Abstract The North Sea has reached an ultra-mature state as a petroleum basin, entering a phase of infrastructure-led exploration in an attempt to extend the economic lives of the main fields and reduce the rate of production decline. At the same time, the transition to a future low-carbon use of the basin is also in progress. As the papers in this volume demonstrate, in order to find, appraise and develop the mostly smaller near-field opportunities as well as making sure to grasp the opportunities of the near-future energy transition, a regional understanding of the North Sea is still critical. Even more so, a cross-border approach is essential because: (1) some of the plays currently being targeted have a clear cross-border element; (2) it allows a comparison of stratigraphic names throughout the entire basin; and (3) it enables explorers to learn lessons from one part of the rift to be applied somewhere else. This volume offers an up-to-date ‘geology-without-borders’ view of the stratigraphy, sedimentology, tectonics and oil-and-gas exploration trends of the entire North Sea Basin. The challenges associated with data continuity and nomenclature differences across median lines are discussed and mitigated. Examples of under-exploited cross-border plays and discoveries are discussed.

Abstract The Devonian–Recent tectono-stratigraphic history of the Northern, Central and Southern North Sea is here reviewed at a regional scale and four novel cross-border pseudo-Wheeler diagrams are presented to summarize the stratigraphic evolution of the cycles of basin fill and uplift/erosion. In this scheme, six first-order megasequence boundaries have been defined, characterized by extensive and long-lasting erosional hiatuses and major coastal regressions: (1) Caledonian (or Base Devonian) Unconformity; (2) Variscan–Saalian (or Base Permian) Unconformity; (3) Mid Cimmerian (or Intra-Aalenian) Unconformity; (4) Late Cimmerian (or Base Cretaceous) Unconformity; (5) Atlantean (or Near-Base Tertiary) Unconformity; and (6) Eridanos (or Mid-Miocene) Unconformity. These surfaces have been linked to regional causal factors ranging from orogenesis-related compressional uplifts, in either active plate margin settings (1) or foreland basin settings (2), to intra-plate dynamically supported uplifts associated with the development of mantle plumes (3, 5 and 6) and the end-of-rifting and associated widespread erosion of tilted fault block crests (4). The aforementioned megasequence boundaries punctuate the geodynamic evolution of the North Sea area and facilitate the subdivision of the entire the North Sea sedimentary basin fill into six megasequences, named here A–F. All of the lithostratigraphic units of the North Sea (formations and members) have been described within the context of this first-order tectono-stratigraphic framework. The correlation powers of certain stratigraphic markers are also compared and contrasted, together with the potential cross-border equivalence of sedimentary units on different sides of the political median lines.

Abstract This paper presents the historical results of onshore and offshore petroleum exploration in the Anglo-Dutch Basin of the Southern North Sea. A total recoverable resource of 220 Tcfe has been discovered within a contiguous area of 85 000 km 2 . 73% of the resource occurs in The Netherlands. The resource is predominantly gas (207 Tcf), sourced from Upper Carboniferous coals, although the youngest play is oil, sourced from Lower Jurassic shales. There are five plays, partitioned by late Permian-age (Zechstein) salt. In terms of discovered resource they are ranked: (1) Rotliegend aeolian–fluvial sandstones (443 gas discoveries, 417 Bcfe average size); (2) Triassic fluvial sandstones (101 gas discoveries, 140 Bcfe average); (3) Lower Cretaceous paralic–shallow-marine sandstones (61 oil discoveries, 29 MMboe average); (4) Westphalian fluvial sandstones (70 gas discoveries, 88 Bcfe average); and (5) Zechstein carbonates (51 gas discoveries, 83 Bcfe average). Although the main Rotliegend fairway is mature, there are probably discoveries yet to be made in the Westphalian and Zechstein plays, and possibly within the Triassic and Lower Cretaceous plays. There is also potential to extend the Rotliegend play beyond where it is proven: for example, along the northern margin of the basin and towards its centre.

Abstract The central–northern North Sea region represents one of the richest and most diverse petroleum basins in the world. This paper presents an analysis of the historical results of exploration in the Central Graben, Outer Moray Firth and Viking Graben, plus surrounding regions, including West of Shetlands. The richness and maturity of the main late Jurassic source rock accounts for almost 100 billion barrels of oil equivalent (boe) recoverable resource in the three North Sea rift arms in reservoirs as old as Devonian and as young as Quaternary. Within a contiguous area of 100 000 km 2 , 29 different proven part plays (area-confined, deposition-specific stratigraphic intervals or play segments) have been analysed in detail. These range in age from Triassic to Eocene, representing 717 discoveries with a total of 78.1 billion boe recoverable resource present in primary reservoirs. The most prolific plays in terms of total resource found are Mid Jurassic paralic sandstones (in the Viking Graben), Upper Cretaceous chalk (in the Central Graben) and Paleocene turbidite sandstones (in all three rift arms). The average discovery rate (historical chance of geological success) across all the plays is 44% with a relatively low variance. No significant improvement is seen in success rate over the last 50 years, despite increased well density, knowledge and huge advances in geophysical data. Of the discoveries, 404 are commercial fields representing 57% of all discoveries but the rate has decreased to 33% over the last 10 years, as average discovery sizes have decreased significantly. Discovery size is highly variable (10 to more than 600 MMboe) but the historical average is 114 MMboe, a world-class value. Dividing discovery recoverable resource size by footprint area produces a useful measure of reservoir yield, MMboe/km 2 , and the median value for the different part plays varies from 1.3 MMboe/km 2 for an Eocene turbidite to 5.3 MMboe/km 2 for a Jurassic turbidite. From a global perspective, the North Sea is not only unusual in the volume of petroleum, diversity of reservoirs and size of discoveries but also for its number of overlapping part plays and for the fact that the largest volume of total resource is found in reservoirs older than the source rock, syn-rift sandstones having been charged from an end-of-rift marine shale.

Abstract The Triassic of the Central North Sea is a continental succession that contains prolific hydrocarbon-bearing fluvial sandstone reservoirs stratigraphically partitioned by mudstones. Within the Skagerrak Formation of the UK sector, hydrocarbon accumulations in the Judy, Joanne and Josephine Sandstone members are top sealed by the Julius, Jonathan and Joshua Mudstone members, respectively. However, UK and Norwegian stratigraphic correlations have been problematical for decades, largely due to biostratigraphic challenges but also due to the non-uniqueness of the lithotypes and because the cross-border stratigraphic nomenclature differs and has yet to be rationalized. This study focuses on mudstones rather than sandstones to unify cross-border correlation efforts at a regional scale. The mudstone members have been characterized by integrating sedimentological, petrophysical and geophysical data. The facies are indicative of playa lakes that frequently desiccated and preserved minor anhydrite. These conditions alternated with periods of marshy, palustrine conditions favourable for the formation of dolostones. Regional correlations have detected lateral facies changes in the mudstones which are important for their seismically mappable extents, resulting palaeogeographies and, ultimately, their competency as intraformational top seals. Significant diachroneity is associated with the lithological transitions at sandstone–mudstone member boundaries and although lithostratigraphic surfaces can be used as timelines over short distances (e.g. within a field), they should not be assumed to represent timelines over longer correlation lengths. Palaeoclimatic trends are interpreted and compared to those of adjacent regions to test the extent and impact of climate change as a predictive allogenic forcing factor on sedimentation. Mudstone member deposition occurred as a result of the retreat of large-scale terminal fluvial systems during a return to more arid ‘background’ climatic conditions. The cause of the member-scale climatic cyclicity observed within the Skagerrak Formation may be related to volcanic activity in large igneous provinces which triggered the episodic progradation of fluvial systems.

Abstract Salt is a crystalline aggregate of the mineral halite, which forms in restricted environments where the hydrodynamic balance is dominated by evaporation. The term is used non-descriptively to incorporate all evaporitic deposits that are mobile in the subsurface. It is the mobility of salt that makes it such an interesting and complex material to study. As a rock, salt is almost unique in that it can deform rapidly under geological conditions, reacting on slopes ≤0.5° dip and behaving much like a viscous fluid. Salt has a negligible yield strength and so is easy to deform, principally by differential sedimentary or tectonic loading. Significant differences in rheology and behavioural characteristics exist between the individual evaporitic deposits. Wet salt deforms largely by diffusion creep, especially under low strain rates and when differential stresses are low. Basins that contain salt therefore evolve and deform more complexly than basins where salt is absent. The addition of halokinetic processes to the geodynamic history of a basin can lead to a plethora of architectures and geometries. The rich variety of resultant morphologies have considerable economic as well as academic interest. Historically, salt has played an important role in petroleum exploration since the Spindletop Dome discovery in Beaumont, Texas in 1906. Today, much of the prime interest in salt tectonics still derives from the petroleum industry because many of the world's largest hydrocarbon provinces reside in salt-related sedimentary basins (e.g. Gulf of Mexico, North Sea, Campos Basin, Lower Congo Basin, Santos Basin and Zagros). An understanding of

Abstract Fluvial incision and terracing produces a scale of architectural complexity that is often overlooked in geological interpretations and the construction of subsurface reservoir models. Results of a 2D seismic forward modelling study demonstrate the difficulties and limitations in resolving terraces seismically. We propose that terraced sequence boundaries fall readily into a subsurface data resolution gap between seismic and core to wireline log scale. Due to limitations in seismic resolution, sequence boundaries are usually interpreted as simple, single surfaces of erosion, for example at the bases of incised-valley fills. However, modern analogues show that sequence boundaries are in fact often compound in nature. The stratigraphic response to pulses of incision and aggradation in an incision-dominant phase can result in flights of stair-stepping terraces. In this paper we recognize two distinct architectural styles: attached and detached terrace flights. Their formation depends on the interplay between the magnitudes of incision versus aggradation. We suggest that terraced sedimentary architectures, the nature of the terrace-fill lithology, and any associated pedogenesis impacts upon mesoscale to macroscale reservoir permeabilities and therefore have implications for subsurface fluid flow. The existence of terraces, in conjunction with the inherent lack of temporal control in fluvial successions, may help to explain correlation difficulties often encountered and to an extent the compartmentalized production behavior of some fluvial hydrocarbon reservoirs.