A crustal magnetic model of Britain obtained by 3D inversion

The national baseline aeromagnetic survey of Britain allows a uniform assessment of the shallow and deep magnetic properties of the British tectonic terranes. The most significant is that associated with destruction of early Palaeozoic oceanic lithosphere across the Iapetus Suture separating Baltica and Avalonia from the Laurentian terranes. Here a formal 3D inversion of a continuous swathe of the data is considered. The study provides a uniform volumetric whole crust assessment extending for over 1000 km. Normally a 3D inversion of magnetic data is controlled using a variety of constraints however this is not appropriate at the crustal scale due to our increasingly imprecise knowledge of lithology at increasingly greater depths. The main crustal interface encountered occurs at the Curie isotherm depth. We demonstrate the behaviour of introducing different magnetic crustal depths and suggest the crustal ‘magnetic depth’ of our models can be independently constrained using global or regional studies of the deep geotherm. Static magnetic data have no inherent depth resolution. Here an empirical ‘1D depth’ weighting and a more formal ‘3D distance’ weighting are assessed. The inversion procedure is regularised to provide stable models appropriate to the data and their errors. To gain confidence when using such a ‘geologically-unconstrained’ inversion, we compare our 3D inversion results with an existing geologically-constrained 2.5D profile inversion across northern Britain. A surprising agreement in the 3D susceptibility magnitudes is observed. The chosen study area traverses 10 British terranes and images their tectonic fabric by way of non-magnetic zones (i.e. susceptibilities <0.0001 to 0.001 SI) and magnetic zones displaying geological relevance and tectonic significance at deeper crustal levels. Here we discuss the more significant 3D model features which, by virtue of a continuous crustal-scale assessment and fitting the data with a high degree of fidelity, provide additional structural insights

Palaeozoic petroleum systems of the Irish Sea

This report synthesises the results of the 21CXRM Palaeozoic project in the Irish Sea to describe the Palaeozoic petroleum systems of that area. One hydrocarbon play system dominates the basin system: Namurian organic-rich marine shales (Bowland Shale Formation) generated oil and gas with a peak during maximum burial of the system in late Jurassic/early Cretaceous time. These hydrocarbons passed to reservoirs in the Triassic Ormskirk Sandstone (Sherwood Sandstone Group) by way of structures generated during the Variscan Orogeny and Cenozoic inversion, resulting in the Morecambe, Hamilton and other gas and oil fields The Palaeozoic study of the wider Irish Sea area has assessed the potential for more widespread petroleum systems situated outside the well-known play, particularly within the Carboniferous. Within the Main Graben system of the East Irish Sea Basin, Coal Measures strata were partially removed following Variscan inversion and early Permian uplift. They are not rich in coals, and not inferred to be a significant source rock. There is some potential in the Millstone Grit and Yoredale sequences, as some shales (particularly those associated with marine bands) are known to have high Total Organic Contents. The source rock potential of shales within the Carboniferous Limestone sequence is poorly constrained by data. A Devonian source rock is unproven and considered unlikely. Potential Namurian source rocks, such as the Yoredale Group, have been largely eroded in the Peel and North Channel basins, considerably reducing their prospectivity, although terrestrial sequences of equivalent age in the Solway Basin may offer better potential. The variable seismic data quality at Carboniferous levels and sparsity of deep well control have led to challenges in interpretation, particularly of the deeper picks. The interpretation of the surfaces contains a strong model-driven element, evidenced by the onshore relationships and areas where seismic picks can be made with the greatest confidence. Based upon the integration of regional seismic mapping with a limited well, source rock and reservoir property dataset, the most prospective parts of the region, outside the Ormskirk conventional gas play, are considered to be: The thick Westphalian sequences preserved in the Eubonia Tilt-Block in Quadrant 109, outside the main Permian-Mesozoic graben system and unaffected by Cenozoic inversion. The presence and quality of seals form a major risk as the Cumbrian Coast Group seal is thin or absent and Carboniferous intraformational seals are required but untested. Based on the limited dataset available in adjacent basins, reservoir quality is also a significant risk. A belt of Variscan inversion structures correlated with structures on the Formby Platform, and Ribbledale Foldbelt onshore, from which hydrocarbons have leaked into the overlying, Ormskirk-hosted Hamilton fields. The biggest risk here is whether reservoirs remain unbreached at the Pre-Permian level, and retain good poroperm characteristics at depths of about 2500 m. A more speculative play lies in the extensive carbonate platform in Quadrant 109 and surrounding the Isle of Man, in reefal facies with enhanced secondary porosity. Here, source rock presence and migration pathways, reservoir properties and seal quality are major risks

The 2020 national seismic hazard model for the United Kingdom

We present updated seismic hazard maps for the United Kingdom (UK) intended for use with the National Annex for the revised edition of Eurocode 8. The last national maps for the UK were produced by Musson and Sargeant (Eurocode 8 seismic hazard zoning maps for the UK. British Geological Survey Report CR/07/125, United Kingdom, 2007). The updated model uses an up-to-date earthquake catalogue for the British Isles, for which the completeness periods have been reassessed, and a modified source model. The hazard model also incorporates some advances in ground motion modelling since 2007, including host-to-target adjustments for the ground motion models selected in the logic tree. For the first time, the new maps are provided for not only peak ground acceleration (PGA) but also spectral acceleration at 0.2 s (SA0.2s) and 1.0 s for 5% damping on rock (time-averaged shear wave velocity for the top 30 m Vs30 ≥ 800 m/s) and four return periods, including 475 and 2475 years. The hazard in most of the UK is generally low and increases slightly in North Wales, the England–Wales border region, and western Scotland. A similar spatial variation is observed for PGA and SA0.2s but the effects are more pronounced for SA0.2s. Hazard curves, uniform hazard spectra, and disaggregation analysis are calculated for selected sites. The new hazard maps are compared with the previous 2007 national maps and the 2013 European hazard maps (Woessner et al. in Bull Earthq Eng 13:3553–3596, 2015). There is a slight increase in PGA from the 2007 maps to this work; whereas the hazard in the updated maps is lower than indicated by the European maps

Seismic interpretation and generation of depth surfaces for Late Palaeozoic strata in the Irish Sea Region

This report describes the methodology and results of a regional seismic interpretation of the basins of the Irish Sea. It does not review the basins of the Celtic Sea. The aim of the interpretation was to map the distribution of Palaeozoic basins and highs, interpreting the key Devonian-Carboniferous surfaces and main structural elements of the area. About 40,000 km of 2D seismic reflection data have been interpreted and tied to key released wells in the project area. The seismic and well data were augmented by donated reports from sponsor companies. A set of 8 depth structure maps of key horizons have been produced for the pre-Permian succession. These maps provide a key dataset to aid assessment of the petroleum systems of the Palaeozoic strata within the study area. The surfaces, supplied digitally at a grid spacing of 5000 m, give a regional view of the topography of the horizons, and comprise: ‘UVAR’ (Variscan Unconformity) beneath Permian and Triassic strata Base Warwickshire Group (late Westphalian - ?Stephanian) Top Namurian (Base Pennine Coal Measures Group) Top Intra-Namurian (Top Bowland Shale in south, Base Millstone Grit elsewhere) Top Visean (Lower Carboniferous) Intra-Visean (amalgamated with Top Middle Border Group in north) Base Carboniferous (amalgamated with Base Clyde Plateau lavas in the North Channel to South-West Arran Sub-Basin) ‘UCAL’ Acadian (Caledonian) Unconformity) It is important to note that the variable data quality and sparsity of deep wells leads to a seismic interpretation which is strongly driven by regional geological models, themselves heavily dependent on inference from the onshore area. This is particularly the case with the deeper Carboniferous horizons which are not penetrated by any well and which may be only weakly reflective. In such cases, picks from better quality data may be interpolated through areas with poor quality data, as a modelled surface, to ensure a continuous surface for gridding. The well dataset has been re-interpreted (Wakefield et al., 2016) before integration with the seismic interpretation. The following general observations are made: The present study has confirmed the Permian-Mesozoic structural framework for the region established by Jackson and Mulholland (1993) and Jackson et al. (1995, 1996, 1997). The basin recognised in Quadrant 109 by Jackson and co-workers (op. cit.) is reinterpreted as a major Carboniferous half-graben structure controlled by a syndepositional fault on its NW side. It continues beneath thin Permo-Triassic cover into the Eubonia Basin and Ogham Platform, preserving a thick Westphalian succession, including inferred Warwickshire Group strata. It is inferred to have continued eastward into the Lagman Basin prior to its tectonic dissection by a combination of Variscan inversion and Permo-Mesozoic graben development along the Keys Fault. A belt of Variscan fold/thrust inversion structures on the Godred Croven Platform is correlated with structures on the Formby Platform and Ribblesdale Foldbelt onshore. CR/16/041 Last modified: 2016/05/30 09:37 10 The area of Carboniferous (undivided) subcrop depicted on mapping by BGS (1994) to north and west of the Isle of Man has been reclassified into Visean and Namurian elements. The presence of significant thicknesses of Carboniferous strata in the southern part of the North Channel is regarded as doubtful, but farther north, in the Larne, Rathlin and South-West Arran Sub-basins, greater thicknesses may be present. A detailed description of tectono-stratigraphic development based on the seismic interpretation is given in Pharaoh et al. (2016b), integrated with the petroleum system analysis

Deep geothermal resource assessment of early carboniferous limestones for Central and Southern Great Britain

Early Carboniferous limestones (ECL) host active geothermal systems in Central and Southern Britain. Equivalent rocks have been successfully developed for geothermal energy in Belgium and the Netherlands, but the ECL has yet to be fully assessed as a geothermal resource in Britain. We use established statistical methods to assess the depth, distribution, and geothermal potential of the ECL in Central and Southern Britain. Total heat in place (HIP) resources of 1415 (P10)–1528 (P90) EJ may be present, with a tentative potential recoverable thermal power of 106–222 GW. Further work is needed to understand the resource by identifying areas with sufficient flow rates for successful development such as enhanced permeability zones around faults, fractures or karsts

Vale of York 3-D borehole interpretation and cross-sections study

The Vale of York between Doncaster and Scunthorpe in the south and York and Bugthorpe in the north is largely underlain by bedrock of the Sherwood Sandstone Group – one of the regions principal aquifers. Significant superficial deposits of Quaternary age overlie the Sherwood Sandstone. This study aims to investigate the nature of these superficial deposits with respect to their relationship with the underlying aquifer. The Vale of York project area represents a varied glaciated terrain, consisting of pro-glacial finegrained sediments, coarser glaciofluvilal sediments and extensive glacial tills. These diverse superficial units vary in thickness throughout the project area. The hydrogeological nature of the natural superficial sequence is consequently highly variable. Units may be considered as aquitards, while others may act as aquifers, providing a potential pathway to the underlying sandstone. The classification of lithologies as aquifer or aquitard is described in detail in this report. To investigate the hydrogeological nature of the superficial sequence, six east-west and three north-south lithostratigraphical cross-sections were constructed. A range of geoscientific information was considered, including existing geological mapping and over 3000 fully attributed and coded boreholes. The cross-sections show a subdivision of the superficial sequence into lithostratigraphical units. Each unit is described in detail in this report. In addition, a series of thematic maps were generated from the lithological component of the digital borehole data. Total superficial aquifer and superficial aquitard maps show how the lithological nature of the superficial sequence varies across the area. Rockhead elevation and superficial thickness maps indicate where the sandstone aquifer outcrops at the ground surface. In summary, four main lithostratigraphical units overlie the Sherwood Sandstone Group aquifer in the project area: a basal sequence of glaciofluvial sand and gravel (interpreted as a superficial aquifer), glaciolacustrine laminated silt & clay (aquitard), glacial till comprising sandy gravelly clay (aquitard), and a cover sequence of fluvial and aeolian sand, clay and peat (aquifer / aquitard). The correlations illustrate that in certain areas, superficial deposits are thin or absent and that in these areas the Sherwood Sandstone aquifer comes directly to ground surface

The Geology and Geophysics of the United Arab Emirates. Volume 6, Geology of the western and central United Arab Emirates

This volume forms part of the product of a multidisciplinary study by the British Geological Survey (BGS), commissioned by the UAE Federal Government, Ministry of Energy, to produce 1:100 000 scale geological maps of the bedrock and superficial geology of the central and western part of the UAE. 1:50 000 scale maps were produced of selected areas near Abu Dhabi and for seven emergent salt domes (mainly on offshore islands). This document therefore follows on from the previous volume (Styles et al., 2006) on the geology of the northern Emirates. The present volume similarly contains information about the findings of the geological mapping and associated specialist research. Much of the central and western part of the UAE is blanketed by Quaternary and recent dune sands, fluvial sediments and sabkhas. The only significant areas of exposed older rocks occur along the coast between Abu Dhabi and As Sila and on the various salt islands. Previous geological maps were made more than 20 years ago and were based largely on air photo interpretation with limited ground-truthing. The rapid economic development in the intervening period has seen a tremendous expansion of the national infrastructure and this has greatly increased the demand for high quality, field-based geological maps. The area was mapped at a scale of 1:50 000 and maps produced at 1:100 000, with the exception of the salt dome islands which were mapped at 1:25 000 scale. An area of the coast around Abu Dhabi is illustrated in two additional maps produced at 1:50 000 scale. Each 1:100 000 scale map is accompanied by a Sheet Explanation that describes the rock sequences in the area covered by that particular map. This report describes the broader features of the main rock groups on a regional basis and includes the results of the specialist laboratory studies that were used to define and interpret the geology of the project area. The field mapping was undertaken in the winter months between October 2008 and March 2011. The mapping was carried out by A R Farrant, R A Ellison, R J Thomas, J W Merritt, S J Price, A J Newell, J Merritt, J R Lee, A B Leslie, H F Burke, R A Smith, K M Goodenough, A Finlayson, C J Jordan and S L B Arkley. The laboratory studies were mostly carried out at the BGS. The petrographic study of the Miocene and Quaternary rocks and sediments was carried out by E R Phillips, whilst the igneous petrology of the Hormuz Complex rocks was undertaken by members of the mapping team. I P Wilkinson undertook micropalaeontologial determinations of both Miocene and recent sediments. The macropalaeontology of the basal Baynunah Formation is based largely on published information provided by Dr Mark Beech, Professor Andrew Hill and Dr Faisal Bibi. Some additional data on the Fars Group was provided by M A Woods. R Knox did the heavy mineral analyses, whilst D Wagner and S Kemp worked on the XRD analysis. The U-Pb zircon and carbonate geochronology was undertaken by M Horstwood, N Roberts and R Parrish at the NERC Isotope Geoscience Laboratories at the BGS, Keyworth. The Optically Stimulated Luminescence (OSL) dating was carried out by Professor G Duller and Dr H Roberts in the Aberystwyth Luminescence Research Laboratory at the Institute of Geography and Earth Sciences, Aberystwyth University. P Turner and C Simpson were responsible for the GIS systems and map layouts. H Holbrook and S Ward drew the majority of the illustrations. W Masterson and A Hill formatted this Memoir and the Sheet Explanations. This volume was compiled by A R Farrant and edited by R J Thomas

An overlooked play? Structure, stratigraphy and hydrocarbon prospectivity of the Carboniferous in the East Irish Sea–North Channel basin complex

Seismic mapping of key Paleozoic surfaces in the East Irish Sea–North Channel region has been incorporated into a review of hydrocarbon prospectivity. The major Carboniferous basinal and inversion elements are identified, allowing an assessment of the principal kitchens for hydrocarbon generation and possible migration paths. A Carboniferous tilt-block is identified beneath the central part of the (Permian–Mesozoic) East Irish Sea Basin (EISB), bounded by carbonate platforms to the south and north. The importance of the Bowland Shale Formation as the key source rock is reaffirmed, the Pennine Coal Measures having been extensively excised following Variscan inversion and pre-Permian erosion. Peak generation from the Bowland source coincided with maximum burial of the system in late Jurassic–early Cretaceous time. Multiphase Variscan inversion generated numerous structural traps whose potential remains underexplored. Leakage of hydrocarbons from these into the overlying Triassic Ormskirk Sandstone reservoirs is likely to have occurred on a number of occasions, but currently unknown is how much resource remains in place below the Base Permian Unconformity. Poor permeability in the Pennsylvanian strata beneath the Triassic fields is a significant risk; the same may not be true in the less deeply buried marginal areas of the EISB, where additional potential plays are present in Mississippian carbonate platforms and latest Pennsylvanian clastic sedimentary rocks. Outside the EISB, the North Channel, Solway and Peel basins also contain Devonian and/or Carboniferous rocks. There have, however, been no discoveries, largely a consequence of the absence of a high-quality source rock and a regional seal comparable to the Mercia Mudstone Group and Permian evaporites of the Cumbrian Coast Group in the EISB

New insights on subsurface energy resources in the Southern North Sea Basin area

The Southern North Sea Basin area, stretching from the UK to the Netherlands, has a rich hydrocarbon exploration and production history. The past, present and expected future hydrocarbon and geothermal exploration trends in this area are discussed for eight key lithostratigraphic intervals, ranging from the Lower Carboniferous to Cenozoic. In the period between 2007 and 2017, a total of 95 new hydrocarbon fields were discovered, particularly in Upper Carboniferous, Rotliegend and Triassic reservoirs. Nineteen geothermal systems were discovered in the Netherlands onshore, mainly targeting aquifers in the Rotliegend and Upper Jurassic/Lower Cretaceous formations. Although the Southern North Sea Basin area is mature in terms of hydrocarbon exploration, it is shown that with existing and new geological insights, additional energy resources are still being proven in new plays such as the basal Upper Rotliegend (Ruby discovery) for natural gas and a new Chalk play for oil. It is predicted that hydrocarbon exploration in the Southern North Sea Basin area will probably experience a slight growth in the coming decade before slowing down, as the energy transition further matures. Geothermal exploration is expected to continue growing in the Netherlands onshore as well as gain more momentum in the UK

Palaeozoic terranes and their lithospheric boundaries within the Trans-European Suture Zone (TESZ): a review

The Trans-European Suture Zone (TESZ) is a broad and complex zone of terrane accretion separating ancient lithosphere of the Baltic Shield and East European Craton (EEC) from the younger lithosphere of western and southern Europe. There is debate about the number of terranes involved, and their origins. The most significant terrane boundaries, originally oceanic sutures, are poorly exposed, and are defined using faunal provinciality. Prominent geophysical lineaments may represent sub-terrane boundaries within composite terranes. The Avalonia Composite Terrane was amalgamated to Baltica in latest ician time. The provenance of the Malopolska and Moravo–Silesian terranes, whose Neoproterozoic basement is suspect with respect to Baltica, but with which they nonetheless share faunal affinities, is still debated. The extent of the Variscide Rhenohercynian Zone defines the southern limit of the early Palaeozoic-accreted terranes. Terranes within the Bohemian Massif exhibit HP metamorphism recording subduction-driven collision prior to incorporation in the TESZ collage, itself associated with a HP event. In Romania, the Saxothuringian Zone (and Rheic Suture) are directly juxtaposed with the EEC, and the crustal structure of the TESZ has been much modified by post-Palaeozoic tectonism. Deep seismic reflection data, where available, show that the oceanic sutures are frequently associated with inclined reflectivity zones (IRZ) in the lithospheric mantle, here inferred to mark relict subduction zones. Interpreting the age of subduction is complicated by offset of originally coplanar mantle and crustal segments of the IRZ, resulting from early orogenic lithospheric delamination at the Moho interface and/or post-orogenic ‘reordering’ processes. The mantle IRZ appear to indicate a N-directed polarity for early Palaeozoic subduction, apparently incompatible with the geological evidence, which indicates subduction of the Tornquist Sea beneath Avalonia, and a large amount of underthrusting of Avalonia by Baltica. Possible explanations for these apparently incompatible observations are evaluated