A quantitative assessment of tectonic stretching in the northern North Sea, the Triassic of the Utsira High and greater Tampen Spur area

The Triassic interval of the northern North Sea contains important hydrocarbon reservoirs and potential targets for CO2 sequestration. However, understanding of the age and distribution of sedimentary facies within the Triassic succession is hampered by the absence of a robust chronostratigraphic framework. Although recent studies for the UK Central North Sea (UK CNS) have defined a robust Triassic chronostratigraphic framework, significant stratigraphic nomenclature differences coupled with lack of dateable palynomorphs within the Triassic continental Hegre Group of the Norwegian Northern North Sea have led to non-biostratigraphic cross-border correlations.Preliminary results from palynology and heavy mineral studies from the Norwegian Northern North Sea within a newly refined tectonostratigraphic framework have prompted a re-examination of the chronostratigraphic correlations between the Central and Northern North Sea. In the greater Tampen Spur area, a lack of active fault movement during sedimentation suggests that deposition of up to 2 km of Triassic Hegre Group sediments is largely climatically controlled and potentially regionally correlatable. Herein we aim to extend the well-established Triassic chronostratigraphic framework of the UK CNS to the Norwegian Northern North Sea area using distinct and age-equivalent Triassic mudstone units.Well data from the Tampen Spur area reveal a regionally correlatable Norian mudstone which represents the Alke formation of the Hegre Group and an additional unassigned Ladinian mudstone package. These mudstone units were deposited on the distal fringes of a major distributive fluvial system sourced from the Norwegian margin. The Alke Formation seals the progradational motif of the Norian Lunde Formation and the Ladinian mudstone seals the Norian-Ladinian sandstone of the Lomvi Formation. The sandstones record the progradation and mudstones the retrogradation of the distributive fluvial system.Given the age and similar petrophysical properties, we propose that the Alke Formation is equivalent to the Jonathan Mudstone Member and the Ladinian Mudstone is equivalent to the Julius Mudstone Member of the Skagerrak Formation in the UK CNS. Thus, the Lunde formation is Josephine equivalent and the Lomvi formation represents the Joanne Member. This layer-cake reservoir configuration is completed with a sandier and locally truncated Raude Formation which is equivalent to the Rhaetian Joshua Mudstone Member of the UK CNS.This preliminary cross-border chronostratigraphic framework unifies the Triassic stratigraphy of the North Sea within a newly refined tectonostratigraphic framework. Regional chronostratigraphic correlations allow a comprehensive paleogeographic reconstruction between the Central and Northern North Sea enlightening all major climatic alternations from Olenekian to Rhaetian.

The assemblages of attached and freeliving epibenthic species in the North Sea are described, based on analysis of samples collected with a small beam trawl. Clustering of survey sites based on the presence or absence of attached species indicated that three regions had characteristic assemblages: the northern North Sea, the central North Sea from 55 to 57°N and the southern North Sea. Clustering of sites based on counts of free-living epibenthic species also revealed that the sites formed three major groups but these corresponded to regions in the north-east North Sea, the northern and western central North Sea and the southern and eastern central North Sea. Species which contributed most to the similarity within and dissimilarity between groups were identified. The environmental factors which best accounted for the grouping of sites were depth, winter temperature and the temperature difference between winter and summer for attached species and depth and the temperature difference between winter and summer for free-living species. The species richness of attached and free-living epibenthic species was higher in the central and northern North Sea than in the south. The number of abundant (Hill's N1) and very abundant (Hill's N2) free-living species also increased from south to north.

Size fractions (0.4 and 0.4–1.0 m) of Brent Group sandstones from the northern North Sea contain mostly illite-smectite mixed layers with kaolinite, whereas the same size fractions of Fulmar Formation sandstones from the south-central North Sea consist of illite-smectite mixed layers with minor chlorite. Transmission electron microscope observations show elongated illite laths or agglomerates consisting of small laths when larger individual laths are lacking. The K-Ar data of the fractions less than 0.4 m of Brent Group samples plot on two arrays in a 40Ar/36Ar vs. 40K/36Ar diagram that have isochron characteristics with ages of 76.5 4.2 and 40.0 1.5 Ma, and initial 40Ar/36Ar ratios of 253 16 and 301 18, respectively. For the Fulmar Formation samples, the data points of the fractions less than 0.2 and less than 0.4 m also fit two isochrons with ages of 76.6 1.4 and 47.9 0.5 Ma and initial 40Ar/36Ar ratios of 359 52 and 304 2, respectively. Some of the coarser 0.4–1.0-m fractions also plot on the two isochrons, but most plot above indicating the presence of detrital components more than 0.4 m. The almost identical ages obtained from illite-type crystals of sandstones with different deposition ages that are located about 600 km (373 mi) apart record two simultaneous illitization episodes. These events were not induced by local burial conditions, but are related to episodic pressure and/or temperature increases in the studied reservoirs, probably induced by hydrocarbon injection. This interpretation is indirectly supported by notably different K-Ar illite ages from cores of a nearby reservoir at hydrostatic pressure. Illite is not as well crystallized as expected for potential crystallization temperatures above 160C measured by fluid-inclusion determinations. In both the northern and south-central North Sea, the two illite generations remain unaffected after crystallization despite continued burial, suggesting notably higher crystallization temperatures than those estimated from geothermal gradients. No recent illite crystallization or alteration is recorded in the K-Ar ages, despite a dramatic regional acceleration of the subsidence in the southern North Sea.

SEMAC I, a large semisubmersible pipelay/ derrick barge designed specifically for use in rough-sea areas, is now under construction and scheduled for completion in 1976. The barge will be capable of installing large diameter pipelines in water depths exceeding 1200 ft. Modifications can be made to the barge for installing pipelines in even greater water depths when the need arises. This paper describes the barge and discusses reasoning behind selection of the hull and equipment on it. INTRODUCTION The need for new and improved offshore pipe1aying equipment has grown rapidly in the past few years because of oil and gas discoveries being made in deeper and rougher water. Most of this recent activity has been in the North Sea, particularly the northern area of the North Sea. Many miles of pipelines will be installed in water depths greater than 400 feet, which is approaching the capability limits of most conventional equipment. SEMAC I will be a major addition to the improved and advanced equipment (Reference 1 and 2) now available in the offshore contracting industry. This equipment is needed for the northern North Sea and for other areas of the world with comparable conditions where discoveries of new oil and gas fields are expected to be made. In addition, discoveries are likely in water depths greatly exceeding 1000 feet where pipelines would be required. Therefore, SEMAC I was designed to initially operate effectively in northern North Sea type water depths and sea conditions; however, the barge has built-in features to allow it to be modified at moderate cost to install pipelines in the very deep water. In addition to meeting the water depth lay capability, the primary objective was to achieve reliability of operation in rough seas. This objective has been the controlling factor in selection of vessel type, pipelay equipment, mechanical and electrical systems, and the stinger. DESCRIPTION OF BARGE SEMAC I utilizes the well-known semisubmersible concept (Figure 1), which provides a motion-reduced mobile platform to conduct operations in the design sea state of l5-ft. significant waves. The twin-pontoon variable draft feature also allows the barge to elevate all exposed deck machinery and other facilities well above the wave crests in the most severe seas without "running for shelter". The large size (433 ft. long overall by 180 ft. wide by 91 ft. deep) provides adequate work and storage space and contributes to its reduced on and increased stability. The goal of the SEMAC I design was to have the smallest barge that could effectively operate in 15 ft. significant seas, yet have the desired motion characteristics, deck capacity, and productivity required. A small vessel is desired because of its lower initial cost and fewer problems associated with operations, such as station keeping. Minimizing station-keeping forces by having a moderate-size barge becomes even more important in deep water where dynamic positioning will be required.

We studied Permo-Triassic rift development in the Northern North Sea using 2D and 3D seismic reflection datasets, gravity data, wireline logs and palynological data. Tectonostratigraphic analysis of four key sub-basins identified six age-constrained packages, differentiated by their seismic characteristics and variations in thickness. Rifting initiated during the latest Permian and displayed significant spatial and temporal variations. Rifting occurred over a c .10 Ma period in the Øygarden, Vette, Tusse, Brage and Ninian West faults, whereas the Sogn Graben was active throughout the Triassic (over a period of 50 Ma). The mapped distribution of Triassic sediments, together with the sedimentation rates calculated from well data, show that regional subsidence prevailed across the entire basin, with enhanced subsidence within the active half-graben. In the Tampen Spur area, the lack of tectonic activity throughout the Triassic period, coupled with an average sediment accumulation rate of 35 m Ma −1 , suggests regional subsidence. In the context of the opening of the Atlantic Ocean, changes in fault orientation from north–south to NE–SW suggest that the Northern North Sea Permo-Triassic rift constitutes an early failed rift event, with strain being accommodated by large NE–SW-oriented faults in the North Atlantic region. Permo-Triassic rift development in the Northern North Sea defined the structural framework for subsequent Mesozoic rifting.

Due to its geographical location, the North Sea is one of the busiest seas worldwide, undergoing increasing pressure due to continuous developments of offshore human activities. These activities affect the North Sea ecosystem, for example by introducing new habitats/species or infrastructure into previously unobstructed environments. At the same time, climate change has already been affecting the North Sea ecosystem, leading to observed changes in species distribution. These changes may also affect physical processes such as stratification. Stratification is one of the main factors influencing primary production, which constitutes the foundation of the marine food web. To be able to mitigate these effects, it is crucial to understand the bottom-up, cumulative impacts of anthropogenic climate change and offshore activities on marine ecosystems. With this goal, we adapted and nested a 3D process-based hydrodynamics and water quality model of the North Sea (3D DCSM-FM) within a global Earth System Model (CMCC-ESM2). We simulated two contrasting future climate change scenarios: one representing the situation of a society focused on global sustainability, with low carbon emissions (SSP1-RCP2.6), and one of a society focused on global markets, with abundant exploitation of fossil-fuels and high carbon emissions (SSP5-RCP8.5). We investigated how the two different scenarios impact important abiotic and water quality variables, up to the end of the century.As expected, our model results show clear surface water temperature increases for both scenarios, above 3°C by 2100. The Northern part of the North Sea is mainly driven by exchange with the Atlantic ocean. In the Northern and Eastern North Sea and in the Dogger Bank, our model simulates an increase in temperature stratification, leading to decreases in near-surface dissolved inorganic nutrient concentrations, chlorophyll-a and growing-season primary production (~-20-30% by 2100 on average in the Northern North Sea). The Southern part of the North Sea, especially along the coast, is driven by an along-coast current from the English channel and large, nutrient-rich freshwater inputs. The Southern North Sea shows a more spatially-variable response to the simulated scenarios in terms of nutrient concentrations, chlorophyll-a and primary production, with areas of increase and areas of decrease. Overall, the Southern North Sea shows a small increase in growing-season primary production for scenario SSP1-RCP2.6 by 2100 (+6%), while it shows a decrease for SSP5-RCP8.5 (-14%).Our model offers the resolution to understand the effects of local pressures within a globally changing climate. Such tools are crucial to support the management of future offshore activities, ensure their long-term effectiveness and minimize their impacts on the ecosystem.

During a North Sea survey in late spring 1986, zooplankton biomass (g dry weight/m2) as well as pollutant concentrations in zooplankton (ng/g dry weight) were determined at 127 stations. On the basis of these data, the load of several cyclic organochlorines in zooplankton (ng/m2) was estimated. It appeared that the highest loads of these pollutants were incorporated in the large zooplankton stocks of the northern and central North Sea, north of 56°N, whereas the highest concentrations were found in zooplankton of the southern North Sea. An explanation for the accumulation of pollutants in sediments as well as in benthos organisms of the central and northern North Sea as found by several authors is discussed in connection with the assumption of a considerably high vertical flux of organic material in these areas. The calculation of total zooplankton content in the North Sea during the late spring survey amounted to about 5 million tons dry weight. This biomass was estimated to contain about 2.6 t PCBs, 19.3 kg p,p′-DDE, 12 kg τ-HCH, 6.4 kg HCB und 5.6 kg α-HCH. These results agree with those on other compartments of the North Sea ecosystem (benthos, fish) found in recent literature. The estimated annual turnover of PCBs in zooplankton (15.2 t) is of the same order of magnitude as the estimated amount transported yearly into the North Sea by rivers and the atmosphere (6–13 t).

Abstractde Castro, C., Wright, P. J., Millar, C. P., and Holmes, S. J. 2013. Evidence for substock dynamics within whiting (Merlangius merlangus) management regions. – ICES Journal of Marine Science, 70: 1118–1127. Whiting in the North Sea and Eastern Channel is currently assessed as a single management unit. However, several studies suggest that this stock may be comprised of more than one subpopulation within a larger metapopulation. A key characteristic of metapopulations is asynchrony in the dynamics of component subpopulations. In this study, indices of recruitment and spawning–stock biomass (SSB) were developed to test for asynchrony across putative subpopulations in the North Sea and west of Scotland. Differences in SSB and recruitment trends were detected, consistent with expectations from metapopulation dynamics. At least three different subpopulation components (southern and northern North Sea, and west of Scotland) were indicated on the basis of differing trends. Analysis of spatial distribution suggested that the boundary between the northern and southern North Sea subpopulations was associated with the change in bathymetry that extended from the coast of Norfolk in England to the southern tip of Norway. The current management system for whiting in the North Sea assumes a unit stock, which is contrary to current sources of biological evidence and seems inappropriate. Consideration of a north–south split along the boundary detected should be beneficial for both assessment and management of the resource.

Catches of European hake (Merluccius merluccius) in the North Sea have increased substantially during the last decade, even though there is no directed commercial fishery of hake in this area. We analysed the spatial distributions of hake in the northern the parts of its range, (where it is less well-studied), using ICES international bottom trawl survey data from 1997 to 2015. We examine length-frequency distributions for (i) distinct modes enabling the assignment of fish into categories which likely corresponded to the ages 1, 2, and 3+ and (ii) patterns of seasonal spatial distribution for the different groups. Age categories 1 and 2 fish were most abundant in the northern North Sea, and appear to remain in the North Sea until 2 years of age, when they move into deeper waters. Their distribution has expanded into the western-central North Sea in the last decade. Age category 3+ fish were most abundant in the northern and central North Sea during summer, indicating a seasonal influx of large individuals into this area likely associated with spawning activity. The distribution of these older fish has gradually expanded westward in both seasons.

During five annual ground fish surveys of the North Sea, all the benthic invertebrates trawled at 48 primary stations were recorded. The data were subjected to classification analysis, which showed a basic division between northern and southern North Sea benthos. The southern North Sea was further divided into three benthic regions, and the northern North Sea into four benthic regions (including one to the west of Shetland). The factors influencing the faunal assemblages in the various regions were discussed.

Estimating surge in extreme North Sea storms

Growth of North Sea cod, Gadus morhua

The population structure of harbour porpoises from British and adjacent waters was studied by examining variability in a 200 bp (base pair) section of the control region of mitochondrial DNA (mtDNA) extracted from 327 animals. This region contained 20 variable sites giving rise to 24 different haplotypes. Mean nucleotide diversity between all pairs of haplotypes was 0.81% (range 0-4%). The most common haplotype occurred in 63% of the samples and was recorded in all geographical areas; several other haplotypes were present in two or more of the sampling locations. This suggests considerable historical interconnections among populations, probably through gene flow. However, there were significant differences (p < 0.05) as determined by AMOVA (Analysis of Molecular Variance, Excoffier et al. 1992), between porpoises from the northern and southern North Sea, and between the northern North Sea and the Celtic/Irish Sea. The differences were predominantly due to variation among females. This sex-related difference in population genetic structure suggests that males disperse more than females. This has important consequences for evaluating the consequences of incidental catches of porpoises by fisheries in these seas since there may be a greater impact on local populations than is implied by simple calculations of mortality.

Northern limit of the “Brent delta” at the Tampen Spur —a sequence stratigraphic approach for sandstone prediction

Long-term wave statistics have been generated using a deep-water numerical wave model based on a parametric method first developed from the Joint North parametric method first developed from the Joint North sea wave Project (JONSWAP). The input to the model consists of wind data from a subset of severe storms in the northern North Sea over a period 1966–1976. The wave model was compared with wave measurements at two stations. Extreme value wave statistics have been derived using the statistics from the independent storm events during the hindcast period and taking into account possible secular changes in the wind field extremes. The results are compared with wave height estimates obtained in an independent study. Introduction In engineering studies concerned with the design and certification of offshore structures it is essential to have probability estimates of the most extreme wave conditions expected during the lifetime of the structure which is typically 20 years. Reliable estimates of extreme conditions require a time base of at least 10 years from which to extrapolate to return periods of 25 years or more. Measured wave data are available at only a few stations in the northern North Sea and furthermore the data are generally intermittent or only cover a few years (see Draper). The only method available at the present time for constructing wave statistics in the area being considered has therefore to be based on a wave hindcast approach using a data base of historical wind fields. Numerical hindcast procedures are being increasingly used in sea areas where the sparse nature of wave measurements, both in time and space, make it difficult to obtain reliable design wave information. For example, in the Gulf of Mexico hurricane wave conditions have been hindcast by Patterson, and by Cardone, Pierson and Ward. Resio and Vincents have used the formulation of Barnett's wave model to derive design wave information for the Great Lakes based on carefully estimated wind fields over the past two decades. A wave hindcast study is presently being carried out by the Norwegian Meteorological Institute (Haland and Smaland) for the Norwegian continental shelf area. The purpose of this study, the North Sea wave Model (NORSWAN) project, was to generate long-term wave statistics in the North Sea and adjacent areas using hindcast wave data from severe storms occuring during the period 1966 to early 1976. The 42 storms chosen for the hindcast study were representative of all gales occurring during the period under consideration so that a representative statistical base was available for extreme value analysis. A deep water hybrid parametrical wave model (Gunther et al.) was chosen to parametrical wave model (Gunther et al.) was chosen to hindcast the wave data the wave model was checked against wave measurements at two stations near the British Isles. Finally long-term wave statistics were estimated by a storm-model approach taking into account possible changes in the wind field extremes over the last one hundred years. Details of the NORSWAM project together with the derivation of extreme value wave height estimates have been given by Ewing, Weare and Worthington. WIND FIELD SPECIFICATION A most important part of all wave hindcast studies is that concerned with the accurate specification of the wind fields over the ocean. A full description of the way the wind fields were analysed has already been given by Harding and Binding. However, as this aspect of the study is of central importance in the accuracy of the hindcast results and in the estimation of Long-term extremes, a brief description of this work will be given here. The Daily Weather Reports of the British Meteorological Office were examined for the period 1966 to early 1976 for the occurrences of gales in the northern North Sea. (Synoptic weather charts are available for periods earlier than 1966 but the data were considered too limited for the accurate reconstruction of wind fields). P. 87