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 Zattin
Correlatore: 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
6.2. Interpretation
51
55
3
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 centrosettentrionale 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
5
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.
6
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 paleoenvironment 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 pressuredepth 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.
7
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 evolution
The 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. Paleozoic
During 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.
Triassic
Permo-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
14
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.
Cretaceous
The 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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16
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 intraOligocene 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.
19
3. Stratigraphy
3.1. Paleozoic
Metamorphic 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.
Triassic
This 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.
21
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.
Jurassic
In 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.
22
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.
Cretaceous
Rifting 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
24
(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. Cenozoic
During 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 reservoir
The 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 oilwater-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 crossstratified sandstone and thinner beds of planar parallel-stratified and ripple crosslaminated 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 wavedominated, 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) deltafront 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 freshwater 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
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 LateMiddle 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
41
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 oilwater 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
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 (13521488), 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
48
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
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.
50
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.
51
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 reestablishment 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 tidedominated 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.
52
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 wavedominated 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 fineto 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.
53
Figure 17 – Palaeogeographical reconstruction for the Sognefjord composite sequence.
54
6.2. Interpretation
Marine mudstone with organic matter of the Draupne Formation and shalesiltstone 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
55
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).
56
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)
57
Figure 21 - Top Dunlin fm. Structural map and gas-oil contact depth associated with the
wellbores
58
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.
59
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.
60
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.
61
62
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