sedimentation and resulted in thin-skinned faulting.

Sealing of fluid pathways in overpressure cells: a case study from the Buntsandstein in the Lower Saxony Basin (NW Germany)

We studied veins in the Triassic Buntsandstein of the Lower Saxony Basin (NW Germany) with the aim of quantifying the evolution of in-situ stress, fluids and material transport. Different generations of veins are observed. The first generation formed in weakly consolidated rocks without a significant increase in fracture permeability and was filled syntectonically with fibrous calcite and blocky to elongate-blocky quartz. The stable isotopic signature (δ18O and δ13C) indicates that the calcite veins precipitated from connate water at temperatures of 55–122°C. The second vein generation was syntectonically filled with blocky anhydrite, which grew in open fractures. Fluid inclusions indicate that the anhydrite veins precipitated at a minimum temperature of 150°C from hypersaline brines. Based on δ34S measurements, the source of the sulphate was found in the underlying Zechstein evaporites. The macro- and microstructures indicate that all veins were formed during subsidence and that the anhydrite veins were formed under conditions of overpressure, generated by inflation rather than non-equilibrium compaction. The large amount of fluids which are formed by the dehydrating gypsum in the underlying Zechstein and are released into the Buntsandstein during progressive burial form a likely source of overpressures and the anhydrite forming fluids.

Influence of upwelling Zechstein sulphate on the concentration and isotope signature of sedimentary sulphides in a fluvioglacial sand aquifer

Very low concentrations of total S, mainly sedimentary sulphides, were quantitatively extracted from Quaternary sands of the Elbe Basin, using HNO 3, Br 2 and HCl, to distinguish 3 aquifer zones: • an upper aerobic section, containing low concentrations (only a few ppm) of non-sulphidic S compounds, • the central and lower part of the aquifer, dominated by 34S-depleted sedimentary Fe sulphides, formed by reduction of infiltrating SO 4, derived from groundwater recharge, and • the lowest 5–10 m of the aquifer, containing high concentrations of 34S-enriched sulphides. The latter originated from dissolved Zechstein SO 4, which was reduced during upwelling through the organic-rich Tertiary aquiclude. H 2S and HS − reacted and precipitated with Fe and other metal ions shortly after migration into the C org-poor Quaternary aquifer. The sulphides yield valuable information concerning the ascent of confined saline solutions from isolated Zechstein evaporites inside the “Mühlberger Graben”, which is covered by Cenozoic sediments and whose extension and boundaries are therefore not well defined. Only a few locations, close to faults and geological windows, show deep-water admixture sufficiently strong to cause visible changes in hydrochemistry and isotopic ratios of SO 4 and DIC directly above the base of the Quaternary. Sulphides showing different origins may possibly be used in other areas to provide information concerning underlying geology and hydrodynamics.

Fluid systems and mineralization in the north German and Polish basin

AbstractThe North European Basin hosts mineral deposits like the Kupferschiefer and the Mississippi Valley Type deposits in the Silesian sub‐basin in Poland. The basement to this basin, exposed in the Harz Mts and in the Flechtingen and Calvörde Blocks, contains Mesozoic Pb–Zn vein mineralization and barite–fluorite deposits as well as massive hematite veins in the Rotliegend volcanics. A comparison of the mineralizing models of these deposits with results from a basin‐wide petrographic, fluid inclusion and stable isotope study shows that the genesis of the mineral deposits can be explained by fluid systems that were active during different stages of basin evolution. These comprise syn‐ to post‐magmatic fluids derived from or mobilized in the course of the Rotliegend magmatism, fluids convecting in the Rotliegend units during the extensional basin subsidence in the Permo‐Triassic and originating from progressive devolatilization of the basin sequence and fluids derived from the overlying Zechstein evaporites. Deep‐reaching fault systems developing during the Cretaceous tectonic reactivation enhanced fluid percolation from the surface to the deep sections of the basin sequence. Identification and correlation of these fluids across the basin and in the mineralizations provide the base for a basin‐wide metallogenetic model.

Large-scale carbon isotope fractionation in evaporites and the generation of extremely 13C-enriched methane

Petrographic, fluid-inclusion, geochemical, and gas stable isotope data are reported here for a Permian Zechstein evaporite sequence. This deposit is a geochemically unaltered sequence. Bromine concentrations show a continuous evaporation profile with little postdepositional alteration in halite chemistry. Bacterial fermentation gases, identified in primary inclusions, change from an N 2 -H 2 S composition in the lower-middle halite series to a CH 4 -H 2 composition in the upper halite and potash series. Carbon isotope results for CH 4 show a 1 3 C enrichment up-sequence from typical biogenic values of -45‰ to -50‰ to extremely unusual 1 3 C-enriched values as high as +21‰. The δD values for these 1 3 C-enriched CH 4 gases range from -240‰ to -377‰. A model is proposed for the formation of the CH 4 gases whereby the dominant isotopic fractionation process controlling the system was evaporation of the brines. This generated a progressive 1 3 C enrichment in the carbon in the residual brines due to preferential loss of 1 2 CO 2 to the atmosphere. The resulting CH 4 generated in the sediments, as evaporation and precipitation advanced, recorded this 1 3 C enrichment in the carbon reservoir. Therefore, the isotopic profile observed in this sequence today represents a primary feature with little evidence for postdepositional migration.

Geochemistry and mineralogy of Rotliegend metavolcanic mafic rocks from Poland: pervasive low-grade metamorphism versus parent rock signature

Abstract Permian volcanic rocks from the Gorzów Wielkopolski region (NW Poland), although pervasively altered by low-grade metamorphism, still preserve the geochemical characteristics of continental mafic volcanic rocks formed by partial melting of an enriched mantle source. The metamorphic assemblage comprises corrensite, pumpellyite, laumontite, quartz and chalcedony, albite, calcite and solid bitumen (major components). Petrological studies combined with microthermometric determinations indicate a low-pressure zeolite-greenschist-facies transitional zone metamorphic grade with a clockwise (pressure-temperature) P-T path: earliest event 140–210 °C and 630–760 bar; metamorphic peak 220–300 °C and 950 bar; youngest episode: T ⩾ 130 °C and 630–760 bar. The metamorphism of the Rotliegend volcanic rocks is generally ascribed to penetration of upwelling fluids released from clastic rocks underlying the extrusive Permian unit. However, the ubiquitous occurrence of anhydrite in the altered volcanic rocks suggests an influence of pore water from the overlying Zechstein evaporite sequence. The source of metamorphic heat can be tentatively assigned to abnormal heat flow and/or exothermic reactions during magmatic mineral alteration processes. Dating of metamorphism in neighbouring areas suggests an Upper Jurassic thermal event related to the upwelling of a mantle diapir during the initiation and early evolution of the North Atlantic rift.

Stable isotope relationships of mineralisation in theNorth Eastern German Basin

Stable isotope systematics ( δ 34S, δ 18O, δ 13C) of carbonates, sulphates and sulphides of hydrothermal veinsand fracture infills in the North Eastern German Basin (NEGB) give evidence of a complex history of fluid input from at least two sources: fluids equilibrated with the Zechstein evaporites and fluids ascending from deeper sources such as the Rotliegend and Upper Carboniferous volcanics. The different fluid events can be related to mineralising events of vein deposits in the Harz Mountains and the polymetallic Kupferschiefer mineralisations.

The Corvette Field, Block 49/24, UK Southern North Sea

abstract Corvette is a small prolific gas reservoir with reserves of 211 BSCF located on the Indefatigable Shelf in the Southern North Sea. The reservoir is the Permian Rotliegend aeolian sandstone, capped by Zechstein evaporites and sourced from the Carboniferous Coal Measures. The structure is a 'pop up' between the Gawain and Baird Fields. The field was discovered in 1996 and brought on production in 1999, with gas being evacuated via the Leman field to the Shell/Esso Bacton gas terminal.

The Barque Field, Blocks 48/13a, 48/14, UK North Sea

Abstract The Barque Field is associated with some of the earliest gas discoveries in the southern North Sea. In the Sole Pit area the reservoir, the Rotliegend Group Leman Sandstone Formation of Lower Permian age, occurs between Carboniferous Coal Measures, which source the gas, and Zechstein evaporites which form an excellent seal. Primarily aeolian, the sandstone has generally very low permeability resulting from deep burial of the Sole Pit Trough. The deepest burial and hence the maximum diagenetic damage to the reservoir was achieved in the early Late Cretaceous prior to two phases of inversion in Late Cretaceous and Mid-Tertiary. Compaction and diagenesis reduced reservoir permeability to such an extent that parts of the field would be non-productive were it not for the presence of effective natural fracture zones and well stimulation by hydraulic fracturing techniques. Though appraisal and evaluation have been relatively extensive, remaining uncertainties dictated a conservative development in conjunction with the adjacent Clipper Field. A selected initial area was developed for first gas in October 1990. Good reservoir performance from horizontal wells led to later development of the whole field.

The Sean North, Sean South and Sean East Fields, Block 49/25a, UK North Sea

Abstract Sean North, Sean South and Sean East are small prolific gas fields located on the Indefatigable Shelf in the Southern North Sea. They, like most of the other fields in the area, have a Carboniferous source, a Rotliegend aeolian sandstone reservoir and a Zechstein evaporite cap rock. Sean North and South have been developed to fulfil a peak-shaving role, being produced for only a few days per year in times of high gas demand when they can produce at rates of up to 600 MMSCF/D. East Sean is sold to the direct market. Reserves for the fields are 234 BCF (North), 488 BCF (South) and 127 BCF (East).

Structure and quantification of processes controlling the evolution of the inverted NE-German Basin

A NE–SW reflection seismic transect crossing the entire NE-German Basin (NEGB) was investigated in detail to study the interaction between tectonics and sedimentation during the multiphase evolution of the region. By restoring a geological section across the post-Rotliegend, the kinematic evolution was reconstructed and rates of important geological processes such as erosion or basin shortening were quantified. The present day structural style of the NEGB is the result of several major deformation events: (1) regional subsidence accompanied by local thin-skinned E–W extension during the Permo-Triassic/Jurassic was followed by basin inversions at the (2) Jurassic/Cretaceous, and at the (3) Cretaceous/Tertiary boundary. Zechstein salt played an important role since mobile Zechstein evaporites decoupled the supra-salt from the sub-salt succession and allowed the transmission of deformation over a large distance. Of major significance to the Late Cretaceous structural development was the crustal-scale upthrusting of the Calfoerde Block along the Gardelegen Fault at the southern basin margin. The accompanying horizontal shortening has a minimum value of 9 km, and led to the development of thrusts and folds within the post-Zechstein succession.