Chapter 1. Introduction

Supplementary material: A spreadsheet tabulation of identified sequences and lithostratigraphic units (depths and thicknesses) in wells referenced in this Memoir in MS Excel format is available at https://doi.org/10.6084/m9.figshare.c.6695477

Since the 1980s, sequence stratigraphy has become increasingly used as a powerful tool in basin analysis, as a means of interpreting subsurface stratigraphic relationships, that has proved to be more consistently reliable than lithostratigraphy or biostratigraphy. Sequence stratigraphy provides a method of calibrating and linking relationships evident from seismic data to well data, leading to the formulation of new play concepts. The method also provides an element of lithological prediction in undrilled areas.

The application of sequence stratigraphic analysis has been embraced and, indeed, driven by the hydrocarbon exploration and development industry. The bulk of the information and interpretations included in this memoir have been generated and derived from the search for hydrocarbon resources in the development of the North Sea Basin, a major province of world significance. The hydrocarbons discovered and developed have provided great economic benefit to the countries that border the North Sea region, and the contribution made by the hydrocarbon industry to the development of sequence stratigraphy has been substantial. The stimulation of the development of sequence stratigraphy by the hydrocarbon exploration and development industry parallels the similar advancement of other essential subsurface evaluation techniques such as seismic acquisition and interpretation, reservoir modelling, structural geology, basin analysis, and petrophysical analysis, among many other disciplines. It is, however, a symbiotic relationship, whereby advances in the science have benefitted the efficiency of the search and recovery of the basin's hydrocarbon resources, as shown by the examples and case studies described in this memoir.

We now live in an era in which society intends to become less reliant on fossil fuels such as oil and gas, and aims to move towards achieving net zero carbon (CO₂) emissions. In the case of the UK, the country is legally committed to achieving net zero by the year 2050. However, official forecasts from the UK North Sea Transition Authority (a renaming of the Oil and Gas Authority (OGA)) indicate that oil and gas will remain a vital part of the UK's energy mix for several decades to come, and it is therefore important to maximize production of oil and gas from the remaining resources in the North Sea Basin. This imperative has become even more critical in 2022 with the energy crisis, creating the desire of the UK Government to maximize domestic oil and gas reserves. With this aim in mind, techniques such as sequence stratigraphy will continue to play an important role in developing the remaining hydrocarbon resources of the basin as efficiently as possible, while at the same time moving through the energy transition towards the net zero target. Furthermore, those technological advancements stimulated by the hydrocarbon industry, as noted above, will enhance the development of alternative energy sources such as geothermal energy (particularly sourced from deep aquifers). The requirement to sequester CO₂, or use it for enhanced oil recovery (EOR), as part of carbon capture, utilization and storage (CCUS) projects will also benefit significantly from the aforementioned techniques and the enhancements of subsurface geological relationships that derive from the application of sequence stratigraphic methodologies.

The North Sea Basin is often considered to be a mature hydrocarbon province; however, discoveries have continued to be made in recent years in Jurassic reservoirs. Complex structuration and rising creaming curve trajectories for the Jurassic (Eriksen et al. 2003; Vining et al. 2005; Erratt et al. 2010) suggest that significant hydrocarbon exploration potential remains in this interval, as has been demonstrated by many recent discoveries (see later in this chapter for examples). The application of sequence stratigraphic methodologies, both in exploration and field development, is pivotal to unlocking the next generation hydrocarbon discoveries in this important hydrocarbon province.

This publication contains a full documentation of North Sea Jurassic sequences, and builds on previously published work by the authors and other researchers, formerly of BP, and subsequently applied and extended within other organizations. The geographical scope of this publication is focused upon the area covered by the UK, Norway and Denmark sectors of the North Sea Basin, with correlation to offshore Netherlands, offshore Ireland and onshore UK where appropriate.

This study confirms the basic concepts of sequence stratigraphy, particularly the cyclicity of Jurassic sedimentation and the development of stratigraphic breaks at cycle boundaries. However, in a tectonically active basin such as the North Sea, some of the concepts of sequence stratigraphy that have been established for passive-margin settings are not always appropriate for the Jurassic of this basin. This memoir considers the likely causes of sequence generation, particularly in relation to the tectonic evolution of the area during Jurassic–earliest Cretaceous time.

Thirty-nine third-order (see below for a discussion of sequence scale) stratigraphic sequences/subsequences (plus one additional potential sequence) can be recognized in the North Sea Jurassic. These sequences are predominantly defined in relation to maximum flooding surfaces, although a number of depositional sequence boundaries and transgressive surfaces have also been recognized, two of which are utilized as sequence-bounding surfaces (base J73 and top J76). Many of these key surfaces are developed as unconformities in certain areas and, in total, 18 significant unconformities have been identified. The interpreted stratigraphic positions of the defined sequence boundaries relative to standard stages and ammonite zones are shown in Table 1.1. Also shown are the approximate, inferred geochronological ages of the sequence boundaries relative to the most recent geological timescale of Hesselbo et al. (2020). Correlation to the most recent published geological timescale should be regarded as approximate and will be subject to reinterpretation as future, revised timescales are published.

The work presented in this memoir has been in development for around 40 years, since the authors first began detailed stratigraphic studies on the North Sea Jurassic in the early 1980s, initially as biostratigraphic consultants (at Robertson Research) and subsequently as biostratigraphers with Britoil (PC) and BP (MAP and PC). As a result of those early studies, it soon became apparent that it was possible to recognize particular units within the Jurassic that were consistently transgressive in nature, and which onlapped onto structural highs. In addition, significant unconformities were regularly recognizable, as indicated by missing biozones evident from the biostratigraphic studies and well correlations that were being carried out to support hydrocarbon exploration and development activities. The unconformities seemed to be developed synchronously across a range of blocks and sub-basins across wide areas of the North Sea Basin, and some matched the seismic horizons being mapped by the geophysicists in the exploration and development teams. Similar depositional patterns in the North Sea Basin, some of which were also expressed in the stratigraphic successions of onshore Britain, were recognized as a set of ‘major sedimentary events (transgressions and regressions)’ (Rawson and Riley 1982), this being the first published work to recognize such regional changes in the North Sea Basin. The transgressive–regressive events of these authors, as recognized in the Upper Jurassic–Lower Cretaceous interval, correspond to unconformities and associated maximum flooding surfaces, of regional extent, all of which can be related to the boundaries of particular J sequences as originally defined and further described in this memoir.

The Partington et al. (1993a) North Sea J sequence biostratigraphic calibration paper was extracted from an earlier version of the present publication, which was too large to be published in full in the 4th Petroleum Geology of Northwest Europe conference proceedings. The original main aim of this current publication, therefore, was to provide detailed descriptions and definitions of the North Sea J sequences that were the basis of several ‘BP’ papers in the early 1990s (e.g. Mitchener et al. 1992; Partington et al. 1993a, b; Rattey and Hayward 1993). Because the original J sequences were not defined in detail in the latter publications, this has led to some confusion in the application of the original sequence definitions. The present memoir is intended to provide this full documentation, supported by data from a significant number of reference wells, seismic lines¹, core information and outcrop studies. In compiling one of the recent geological timescales, Gradstein et al. (2012, p. 14) stated that ‘at the time of writing, no academic manual was in place that systematically listed sequences and sequence boundaries in a stratigraphic database with type sections and type events’. The current work should address some of these requirements, at least for the North Sea Jurassic.

In addition, the current publication reviews the most relevant of the many papers that have been published on the North Sea Jurassic since the original J sequence definitions in 1992–93 and has used this large body of new knowledge to take a view not only on how applicable the J sequence scheme is and to update it accordingly but also, importantly, to consider what were the likely controls on the undoubted cyclic depositional history of the Jurassic of the basin. Several figures from previous publications have been reproduced in the following chapters to reflect the legacy work that has been carried out on the memoir subject area and to place prior evaluations into the context of the current interpretations.

It is evident that while there are variations in basin evolution during the Jurassic across the study area, the existence of different lithostratigraphic schemes in all the countries that share the North Sea Basin (UK, Norway, Denmark and The Netherlands) obscures a clear view of such changes. The application of a consistent set of stratigraphic sequences across this area, however, aids the understanding of the basin evolution of the whole region. This applies, for example, to the understanding of the regional controls on sequence development, timing of tectonic events, correlation of source rock intervals and reservoir developments. A good example of this is the similarity of the trends that have been observed in the evolution during the Middle–Late Jurassic in offshore Netherlands, Germany and Denmark, in the studies of Bouroullec et al. (2018) and Verreussel et al. (2018), and particularly in the initial Late Jurassic phase of rifting but which diverged considerably in the last phase (latest Tithonian and Berriasian). Nevertheless, it is still evident that the major regional changes in tectonic evolution (e.g. across The Netherlands, Germany–Denmark region) can be correlated with geological changes across the rest of the North Sea Basin, even though details vary from area to area. It is this consistent pattern of sequence boundaries (in the broadest sense) across the region that is the basis for the recognition of the J sequences described in this memoir.

The basic principles and definitions of sequence stratigraphy are described in numerous publications (e.g. Vail et al. 1977; Loutit et al. 1988; Posamentier and Vail 1988; Van Wagoner et al. 1988, 1990; Emery and Myers 1996; Embry 2009, among others) and are not reiterated in detail herein. However, the current publication does document a pragmatic approach to sequence stratigraphy and the particular techniques that have been used by the current and other authors in the Jurassic of the North Sea Basin. A review and discussion of sequence methodologies in tectonically active basins such as the North Sea is provided (Chapters 2, 8 and 12), some aspects of which are not covered in those sequence stratigraphic publications cited above.

To reflect the global surge in the studies of exploration provinces and plays adopting a sequence stratigraphic approach, there have been several attempts to document and standardize the practical methodologies and terminology (e.g. Emery and Myers 1996; Catuneanu 2003, 2006, 2012; Abreu 2007; Catuneanu et al. 2009, 2010, 2011; Neal and Abreu 2009; Abreu et al. 2010, 2014; Martins-Neto and Catuneanu 2010; Neal et al. 2016). At the same time, there have been valuable documentations and discussions of the range of different approaches to sequence stratigraphy that now exist, including a critique of these methodologies and practical problems concerning their application (e.g. Embry 1995, 2009; Catuneanu 2006; Embry et al. 2007; Holbrook and Bhattacharya 2012). Several publications that are particularly critical of the original Exxon approach to sequence stratigraphy (e.g. Vail et al. 1977) have been published by Miall (1986, 1992, 2010). These different approaches, and their relevance to the North Sea Jurassic, are reviewed in Chapter 2.

Sequence stratigraphic concepts are discussed in Chapter 2 in the context of a review of the historical development of thought on sequence stratigraphy and its relevance to the understanding of sequence expression in the North Sea Basin. In addition, published sequence stratigraphic models and approaches are discussed in relation to the nature of sequence development in the North Sea Basin. This includes a discussion of the merits and applicability to the North Sea Jurassic, the three different sequence stratigraphic approaches that have been established, based upon the three key stratal surfaces: the maximum flooding surface, the depositional sequence boundary and the transgressive surface.

A significant aspect of the Partington et al. (1993a) paper was the inclusion of pull-out enclosure diagrams to the conference proceedings that gave details of the biostratigraphic zonation used to characterize and correlate the sequences. Those biozonation schemes are described, updated and defined in this memoir (Chapter 13), related to which are improvements to the biostratigraphic calibrations of the described sequences (Chapters 3–5).

As stated by Underhill (1999, p. 4), the original J sequence scheme, as documented in the publications cited above, ‘has been found to be extremely robust and remains the most widely applied template for sequence stratigraphic correlations in the North Sea’. Since the original definition of the J sequences, many authors have applied this sequence scheme to particular areas of the basin, and a number of these studies are fully referenced and compared in this publication (Chapters 3–5).

In addition, the present memoir (Chapter 7) reviews and compares with different sequence stratigraphic schemes that have been applied to the North Sea Jurassic but which have not utilized the published J sequences: for example, Carruthers et al. (1996), Harker and Rieuf (1996), de Graciansky et al. (1998), Hardenbol et al. (1998), Duxbury et al. (1999), Jeremiah and Nicholson (1999), Andsbjerg and Dybkjær (2003), Fraser et al. (2003), Herngreen et al. (2003) and Verreussel et al. (2018), among others. One aim of this is to assess to what extent the original J sequence scheme published in the early 1990s remains applicable in the North Sea Basin and to try to see what common patterns of sequence development can be discerned from other published schemes. Furthermore, these additional published schemes, which have generally been applied for particular subregions or developed fields, often describe additional sequences or key surfaces to those formally recognized in the regional J sequence scheme; in this memoir, these are reviewed to see if these additional surfaces are of sufficiently regional extent to warrant the introduction of a new sequence into the J sequence scheme.

The geographical focus of this publication is the North Sea Basin, primarily the area covered by the UK, Norway and Denmark sectors (Figs 1.1 & 1.2). Within the UK offshore area, most of the focus is on the Central and Northern North Sea, where detailed studies of the J sequences have been made. The West of Shetland, Atlantic Margin, English Channel and South West Approaches areas of the UK offshore area are outside the geographical scope of this review. Some reference is also made to the Southern North Sea area, in which thick successions of Jurassic sediments are present, particularly the Lower Jurassic because offshore sections of this age are generally very sparsely developed in the Central North Sea area. Modern, comprehensive data are not available to the authors from offshore Netherlands and therefore this part of the North Sea is not referred to in detail here, other than to cross-refer to key published works from this area.

The nomenclature of structural elements, including basin, sub-basin and platform areas, varies between authors and country jurisdictions. For the most part, the structural elements described by Fraser et al. (2003) have been followed herein, and the geographical scope of the latter publication is a close match to that of the current memoir. Figures 1.1 and 1.2 show the main structural elements referred to in this memoir. Figure 1.2 shows the superposition of the main Jurassic depositional areas in the North Sea region compared to a Base Cretaceous seismic horizon grid, which reflects the underlying Jurassic depositional areas (from Fraser et al. 2003; Millennium Atlas GIS 2003).

References are also made to adjacent geographical areas where relevant, such as onshore UK, basins west of the UK, such as the Celtic Sea, and the Slyne Basin of offshore Ireland. The West of Shetland area, offshore the UK is not referred to in this publication, due to the attenuated and generally poorly developed Jurassic successions in this area that preclude the application of sequence stratigraphic studies at the present time.

The structure of this memoir is as follows:

• Chapter 1 provides background to the J sequences, outlines the scope of the study, and considers matters including the definition of the Jurassic System and the lithostratigraphic schemes currently in use in the region.

• Chapter 2 provides a review and discussion of sequence stratigraphic concepts and methodologies relevant to the Jurassic of the North Sea Basin, including a review of the historical application of sequence stratigraphic approaches to the North Sea Jurassic.

• Chapters 3–5 contain the detailed individual descriptions and definitions of the J sequences, at second- and third-order level, defining each in terms of age, biostratigraphic characterization, definition of basal boundary, seismic expression, key surfaces and correlation with lithostratigraphic subdivisions. A comparison with previously published work and also any known correlations with onshore UK successions are made for each sequence. A set of reference wells is provided for each defined sequence.

• Chapter 6 discusses the seismic expression of North Sea Jurassic sequences, and includes a discussion of the Base Cretaceous Unconformity.

• Chapter 7 provides comparison discussions of previously published Jurassic sequence stratigraphy schemes.

• Chapter 8 discusses controls on the development of North Sea Jurassic sequences.

• Chapter 9 discusses the application of sequence stratigraphy in the exploration of North Sea Jurassic stratigraphic traps.

• Chapter 10 discusses selected North Sea hydrocarbon fields, with Jurassic reservoirs, in a sequence stratigraphic context, and also touches on issues related to carbon capture and storage.

• Chapter 11 provides a lexicon of North Sea Jurassic lithostratigraphic units, and includes definitions and descriptions of some new units, illustrated with reference well displays.

• Chapter 12 sets out a well sequence stratigraphic interpretation methodology.

• Chapter 13 describes the biozonation scheme that has been applied to the chronostratigraphic interpretation and characterization of the J sequences.

• Chapter 14 provides a summary and conclusions.

The North Sea Graben is one of the world's great petroleum provinces. The oil and gas accumulations found there occur in a variety of structural settings and within reservoir rocks of various of ages, but almost all originated from shales that were deposited during a relatively brief stratigraphic interval encompassing Late Jurassic to earliest Cretaceous time

(Gautier 2005, p. 2).

After nearly 60 years of hydrocarbon exploration² in the North Sea Basin, oil and gas reserves have been found in reservoirs of a great variety of stratigraphic ages across numerous play configurations. The most significant proportion of proven hydrocarbon resources in the basin is reservoired in, and sourced by, sediments of Jurassic age, as noted in the quote above. Of the 100 × 10⁹ barrels of oil equivalent (boe) of total resources³ discovered in the North Sea Basin, as of 1996, Spencer et al. (1996) estimated that 49.7 × 10⁹ boe, in 243 pools, were contained in Jurassic reservoirs (49.7%). Furthermore, 71% of all oil and gas resources known at that time had been sourced from Jurassic sediments. It was later estimated that 40% of the total discovered reserves/resources in the Central and Northern North Sea are contained in Lower–Middle Jurassic reservoirs and 25% in the Upper Jurassic–Lower Cretaceous interval (Eriksen et al. 2003).

In a more recent North Sea Basin-wide play resource assessment, Quirk and Archer (2020) provided some key statements regarding the importance of the Jurassic-related resources (comments in italics below are from the current author):

• The reservoir type with the largest total discovered petroleum resource of all North Sea reservoirs is Middle Jurassic paralic sandstones; this includes the Brent (and Sleipner) play in the North Viking Graben region (20 Bboe in Norway and UK), and the Hugin Sandstone in the Central and South Viking Graben (2 Bboe in the UK and Norway) [Note that the Hugin Sandstone ranges into the Upper Oxfordian in some areas and in many cases is of fully marine origin]:

  • of these Jurassic paralic sandstone reservoirs, these are better represented in Norway than in the UK offshore; and

  • Middle Jurassic paralic sandstone discoveries are absent from the Outer Moray Firth Basin.

• Lower Jurassic discoveries are only found in the Viking Graben. These include the Statfjord Formation and Cook Formation sandstones.

• Upper Jurassic plays rank fourth in terms of resource size behind the Middle Jurassic paralic sandstones, Paleocene turbidite sandstones and Upper Cretaceous Chalk reservoirs. Upper Jurassic plays can be subdivided into five specific types, which, in descending resource size order, are:

  • Fulmar–Piper–Ula shallow-marine sandstones, Outer Moray Firth–Central Graben (8 Bboe in UK and Norway);

  • Sognefjord Sandstone shallow-marine sandstones on the Horda Platform (6 Bboe to date);

  • Vik shallow-marine sandstones around the Utsira High in Norway (3 Bboe) [note that these are very comparable to other shallow-marine sandstones developed in the UK area, some of which are grouped in the Fulmar–Piper–Ula play type];

  • Brae–Heather–Draupne turbidite sandstones of the Viking Graben (4 Bboe), mostly in the UK; and

  • Burns–Galley–Claymore turbidite sandstones of the Moray Firth in the UK (2 Bboe) [this type includes, in addition, the Buzzard and Ettrick sandstones].

• Under-explored plays; proven Jurassic plays that have significant yet-to-find potential, with relatively low numbers of exploration wells, low densities of discoveries and irregular discovery size distributions, which include; Lower Jurassic plays in the Norwegian Viking Graben; Middle Jurassic paralic sandstones in the Inner Moray Firth and in the Egersund Basin (Norway); Upper and Middle Jurassic paralic sandstones in the Norwegian–Danish part of the Central Graben plus adjacent highs; and Upper Jurassic turbidites in the Central Graben and Upper Jurassic paralic sandstones on the Norwegian side of the Viking Graben [note that many of these ‘paralic’ categories are more often shallow marine than paralic].

In addition to these large discoveries, other finds that have been made in Jurassic reservoirs in more recent years include the Jackdaw discovery (with well 30/2a-6, drilled by BG in 2005) with gas and condensate reservoired in intra-Heather Sandstone, the King Lear discovery (Norway Block 2/4, with well N 2/4-21, drilled by Statoil in 2012: Jones et al. 2013), reservoired in Eldfisk Sandstone (probable J64 sequence), the Tybalt discovery (UK Block 211/8b, with well 211/8c-4 and 211/8c-4Z, drilled by Valiant Petroleum in 2010), reservoired in J64 Magnus Sandstone, and the Cladhan discovery (UK Block 210/29), with well 210/29-4, drilled by Sterling Resources in 2008 (Williams et al. 2020), reservoired in Ptarmigan, Magnus and Home sandstones (J62–J72 sequences). The Jackdaw discovery is reported to contain reserves of between 120 and 250 MMboe, a revised development plan for which was submitted by current operator Shell in 2022. In 2019, CNOOC announced that the 22/21c-13 well, drilled on the Glengorm Prospect, located in offshore UK Central North Sea (west Central Graben) encountered gas and condensate, it is understood, in the Oxfordian J54 Freshney Sandstone Member, with estimated recoverable resources of 250 Mmboe of gas and condensate (according to partner Total).

In the Danish sector, the discoveries at Hejre-1 (drilled in 2001 by Phillips, reservoired in J62 ?Ravn Sandstone), Svane-1 (drilled in 2001 by ConocoPhillips, J63–J64 deep-marine sandstone) and Xana-1X (drilled in 2015 by Maersk, Upper Jurassic sandstones) have demonstrated the prospectivity of the Upper Jurassic in this sector.

In addition to new, exploration wildcat discoveries, further finds continue to be made in Jurassic reservoirs at near-field locations close to existing producing fields. For example, in 2014, Statoil (now Equinor) and its partners discovered gas in the Askja West Prospect and an oil discovery in the Askja East Prospect with wells N 30/11-9S and N 30/11-9A, located between the Oseberg and Frigg fields, reservoired in Upper and Middle Jurassic rocks. Equinor estimated the hydrocarbon volumes in Askja West and Askja East to be in the range of 19–44 Mmboe recoverable. Also in Norway, DONG Exploration & Production Norge discovered oil and gas in 2013 with well N 3/7-8S, in Middle Jurassic reservoir rocks (the Lulu and Bryne formations); preliminary estimates suggest that the size of the discovery is between 8.4 Mmboe and 16.8 Mmboe (Norwegian Petroleum Directorate). In 2016, Faroe Petroleum Norge announced the discovery of between 43 and 80 Mmboe recoverable with the N 31/7-1–N 31/7-1A wells, south of the Brage Field, with hydrocarbons reservoired in the Middle Jurassic Fensfjord Sandstone Member.

In 2017, Statoil (now Equinor) discovered oil in the Verbier Prospect, with the UK 20/5b-13Z well, located in the Outer Moray Firth Basin, of the UK Central North Sea, close to the Buchan and Tweedsmuir South oilfields, with estimates of gross recoverable resources of between 25 and 130 Mmboe (Jersey Oil & Gas website: https://www.jerseyoilandgas.com/our-assets/verbier/). The hydrocarbons in Verbier are reservoired in Upper Jurassic deep-marine sandstones, equivalent to the Buzzard Sandstone.

In 2020, Jersey Oil & Gas became the operator and announced that Verbier will be developed as part of the Greater Buchan Area (‘GBA’) hub. This project will aim to develop Verbier, and tie in several other known discoveries in the area (including a number with Jurassic reservoirs such as the Glenn discovery), as well as explore for new Jurassic prospectivity, with the aim to tie back into the existing infrastructure in the area (the Greater Buchan Area hub). This project is a good illustration of the strategy of exploring in areas where infrastructure is still available, into which future production may be tied back.

In 2020, Neptune and partners discovered estimated recoverable resources of between 40 and 120 Mmboe with the N 34/4-15S well reservoired in the Rannoch Formation (Brent Group), at the Dugong discovery. This was followed by the discovery of oil in well N 34/4-15A in intra-Draupne Sandstone (at the Sjøpølse Prospect). Further discoveries in offshore Norway during 2020 that proved the presence of hydrocarbons in Jurassic reservoirs included MOL's N 25/8-19 Iving/Evra well (resources of 12–71 Mmboe, partly in the Lower Jurassic), Equinor's N 15/3-12 Sigrun East Middle Jurassic discovery (resources of 7–12 Mmboe), and Equinor's N 30/2-5 Atlantis Middle Jurassic (Brent Group) discovery (with resources of 19–63 Mmboe) and N 34/7-E-4 AH Lomre discovery in the Middle Jurassic in the North Viking Graben. Lomre was a sidetrack from a previous development well at the Snorre Field and has resources of 5.5–9.5 Mmboe (https://www.westwoodenergy.com/news/infographics/westwoods-nw-europe-exploration-time-lapse). In March 2020, Total announced that the Isabella 30/12d-11 well, located in the Central North Sea offshore the UK, encountered hydrocarbons in Upper Jurassic and Triassic sandstone reservoirs (with resources of 63.4 Mmbo and 499.2 Bcfg).

In 2021, Equinor and partners announced a discovery north of the Troll Field, with the N 31/2-22S well (Blasto discovery), which proved an estimated recoverable resource of between 75 and 120 Mmboe reservoired in the Sognefjord Sandstone, which is the main reservoir unit in the nearby Troll West and Troll East fields. Also in 2021, exploration well N 31/1-2 S and appraisal well N 31/1-2 A, 10 km NW of the Troll Field, operated by Equinor, proved the presence of petroleum in the Middle Jurassic Brent Group (Oseberg and Etive formations) and in the Lower Jurassic Dunlin Group (Drake Formation). Equinor has pursued the Lower Jurassic Cook Formation sandstone play in the Tampen Spur area where it made two discoveries. In 2012, oil was discovered with the N 34/6-2S well, the Garantiana discovery (https://www.norskpetroleum.no/en/facts/discoveries/346-2-s-garantiana/), which was followed by the drilling of the N 34/6-5S well (Garantiana West discovery) in 2021 (https://www.equinor.com/news/archive/20210614-oil-discovery-near-visund). This play will be the subject of further drilling in 2022.

Consistent with the succession of discoveries outlined above, creaming curves for the Upper Jurassic in the North Sea Basin during and up to the end of the first decade of this century (Eriksen et al. 2003; Vining et al. 2005; Erratt et al. 2010) showed a rising trajectory, suggesting that there were significant yet-to-find reserves in this interval at that time. Quirk and Archer (2020, fig. 14) displayed a creaming curve for all Jurassic plays in the Central Graben, Outer Moray Firth and Viking Graben that also showed a continued rising trend. The analysis of Stoker et al. (2006) suggested that the discovery curve data for the Upper Jurassic deep-water play in the UK Continental Shelf (UKCS) was not yet (at that time) mature, and cited the rejuvenation of this play following the discovery of the Buzzard Field in 2001. In contrast, the creaming curve for the Lower–Middle Jurassic play, particularly in the UK (Eriksen et al. 2003, fig. 20.25), is now level or flattening (Quirk and Archer 2020, fig. 7), suggesting more limited resource growth potential in reservoirs of that age and that the play may be in its mature phase at the present time. Nevertheless, in this play, there is considered to be substantial remaining hydrocarbon potential in known fields, and this has stimulated the continued development of sequence stratigraphic approaches, combined with detailed facies analysis in the Brent Province (North Viking Graben, UK and Norway) in particular (Hampson et al. 2004; Went et al. 2013), as is discussed further in this memoir (see Chapters 3 and 9). In a more recent analysis, over the period 2010–end 2020, of all the hydrocarbons discovered in the UKCS and Norwegian Continental Shelf (NCS) (i.e. a wider area than the North Sea Basin alone), the highest hydrocarbon volumes were discovered in the Upper Jurassic play (amounting to 3.2 Bboe), while the Middle Jurassic play added 2.1 Bboe, much of which was in Norway (Moseley 2020). In terms of hydrocarbon production, as of July 2021, two of the three top producing UK fields, Franklin Field (81 Mboe/d) and Buzzard Field (80 Mboe/d) (UK North Sea Transition Authority: https://data-ogauthority.opendata.arcgis.com/pages/production), were producing from Jurassic reservoirs. In 2020, the top two producing fields in offshore Norway were Troll (260 Mmboe total annual production) and Johan Sverdrup (164 Mmboe total annual production) (Norwegian Petroleum Directorate: https://www.norskpetroleum.no/en/facts/historical-production/#per-field-in-2020). Both of these fields produce largely from Jurassic reservoirs. The Buzzard, Elgin and Troll fields, and the sequence stratigraphy of their reservoir successions, are discussed further in Chapter 9. A well from the Johan Sverdrup Field, N 16/3-6, and its reservoir development are discussed in Chapter 5 (see Fig. 5.19).

While most known discoveries and fields are trapped in conventional structural closures, some significant reserves have been proven in stratigraphic traps, the most significant being the Buzzard Field as mentioned above. Structural traps continue to be explored successfully, as noted previously; however, a significant number of stratigraphic prospects have been identified in various parts of the North Sea Basin in several different play configurations. Should economic circumstances allow (e.g. favourable tax terms and oil price), it is expected that a number of these will be drilled to prove up significant further hydrocarbon resource in the Jurassic. The application of sequence stratigraphy to the evaluation of stratigraphic traps is discussed in more detail in Chapter 10 with reference to several known Jurassic stratigraphic traps that have yet to be drilled.

In contrast to other parts of the stratigraphic column, the Jurassic stratigraphy of the North Sea Basin is complicated by the intense tectonic activity that took place during the period, particularly during the Late Jurassic. It is evident that sequence development during the Jurassic was at least partially controlled by structural changes (see Chapters 2 and 8) and the ensuing stratigraphy from area to area is, as a result, highly complex. Not only was there faulting and inversion but intimately bound with this was the movement of Permian (Zechstein) halite that appears to have been partly synchronous with Jurassic sedimentation, particularly in the Central North Sea (UK, Norway, Denmark and Dutch sectors). One effect of this is that the hydrocarbon play potential of the whole North Sea Basin, across several international sectors, has not been fully realized, as is demonstrated by the significant discoveries that continue to be made in Jurassic reservoirs. As the creaming curves show (see above), the Jurassic, particularly the Upper Jurassic play, is not yet mature and exploration potential remains, particularly in areas such as the deep Central Graben and the basin flanks.

Notwithstanding the above remarks, there is no doubt that the UK sector of the North Sea Basin overall, across all plays, is in a mature phase for hydrocarbon exploration. Some fields are now in their late life phase, and others, some of which are described herein, have already been decommissioned. It is hoped, that, despite the relative maturity of parts of the North Sea Basin for hydrocarbon exploration, that the current publication will prove a useful tool for those geoscientists whose role will be to realize the remaining potential of the Jurassic plays in this region, both within the UKCS and other sectors. In addition, the optimization of the reserves from the many fields with Jurassic reservoirs should benefit from the information and concepts in this work, and their impact on enhanced reservoir zonation, reservoir modelling and production optimization. The application of sequence stratigraphic concepts in field development is discussed in more detail, with reference to a selection of fields with Jurassic reservoirs, in Chapter 9.

While official forecasts from the UK North Sea Transition Authority (NSTA: prior to 2022 known as the Oil and Gas Authority (OGA)) suggest that oil and gas will remain a vital part of the UK's energy mix for several decades to come, the British Government has committed to the UK becoming carbon neutral (‘net zero’) by 2050. As an important part of this, there is the aim to develop alternative energy sources such as wind, solar and hydrogen, in particular. In order to achieve this target, in early 2020 the OGA challenged the UK oil and gas industry to commit to production emission-reduction targets. The NSTA is intending to implement strategic changes that have the potential to make a significant contribution to achieving net zero; both through carbon capture, utilization and storage (CCUS) and CCUS plus hydrogen. The Norwegian government too is encouraging CO₂ sequestration, and has been the location of the world's first CCS project, at the Sleipner Field. Norway has recently become the site of a new CCUS project, the Northern Lights Project, utilizing a Jurassic reservoir sandstone for CO₂ capture and storage (see Chapter 9 for further discussion, including reference to the potential application of sequence stratigraphic approaches in this project).

A major part of the current memoir (Chapters 3, 4 and 5) comprises detailed descriptions and definitions of the North Sea J sequences that had been proposed previously, but never fully defined, in several papers published in the early 1990s (Mitchener et al. 1992; Partington et al. 1993a, b; Rattey and Hayward 1993). In this memoir, each J stratigraphic sequence is described with reference to its biostratigraphic calibration (and correlation with standard chronostratigraphy), basal boundary definition (in well sections, with referral to reference wells), relationship with previously published sequences (both offshore and onshore), and development of key surfaces within sequence, seismic expression, distribution and lithological and lithostratigraphic character.

A total of 85 reference wells are displayed in the memoir (56 UK wells, 22 Norway wells and seven Denmark wells), showing chronostratigraphy, lithostratigraphy, wireline logs (gamma ray and sonic), interpreted J sequences and, in many cases, key biostratigraphic markers (with biozones indicated). The reference wells illustrate sequence development, together with their boundaries. The locations of the reference wells included in the memoir are shown in Figure 1.3. The reference wells, and the sequences and lithostratigraphic units to which they refer, and the figures in which they are illustrated, are listed in Table 1.2.

The biozones that have been recognized in the reference wells, and the key biostratigraphic markers, are shown in many of the reference well displays to support the sequence boundary definitions. The selection of reference wells has not been straightforward, because, due to the complexity of depositional patterns that are clearly affected by local and regional tectonics, literally no two wells show the same succession, and it is therefore difficult to select wells that show typical developments of individual or groups of sequences. This is particularly true of the Upper Jurassic–Lower Cretaceous successions, which are highly variable due to active syndepositional tectonics. Lower Jurassic successions, by contrast, are more uniform in their development.

Reference wells have been selected that demonstrate clear sequence developments where the sequences and their boundaries are well displayed both on wireline logs and, where possible, with good biostratigraphic data. In addition, reference wells have been chosen that show a range of reservoir developments and from several different sub-basins across the North Sea depositional area. The sequence stratigraphic and lithostratigraphic units shown in these wells are interpretative and it is accepted that different interpretations of the displayed well sections are possible. The chronostratigraphic intervals shown on the well displays are interpretative and are based on a combination of biostratigraphic data and sequence boundary interpretations, each of which carries age inferences. In some cases, therefore, these age boundaries are drawn between biostratigraphic data points, having been placed at interpreted genetic sequence boundaries placed on wireline log features.

Twenty-six well correlation panels⁴ are provided that demonstrate the extent of sequences and the lateral lithostratigraphic relationships, sometimes across offshore country jurisdiction sector boundaries. The locations of the correlation panels displayed are shown in Figure 1.4 

Comparisons of the North Sea Jurassic sequences with onshore outcrop sections, particularly from the UK, are made where appropriate. The apparent recognition of the maximum flooding surface condensed sections and stratigraphic breaks in both the onshore and offshore areas attest to their regional stratigraphic significance beyond the North Sea Basin.

A comparison of the well sequences with seismic sequences is made in 17 illustrated seismic lines, both from previously published work and supplemented by newly illustrated seismic lines, including ties to selected reference wells. The locations of the illustrated seismic lines are shown in Figure 1.5 

For each defined sequence, the seismic expression is discussed. Many of the important maximum flooding surfaces are developed as distinct onlap and downlap surfaces on seismic profiles. In the North Sea Basin, it is difficult to map the sequences over a wide area from seismic data alone with any degree of confidence. This is due partly to structural compartmentalization, which subdivides the region into discrete sub-basins, separated by terraces and highs. A further problem is seismic data quality, which is often insufficient to resolve many of the sequences at Jurassic level, particularly in legacy datasets. In the deeper parts of the North Sea Basin, poorly imaged deep structures and lack of well ties contribute to the high level of subjectivity associated with seismic horizon picks below the Base Cretaceous level. Typically, sequences are only mappable seismically within sub-basins, where the thickness of the section is sufficient. This has important implications for regional seismic interpretations and means that seismic data alone cannot provide sufficient level of detail for regional mapping. Integration with data that have a finer resolution, such as well data, is therefore essential. For these reasons, sequence stratigraphy evaluations of the Jurassic of the North Sea need to be based primarily on well interpretations and correlations. The ensuing well-based dataset can then be used to calibrate the seismic data, where seismic resolution is sufficiently clear. Seventeen seismic lines illustrating sequences and well ties are provided in the memoir, including a set of figures in Chapter 6, together with other figures included in the relevant sequence description in Chapters 4 and 5, plus selected seismic lines in regard to hydrocarbon fields discussed in Chapter 9, while other lines are discussed in regard to the definition of stratigraphic traps in Chapter 10.

The sequence stratigraphy scheme described in this publication recognizes 39 third-order stratigraphic sequences (see below for further discussions). The Hettangian–Aalenian, Aalenian–Callovian and Callovian–Berriasian parts of the scheme applied to this study are shown in Figures 13.1 and 13.2. These diagrams show the correlation of the bounding, maximum flooding surfaces of the sequences (plus intra-sequence surfaces such as depositional sequence boundaries and transgressive surfaces) and their correspondence with the biozones, chronostratigraphic ages and geochronological ages (against the timescale of Gradstein et al. 2020). Zonal microfossil index taxa are summarized in these figures (providing refinements and updates of versions of charts published in Partington et al. 1993a), and the biozones that calibrate the sequences are defined in Chapter 13.

It is possible to suggest ammonite zone calibrations for many of the maximum flooding surfaces that define the base of each genetic stratigraphic sequence. The ammonite zone calibrations are given in the sequence descriptions, as far as is known, on the basis of inferred correlation of the offshore biostratigraphic markers (described in Chapter 13) with the ammonite zones. In several instances, the suggested correlations of genetic stratigraphic sequence boundaries with the ammonite zones have been modified compared to the original definitions in Partington et al. (1993a). In addition, some changes have been made to the standard ammonite zonation schemes: for example, in the Lower⁵ Jurassic, the Levesquei Zone is no longer used and the Falciferum Zone has been replaced by the Serpentinum Zone. These two levels contain two defined bases of genetic stratigraphic sequences, namely the base of the J17 sequence and the base of the J18 sequence.

Some workers (e.g. Partington et al. 1993a, b; Underhill and Partington 1993) have used the inferred ammonite zone name as an epithet for the maximum flooding surface (e.g. the ‘Baylei MFS’). However, as some of these calibrations to ammonite zones are uncertain (and some of the ammonite zones themselves have changed names, see above), the current authors discontinue to reference the maximum flooding surfaces by ammonite zone epithets. Other workers have named key surfaces after geochronometric dates: for instance, Fjellanger et al. (1996) named flooding surfaces and sequence boundaries in the Middle Jurassic Brent Group as follows ‘SB 169 Ma’, ‘MFS 167 Ma’ etc. Geochronological timescales are frequently revised (e.g. Gradstein et al. 2004, 2012, 2020; Ogg et al. 2016; Hesselbo et al. 2020), however, and such geochronologically based names will inevitably, therefore, become out of date. Jeremiah and Nicholson (1999) named flooding surfaces in the Upper Jurassic of the UK Central North Sea after associated dinocyst markers (e.g. Pannosum MFS); however, microfossil names also change occasionally due to taxonomic revisions. For these reasons, it is considered preferable to refer to sequence surfaces simply by their sequence name, such as ‘the base J62 maximum flooding surface’.

When the original J sequences were published, they were given even numbers (e.g. J12, J14, J16) to allow the insertion of additional sequences (using odd numbers: e.g. J13, J17) that were expected to be recognized in the future by further work. Nevertheless, some additional sequences were numbered by the original authors (Partington et al. 1993a), such as J16a and J16b, J54a, J54b, J66a and J66b. To redress this, such sequences are renamed in the current publication: for example, J16b becomes J17, J54a becomes J54 and J54b becomes J55.

The succession of documented J sequences differs from that which was previously published in Partington et al. (1993a) in several respects (see points 1–10 below). These differences represent revisions introduced in the light of further study, including renaming (points 2, 4, 5 and 7), refinements of calibration (points 3, 8, 11 and 12), approach (points 8 and 11) or recognition of additional sequences (points 1, 9 and 10). In the cases of points (8) and (13), we have chosen to place boundaries at two depositional sequence boundaries, which are very prominent features both regionally and locally in wells and on seismic sections. Although these two instances represent departures from the accepted pattern of picking sequence-bounding surfaces at maximum flooding surfaces, they reflect the pragmatic approach adopted in this study, which is to identify and map the most prominent and readily identifiable features that correspond to proven unconformities:

1. The former J02 sequence is subdivided into the J1 and J2 sequences, and the former J04 sequence is subdivided into the J3 and a revised J4 sequence.

2. The former J12 sequence is subdivided into the J12 and J13 sequences.

3. The J16 and J17 sequences correspond to the former J16a and J16b sequences.

4. The base of the J52 sequence is placed at the base of the Middle Oxfordian (base of the Densiplicatum Zone) rather than within the Middle Oxfordian (intra-Densiplicatum Zone).

5. The base of the original J54a sequence is placed at the base of the Upper Oxfordian (base of the Glosense Zone) rather than within the Glosense Zone. This sequence is renamed the J54 sequence.

6. The J55 sequence replaces the former J54b sequence.

7. J64 is subdivided into the J64a and J64b sequences. J64 was not previously subdivided.

8. Previously, J66a and J66b sequences were recognized. In this publication, the former is referred to as J65 and the latter as J66, to reflect the demonstrable regional extent of the J65 sequence.

9. The base of J73 is placed at a higher position than previously, at a major regional unconformity. The revised base J72b corresponds to the base of J73 of former usage. The base J72b MFS is a subtle feature that is often difficult to identify. The base J73 as picked in this study is a very prominent feature over the whole region.

10. An additional MFS is identified, defining the base of J72c.

11. An additional MFS is identified, defining the base of J73b.

12. The base of the J74 sequence is placed in the Lamplughi Zone (Early Berriasian), one ammonite zone higher than it was placed previously.

13. The top of J76–base of K10 is placed at the base of the Cromer Knoll Group. This corresponds to the prominent ‘Base Cretaceous’ or ‘Near Base Cretaceous’ seismic horizon that is identified consistently throughout the North Sea Basin. The present authors thus use this horizon as the sequence-bounding surface, rather than the potential minor maximum flooding surface beneath (top of J76 of former usage), which is much more subtle and not demonstrably flooding in nature.

The sequence scheme described in this memoir may be used as the basis for an integrated approach to studies of reservoirs, source rocks and seals from well data. Each of these factors is influenced by the significant regional palaeoenvironmental changes that are expressed in the rocks as stratigraphic sequences. These same changes are those that are also mapped from seismic data by explorationists. The understanding and predictability of intra-reservoir layering and intra-formational seals will also result from the improved sequence stratigraphic insight arising from the new scheme and information.

Thus, the well-based sequence stratigraphic methodology provides a powerful unifying stratigraphic medium of communication and utility in the petroleum and related subsurface industries. It has been applied at all scales ranging from basin-wide to more focused appraisal and development studies. While the scheme described in the paper is primarily of application to the NW European offshore area, the methodology of sequence recognition and utility has general applicability in this and other basins of different stratigraphic age and tectonic settings.

The authors have benefitted from having worked on many licence blocks, wells and seismic datasets over 40 years in the North Sea Basin, across multiple projects. These have ranged from large, in-house projects within major oil companies (Britoil, BP and Shell) to regional speculative projects (Integrated Exploration and Development Services (IEDS), two studies) to regional studies and smaller-scale, more block-specific studies for consulting clients (Merlin Energy Resources). This has provided access to many wells and seismic datasets over this period. For the current memoir, a selection of key 85 reference wells (56 UK wells, 22 Norway wells and seven Denmark wells) is displayed in the memoir, and 17 seismic lines have been chosen in order to demonstrate the validity of the defined J sequences and to demonstrate their application across the region. Only if a regional extent can be demonstrated are the J sequences considered valid. There are instances in which additional sequences may be recognized in some areas (e.g. J61), however, these are not demonstrable on a true regional scale and are not formally included in the J sequence scheme.

For this memoir, the authors were provided with access to a large UK offshore well dataset by TGS, including digital wireline logs and reports, that was utilized in the preparation of this publication. Many of the digital wireline logs from UK wells displayed in this memoir were provided by TGS. In 2019, the UK Oil and Gas Authority (now the North Sea Transition Authority) launched the online National Data Repository (NDR) (https://ndr.ogauthority.co.uk), including free access to digital well logs, reports and other data from more than 12 000 released wells. Previously, these data had only been available for purchase via release agents (TGS, IHS Markit and CGG). As a result of this, additional information from the NDR has been accessed for this project to supplement the data provided by TGS. In addition to well data, the NDR also provides free access to pre- and post-stack data from disclosed (released) proprietary seismic surveys and field seismic surveys.

For Norway, however, free data are generally not available (although a small number of reports and logs are available for no cost on the Norwegian Petroleum Directorate (NPD) website), and this study has relied upon MOL Norge and Source Energy to provide access to selected Norway well and seismic data. In addition, the Geological Survey of Denmark and Greenland donated well data from offshore Denmark from selected wells and boreholes to this study free of charge.

Further to the above, some sequence data are published in many papers that have been issued since the original J sequences were defined, and some of this information has been incorporated into the well interpretations that are displayed in this memoir.

While this is primarily a sequence stratigraphic publication, lithostratigraphy provides an important language of geological communication using which rock units are named and recognized. In addition, the relationship between the sequences and known lithostratigraphic units requires description and documentation. Until the 1990s, most of the stratigraphic schemes employed in the Jurassic of the North Sea had been lithostratigraphically based. Utilized lithostratigraphic nomenclature has been largely either in-house, company-based (including multiple local schemes for particular fields or areas) or country-specific lithostratigraphic schemes such as those of Michelsen (1978), Jensen et al. (1986), Michelsen et al. (2003) (Danish sector), Vollset and Doré (1984) (Norwegian sector), Nederlandse Aardolie Maatschappij BV and Rijks Geologische Dienst (1980), Munsterman et al. (2012) (Dutch sector), and Rhys (1974), Deegan and Scull (1977), Richards et al. (1993) and Lott and Knox (1994) (UK sector). The scheme of Richards et al. (1993) was developed at a time when sequence stratigraphic ideas were at the forefront of exploration thinking in the UK sector; however, the scheme did not take onboard any of the refinements that were possible in terms of lithostratigraphic subdivision, had sequence stratigraphy been applied. The result of this is that many named lithostratigraphic units in the Richards et al. (1993) scheme are very broad and span multiple J sequences. It is only by the application of sequence stratigraphy that the true complexity of the stratigraphic succession and the controls on its development can be understood.

The result of the development of different lithostratigraphic schemes for separate sectors of the North Sea Basin has been a proliferation of different terms, and a lack of a clear and unifying, true basin-wide stratigraphic framework. This has led to considerable confusion, poor communication across national boundaries, and a lack of appreciation of the distribution and controls on stratigraphic relationships for numerous important Jurassic siliciclastic reservoirs. One of the major hindrances to the achievement of a regional understanding of the Jurassic is the tendency for each sector of the North Sea to define a different set of lithostratigraphic names, for what often clearly represents the same, contiguous stratigraphic units, with name changes occurring at offshore median lines. For example, the Kimmeridge Clay Formation of the UK offshore area clearly continues into offshore Norway as the Draupne Formation (Viking Graben), the Farsund and Mandal formations (Central North Sea), and into offshore Denmark as the Farsund Formation. While there are evidently some regional lithostratigraphic variations that do occur, nevertheless, many of the lithological similarities are undoubtedly obscured by the name changes that have been imposed at median lines. The application of sequence stratigraphy and the recognition of a consistent set of sequences across these different depositional areas allow a much improved understanding of the development of the whole region during the Jurassic.

Lithostratigraphic definitions, type wells and biostratigraphic calibrations for the main defined Jurassic lithostratigraphic units are described in Chapter 11. This chapter also discusses some lithostratigraphic problems and provides clarifications regarding the definition of particular units. In addition, some new lithostratigraphic terms are introduced in this section.

A chronostratigraphic unit is a body of rock that serves as a material reference for all constituent rocks formed during the same span of time, and includes terms such as system, series, stage and substage (North American Commission on Stratigraphic Nomenclature 2005). Formal subdivisions of such units are designated as, for example, Lower, Middle and Upper (e.g. Upper Jurassic). Geochronological (or geochronologic) units are divisions of time that correspond to the time span of an established chronostratigraphic units (North American Commission on Stratigraphic Nomenclature 2005), and include terms such as period, epoch and age. Formal subdivisions of such units are designated as, for example, Early, Middle and Late (e.g. Late Jurassic). As advised by Haile (1987), and followed here, chronostratigraphic units (e.g. Lower/Upper) are used for attribution of age to rocks, lithostratigraphic units, biozones, unconformities and seismic reflectors. Time (geochronological) units (e.g. Early/Late) should be used for describing the time of occurrence of geological events, such as periods of erosion, transgression and tectonism. In the present memoir, chronostratigraphic unit and geochronological unit terminology has been used, sometimes in the same narrative, where it is useful to distinguish bodies of rock from the time intervals that they represent. Some of the figures in the memoir show geochronological time units or geochronometric units (absolute time units) in columns alongside rock-based units, such as lithostratigraphic units; this is considered an acceptable practice that is commonly used in geological publications, for instance in Gradstein et al. (2020).

The geochronological timescale and chronostratigraphic framework (standard ammonite zones⁶, substages and stages) applied in the present publication essentially follow Hesselbo et al. (2020). For certain intervals, the following modifications are utilized:

• In the Lower Jurassic, the ammonite zones of Page (2003) are followed. This scheme uses the standard European Margaritatus and Spinatum zones rather than the Lavinianum, Algovianum and Emaciatum zones.

• For the Middle Jurassic, the chronostratigraphy scheme follows the European Subboreal primary standard scheme as described by Callomon (2003). Five ammonite zones, including the Scissum Zone, are used in the Aalenian, compared to four in Hesselbo et al. (2020). Contrary to the latter publication, no substages are recognized in the Aalenian.

• No substages are recognized in the Kimmeridgian.

• The base of the Tithonian is placed slightly higher than in Hesselbo et al. (2020), at the base of the Elegans Zone.

• In the Tithonian, the Oppressus ammonite zone, as utilized in the chronostratigraphic schemes of previous authors, is discontinued, based on the considerations outlined below.

Ever since it was first erected by Casey (1973), the Oppressus Zone has been used consistently by British stratigraphers, including ammonite workers (e.g. Wimbledon and Cope 1978; Wimbledon in Cope et al. 1980a). Its usage has therefore been followed by other stratigraphers working in the North Sea and the UK area, and has been included in all published biozonation schemes pertaining to this area (e.g. Rawson and Riley 1982; Riding and Thomas 1992; Partington et al. 1993a; Duxbury et al. 1999; Poulsen and Riding 2003) and in many unpublished industry biozonation schemes also. However, Wimbledon (1984, p. 541) has questioned the validity of the species Paracrespedites oppressus Casey, and therefore of the Oppressus Zone, based on an examination of Casey's original type material, stating that:

[T]he …‘oppressus Zone’ fauna (Casey 1973) is not here regarded as any more than an assortment of unassigned and perhaps unassignable inner whorls of larger Titanites … The ‘oppressus Zone’, and the fauna it is supposed to contain in Dorset and in eastern England is fraught with such confusion that its use is not continued herein.

All the ‘Oppressus Zone’ ammonite specimens from the Sandringham Sands Roxham Beds (Casey 1973) in the British Geological Survey collections have been studied by W.A. Wimbledon, who states that ‘they are mostly small, poorly preserved pavloviids (frequently with complete pyrite overgrowth) and often unidentifiable. None indicates an age younger than the Kerberus Zone’ (W.A. Wimbledon pers. comm. 2022), above which, in Norfolk and Lincolnshire, there is a significant break in the succession. Wimbledon (1984) pointed out that the reported fauna of the ‘Oppressus Zone’ in eastern England is largely derived from glacial erratics of basal Spilsby–Sandringham beds; nevertheless, the ammonite taxa recorded appear to comprise a consistent assemblage that probably represents the faunas of two distinct ages within the Okusensis or Kerberus zones.

For these reasons the Oppressus Zone is no longer used in publications on Upper Jurassic ammonite zones (e.g. Wimbledon 2008; Harding et al. 2011; Wimbledon et al. 2011), including the current work, despite its continued inclusion in compilations such as the ‘standard’ geological timescales (see Gradstein et al. 2012; Ogg et al. 2016; Hesselbo et al. 2020).

Cope (2020) noted that the Anguiformis Zone appears unknown outside of south Dorset (apart from a possible occurrence in the Vale of Wardour in Wiltshire), and over the South Midlands rocks of this age are in Purbeck Group facies (Wimbledon 1980). As the zonal index Titanites anguiformis is absent from eastern England, Cope (2020) considered it possible that the Primitivus Zone of eastern England is the correlative of the Anguiformis Zone of southern England. This belies the fact that there is a succession of palynology bioevents through the Anguiformis, ‘Oppressus’ and Primitivus zones in England that is replicated in offshore well successions, and which is used herein to define the PJ43–PJ46 dinocyst biozones (see Chapter 13). For the moment, therefore, the Anguiformis and Primitivus zones are retained in this memoir, although the Oppressus Zone is discontinued.

Biostratigraphy provides an essential means of characterization and correlation of North Sea Jurassic sequences. The discipline also provides the primary basis of the chronostratigraphic framework upon which the whole sequence stratigraphic, lithostratigraphic and seismic stratigraphic edifice, and therefore the whole understanding of the basin and its evolution through time, is based.

The understanding of the stratigraphy of the North Sea Jurassic is based fundamentally on well sections and their correlation both to other wells and into undrilled areas on the basis of seismic interpretations. For the Jurassic, the standard biozonal fossil group is the ammonites, based upon which a standard scheme of zones and subzones has been defined (see Cope et al. 1980a, b; Gradstein et al. 2004, 2012; Ogg et al. 2016; Hesselbo et al. 2020); however, in the North Sea, ammonites are only rarely recorded from well sections, where, due to the nature of drilling and the destruction of larger fossils by the drilling process, ammonites can only be recovered from cores.

Core samples are relatively rare, although in them ammonites may occasionally be found; however, these occurrences are too sporadic to provide a reliable basis for the stratigraphic subdivision of well successions. The standard well sample types, ditch cuttings and sidewall cores are too small to allow whole ammonites to be recovered, and therefore the biozonal framework for the North Sea is based on microfossils that are small enough to be preserved whole in these types of samples. Indeed, microfossils may achieve high abundance in cuttings, sidewall cores and core samples, enabling, in general, a good degree of chronostratigraphic resolution to be achieved through the Jurassic.

Correlation of the offshore biostratigraphic succession with the standard zones does rely on the ties to the ammonite succession, either in the occasional ammonite recovered from well cores or, more usually, comparison with the known relationships between the key microfossil markers seen in offshore sections to the same markers in onshore sections, calibrated to the ammonite zones and subzones. For the North Sea area, such correlations are based on the recorded microfossil distributions, tied to ammonite biozones in Jurassic outcrops and boreholes from onshore Britain and, to a lesser extent, onshore northern France and Greenland, these being the only two areas of Jurassic outcrop adjacent to the North Sea Basin. Those instances in which ammonites have been recorded from North Sea cored sections are referred to in the sequence descriptions in the main part of this memoir (Chapters 3–5). These include ammonite records from the Outer Moray Firth (Boldy and Brealey 1990; Hesketh and Underhill 2002), and the Northern and Central North Sea (Callomon 1975, 1979, 2003; Morton et al. 2020). In addition, there are important ammonite records in cores described in released well completion reports, including those from UK Central North Sea wells 22/11-4, 21/2-5 (see discussions of the J54 and J55 sequences in Chapter 5) and 29/10-2 (see also the discussion in Jeremiah and Nicholson 1999). Biostratigraphic calibration of the J74 and J76 sequences in offshore Denmark is provided by ammonite records from the E-1 well documented by Birkelund et al. (1983) and Jensen et al. (1986). Charnock et al. (2001) reported an ammonite record that aids the calibration of the J16 sequence in the Dunlin Group of the Norwegian North Viking Graben.

While ammonites are the main fossil group upon which the standard chronostratigraphy is based in the Jurassic–earliest Cretaceous interval, this group is not without its problems as witnessed by the problematic use of the ‘Oppressus Zone’ as noted above, and the major provincialism of ammonite zonations around the Jurassic/Cretaceous boundary across the Boreal and Tethyan regions (Hunt 2004). There are also issues related to different workers referring the same ammonite specimens to different species, as is apparent from the works of Boldy and Brealey (1990) compared to Hesketh and Underhill (2002).

It is possible to tie the offshore microfossil-based successions to ammonite-dated outcrops in the UK and other European countries for most of the Jurassic interval, and these correlations are discussed in detail in Chapter 13. Particularly problematic is the Upper Tithonian–Lower Berriasian interval and how the offshore successions correlate with onshore UK sections over this section. It does seem that many of the key North Sea dinocyst species are either absent from the southern England sections or have different stratigraphic ranges over this interval. In addition, some North Sea dinocyst markers have previously been tied to ammonite records from eastern England, in the Casey collection, of which some of the ammonite identifications are now considered to be unreliable (see the discussion of the Oppressus Zone above). For these reasons, a reassessment of the onshore dinocyst–ammonite records, together with resampling of key onshore sections, is required in order to allow a re-evaluation of the offshore to onshore correlations. Until this work is completed, it should be realized that the ammonite zone–dinocyst calibrations set out in the current publication, particularly over the Upper Tithonian–Lower Berriasian interval, may be subject to future revision.

Zonal microfossil index taxa are summarized in Figures 13.1, 13.2 and 13.3 (providing refinement and updates of versions of charts published in Partington et al. 1993a), and the biozones that calibrate the sequences are defined in Chapter 13. The biozonation scheme utilized and described in the present publication is an updated version of that of Partington et al. (1993a), which was, at the time of publication, essentially developed in BP by the authors in collaboration with many former colleagues (see the acknowledgements section in Partington et al. 1993a). This biozonation scheme is based on a set of palynology (PJ) and microfaunal (MJ) biozones. Further work by the authors has modified the scheme, in addition to which significant additional biostratigraphic studies by other researchers have since been carried out, key results from which are incorporated into the present work. Furthermore, many more wells have been drilled, substantially improving the available biostratigraphic database. The PJ and MJ biozones were not formally defined by Partington et al. (1993a) but were illustrated on large charts correlated with the chronostratigraphic scheme in use at that time, and tied to the J sequences and their boundaries. These biozones (and the defining marker species) are defined in more detail in the current publication (see Chapter 13), and their relationships to the J sequences are discussed in the individual J sequence descriptions (Chapters 3–5) and also cited in relation to lithostratigraphic units (Chapter 11). Full reference is made in Chapter 13 to North Sea Jurassic biozonation schemes previously defined by other authors, including Davey (1979), Riley and Fenton (1982), Riley et al. (1989) and Duxbury et al. (1999), for the UK offshore area, Dybkjær (1991) for offshore Denmark, and Herngreen et al. (1988, 2000) for offshore Netherlands.

The microfossil group in the North Sea Jurassic offering the greatest degree of biostratigraphical resolution is dinoflagellate cysts (dinocysts). They have been documented in many publications from both ammonite-zone-calibrated onshore outcrops (e.g. Woollam and Riding 1983) and onshore boreholes, as well as offshore well sections (see below for references to key works). Dinocysts provide the highest degree of stratigraphic resolution in the Jurassic as a whole; however, being of organic composition, they do become degraded (oxidized) by high thermal maturity, hampering their utility in deeply buried graben areas of thick Upper Jurassic sedimentation. Price et al. (1993) found that the degree of degradation of the palynological content of the Jurassic in the Central Graben area varied with variations in heat flow related to salt domes (in which local heat flow is enhanced); as a result, degradation of the organic-walled microfossils is seen at depths ranging between around 12 000 ft (3650 m) and 14 000 ft (4270 m) in the Central North Sea. In such deeply buried areas, other types of microfossils, such as foraminifera, ostracoda and radiolaria, provide additional biostratigraphic control, albeit in some intervals offering lesser degrees of resolution than offered by palynology where non-degraded by thermal effects related to burial depth.

Several important biostratigraphic studies, published since 1993, are worthy of note and their results have been incorporated into the current work (see Chapter 13 for a further discussion in relation to defined biozones and key biostratigraphic markers providing chronostratigraphic calibration for the sequences). A review of key publications documenting North Sea biozonation schemes is also included in Chapter 13.

As pointed out by Turner et al. (2018), however, while biostratigraphic (specifically, palynostratigraphic) studies have been carried out on many well sections in the North Sea over 40 years, there remains a mismatch between the interpreted stratigraphic positions and ties to the standard chronostratigraphy between different biostratigraphic contractors and authors. This may impart uncertainty as to which sequence should be designated from the palynological analysis for a particular well section, and also casts doubt on reservoir correlations based on interpretations of age in different wells by differing authors, which can lead to uncertainties in mapping the extents of reservoirs (Turner et al. 2018). It is important, therefore, to cite the depth and name of the marker taxa rather than the interpreted ages. The current publication attempts to define the age interpretations of the biozones and sequence boundaries more definitively than previously, and this hopefully will help to overcome the shortcomings of differing interpretations of biozonation schemes and ties to sequence development and the standard chronostratigraphic timescale. However, it is accepted that biostratigraphic datasets from wells, whilst being of vital importance for subsurface evaluations, are always imperfect and, by nature, variable from well to well and from interpreter to interpreter (whether contractor or oil company personnel). Some of the reasons for this include variable sample quality, variable sample processing techniques, variable levels of biostratigraphic expertise, variable levels of detailed study undertaken (e.g. qualitative v. quantitative analysis), vintage of data (the most recent data are generally the most detailed) and reporting detail (reports vary greatly in level of reported detail, some include distribution charts for instance). One of the most critical, yet often overlooked, aspects affecting data quality is sample spacing.

In the biostratigraphic discussions, the term first downhole occurrence encountered while drilling (i.e. uphole disappearance) is abbreviated to FDO and last downhole occurrence (appearance or inception) to LDO.

The Jurassic System is unusual in that its upper and lower boundaries are the last system boundaries to be defined formally by reference to a selected Global Stratotype Section and Point (GSSP). Since the North Sea J sequences were initially published (Mitchener et al. 1992; Partington et al. 1993a, b; Rattey and Hayward 1993), the basal boundary of the Jurassic has been defined and considerable progress has also been made regarding a definition of the upper boundary of the system. In both cases, these upper and lower boundaries are somewhat different in stratigraphic position to how they were understood in the early 1990s, and therefore merit summarizing here.

Formal definition of the Triassic/Jurassic boundary at a Global Stratotype Section and Point (GSSP) for the base of the Jurassic System proved to be difficult and protracted. A Working Group was established by the International Subcommission on Jurassic Stratigraphy (ISJS) in 1984 to investigate and document possible candidate sites and eventually select one to be proposed as GSSP. However, it was not until 2010 that the Kuhjoch section in Austria was ratified as the GSSP by the International Union of Geological Sciences (IUGS) (Morton 2012). The acceptance of Psiloceras spelae as the oldest Jurassic ammonite species below P. planorbis (which had been previously used to define the base of the Jurassic in Britain: Warrington et al. 1980; Page 2003) allows the introduction of a new ammonite zone, the Tilmanni Zone, as the basal zone of the Jurassic (Hillebrandt and Krystyn 2009).

In British sections, it is not possible to recognize the base of the Jurassic according to the new ammonite biozone/zone definition, due to the absence of the ammonite Psiloceras spelae and related forms from this country. P. spelae has, however, been recorded from near the base of the Fjerritslev Formation in the Rødby-1 borehole, onshore Denmark (Lindström et al. 2016). Placement of the base of the Jurassic can now only be recognized in the UK by the use of a proxy for the boundary. In the boundary stratotype section, at Kuhjoch (Austria), and other sections in Europe and North America, two organic carbon isotope excursions (CIEs) are developed, known as the short ‘initial’ negative CIE of 5‰, which is separated from the succeeding and longer ‘main’ negative CIE by a 3.5‰ positive excursion (Ruhl et al. 2009). At Kuhjoch, and other nearby sections in the Northern Calcareous Alps (Eiberg Basin), the incoming (evolutionary appearance) of P. spelae, which defines the base of the Jurassic, occurs in the positive excursion interval between the two negative excursions (Ruhl et al. 2009, fig. 5). In Britain, from which P. spelae is absent, it is possible to infer a level for the placement of the boundary by correlation of the carbon isotope curves, as shown by Clémence et al. (2010, fig. 2), approximating with the Lias Group (Pre-Planorbis Beds)/Penarth Group (White Lias Formation, Langport Member) boundary. Notably, this level agrees with the placement of the base of the Jurassic in Britain favoured many years ago by George et al. (1969) at the base of the basal Lias Group paper shale.

This level in onshore Britain is associated with the well-documented marine transgression that follows a worldwide sea-level fall in the latest Triassic (Rhaetian). This transgressive event is recognized in this publication as the base J00 (J1) MFS; however, due to a combination of incomplete stratigraphy and inappropriate facies, this is rarely recognizable in the Lower Jurassic successions preserved in the North Sea area (see Chapter 3 for a detailed description and occurrences in the region) but is well exposed in onshore sections, for example, in SW Britain (Figs 3.7 & 3.8).

Global correlation and definition of the Jurassic/Cretaceous (J/K) boundary interval has always proved problematical due to intense and widespread faunal provincialism throughout the northern hemisphere. The profound endemism of the latest Jurassic–earliest Cretaceous ammonite faunas has led to a complex stage terminology for the terminal Jurassic interval; thus, the stages Portlandian, Purbeckian, Tithonian and Volgian have been variously applied from England to France, Poland, Greenland, the Russian Platform/Siberia and the Submediterranean/Tethyan Province, each area having different ammonite zonations reflecting the faunal provincialism. To simplify the terminology of the terminal Jurassic, and after much debate at various Jurassic symposia, in 1990, the International Commission on Stratigraphy formally ratified the Tithonian as the terminal Jurassic stage, with the Berriasian being the basal Cretaceous stage (Cope 2008; Harding et al. 2011; Wimbledon et al. 2011).

In areas where the Volgian was historically used (i.e. Russia and Poland), it had been customary to combine its use with the succeeding Ryazanian Stage. However, it is now recognized that the base of the Ryazanian correlates with a level high up in the Primary Standard Berriasian Stage and therefore that the Upper Volgian is of earliest Cretaceous age (Cope 2013) (Fig. 1.6). By definition, a Stage cannot belong to two Systems, and the Volgian was therefore removed from the geological timescale according to the guidelines of the Interdepartmental Stratigraphic Committee of the Russian Federation (ISC RF) (Zhamoida and Prozorovskaya 1997; Zakharov and Rogov 2008; Cope 2013). Nevertheless, the use of the Volgian (and Ryazanian) continues in areas where it has traditionally been used: for example, northern Eurasia and the Arctic shelves (Zakharov et al. 2014). As a consequence, there is a view promulgated by Russian geologists (e.g. Zakharov and Rogov 2008) that this continued usage of the Volgian Stage over a wide area may provide the basis for an appeal to the International Commission on Stratigraphy for the restoration of the stage, parallel to the Tithonian, in the international timescale.

For many years the terminal stage of the Jurassic in the North Sea hydrocarbon province has been regarded by most workers and operating companies as the Volgian (although, notably, Shell has always used the Portlandian as the terminal stage in the North Sea area) (Fig. 1.6). The use of the Volgian Stage in the North Sea essentially stems from work by Casey (1971, 1973), who identified ammonites of the Upper Volgian in East Anglia. Casey (1971) continued to use Portlandian for southern England, but by 1973 was aware that the base of the Berriasian was older than the base of the Ryazanian and wrote (Casey 1973, p. 229):

Should the … postulated overlap of that Stage [Berriasian] and the Volgian be confirmed, the name of the terminal Jurassic Stage in the Boreal Realm would need further consideration.

Casey's doubts on the suitability of both Volgian and Ryazanian continued, for he said (Casey 1973, p. 228), ‘Their [Ryazanian] condensed and transgressive characters render the Ryazan Beds even less suitable as a stratigraphical standard than the Volgian of the same region’. Despite these comments, Riley (1977) proposed that in the North Sea Basin the stages Kimmeridgian, Volgian and Ryazanian should be used, and usage of Portlandian and Berriasian be abandoned. The result was that stratigraphers and commercial biostratigraphic contractors working on the North Sea (e.g. Robertson Research 1978; Rawson and Riley 1982) began to use Volgian as the preferred uppermost Jurassic stage, a practice that has continued to the present day, with most companies and contractors using the Volgian as the terminal Jurassic stage and the Ryazanian as the initial Cretaceous stage (for a rare exception see Turner et al. 2018, in which Tithonian is applied in the Brae area of the South Viking Graben). The companies Shell and BP did, however, formerly use the terms Kimmeridgian, Portlandian and Berriasian in the North Sea, as can be seen on the many composite logs from wells drilled by these companies. BP, however, when it published the Upper Jurassic–Lower Cretaceous J sequence stratigraphy in 1993 (Partington et al. 1993a, b; Rattey and Hayward 1993), adopted the usage of Volgian and Ryazanian within the company. Use of Volgian and Ryazanian, rather than Tithonian and Berriasian, has also been the standard practice in East Greenland by many workers (e.g. Surlyk 2003; Alsen and Piasecki 2018).

For the North Sea, to reflect the formal international usage and to avoid the confusion of usage of different stage nomenclature, however, the Tithonian is used as the terminal Jurassic stage herein, largely equivalent to the Lower and Middle Volgian of previous usage, sitting above a short Kimmeridgian (sensu Gallico). Similarly, the Berriasian is recognized as the initial Cretaceous stage. However, it is appreciated that correlation from Tithonian sections in the Tethyan type area into the UK area is difficult. For example, the placement of the base of the Tithonian in UK sections is uncertain due to the lack of characteristic lowermost Tithonian ammonite species in the UK sections such as those in Dorset. In the most recent geological timescale (Hesselbo et al. 2020), the base of the Tithonian is placed within the Autissiodorensis Zone, at the level of the incoming (LDO) of the ammonite genus Gravesia. However, claimed identifications of Gravesia taxa within the Autissiodorensis Zone in southern England, which would suggest placement of the base of the stage at that level, are not considered to be reliable (J.C.W. Cope pers. comm. 2020). An alternative placement for the base of the Tithonian/top of the Kimmeridgian is the extinction (FDO) of the ammonite genus Aulacostephanus, defining the top of the Autissiodorensis Zone (Cox and Gallois 1981; Gallois and Etches 2010).

In terms of the affinity of the North Sea faunas, Cope (2008) stated that the ammonite faunas of the North Sea are primarily of Kimmeridgian and Portlandian affinity, and the only Volgian ammonite faunas in the area are those of Late Volgian character (which are now considered to be of Early Cretaceous, Berriasian age, see below). The same is not true of the microfaunas, however, and boreal type radiolarian assemblages are extensive as far south as the UK Central North Sea and offshore Denmark, which are in some respects comparable with those recovered from the type Volgian.

While the Tithonian has now been adopted as the Primary Standard terminal Jurassic stage, ammonite faunas in the Tithonian successions of southern Europe undeniably differ fundamentally from those of NW Europe, including onshore Britain, thus the question arises regarding how to correlate between the two regions. Cope (2013) stated that ‘there is not one Tithonian Standard Ammonite Zone (zone) recognisable in Britain, but there are eight Bolonian and five Portlandian ammonite Standard Zones currently recognised’. On this basis, Cope (2013) advocated the use of Bolonian and Portlandian as Secondary Standard Stages to the Tithonian in Britain. In this country, the Kimmeridgian has always been used in its extended sense (sensu Anglico) to equate with all strata between the Oxfordian and the base of the Portlandian; while in France, the Kimmeridgian has been used differently, to equate to a shorter interval (sensu Gallico), with both British and French workers claiming to be following d'Orbigny (1842–51), who originally named the Portlandian and Kimmeridgian stages. As pointed out by Cope (2013), because the Tithonian Standard Stage rests on the shorter Kimmeridgian (sensu Gallico), the latter stage can only be used in its shorter meaning. So, if using the secondary stage name Portlandian in southern Britain, an additional name is required for the interval that equates to the upper part of the former longer Kimmeridgian (sensu Anglico). Blake (1881) had originally proposed the term ‘Bolonian’ for this interval, and since the formalization of Tithonian as the terminal Jurassic stage, Cope (1993, 2013) has argued for the use of Bolonian in Britain when the Portlandian is also used, although the use of the Bolonian stage name has generally not found favour with other workers. Use of Tithonian for British successions, however, removes the need to use the Bolonian as a local stage name. In the North Sea Basin, there is no single instance known to the authors of any company, consultant or researcher using the Bolonian stage name, and accordingly this name is not used in this memoir. Furthermore, in the Geological Time Scale 2020 publication (Hesselbo et al. 2020), the secondary standard terms Bolonian and Portlandian are not utilized, and neither is their usage supported by the International Commission on Stratigraphy.

Since 2007, the Berriasian Working Group of the International Subcommission on Cretaceous Stratigraphy (ISCS) has been working towards the definition of a boundary stratotype section for the base of the Berriasian Stage (and therefore of the base of the Cretaceous System) (see Wimbledon et al. 2011; Schnabl et al. 2015). As a result of this work, a recommended basal stratotype section was chosen, and the proposed criteria (biostratigraphic and magnetostratigraphic isotopic) for the Jurassic/Cretaceous boundary definition described by the working group (Wimbledon et al. 2020a, b). The proposal of the working group was that the key biostratigraphic definition for the base of the Berriasian is based on calpionellids, with the base of the Calpionella alpina Subzone proposed as the primary boundary marker. The proposal (Wimbledon et al. 2020a, b) also documented other biostratigraphic proxies for the definition of the boundary. The base of the C. alpina Subzone lies above the base of the Berriasian as previously defined: that is, within rather than at the base of the Jacobi Zone that was previously considered to be the basal zone of the stage (Schnabl et al. 2015, fig. 1). This level is within the Upper Volgian as originally defined and recognized in previous North Sea studies, including the present authors (Partington et al. 1993a) and many other workers. This position for the boundary has been reported by Hesselbo et al. (2020, although they drew the base of the Berriasian in the Lamplughi Zone rather than in the Preplicomphalus Zone as shown by Wimbledon et al. 2020a, fig. 5) in the most recent geological timescale and has been applied in the present memoir (Fig. 1.6). However, the proposal by Wimbledon et al. (2020a, b) for the definition of the base of the Berriasian Stage was not accepted by the International Commission on Stratigraphy and, consequently, the base of this stage, and that of the Cretaceous, remains undefined at the present time.

With the position of the base of the Berriasian as outlined above, there is uncertainty as to where the boundary should be placed in basins such as the North Sea where neither calpionellids nor Berriasian ammonites are known to occur. Calpionellids are distinctively Tethyan taxa and have not been recorded in the North Sea. This, allied to the general lack of ammonites from most well sections, requires that the correlations of subsurface sections with the standard chronostratigraphy over the Jurassic/Cretaceous boundary interval will rely on the recognition of proxies, comprising key palynology (dinocysts) marker species, supported by radiolaria and occasional foraminiferal taxa. What is clear, however, is that the Tithonian should now be used as the terminal Jurassic stage, and the Berriasian as the initial Cretaceous stage, in the North Sea Basin. The relationship between these and the key North Sea biostratigraphic marker taxa is shown in Figure 1.7. This applies to the whole of the UK, Norway and Denmark sectors of the North Sea Basin. In this area, the sediments around the boundary are of fully marine origin and are rich in microflora and microfauna, some of which are also found in onshore sections in which ammonite zones are defined; this correlation allows these zones to be inferred by correlation to the offshore area.

Note that in the above discussion, all cited North Sea authors have utilized the Boreal stages of Volgian and Ryazanian, which are still in common usage in the basin; these have been equated here with the Tithonian/Berriasian terminology and are shown in Figure 1.6 

The definition of the top of the Tithonian, using palynological and micropalaeontological criteria, is discussed in Chapter 5 of this memoir (see the discussion of the J73 sequence in this chapter). The base of the Berriasian appears to fall either within or at the base of the J73 sequence. This level is some way below the ‘Base Cretaceous’ seismic horizon (see Chapter 6 for further discussion of this seismic horizon).

The author is grateful to Merlin Energy Resources Ltd for provision of facilities, access to software, and for encouragement and permission to publish.

The author is grateful to Beagle Consulting for permission to reproduce images compiled from the Millennium Atlas GIS (Figs 1.1 and 1.2). Georgina Lockham, of Merlin Energy Resources Ltd, is thanked for generating location maps that are included in several of the figures in the memoir, including Figures 1.3, 1.4 and 1.5. and several of the location inset maps on other figures. Georgina also updated the maps in Figures 1.1 and 1.2 with revised field outlines accessed from the North Sea Transition Authority and the Norwegian Petroleum Directorate.

The author acknowledges the detailed reviews provided by John Gregory and a second anonymous reviewer on the manuscript. The final version was substantially improved as a result of their helpful and insightful comments.

The author declares that he has no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

PC: conceptualization (lead), formal analysis (lead), investigation (lead), methodology (lead), project administration (lead), resources (lead), software (lead), writing – original draft (lead), writing – review & editing (lead).

This research received no specific grant from any funding agency in the public, commercial, or not-for-profit sectors.

No datasets were generated in this introductory chapter.