Geological controls on petroleum plays and future opportunities in the North Sea Rift Super Basin - GeoScienceWorld
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Geological controls on AUTHORS
petroleum plays and future John R. Underhill ~ Centre for
Exploration Geoscience, Institute of
GeoEnergy Engineering, School of Energy,
opportunities in the North Sea Geoscience, Infrastructure & Society,
Heriot-Watt University, Edinburgh Campus,
Rift Super Basin Riccarton, Edinburgh, United Kingdom;
present address: Interdisciplinary Centre for
John R. Underhill and Nick Richardson Energy Transition, School of Geosciences,
Meston Building, King’s College, Aberdeen
University, Aberdeen, United Kingdom;
john.underhill@abdn.ac.uk
ABSTRACT John R. Underhill is the director of the
The North Sea Super Basin is a trilete rift system located in the Interdisciplinary Centre for Energy Transition
and professor of geoscience and energy
maritime waters of the United Kingdom, Norway, Denmark, Ger-
transition at Aberdeen University, Scotland.
many, and the Netherlands. Created after a phase of regional ther- He is the academic executive director of the
mal doming in the Middle Jurassic, the rift basin consists of the Centers of Doctoral Training in Oil & Gas and
Viking Graben, Central Graben, and Moray Firth Basin and their in GeoNetZero. He populates the Scottish
surrounding platform areas. Synrift extensional activity occurred Science Advisory Council and United
during the Upper Jurassic followed by postrift thermal subsidence Kingdom’s (UK) Exploration Task Force. John
from the Cretaceous to the present day. The basin’s main Upper is a recognized expert on the North Sea Basin
Jurassic (Humber or Viking Group) source rocks were deposited and is leading efforts to repurpose it for
carbon storage and the energy transition. He
contemporaneously with rifting. Their subsequent subsidence his-
has been an AAPG member for almost 40
tory led to progressive maturation, initially focused in the grabens, years during which time he has received
but becoming ever more extensive with time. Maximum burial of AAPG’s George C. Matson, Grover E. Murray
the Upper Jurassic occurs at the present day leading to the efficient Distinguished Educator, and Ziad Beydoun
charge of a diverse array of overlapping plays. The only exception awards as well as the Geological Society’s Lyell
occurs in western parts of the Moray Firth rift arm, where Ceno- Medal and their Energy Group’s Silver Medal.
zoic uplift arrested the maturation of Upper Jurassic source inter- He is the corresponding author of this paper.
vals. However, Middle Jurassic (paralic), Lower Jurassic (marine), Nick Richardson ~ Oil & Gas Authority
and Middle Devonian (continental) lacustrine source rocks (OGA), Aberdeen, United Kingdom;
reached maturity there too and created an additional local petro- Nick.Richardson@ogauthority.co.uk
leum system. The North Sea’s reservoirs span the entire geological Nick Richardson leads the Exploration and
column and all of the post-, syn- and prerift megasequences. New Ventures team for the UK OGA. He has a
Despite being stratigraphically older, the Devonian–Middle Juras- B.A. degree in geology from Oxford University,
sic (prerift) reservoirs receive charge because the Upper Jurassic an M.Sc. degree from The University of
source rocks lie structurally deeper in the graben. Edinburgh, and a Ph.D. from ETH Zurich. Nick
Since oil first flowed from the North Sea in 1975, 15 fields have worked for Shell, Maersk Oil, and Dana
been found to contain more than 1 billion BOE in recoverable Petroleum prior to joining the OGA in 2016.
reserves and more than 95 billion BOE have been extracted to
ACKNOWLEDGMENTS
date, making it one of the most significant petroleum basins in
This paper was first presented at the AAPG
the world. Production from the area allowed the United Kingdom Super Basins Conference in Houston in March
and Norway to exceed their energy needs and become net 2018. J.R.U. acknowledges Charles Sternbach,
Claudio Bartolini, and AAPG Editor Robert K.
Copyright ©2022. The American Association of Petroleum Geologists. All rights reserved. Merrill for their encouragement and invitation
Gold Open Access. This paper is published under the terms of the CC-BY license. to publish the outcomes of the study. We
Manuscript received May 22, 2020; provisional acceptance July 1, 2020; revised manuscript received thank Duncan Erratt and the editors of the
May 6, 2021; final acceptance May 7, 2021.
DOI:10.1306/07132120084
AAPG Bulletin, v. 106, no. 3 (March 2022), pp. 573–631 573
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thematic set for their constructive and helpful exporters for two decades. However, rates have declined signifi-
reviews. The paper builds upon the work cantly, and the rift system is now considered to be a mature petro-
undertaken by numerous students based in leum province. Despite the smaller volumes and reduced size of
the Grant Institute of Earth Science at The
accumulations, companies continue to seek, identify, and drill
University of Edinburgh and in the Centre for
prospects that will extend the life of the basin. The continued
Exploration Geoscience in the Institute of
GeoEnergy Engineering at Heriot-Watt exploration activity and field development has led to an estimate
University. Shell is thanked for financial that approximately 10–20 billion bbl of oil equivalent remains
support for the Wouter Hoogeveen to be developed. With many fields now depleted, efforts are
Laboratory in which some of the work was increasingly focused upon decommissioning and the repurposing
undertaken and for supporting J.R.U.’s and reemergence of the basin for an alternative (renewable) low-
participation in the AAPG Super Basins carbon future that faces the energy transition and challenge to
meeting. Paul Renaut (Sparos Graphics) is
meet stringent net zero emission targets. These include the evalu-
thanked for drafting the diagrams. The UK
ation of safe subsurface storage sites for carbon dioxide, hydrogen,
OGA, Norwegian Petroleum Directorate, and
Danish Energy Agency are thanked for the methane, and compressed air, and the development of new
publication of analyses derived from the energy integration projects such as coupled blue hydrogen, green
resource assessments for their respective hydrogen, platform electrification using wind turbines, geother-
offshore areas, available on their websites mal energy, gas-to-wire, and geothermal initiatives. The latter
under the relevant licenses. We thank the staff will all become increasingly important as the basin addresses
who support the OGA’s National Data the energy transition and challenge to meet net zero emission
Repository, whose dedication has enabled the
targets.
public release of UK data on which many of
our interpretations are based. Nick
Richardson’s contribution has been approved
INTRODUCTION AND CONTEXT
by the operations director of the OGA. The
authors also acknowledge and credit the staff
of the OGA and predecessor organizations Super basins are defined as those sedimentary basins from which
(Department for Business, Energy and more than 5 billion BOE have been produced and at least 5 billion
Industrial Strategy; Department of Energy and BOE of reserves and undeveloped resources have also been identi-
Climate Change; Department for Business, fied (Sternbach, 2018, 2020). As such, they host the most prolific
Enterprise and Regulatory Reform; and efficient petroleum systems in the world. Several prospective
Department of Trade and Industry, etc.) for the sedimentary basins occur in the northwestern European continen-
skill and care that they have exercised, and
tal shelf, yet only two meet the criteria that allows them to be clas-
continue to exercise, in maximizing the value
of the North Sea and wider United Kingdom sified as global super basins. One is the Anglo-Polish trough (also
Continental Shelf for the benefit of the nation. referred to as the Southern Permian Basin), which stretches from
eastern England to Poland via the Netherlands, Germany, and Den-
mark (Figure 1). The other is the North Sea rift system, which
largely straddles the international borders of the United Kingdom,
Norway, and Denmark (Figure 1). The hydrocarbon volumes
extracted from both of these super basins places them in the top
25 productive basins of all time.
The aim of this paper is to describe the key factors that control
and underpin the exploration history, petroleum systems, and vol-
umes that lead to the North Sea rift being classified as a global super
basin. The basin’s geological success is founded on the presence of
one major and several minor source rocks (Cornford, 1998), multi-
ple reservoir play levels, occurrence of excellent regional top, and
intraformational seals. The oil and gas traps display a variety of
structural styles, an ideal timing of maturation after trap formation,
and the occurrence of highly efficient migration pathways. The
result of the favorable disposition of the geological elements has
574 The North Sea Rift Super Basin
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10°W 5°W 0° 5°E 10°E 15°E 20°E 25°E 30°E
NPB - Northern Permian Basin
MNSH - Mid North Sea High
ben
West Shetland MVS - Midland Valley of Scotland
Basin EB - Egersund Basin
Viking Gra
60°N OMF - Outer Moray Firth
NORTH SEA RIFT
SUPERBASIN
Inner Moray Firth
OMF EB
Ce
NPB
nt
PB
ra
MVS N
lG
ra
be
Ringkøbing - Fy
MNSH n High
n
55°N
Anglo - Polish Su
East Irish per Basin
Sea Basin East Midlands
Province
Groningen
Southern
Ireland Weald Basin
Basin
50°N Wessex Basin
0 km 500
Figure 1. Map showing the geographical extent of the main petroleum systems of the northwestern European continental shelf and the
position of the North Sea Rift Super Basin in relation to the Anglo-Polish Super Basin (or Southern Permian Basin), its Northern Permian Basin
counterpart, and other significant petroleum systems such as the West Shetlands, East Irish Sea, Wessex, and Weald Basins. The North Sea rift’s
petroleum system encompasses the three rift arms (Viking Graben, Central Graben and Moray Firth) as well as the Egersund Basin and Inner
Moray Firth outliers.
led to successful development of clastic and carbonate Vail et al. 1984; Underhill 1991a, b; Galloway et al.,
reservoirs, the stratigraphy for which spans the Phaner- 1993; Partington et al., 1993a, b; Rattey and Hayward,
ozoic (Figure 2) throughout the rift system (Figure 3). 1993), plume-related volcanism and rifting (Underhill
As well as its immediate impact for the economies and Partington, 1993, 1994), salt tectonics (halokine-
of the United Kingdom, Norway, and Denmark, sis; Penge et al., 1993; Davison et al., 2000a, b), clastic
exploration for and production from the North Sea depositional systems (Johnson and Stewart, 1985),
has been a major stimulus for understanding the and injectites (Hurst et al., 2005), among others. The
generic development of rift systems. The knowledge stratigraphy, geological evolution and exploration his-
obtained through seismic acquisition and drilling activ- tory of the North Sea has been comprehensively docu-
ity in the super basin has been deployed to great effect mented in a series of landmark publications (Wood-
in other extensional settings (Davison and Underhill, land 1975; Illing and Hobson 1981; Rønnevik et al.,
2012), and several seminal papers resulted from the 1983; Brooks and Glennie 1987; Abbots, 1991; Hard-
studies in the North Sea that have had global applica- man, 1992; Parker 1993; Steel et al., 1995; Glennie,
bility to rift systems. Examples include those with 1998a; Fleet and Boldy 1999; Evans et al., 2003;
impact on sedimentary basin evolution (e.g., McKen- Gluyas and Hitchens, 2003; Dore and Vining, 2005;
zie, 1978), subsidence and compaction trends (Sclater Vining and Pickering, 2010; Bowman and Levell,
and Christie, 1980; White and Latin, 1993; Nadin 2018; Goffey and Gluyas, 2020), each of which con-
et al., 1995), seismic and sequence stratigraphy (e.g., tain an extensive set of references.
UNDERHILL AND RICHARDSON 575
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Age in Millions of Years
0
CENOZOIC
Neogene
POSTRIFT MEGASEQUENCE
Paleogene
O
R
E
Uplift Balder Fm
R
P
66 Ma
Shetland
Upper
Chalk R
Group Group
Cretaceous
Plenus Marl
100 R
INTRAPLATE SETTING
Cromer Knoll Group
Lower
MESOZOIC
SYNRIFT Humber Group SR R
Upper
R
R
Jurassic
Lower Middle
Fladen Group
SR R
Doming R
Brent Group
200 Dunlin Group R
SR R
Banks Group R
ity
Humber/Viking Group
form
Triassic Uncon Hegre Group R
251 Ma
Zechstein Group
merian
R
Permian Rotliegend Group R
Mid Cim
Upper
Carbonifeous
UPPER PALEOZOIC
Variscan
VARISCAN PLATE CYCLE
PRERIFT MEGASEQUENCE
300
Unconformity
SR
Lower
SR
R
Devonian
Eday Shales
SR
400
419 Ma
Caledonian
CALEDONIAN PLATE CYCLE
Silurian Unconformity
LEGEND
LOWER PALEOZOIC
R Reservoir
Ordovician
SR Source Rock
NW Scotland
Contractional Tectonics
Succession
Foreland
Cambrian
500 Extensional Tectonics
541 Ma
PRECAMBRIAN BASEMENT
Lewisian Fractured
AND ITS TORRIDONIAN COVER Basement
Moine Schist
Mudstone Chalk Deformed Metamorphics
Sandstone Volcanics Igneous Basement
Shale Evaporites Unconformity (Missing Section)
Conglomerate Heterolithic Clastic Sequences
Figure 2. Stratigraphic column depicting the main plate cycles, unconformity-bound tectono-stratigraphic (pre-, syn-, and postrift) megase-
quences, the occurrence of key source rock intervals, and the predominant reservoir plays in the basin. The prospective sedimentary reservoirs
range in age from the Devonian to the Eocene. Although they are predominantly clastic depositional systems, upper Permian (Zechstein Group)
and Upper Cretaceous (Chalk Group) carbonates also make a contribution to production in the central North Sea. E 5 Eocene; O 5 Oligocene; P
5 Paleocene.
576 The North Sea Rift Super Basin
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Legend
62°N
Stratigraphic Age
Quaternary
Neogene Magnus
Snorre
Paleogene
Upper Cretaeous
Vi ki ng Grab
Statfjord Gullfaks
Lower Cretaceous
Brent Kvitebjørn 61°N
Upper Jurassic
Ninian
Middle Jurassic Troll
Lower Jurassic Oseberg
en
Triassic
Paleozoic
60°N
Frigg
Beryl
DEVONIAN SOURCED INNER Grane
MORAY FIRTH PETROLEUM
SYSTEM ‘OUTLIER’ 59°N
Johan
Scapa & Sverdrup
Claymore
Brae
Moray Firth
Piper Sleipner
GEOGRAPHIC EXTENT OF THE
Captain NORTH SEA RIFT’S UPPER
Goldeneye JURASSIC SOURCED
Beatrice
PETROLEUM SYSTEM
58°N
Forties
Buzzard
57°N
Ekofisk
Fulmar
Eldfisk
Symbol Size Ce
ntr Valhall
Discovered Volumes al
Gr Argyll
Resources (2P) ab 56°N
10 BBOE en
N Tyra
5 BBOE
Halfdan
Dan
2 BBOE
1 BBOE 0 50 100 km
500 MMBOE
100 MMBOE 55°N
20 MMBOE
4°W 2°W 0° 2°E 4°E 6°E
Figure 3. The geographical extent of the distribution of fields classified by reservoir age and size with the field volumes illustrated by relative
diameter of the circles. Fifteen of the rift’s fields contain recoverable reserves of more than 1 billion BOE (BBOE), and a further 10 have recov-
erable reserves in excess of 700 million BOE (MMBOE). Seven of the largest fields sit in the Norwegian Sector: Troll, which contains more than
11 BBOE, Ekofisk (4.6 billion [B] barrels), Statfjord (4.4B), Oseberg (3.4B), Johan Sverdrup (2.7B), Gullfaks (2.5B), and Snorre (2B). The other
three lie in the United Kingdom sector: Forties (2.9M), Brent (2.7B), and Ninian (1.3B). As well as showing where the largest fields reside, the
diagram serves to demonstrate that the fields hosting Middle Jurassic reservoirs dominate the North Viking Graben and Paleocene ones are the
most significant in the Central Graben. The mapped extent of the Upper Jurassic Humber Group sourced petroleum system and the Devonian-
sourced “outliers” in the Inner Moray Firth rift arm and Egersund Basin are highlighted. 2P 5 proven plus probable reserves.
UNDERHILL AND RICHARDSON 577
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Significant advances in technology through was not in place and only a few isolated wells were
enhanced seismic acquisition, processing, and imaging drilled in undisputed nearshore “territorial” waters
together with improved drilling methods have that extended 5–20 km from the coast. Control over
contributed to an ever-increasing understanding of deeper waters that extended to the edge of the conti-
the subsurface geology. The progressive sharpening nental shelf, defined by the 200-m isobath, was con-
of geological concepts resulting from the research in tested by nation states. Because the North Sea lay in
the North Sea combined with technological advances shallower depths, the countries with a coastal
and the construction of robust engineering networks of boundary followed rules laid down by the 1958
fields and pipeline infrastructure provided the platform Geneva Convention, whereby equidistance outward
upon which to explore and develop the super basin. The from the nearest opposed coastline defined the median
very same solid technical foundation, engineering, and line. Significantly, that agreement meant that seismic
drilling technology and infrastructure now offers new surveys could be acquired prior to licensing arrange-
opportunities to extend its life and major efforts are ments being in place and the first offshore survey was
not only going in to near-field exploration (NFE) or shot in Danish waters in 1963 using 50-lb dynamite
infrastructure-led exploration but also toward repurpos- charges, and some 14,000 km had already been
ing the basin to face the low-carbon energy transition in acquired by 1967 (Childs and Reed, 1975).
an effort to meet net zero emissions targets. The Continental Shelf Act (Legislation.gov.uk,
Finally, having largely stable and attractive fiscal 1964) was passed by the United Kingdom Parliament
(tax) regimes (Brzozowska et al. 2003), regular licens- in 1964 and set out the rules for offshore licensing at
ing rounds, a relatively rapid turnover of fallow acreage the time. Similar laws were ratified in Denmark and
leasehold, a governmental demand for indigenous oil Germany in the same year, in Norway in 1965, and
and gas to guarantee secure supply, and the public finally, in the Netherlands in 1968. Although there
release of data have all helped to promote its explora- was consistency in defining quadrants by 1 of latitude
tion, appraisal, development, and production history and 1 of longitude, there were important differences
(Brennand, 1984; Brennand et al., 1990). in the shape and size of individual license blocks. The
United Kingdom subdivided their quadrants into 30
blocks of approximately 200 km2; Norway decided
EARLY HISTORY OF LICENSING AND to have larger subdivisions and defined 12 blocks in
EXPLORATION each quadrant, each of which are approximately 500
km2. Denmark settled upon 32 blocks per quadrant,
There had been a long history of exploration success in and the Netherlands and Germany both went for 18.
several onshore areas of northwestern Europe prior to Having agreed the size of license blocks and a pro-
the development of the North Sea. Over the course of cedure for exploration and exploitation in offshore
the twentieth century, petroleum systems were well waters, the United Kingdom led the way by launching
established in the East Midlands, Midland Valley the First Seaward Licensing Round on September 17,
of Scotland, and Lancashire in the United Kingdom 1964. The outcome of the applications saw 53 licenses
(Figure 1) (Lees and Cox, 1937; Lees and Taitt, consisting of 394 blocks awarded to 51 companies that
1945) as well as parts of Germany and the Nether- formed part of 22 joint venture consortia. The first
lands. The latter provided the catalyst for offshore Norwegian licensing round for offshore exploration
exploration in the North Sea with the drilling of the was held in 1965 and included 278 of the 346 blocks
Slochteren-1 well in 1959 and Ten Boer-1 well 4 yr available for award. The first offshore licensing round
later that discovered and appraised the giant Gro- in the Netherlands was held in 1968. By way of con-
ningen gas field in northern Holland (Stauble and Mil- trast, in Denmark, the first and sole initial concession
nus, 1970; de Jager and Visser, 2017). Its discovery was granted to A. P. Møller-Maersk back in 1962 and
renewed speculation that oil and gas reserves might covered the entire offshore area. That was subse-
be present in offshore basins beneath the North Sea. quently amended in 1981 and paved the way for the
Despite new-found interest in moving exploration first Danish offshore licensing round to take place in
activity offshore, there was no practical way of doing so 1984. No formal licensing rounds have been held in
because the prerequisite legislation to govern licensing offshore waters of Germany and individuals, corporate
578 The North Sea Rift Super Basin
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bodies or commercial partnerships have been able to As a consequence, licenses were offered over large
apply at any time. swaths of the United Kingdom Continental Shelf
The first well drilled on United Kingdom licensed (UKCS).
acreage was Amoseas’ 38/29-1 well, which was Although very few of the blocks in the northern
spudded in the Dogger Bank area on December 26, North Sea initially attracted bids, one exception was
1964. Drilled some 200 km east of the English coast, the Shell/Esso joint venture, who bid for, and were
in what is now recognized as the mid North Sea high awarded, block 211/29 in the third Licensing Round
(i.e., outwith the North Sea rift system as it would in 1970, with a commitment to acquire seismic data
now be defined), it was plugged and abandoned as a and drill a well. Early explorers had little understanding
dry hole. Twelve other wells followed, all of which of the stratigraphy that they might encounter. How-
were drilled farther south in the shallow waters of ever, rather than this being a deterrent, there was gen-
the southern North Sea. The fourth well, British Petro- uine excitement and much speculation in the Shell/
leum’s (BP) 48/6-1, discovered gas in Permian (Rotlie- Esso joint venture as to what the drill bit might encoun-
gende) sandstones (in what was to become the West ter especially in a seismically opaque package lying
Sole field) in 1965, thus paving the way for the offshore beneath a prominent unconformity (originally termed
part of the Anglo-Polish Basin to be opened up. It was the “X horizon” by operators in the early years of explo-
soon followed by gas discoveries by Conoco (Viking), ration). Some interpreters suggested that the surface
Shell and Amoco (Leman and Indefatigable), and Phil- represented a nonconformity and predicted that wells
lips Petroleum (Ann and Deborah). Arco made a fur- would encounter nonprospective igneous or metamor-
ther discovery in Triassic sandstones in 1966 (Hewett phic basement below similar to the Scottish Highlands
field; Cumming and Wyndham, 1975). and Norway either side of the North Sea (Bowen,
Exploration moved north into the central North 1975). Those of a more optimistic mindset drew
Sea and led to the first oil discoveries being made in encouragement from the fact that the initial seismic
the North Sea rift system itself in Danish waters at data showed some dipping reflectors beneath the
Anne (1966), Roar and Tyra (1968), and Arne unconformity (Bowen, 1975, 1992). They also drew
(1969). In Norway, early discoveries made at Valhall attention to best-fit restorations of the Atlantic (e.g.,
(1967) and Cod (1968) were followed by success at Bullard et al., 1965), which placed the area closer to
the giant Ekofisk field by Phillips Petroleum in Decem- rift systems of East Greenland (Haller, 1971), where
ber 1969. The latter discovered oil in lower Cenozoic many of the stratigraphic, sedimentary, and structural
(Danian) and Upper Cretaceous carbonate reservoirs components of a prospective petroleum system were
contained in a broad, open anticline created by salt recognized (Birkelund and Perch-Nielsen, 1969; Sur-
movement (halokinesis) of upper Permian (Zechstein lyk et al., 1971, 1974; Surlyk and Birkelund, 1972; Sur-
Group) evaporites. lyk, 1978).
The first oil to be discovered in the United King- Drilled in 1971, at a location some 500 km north-
dom sector came when Amoco drilled Paleocene northeast of Aberdeen, the 211/29-1 well was at the
clastics at the Montrose field and BP followed suit by time the most northerly offshore exploration well. Its
discovering the giant Forties field in the same reservoir target was an elongate four-way closure of the
interval in October 1970. In the same month, Shell dis- so-called X horizon defined using a sparse grid of
covered oil in upper Paleozoic (Permian) reservoirs at two-dimensional seismic data (Bowen, 1975). Its out-
Auk field, and the following year, Hamilton did like- come was to confound the skeptics, however, since it
wise at the Argyll field (Pennington, 1975). Because proved that major oil-bearing Middle Jurassic clastic
it was oil rather than gas that was discovered, all of reservoirs lay beneath the “X,” which was rechristened
these discoveries proved to be the game changer that and thereafter referred to as the Base Cretaceous
really triggered the appetite for further frontier explo- unconformity. With the fourth Licensing Round
ration in deeper, less hospitable waters of the northern imminent, the well was quickly completed in the
North Sea. So, with an active oil-prone petroleum sys- Lower Jurassic and kept on a very strict tight-hole sta-
tem proven and multiple levels to explore, the race to tus so as not to alert competitor companies to the sig-
acquire acreage in the North Sea gathered pace and nificance of the find. The subsequent appraisal and
subsequent licensing rounds were keenly contested. testing only occurred in 211/29-2, which not only
UNDERHILL AND RICHARDSON 579
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found a 545 ft oil column but also located a deeper which to extend geological observations and interpre-
Triassic–Lower Jurassic (Statfjord Formation) reser- tations from onshore outcrops to the subsurface of
voir, albeit water-bearing at this location. The latter the North Sea (e.g., Woollam and Riding, 1983; Voll-
was to add pay at Brent and prove to be a major reser- set and Dore, 1984; Partington et al., 1993a; Hesketh
voir in other fields across the province including the and Underhill, 2002), and the definition of new lith-
Statfjord field from which it took its name. ostratigraphic units in the offshore (e.g., Rhys, 1974;
The Brent discovery stimulated exploration activ- Deegan and Skull, 1977 etc.). Conversely, the inte-
ity for similar targets in the East Shetland Basin on the gration of the seismic, well, and core data acquired
western flank of the North Viking Graben during the in the pursuit of petroleum reserves in the buried
aforementioned fourth United Kingdom offshore rift has added massive value to the understanding of
licensing round. That round remains the only one in the tectonic, stratigraphic, and sedimentological
which financial bids were required and opened in development and evolution of northwestern Europe.
public. Such was Shell/Esso’s desire for more prime The rich database also provides the foundation
acreage in the same area that they tabled a bid of £21 upon which to evaluate sites for safe subsurface
million to secure block 211/21, which subsequently storage of carbon dioxide (CO2), hydrogen, and
proved to be some £13 million higher than the next methane gas as the basin evolves to face a low-
nearest bid for it. Encouraged by the success seen in carbon future.
the United Kingdom northern North Sea, the Norwe- Integration of the subsurface data from onshore
gian authorities made a special award of blocks 33/9 and offshore areas with field studies has demonstrated
and 33/12 in the area in August 1973. The subsequent that the North Sea domain was preceded by and sited
drilling campaign led to the discovery of numerous upon two complete Phanerozoic (Caledonian and
giant fields either side of, and in some cases straddling, Variscan) plate tectonic cycles (Figure 2), both of
the border between the United Kingdom and Norway which involved the construction (rift-drift), subduc-
in what became known as the “Brent province.” These tion, accretion of major (Iapetus and Rheic) oceans
included the discovery of Cormorant (1972), Thistle and their destruction through continental collision
(1972), Ninian (1974), Statfjord (1974), Snorre and mountain building (Ziegler, 1982, 1990a; Glennie
(1974), Heather (1976), and Gulfaks (1978) in the and Underhill 1998; Underhill, 2003).
northern North Sea. Major discoveries were also Since the end of the (Permian–Carboniferous)
made during the same period at Beryl (1972) in the Variscan orogeny, the area has lain in an intraplate set-
South Viking Graben, and Beatrice (1976) in the Inner ting and, hence, only been affected by intraplate pro-
Moray Firth. cesses that have primarily taken the form of extension
Despite being found after the initial oil discoveries and contraction (structural inversion) caused by far-
in the province, the honor of being the first North Sea field (intraplate) stresses driven by Alpine (Tethyan)
oil producer belongs to the Permian (Rotliegende and and Atlantic deformation. The early Cenozoic could
Zechstein Group) reservoirs of Argyll field, with the first arguably be considered an exception to this general
oil on stream via a tanker in the Thames Estuary in June intraplate setting, however, since the stratigraphic
1975. Development of Paleogene reservoirs of the cen- development of the British Isles was more closely asso-
tral North Sea occurred in parallel to Argyll though and ciated with the opening of the north Atlantic Ocean
soon led to oil flowing through the Forties Pipeline Sys- that propagated north at that time. Sea-floor spreading
tem to Cruden Bay and Hound Point near Edinburgh was initiated in the early Eocene, leading to creation
later the same year. Oil from the Brent province came of oceanic crust, and continental drift between Green-
on stream on November 25, 1978, through the Sullom land and northwestern Europe, a process that
Voe terminal on the Shetland Islands. continues to the present day. The main tectonic conse-
quences largely take the form of diffuse extension and
structural inversion caused by the compressional reac-
GEOLOGICAL EVOLUTION tivation of former normal faults and deformation
caused by thermal effects associated with the genera-
The exploration activity that has occurred over the tion and decay of mantle plumes under the North
past six decades has provided a wealth of data upon Sea and in Iceland.
580 The North Sea Rift Super Basin
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by guest
The Caledonian Plate Cycle controlled by the occurrence and reactivation of Cale-
donian and other basement lineaments together with
The early Paleozoic history of the British Isles and the presence of the aforementioned granitic plutons
North Sea areas was dominated by the late (Underhill et al., 1988; Corfield et al., 1996). An
Cambrian–late Silurian, Athollian (former Grampian), east-west–striking passive continental margin devel-
and Caledonian orogenies, which are the tectonic prod- oped across much of southern parts of England and
ucts of continent-ocean, continent-continent collision, Wales that consisted of south-dipping normal fault sys-
and major transpression (Glennie and Underhill, tems (Glennie and Underhill, 1998). To the north, a
1998). Prior to these events, the North Sea area com- series of intracontinental extensional half graben char-
prised three different continental fragments (Avalonia, acterized the East Midlands and northern England.
Baltica, and Laurentia) that were widely separated by Where the basement lineaments or the margins of Cal-
the southwest-northeast–striking Iapetus ocean and edonian granitic intrusions were not favorably oriented
the northwest-southeast–striking Tornquist Sea. Clo- to take up dip-slip extensional displacement, strike- or
sure of the Iapetus ocean is thought to have been dia- oblique-slip deformation resulted across northern
chronous and achieved by both northwest- and Britain (Underhill et al., 1988, 2008; Corfield et al.,
southeast-directed subduction (Phillips et al., 1976) 1996).
and continental suturing and resulted in the creation Continental sequences dominate the synrift depo-
of the Laurussia mega-continent. centers of northern Britain (e.g., the Midland Valley of
The collisional processes led to the formation of a Scotland; Underhill et al., 2008), reflecting their prox-
major (Caledonian) mountain range that stretched imity to the Caledonian Mountains from which they
from the southern United States to eastern Canada werederived.Moresoutherlydepocentersarecharacter-
(Appalachians) through northern Britain to the north- ized by marine sequences, with those of northern
ern end of the Greenland-Scandinavia craton. The line England dominated by fine-grained clastics (e.g., the
of closure is marked by a northeast-southwest-striking Bowland and Hodder Shales) that form source rocks
suture that can be traced from the Shannon Estuary in thatchargepetroleum systems intheEast IrishSea Basin,
western Ireland through Northern Ireland, beneath East Midlands, and southern North Sea (Besly, 2018).
the Solway Firth and Northumberland trough to the The same units also create shale gas targets (e.g., in Lan-
northeastern coast of England. The so-called “Iapetus cashire, Yorkshire, and Nottinghamshire). Farther
suture” passes out into the North Sea, to meet its Torn- south, the lower Carboniferous depocenters are domi-
quist counterpart at a triple junction in the central nated by carbonates that line the northern margin of
North Sea before reappearing in Norway. Each plate the Rheic ocean and pass southward into basinal mud-
is characterized by different crustal rheology, some- stones of Devon and Cornwall (Ziegler, 1982, 1990a, b).
thing that is readily apparent from seismic tomography The Variscan orogeny marked the subduction,
(Crowder et al., 2020). Reactivation of the structural accretion, and eventual closure of the Rheic ocean and
trends was to be a significant factor in later deforma- the creation of the supercontinent Pangaea. It led to
tion. The final stages of Caledonian collision were the former passive continental margin being telescoped
accompanied by the intrusion of Early Devonian gran- to form a mountain belt and a northward-tapering, flex-
ites that subsequently played an important role in cre- ural foreland (foredeep) basin, the depocenter for
ating extensional fault blocks during the periods of which stretched from southwestern Ireland through
Carboniferous and Late Jurassic extension. southern Wales, to Kent, the Ardennes of Belgium
and beyond. The effects of the contractional deforma-
The Variscan Plate Cycle tionand associatedmetamorphism placeeffectivelimits
on the southern extent of Devonian and Carboniferous
The Variscan plate cycle lasted from the Devonian to play fairways leading to the exploration potential being
the late Carboniferous. It began with a phase of Devo- severely challenged and extremely limited in south-
nian rifting initially driven by intramontane collapse of western England and the Western Approaches.
the Caledonian Mountains and the creation of the Farther north, the intracratonic extensional and
Rheic ocean to the south. The structural configuration strike-slip basins of central and northern parts of the
of the British area appears to have been partially United Kingdom, such as the Midland Valley of
UNDERHILL AND RICHARDSON 581
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by guestScotland, Bowland Basin, Widmerpool Gulf, Edale initially dominated by postrift thermal subsidence,
trough, and Northumberland Basin, accommodated which was accompanied by the development of wide-
Variscan deformation through the contractional reac- spread salinas and mudflats ascribed to the Mercia
tivation (structural inversion) of the former exten- Mudstone Group onshore and the equivalent units
sional faults and regional uplift (Underhill et al., that are ascribed to the Haisborough Group offshore
1988, 2008; Fraser and Gawthorpe, 1990; Underhill (e.g., Rot, Muschelkalk, and Keuper halites and
and Brodie, 1993; Corfield et al., 1996; Anderson mudstones).
and Underhill, 2020). Footwall closures, inversion Continued subsidence led to marine waters
structures, intrabasinal folds, and stratigraphic trunca- returning to the northwestern European continental
tion that resulted from the extensional and subsequent interior in the Lower Jurassic (Hettangian) for the first
contractional events contribute to the formation of time since the upper Carboniferous, a period of more
prospective traps in the Carboniferous play (e.g., in than 115 m.y. The transgression was more pronounced
the East Midlands and southern North Sea; Fraser in areas where extension and growth faulting occurred
and Gawthorpe, 1990; Corfield et al., 1996). such as in the Cleveland, Weald, and Wessex Basins
and on the Ninian-Hutton fault, which controlled
Intraplate Deformation thickness and facies distribution of the Statfjord and
Dunlin Formations in the East Shetland Basin. Data
The early Permian extension led to extensive, rift- from basins in Denmark and offshore Britain suggest
related volcanism and igneous intrusion across north- that marine incursion came from the south, with grad-
western Europe in Norway (the Oslo graben), Poland, ual onlap of marine strata to the north. This regional
Germany, the Netherlands, the Midland Valley of pattern supports the interpretation of a southerly dip-
Scotland, the occurrence of the Whin Sill igneous com- ping paleoslope toward a shoreline that was located up
plex across northeastern England, and volcaniclastic to 350 km southward of proximal fluvial environments
sediments in Devon (the Exeter Volcanic Series). in the North Viking Graben (Ryseth, 2001).
Two, elongate, east-west–striking intracratonic The period of Triassic–Early Jurassic postrift
basins were created, separated by the mid North subsidence and transgression was terminated by a
Sea high (Glennie, 1998b; Ziegler 1982, 1990a, b; phase of latest Early Jurassic–Middle Jurassic doming,
Brackenridge et al., 2020). Commonly referred to as which reset the whole depositional system and con-
the Northern and Southern Permian (or Anglo-Polish) trolled subsequent basin rifting and thermal subsi-
Basins, their development initially hosted an extensive dence history (Ziegler, 1982, 1990a, b, 1992; Under-
succession of aeolian and fluvial red beds belonging to hill and Partington, 1993, 1994). Accompanied by
the Rotliegende Group that gave way to a succession of igneous activity (the Rattray Volcanic Series; Dixon
carbonate-evaporite cycles ascribed to the Zechstein et al. 1981; Quirie et al., 2019), the uplift was centered
Group. Although the latter forms an extensive top on the central North Sea and is interpreted to have
seal for Rotliegende reservoirs in the area, basin- resulted from the development of a warm, diffuse,
margin Zechstein Group carbonates also form reser- and transient plume head that created “the North
voirs locally in the North Sea rift, as exemplified by Sea dome” (Figure 4) (Underhill and Partington,
production at Auk and Argyll in the Central Graben 1993, 1994). Ammonite faunas demonstrate that the
(Brennand and Van Veen, 1975; Trewin et al., 2004). dome created a barrier between Arctic and Tethyan
Extensional activity was renewed in the Triassic, (sub-Mediterranean) waters, which complicated
with the development of numerous half-graben depo- regional stratigraphic correlations (Morton et al.,
centers in areas such as the East Irish Sea Basin, Wessex 2020). The North Sea dome also created a quaquaver-
Basin, Worcester graben, Cheshire Basin, and Vale of sal pattern of drainage was established with the pro-
Eden. The United Kingdom basins form part of a wider gressive outward progradation of significant volumes
pattern of distributed rifting that extends across the of fluvio-deltaic sediments derived from erosion of
North American seaboard (e.g., Newark Basin) and central areas (Figure 4). The resultant major deltaic
Norway (e.g., the Stord and Egersund Basins, where wedge created the main Brent Group reservoir play
there was accompanying igneous activity). The Trias- fairway in the East Shetland Basin of the northern
sic Period of extensional basin development was North Sea (Budding and Inglin, 1981; Eynon, 1981;
582 The North Sea Rift Super Basin
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by guestby guest
5°W 0° 5°E 10°E 5°W 0° 5°E 10°E
s
cie
Corr n Basi
ela
ti ity lenia nW a
ve rm a Aa ar tF
C on fo Intr d S hi f
Horda n Horda
nia ian
Platform alo E Bajoc Platform
LA
in Facies
L Bajocian
l Sediment Input
Basinward Shift
a
Loc
L Bajocian
60°N 60°N
Fenno-Scandian Fenno-Scandian
F Shield F Shield
e
en
n
East Shetland East Shetland Bathonian
no
Platform Platform no
-Sc
-Sc
Halibut
an
an
Horst
di a
di a
n
n
B
B
Forties Volcanic or or
Centre de de
r Zo r Zo
ne ne
E Kimmeridgian
Tr i ass
E Kimmeridgian L Oxfordian
ic S
Basinward L Callovian
Shift in Facies
upc
E-M Callovian
ro
SINEMURIAN
p
Bathonian/E Callovian
E Oxfordian
OP
M Oxfordian
BC R
L Oxfordian
SU
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T R IA
Mid North Sea SSIC OR BASE M E N T
Mid North Sea E Kimmeridgian
P
High RO High
HETTANGIAN S U B C Ringkøbing Ringkøbing
55°N Fyn High 55°N Fyn High
SINE
MURI
AN S Aalenian
UBCR O P
Bathonian
PLE
INSB E Kimmeridgian
Pennine ACHIAN SUBCRO P Pennine
i on
at
High High c
ru n
im it of T
ter L
P ROP Ou
RO SUBC E Kimmeridgian
AN
BC LENI
AA
AN RA
NT SU
CI I
AR
TO IT TO
LI M Bathonian
ity
ne
chro
Dia
UNDERHILL
Lond Lond
on-B on-B
AND
r r
Poor Data Area Mass abant
if
Mass abant
if
Poor Data Area
50 Basinward Shift in Facies 50
300 km
N 300 km
N
0 SCALE 0 SCALE
RICHARDSON
Figure 4. Diagram showing the extent of the Middle Jurassic North Sea dome and record of subsequent transgression along the nascent rift arms (modified after Underhill and Partington,
1993, 1994). E 5 Early; L 5 Late; M 5 Middle.
583Graue et al., 1987; Helland-Hansen et al., 1992) on the 0° 4°E 6°E
Ac
northern flank of the North Sea dome (Figure 4) A 0 Scale 200km
(Underhill and Partington, 1993, 1994). 60°N
The initial deflation of the dome occurred in the
Middle to Upper Jurassic (Bathonian–Oxfordian) fol- Bc
FENNOSCANDIAN
SHIELD
B
lowing igneous and volcanic activity (Underhill and
0
Partington, 1993, 1994). Its collapse led to the progres- 58°N
1k
m
sive yet punctuated (subseismic) onlap along the Cc
C
nascent rift arms prior to more substantive (seismic-
2k
2k 3k
m
m m
scale) normal faulting (Figure 4) (Underhill and Part- Denmark
ington, 1993, 1994), the effect of which was to create 56°N
a triple junction intersection at the center of the
1k
m
thermal dome. The trilete rift system’s formation,
deflation, and collapse not only drove Late Jurassic to
0
earliest Cretaceous extensional tectonics but also led 54°N
to deep-water sedimentation along the graben axes
(Underhill and Partington, 1993, 1994). N
Seismic interpretations ably demonstrate that the England
52°N
Viking Graben, Central Graben, and Moray Firth were Paleozoic &
Cenozoic Mesozoic
all characterized internally by extensional fault-block Precambrian
rotations leading to the formation of major three-way Figure 5. Depth to Base Cenozoic showing the elongate nature of
fault-bound structural traps (Beach, 1984; Badley the North Sea Basin’s postrift fill. The lines of section correspond to
et al., 1988; Yielding, 1990; Underhill, 1991b; Yield- the cross sections shown in Figure 6.
ing et al., 1992; Underhill, 1998, 2003). The clear
inference taken from this is that each rift arm was dom- development of a saucer-shaped basin superimposed
inated by dip-slip extension (Davies et al., 2001), on the three rift arms (Figure 5) to create a typical
meaning that there is no need to accommodate the “steer’s horn” cross-sectional basin geometry (Figure
deformation in a single regional slip vector (cf. Roberts 6). The synrift-postrift boundary is commonly taken
et al., 1990; Bartholomew et al., 1993; Erratt et al., to be marked by the Base Cretaceous unconformity.
1999), thus ruling out the need to appeal to major Although it is a very prominent seismic horizon event,
strike- or oblique-slip movement in two of the three its name was ascribed as a consequence of the early
rift arms. wells, and more detailed studies now demonstrate
The occurrence and strike of the Permian–Triassic that it does not coincide with the Jurassic–
faults had long been supposed to have contributed to Cretaceous boundary and is not even an unconformity
Upper Jurassic fault activity through reactivation. How- in the basin depocenters. Instead, it represents a con-
ever, improvement in seismic technologies and imaging densed stratigraphic interval in the rift arms and the
haverevealedthatthetwosetsof extensionalfaultscom- erosional unconformity is confined to footwall highs
monly have a different strike and dip polarity, implying and the basin margins (Rawson and Riley, 1982).
thattheywerelargelyindependentofoneanother(Tom- Source rocks belonging to the Upper Jurassic,
asso et al., 2008) and only a few Upper Jurassic faults can Kimmeridge Clay, and Heather Formations of the
be attributed to the reactivation of Permian–Triassic Humber, Viking, and Vestland Groups were buried
faults.Asaresult,thetrileteNorthSeariftisnowthought sufficiently deeply to mature during the postrift phase
to be largely independent from and superimposed on of subsidence (Cornford, 1998). That led, in turn, to
older structures with the thickest Triassic depocenters petroleum migration out from the deepest parts of
(e.g., the Stord Basin) occurring to the east under the the basin to charge reservoirs contained within sealed
Horda platform (Tomasso et al., 2008). traps through updip fill-and-spill that has made the
The phase of Late Jurassic extensional basin devel- North Sea Basin the prolific oil and gas province that
opment was followed by a phase of Cretaceous– it has become (Goff, 1983; Burley, 1993). Additional
Cenozoic postrift thermal subsidence leading to the source rock potential of Middle Devonian and Lower
584 The North Sea Rift Super Basin
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by guestA A„
WNW ESE
Norway
EAST
SHETLAND EAST SHETLAND BASIN VIKING GRABEN HORDA PLATFORM NS Ac
PLATFORM A
n be
Shetland Is.
0
Viking Gra
Sea Bed
60°N
Cenozoic Base Cretaceous
Unconformity Orkney Is.
B
Quaternary Troll
B ase Cenozoic POSTRIFT Egersund Bc
Base Cretaceous MEGASEQUENCE Basin
Unconformity 2500ft Moray
Ninian Firth No
Alwyn Upper Cretaceous rth
er Cc
Huldra Ba n P
in
Scotland sin erm
as
Lower Cretaceous
Ce
ian
B
C
rm rn
n
ian
Pe rthe
tra
Upper Jurassic
SYNRIFT
lG
No
Middle Jurassic
ra
5000ft
b
PRERIFT
en
Triassic
1000 ft
Base Cenozoic Midland Valley
of Scotland Mid North
100 kms Sea High
55°N
England
0° 5°E
B B„
NW GUDRUN SE
EAST SHETLAND SOUTH VIKING TERRACE
PLATFORM GRABEN UTSIRA HIGH LING GRABEN ANCA GRABEN
Sea Bed 0
Cenozoic Base Cretaceous Quaternary
Base Cretaceous Unconformity
Base C Unconformity Johan
enozoic POSTRIFT Sverdrup
MEGASEQUENCE
Avgvald 2500ft
Graben
Haug
Avaldnes
Upper Cretaceous High
alan
Lower Cretaceous
dH
SYNRIFT
ig h
Upper Jurassic
Middle Jurassic 5000ft
1000 ft
Triassic
Base Cenozoic
PRERIFT 100 kms
C C„
SW NE
WEST CENTRAL SHELF CENTRAL GRABEN NORWEGIAN (EASTERN) PLATFORM
FORTIES
WEST MONTROSE EAST
CENTRAL HIGH CENTRAL JAEREN HIGH EGERSUND BASIN
GRABEN HIGH
0
Base Cretaceous Unconformity
Base Cretaceous Cenozoic
Unconformity
Salt Diapir Base Cenozoic T
Quaternary POSTRIFT SYNRIF
MEGASEQUENCE
Forties
Upper Cretaceous 2500ft
Lower Cretaceous
SY
N SY
Upper Jurassic N Middle Jurassic
PRE
Triassic
PRE
FORTIES 5000ft
1000 ft
MONTROSE Base Cenozoic
WESTERN HIGH EASTERN
TROUGH TROUGH 100 kms
Figure 6. Three regional west-east–striking (dip) cross sections across the North Sea rift system illustrate the basin’s three main (pre-, syn-,
and postrift) tectono-stratigraphic megasequences. The main rift-related platform-graben structural domains such as the Shetland platform,
the East Shetland Basin, the Viking Graben, the Horda platform and Norwegian shelf, Utsira high, Stord Basin, Western platform, Western
trough, Forties Montrose high, Eastern trough, Jaeren high and Egersund Basin are all highlighted. The cross sections serve to illustrate
how migration out from the stratigraphically younger, but structurally deeper Upper Jurassic (synrift) source rock can fill-and-spill into rota-
tional fault blocks housing prerift reservoirs, their synrift counterparts in half-graben hanging-wall depocenters and into reservoirs belonging to
the typical “steer’s horn” postrift section too.
UNDERHILL AND RICHARDSON 585
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by guestand Middle Jurassic occurs in the Inner Moray Firth, The creation of oceanic crust between Greenland
where it cosourced waxy crude found in the Beatrice and northwestern Europe during the Eocene, com-
field and its satellites. bined with the effects of Tethyan (Alpine and Pyre-
The development of the Atlantic Ocean had a pro- neean) collision (Ziegler, 1987), has led to additional
found effect on the northwestern European shelf in phases of compressional on the Atlantic margin (e.g.,
general and the North Sea in particular. Cretaceous Tuitt et al., 2010) and in the neighboring plate interiors
extension linked to the northeastward propagation thereafter. Numerous Mesozoic Basins experienced
and eventual onset of sea-floor spreading in the north basin inversion with the formation of major anticlines
Atlantic Ocean in the early Eocene led to a series of as a consequence (e.g., the Wessex Basin [Colter and
northeast-southwest–striking sedimentary basins Harvard, 1981; Underhill and Paterson, 1998; Under-
being formed, the most notable of which is the hill and Stoneley, 1998], the Weald Basin [Butler and
Faroe-Shetland Basin (Lamers and Carmichael, Pullan 1990], the Cleveland Basin, and in the southern
1999). Although that basin is also prospective and con- North Sea [e.g., Glennie and Boegner, 1981; Ziegler,
tains the largest single oil pool in the United Kingdom 1982, 1990a, b; Van Hoorn, 1987; Badley et al.,
in the 7 billion bbl Clair field (Coney et al., 1993; John- 1989]). Although the North Sea appears to have
ston et al., 1995; Witt et al., 2010), it has a distinctive escaped the most significant effects of the deformation,
structural history and its total volumes fall well short of it is still characterized by punctuated subsidence, local-
those in the North Sea. ized uplift, and fault reactivation.
The Magnus field represents an example of a struc-
ture lying in the North Sea Rift Super Basin that The Overprint and Impact of Climate
resulted from the postdepositional footwall uplift of Change, Continental Drift, and Eustasy
Upper Jurassic deep-water clastic reservoirs driven
by Cretaceous extension. The far-field effects of Atlan- Although the aforementioned tectonic history and
tic rifting also drove structural inversion during the consequent sedimentary responses are significant, it is
Lower Cretaceous and the formation of anticlinal traps important to underline that the changing pattern of
during the Lower Cretaceous including those contain- crustal fragmentation and reunification occurred
ing prospective Upper Jurassic (synrift) clastics of the against a backdrop of an overall slow northward passive
Brae trend in the South Viking Graben. drift of the continents with consequences for climate.
The subsequent development of the Iceland hot- This drift took the North Sea area from south of the
spot, the northward propagation of the North Atlantic, equator prior to the Carboniferous to its present loca-
and instigation of sea-floor spreading between north- tion over halfway from the equator to the northern
western Europe and Greenland have all impacted the pole (Habicht, 1979; Smith et al., 1981). The inexora-
North Sea. The inception of the mantle plume led to ble northward-drift had a pronounced effect on fauna
the creation of the North Atlantic Large Igneous Prov- and on sedimentation as the area passed through suc-
ince (Mussett et al., 1988) and significant igneous cessive latitudes and climatic belts.
underplating of the continental crust. Underplating The development of major ice sheets in the polar
led to Cenozoic exhumation of large parts of the British regions provided an additional impact on climate
Isles and punctuation of the postrift subsidence by that modified the effects of Britain’s northerly drift.
periods of uplift in western areas (Nadin et al., 1995) During the Phanerozoic, the Earth’s climate appears
that is particularly well documented in the western to have oscillated between a state of global warming
part of the North Sea rift (the Inner Moray Firth; Hillis and incubation (“greenhouse conditions”) and one of
et al., 1994; Thomson and Underhill, 1993), where global cooling and refrigeration during which ice sheets
progressively older subcrop patterns occur (Guari- grew and dominated the poles (“icehouse conditions”).
guata-Rojas and Underhill, 2017). The associated fault In total, seven oscillations appear to have occurred
reactivation also added to the structural complexity with greenhouse conditions prevailing during the early
and was largely detrimental to petroleum prospectivity Cambrian–Late Ordovician (570–458 Ma), early
(Argent et al., 2000) and affects the carbon storage Silurian–early Carboniferous (428–333 Ma), and late
potential of saline aquifers in the area (Guariguata- Permian–early Cenozoic (258–55 Ma). Conversely,
Rojas and Underhill, 2017). icehouse climate affected the late Proterozoic–early
586 The North Sea Rift Super Basin
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by guestPhanerozoic (800–570 Ma), Late Ordovician–early (Draupne) and Heather Formations, which form com-
Silurian (458–428 Ma), early Carboniferous–late ponent parts of the Humber (Viking) Group (Barnard
Permian (333–258 Ma), and early Cenozoic onward and Cooper, 1981; Chung et al., 1992; Cornford,
(55 Ma to the present day). 1998; Isaksen et al., 2002). Deposition of the Upper
Several factors also combined to control global Jurassic source intervals was contemporaneous with
(eustatic) sea levels through the Phanerozoic, but Upper Jurassic synrift fault activity and their gentle
they were particularly responsive to changes in the (postrift) subsidence led to thermal maturation and
size of ocean basins resulting from variations in petroleum migration occurring from the early Ceno-
sea-floor spreading rates, periods of continental defor- zoic to the present day. The effect was to set up a highly
mation, and times of high sediment supply. In combi- efficient petroleum system in the North Sea Basin,
nation, climate and eustasy influenced the nature of whereby extensional fault blocks created traps con-
sedimentation by determining the extent to which taining reservoir-seal pairs that received its petroleum
continental land masses were flooded, the depth of charge from neighboring kitchen areas (Goff, 1983;
water in oceans and the degree of water circulation Burley, 1993).
within and between basins. The variation in global Although the Upper Jurassic sources dominate,
sea level has largely mirrored the climate through there is a recognition from geochemical analyses that
time as exemplified by the glacio-eustatic cyclothems other sources also contribute to some fields. High total
that controlled Carboniferous deposition and Late organic carbon characterizes parts of the Lower Juras-
Cretaceous warming that drove global sea levels to sic (Dunlin and Lias Groups), Permian (Zechstein
their highest levels during the Phanerozoic with the Group), Carboniferous (Dinantian and Namurian)
creation of marine seaways and chalk deposition over Bowland Shale, and the Middle Devonian sedimentary
large areas of the continental interior. sequences. Additional source intervals include the
Middle Jurassic Sleipner and Brora Coal Formations,
the upper Carboniferous (Pennsylvanian) Coal Meas-
THE NORTH SEA RIFT’S PETROLEUM SYSTEM ures Group, the lower Carboniferous (Mississippian)
Scremerston Coal Group of northeastern England,
A petroleum system encompasses all of the essential the West Lothian Oil Shale in the Midland Valley of
elements (source, reservoir, seal, and overburden Scotland (Underhill et al., 2008), and the Middle
rock) processes (trap formation, generation-migra- Devonian lacustrine (fish-bearing) Eday Shale clay-
tion-accumulation) and all genetically related petro- stones of the Orcadian Basin. The latter makes a signif-
leum that originated from one pod of active source icant contribution in western parts of the Moray Firth
rock and occurs in shows, seeps, or accumulations rift, where Upper Jurassic source intervals are too shal-
(Magoon and Dow, 1994). The geographical extent low and cool to be mature, creating a petroleum system
of a petroleum system is commonly delineated by a that charges the Beatrice field and its satellites (Peters
line within which all fields, discoveries, shows, and et al., 1989). The occurrence of the waxy lacustrine
seeps ascribed to a specific source rock occur. The fol- source rocks even necessitated the construction of a
lowing sections describe the key elements of the North dedicated refinery at Nigg to handle the waxy crude
Sea rift’s petroleum system and the controls on its main derived from the Middle Devonian source rocks.
reservoir play fairways. Comparison between the three
rift arms highlights significant differences in the distri- Sedimentary Play Fairways
bution of the prospective reservoirs, the controls on
which we will attempt to explain. Paleozoic and Older Reservoirs
A variety of upper Paleozoic and older rock formations
Source Rocks host petroleum in several parts of the North Sea
(Figure 7), albeit without forming any giant fields in
The key to the success of any petroleum province is the their own right. Their distribution is almost exclusively
occurrence and maturity of an extensive, thick, and confined to upstanding areas lying on the graben flanks
rich source rock. In the North Sea rift, pride of place or in intrabasinal horst blocks, especially those that lie
goes to source horizons within the Kimmeridge Clay within the inner confines of the uplift resulting from
UNDERHILL AND RICHARDSON 587
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