UNIVERSITÀ DEGLI STUDI DI PADOVA

FACOLTÀ DI SCIENZE MM. FF. NN.

Dipartimento di Geoscienze

Direttore Prof.sa Cristina Stefani

TESI DI LAUREA MAGISTRALE

IN

GEOLOGIA E GEOLOGIA TECNICA

HYDRODYNAMIC MODEL OF THE TROLL

FIELD RESERVOIR

Relatore: Prof. Massimiliano ZattinCorrelatore: Dott.re Domenico Grigo

Laureando: Giacomo Mangano

ANNO ACCADEMICO 2013 / 2014

2

Summmary

Riassunto

Abstract

1. Geographical setting 9

2. Structural evolution

2.1. Paleozoic 11

2.2. Mesozoic 13

2.3. Cenozoic 17

3. Stratigraphy

3.1. Paleozoic 20

3.2. Mesozoic 21

3.3. Cenozoic 26

4. The Sognefjord Formation 28

5. Data collection 38

6. Depositional model and interpretation

6.1. Depositional model and

Palaeogeographical setting of

the Sognefjord Formation

51

3

6.2. Interpretation

55

7. Conclusions

61

References 63

4

Riassunto

Il principale scopo di questo lavoro è stato quello di ricostruire il modello

idrodinamico del Troll Field Reservoir, integrando il modello deposizionale e

strutturale con dati di pressione dei fluidi di formazione.

Il Troll Field, localizzato all’interno della Horda Platform, nel settore centro-

settentrionale del Mar del Nord, è uno dei principali campi petroliferi offshore in

tutto il mondo.

Una prima parte, basata su ricerca bibliografica, ha messo in rilievo i principali

momenti dell’evoluzione strutturale e stratigrafica del North Sea Basin, dal

Paleozoico all’attuale.

E’ stata fatta quindi una descrizione delle principali facies e del modello

deposizionale della Sognefjord Formation, principale reservoir dell’intero

giacimento. Questa unità sedimentaria è rappresentata da una ciclica

progradazione verso il bacino di cordoni litorali, con intercalazioni di livelli

argillosi associati ad eventi d’inondazione e fiancheggiati da depositi costieri a

granulometria siltosa influenzati da regime tidale.

Una tipica sezione della Sognefjord Formation è rappresentata da arenarie di

offshore, bioturbate, con granulometria medio-fine, che passano verso l’alto ad

arenarie di shoreface-foreshore a granulometria progressivamente più grossa. In

sezione sismica, le geometrie offlapping rappresentano la progradazione dei

cordoni litorali, che verso ovest evolvono in forme ondulate associate a depositi

costieri.

La successione sedimentaria dell’attuale Troll Field è interessata da un sistema di

faglie che si sono sviluppate durante il Kimmerigiano. Tali faglie hanno dislocato

il reservoir in varie unità.

La parte fondamentale di questo lavoro è consistita nella raccolta di una serie di

dati relativi ai fluidi di formazione in sette pozzi. L’esame delle relazioni tra

pressione e profondità ha consentito di quantificare le sovrappressioni, definite in

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base allo shift dei gradienti dei fluidi di formazione rispetto a quello idrostatico

teorico.

Sono stati misurati anche i contatti gas-olio e olio-acqua, le cui variazioni di quota

tra un pozzo e l’altro hanno suggerito un primo indizio di ridotta comunicabilità

laterale.

La presenza di un regime di sovrappressione è giustificato dalle formazioni

scarsamente permeabili che confinano superiormente e lateralmente la Sognefjord

Formation. Queste unità non permettono infatti il drenaggio dei fluidi.

Una volta definite le differenze di sovrappressione, lo sviluppo del modello

strutturale e deposizionale ha permesso di considerare lo “shale smearing” quale

principale fattore di controllo sulla permeabilità della faglia e quindi responsabile

delle differenze di sovrappressione.

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Abstract

The main goal of this work is to provide a description of the Troll Field reservoir

hydrodynamic model, integrating the depositional and structural model with

formation fluid pressure data.

First of all, an overall overview of the North Sea Basin inception, with structural

and stratigraphy evolution, is given, being followed by a description of facies and

paleo-environments of the Sognefjord Formation.

The Sognefjord Formation is an important reservoir for oil and its paleo-

environment is given by cyclic progradation of spit system, with intervening

flooding events and tide-dominate coastal plains eastwards.

A typical vertical section of the Sognefjord formation is characterized by offshore

fine- to medium-graded bioturbated transition sandstones, passing upward to

lower shoreface sandstones, up to shoreface and foreshore coarse-graded

sandstone. Intervening thin siltstone intervals mark flooding events. The

offlapping geometry of this succession reflects the progradation of spit system,

which is flanked eastwards by muddy- tide-dominated coastal plain deposits with

ondulatory geometric in seismic section.

In tidal facies heterolithic deposits were developed in tidal flats or tidal channel,

whereas seawards mouth bars, sandy ridges, muddy shelf and prodelta deposits

can be found in some area.

A faults swarm within the Troll Field developed during the Kimmeridgian and

affected the reservoir units.

After describing Troll Field paleoenvironment, a batch of formation fluid pressure

data have been collected from seven wellbores and then plotted in a pressure-

depth diagram. Fluid pressure gradients have been calculated and then compared

with theoretical hydrostatic ones. The quantification of the shift between the

formation fluid pressure data and theoretical hydrostatic gradient has allowed to

calculate overpressure values.

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Both gas-oil- and oil-water-contact have also been measured, whose related

depths have provided a first clue of poor laterally communication.

The presence of this overpressure system is justified by existence of the lateral

and overlying, poorly porous-permeable formations. These units do not enable

fluids to drain.

Once differences of overpressure among wellbores have been defined, a

depositional and structural model has been developed. This model takes into

account the alternation of sandstone and siltstone that point to a “shale smearing”

model, considered to be the main control factor of the seal efficiency and

responsible of overpressure differences.

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1. Geographical setting

The giant Troll oil and gas Field, which is the second largest gas field discovered

offshore Europe (Bolle, 1990), is located 60 km off the west coast of Norway, in

the Horda Platform area, one of the most important structural elements of the

North Sea Basin. The field covers four Norwegian North Sea blocks (31/2, 31/3,

31/5, and 31/6) and has an areal extent of approximately 770 km2. The examined

field is bordered by the Sogn Graben northwards, the Viking Graben eastwards,

the Norwegian-Danish Basin southwards and the Norwegian mainland eastwards.

At large scale, the More Basin and Faroe Basin limit the field northwards and

north-westwards respectively (Fig. 1).

Figure 1 – Major structural elements of the North Sea Basin.

9

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2. Structural evolutionThe significant processes which contributed to the North Sea basin inception may

be considered starting from the Paleozoic. Caledonian orogeny during

Ordovician-Devonian, the Permo-Triassic and late Jurassic rifting phases are

responsible for the major extensional structures of the basin. Finally, the late

Cretaceous-early Tertiary north-south compression is well recorded in the Central

Graben as inversion structure.

2.1. PaleozoicDuring the Caledonian orogeny, the Baltica collided with Laurentia. This event

was caused by the Late Ordovician closure of the Iapetus ocean. The Caledonian

Deformation Front (CDF), with a NE-SW trend from the southern Norwegian

mainland to the southern North Sea Basin, represents the outer limit of

Caledonian orogeny deformation against the cratonic foreland of Baltica (Fig. 2).

During the Devonian, the area was part of a tensional basin developed along the

north German-Polish Caledonides (Ziegler, 1990a), whereas the Variscan foreland

basin developed with the late onset of the Variscan orogenic cycle.

The Devonian-Carboniferous was affected by uplift as is proved by the respective,

truncated succession.

In the Late Carboniferous-Early Permian, northwest Europe was affected by a

Variscan faulting system. Variscan orogen collapsed, with the coeval

development of a widespread NW-SE and conjugate NE-SW system of fractures.

This process may have been the outcome of a right-lateral reorientation of the

movement between the former Laurussia and Gondawana (Ziegler, 1990). These

events triggered magmatism, as proved by dikes swarms and sills as well as tuffs

and basaltic lavas (Dixon et al. 1981; Sorensen and Martinsen 1987); the volcanic

activity was followed by subsidence, which caused a system of associated horsts

and grabens mainly in the area over the North Germany.

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Mid-Permian extension is well documented in East Greenland (Surlyk, et al.

1984). These extensional structures may have propagated southwards and

contributed to fracture the Viking and Central Grabens.

Permo-Triassic transition had a time of 90 Ma and the rates of the subsidence

reached 220 m per million years (Menning, 1991). Some of this subsidence may

be associated with the rapid crustal loading of Rotliegend basins (Glennie, 1990).

Figure 2 – Main trends of the Caledonian orogeny..

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2.2. Mesozoic

The extensional events, which began during the Permian, continued during this

time and completed the structure of the North Sea Basin.

TriassicPermo-Triassic rifting splitted Pangea into Laurasia and Gondwana and was

followed by post-rift thermal subsidence, while Middle Jurassic doming created a

failed rift system.

Subsequently, the regional extension led to the creation of the North Atlantic

Ocean, and the later Alpine orogeny caused important phases of basin inversion

(Ziegler, 1990).

During the Triassic, the extensional activity that had begun during the Permian

continued in the central-northern North Sea and the inception of the Mid-North

Sea High separated it from the southernmost North Sea (Ziegler, 1982).The huge

area was composed by several halfgrabens, whose extension was approximately

20-30 km wide; they gave rise to a rift valley with an extension up to 400 km

wide. The present-day Shetland Islands bounds the rift valley westwards and the

Oyagarden Fault eastwards.

During the Early Triassic, the crustal extension and subsidence led to the creation

of several deep marine rift basins.

The rifting activity slowed down during the Middle Triassic. The Boreal sea

limiting Pangea northern margin was not affected by rifting activity but by global

sea-level change due to lithospheric plate movements. Crustal movements caused

the inception of the islands, which led to obstructions for westwards sediment

transport from the Uralian mountain belt.

In the Late Triassic, the mainland Norway was affected by uplift.

Jurassic During the early Jurassic, a slow subsidence, caused by thermal cooling following

the Permo-Trias rifting (Badley, 1988), weakly affected the whole area of the

Viking Graben, increasing from the basin margins (10 m/Ma) to the graben axis

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(10-15 m/Ma). This subsidence decreased through time, above all slowing down

during the Baiocian (except in the Sogn Graben area).

Graben margin is characterized by continental or nearshore upper Triassic

sediments (Mercier, 1986), whose thickness distribution was influenced by

geographical variations of the subsidence giving rise to a saucer-shaped geometry.

The zone of the maximum late Triassic and Liassic post rift tectonic subsidence

corresponds to the axis of the Permo-Triassic rifting, which is also the axis of the

forthcoming late Jurassic rift.

The rifting begun in the North Sea during the Middle Jurassic, but it increased its

activity during the Late Jurassic. This process led to the inception of the Viking

Graben in the north and the Central Graben in the south. The rifting resulted in a

creation of a huge rift valley comprising a few major faults and fault terraces

elsewhere; these structural elements consist of series of rotated fault blocks with a

width of tens of kilometers and a length of several kilometers, having their roots

deep in the crust. Currently, they are tilted and link the shallow platforms whit the

deep rift valley; this geometry is a result of later thermal subsidence. The axis of

the rifting is estimated to be located beneath the Viking Graben, assuming the

homogenous condition of stretching (Gilner, 1987, Lippard 1992).

Figure 3 – Tipycal cross section W-E of the northern North Sea Basin.

Most of the rift structure was submerged, but sometime fault blocks emerged

above sea level forming narrow, elongate islands.

The late Jurassic rifting affected the northern part between Greenland and

Norway. Its axis is buried beneath several kilometers of Cretaceous sediments

within the Ras Basin. Late Jurassic rifting led to the formation of half grabens in

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the northern East Greenland, whereas the southern part was only gently tilted. The

rift structure extends into the southern part of Barents Sea, linking with an

embryonic spreading centre in the Artic Ocean through a transverse fault.

During this time, a slight and homogeneous subsidence affected the Horda

Platform (10 m/Ma) as well as the Gulfaks-Snorre block; anyway, the

development of the main graben was related to significantly higher subsidence

(from 10-30 m/Ma to 20-40 m/Ma).

Rifting in the Barents Sea begun in the latest Jurassic and increased during the

Early Cretaceous; this event fragmented the whole Middle Jurassic shelf from

North Sea to Barents Sea, with formation of a vast sea with elongated islands.

CretaceousThe onset of Cretaceous marks the end of the rifting and it is characterized by

cooling and thermal subsidence. This led to the burial of the block faulted terrace

province located on the basin margins and the infilling of the exensive grabens

along the rift axis. Some restricted, saucer-shaped depressions within the

platforms represent an adaptive response to tensional crustal forces beyond the

margins of the major rift structures, after the latter became inactive during the

Early Cretaceous. During the Early Cretaceous, the focus of the rifting moved into

the More and Voring Basins. The magnitude of extension led to a reduction of the

More Basin crystalline crust to a few kilometers and caused the formation of new

oceanic crust. (Brekke, 2000; Skogseid et al.,2000). This extensional event gave

rise to regional depressions along the rift axis, where the crust underwent the

maximum thinning. This process led to thermal subsidence, which kept pace with

basin infilling. Barents Sea was also affected by rifting during the Early

Cretaceous (Gabrielsen et al., 1990), forming two divergent rift arms; shallow

depression developed on the marginal platforms. In the Late Cretaceous, large

parts of the northern Barents Sea uplifted, causing widespread erosion. A global

rise in sea level characterizes the Cretaceous leading to submersion of lowlands

and large part of mainland Scandinavia.

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2.3. Cenozoic

Paleocene and early Eocene were marked by the opening of the north Atlantic

Ocean between Norway and Greenland, as testified by the smectites in the

Cenozoic North Sea sediments originated by transformation of volcanic ash from

eruptions.

The major event which affected the North Sea Basin during the Cenozoic was the

Oligocene uplift, which led to raise the sediment source.

This event is proved by a change in the sediment characteristics from Eocene to

Oligocene successions (Spjeldnoes, 1975).

The Paleocene and Eocene succession is characterized by clays, with small

amounts of silt and mainly composited by quartz and muscovite. Their thickness,

distribution and composition are homogeneous throughout the whole area

(Nielsen et al., 1986). The thickness distribution of the Eocene sediments suggests

that tectonic structures in the basement were active, and intervals with reworked

older sediments indicate an easterly located source area.

A massive southwestward progradation of sediments during the Oligocene

indicates the presence of a new source area exposed for erosion towards the

northeast. This is also reflected in the mineralogical variations and kaolinite

stability observed in parts of the late Oligocene succession.

Sediment transport directions and the sediment characteristics indicate the

exposure of land to east of the North Sea Basin during the Oligocene. The basin

was influenced by regional tectonic movements along weak crustal zones. These

zones were probably reactivated during accommodation of strain from an

interaction of the Alpine Orogeny and the opening the North Atlantic.

The base of Oligocene succession is characterized by clayey silt with mica and

higher content of illite and kaolinite than in the underlying smectite-dominated

Eocene deposits. Furthermore facies distribution indicates a pronounced

progradation from the northeast during the Oligocene.

However, inversion tectonics in the Central Trough, the erosional pattern at Top

Chalk, and the occurrence of Upper Paleocene sandy intervals prove that the

17

tectonic uplift of eastern margin of the North Sea Basin took place during the

Palaeogene.

Horda Platform

The Horda Platform, as previously mentioned, is the structural element which

links the western margin of Norwegian mainland to Viking Graben, which on its

turn, is separated by a marginal fault zone on the platform western edge. Its

structural evolution can be taken into account starting from the Paleocene; indeed,

the significant Mesozoic faulting, which occurred on Viking Graben, weakly

affected the Horda Platform so that its layers were not deformed.

Troll Field

During the Cenozoic the Troll Field was affected by faulting. Its Oligocene

succession is composed by several faults which were generated by overpressure

build up, volumetric contraction during the mudstone compaction (Cartwright and

Lonergan,1996) and gravity sliding, on its turn, triggered by middle Miocene

tilting.

The most prominent structural elements within this area are N-S, NW-NE, and

NNW-SSE striking faults, all of which are extensional with a planar geometry

even if they assume a gentle listric shapes at depth (Rùnnevik and Johansen,

1984; Birtles, 1986; Hellem et al., 1986; Gabrielsen and Koestler, 1987; Gray,

1987; Badley et al., 1988; Bolle, 1990; Gabrielsen et al., 1990; Horstad and

Larter, 1997).

The current mean dip angle is 73°, and the highest density of the faults

distribution is located in the western part.

The lowermost part of the Oligocene sequence, which is also marked by incipient

clay pillow and diapirs, shows a chaotic structure related to gravitational

instability. The clay pillow and diapiris are cut through by extensional faults with

short displacements and which dip toward the N, SW and E.

The lowest limit of the Oligocene sequence is characterized by an angular intra-

Oligocene unconformity, as well as the uppermost sequence boundary. The late

Oligocene sequence has a thickness of 0 m starting from eastern part, increasing

18

towards west up to 475 m with onlap structures against intra-Oligocene

unconformity.

The faults swarm seems to cut locally the Eocene sequence besides the underlying

Oligocene; sometimes these faults can reach the Mesozoic units and are linked

with the major faults, which compartmentalize the Horda Platform.

It is therefore suggested that Oligocene activity reactivated the Mesozoic faults.

The fault activity which affected the top Oligocene unconformity occurred earlier

than deposition of Pliocene sedimentary sequence, as it is proved by the base of

the Pliocene.

Intraformational faults, which are observed in fine sediments, can be generated by

several processes: seismic activity which trigs gravity sliding (Rundeberg, 1989),

differential subsidence along older fault trends (Clause and Korstgard, 1993),

downslope gravity sliding related to uplift and tilting (Lippard and Fanavoll,

1992; Higgs and McClay, 1993) and fluid overpressure and associated density

inversion (Henriet et al., 1991; Cartwright, 1994a, b; Cartwright and Lonergan,

1996; Gregersen et al., 1998; Lonergan et al., 1998a)

It is also noted that the dominant fault trend is parallel to the Oligocene-Miocene

remote stress related to ridge-push (Vagnes et al.,1998). This can indicate that the

fault activity was affected by far-field plate tectonic stress which involved the

Northern European plate. The plate tectonic stress is composed by several

components: Alpine compression and its secondary stresses (Letouzey, 1986;

Bergerat, 1987; Le Pichon et al., 1988; Kooi et al., 1989; VaÊ gnes et al., 1998)

and NW-SE to NNW-SSE directed ridge-push associated with the North-Atlantic

mid-oceanic ridge-system (Kooi et al., 1989; DoreÁ et al., 1997; VaÊ gnes et al.,

1998). Furthermore the doming of the south-central Norway and the subsidence

of the North Sea Basin contributed to the total stress field (e.g. Rohrman and van

der Beek, 1996; Rohrman et al., 1996; (Hall and White, 1994; Jordt et al., 1995;

Nadin and Kusznir, 1995). Its resultant stress vector was oriented NE-SW.

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3. Stratigraphy

3.1. PaleozoicMetamorphic and intrusive rocks of Caledonian age form the basement for most

of the North Sea area. The Caledonian basement is a complex of gneisses and

granulites of medium to high metamorphic grade. The metamorphism is related

to the Ordovician-Silurian phases of Caledonian orogeny. The basement

incorporates metasediments, metavolcanics and metamorphosed layered basic and

anorthositic bodies, with various granites and pegmatites.

The age varies from 750 - 700 Ma to 418-350 Ma; eclogites with coesite and

microdiamonds have been found into Devonian rocks at a crustal depth of 125

km.

Cambrian succession characterizes the Baltoscandian platform and it is dominated

by siliciclastics, even if black shales mark the middle and late Cambrian and early

Tremadocian (Andersen et al., 1986). Carbonate deposition belongs to the

Ordovician and towards the west there is a change into turbidites, in response to

early orogenic activity.

Devonian and Carboniferous deposition transgressed from the south over the

eroded Caledonides and reached maximum thickness in the southern North Sea,

an area which formed part of Variscan foredeep.

The deposition of the Old Red Sandstone spans the late Silurian to the early

Carboniferous, as a result of post-orogenic collapse structures, following

overthickening of the Caledonian crust (Brewer ans Smythe, 1984; Enfield and

Coward, 1986; Seguret et al., 1989). The Old Red Sandstone has been drilled in

several wells in the North Sea and is dominated by alluvial fan, braid plain, fluvial

and lacustrine red beds. This succession testified a semi-arid climate and can be

recognized on both sides of the Viking Graben.

During the Permian, North Sea rifting initiated, possibly coeval with rotation of

the north-trending series of en echelon half grabens as well as intra-Variscan

basins such as the Western Approaches and Celtic Sea Basins. This interpretation

20

could be supported in the North Sea area by the occurrence of volcanism in the

Central, Horn and Oslo Grabens, and by preservation of Zachstein halite within

South Viking Graben together with Rotlegend dune sands. Permian sediments

were coevally deposited with the New Red Sandstones in the Southern North Sea

Basin.

3.2. Mesozoic

Mesozoic era is an important step of North Sea Basin infilling, whose sedimentary

structures represent syn- and post-rift succession.

TriassicThis time is characterized by the continuation of Permiam rifting (Ziegler, 1988;

Seidler et., 2004). This led to the development of continental conditions in the

central-northern North Sea and new generated accommodation space was mostly

filled with alluvial sand, gravel and mud sediments (Steel,1993; Fisher and

Mudge, 1998; Goldsmith et al., 2003; Lervik, 2006). Triassic overburden and

faulting mobilized the thick Permian salt deposits, forming salt pillows and

diapirs. These structures led the seafloor to push upwards controlling the sediment

distribution and its depositional patterns.

In the eastern Danish part, the deposition of redbeds took place in extensive

floodplains in arid desert. The Lower Triassic sedimentary succession has a

thickness of 800 m, belonging to Bacton Group. The Middle Triassic succession is

characterized by halite, anhydrite and clay, which were deposited in shallow

coastal environments (Lolland and Jylland Groups). During the Late Triassic the

brackish sea transgressed the shallow area along the north-eastern basin margin,

giving rise to sandy delta deposits.

The Early Triassic sedimentary succession in the Norwegian Sea is composed by

marine sand and mud (Brekke et al., 2001; Seidler et al., 2004; Nystuen et al.,

2006).

During the Middle Triassic, the cessation of the rifting led the basin to be filled by

sand and mud redbeds, which were deposited between Norway and Greenland.

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In the Late Triassic, new uplift of mainland Norway increased the sediment

supply towards the Norwegian Sea Basin; the area again passed to shallow sea,

and the marine mud and salt mark the lowermost Upper Triassic succession.

During the end of the Triassic, the Norwegian continental shelf became a dry

land.

JurassicIn the northern part of northern North Sea Basin, the lowermost part of Jurassic

succession is mostly made by fluvial sandstone and mudstone with thick coal

units. At the transition with Middle Jurassic, the southern part of the northern

North Sea was affected by uplift and erosion; as a result, a hiatus characterizes the

end of lower Jurassic succession.

Both the North Sea and Norwegian sea during Early Jurassic are shallow marine

basins with a thickness rarely more than 100 m deep and tidally influenced.

The Early Jurassic succession of the northern North Sea Basin is characterized by

fluvial sandstone and mudstone in the lowest part belonging to Statfjord group,

whereas shoreline and shallow marine sandstone and marine mudstone of Dunlin

group characterize the upper part (Steel, 1993; Husmo et al., 2002).

The Early Jurassic of mid-Norwegian shelf is represented by fluvial shallow

marine mudstones and sandstone with a thickness of 700 m; they are the result of

the transgressed coastal plains and belong to the Bat Group (Gjelberg et al., 1987;

Johannessen and Nottvedt, 2006).

The Middle Jurassic of the northern North Sea Basin is marked by the Brent

group, which includes sandstone and mudstone as a result of the erosion of the

North Sea Dome.

The Lower and Middle Jurassic succession of northern North Sea Basin have a

thickness of less than 1 km along the basin margin and more than 2 km in the

depocenter, due to compaction driven subsidence in the Permian-Triassic rift

system. Along the margin of the Mid-Norwegian shelf, Middle Jurassic sediments

have a thickness of few hundred meters. Middle Jurassic deposits in the

Norwegian Sea are sand-rich as a result of advancing coastal plains and great

delta system.

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Sedimentation of Late Jurassic is characterized by fine-grained and organic-rich

mud as the Middle Jurassic floodplains were encroached by the sea. This led to an

accumulation of mudstone belonging to Viking group with a thickness up to 1 km.

Sandstones resulted from deltaic progradation, local sedimentation around

emergent islands and from submarine fan deposition along the rift structure

(Nottvedt et al. 2000; Fraser et al., 2002). Central rift provinces were supplied

only with mud as the western flank acted as sink trapping the coarse grains.

23

Figure 4 – Mesozoic lithostratigraphy of the North Sea Basin.

CretaceousRifting in the North Sea ceased at the beginning of Cretaceous, which was

characterized by burial of the faulted blocks and continuos infilling of the

extensive basins located along the rift axis.

Mudstones and marls dominate the northern North Sea Lower Cretaceous

succession, whose thickness varies from some hundred of meters up to 1 km

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(Cromer Knoll Group) (Oakman and Partington, 1998; Copestake et al., 2003;

Brekke and Olaussen, 2006). In the Late Cretaceous, the sea level rise encroached

much of mainland Scandinavia and cut off the terrestrial sediment supply; this led

the deposition of mudstone and limestone of the Shetland Group. The Chalk

Group dominates the uppermost Cretaceous sequence of Danish and southern

Norwegian Basin. It was formed by accumulation of more than 2 km of

calcareous coccolith ooz (Surlyk et al., 2003). These sediments represent the

major reservoir for oil and gas, especially above salt structures. The entire

Cretaceous mid-Norwegian shelf succession is dominated by shallow marine

mudstone with interbedded sandstone, belonging to the Cromer Knoll Group.

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3.3. CenozoicDuring Cenozoic the North Sea was an epicontinental basin, limited by

continental areas along southern Scandinavia eastwards, British Isles westwards,

Central Europe southwards.

Erosional products coming from elevated areas of Shetland and Scandinavian

continental platform were deposited into the deep marine North Sea Basin during

the Late Palaeocene.

Palaeocene-Eocene transition was marked by regional transgression of the basin

margins with explosive volcanism in the west and followed by regional tectonic

subsidence to the east and south-east. Fine-grained sediments generated by the

erosion of basaltic lavas along the Atlantic continental margin in the north were

deposited into the basin from the East Shetland Platform during the Middle and

Late Eocene.

In the late Eocene the uplift resumed and caused the erosion or starvation in large

parts of the northern North Sea.

During the transition Eocene-Oligocene, uplift affected again the southern

Norway so that coarse grained sediments were deposited towards the west along

the Oygarden Fault Zone and towards the south in the Norwegian-Danish Basin.

At the same time a growth of glacial ice-sheets occurred, causing a global sea

level fall.

Mid-Oligocene was marked by a change of strata stacking pattern from margin

progradation to widespread aggradation as a result of sea-level rise associated to a

reduced sediment supply.

In the latest Oligocene an influx of sand-rich sediment coming from the East

Shetland Platform occurred, being caused by uplift in the north-west area.

The time from Miocene to Recent was marked by sediment deposition with a

mean thickness greater than 1500 m (Rundberg, 1989; Jordt et al., 1995).

Marginal uplift and regional subsidence in the Viking Graben area controlled the

depositional system during the late Cenozoic (Rundberg, 1989; Jordt et al., 1995;

Gregersen et al., 1997; Fyfe et al., 2003).

26

Miocene begun with shallow marine sands (Utsira formation), followed by the

development of large prograding clinoforms (shaly sands) coming from the

progressively uplifted Norwegian margin.

Mid-Miocene was characterized by a sedimentation break with an erosion of the

underlying deposits (Rundenberg, 1989; Jordt et al., 1995; Gregersen et., 1997;

Fyfe et al.,2003)

Deposition of quartz-rich fine-to-medium grained sand was caused by denudation

of the northern North Sea basin margins. A depositional system with sands

prograding eastwards is located along the eastern margin of the East Shetland

Platform. Sediment supply from Scandinavian area led to the creation of basin

marginal deltas with sand/silts at the East Shetland Platform and Norwegian

margin (Gregersen et al., 19997).

During the Pliocene, deposition of argillaceous sediments occurred. Ice sheet

movements at the western Norwegian margin inner shelf have been proposed as a

cause for late Pliocene shelf-margin progradation (Henriksen and Vorren, 1996);

a large amounts of clastic material were glacially transported to the coastline. The

other important Pliocene sediment source was the entry of the Baltic river system

into the basin. Uplift caused the erosion of the basin marginal areas and probably

increased the sediment supply, contributing to the progradation of systems.

Pleistocene glaciations resulted in erosional unconformity and removed the upper

part of the prograding Pliocene deposits in the northern North Sea. The maximum

extension of large ice sheets is marked by a broad NW-SE oriented topographic

depression (Norwegian Trench), which is still the main feature of the present day

bathymetry.

The following phases of glaciations/deglaciations caused the formation of tills

interbedded with shallow-marine clays (Sejrup et al., 1991).

Therefore, the current geometry of the Cenozoic sequences is a result of tectonic

uplift occurred from Oligocene to Pliocene, and further uplift related to late

Pliocene-Pleistocene glacial erosion and isostatic adjustments.

27

4. The Sognefjord Formation reservoirThe Troll field represents the largest petroleum discovery within the entire North

Sea area, with 74% of the accumulated petroleum present as dry gas and 26% as a

heavy biodegraded oil leg.

According to the oil and gas distribution and the thickness of oil column, the field

has been divided into three main provinces. These provinces are characterized by

a restricted lateral communication, which is suggested by a variation of the oil-

water-contact across the whole area.

The Troll West oil province has an oil column of 22-26 m, which passes to 12-14

m in the Troll West gas province and decreases up to 0-4 m in the Troll Est. The

maximum thickness of the gas column is 230 m, and the total amount of

petroleum accumulated in the entire area is 2245x106 tons oil equivalents, 74% of

which is gas.

The Troll West reservoir formed on the edge of the Horda Platform during the

Late Jurassic rift event and it is characterized by offlapping sandstone units with

intervening finer-grained deposits. The reservoir succession contains three

composite sequences, the lower two of which belong to the Sognefjord Formation

and the upper one is part of Draupne Formation.

Petroleum has been found into the Middle-Upper Jurassic shallow-marine

sandstone of the Sognefjord and Fensfjord formations, which interfinger with

shale and silt of the Heather formation, forming pinch out structures (Fig. 5)

These shales, according to their location relative to the Fensfjord and Sognefjord

formations, have been named Heather A, B and C (Hellem et al., 1986).

28

Figure 5 - Middle-Upper Jurassic lithostratigraphy of the Troll Field; red circle indicates where the Sognefjord and Fensfjord Formations are located.

The sandstones of the Sognefjord reservoir, are poorly consolidated with a

thickness of 200 m and calcite-cemented horizons in all reservoir units.

The porosity of the reservoir may reach up to 34% and its permeability varies

from a few millidarcy to above 10 d (Gray, 1987).

The seal of the petroleum system is given by Upper Jurassic-lower Tertiary

mudstone (Hellem et al., 1986); the field had not an efficient seal until the

Tertiary.

The migration of petroleum is controlled by the dominating faults swarm, whose

main trends are north-south and northwest-southeast (Gray, 1987).

The Sognefjord Formation is interpreted as a shoreline-attached tidally-influenced

shelf complex (Gibbons, Hellem, Kjemperud, Nio, Vebenstad, 1993). The main

paleoenvironment is compounded by spit system, flanked by tide-dominated

29

deltaic system landwards (Fig. 6a). Clinoform geometry passing into ondulatory

shape landwards reflects the environments previously mentioned (Fig. 6b)

Figure 6 – a) Satellite image from Cape Lopez, Gabon, which similarly displays paleomorphology of the Troll Field. b) Seismic section of clinoform geometry, which reflects the spit system progradation, by passing eastwards into the ondulatory shape, which in turn belongs to the coastal deposits.

This formation deposited from the Late Callovian to early Volgian during a

general rise in sea-level. The main evidence is provided by high-sinuosity and

30

low-gradient estuarine channels; the sea-level rise decreased the slope-gradient

and at the same time increased the tidal action into the shallower part of the basin.

It also contains progradational geometries associated with high system tract and

aggradational geometries associated with transgressive system tract, which reflect

a sea-level rise.

During a later stage of transgressive system tract, the Sognefjord Formation was

affected by high erosion rates, which led to a higher sediment supply and to a

prograding ebb-delta (Vail et al., 1987), therefore causing an offlap geometry

upon the shelf.

The sediments are characterized by a cyclic nature with a repeated succession of

micaceous, silty, very fine to fine-grained sandstones and mineralogically mature,

clean, coarse-grained sandstones (Grey, 1987; Hellem et al., 1986). This cycled

structure has been associated to fluctuations in sea-level.

The examined succession shows a typical depositional cycle both with

progradational and transgressive components. At the base, fine micaceous

sediments reflect low energy environment, frequently offshore, whereas upwards

medium-to-coarse, clean sands represent coastal progradation over shallow shelf

(Fig. 7)

31

Figure 7 – Typical Sognefjord succession, which reflects an alternation of facies due to sea level fluctuations.

The maximum sea-level rise led to formation of carbonate horizons, which are

associated with maximum flooding surface. These horizons are both nodules and

stratiform layers with a thickness of 0.5-5 m and represent the 10% on average of

the reservoir.

The underlying formation is represented by the Fensfjord Formation, of Callovian

– Oxfordian age, and consists of fining upwards with scattered sands, whereas the

32

fine-grained silty units referred as Heather C limits the Sognfjord Formation

eastwards (Stewart et al., 1995).

The Troll Field is divided into three provinces (Hellem et al., 1986): a

northwestern, a central, and an east-southern province. The northwestern province

belongs to open marine shelf area, the central province represents the most

proximal offshore with prograding ebb-delta lobes, and the southeastern province

developed into sheltered, enclosed, partially inshore tidally-dominated

environment.

The Troll Field reservoir includes several facies, each of which represents a type

of depositional environment:

a) low-energy marine;

b) moderate-energy marine

c) high-energy marine;

d) tide-influenced marine;

e) storm region.

Ten type of facies have been recognized into the Sognefjord Formation, which in

turn, have been grouped into four facies association.

Shelf facies association

Facies 1: Offshore transition fines. Bioturbated mudstones to very fine-grained

sandstones. These deposits are moderately to poorly sorted, with a large

proportion of mud and mica flakes within sandstones, and sand grains in

mudstones.

Facies 2: Event deposits. Isolated beds of fine to coarse-grained sandstones with

intercalated mud-rich deposits. These facies units vary their thickness from few

meters to more than 30 meters, with interbedded bioturbated silty mudstone and

very fine sand of facies 1. These sediments are probably related with high-energy

sediment-transport events, like flood, storm and tsunami which led sand and fine

gravel into an environment characterized by low suspension fallout (Myrow &

Southard 1996).

33

Spit and strandplain facies association

Facies 3: Rip channel deposits. Erosively based normally graded sandstones,

interbedded with either swash-bar sandstone and inter-bar trough facies, or

bioturbated silty sandstones of offshore transition facies association.

Facies 4: Swash-bar and inter-bar deposits. Sets of low-angle parallel stratified

and planar cross-stratified sanstones interbedded with thick units of trough cross-

stratified sandstone and thinner beds of planar parallel-stratified and ripple cross-

laminated sandstone.

Facies 5: beach deposits. Low-angle parallel-stratified sandstone units, which

compounds are well sorted with an average grain-size of medium sand.

Delta-front facies association

Facies 6: Distribution channel deposits. Poorly sorted sandstones were found in

the uppermost parts of the coarsening-upwards units of the northeastern parts of

the Troll West.

Facies 7: Mouth-bar deposits. Gravelly to fine-sandstone beds with a decimeters

thickness; they are poorly sorted and a planar-parallel geometry with intercalated

trough cross-stratified and low-angle stratified sandstone of facies 4 and 5.

Tidal facies association

Facies 8: Tidal channel deposits. Upwards-fining erosively based medium to

coarse-grained sandstones with a thickness between 1-9,5 m. Sub-rounded

mudclasts and well-rounded extraclastes are very common with a maximum

thickness of 50 cm.

Facies 9: Tidal delta and sand-ridge deposits. Coarsening-upwards units with

basally lenticular-bedded mudstone and mud-draped current ripples, bioturbated,

characterized by plant fragments. The middle part is dominated by herringbone

cross-lamination and wavy bedding. The grain size is from silt to medium-grained

sand with scattered granules. The uppermost part is characterized by well-sorted

coarse-grained sand with a sigmoidal cross-bedding and plane-parallel

stratification.

34

Facies 10: Tidal flat deposits. Bedsets of ripple cross-laminated very fine- to

medium-grained sand and lenticular bedded to laminated silty mud interbedded

with facies 8 and 9. The bedsests show a rhythmic bedding (Dreyer, 1992), by

forming a partition of facies into sandier (reservoir) and muddier (non-reservoir)

units.

Depositional System

The characteristics of the facies and facies associations are typical of wave-

dominated, fluvial-dominated and tidal-dominated environments. Therefore, four

depositional system developed on the Horda Platform during the Late Jurassic

(Fisher & McGowen, 1967; Brown & Fisher 1977): 1) a shelf system; 2) delta-

front system; 3) strandplain and spit system; 4) tidal system.

Shelf system

Offshore transition and lower shoreface deposits (facies 1 and 2) marked by

interbedding of sandy and fine-grained background sediments. Progradation of the

shelf seawards resulted in coarsening-upwards clinoform geometry, capped by

sand-grade deposits of either strandplain/spit or delta-front depositional system.

During transgression, the shelf system developed landwards and received finer

sediment supply as the shoreline retreated and accommodation space increased;

thus, the retreated of the shelf depositional system resulted in fining-upwards

succession, overlying a transgressive ravinement surface formed by wave

winnowing at the base of the retreating shoreface.

Delta-front system

Mouth-bar deposits overlain by fluvial channels and transgressive belong to this

type of system. This vertical stacking of deposits indicates a shoreline

progradation. In more distal parts they either do not accumulated or are not

preserved due to transgressive ravinement.

Strandplain/spit system

This system occurs mainly above the shelf depositional system in regressive

succession with a coarsening-upwards trend. It can also be intercalated with the

35

tidal inshore system in aggradation-dominated and transgressive succession. The

sediment dispersal is longshore (Duke, 1990), so that strike-elongated distribution

of sandbodies developed as a result. This depositional system represents the main

reservoir units in Troll West.

Tidal system

This depositional system is mainly present in the “heterolitic” reservoir zones, in

the eastern part of the study area, but extend into central part during condition of

maximum progradation. Tidal system represents backbasin part protected by

spitsystem: in the northeast area the system is marked by tidal channel and tidal

flat deposits interpreted as prograding ebb-tidal deltas; southwards tidal sand bars

increase their distribution. Facies of this system display both brackish- and fresh-

water fauna. The first one means that the bay communicated with the sea, whereas

the second one also indicates a prolonged disconnection with the sea because of

the extensive spit barriers.

36

Figure 8 - Generalized depositional model of the Troll Field. Spit system are found in the western part of the area, whereas the mouth bars developed where distributaries debouched into the sea. Clinoform were generated by the progradation of the spit system seawards and muddy shelf deposited in the southern part of this repository. The environment landwards is represented by tide-dominated coastal deposits.

37

5. Data collection

38

WELLBORE 31/2-5

Figure 9 – Fluid formation gradients of the wellbore 31/2-5, plotted in a pressure-depth diagram.

Well 31/2-5 testifies that the Late Jurassic Sognefjord Formation sandstone

reservoir is well developed in the western part of the block 31/2. The fluid

formation column consists of 43 m of gas from 1536 m to 1579 m, followed by a

21 m thick oil column up to 1600 m where OWC has been found. Good oil shows

continued down to 1644.5 m. The GOC was found at the same depth as seen in

the other wells in the area. This well is characterized by the presence of an oil

39

section thicker than that one found in any of the other wells, and it belongs to a

part of reservoir with very good clean sand. Paleocene claystones represents the

seal over the structure.

40

WELLBORE 31/2-1

41

Figure 10 - Fluid formation gradients of the wellbore 31/2-1, plotted in a pressure-depth diagram.

In the well 31/2-1, a column of 134.5 meters of gas has been found in the Late-

Middle Jurassic sequence in the so-called Flathead A structure. The reservoir

consists in good-moderate quality coastal-shallow marine sands with the top at

1439,5 m. Oil bleeding occurs from cores in the section between 1567 m and 1597

m, below the gas. It is possible that gas-oil- and oil-water-contact occur in this

zone. Pressure date show that gas-water-contact effectively exists at 1574 m,

where intersection of the extrapolated gas and water pressure gradients occurs.

42

WELLBORE 31/2-2

Figure 11 - Fluid formation gradients of the wellbore 31/2-2, plotted in a pressure-depth diagram.

In the well 31/2-2 gas column was found from 1544 (Sognefjord Formation top

reservoir) to 1579, depth at which there is the gas-oil contact; oil-bearing was

from 1579 to 1591, where oil-water contact was esteemed. Log defined both

contacts, which in turn, were confirmed by pressure gradients.

43

WELLBORE 31/3-1

Figure 12 - Fluid formation gradients of the wellbore 31/3-1, plotted in a pressure-depth diagram.

In the well 31/3-1 a gas column goes through a section of 220 m comprising the

Sognefjord, Heather and Fensfjord Formations with the Top reservoir at 1351.5

m. The gas column is followed by 4 m of oil section down to 1576 m, where oil-

water contact is defined. Hydrocarbons were not found either in the Early Jurassic

or Late Triassic.

44

WELLBORE 31/2-3

Figure 13 - Fluid formation gradients of the wellbore 31/2-3, plotted in a pressure-depth diagram.

In this well the hydrocarbons were found into the Late Jurassic sandstone

reservoir, which were also developed in the northerly part of the structure. Top

structure is at 1384 m; a gas column of 189 m starts from the top down to 1573 m.

The good clean sandstones of the Sognefjord Formation represents the uppermost

part of 120 m of the reservoir, where the gas was found, while the lowermost part

is marked by micaceous and poor reservoir sand of the Heather and Fensfjord

Formations. The gas column is followed by a 12 m thick oil zone, which is

45

contained in a very micaceous and poorly developed reservoir. Palaeocene

claystones with an unconformity surface lie over the reservoir, so that they may

act like a effective seal for the reservoir. Oil- water contact is defined at 1585 m.

46

WELLBORE 31/5-2

Figure 14 - Fluid formation gradients of the wellbore 31/5-2, plotted in a pressure-depth diagram.

In the well 31/5-2, the top reservoir is defined at 1521 m. The gas column start

from 1521 m and ends at 1569 m with 30% porosity and 15 % water saturation.

The oil zone is defined between 1569 and 1582 with 27% porosity and 30% water

saturations.

47

WELLBORE 31/6-1

48

Figure 15 - Fluid formation gradients of the wellbore 31/6-1, plotted in a pressure-depth diagram.

A gas zone from 1352 to 1571.5 m involves the Sognefjord Formation (1352-

1488), the Heater Formation Unit B (1488-1517.5 m) and the upper part of the

Fensfjord Formation. This column is followed by 3 m of oil zone down to 1574,

where oil-water contact is defined. Very fine to fine-grained sandstones,

occasionally medium to coarse with calcite cemented horizons, represents the

hydrocarbons reservoirs. The average porosity is of 28.7% and average water

saturation is of 19.1%.

Homogenous, moderate brown to red brown, micromicaceous, silty shales

characterize the basal part of Triassic rocks (Hegre group), which were

encountered at 2155.5 m. They are predominantly much more calcareous than the

overlying interval. This interval belongs to Scythian-Ansian. Water column was

found in the middle to lower Jurassic and Triassic sandstones.

49

50

6. Depositional model and interpretation

This work permitted to identify a system of pressure anomalies inside of the Troll

Field reservoir.

Fluid pressure is proportional to depth and its density. Locally, pressure anomalies

which move away from predictable hydrostatic trend, can be found and are

described as “overpressures”.

It is important to predict pressure system to prevent blast phenomena during

drilling operations; furthermore, calculation of overpressure provides an useful

tool to define the reservoir compartmentalization and therefore hydrocarbon

simulation.

Origin of overpressure may be given by the following processes: smectite

transformation into illite, (with releasing of water molecules); an increasing of

fluid volume, subsequent to a temperature increasing; generation of hydrocarbons;

tectonic thrust and high sedimentation rates.

All these processes, in case of sediment with low permeability, generate

overpressure for the impossibility of fluids to move out of the systems

compensating the increase fluid volume or the decrease of porosity. Then the

excess pressure are transmitted to the rocks with higher permeability where they

can be preserved only if the system is compartmentalized by permeability barriers.

51

6.1. Depositional model and palaeogeographical

setting of the Sognefjord Formation

The Sognefjord Formation can be divided in three composite sequence, which in

turn are divided in basic sequence named 2-, 3-, 4-, 5-, 6-series (figure 16).

Figure 16 – Typical section of the Sognefjord Formation, which reflects the seaward progradation of spit system, flanked by coastal heterolithic deposits. Three composite sequence (arrowed) characterize the Sognefjord Formation, with the uppermost belonging to the Draupne Formation. The red lines separate basic series (2-, 3-, 4-, 5- and 6-). –m units indicate more marine conditions, whereas –c units more continental conditions; -Het units indicate heterolithic deposits. Lithology: yellow: coarse sandstone-dominated; blue: mudstone (darker blue means high organic matter content); red: offshore deposits; darker green: more continental conditions.

52

2-series

These series represent the initial progradation of the spit system with two

sandstones wedge (2Ac-2Bc), associated with high system tract; the basal part

2Am reflects the transgressive conditions, as well as the 2Bm unit. Facies 1 of

offshore transition and facies 7 of mouth-bar can be mostly recognized in these

series.

3-series

The lowermost part of these series (3Am-3Ac) is associated with transgressive

system tract by causing the backstepping of 2-series units. During the re-

establishment of coarser-grained 2-series units, the paleogeography of the basin

changes significantly: because of the tilting of Horda Platform and the stronger

sea currents with a N-S trend, the elongated spit systems develop, with tide-

dominated backbasin environment eastwards placed (fig. 17). Typical succession

is represented by offshore transition facies association, passing upwards into

coarser, clean sands, and flanked by brackish-water and mouth bar deposits

landward.

3Bm-3Bc units are attributed to spit system progradation (fig. 16), as it is

displayed in seismic section by clinoform geometry (fig. 6b). 3Bm reflects marine

conditions, whereas 3Bc more continental conditions. These units are referred to

Highstand system tracts.

During the development of 3Cm-3Cc units a pronounced progradation westwards

and southwards of the spit systems occurs with a significant incision in estuarine

setting, localized in the southern part. This incision is displayed in the seismic

section by discontinuos channel-like features in the west; it is represented by 3Het

(fig. 16)

3Dm-3Dc units indicate extensive prograding spit systems associated with

lowstand system tract.

4-series

4Am muddy interval unit separates the lower and upper part of the Sognefjord

Formation and represents a flooding surface.

53

4Ac unit is characterized by coarsening-upwards succession associated to a

wave-, storm-dominated delta setting. Facies 2 and 5 have been found in this unit,

which in turn, indicate high system tract.

4Bm-4Bc sansndstone wedges are associated either with a late high system tract

or early forced regressive system tract. Found facies can be linked with wave-

dominated shoreline-shoreface environment.

Significant progradation basinwards is represented by 4Cm-4Cc and 4Dm-4Dc

units with a deep incision of 30 m around; the latter also involves 3-series unit, by

leading to the inception of an unconformity surface. This intra-unconformity

developed during the deposition of 4Cc unit. Paleogeographical setting is marked

by a muddy, tidal back-basin environment with several distributary tide-channel,

by interfingering laterally with bars and sandstone ridge. Heterolithic sediments

marked the eastsouthern block. Although the seismic data are inconclusive, the

geometry of the 4-series progradation has been maintened southwards.

5-series

5Am siltstone unit represents flooding event, by reflecting from nearshore

conditions to offshore transition environment.

5Ac and 5Bc sandstone wedge are associated with progradation of the shoreline

basiwards during the normal regression, 5Cc unit is marked by aggradational

components. Weak flooding events are represented by siltstone-dominated 5Bm

and 5Cm units. Paleogeographical setting points out a progradation of heterolithic

facies and a drowing of the tidal channel southwards, and the development of

delta progradation in northern area.

6-series

6-series units, which belong to Kimmeridgian-Volgian, are separated by the

underlying 5-series units by an unconformity. The first 6Am unit consists in fine-

to medium-sandstones-dominated, overalain by a blocky coarsening-upwards

sands.

6-series units have been placed during the Horda Platform tilting and represent the

syn-rift deposits associated with a reworking of 5-series sandstones.

54

Figure 17 – Palaeogeographical reconstruction for the Sognefjord composite sequence.

55

6.2. InterpretationMarine mudstone with organic matter of the Draupne Formation and shale-

siltstone of the Heather formation, bounds the Sognefjord Formation above, below

and laterally westwards respectively (fig. 18).

Figure 18 – Lithostratigraphy of the Middle-Upper Jurassic indicating which formations bound the Sognefjord Formation.

Muddy-dominated coastal deposits limit the Sognefjord Formation landwards (fig.

16). These poorly porous/permeable formations validate the presence of

overpressure system into the Sognefjord Formation, being embedded in a shaly

sequence.

In addition, a difference of overpressure among the wells within the Sognefjord

Formation exists. As a matter of fact during the Kimmeridgian, the Horda

56

Platform was affected by tilting, so that a faults swarm (fig. 19) displaces the

reservoir units, where the pressure data have been recorded.

Figure 19 – Top Dunlin Fm. Structural map and Overpressure values (kg/cm2) for each of the wellbores

A system of NE-SW post-depositional faults, which offset or terminate spit

systems, integrated with main faults, compartmentalizes the reservoir (fig. 20).

57

Figure 20 – Map covering the most of the study area. The lighter colors indicate the breakpoints of the clinoforms associated with spit system, whereas the dark-coloured lineaments offsetting or terminating the spit lineaments are post-depositional faults.

A reduced lateral communication is also suggested by a variation of both GOC

and OWC (fig. 21-22)

58

Figure 21 - Top Dunlin fm. Structural map and gas-oil contact depth associated with the wellbores

59

Figure 22 - Top Dunlin fm. Structural map and oil-water contact depth associated with the wellbores

The primary control on the seal behavior under pressure conditions is probably

due to the shale/clay content on the fault zone. The analyzed sand-dominated

succession is characterized by interbedded thin siltstone layers, which have a

plastic behavior. A fault, which involves a typical mud-sandstone succession,

leads the clay to smear on the fault zone, by acting like a seal and preventing

hydraulic continuity. This phenomenon is named “shale smearing”. This is

particularly valid for the minor faults. Moreover, on the major North-South faults

with a greater thrown, the uppermost Draupne Formation also facilitates a direct

sandstone-shale lateral juxtapposition.

60

Figure 23 – a) How a “shale smearing” model affects ideally a sandy-shaly succession; b) the smeared shale on the fault plain prevents the formation fluids to migrate through the carrier.

Between the wells 31/3-1 and 31/6-1 no overpressure difference occurs; this fact

can be explained because there are no faults, which separate wells; furthermore,

sand-dominated heterolithic deposits are likely to characterize this part of the

reservoir.

As a whole, the observed differences can be due to the combination of the sealing

effect of faults, determining a partial compartmentalization of the reservoir.

61

7. Conclusions

The Troll Field is one of the greatest offshore hydrocarbon fields in the world.

Most of the oil is retained into the sandstones of the Sognefjord Formation, whose

paleo-environment is mainly given by a cyclic progradation of spit system

westwards, flanked by muddy-, tide-dominated coastal plain eastwards. The

sedimentary succession is affected by a faults swarm, which deploys the reservoir

units.

This work is based on collection and analysis of data from 7 wellbores and

permitted to obtain the following results:

An overpressure system has been identified, by elaborating in a pressure-depth

diagram the fluid formation gradients and comparing them with theoretical

hydrostatic one; both gas-oil- and oil-water-contacts have also been measured and

provide a first clue of reduced lateral communication.

Upwards and laterally neighboring marine-mudstone, shale-siltstone formations

and muddy-dominated deposits are consistent with the overpressure system within

the Sognefjord Formation.

Significant differences of overpressure have been recorded among the different

wellbores.

Depositional model, integrated with structural one, reveals that “Shale smearing”

method can be used to describe the seal efficiency and the differences of

overpressure.

62

63

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