164723621 a sedimentological approach to refining reservoir architecture in a mature hydrocarbon...

7/22/2019 164723621 a Sedimentological Approach to Refining Reservoir Architecture in a Mature Hydrocarbon Province the Brent Province UK North Sea

1/34

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

Marine and Petroleum Geology 21 (2004) 457484

www.elsevier.com/locate/marpetgeo
http://www.elsevier.com/locate/marpetgeohttp://www.elsevier.com/locate/marpetgeo


2/34

G.J. Hampson et al. / Marine and Petroleum Geology 21 (2004) 457484458


3/34

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;

G.J. Hampson et al. / Marine and Petroleum Geology 21 (2004) 457484 459


4/34

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



5/34

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



6/34

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)



7/34bioturbated and upward-coarsening character of the succes- episodic sand deposition from unidirectional currents with

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).



8/34



9/34

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



10/34



11/34



12/34



13/34

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



14/34

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



15/34

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



16/34

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



17/34



18/34

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



19/34



20/34



21/34

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

http://-/?-http://-/?-


22/34

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

http://-/?-http://-/?-http://-/?-http://-/?-


23/34

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

http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-


24/34

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

http://-/?-http://-/?-


25/34

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

http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-


26/34

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



27/34

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

Tomasso, John Underhill (University of Edinburgh), Sarah

Davies, S. J., Dawers, N. H., McLeod, A. E., & Underhill, J. R. (2000). The

structural and sedimentological evolution of early syn-rift successions:

The Middle Jurassic Tarbert Formation, North Sea.Basin Research,12,

343365.

Davies, R. J., Turner, J. D., & Underhill, J. R. (2001). Sequential dip-slip

fault movement during rifting: A new model for the evolution of the

Jurassic trilete North Sea rift system. Petroleum Geoscience, 7,

371388.

Dawers, N. H., & Underhill, J. R. (2000). The role of fault interaction and

linkage in controlling synrift stratigraphic sequences: Late Jurassic,

Statfjord East area, northern North Sea. American Association ofPetroleum Geologists Bulletin, 84, 4564.

Deegan, C. E., & Scull, B. J. (1977). A standard lithostratigraphic

nomenclature for the central and northern North Sea. Report of the

Institute of Geological Sciences, 77/25.

Frseth, R. B. (1996). Interaction of Permo-Triassic and Jurassic

extensional fault-blocks during the development of the northern

North Sea.Journal of the Geological Society of London,153, 931944.

Fjellganger, E., Olsen, T. R., & Rubino, J. L. (1996). Sequence stratigraphy

and palaeogeography of the Middle Jurassic Brent and Vestland deltaic

systems, northern North Sea. Norsk Geologisk Tidsskrift, 76, 75 106.

Flint, S. S., Knight, S., & Tilbrook, A. (1998). Application of high-

resolution sequence stratigraphy to the Northwest Hutton Field,

northern North Sea: Implications for management of a mature Brent

Group field. Bulletin of the American Association of Petroleum

Geologists, 82, 14161436.

Goldring, R., Pollard, J. E., & Taylor, A. M. (1991). Anconichnus

horizontalis: A pervasive ichnofabric-forming trace fossil in post-

Palaeozoic offshore siliciclastic facies Palaios 6 250263



28/34

developments and applied aspects (pp. 249310).Geological Society of

London, Special Publication 18.

Lee, M. J., & Hwang, Y. J. (1993). Tectonic evolution and structural styles

of the East Shetland Basin. In J. R. Parker (Ed.), (pp. 11371149).

Petroleum Geology of Northwest Europe: Proceedings of the fourth

conference.

Livera, S. E. (1989). Facies associations and sand-body geometries in the

Ness Formation of the Brent Group, Brent Field. In M. K. G. Whateley,

& K. T. Pickering (Eds.), Deltas: Sites and traps for fossil fuels (pp.

269286).Geological Society of London, Special Publication 41 .

Livera, S. E., & Caline, B. (1990). The sedimentology of the Brent Group inthe Cormorant Block IV oilfield. Journal of Petroleum Geology, 13,

367396.

MacEachern, J. A., Raychaudhuri, I., & Pemberton, S. G. (1992).

Stratigraphic applications of the Glossifungites ichnofacies: Delineating

discontinuities in the rock record. In S. G. Pemberton (Ed.),

Applications of ichnology to petroleum exploration (pp. 169 198).

Society of Economic Palaeontologists and Mineralogists, Core Work-

shop 17.

McCarthy, P. J., Faccini, U. F., & Plint, A. G. (1999). Evolution of an

ancient coastal plain: Palaeosols, interfluves and alluvial architecture in

a sequence stratigraphic framework, Cenomanian Dunvegan For-mation, NE British Columbia, Canada. Sedimentology, 46, 861891.

McLeod, A. E., Dawers, N. H., & Underhill, J. R. (2000). The propogation

and linkage of normal faults: insights from the Strathspey Brent

Statfjord fault array, northern North Sea.Basin Research,12, 263284.

Mearns, E. W. (1992). Samariumneodymium isotopic constraints on the

provenance of theBrentGroup.In A. C. Morton, R. S. Haszeldine, M. R.

Giles, & S. Brown (Eds.), Geology of the Brent Group (pp. 213 225).

Retallack, G. J. (1990). Soils of the past. London: Unwin Hyman, p. 520.

Reynolds, A. D. (1995). Sedimentology and sequence stratigraphy of the

Thistle Field, northern North Sea. In R. J. Steel, V. L. Felt, J. P.

Johannessen, & C. Mathieu (Eds.), Sequence stratigraphy on the

Northwest European margin (pp. 257271). Norwegian Petroleum

Directorate (NPF), Special Publication 5.

Richards, P. C., & Brown, S. (1986). Shoreface storm deposits in the

Rannoch Formation (Middle Jurassic), North West Hutton oilfield.

Scottish Journal of Geology, 22, 367375.

Roberts, A. M., Kusznir, N. J., Walker, I. M., & Dorn-Lopez, D. (1995).

Quantitative analysis of Triassic extension in the northern VikingGraben.Journal of the Geological Society of London, 152, 1526.

Rnning, K., & Steel, R. J. (1987). Depositional sequences within a

transgressive Reservoir sandstone unit: The Middle Jurassic Tarbert

Formation, Hild Area, northern North Sea. In J. Kleppe, E. W. Berg,

A. T. Buller, O. Hjelmeland, & O. Torstaeter (Eds.),North Sea oil and

gas reservoirs (pp. 169176). London: Graham and Trotman.

Sawyer, M. J., & Keegan, J. B. (1996). Use of palynofacies characterization

in sand-dominated sequences, Brent Group, Ninian Field, UK North

Sea. Petroleum Geoscience, 2, 289297.

Scott, E. S. (1992). The palaeoenvironments and dynamics of the

RannochEtive nearshore and coastal succession, Brent Group,northern North Sea. In A. C. Morton, R. S. Haszeldine, M. R. Giles,

& S. Brown (Eds.), Geology of the Brent Group (pp. 129147).

Geological Society of London, Special Publication 61.

Simon Petroleum Technology Ltd (1994). The Brent DeltaAn integrated

sedimentological and palynological study of the Brent Group between

latitudes 598N and 628N, UK and Norwegian sectors, North Sea.

Unpublished report for Shell UK Expro.



29/34

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


30/34

Fig. 11B (continued)


31/34

Fig. 11C (continued)


32/34

Fig. 11D (continued)


33/34

Fig. 11E (continued)


34/34

Fig. 11F (continued)

164723621 a sedimentological approach to refining reservoir architecture in a mature hydrocarbon...

Documents