Mid North Sea Research Articles

The search for “Jenyon’s Channel”: The missing link between the Permian basins in the North Sea

The Mid North Sea High (MNSH) Seaway, otherwise known as Jenyon's Channel, was the only major marine connection between the Northern and Southern Zechstein Basins during the latest Permian. Current understanding favours a model where marine replenishment began with a northern connection to the Panthalassic Ocean (along a network of basins that were precursors to the North Atlantic Rift System), after which marine water passed into the Northern Zechstein Basin, travelled through the MNSH Seaway before finally reaching the Southern Zechstein Basin. This study delineates evaporite formations in an extensive petrophysical dataset to analyse marine connections and whether they influenced Zechstein facies distribution. The first four Zechstein Cycles (Z1-Z4) are identified on the MNSH platform which shows that this structure was always covered by a thin water column during sea-level highstand; however, during Z2 sea-level lowstand, the MNSH Seaway provided the main connection between the basins. As relative sea-level fell, the MNSH Seaway became increasingly constricted resulting in hypersalinity due to limited volumes of marine water reaching the Southern Zechstein Basin. This system precipitated vast volumes of Z2 (late Wuchiapingian) halite in the Southern Zechstein Basin, whereas this facies remains reduced in the Northern Zechstein Basin. In Z3 and Z4 (Changhsingian) times relative average sea-level was higher resulting in sustained communication between the basins, even in times of sea-level lowstand. This is evidenced through a regionally traceable blanket of Z3 halite, along with occasional examples of Z2 carbonate platforms becoming entombed in Z3 halite. This work provides revised palaeoenvironmental understanding for the latest Permian. The workflow could also aid in the study of other oceanic gateways with further implications for hydrocarbon exploration efforts and emerging energy transition technologies such as subsurface storage on the UK Continental Shelf.

Gravity imaging of sub-Zechstein geological structures in the UK sector of the North Sea using the gravity layer stripping method

Extensive geophysical databases, covering the UK sector of the North Sea, have been used to gravity layer strip the sedimentary layers down to the base Zechstein so that the gravity response of the Carboniferous and deeper strata can be identified and structurally interpreted. To achieve this, the average bulk density grids for each layer were derived using Gardner's functions derived from well velocity and density logs. The resulting residual gravity response of each layer and the Moho response were then removed from the Free air gravity anomaly to generate the isostatic gravity response of the crust below the base Zechstein. This gravity response was used to re-evaluate the British Geological Survey interpretation over the Mid North Sea High (MNSH) and was able to identify the same crustal structures. Using the tilt derivative method, a positive gravity anomaly was found to parallel the central fracture zone that forms a northern extension of the Dowsing fault zone. This anomaly can be traced north across the MNSH with offsets coinciding with the WSW–ENE basement lineaments. To the south, the southern North Sea basin is well defined by the stratigraphic layer depth and thickness maps as well as the residual gravity maps which identify the structures associated with the low-density Carboniferous Coal Measures.

THE ZECHSTEIN Z2 HAUPTDOLOMIT PLATFORM IN THE SOUTHERN UK MID NORTH SEA HIGH AND ITS ASSOCIATED PETROLEUM PLAYS, POTENTIAL AND PROSPECTIVITY

The Mid North Sea High (MNSH) region represents one of the least explored areas for the Late Permian Zechstein Hauptdolomit play in the Southern Permian Basin although some of the first offshore wells drilled in the UK were located here. In other parts of the basin such as onshore Poland, the Hauptdolomit Formation (“Hauptdolomit”) is an active and attractive exploration target, with oil and gas production from commercial‐sized fields. In the UK, the play has been overshadowed by drilling campaigns in areas to the south of the MNSH which tested plays in the underlying Rotliegend and Carboniferous successions. However, with these areas now in decline, there is increased exploration interest in the Hauptdolomit in the MNSH region, particularly since 2019 when 3D seismic data were acquired and the first hydrocarbon discovery was made at Ossian (well 42/04‐01/1Z). Geochemical data from the latter discovery have pointed to the presence of a prolific petroleum system with the potential for Hauptdolomit reservoirs to be charged both by Zechstein‐generated oils and Carboniferous condensate/gas. With regard to hydrocarbon migration and preservation in the southern MNSH, a detailed evaluation of the effects of the Mid Miocene Unconformity has allowed for a greater understanding of the main factors controlling hydrocarbon preservation and remigration. Reservoir characterization of the Hauptdolomit play has been achieved by integrating petrographic microfacies analyses, core data and petrophysical interpretations. The most important factors controlling reservoir quality are the presence and extent of anhydrite cementation and the presence of high energy shoal facies. Thicker and coarser grained shoal facies are expected to occur along the yet‐to‐be explored Orchard platform margin where numerous prospects have been mapped and refined using recently acquired 3D seismic data.

SEDIMENTOLOGY, PALAEOGEOGRAPHY AND DIAGENESIS OF THE UPPER PERMIAN (Z2) HAUPTDOLOMIT FORMATION ON THE SOUTHERN MARGIN OF THE MID NORTH SEA HIGH AND IMPLICATIONS FOR RESERVOIR PROSPECTIVITY

This paper provides an updated understanding of the reservoir stratigraphy, sedimentology, palaeogeography and diagenesis of the Upper Permian Hauptdolomit Formation of the Zechstein Supergroup (“Hauptdolomit”) in a study area on the southern margin of the Mid North Sea High. The paper is based on the examination and description of core and cuttings data from 25 wells which were integrated with observations based on existing and new 3D seismic.Based on thin‐section petrography of cuttings and core from the wells studied, it is evident that Hauptdolomit microfacies are distributed in a relatively predictable way, and well‐defined platform interior, platform margin, slope and basin settings can be distinguished. Platform margins are typically characterised by the development of ooid shoals and, to a lesser‐extent, by microbial build‐ups. High‐energy back‐shoal settings are characterised by a more complex combination of peloid grainstones, thrombolitic and microbial build‐ups, and fine crystalline dolomites. Lower energy lagoons which developed further behind the platform margin are characterised by a variety of microfacies types; fine crystalline dolomites are common in this setting as well as peloidal facies and local microbial build‐ups. Intertidal and supratidal settings are typified by increased proportions of anhydrite and the development of laminated microbial bindstones (stromatolites). Platform margins are in general relatively steep and pass into slope and basinal settings. Only a few wells have penetrated Hauptdolomit successions deposited in a slope setting, and these successions are characterised by a range of resedimented shallow‐water facies together with low‐energy laminated dolomicrites and fine crystalline dolomites. Slope zones in the study area are interpreted from seismic data to be typically 1‐1.5 km in width. Basinal Hauptdolomit deposits have been strongly affected by post‐depositional diagenesis and are dedolomitised to variable degrees. The original depositional facies are rarely preserved.Diagenetic studies show that dolomitisation has affected almost the entire Hauptdolomit Formation throughout the study area in both basinal and platform settings. The dolomite is considered to result from seepage‐reflux processes and is an early diagenetic phase. Mouldic porosity is present in many facies types as a result of dissolution, especially in ooid grainstones, thrombolitic build‐ups and peloidal facies. The dissolution cannot be associated with any one diagenetic phase but was most likely a result of the dolomitisation process itself. Stable isotope analyses indicate that all dolomites were precipitated from Permian marine‐derived pore fluids. Fluid inclusion analyses of dolomite cements indicate that cementation continued into the burial realm. Anhydrite cementation occurs in two phases: early anhydrite precipitation was associated with dolomitisation, and can be distinguished from a later, pore‐filling cement which is highly detrimental to reservoir quality.The Hauptdolomit succession in basinal wells (and in some slope wells) in the study area has undergone significant dedolomitisation. Dedolomitisation was a shallow burial process which affected precursor dolomites, whereby excess calcium from the transition of gypsum to anhydrite during burial combined with CO2 and organic acids derived from basinal sediments. The process was triggered by excess calcium reacting with excess carbonate ions from dissolution.3D seismic volumes supplemented by numerous 2D lines were available in the study area and allowed an interpretation to be made of Hauptdolomit gross depositional settings; platform margins and base‐of‐slope polygons were mapped, with the greatest confidence in areas of 3D seismic. The basin, slope and platform settings were distinguished using seismic data integrated with the results of micro‐facies analysis and incorporating seismic‐to‐well ties. The data shows that large parts of the study area are characterised by the presence of polyhalites within the overlying (Z2) Stassfurt Halite Formation, which may create particular seismic geometries at the Hauptdolomit slope. These are interpreted to be intra‐Stassfurt Halite features, providing an alternative model to the thickened, prograded Hauptdolomit which has been suggested in previous publications.Because few wells drilled in the study area had the Hauptdolomit as the primary target, cores were limited but significant data was obtained from cuttings analyses. More than 400 thin sections were evaluated, allowing depositional models based on microfacies observations to be developed, verifying the seismic‐scale observations.

Can thin coals be resolved in seismic data? An example of the challenges for Carboniferous (Visean to Westphalian) source-rock definition across the Elbow Spit Platform (Netherlands) and the Mid North Sea High (UK) through well synthetic modelling

Abstract The ability to predict coal units from seismic data has become important for hydrocarbon exploration, especially in areas away from traditional play sweetspots. An understanding of coal distribution/thickness from seismic data can influence the risking of basin source-rock presence/charge efficiency. A selection of wells is presented from the Carboniferous of the Elbow Spit Platform (Netherlands), the Mid North Sea High (UK) and adjacent areas. These wells penetrated various coal thicknesses, as well as sandstones, mudstones and thin limestones in different stratigraphic configurations. For each well, full-stack synthetic seismograms (acoustic impedance domain) using a conventional seismic wavelet are generated along with supporting rock physics. This analysis suggests, as a guideline, that coal thickness needs to meet a threshold of 25–35% of loop thickness to be uniquely distinguishable from the surrounding lithologies (discernible soft kick). If the coals are thinner than this threshold, they will be comparable to soft sandstone-related kicks, although some exceptions exist where thin coals directly overlie soft sandstone intervals. The exact percentage value will depend on seismic-data quality and rock properties. This study highlights the need for careful consideration of thin-coal seismic expression when devising interpretation strategies and risking source-rock presence.

A new and working petroleum source rock on the UK Continental Shelf (Upper Permian, offshore Yorkshire)

We report on the discovery of oil from the Boulby Mine and its likely productive source rock from Yorkshire in NE England, located to the west (<30 km) of the newly licensed petroleum exploration areas in the vicinity of the Mid-North Sea High. Oil samples from the mine, dripping out of halite in the roof, have likely been generated from Zechstein Group Kirkham Abbey Formation (KAF) sapropelic carbonate rock as indicated by aliphatic and aromatic hydrocarbon biomarkers. Other potential source rocks of Carboniferous (Westphalian, Namurian, Viséan coals and mudrocks) and Jurassic (the Jet Rock, Bituminous Shales, Kimmeridge Clay Formation) age are ruled out on the basis of organic geochemical data. Boulby oil was generated in the peak-to-late oil-window and it is characterised by the high abundance of C32 and C34 homohopanes, slight even-over-odd predominance (EOP) of C20-25n-alkanes indicating restricted carbonate-evaporite depositional conditions, and C29 ethyl-diacholestane 20 S likely implying a clay-rich source rock. The structural framework and tectonic history of the Permian strata reveal the presence of several fault systems which served as conduits for migrating petroleum. Similar Zechstein-sourced oil is known from Poland and Germany, but the occurrence at Boulby is the first positive identification of oil derived from Zechstein source rock in the North Sea area. The Boulby oil is reservoired in Zechstein 3 (Z3) Brotherton Formation dolomite and sealed by Z3 evaporite rocks. The proven oil occurrence at Boulby has significant implications in terms of reducing the risk of a lack of oil mature source rock for acreage offered in the neighbouring North Sea during the UK's 30th and 31st licensing rounds.

Structural evolution of the Breagh area: implications for carboniferous prospectivity of the Mid North Sea High, Southern North Sea

Exploration success at Breagh demonstrates that western parts of the Mid North Sea High area are prospective despite the absence of an Upper Permian (Rotliegend Group) Leman Sandstone Formation reservoir and source rocks belonging to the Upper Carboniferous Westphalian Coal Measures Group. Detailed seismic and well interpretation shows that the Breagh trap was a long-lived footwall high, the prospectivity of which was enhanced by Variscan folding and uplift, leading to the truncation (subcrop) of Lower Carboniferous reservoirs beneath the Base Permian Unconformity. Its drape (supra-crop) by Upper Permian (Zechstein Super Group) evaporites creates the seal. The complexity of its overburden means that an accurate picture of the Breagh structure only emerges after accurate depth-conversion that takes the effects of the Mesozoic graben into account. Pronounced easterly tilting during the Cenozoic affected the area and controlled gas migration into the structure from palaeostructures lying to the east. However, evidence that Breagh is not filled to spill point (underfill) suggests that charge limitation remained an issue. The study demonstrates that a poorly-documented and under-explored Lower Carboniferous play exists in Southern North Sea, which relies upon careful structural mapping and basin modelling to be undertaken for the play to be understood and its further potential to be realized. Thematic collection: This article is part of the Under-explored plays and frontier basins of the UK continental shelf collection available at: https://www.lyellcollection.org/cc/under-explored-plays-and-frontier-basins-of-the-uk-continental-shelf

Structural and stratigraphic evolution of the Mid North Sea High region of the UK Continental Shelf

Interpretation of newly acquired seismic and legacy well data has led to a greater understanding of the Upper Paleozoic–Recent geological evolution of the Mid North Sea High (MNSH), an under-explored region of the North Sea. The position of granite-cored blocks controlled the distribution of Devono-Carboniferous highs and basins before Variscan uplift led to peneplanation and the creation of the Base Permian Unconformity. The MNSH became the dominant feature during the Permian when it formed a west–east-striking ridge between the Southern and Northern Permian basins. Following a period of non-deposition, sedimentation was renewed in the Late Permian–Triassic before Middle Jurassic doming caused uplift to the NE. Subsequent Late Jurassic North Sea rifting transected the MNSH to create the Western Platform between the Central Graben and Moray Firth rift arms. Following Cretaceous post-rift deposition, the area experienced a significant easterly tilt in the Cenozoic that led to the demise of the MNSH as a prominent topographical feature. The tectonic and stratigraphic evolution exerts a strong control over reservoir facies distribution, source-rock deposition and maturation. However, the area is not barren of petroleum potential. Despite the lack of Upper Carboniferous source rocks over large areas, hydrocarbon potential is evident through shows in legacy wells, indicating the Lower Carboniferous as a potential source rock. Cenozoic uplift to the west imparted a regional tilt, the effects of which remains key to unlocking the area's prospectivity since it reconfigured structures and formed remigration pathways from Lower Carboniferous and Jurassic source rocks. Thematic collection: This article is part of the Under-explored plays and frontier basins of the UK continental shelf collection available at: https://www.lyellcollection.org/cc/under-explored-plays-and-frontier-basins-of-the-uk-continental-shelf

E&amp;P Notes (August 2019)

E&amp;P Notes (August 2019)

Upslope‐climbing shelf‐edge clinoforms and the stepwise evolution of the northern European glaciation (lower Pleistocene Eridanos Delta system, U.K. North Sea): When sediment supply overwhelms accommodation

AbstractClinoforms are basinward‐dipping and accreting palaeo‐bathymetric profiles that record palaeo‐environmental conditions and processes; thus, clinothems represent natural palaeo‐archives. Here, we document shelf‐edge scale clinoform sets which prograded through the entire width of an epicontinental marine basin (ca. 400 km), eventually encroaching onto the opposite basin flank, where they started to prograde upslope and landward, in defiance of gravity (“upslope‐climbing clinoforms”). The giant westward‐prograding Eridanos muddy shelf‐edge clinothem originated from the Baltic hinterland in the Oligocene and achieved maximum regression in the Early Pleistocene, on the UK Central Graben (CG) and Mid North Sea High (MNSH), after crossing the whole North Sea mesopelagic depocentre and causing near complete basin infill. Here we integrate well and seismic data through the MNSH and CG and examine the Eridanos final heyday and demise, identifying five clinothem complexes (A1, A2, A3, B and C) and six depositional sequence boundaries (SB1 to SB6) in the Miocene‐Recent section. Tectonic and climatic events drove the recent evolution of this system. Early Pleistocene climate cooling, in particular, resulted in a stepwise increase in sediment supply. This climaxed in the earliest Calabrian, following a likely Eburonian eustatic fall (=SB3) when the Eridanos clastic wedge was restructured from a 100–300 m thick compound shelf‐edge and delta system to a “hybrid” shelf‐edge delta at sequence boundary SB3 (ca. 1.75 Ma). In the ca. 40 kyr that followed SB3, a progradation rate peak (&gt;1,000 m/kyr) is associated with clinoforms starting to accrete upslope, onto the east‐dipping slope between CG and MNSH. This “upslope‐climbing clinoform” phase was quickly followed by the maximum regression and final retreat of the Eridanos system in the Early Calabrian (=SB4), likely as the result of climate‐driven changes in the Baltic hinterland and/or delta auto‐retreat. To our knowledge, this contributions represents the first documentation of “upslope‐climbing clinoforms” recorded in the stratigraphic record.

Polyphase tectonic inversion and its role in controlling hydrocarbon prospectivity in the Greater East Shetland Platform and Mid North Sea High, UK

Abstract Thick Paleozoic successions are buried under the Greater East Shetland Platform (ESP) and Mid North Sea High (MNSH), two large underexplored platform regions flanking the structural depocentres of the North Sea. Here, newly acquired broadband seismic data are interpreted to provide a novel assessment of the regional tectonostratigraphic evolution and its influence on hydrocarbon prospectivity. Numerous working reservoir units are present over these two frontier areas, together with large Paleozoic traps. Hydrocarbon charge occurs either via a likely maximum 30–40 km lateral migration from the Jurassic/Carboniferous basinal source kitchens or, possibly, via vertical/lateral migration from deeper Devono-Carboniferous source intervals. The two regions underwent a largely similar evolution, consisting of at least eight successive switch-overs between regional compression/uplift and extension/subsidence in the last 420 myr. However, on the Greater MNSH, the lack of significant Permo-Triassic rifting probably resulted in too little subsidence for the lower Carboniferous interval to reach sufficient burial depth for gas maturation. Seep and fluid escape data suggest a working ‘deep’ source in the Greater ESP. Here, the presence of localized Permo-Triassic intra-platform grabens and half-grabens provided sufficient subsidence for the oil-prone middle Devonian unit to eventually enter the oil maturation window and faults provide easy conduits for the upwards migration of oil.

Revised stratigraphic framework of pre-Westphalian Carboniferous petroleum system elements from the Outer Moray Firth to the Silverpit Basin, North Sea, UK

Abstract Spatially and temporally variable Tournaisian to Namurian Carboniferous fluvial, fluvio-deltaic, platform carbonate and shale-dominated basin sedimentary successions up to 3.5 km thick are preserved in a complex series of basins from the Outer Moray Firth (Quadrant 14) to the Silverpit Basin (Quadrant 44). Differences in stratigraphic nomenclature in the areas surrounding the Mid North Sea High and onshore, combined with sparse biostratigraphic data, have hindered the systematic regional understanding of the timing and controls on stacked source and reservoir rock intervals. Over 125 well reinterpretations, tied to seismic interpretations, provide evidence of the inception and extent of a delta system. Regional time slices highlight a long-lived laterally equivalent basinal, mud-rich succession across Quadrants 41–44. They also show that the area from the Outer Moray Firth to the Silverpit Basin was part of the same sedimentary system up to at least Namurian times. All of this is placed within a simplified stratigraphic framework.

The role of palaeorelief in the control of Permian facies distribution over the Mid North Sea High, UK Continental Shelf

Abstract The Mid North Sea High (MNSH) consists of a ridge of Paleozoic strata located in the centre of the North Sea between 55° N and 56° N. In 2010, interest in the Permian of the MNSH was revived by the discovery in Quadrant 44 of the Cygnus gas field. This study focuses on the Zechstein carbonates of the MNSH and uses play concepts that draw an analogy with the Zechstein oil and gas fields discovered in Denmark and Poland, and also with the Wissey gas field in Quadrant 53. New 2D seismic establishes the presence of a significant Zechstein reef that blocks the southern entrance of the Jenyon Gap onto the MNSH. Seismic data show that the reef developed in stages and its presence can be inferred from the occurrence of isolated lagoons within the centre of the build-up. The barrier reef's existence explains both the presence of the rare hygroscopic mineral tachyhydrite in the centre of the MNSH and also the observed isopach difference in the Zechstein cycles over the MNSH, as the barrier restricted marine-water ingress onto the MNSH and allowed the creation of a ‘crystal-starved basin’, the evaporitic equivalent of a sediment-starved basin.

Repeated post-Caledonian intra-cratonic rifting in the central North Sea: Evidence from the volcanic record in the Embla oil field

Abstract Intrusive and extrusive mafic igneous rocks in the Embla oil field, central North Sea, testify to repeated post-Caledonian magmatism on the northern flank of the Mid North Sea High. The igneous rocks are highly clay- and carbonate-altered, but retain their High Field Strength element signatures on the sample scale. These signatures are used to group, classify and investigate the tectonic significance of the rocks. Three magmatic events are identified. Late Devonian transitional basalts are interpreted as part of a bimodal volcanic assemblage that includes ca. 375 Ma alkali rhyolites, suggested to record rifting in a proto-Central Graben. Early Permian volcanic and hypabyssal alkaline rocks in the Embla oil field display lamprophyric traits and represent low degree melts. They likely correlate either to ca. 300 Ma lamprophyre magmatism leading up to, or 298-292 Ma alkaline magmatism in the Midland Valley - Southern Uplands coeval with the northwest European magmatic flare-up at the Permo-Carboniferous boundary. Kimmeridgian-Tithonian submarine tholeiitic basalts in the Embla oil field post-date the alkaline continental mid Jurassic magmatism associated with doming and initial rifting in the North Sea, and are suggested to represent a hitherto unknown volcanic expression of the late Jurassic main rift phase in the Central Graben.

Carboniferous petroleum systems around the Mid North Sea High, UK

The existence of an extensive, prospective Visean–Namurian Carboniferous petroleum system in and around the hitherto under-explored Mid North Sea High is documented. Evidence is drawn from integrated analysis of over 50,000 line kilometres of seismic data and well data, together with regional source rock screening, reservoir analysis and basin modelling.Visean-Namurian marine and non-marine mudstones and coal source rocks are interbedded within fluvial and marine reservoir sandstones within a stacked succession up to 5 km thick. Source rocks are dominantly gas-prone with oil-prone intervals, and have reached oil to gas maturity levels dependent on location. Burial/thermal history modelling indicates a kitchen area on the southern margin of the Mid North Sea High with northwest and northeastwards migration of gas and oil during Mesozoic and Cenozoic times. A variety of structural and stratigraphic traps are possible, including intraformational Carboniferous traps, with a regional seal at Permian (Zechstein) level. Synthesis of many previously unpublished datasets demonstrates the Visean–Namurian play south of the Mid North Sea High as part of an extensive petroleum system from the East Irish Sea, across onshore to offshore UK and into The Netherlands sector of the North Sea. The purpose of this synthesis is to highlight future exploration opportunities beneath and northwards of the productive Westphalian Southern North Sea gas basin, and to begin to de-risk the petroleum systems that are exemplified by the lower Carboniferous Breagh Field in a frontier area of the mature North Sea province.

A Statistical Learning Approach to Model the Uncertainties in Reservoir Quality for the Assessment of CO2 Storage Performance in the Lower Permian Rotliegend Group in the Mid North Sea High Area

It has been identified that the Rotliegend sandstone reservoir in the Mid North Sea High region, in the UK Quadrants 27-29, has a large-scale CO2 storage potential of national importance. In this paper, the authors develop a reservoir model using extensive datasets available from seismic interpretations and core analysis. An advanced statistical learning approach was applied to characterise the uncertainties in the spatial distribution of reservoir quality. The model was used to assess the CO2 injection performance and the preliminary results obtained thusfar indicate promise in the available storage capacities.

New insights into the unexploited reservoir potential of the Mid North Sea High (UKCS quadrants 35–38 and 41–43): a newly described intra-Zechstein sulphate–carbonate platform complex

Abstract The Mid North Sea High (MNSH) is located on the UKCS in quadrants 35–38 and 41–43. It is a large structural high that is flanked by the mature hydrocarbon provinces of the Central North Sea (CNS) to the NE and the Southern North Sea (SNS) to the SE. In the MNSH region, the source and reservoir intervals that characterize the SNS (Westphalian, Lower Permian) are absent and therefore the area is relatively underexplored compared to the SNS Basin ( c . one well per 1000 km 2 ). Nevertheless, two discoveries in Dinantian reservoirs (Breagh and Crosgan) prove that a working petroleum system is present, potentially charged either via lateral migration from the SNS or from within the lower Carboniferous itself. Additionally, gas was found in the Z2 carbonate (lower Zechstein Group) in Crosgan, with numerous other wells in the area reporting hydrocarbon shows in this unit. The results of the interpretation of recently acquired 2D and 3D seismic reflection datasets over parts of UKCS quadrants 36, 37 and 42 are presented and provide insight into both the geology and prospectivity of this frontier area. This study suggests that intra-Zechstein clinoform foresets represent an attractive, hitherto overlooked, exploration target. The Zechstein Group sits on a major unconformity, probably reflecting Variscan-related inversion and structural uplift. Below it, fault blocks and faulted folds occur, containing pre-Westphalian Carboniferous and Devonian sediments, both of which contain potential reservoirs. In the lower Zechstein, a large build-up is observed, covering a total area of 2284 km 2 . This is bounded on its margins by seismically defined clinoforms, with maximum thicknesses of 0.12 s two-way time ( c. 240–330 m). This rigid, near-tabular unit is clearly distinguished from the overlying deformed upper Zechstein evaporites. In map-view, a series of embayments and promontories are observed at the build-up margins. Borehole data and comparisons with nearby discoveries (e.g. Crosgan) suggest this build-up to represent a Z1–Z2 sulphate–carbonate platform, capped by a minor Z3 carbonate platform. Interpreted smaller pinnacle build-ups are observed away from the main bank. The seismic character, geometry, size and inferred composition of this newly described Zechstein platform are similar to those of platforms hosting notable hydrocarbon discoveries in other parts of the Southern Permian Basin. The closest of these discoveries to the study area is Crosgan, which is characterized by the Z2 carbonate clinothem (Hauptdolomit Formation) as a proven reservoir.

Megaspores and associated palynofloras of Middle Jurassic fluvio-deltaic sequences in North Yorkshire and the northern North Sea: a biofacies-based approach to palaeoenvironmental analysis and modelling

Outcrops of the Early Bajocian, Gristhorpe Member (Ravenscar Group) in North Yorkshire have been subjected to a multi-disciplinary analysis which establishes a key reference section at Yons Nab. Eight micro-biofaces types (MBs) are defined based on counts of transitional marine microfaunas, megaspores and plant-derived debris. Associated palynofloral associations (PAs) are used to qualify the micro-biofacies defined to provide an integrated model for interpreting salinity–freshwater trends associated with deltaic progradation and retreat. Megaspore and small-spore trends within the delta-plain are shown to have been closely linked to peat-mires forming in topographic lows, with variations across the delta-plain attributable to the presence of the semi-permanent and seasonal water tables. A reconstruction of the floodplain–peat-mire depositional setting is proposed where the composition of host vegetation (e.g. ferns, lycopsids) varied in relation to groundwater levels. Biostratigraphic correlation within the Gristhorpe Member is attempted using the base of ‘established’ delta-plain deposition (megaspore dominant MBs/spore-dominant PAs) and the top and base agglutinated foraminifera-dominant MBs (brackish). Using these criteria the Gristhorpe Member is modelled in terms of a southward progradation of delta-plain facies into a bay-fill sequence, which persisted at Yons Nab. With the delta-plain established, the MB/PA trends and sedimentology suggest a northward shift towards the higher delta-plain, with seasonal standing water only. The MB/PA model was tested on core samples from the coeval lower Brent Group from northern North Sea well 34/10-B-12 (Gullfaks Field); MB/PA trends are shown to compare closely with those of the Gristhorpe Member at Yons Nab, with the same megaspore/small-spore associations developed within similar lithofacies. On this basis transitional marine, bay-fill and delta-plain deposition are defined in the cored section. These comparative data suggest that the dispersal of megaspore and miospore host floras occurred effectively across the Mid North Sea High, and that they colonized the same niche habitats associated with bay-fill and delta-plain development.

An ‘Intermediate Blockade’? British North Sea Strategy, 1912–1914

Historians have argued that, between 1912 and 1914, Britain’s naval leadership projected a so-called ‘intermediate blockade’, a line of vessels strung across the mid-North Sea. This strategy has been widely criticized as impractical and unrealistic. However, this article demonstrates that the Admiralty never projected such an approach. Rather, the naval leadership intended to adopt a system of mid-North Sea patrols during this period. By misunderstanding Admiralty policy before 1914, historians have been unable to ascertain that these patrols were resurrected in late 1914 and played an important part in the Royal Navy’s wartime strategy.

Role of forced regression in controlling Brent Group reservoir architecture and prospectivity in the northern North Sea

The Middle Jurassic Brent Group of the northern North Sea presents a mature and highly productive reservoir play fairway where a combination of effective facies analysis and depositional sequence stratigraphy offers real potential to optimize exploitation. The north of the Brent province differs from classically studied southern areas in being dominated by marginal-marine delta-front facies. A core- and log-based study of 37 wells around the Don fields was performed to establish a sequence stratigraphic framework, map facies and thereby describe facies architecture. The results demonstrate that reservoir quality and productivity are regionally and locally controlled by facies. Of particular interest are intervals of fluvio-estuarine channel to sharp-based shoreface sandstone that formed during sea-level lowstands, since it is these packages that boost well productivity but, conversely, also increase the risk of early water breakthrough on production. Analogy with the Saloum Delta of Senegal highlights the importance of rapid and continuous barrier migration and destruction in controlling the deflection and switching of fluvio-estuarine channels, explaining also the preferential preservation of channel-floor deposits over continuously eroded barrier and delta-top facies. Interpretations suggest that deposition in the study area was dominated by punctuated progradation of the Brent Delta, as periods of delta plain incision alternated with episodes of base-level rise and delta aggradation/progradation. A model of regression for the Brent Delta is presented, where the Rannoch, Etive and Ness formations are an amalgam of highstand, falling stage and lowstand systems tract deposits, and the Tarbert Formation is a transgressive systems tract deposit, with the delta responding to regional relative sea-level changes driven by uplift and deflation of the mid North Sea dome. The prograding Brent Delta is characterized as a succession of attached shorefaces formed by alternating periods of normal and forced regression. Significantly, this explains the long distance (&gt;200 km) build out of the Brent Delta and the continued presence of coarse-sandstone packages, as well as the potential for high-quality reservoirs even in the distal reaches of the system. It also suggests that there is limited potential for lowstand fan plays beyond the northernmost tip of the delta.