Silver Pit Research Articles

Extent and variability of Mesozoic‐Cenozoic multi‐stage salt diapirs in the Southern Permian Basin, Southern North Sea

AbstractSalt structures can be used as an archive for tectonic and depositional processes as all salt structures respond distinctively. Few salt tectonic studies have investigated the evolution of multi‐stage salt structures, nevertheless, no previous study had systematically identified, mapped, nor classified, the evolution of multi‐stage salt structures in a regional study. Decades of hydrocarbon exploration and the heavily dense 3D seismic data available, make the Southern North Sea one of the best natural laboratories to investigate the evolution of salt structures. The Southern North Sea salt basin is a Late Permian Zechstein salt mega‐basin containing a myriad of salt structures. The complex tectonic evolution of the Southern North Sea created diverse Mesozoic structural sub‐basins with different tectonostratigraphic evolutions. We defined a nomenclature, linked to the mega‐stratigraphic sequences, for the classification of salt structures. We used a Two‐Way‐Travel‐Time 3D seismic reflection dataset and time‐thickness variations around salt structures to systematically analyse the evolution of salt structures across the diverse structural sub‐basins of the Southern North Sea. Multi‐stage salt diapirs were triggered halokinetically in the Early Triassic and are linked to regional palaeo‐depocentres controlled by the sub‐Zechstein structural configuration. Multi‐stage salt diapirs in the different sub‐basins evolved through three different regional phases and up to five distinctive local stages. The most complex salt diapirs developed in the Central Graben, Sole Pit High and Silver Pit Basin, where multi‐stage salt diapirs showed 4–5 local stages of salt diapirism. The multi‐stage evolution of salt structures should be thoroughly investigated to reduce risks and uncertainties in the energy sector and net zero projects, such as in carbon capturing and storage projects, and energy storage in man‐made salt caverns.

The Bytham river reconsidered

The English Midlands is one of the most important regions for British Pleistocene geology. The Middle Pleistocene glacial sediments of the region, termed the Wolston Formation, overlie deposits of a pre-existing river system, the Baginton Formation. The latter are characteristically composed of quartz-rich sediment derived from underlying Triassic bedrock. This SW–NE aligned proto-Soar system was overridden by the Wolstonian ice. Its lack of terraces or discrete sedimentary units implies that the river can only have existed for a limited period. The deposits accumulated during a period of climate change from boreal (?interstadial) to subarctic conditions. A reinterpretation of these sediments as the headwaters of a pre-Anglian ‘Bytham river’, aligned towards East Anglia across the Fenland, where they were linked to the Ingham Formation deposits, found favour for two decades. This was especially so when the Wolston Formation sediments were reassigned to the earlier, Anglian Stage. However, recent work has shown that the ‘Bytham river’ could not have existed in the form suggested by some authors since pre-Anglian-age fluvial sediments are absent from the eastern Fenland margin, except possibly certain deposits at High Lodge, Suffolk. Therefore a re-examination of the evidence is required.It is concluded that the Wolstonian glaciation (late Saalian, Marine Isotope Stage [MIS] 6) indeed post-dates that during the Anglian Stage (Elsterian, MIS 12). In addition, it appears that the Baginton (proto-Soar) stream, as originally conceived, almost certainly did not exist before the early Wolstonian (early Saalian) Stage, following a significant period of fluvial incision. Rather than being aligned towards East Anglia, the substantial proto-Soar flowed into south Lincolnshire and thence potentially to the North Sea via the Wash and the Silver Pit depression. By contrast, the pre-Anglian Ingham Formation deposits of central East Anglia were almost certainly deposited by a separate proto-Trent river that drained the East Midlands over an extended period as a left-bank Thames' tributary. Together these conclusions reinforce the view that the main drainage alignment in southern England during the pre-Anglian period was NW to SE parallel to the regional tilt of the landmass.

The Quaternary history of the Wash fluvial network, UK

This paper appraises and compares the Middle–Upper Pleistocene sedimentary sequences preserved in the fluvial systems draining into the Fenland Basin and the Wash estuary. Of the main Fenland rivers the longest records, which extend back to the initial Anglian (glacial) formation of the basin, are found in the Great Ouse and its tributaries, particularly the Cam and the Nar. These sequences preserve sediments representing all four post-Anglian interglacials. The Nene also has an extensive post-Anglian history, with evidence for a Hoxnian estuary that is presumed to have been the precursor of the Wash. North of the Nene, however, the Welland and Witham (proto-Trent) have relatively short sequences, which are thought to commence with a later (post-Anglian–pre-Devensian) glaciation that affected Lincolnshire and fed the previously-recognized Tottenhill outwash delta south of Kings Lynn. Prior to Devensian deglaciation the Witham valley was occupied by the Trent, which was the trunk river of the late Middle Pleistocene Wash system. During periods of low sea level the river would have extended north-eastwards across what is now the floor of the North Sea, possibly via the Inner Silver Pit. Several of the central Fenland sequences show evidence of infrequent terrace formation during the late Quaternary, although this might in part be due to poor vertical separation between terraces, so that differentiating them has been difficult; this has been exacerbated by mixed biostratigraphical signals due to the preservation of sediments representing more than one interglacial beneath a single terrace surface. In several of the systems there is evidence for valley rejuvenation to the lowest terrace or valley-floor level during the MIS 4–3 transition. The observed differences within what, during the predominant periods of lower sea level, would have been a single Wash river system are difficult to explain.

Impact of long-term benthic trawl disturbance on sediment sorting and biogeochemistry in the southern North Sea

Benthic trawling has a recognised impact on sediment whole organism communities, yet little is known about its impact on sediment biogeochemistry. On 2 cruises in October 2001 and July 2002, we measured sediment characteristics (particle size distribution, porosity and organic matter (OM)) and sediment metabolism (oxygen uptake, denitrification, sulphate reduction and sediment- water nutrient exchange) along gradients of trawling activity at 14 sites in 2 regions of the southern North Sea: one with low tidal disturbance but high trawling disturbance (Outer Silver Pit, OSP), the other with high tidal disturbance but lower trawling disturbance (Thames). There was no measurable impact of trawling activity on oxygen uptake, denitrification or nutrient exchange in either region. In contrast, at the high trawling disturbance sites in the OSP only, there was both a shift in particle size distribution (towards fines) and a greater rate of sulphate reduction (volume specific rates 0.83 and 0.49 nmol SO4 2- cm -3 h -1 at high and low impact sites, respectively), but not in the Thames. In addi- tion, areal rates of sulphate reduction were positively correlated with both silt content (i.e. associated with organics) and long-term trawling in the OSP but not in the Thames. Biogeochemical processes in the upper layers of sediment, both oxic and suboxic, seemed unaffected by trawling in the long- term. In deeper anoxic sediment, however, mineralisation via sulphate reduction may be stimulated by the extra disturbance, at least in areas where tidal energy is slight.

Sediment Deposition in Offshore Deeps of the Western North Sea: Questions for Models

A three-dimensional hydrodynamic model is used to investigate the potential for offshore bathymetric deeps in the North Sea to act as fine sediment traps. Simple conditions are established, representative of the observed conditions, and the response of the model investigated. Two deeps are considered here, Inner Silver Pit and Sole Pit. The model study suggests that in Inner Silver Pit deposition of fine sediment is only possible under weak tidal (neap) flows. Under all other conditions the flow is too energetic to allow deposition. A transport path for fine sediment across Inner Silver Pit is established but does not significantly modify the sediment dynamics in the pit. Experiments including wave action show that 50-year return period waves induce bed shear velocities sufficiently large to remove coarse sediment. For Sole Pit a cycle of deposition followed by erosion at spring tides will inhibit deposition in the pit. Long term accretion of fine sediment does not seem possible in either pit.

Trawling disturbance can modify benthic production processes

Summary Trawling disturbance has wide‐ranging impacts on the marine environment and is well known to modify benthic habitat and community structure. This has led to speculation about the positive and negative impacts of trawling on ecosystem processes such as production. Existing theory suggests that frequent trawling disturbance may lead to the proliferation of smaller benthic species, with faster life histories, because they can withstand the mortality imposed by trawling and benefit from reduced competition or predation as populations of larger species are depleted. Since smaller species are more productive, trawling disturbance may ‘farm the sea’, with knock‐on benefits for consumers, including fish populations. We conducted the first large‐scale studies of trawling effects on benthic production across quantified gradients of trawling disturbance on real fishing grounds in two regions (Silver Pit and Hills) of the North Sea. There were 27‐ and 10‐fold differences in levels of beam trawl disturbance among the Silver Pit and Hills sites, respectively. Size structure was described using normalized size‐spectra, and the slopes and intercepts of these spectra were related to levels of trawling disturbance. Production was estimated from the size spectra, using a new allometric relationship between body mass and the production to biomass (P:B) ratio of marine invertebrates. The general validity of the relationship was confirmed using a phylogenetic comparative approach. In the Silver Pit region, trawling led to significant decreases in infaunal biomass and production. The abundance of larger individuals was depleted more than smaller ones, as reflected by the positive relationship between the slope of the normalized size spectra and trawling disturbance. The effects of trawling disturbance were not significant in the epifaunal community. In the Hills region, where the range of trawling disturbance was lower, trawling disturbance did not have significant effects on biomass or production. In the Silver Pit, relative infaunal production (production per unit biomass) rose with increased trawling disturbance. This was attributable largely to the dominance of smaller animals in the disturbed communities. The increase in relative production did not compensate for the loss of total production that resulted from the depletion of large individuals. There was some evidence for the proliferation of small polychaetes at moderate levels of disturbance, but at higher levels of disturbance their biomass and production fell. We conclude that reported increases in the biomass and production of small infaunal invertebrates in the North Sea are attributable largely to recent increases in primary production that were driven by climate change, and not to the effects of trawling disturbance.

Impacts of trawling disturbance on the trophic structure of benthic invertebrate communities

Bottom trawling causes chronic and widespread disturbance to the seabed in shallow shelf seas and could lead to changes in the trophic structure and function of benthic communities, with important implications for the processing of primary production and the wider functioning of the marine ecosystem. We studied the effects of bottom trawling on the trophic structure of infaunal and epifaunal benthic communities in 2 regions (Silver Pit and Hills) of the central North Sea. Within each region, we quantified long-term (over 5 yr) differences in trawling disturbance at a series of sites (using sightings data from fishery protection flights), and related this to differences in the biomass and trophic structure of the benthic community. There were 27- and 10-fold differences in levels of beam trawl disturbance among the Silver Pit and Hills sites respectively, and we estimated that the frequency with which the entire area of the sites was trawled ranged from 0.2 to 6.5 times yr -1 in the Silver Pit and 0.2 to 2.3 times yr -1 in the Hills. The impacts of fishing were most pronounced in the Sil- ver Pit region, where the range of trawling disturbance was greater. Infaunal and epifaunal biomass decreased significantly with trawling disturbance. Within the infauna, there were highly significant decreases in the biomass of bivalves and spatangoids (burrowing sea-urchins) but no significant change in polychaetes. Relationships between trophic level (estimated using nitrogen stable isotope composition, δ 15 N) and body mass (as log2 size classes) were rarely significant, implying that the larger individuals in this community did not consistently prey on the smaller ones. For epifauna, the relationships were significant, but the slopes or intercepts of the fitted linear regressions were not significantly related to trawling disturbance. Moreover, mean δ 15 N of the sampled infaunal and epi- faunal communities were remarkably consistent across sites and not significantly related to trawling disturbance. Our results suggest that chronic trawling disturbance led to dramatic reductions in the biomass of infauna and epifauna, but these reductions were not reflected in changes to the mean trophic level of the community, or the relationships between the trophic levels of different sizes of epifauna. The trophic structure of intensively trawled benthic invertebrate communities may be a robust feature of this marine ecosystem, thus ensuring the efficient processing of production within those animals that have sufficiently high intrinsic rates of population increase to withstand the levels of mortality imposed by trawling.

Long-term changes in the benthic communities on North Sea fishing grounds

The North Sea has been subjected to fishing activity for many centuries. However, improvements in both fishing vessels and trawling gears since the early 1900s have meant that fishing intensity has increased. A resultant increase in the areas trawled and the use of heavier and potentially more destructive gears probably had effects on the marine community. Information on benthic communities within the North Sea, from both published and unpublished sources, has been compiled to provide a long-term data set of changes in the marine benthos on five selected fishing grounds over 60 years. In two of these (Dogger Bank and Inner Shoal), there was no significant difference in community composition between the early 1920s and late 1980s. In the remaining three areas (Dowsing Shoal, Great Silver Pit, and Fisher Bank) significant differences were observed. However, these were the result of changes in abundance of many taxa rather than large-scale losses of sensitive organisms. These results suggest that fishing has influenced benthic communities in the North Sea. The possibility remains that fishing-induced changes had occurred at the Dogger Bank and Inner Shoal prior to the 1920s.

Foraging ranges, diets and feeding locations of gannets Morus bassanus in the North Sea:evidence from satellite telemetry

We used satellite telemetry to examine the foraging ranges, feeding locations and travel speeds of 17 chick-rearing gannets Morus bassanus from the Bass Rock, SE Scotland. Regurgitates indicated that birds at the colony exploited a wide range of prey, frequently including 0-group sandeels (<10 cm in length) and mature mackerel and herring (up to 33 cm) in the diet. The maximum foraging range was 540 km, and the mean distance to the furthest point from the colony on any one trip was 232 km. Destinations of foraging trips covered a wide area of the North Sea, with a non-random distribution and a higher than expected proportion of trips to the NE (generally in the vicinity of Buchan Deep and Halibut Bank) and to the SE (mostly between Farne Deep, Dogger Bank and Outer Silver Pit) of the colony. Foraging trips lasted 13 to 84 h, and trip duration explained 94% of the variance in maximum distance from the colony, indicating that distance travelled could be predicted with a high degree of accuracy from time spent at sea. However, the average speed of travel during foraging trips (15 km h-1) was considerably lower than maximum ground speed (~55 km h-1 in most cases). The results of this study suggest that gannets breeding at the Bass Rock utilize a wide range of species and sizes of prey over a large area of the North Sea, and that they focus their activity on bathymetric features that are probably associated with high primary production. Further data are now required to examine the foraging ranges and feeding locations of gannets in different oceanographic regions in order to obtain a broader understanding of how gannets make use of different marine environments.

Foraminiferal isoleucine epimerization determinations from the Nar Valley Clay, Norfolk, UK: implications for Quaternary correlations in the southern North Sea basin

Fully temperate freshwater, brackish and marine sediments overlying Anglian till and glacilacustrine sediments in the Nar Valley area of northwest Norfolk, UK, have been attributed to the Middle Pleistocene Hoxnian temperate stage on palynological grounds, and basal peats associated with this sequence have been recently correlated with oxygen isotope stage 9 on the basis of a series of 230Th/238U dates (mean 317±14 ka). At Tottenhill these sediments (Nar Valley Freshwater Beds, Nar Valley Clay) underlie a deltaic complex attributed to the Wolstonian ice margin. The lithostratigraphical relations between the major formations in the Nar Valley, and the pollen stratigraphy of the fully temperate sequence, are very similar to the Pleistocene sequence in the Inner Silver Pit area of the southern North Sea, and correlation has been proposed between the successions described from these two localities. However, the Inner Silver Pit sequence has yielded aminostratigraphic data consistent with isotopic stage 11.Benthic foraminiferal assemblages and foraminiferal amino-acid determinations have been investigated from the Nar Valley Clay in order to test further the palaeoenvironmental setting of the sequence and to help resolve the age of the sequence and correlation with the Inner Silver Pit interglacial. The foraminiferal assemblages support previous sedimentological and palaeontological evidence for a transgressive tendency within this sequence. Multiple isoleucine epimerization determinations on Ammonia beccarii and Aubignyna perlucida from five levels within the Nar Valley Clay give mean aIle/Ile ratios of 0.135 and 0.111, respectively. The A. beccarii ratios are much lower than mean aIle/Ile ratios on equivalent species from the interglacial sequence in the Inner Silver Pit (upper Sand Hole Formation), which are close to 0.2, and the two datasets fail to overlap at the 1σ level. The new aminostratigraphic ratios indicate correlation of the Nar Valley Clay with oxygen isotope stage 9, and therefore support the pre-existing 230Th/238U data.These results suggest that two temperate stages of Hoxnian palynological affinity are present in the Quaternary record of East Anglia and the southern North Sea basin, a conclusion consistent with independent new U-series data from other Hoxnian sites in East Anglia. An alternative model in which the amino-acid ratios are explained as a function of different post-depositional thermal histories, related to length of cover by ice and water, is discussed but considered unlikely. The conclusions have important implications for the timing and number of glacial events in and around the southern North Sea basin, and help to resolve discrepancies in relative sea-level histories and biogeography in temperate sequences hitherto accommodated within a single stage.

A Middle Pleistocene glacial–interglacial succession in the Inner Silver Pit, southern North Sea: foraminiferal stratigraphy and amino acid geochronology

Benthic foraminiferal data from the depth interval between 42.0 and 30.5 m in the Inner Silver Pit core 81/52A in the southern North Sea has revealed a glacial–interglacial succession comprising a 5 m thick interglacial deposit. The foraminiferal assemblages initially indicate ice-proximal conditions (Zone 52-1) from a deglaciation period. A transitional zone (Zone 52-2) with faunal indication of relatively deeper water and slightly higher but still subarctic temperature conditions, is followed by a true interglacial succession (called the Inner Silver Pit Interglacial) with increased faunal diversity and with a high content of warm water species (Zone 52-3). A decrease in water depth is registered towards the top of the interglacial succession. The assemblage in the topmost sample (Zone 52-4) indicates a return to high arctic glacial marine conditions. Foraminiferal faunas in three vibrocores obtained from the slopes of the neighbouring Inner Silver Pit valley are correlated with the interglacial assemblages in Zone 52-3 and with the succeeding glacial marine Zone 52-4 of the Inner Silver Pit core 81/52A. Based on the degree of isoleucine epimerisation in benthic foraminifera the interglacial deposit is referred to the Middle Pleistocene, and a correlation with oxygen isotope stage 9 is suggested.

A middle pleistocene shallow marine interglacial sequence, inner silver pit, southern north sea: pollen and dinoflagellate cyst stratigraphy and sea-level history

The Sand Hole Formation is a Pleistocene seismic–stratigraphic unit of restricted distribution in the Inner Silver Pit, southern North Sea. Pollen and dinoflagellate cyst analyses of this formation, penetrated by British Geological Survey borehole 81/52A and three juxtaposed vibrocores, enables division of the formation into two sequences of contrasting palaeoclimatic affinity. The lower sequence contains poorly-preserved pollen assemblages of low concentration dominated by residual reworked elements, and is coincident with cold, shallow marine dinoflagellate cyst and foraminiferal assemblages. This sequence overlies till and glacimarine sediments of the Anglian/Elsterian Swarte Bank Formation, and is also interpreted as late Anglian/Elsterian in age. The upper sequence contains well-preserved, high concentration, pollen spectra recording the latter half of an interglacial stage with Hoxnian affinities. The strong terrigenous signal preserved in this sequence suggests a subtidal depositional setting within a region of freshwater influence; correlation of the vibrocores with the longer record preserved in borehole 81/52A indicates that interglacial sedimentation rates increase southwards towards a suggested terrigenous source in the Wash area. The first half of the interglacial cycle is missing, indicating that a hiatus is present between the two sequences preserved within the Sand Hole Formation, but the termination of the interglacial is clearly preserved. The spatial and temporal distribution of the sediments and bounding unconformities in the region are consistent in recording the transgressive ravinement surface, highstand systems tract and subsequent regression within an interglacial eustatic cycle. The character, age and elevation of the preserved facies is typical of a glacio-isostatically controlled emergence cycle and lends support to the notion that this temperate stage (oxygen isotope stage 9) immediately followed extensive glaciation during the late Anglian/Elsterian.

Palaeoecology and geochemistry of shallow marine ostracoda from the sand hole formation, inner silver pit, southern north sea

Ostracods were obtained from the Sand Hole and Swarte Bank Formations of Tappin (1991)in BGS borehole 81/52A, cored from the Inner Silver Pit, southern North Sea. The faunal assemblages were dominated by Sarsicytheridea punctillata, Elofsonella concinna and Acanthocythereis dunelmensis. The assemblages recovered reflect a transition from glaciomarine conditions in the Swarte Bank Formation up into a cold-temperate marine environment in the Sand Hole Formation with deteriorating temperatures indicated at the top of the interval studied. The palaeoecology of the ostracod assemblages recovered is compared to foraminiferal data from the same section and to ostracod data from other Hoxnian/Holsteinian sites in north western Europe. These data are also discussed in the light of evidence indicating that there was a barrier across the southern North Sea when the sediments examined were deposited. Sarsicytheridea were used for trace element (Mg : Ca, Sr : Ca) and stable isotope (δ 18O, δ 13C) analyses. A calibration equation for the calculation of temperature from Mg : Ca ratio was successfully established from analysis of modern Sarsicytheridea. Modern Sr : Ca data could not be used to establish a calibration for salinity since there was too much scatter in the data. Ostracods were also analysed to examine the changing ratios of Mg and Sr as a proxy for temperature and salinity changes in the Hoxnian of the southern North Sea. Mg : Ca data indicated a deterioration in temperature from 36.20 m to the top of the section studied of 9°C. Oxygen stable isotopic analyses from the same species, measured through the same core intervals, did not exhibit a trend, but their values were indicative of normal marine salinities. Carbon stable isotope analyses, performed at the same time as the oxygen analyses, indicate that there may have been high productivity in the region of deposition of the Sand Hole Formation, implying deposition in a region of freshwater influence.

Inversion and the development of the Murdoch Block, Silver Pit Basin, Southern North Sea&amp;nbsp; :ABSTRACT

Inversion and the development of the Murdoch Block, Silver Pit Basin, Southern North Sea&amp;nbsp; :ABSTRACT

Tertiary extensional fault systems on the western margin of the North Sea basin

Major thin-skinned fault systems of Tertiary age, detaching on Permian salt, are present on the platform west of the Central Graben in the Central and Southern North Sea. In plan view the fault systems describe two large arcs which are separated by the Mid North Sea High, partially encircling the West Central Shelf of the Central North Sea and the Silver Pit Basin in the Southern North Sea. The fault trends broadly follow the strike of base salt topography. The geometry of individual faults reflects lateral variation in the role of basement faults and pre-existing Mesozoic thin-skinned extensional structures. Dominant down-to-the-basin fault polarity suggests that the fault systems represent the limit of a gravity-driven slip zone. Extension is largely balanced by down-dip growth of fold structures. Further implications for hydrocarbon exploration include growth of roll-over fold closures adjacent to the fault zones and creation of a tectonic barrier to up-dip fluid migration.

Permo-Carboniferous plays of the Silver Pit Basin

The Silver Pit Basin is situated in the northern part of the southern Gas Basin, and is separated from the Sole Pit Basin to the south by the Outer Silver Pit Fault. Over much of the basin up to 400 m of Silverpit Formation interbedded shales and evaporites are present. The absence of the Leman Sandstone reservoir over much of the basin has meant that gas discoveries predominate in the Carboniferous (e.g. Caister, Murdoch) rather than in the Permian, as in the Sole Pit Basin. In the Silver Pit Basin and its immediate margins, hydrocarbons have been discovered in Paleozoic reservoirs ranging from Dinantian sandstones to Zechstein carbonates. Within the Carboniferous the most significant reservoirs are fluvio-deltaic sandstones within the Westphalian and Upper Namurian. The best quality reservoirs are generally found in the coarsest sandstones, though locally the development of fracture porosity and secondary porosity due to leaching can be significant. The reservoir potential of the Permian is only significant around the margins of the Silver Pit Basin. On the southern margin, at the feather edge of the main Rotliegend Leman Sandstone fairway, significant gas accumulations are present in aeolian and mixed aeolian/sabkha facies. On the northern margins the presence of Zechstein intra-basinal highs resulted in the local deposition of carbonates which may also have reservoir potential. The main source rocks are coals and shales within the Carboniferous, predominantly in the Westphalian, but also to a lesser extent in the Namurian and Dinantian. The main kitchen area is in Quadrant 44, where significant gas generation has occurred since the Cretaceous. Migration and hydrocarbon charging is relatively straightforward for the Leman Sandstone and Carboniferous sandstone reservoirs. For Zechstein carbonate reservoirs migration via basin margin faults facilitates hydrocarbon charging. The most significant regional seal is provided by the Permian sequence of Silverpit shales and overlying Zechstein evaporites. Carboniferous shales probably also act as seals on a local or semi-regional scale, supplementing the regional seal and contributing to deep intra-Carboniferous traps. Most discoveries have been made in reservoirs immediately beneath the Base Permian Unconformity and generally the prospective areas are best identified by mapping at Base Zechstein/Base Permian horizons. The most prospective structures are associated with the major northwest-southeast-trending structural highs, often as ‘pop-up’ features. Most accumulations are predominantly structurally closed; however, for the ‘feather-edge’ Rotliegend play, closure to the northwest can have a stratigraphic component.

The late Carboniferous ‘Barren Red Bed’ play of the Silver Pit area, Southern North Sea

A multidisciplinary study of the gas-bearing Westphalian red bed sequence of the Southern North Sea is presented. Comparison with analogous fades in Carboniferous sequences in the UK onshore and the eastern Netherlands suggests that two distinct alluvial red bed units are present, one of Westphalian C and one of Westphalian D age. Development of red pigment in these units was largely the result of contemporaneous pedogenesis. Interpretation of the stratigraphy is complicated by the widespread occurrence of reddening caused by penetrative weathering, both during the Carboniferous and related to the formation of the sub-Permian unconformity. Gas reservoirs are present in sandstones in the Westphalian C red bed unit. This unit has higher net:gross ratios and better poroperm characteristics than the underlying Coal Measures. The better reservoir quality in the red bed unit is largely influenced by late diagenetic alteration related to hydrocarbon migration. A sedimentological and diagenetic model for the formation of the Westphalian C red bed unit implies that this unit is likely to have a distinctive reservoir architecture, which, together with better reservoir qualities, distinguishes it from the hydrocarbon play in the underlying Coal Measures.

The Caister Fields, Block 44/23a, UK North Sea

The Caister Gas Fields are located in Block 44/23a, within the Silver Pit Basin area of the Southern North Sea Basin and comprise a Bunter accumulation overlying a Carboniferous accumulation. The Carboniferous structure is a faulted northwest–southeast-trending anticline, bounded to the northeast by a major reverse fault. The reservoir is formed of early Westphalian B braided channel sandstones, the thickness of which varies from 20–50 m over the field. Porosity is fair with an average of 11%. Average horizontal permeability is 5 mD, although permeabilities up to 400 mD are measured. The Bunter structure is a simple, unfaulted, elongate northwest–southeast-trending dome, with seal provided by the overlying Haisborough Group saliferous shales. The sheet-flood reservoir sandstones, with a thickness of 150 m over the field, have an average porosity of 21% and an average horizontal permeability of 100 mD. Combined initial gas in place is estimated at approximately 400 BCF with recoverable reserves in the order of 300 BCF.

Introduction and review

The aim of this session of the conference was to elucidate the nature of the Permo-Carboniferous and older plays in the producing areas of NW Europe and to suggest areas and play types where new discoveries could be made. The introductory paper by Gerard et al. describes the main producing areas: Southern Permian Basin, Timan-Pechora and the Baltic Basin. Source-rock distribution from Cambrian to Permian times is discussed in terms of eustatic and super-continent cycles, while the obvious relation of areas of production to areas without late tectonic deformation is described. For the Southern Permian Basin, the general hydrocarbon habitat and the development of the Southern North Sea is well illustrated by Hollywood and Whorlow's paper on multiphase structural development and the review by Bailey et al. of the plays of the Silver Pit Basin. The paper on the Caister Field by Ritchie and Pratsides illustrates the details of stacked structural plays with reservoirs of Triassic and Carboniferous age, while the papers by Turner et al. on the Ravernspurn North Field and by Burri et al. on the Rotliegend of NW Germany highlight the complexity of some traps.These two key papers on Rotliegend fields demonstrate the relationships between diagenesis, stratigraphy and structural history with the presence of good reservoir and the definition of subtle trap geometries. The approaches used by these authors point the way to future exploration evaluation as the Southern Permian Basin becomes increasingly mature. That the detailed stratigraphic and sedimentological frame-works necessary in the search for subtle . . .

Dissolved and particulate trace metals in surface waters over the Dogger Bank, Central North Sea

Samples of surface water were collected from 14 stations over the southern Dogger Bank and the Outer Silver Pit in November 1989. Analyses were carried out for dissolved Mn, Fe, Co, Ni, Cu, Zn, Cd, and Pb by solvent extraction followed by graphite furnace atomic absorption spectrophotometry (GFAAS), and for Hg (total and reactive) by cold vapour AAS (CVAAS). Samples of suspended particulate material, isolated by filtration were analysed for Al, Cr, Ni, Cu, Zn, Cd, and Pb by GFAAS following digestion with hydrofluoric acid, and for Hg by atomic fluorescence following digestion with concentrated nitric and hydrochloric acids. Concentrations of dissolved trace metals were in good agreement with those reported previously for the Central North Sea (Mn, Co, Ni, Cu, Cd) and offshore waters (Pb, Hg). The concentrations of trace metals in suspended particulate material were similar to, or lower than, those reported by other workers. Elevated concentrations of some trace metals were seen in both phases following a storm.