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A sedimentological approach to refining reservoir architecture in a maturehydrocarbon province: the Brent Province, UK North Sea
Gary J. Hampson*, Peter J. Sixsmith, Howard D. Johnson
Department of Earth Science and Engineering, Imperial College, Prince Consort Road, London SW7 2BP, UK
Received 5 April 2003; received in revised form 26 July 2003; accepted 29 July 2003
Abstract
Improved reservoir characterisation in the mature Brent Province of the North Sea, aimed at maximising both in-field and near-field
hydrocarbon potential, requires a clearer understanding of sub-seismic stratigraphy and facies distributions. In this context, we present
a regional, high-resolution sequence stratigraphic framework for the Brent Group, UK North Sea based on extensive sedimentological
re-interpretation of core and wireline-log data, combined with palynostratigraphy and published literature. This framework is used to
place individual reservoirs in an appropriate regional context, thus resulting in the identification of subtle sedimentological and
t t t ti hi f t f i hit t th t h b i l l k d W h i th f ll i i i ht
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reservoir architecture and their links to the regional, Middle
Jurassic structural evolution of the northern North Sea.
2. Geological setting
2.1. Structural evolution of the northern North Sea
The northern North Sea has undergone a complexstructural evolution that encompasses three discrete episodes
of rifting separated by periods of tectonic quiescence
(Fig. 2A;Lee & Hwang, 1993; Rattey & Heyward, 1993).
The first rifting episode occurred as part of the Caledonian
Orogeny during the Devonian, and resulted in the generation
of NESW trending structures that are exemplified by the
Tern Eider Horst feature (Fig. 2A and B; Johnson &
Dingwall, 1981; Lee & Hwang, 1993). The second rifting
episode occurred duringthe Triassic (Fig.2AandB; Frseth,1996; Lee & Hwang, 1993; Roberts, Kusznir, Walker, &
Dorn-Lopez, 1995), and has previously been difficult to
interpret because of overprinting by late Jurassic structures.
However, recent interpretation of high-quality 3D seismic
volumes suggests that this rifting episode produced a seriesof
half-grabens bounded by NNESSW trending, west-facing
normal faults (John Underhill, pers. comm.). Although the
spatial development of the Triassic basin is broadly
coincident with later Jurassic depocentres in the northern
North Sea (Lee & Hwang, 1993; Rattey & Heyward, 1993;
Roberts et al., 1995), there are some significant differences in
faultorientation and spacing between the two rifting episodes
(Fig.2B and C; Frseth, 1996; John Underhill, pers. comm.).
The Brent Group was deposited during the phase of post-
rift subsidence that followed Triassic rifting (Fig. 2A). Brent
Group deposition is coincident with thermal doming above a
transient mantle plume in the central North Sea and
subsequent deflation of this dome as the thermal anomaly
produced by the mantle plume dissipated (Fig. 2A; Underhill
& Partington, 1993). It has been suggested that deflation of
the thermal dome introduced the regional tensional regime
that caused development of the trilete North Sea rift basin in
the late Jurassic (Davies, Turner, & Underhill, 2001). In thenorthern North Sea, late Jurassic rifting produced a series of
half-grabens bounded by NS trending normal faults (Fig.
2A and C; Davies et al., 2001). Detailed interpretation of
these fault systems suggest that they were initiated during
deposition of the youngest Brent Group strata, but they grew
and linked predominantly during post-Brent times (Fig. 2B;
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Davies, Dawers, McLeod, & Underhill, 2000; Dawers &
Underhill, 2000; McLeod, Dawers, & Underhill, 2000).
A good understanding of the complex structural evolution
outlined above (Fig.2) is critical in determining thestructural
setting of the northern North Sea during deposition of the
Brent Group in the Middle Jurassic. Most importantly, many
of thelate Jurassic extensional faultsthat define the structural
traps of all current Brent Group reservoirs (Fig. 2C) were not
present during Brent Group deposition (Dawers & Underhill,2000; McLeod et al., 2000). Instead, Brent Group deposition
was predominantly influenced by passive differential sub-
sidence across older, basement structures and active exten-
sion acrossa small number of pre-existing fault systems (Fig.
2B). The latter may have formed in response to thermal
doming and relaxation during the Lower and Middle Jurassic
(Underhill & Partington, 1993). Latest Brent Group depo-
sition may have been influenced locally by initiation of the
fault arrays that grew and linked into major rift structures
during the late Jurassic, particularly along the southern part
of the North Alwyn Brent-Statfjord fault system (Fig. 2B;
Davies et al., 2000; McLeod et al., 2000). Key structures
controlling Brent Group deposition include: (1) the Tern
Eider Horst; a basement-involved Caledonian trend whose
northwestern boundary is marked by Triassic rift faults and
and Tarbert Formations (Fig. 3;Deegan & Scull, 1977). The
Broom Formation and its Norwegian equivalent, the Oseberg
Formation, comprise coarse-grained shallow marine deposits
that are now widely considered to be genetically unrelated to
the overlying Formations (Fig. 3; Fjellganger et al., 1996;
Helland-Hansen et al., 1992; Mitchener et al., 1992). The
upper four Formations record a major regressivetransgres-
sive episode in which the Rannoch, Etive and Lower Ness
Formations represent overall regression of a wave-dominateddeltaic coastline and the Upper Ness and Tarbert Formations
record subsequent transgression (e.g. Brown, Richards, &
Thomson, 1987). More detailedinterpretationsof the regional
Brent Group succession have recognised a variablenumber of
low-frequency (1 10 Ma) stratigraphic cycles that are super-
imposed on the major regressivetransgressive episode (e.g.
Fjellganger et al., 1996; Graue et al., 1987; Helland-Hansen
et al., 1992; Mitchener et al., 1992).
2.3. Sediment provenance
Heavy mineral suites and isotopic data both suggest that
the Brent Group was derived from multiple provenance
areas (Hamilton, Fallick, & MacIntyre, 1987; Mearns, 1992;
Mitchener et al., 1992; Morton, 1992). These data indicate
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present a brief summary of previous facies analyses,
combined with our own observations. The resulting facies
association scheme is summarised inTable 2, and forms the
basis for recognition of facies discontinuities across key
sequence stratigraphic surfaces. The robust and consistent
recognition of such surfaces in cores and wireline logs is
essential for high-resolution (i.e. sub-seismic) application of
sequence stratigraphic methods.
4.1. Facies associations
A number of facies associations have been identified in
core, based on lithology, primary sedimentary structures,
bioturbation fabric and the nature of bedding contacts with
underlying and overlying units (Table 2). Facies associ-
ations represent a variety of shallow marine, marginal
marine and non-marine environments that may be classified
into three broad groups (Table 2): (1) weakly wave-influenced shallow marine, (2) wave-dominated shallow
marine and marginal marine, and (3) lagoonal and non-
marine. The last two groups of facies associations occurwithin the Rannoch, Etive and Ness Formations and can be
readily accommodated in the widely used facies model
developed for the Brent Group by Budding and Inglin
(1981) Fi 5) Th f i i i h b
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Table 2
Summary sedimentology of facies associations in the Brent Group
Lithofacies association Lithology and sedimentary structures Bioturbation
1. Weakly wave-influenced shallow marine facies associations (Broom Formation, Tarbert Formation)
1.1. Offshore transition (OT) Mudstone and siltstone with rare (,25%) laminae
and beds of very fine- to fine-grained sandstone.
Parallel lamination, wave and current ripple
cross-lamination
Moderate to intense (BI 35; Planolites,
Terebellina, Paleophycus, Rosselia,
Thalassinoides , Teichichnus, Asterosoma,
mud-filledArenicolites)1.2. Distal tide-influenced sheet
sandstone (dTSS)
Micaceous, lower fine-grained to lower medium-
grained silty sandstones (80100%). Intense
bioturbation almost completely obscures primary
physical structures, but silty and micaceous laminations
at 1 5 cm spacing record original bed tops.
Rare current ripples and cross-beds. Rare marine
body fossils (e.g. belemnites)
High to complete (BI 46: Anconichnus,
Thalassinoides , Arenicolites, Skolithos, Planolites,
Palaeophycus, Ophiomorpha, Teichichnus,
Cylindrichnus). Teichichnus-Anchonichnus
ichnofabric ofGoldring et al. (1991).
1.3. Proximal tide-influenced sheet
sandstone (pTSS)
Micaceous, lower fine-grained to lower medium-
grained silty sandstones (100%). Trough and
tabular cross-beds with intensely bioturbated
bed tops. Bimodal grain-size distribution and claydrapes in some cross-beds. Rare planar lamination
and current ripples. Abundant drifted plant material
Moderate to intense (BI 35; Thalassinoides,
Arenicolites, Palaeophycus, Cylindrichnus,
Ophiomorpha)
1.4. Tide-influenced channel-fill
sandstone (TCS)
Well-sorted to moderately sorted lower medium- to
coarse-grained sandstones (100%) in erosively based
bodies 220 m thick. Trough and tabular cross-beds
with sparsely bioturbated bed tops. Bimodal grain-size
distribution and clay drapes in some cross-beds
Absent to low (BI 02; Ophiomorpha,
Thalassinoides , Arenicolites)
2 Wave dominated shallow marine and marginal marine facies associations (Rannoch Formation Etive Formation Tarbert Formation)
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Table 2 (continued)
Lithofacies association Lithology and sedimentary structures Bioturbation
3.2. Fluvial/wave-influenced delta
(lagoonal shoreface, LS)
Upward-coarsening succession of very fine to
lower coarse-grained sandstones (50100%) with
mudstone and siltstone interbeds in its lower part.
Sandstone beds thicken upwards. Wave and current
ripple cross-lamination, low-angle cross-lamination,
climbing current ripples, planar-parallel lamination
and cross-beds in sandstone beds
Generally sparse to moderate (BI 13;
Skolithos, Thalassinoides, Arenicolites,
Teichichnus)
3.3. Fluvial channel-fill (F) Poorly to moderately sorted, very fine- to coarse-
grained sandstone (80100%) in erosively based
bodies 515 m thick. Amalgamation into thick
(1550 m) multistorey channel-fill complex.
Cross-beds and current ripples
Generally absent (BI 0), but locally
sparse to moderate (BI 13; Taenidium,
Planolites, mud-filled Arenicolites)
3.4. Aggradational floodplain (AF) Mudstone and siltstone with very fine- to medium-
grained sandstone laminae and beds 1200 cm
thick (1080%). Cross-beds, current ripples,
dewatering structures and soft-sediment folds.
Isolated roots and/or pedogenic features arepervasive
Absent to moderate (BI 03; Planolites,
mud-filled Arenicolites)
3.5. Coal Coal and carbonaceous shale Absent (BI 0)
Bioturbation is described using the bioturbation index (BI) scheme ofTaylor and Goldring (1993).
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to pass down-dip into, regressive tide-influenced sheet
sandstones. We refer to these successions as tide-influenced
channel-fill sandstones.
4.3. Sequence boundaries and forced regressive deposits
Major, low-frequency sequence boundaries within the
Brent Group are interpreted at the base of the Broom
Formation (SB100) and at, or near, the base of the TarbertFormation (SB1000; Figs. 6AC and 7; Mitchener et al.,
1992; Morton, 1992). Both surfaces are characterised by an
abrupt influx of coarse-grained, extrabasinal material from
the basin flanks (Mitchener et al., 1992; Morton, 1992), and
biostratigraphic data imply a significant time gap (.1 Ma)
across both surfaces over much of the East Shetland Basin.
The base-Broom sequence boundary (SB100) records a
major basinward shift (.100 km) in shallow marine
sedimentation, and is interpreted to overlie a regionallyextensive angular unconformity produced by thermal
doming in the central North Sea (Underhill & Partington,
1993). The origin of the base-Tarbert sequence boundary
(SB1000) is more cryptic and its regional extent and
morphology are described later. Both sequence boundaries
are overlain by depositional systems composed of the
kl i fl d h ll i f i i i
SB700;Fig. 7). In cores and wireline logs, each candidate
valley fill is characterised by the stacking of single-storey
channel-fill bodies into a considerably thicker (ca. 30 m),
multistorey body, which commonly has a distinctive
internal facies architecture that is interpreted to reflect
increasing accommodation space during valley filling. For
example, candidate valley fills in the Etive and Ness
Formations generally have a lower, fluvial component
(facies 3.3; Table 2) and an upper, tide-influencedcomponent (facies 2.5; Table 2). Candidate valley fills
generally lack an abrupt basinward shift in facies across
their bases, but in areas of dense well-spacing the candidate
valleys can be demonstrated to be laterally discontinuous
and deeply erosive (e.g. in the Cormorant Field;Jennette &
Riley, 1996). In some cases, the valleys erode through
underlying stratigraphic markers, such as field-wide coal
seams. Several candidate valley fills also have a basal lag of
anomalously coarse-grained, extrabasinal sand and pebbles(Fig. 8A). A number of candidate valley fills in the Etive
Formation comprise stacked, tidal inlet/estuarine channel-
fill (facies 2.5;Table 2) and barrier sandstones (facies 2.4;
Table 2) above a coarse-grained, extrabasinal lag. In these
cases, the basal lag is the only feature that allows a valley-
fill origin to be interpreted, rather than an unconfined
d i l b i A ll fill i i f
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Fig. 8. Photographs showing sedimentological aspects of sequence boundaries in the Ness Formation. (A) Basal part of a multistorey, fluvial channel-fill
sandstone in the Upper Ness Formation (9708
0
in 3/4-12;Fig. 7A), containing granule-sized, lithic extraclasts (labelled e) and mudstone intraclasts (labelledi). This sandstone is interpreted to overlie a sequence boundary (SB600). (B) Carbonaceous root traces overprinting floodplain deposits (97410 in 3/4-12;Fig.
7A). The roots are interpreted as part of a poorly developed gley palaeosol, which is typical of floodplain deposits within the Ness Formation. (C) Distinctive
palaeosol in the Upper Ness Formation (97350 in 3/4-12;Fig. 7A), which comprises mottled red-brown siltstone containing green-coloured root traces (labelled
r) and calcite-filled, post-depositional fractures (labelled c) around rhizoconcretions. The high degree of mottling and abundance of rhizoconcretions
implies prolonged soil formation, while the pervasive red-brown colour and scarcity of carbonaceous material suggests soil development under oxidising
conditions above the water table. In the context of the coal-prone Ness Formation, this palaeosol records anomalous groundwater drainage conditions and is
interpreted to mark a sequence boundary (SB600). (D) Another distinctive palaeosol, containing abundant red siderite rhizoconcretions and rare carbonaceous
root traces overprinting pale grey sitstones, in the Upper Ness Formation (9509 0 in 3/4-9). This palaeosol is also interpreted to record pedogenesis under
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seam. These upward-fining intervals commonly contain
sharp-based, well-sorted, medium-grained sandstones con-
taining a limited assemblage of trace fossils (Ophiomorpha,
Thalassinoides, Skolithos, Arenicolites, Palaeophycus;
Fig. 9A C). The textural maturity of the sandstones implies
that they have been reworked from a shoreface or barrier,
while the trace fossil assemblage has a marginal marine
affilinity. Several of the sandstones have unlined Thalassi-
noides burrows at their base, which are interpreted toconstitute a firmground,Glossifungitesichnofacies (MacEa-
chern, Raychaudhuri, & Pemberton, 1992;Fig. 9A C). This
interpretation implies that the surfaces were exhumed by
transgressive erosion. In combination, the features
described above suggest that barrier sands were reworked
during transgression, either in large washover fan systems or
as a result of barrier breaching and destruction during its
rapid retreat. The fining-upward intervals that contain these
sandstones represent transgressive deposition that culmi-
nated in development of a lagoonal flooding surface.
The development of deep (up to 20 m) tidal channel
fills (facies 2.5; Table 2) is associated with several
flooding surfaces in the uppermost Ness and lowermost
Tarbert Formations. These deep channels overlie trans-
gressive erosion surfaces and erode deeply into underlying
lagoonal deposits (Fig. 9E). Flooding surfaces within the
Etive Formation are more difficult to identify, particularlyin successions dominated by channel-fill sandstones that
erode underlying strata. However, several flooding
surfaces juxtapose the deposits of barrier systems (facies
2.42.6; Table 2) above coal seams (facies 3.5; Table 2)
or rooted horizons that in turn overlie older barrier system
deposits (facies 2.42.6; Table 2). In the northern part of
the study area, the Etive Formation comprises several
stacked barrier systems each bounded by such flooding
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surfaces (e.g. Fig. 7D; Brown & Richards, 1989;
Reynolds, 1995). These successions imply that the
EtiveNess system was largely aggradational.
4.5. Core to wireline-log calibration
Widespread use of the facies scheme (Table 2) and
sequence stratigraphic interpretations described above relies
on the accurate calibration of core and wireline-log data.Several facies associations possess a distinctive wireline-log
character, particularly the lagoonal and coastal plain facies
associations of the Ness Formation, which generally display
pronounced variations in lithology and consequent wireline-
log response (e.g. Bryant & Livera, 1991; Livera, 1989).
However, differentiating fluvial channel-fill sandstones
(facies 3.3; Table 2) and estuarine channel-fill sandstones
(facies 2.5;Table 2), which most likely occur within incised
valleys, is not possible from wireline-log data alone, butrequires core interpretation (e.g. Flint et al., 1998). The
various sand-dominated, shallow marine facies of the
Broom, Rannoch, Etive and Tarbert Formations possess
less distinctive wireline-log characteristics. Concentrations
of heavy minerals are diagnostic of wave-dominated
foreshore deposits (facies 2.3; Table 2), and thus sand-
spikes (e.g. at 107680 in well 211/19-6;Fig. 7D) are likely
to comprise wave-dominated facies (facies 2.12.6; Table
2). Tidal inlet/estuarine and fluvial channel-fill deposits
(facies 2.5 and 3.3, respectively; Table 2) in the Etive
Formation are difficult to distinguish, because each is
characterised by abrupt increases in sand content, grain size
and porosity across their bases. In our experience, detailed
interpretation of the Broom, Rannoch and Tarbert For-
mations in uncored wells must rely heavily on calibrationwith nearby cored wells. Inter-well correlation using key
sequence stratigraphic surfaces is essential to such cali-
bration, because this methodology provides a framework in
which core-based facies trends may be extrapolated using
appropriate depositional models.
5. Regional high-resolution sequence stratigraphic
framework
The key surfaces described above, and the units that
they bound, have been correlated to construct a regional,
high-resolution sequence stratigraphic framework. Corre-
lation between the wells selected for this study (Fig. 4)
was constrained by a regional, palynologically based
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Fig. 10. Summary of the high-resolution sequence stratigraphic framework presented in this paper. Absolute ages and the regional North Sea stratigraphic
scheme (3rd order cycles) are after Mitchener et al. (1992) and Rattey and Heyward (1993) . Low-frequency sequence boundaries occur at the base of the
Broom and Tarbert formations (SB100 and SB1000, respectively). Seven higher frequency sequence boundaries occur in the RannochEtiveNess interval
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Their distribution and thickness variations imply that
sediment was transported from the southwest of the study
area and along the southern part of the NinianHutton
Dunlin fault hangingwall (e.g. Fig. 13A and B). Isopach
maps and palaeogeographic reconstructions also suggest
that there was a third sediment transport route along the
northern margin of the TernEider Horst, which acted as
an intra-basin high (Figs. 12A and 13A and B). An
unusually thick succession is observed locally in the area
of the Cormorant Block IV Field, along the eastern margin
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a multistorey channel-fill body with a lower, fluvial
component (facies 3.3; Table 2) and an upper, tide-
influenced component (facies 2.5;Table 2). These features
imply an incised valley-fill origin. Etive valleys are
interpreted to incise into the deposits of an unconfined
aggradational barrier system, which lack a basal extra-
basinal lag. We interpret the initiation of Etive fluvial
incision to have been coincident with progradation of the
sharp-based Rannoch shoreface in the northern part of thestudy area, with Etive valleys acting as conduits for
sediment bypass to the forced-regressive Rannoch shoreface
(Fig. 13D). The widespread extent of Etive valley-fill
systems implies a prolonged period of incision and valley-
widening, and possibly several discrete episodes of incision.
The lack of lateral facies variability in the RannochEtive
succession (Figs. 11, 13C and D) makes it difficult to
interpret sediment transport routes and shoreline prograda-
tion directions, but studies in individual fields imply atributary system of westeast-trending Etive valleys (Jenn-
ette & Riley, 1996; Livera, 1989). The Rannoch Etive
succession also exhibits gradual thickness variations across
the study area, thickening abruptly only in the hangingwall
of the NinianHuttonDunlin fault system (Fig. 12B).
5 3 FS300 FS400 (L N F i )
3/4-12,Fig. 7A). Two incised valley systems are interpreted
within the Lower Ness Formation (SB350, SB400; Fig. 10),
but it is difficult to reconstruct their palaeogeography. In both
cases, west east-trending valleys are documented in the
Tern Field, on the TernEider Horst (Jennette & Riley,
1996), and east of the NinianHuttonDunlin fault system
(e.g. Livera, 1989; Fig. 13F). It appears likely that thedominant sediment transport routes were along the northern
margin of the TernEider Horst and from southwest of thehangingwall of the Ninian Hutton Dunlin fault system
(Fig. 13F). Southward-retreating transgressive shorelines
(e.g. FS300 inFig. 13E, FS350 inFig. 13G) may result from
the abandonment of the northerly sediment transport route,
whereas westward-retreating transgressive shorelines (e.g.
FS400 in Fig. 13H) may reflect abandonment of both
sediment transport routes. Differential subsidence across
the Ninian HuttonDunlin fault system appears to have no
significant influence on any of the palaeogeographiesdescribed above (Fig. 13E H).
5.4. FS400FS500 (Middle Ness Formation)
The Middle Ness Formation is bounded at its base by the
base-Mid-NessShale flooding surface (FS400; Fig. 10) and
i b i fl di f b h Mid
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and fluvial channel-fill deposits (facies 3.3,3.4;Table 2) in
the southern part of the study area. Consequently, we
interpret these strata to be the most proximal part of the
Brent Group succession. They contain a high abundance of
fluvial channel-fill sandstones (facies 3.3,3.4;Table 2) that
can be subdivided into three stratigraphically discrete,
laterally extensive, multistorey channel-belt deposits in the
hangingwall of the Ninian HuttonDunlin fault system.
Each of these channel-belt deposits is tentatively interpretedto overlie a sequence boundary (SB550, SB600, SB700;
Figs. 10 and 11E). It is not clear from our correlations
whether these channel belts are confined to incised valleys,
although each appears to locally erode out lagoonal
mudstones and minor flooding surfaces (Fig. 11E). Each
channel belt trends west east (e.g. Fig. 13J and K),
implying that sediment was dominantly transported from
the southwest of the study area and was not influenced by
differential subsidence across the NinianHuttonDunlinfault system.
The upper part of the Upper Ness Formation (FS800
SB1000;Fig. 10) is dominated by lagoonal deposits (facies
3.1,3.2; Table 2) in the southern part of the study area and a
southward-retreating, wave-dominated shoreface and bar-
rier system (facies 2.1 2.6;Table 2) in the northern part of
h d Fl di f FS800 i i d i h
wells 211/23-2, 211/23-DA27, 211/19-1, 211/19-5, 211/19-
3 and 211/19-6 inFig. 11F). Using 3D seismic data,Davies
et al. (2000) and McLeod et al. (2000) have interpreted
similar stratigraphic relationships in the Upper Brent Group
(above flooding surface FS800) as the result of small
depocentres and intra-basinal highs created by the initiation
of major rift fault arrays (e.g. the North AlwynBrent
Statfjord fault system inFig. 2;McLeod et al., 2000). Our
interpretations of local angular stratigraphic relationshipsare consistent with this model.
5.6. SB1000 FS1200 (Tarbert Formation)
The base of the Tarbert Formation is interpreted as a
major sequence boundary (SB1000; Figs. 10 and 11)
marked by an abrupt influx of coarse-grained, extrabasinal
material (Mitchener et al., 1992; Morton, 1992) and a
significant time gap (.1 Ma). There is also an abruptchange in facies character across the surface, from lagoonal
and wave-dominated shoreface and barrier deposits (facies
2.12.6;Table 2) to tide-influenced channel-fill and sheet
sandstones (facies 1.2 1.4; Table 2). The top of the
formation is defined by the transition into overlying offshore
Heather Shales (Fig. 10). This transition is diachronous, and
i hi h d h f h T b F i
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211/19-6 inFig. 7B and D). Upward-coarsening successions
are thicker and more common in the south-western part of
the study area (e.g. wells 2/5-17, 2/5-3, 3/1-1, 3/2-3 and 3/2-
4 inFigs. 7B and 11A) and in the immediate hangingwall of
the NinianHuttonDunlin fault system (e.g. well 3/3-8 in
Fig. 11A,well 211/18-5 in Fig. 11B, and well 211/24-5 in
Fig. 11C), as reflected in isopach maps of the lower part of
the Tarbert Formation (Fig. 12F). These retrogradationally
stacked successions record net transgression of the Tarbertdepositional system, culminating in flooding surface
FS1050 and the deposition of offshore shales over the
western and northern part of the study area (Fig. 11). Their
distribution and thickness variations imply that sediment
was transported from the southwest of the study area and
along the southern part of the NinianHuttonDunlin fault
hangingwall (Figs. 12F and 13O). Isopach maps and
palaeogeographic reconstructions suggest that there was a
third sediment transport route along the northern margin ofthe TernEider Horst (Fig. 13O) and a local depocentre in
the area of the Cormorant Block IV Field, along the
southern margin of the Tern Eider Horst (e.g. wells 211/21-
5 inFigs. 11C and 12F).
In the hangingwall of the southern part of the Ninian
HuttonDunlin fault system (e.g. wells 3/3-8, 3/4-8, 3/9-4,
3/10 1 3/4 12 d 3/4 9 i Fi 11A d E) fl di
boundary SB1200 and flooding surface FS1200 (e.g. well
210/20-1, 210/20-2 and 211/16-6 in Fig. 11D). Such
ironstones require extended physical reworking in areas of
clastic sediment starvation (Young, 1989) and have been
documented at similar sediment-starved sequence bound-
aries that underwent subsequent transgressive reworking
(e.g.Taylor, Simo, Yokum, & Leckie, 2002).
6. Discussion: the added value of an integrated regional-
to reservoir-scale approach
The high-resolution sequence stratigraphic framework
summarised above integrates core-scale sedimentology,
reservoir-scale facies architecture and regional stratigraphy.
The extensive use of core data is important, because it
allows sedimentological and sequence stratigraphic
interpretations to be constructed from first principles.The integrated regional- to reservoir-scale approach aids
identification of subtle intra- and inter-reservoir features,
which are not evident via the detailed study of individualreservoirs in isolation. This approach is particularly
valuable in a mature hydrocarbon province, such as the
UK Brent Province, because it generates new insights,
d d l h ill lik l ib i d
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the Broom and Tarbert Formations are not explained by
these models. Instead, these two reservoir intervals have
been described only within individual fields, resulting in
apparently contrasting interpretations that have limited
predictive value outside of a specific reservoir. For
example, the Tarbert Formation has been variously
interpreted as a tidal valley-fill sandbody (Jennette &
Riley, 1996 in the Eider Field; Flint et al., 1998 in the
Northwest Hutton Field), a series of retrogradationallystacked barrier sandstones and lagoonal mudstones (Rn-
ning & Steel, 1987 in the North Alwyn, Alwyn and Hild
Fields) that locally contains a valley-fill sandbody
(Bruaset, Batevik, Jakobsen, & Helland-Hansen, 1999 in
the Gullfaks Field; Davies et al., 2000 in the Snorre and
Tordis Fields), and as a complex series of wave- and tide-
dominated sandstones (Reynolds, 1995 in the Thistle
Field). Although each of these interpretations is essentially
correct for a specific reservoir, our work indicates that theyapply to depositional systems developed in different
stratigraphic intervals. For example, the Tarbert Formation
comprises an estuarine channel-fill sandbody developed
within a wave-dominated barrier-shoreface system and
deposited below flooding surface FS850 in the Eider Field
(e.g. 89800 90230 in well 211/16-6,Figs. 7C and 11D), and
i f k d id i fl d h d
the locally restricted, upper part of the formation (FS1050
FS1200; Fig. 10) comprises two cycles of regression and
subsequent transgression of the same depositional system
(Figs. 11A, E and 13P). Retrogradationally stacked, wave-
dominated barrier and lagoonal deposits that are genetically
affiliated to the Ness Formation (FS800 SB1000;Fig. 10)
and that underlie the regional base-Tarbert sequence
boundary (SB1000; Fig. 10) are allocated to the Tarbert
Formation in some fields on the basis of lithostratigraphy.Such a lithostratigraphic approach to regional correlation
produces erroneous palaeogeographic reconstructions that
are difficult to reconcile with current sedimentological
models of wave- and tide-influenced depositional systems.
6.3. Tectono-stratigraphic controls on reservoir
architecture
The tectono-stratigraphic evolution of the Brent Provinceexerts a significant, but underappreciated, influence on
reservoir architecture. We interpret a threefold hierarchy of
tectono-stratigraphic controls, outlined below.
At the first-order scale, the interplay between two
variables exerted a fundamental control on palaeo-geomor-
phology, sediment transport routes and sedimentary process
i (1) di l d (2) h
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relative to differential accommodation generation, was
characterised by the regressive to aggradational Rannoch,
Etive and Ness Formations (FS200 SB1000; Fig. 10).
Despite significant differential subsidence across the
NinianHuttonDunlin fault system and the TernEider
Horst (Fig. 12B E), these features did not strongly influence
palaeogeographic trends and sediment supply routes during
these times (Fig. 13CN). Thus, we infer that sediment
supply was sufficient to continuously infill the differentialaccommodation created across these structures, so that they
had no surface, geomorphological expression. The wave-
dominated character of RannochEtive shoreline systems
reflects deposition in an unconfined, open basin with large
wave fetch. This facies character is also consistent with the
subdued surface expression of underlying structural features.
The thick (.100 m), widespread character of the Rannoch
Etive Ness succession (FS200 SB1000) implies that
regional tectonic subsidence across the entire Brent Provincewas more rapid than during deposition of the underlying
Broom Formation (SB100 FS200), which represents a
similar timespan (6 8 Ma,Fig. 10).
At the second-order scale, the key sequence strati-
graphic surfaces identified in our study (Fig. 10) define
unconformity-bounded sequences that extend across the
UK B P i (Fi 11 13) Wi h h i f
the base-Tarbert sequence boundary (up to 5 Ma;Fig. 10),
its angular character and the influx of extrabasinal material
above it are consistent with a phase of renewed thermal
doming. Given the limited current knowledge of the
regional distribution of this surface outside our study
area, we regard its origin as enigmatic.
At the third-order scale, we interpret a number of small
(,10 km wide), short-lived and localised depocentres. A
series of such depocentres formed along the eastern marginof the TernEider Horst at various stages of Brent Group
deposition (Fig. 12A, D and F). We tentatively interpret
these depocentres to have resulted from episodic movement
of the basement-involved Tern Eider Horst, perhaps in
response to intra-plate stresses associated with thermal
doming and relaxation near the central North Sea triple
junction. The depocentres may have been fault bounded. We
also interpret several local angular stratigraphic relation-
ships in the upper Brent group (FS850FS1200; Fig. 10;e.g. between wells 211/16-6, 211/18-19 and 211/19-6 inFig.
11D,and between wells 211/23-2, 211/23-DA27, 211/19-1,
211/19-5, 211/19-3 and 211/19-6 in Fig. 11F) that are
consistent with models of small (,5 km wide) depocentres
and highs developed in response to rift initiation (Davies
et al., 2000; McLeod et al., 2000). Individual depocentres
t d b th d l t b l d i d t il i
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this pinchout is poorly constrained by our interpretations.
Delineation of these trends is likely to require detailed
stratigraphic interpretation and/or attribute analysis of 3D
seismic data volumes north of the TernEider Horst (Figs. 2
and 4). (3) Localised depocentres filled by the shallow-
marine sandstones of the uppermost Ness and Tarbert
Formations (FS850FS1200;Fig. 10) may provide explora-
tion targets. The location of such depocentres is not
predictable from late Jurassic structure maps (e.g. Fig.2C), but will require detailed and careful tectono-strati-
graphic analysis of 3D seismic data volumes (e.g. Davies
et al., 2000; McLeod et al., 2000).
6.5. Applications to improved in-field recovery
The insights discussed above also have applications to
improved understanding of facies architecture in producing
reservoirs, and thus, via input to reservoir models, topredicting the distribution of remaining oil in place. We
highlight three approaches through which reservoir facies
architecture may be improved. (1) The framework discussed
above may provide a context for improved temporal and
spatial resolution of depositional trends within some
reservoirs, thus leading to refinement of reservoir zonation
d i f i d F l i l d b
initiation. These relationships may be tested using 3D
seismic data, where sufficient vertical resolution is possible,
and focused biostratigraphic analysis.
7. Conclusions
Using an extensive core and wireline-log dataset,
integrated with palynostratigraphy and published literature,we have constructed a high-resolution sequence strati-
graphic framework for the UK Brent Province. This
framework allows temporal and spatial trends in regional
deposition to be interpreted at a higher resolution than
previously possible. The resulting high-resolution interpret-
ations of reservoir distribution are consistent both within
and between established fields.
We interpret a hierarchy of basinwide, unconformity-
bounded sequences within the Brent Group. Regionallyextensive, low-frequency sequence boundaries occur at the
base of the Broom Formation and at, or near to, the base of
the Tarbert Formation. Both sequence boundaries are
associated with regional angular truncation, significant
missing time (up to ca. 5 Ma) and an influx of extrabasinal
material and they are interpreted to be tectonically driven
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changes and angular stratal relationships within several
higher frequency sequences.
The insights gained from an integrated regional- to
reservoir-scale approach may contribute to the identification
of near-field exploration potential and to improved in-field
recovery. The former is achieved via the recognition of
various stratigraphic trapping mechanisms. The latter
involves using the regional sedimentological and tectono-
stratigraphic context to constrain intra-reservoir deposi-tional trends and the choice of analogue datasets in reservoir
model construction, therefore reducing uncertainty in
reservoir characterisation.
Acknowledgements
This work has been funded by Shell UK Expro, and has
benefited from numerous discussions with Shell andExxonMobil geoscientists, in particular Frances Abbotts,
Janet Almond, Duncan Erratt, Nick Hogg, Winfrield
Leopoldt, John Marshall, David Taylor and Steve Taylor.
Additional discussions with Huw Williams, Paul Davies
(Reservoir Geology Consultants), Aileen McLeod, Mark
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Fig. 11. Correlation panels across the East Shetland Basin (see Fig. 4for location): (A) West East panel through the Heather, Lyell, Ninian and North Alwyn Fields; (B) West East panel through the South Cormorant, Northwest Hutton, Hutton and Brent Fields; (C) WestEast panel through the Tern, North
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Fig. 11B (continued)
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Fig. 11C (continued)
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Fig. 11D (continued)
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Fig. 11E (continued)
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Fig. 11F (continued)
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