Reservoir quality of fluvial sandstone reservoirs in salt-walled mini-basins: an example from the Seagull field, Central Graben, North Sea, UK
Reservoir quality of fluvial sandstone reservoirs in salt-walled mini- basins: an example from the Seagull field, Central Graben, North Sea, UK
Stephan Stricker 0 1 2 3 4 5
Stuart J. Jones 0 1 2 3 4 5
Neil Meadows 0 1 2 3 4 5
Leon Bowen 0 1 2 3 4 5
0 Stuart J. Jones
1 & Stephan Stricker
2 Diagenesis Clay minerals Porosity
3 Department of Physics, Durham University , South Road, Durham DH1 3LE , UK
4 RedRock Associates International Limited , 38 Queens Drive, Prenton, Wirral CH43 0RP , UK
5 Neil Meadows
The Triassic fluvial sandstones of the Skagerrak Formation were deposited in a series of salt-walled mini-basins and act as important hydrocarbon reservoirs for several high-pressure, high-temperature (HPHT) fields in the Central Graben, North Sea. The HPHT reservoirs exhibit excellent reservoir quality considering their depth of burial and hence have been of high interest for hydrocarbon exploration. This research uses a multidisciplinary approach to assess the Skagerrak Formation fluvial reservoir quality from the Seagull field incorporating core analysis, petrography, electron microscopy, XRD analysis, fluid inclusion appraisal and burial history modelling. Halokinesis and salt withdrawal at the margin of the saltwalled mini-basin induced early disaggregation bands and fractures at shallow burial and led to increased influx of meteoric water and clay mineral infiltration from overlying sedimentation. The density of disaggregation bands correlates with the occurrence and magnitude of pore-filling authigenic clay minerals, concentrated along the margin of the saltwalled mini-basin. The fluvial channel sandstones of the Skagerrak Formation are subject to strong intra-basinal spatial reservoir quality variations despite diagenesis and low vertical effective stress having played a favourable role in arresting porosity loss.
Reservoir quality; Salt-walled mini-basin; Edited by Jie Hao
Department of Earth Sciences, Durham University, South
Road, Durham DH1 3LE, UK
The eventual quality of clastic reservoirs, such as grain
size, sorting and composition, is initially determined by the
environment of deposition. Depositional settings and their
effects on reservoir quality are of particular interest for
clastic reservoirs, where contemporaneous salt movement
influences sedimentation and the resulting distribution of
(e.g. Fox 1998; Banham and Mountney 2013a, b)
Subsequently, burial-related diagenesis also plays a
significant role for governing reservoir quality, as it can
destroy, preserve or enhance porosity and permeability,
depending on diagenetic pathways.
Recent research has contributed to a deeper
understanding of salt tectonics and related processes in
(e.g. Hudec and Jackson 2007; Archer
et al. 2012; Hudec et al. 2013; Sathar and Jones 2016)
Such studies have increased our understanding of salt
movement, the development of salt-walled mini-basins and
on the effects of coeval halokinesis on sedimentary
processes and facies distribution
(e.g. Fox 1998; Hudec
et al. 2009; Banham and Mountney 2013a, b)
. However, a
detailed understanding of the effects of contemporaneous
salt movement on diagenesis and reservoir quality is yet to
The Skagerrak Formation was deposited in an array of
salt-walled mini-basins within the Central Graben, Central
(e.g. Smith et al. 1993; Bishop 1996; Helgeson
1999; McKie 2014)
and is an important reservoir for
several high-pressure, high-temperature (HPHT; [ 65 MPa
and [ 150 C) hydrocarbon accumulations. This study
utilized channel sandstone samples of the Judy Sandstone
Member of the Skagerrak Formation from the wells
22/292 and 22/29-3 (Seagull field) to identify:
How the salt-walled mini-basin setting controls
reservoir quality in the fluvial sandstones of Triassic
Skagerrak Formation, Central North Sea, UK and:
How contemporaneous salt movement influences
fracturing, clay infiltration and diagenesis to guide
prediction of best reservoir quality in salt-walled mini-basins.
2 Geological setting
2.1 Central Graben of the North Sea
The Central Graben of the North Sea is a prolific
hydrocarbon province of 70–130 km width and 550 km length. It
forms the southern arm of a trilete rift system (i.e. an
incipient ridge–ridge triple junction) in the North Sea with
the Viking Graben (VG) as the northern arm and the Moray
Firth Basin (MFB) as the western arm (Fig. 1a)
and Neumann 2008)
. The Forties-Montrose High and
Josephine Ridge horst blocks divide the Central Graben
into the East Central Graben (ECG) and the West Central
Graben (WCG), flanked by the Norwegian basement in the
east and the UK continental shelf in the west. The rift
system developed in at least two major extension phases,
one during the Permian–Triassic (290–210 Ma) and
another in the Late Jurassic (155–140 Ma)
(Gowers et al.
1985; Taylor 1998)
This study focuses on the 22/29-2 and 22/29-3 wells of
the Seagull field located at the southern end of the
FortiesMontrose High in UK Quadrant 22 (Fig. 1b). The 22/29-2
well is located in a salt-walled mini-basin centre (Fig. 2a,
b), whereas 22/29-3 is located more proximal to a salt wall.
2.2 Skagerrak Formation stratigraphy
The Triassic strata of the Central North Sea area have been
previously described in some detail,
(e.g. Goldsmith et al.
2003; McKie and Audretsch 2005; McKie 2014)
This thick succession of entirely continental strata is
subdivided into the Early Triassic Smith Bank and Bunter
Formations (shales, evaporites and thin sands) and the
Middle to Late Triassic Skagerrak Formation (thickly
interbedded sands and shales). The Middle to Late Triassic
Skagerrak Formation in the Central Graben, North Sea,
comprises 500–1000 m (1640–3280 ft) of predominantly
continental braided and meandering fluvial system deposits
and terminal fluvial fan deposits with lacustrine facies
(McKie and Audretsch 2005; De Jong et al. 2006; Kape
et al. 2010; Stricker et al. 2016b)
. The stratigraphic
nomenclature of the Triassic for the Central Graben was
Goldsmith et al. (1995)
, based on detailed
biostratigraphic and lithostratigraphic correlation of wells
from the Josephine Ridge and was extended and correlated
towards the study area (Fig. 1b) by
McKie and Audretsch
The Skagerrak Formation in the Central Graben is
subdivided into three sand-dominated members (Judy,
Joanne and Josephine) and three mud-dominated members
(Julius, Jonathan and Joshua) (Fig. 3). The sand-dominated
members include sheetflood deposits and multi-story
stacked channel sandbodies
(Goldsmith et al. 1995; McKie
and Audretsch 2005)
, whereas the mud-dominated
members include a variation of non-marine, basin-wide
floodplain and playa lake deposits. The thick and laterally
extensive mud-dominated units provide the main
correlative units for the Skagerrak Formation in the Central
Graben (McKie and Audretsch 2005). The present-day
Triassic stratigraphy of the study area is incomplete due to
deep erosion during the Middle and Late Jurassic
et al. 1999; McKie et al. 2010)
. The stratigraphy comprises
the Early Triassic Smith Bank and Bunter Formations and
the lowermost members of the Skagerrak Formation (Judy
Member and Julius Member)
(McKie and Audretsch 2005;
McKie et al. 2010)
The Skagerrak Formation of the Seagull field comprises
a major sandstone reservoir, the Judy Sandstone Member,
which is bound by the regional shale markers, the Marnock
and Heron Shales, equivalent to the Julius Mudstone
Member and the upper Smith Bank (Fig. 3).
subdivided the Judy Member into a lower
terminal splay-dominated interval and an upper
channelized interval, separated by a shale-prone section. The lower
terminal splay facies is characterized by fine-grained,
planar cross-bedded and ripple-laminated sandstones. In
comparison, the upper interval is dominated by channel-fill
deposits, which are organized into fining upward packages
with coarse lag deposits (usually with ripped-up calcrete
nodules) commonly occurring at the base. Channel-fill
deposits are characterized by well-sorted cross-bedded
sandstones and can be separated into channel and
2.3 Mini-basin development and halokinesis
Triassic sediments of the Central Graben accumulated
directly on top of the Late Permian Zechstein salt within a
series of fault-controlled and salt-controlled mini-basins
(Figs. 2, 4). The Late Permian Zechstein salt strongly
controlled the Triassic deposition by forming salt
withdrawal mini-basins within an overall rift-controlled basinal
(Smith et al. 1993; Bishop 1996; Matthews
et al. 2007)
. The initiation of mini-basin subsidence or
creation of salt-walled mini-basins requires the presence of
salt with a sufficient thickness to allow halokinesis
et al. 2009)
, and a mechanism to initiate halokinesis, i.e.
extension, compression, differential loading or buoyancy
(Banham and Mountney 2013a)
. The halokinesis of the
Zechstein evaporites started during the Early Triassic and
Basin after Brown (1991); b Detailed outline of the study area (in
black box on the regional map) with the hydrocarbon reservoirs of the
area (Egret, Heron, Marnock, Seagull and Skua) and location of the
schematic cross sections (Fig. 2) (dashed lines)
has been associated with rifting in response to the
Hardegsen tectonic event
. Reactivation of the
Permian fault system and localized loading by ongoing
Triassic sedimentation together with uneven salt
distribution resulted in a complex topography
(Hodgson et al.
1992; Smith et al. 1993; Bishop 1996; Matthews et al.
. This led to variations in the mini-basin development
and the creation of accommodation space throughout the
Triassic period, together with the potential for mini-basin
grounding (Fig. 2a). The coeval halokinesis of the
Zechstein salt significantly influenced the depositional
environment of the Triassic sediments in the Central Graben.
Coeval halokinesis controlled the overall subsidence of the
mini-basins, sediment transport pathways and the
reworking of uplifted sediments
(e.g. Banham and Mountney
. Salt withdrawal and reduced salt thickness below
the Seagull mini-basin allowed grounding in the Early to
Middle Triassic on the underlying Rotliegend basement
(Fig. 2a, b) (McKie and Audretsch 2005; McKie et al.
2010). The syn-rift and halokinesis controlled Smith Bank
Formation sediments representing the bulk and basal part
of the mini-basin infill, whereas the overlying post-rift and
halokinesis controlled Skagerrak Formation sediments are
the thinner upper part of the mini-basin infill. Coeval
halokinesis and varying Zechstein salt thickness created
variable accommodation space for the Early and Middle
Triassic sediments within mini-basins and between
different mini-basins (Fig. 2) that has influenced reservoir
thickness and diagenesis, e.g. halite cements related to the
Zechstein salt mobilization
(Nguyen et al. 2013)
Fig. 5 Facies interpretation of the 22/29-2 and 22/29-3 core material,
with the porosity distribution (point count) of the channel sandstone
samples [CF sands (PC)], non-channel sandstone samples [Non-CF
(PC)] and helium porosity from core plugs of the 22/29-2 well (solid)
and the 22/29-3 well (hollow), and a regional Central North Sea
porosity–depth relationship for shaly sandstone by
22/29-2 Non-CF (PC)
22/29-2 CF sands (PC)
22/29-3 Non-CF (PC)
22/29-3 CF sands (PC)
Core samples and thin sections examined in this study have
been selected from channel sandstone facies of the wells
22/29-2 and 22/29-3 (Fig. 5). For the purposes of this study
the sedimentary facies of the sandstone members within the
Skagerrak Formation broadly follow previous published
descriptions of the succession
(Goldsmith et al. 1995, 2003;
McKie and Audretsch 2005; McKie 2014)
. These have
been modified slightly with regard to seminal papers on the
Table 2 Petrographic and point count data for the 22/29-3 sample
set, with sample depth in metre and feet TVDSS, facies type [channel
sand (CF), unconfined fluvial (UCF), lacustrine shoreface (LSF)],
grain size (GS), coated grains with [ 80% coated surface (CG), and
character of dryland fluvial systems
(e.g. Bridge and Lunt
, continental trace fossil assemblages and their
(Hasiotis et al. 2002)
. A total of 65 sandstone
samples have been taken from the cored Skagerrak
Formation intervals of the 22/29-2 well (36) and the 22/29-3
well (29) for the reservoir quality analysis of this study
(Tables 1, 2), and these have been selected entirely from
sandstones ascribed to the fluvial channel facies (CF)
(Tables 1, 2).
Core sample thin sections were used to determine optical
porosity, grain-size distribution and the fraction of
claycoated grains. Porosity and fraction of clay-coated grains
with [ 80% coats were measured on blue
epoxy-impregnated thin sections by point counting with 300 counts per
thin section. Point counting was done using a standard
petrographic microscope (Leica DM2500P and DM750P)
and point counting stage (PETROG—Conway Valley
Systems Limited). Grain-size distribution was analysed by
using the Leica QWin (V. 3.5.0) software on thin section
micrographs. Additional petrographic analysis, i.e.
intergranular volume (IGV)
(Paxton et al. 2002)
cement volume (C)
, was performed
exclusively on confined fluvial channel sandstones (CF)
and measured by point counting, with 300 counts per thin
section. Total cement (C) values comprise intergranular
clay mineral cement, intergranular carbonate cement, as
well as optically visible quartz and feldspar cements.
Thin sections were highly polished to 30 lm and coated
with carbon prior to analysis by a Hitachi SU-70 field
emission gun scanning electron microscope (SEM),
equipped with an energy-dispersive detector (EDS).
Scanning electron microscope analyses of thin section and bulk
rock samples were conducted at 5–20 kV acceleration
voltage with beam currents of 1.0 and 0.6 nA, respectively.
Point analyses had an average duration of 2 min, whereas
line analyses were dependent on length. SEM–EDS was
used for rapid identification of chemical species and
orientation on the sample. Cathodoluminescence analysis has
been undertaken on selected thin sections with visible
quartz overgrowths using a Gata MonoCL system with a
panchromatic imaging mode operated at 8 kV.
The modelled key Skagerrak Formation reservoir member is in bold
Group/formation Water depth Nordland Lark/Horda
3.3 X-ray diffraction analysis
X-ray diffraction (XRD) analysis (semiquantitative bulk
rock and clay fraction) was conducted by X-Ray Mineral
Services Ltd and has been used to identify the clay mineral
quantities, composition and diagenetic alterations within
the two sample sets. Selected bulk rock samples were
deoiled, disaggregated and powdered to a mean particle size
between 5 and 10 lm for the XRD analysis. Samples were
analysed by using a Philips PW1730 Generator at 2H
(theta) angles between 4.5 and 75 , with a step size of
0.06 per second using X-ray radiation from a copper
anode at 40 kV, 40 mA, equipped with a Philips PW1050
Goniometer with graphite monochromator and a PW1170
automatic sample changer. Identification of the minerals
was achieved by using the X-ray Mineral Services Ltd
inhouse ‘Traces’ and ‘Search-Match’ software to compare
the X-ray diffraction pattern with the International Centre
for Diffraction Data PDF-4 Minerals database. The
measured maximum intensity of each mineral was compared to
the standard intensity of the pure mineral. The method does
not take any amorphous content into account, and the
results were normalized to 100% based on the assumption
that the whole mineral content accounted for the
Separation of the clay fraction (\ 2 lm fraction) was
achieved by using ultrasound and centrifugation of
disaggregated fresh rock samples (5 g) in suspension, with
additional evaporation of the fluid content at 80 C. The
samples were analysed as untreated clay, after saturation
with ethylene glycol vapour overnight and following
heating at 380 C for 2 h and 550 C for 1 h. The initial
Table 5 Homogenization
temperatures (Th) of aqueous
(Aqu.) and oil (oil) fluid
inclusions in quartz overgrowth,
feldspathic and carbonate
4174.85 m TVDSS
4185.11 m TVDSS
4190.72 m TVDSS
4203.23 m TVDSS
scan for these four treatments was performed with the same
machine and at 2H angles between 3 and 35 with a step
size of 0.05 per second using X-ray radiation from a
copper anode at 40 kV, 40 mA. The untreated samples
were also analysed between 24 and 27 2H at a step size
of 0.2 per 2 s to further define kaolinite/chlorite peaks.
Interpretation was performed by overlaying the four
diffractograms to identify the clay mineral assemblages
and to assess the effect of the treatment on the clay
minerals. Peak intensities were measured and analysed to
indicate the relative amounts of clay minerals present and
have been referenced to the total amount of clay minerals
present in the bulk rock analysis.
3.4 Fluid inclusion analysis
Microthermometry was conducted on double-polished
detached wafers to determine the conditions of cementation
and evidence for formation water salinity. The wafers were
firstly checked under incident UV on a petrographic
microscope to determine which contain petroleum
inclusions and under transmitted light to determine the
distribution of both aqueous and hydrocarbon fluid inclusions
for subsequent analyses. A Linkam THM600/TS90
heating–cooling stage connected to a Nikon petrographic
microscope was used to obtain temperature data.
Instrumental precision is ± 0.1 C, while accuracy, dependent
on the manufacturer’s stated accuracy for the calibration
standards used (synthetic inclusions and pure organic
compounds), is better than ± 0.1 C, over the range of
temperatures reported here. Routinely available
measurements are homogenization temperatures (Th) and final
melting temperatures (Tm). Homogenization is the
conversion of multiphase inclusion contents to a single phase,
usually at temperatures above room temperature.
Interpreting homogenization temperatures in carbonates,
sulphates and halides can be complicated because aqueous
inclusions can, though not necessarily do, reset to higher
temperature if they are (a) overheated beyond a threshold
which is dependent on the mineral strength and inclusion
geometry (Goldstein and Reynolds 1994), or (b) frozen.
This can occur in the laboratory as well as through
geological processes, so care is taken over the order in which
analyses are made for each rock chip. If resetting has
occurred, larger inclusions may give higher temperatures,
homogenization temperature distributions may show a
high-temperature tail, and data from paragenetically
distinct settings may overlap. Final melting occurs at the
disappearance of the last trace of solid in the inclusion on
heating, usually after cooling an inclusion to well below
room temperature. If ice is the final phase to melt, as in the
present study, salinities are calculated using the equation
Oakes et al. (1990)
3.5 One-dimensional basin modelling
Burial history, temperature and pore pressure evolution of
the Judy Sandstone Member were modelled in one
dimension using Schlumberger’s PetroMod (V. 2012.2)
basin modelling software. Burial history and lithology are
inferred from the present-day well stratigraphy, well log
lithology and lithological description of the modelled units
(Fig. 3 and Table 3). The lithological unit types used in
these models are mainly PetroMod (V. 2012.2) default
lithology types, based on well log descriptions and core
analysis reports for the two wells, and generalized Central
Graben lithology descriptions
(Evans et al. 2003)
Exceptions are the lithology types of the Hod Formation and the
Skagerrak sandstone members. The Hod chalk unit is
modified to represent the North Sea non-reservoir chalk
(Table 4) and to match the compaction trend and low
permeability trend by
Mallon and Swarbrick (2002
Vertical effective stress
Fluid inclusions Qtz OG
Vertical effective stress
Fluid inclusions Qtz OG
Fig. 7 a QFL diagram of the 22/29-3 (black) and 22/29-3 (grey)
channel sandstones (CF), b grain-size distribution of the 22/29-2 (36
samples) and 22/29-3 (29) sample sets by facies (channel sands (CF):
The Skagerrak sandstone member lithology is a mix of
lithologies (80% sand, 10% silt, 10% shale) in combination
with the regional compaction trend for shaly sandstone by
Sclater and Christie (1980)
, to match the general Judy
Sandstone Member characteristics. The thermal upwelling
basement paleo-heat flow model of
Allen and Allen (2005)
was used with 63–110 mW/m2 (average of 80 mW/m2)
during syn-rift phases and 37–66 mW/m2 (average
50 mW/m2) during post-rift phases combined with the
paleo-surface temperature history published by
et al. (2000)
and di Primio and Neumann (2008). The
temperature evolution models are calibrated against
present-day RFT temperature measurements corrected after
Andrews-Speed et al. (1984)
, vitrinite reflectance data,
maximum temperatures obtained from apatite fission-track
analyses and paleo-temperatures obtained from fluid
inclusions in mineral cements (Table 5)
(Swarbrick et al.
2000; di Primio and Neumann 2008)
. The one-dimensional
modelling provides a good insight into overpressure
buildup by disequilibrium compaction and pore fluid
expansion due to increasing temperature. However, the
models do not include other mechanisms for generating
excess pore pressure, such as fluid flow or hydrocarbon
cracking, or cementational porosity loss, and are only able
to take vertical stress into account. The pressure evolution
models are calibrated against measured Skagerrak
Formation porosities (Fig. 3) and carefully adjusted towards
present-day formation pressure measurements by
considering late-stage, high-temperature overpressure
(Osborne and Swarbrick 1997; Isaksen 2004)
dark grey; unconfined fluvial (UCF): medium grey; lacustrine
shoreface (LSF): light grey), with average sample set grain size (Av.)
4 Burial modelling, petrography and diagenesis
4.1 Burial history modelling results
The one-dimensional burial history models of the 22/29-2
and 22/29-3 wells show the evolution of burial depth,
temperature, pore fluid overpressure and VES for the top of
the Judy Sandstone Member throughout the geological
history (Fig. 6). The Judy Sandstone Member is at
maximum burial depth and temperatures ([ 160 C) at present
day (Fig. 6). The Judy Sandstone Member experienced a
long shallow burial phase (* 150 million years) followed
by a phase of rapid burial starting between 90 and 70 Ma to
their present maximum burial depth. The phase of rapid
burial was accompanied by significant temperature and
pore pressure increases. The burial history models show a
burial rate increase from * 90 Ma onwards, leading to a
present-day maximum burial depth of * 4000 m (13,123
ft) below seafloor (Formation top). Modelled reservoir
temperatures and pore fluid overpressures increase
constantly during the rapid burial phase to present-day maxima
of 160 C and * 38 MPa, respectively (Fig. 6). The rapid
overpressure increase in the Skagerrak Formation limits the
increase in vertical effective stress (VES) during the rapid
burial phase to a maximum VES of * 21 MPa at around
* 10 Ma and present-day VES of 11.5 MPa (22/29-2) and
12.5 MPa (22/29-3) (Fig. 6). Fluid inclusion analysis has
identified the late timing of quartz cementation in the
sandstones (Table 5). The aqueous homogenization
temperature (Th) for the quartz cementation is very late in the
burial history (* 10 Ma; Fig. 6) and concurs with the late
rapid overpressure increase (from around 10 Ma). This late
increase in overpressure is interpreted to be caused by
migration of overpressured fluids and hydrocarbon
cracking (Isaksen 2004;
Lines and Auld 2004
; Winefield et al.
Fig. 8 Micrographs and SEM images of a calcrete/dolocrete rip-up
clast with dolomite crystals [22/29-2; 4201.97 m (13,786.0 ft)];
b well-developed authigenic chlorite coating [22/29-2; 4126.47 m
(13,538.33 ft)]; c well-developed authigenic illite coating [22/29-3;
4206.09 m (13,799.5 ft)]; d authigenic clay mineral coating,
2005), leading to the late VES reduction
Stricker et al. 2016a, b)
The 65 samples from the two samples sets (36 from
22/292 and 29 from 22/29-3) vary compositionally within a
Table 7 Clay fraction XRD results for the channel sandstone samples
from the 22/29-2 and the 22/29-3 sample set, with the weight
percentage of the clay fraction (\ 2 lm), the weight percentage of the
clay minerals relative to the size fraction for illite/smectite, illite,
chlorite, quartz and barite, the mixed-layer ordering of illite/smectite
narrow range of arkosic arenites (Fig. 7a). The grain sizes
vary from very fine-grained to medium-grained sand
(Fig. 7b). The sample sets show a wide range of optical
porosity with 2% to 19% and 0% to 7.7% for the 22/29-2
and 22/29-3, respectively, with the higher porosities
occurring in the 22/29-2 sample set (Fig. 5). Point count
porosity (PC) is complemented by helium core plug
porosities for the core sections (measured after Boyle’s
law). The higher helium core plug porosities indicate the
presence of significant microporosity in the sample sets
Grain-size variability and sorting is limited across all
facies, with most of the sandstones being fine to very
finegrained sand (Fig. 7b)
(Nguyen et al. 2013; Cui et al.
. Confined fluvial channel sands (CF) and unconfined
fluvial sands (UCF) range from very fine to
mediumgrained sand (Fig. 7b), with the medium grain-sized
samples of the 22/29-2 sample set represent coarser grained
basal parts of the confined channel and unconfined sheet
Clay mineral cement
Quartz & Feldspar
Fig. 9 Intergranular volume (IGV) of the confined fluvial channel
sandstone (CF) samples of 22/29-2 and 22/29-3, subdivided into
intergranular clay mineral cements (black), intergranular carbonate
cements (white), porosity (dark grey) and quartz and feldspar cements
(dotted), based on point counting. Samples names refer to true vertical
depth below mean sea level in metre (m TVDSS)
sandbodies. The nine lacustrine shoreface samples (LSF)
show in comparison with the confined fluvial channel sands
(CF) and unconfined fluvial sands (UCF) marginally finer
grain sizes of very fine to fine-grained sand (Fig. 7b and
Tables 1, 2).
4.3 Diagenetic cements and grain coatings
The diagenetic history of Skagerrak Formation reservoirs
has been described by several authors
(e.g. Smith et al.
1993; Weibel 1999; Kape et al. 2010; Nguyen et al. 2013;
Stricker and Jones 2016; Stricker et al. 2016b)
, with the
main diagenetic cements including localized carbonate,
authigenic clay mineral, quartz, feldspar and halite
cements. Carbonate cements are reported to be localized
throughout the Skagerrak Formation in the Central Graben
(McKie and Audretsch 2005; McKie and Williams 2009;
Nguyen et al. 2013; Cui et al. 2017)
and are scarce in the
Seagull sample sets. Carbonate cement is present as
deformed carbonate rip-up clasts due to mechanical
compaction (Fig. 8a) or as pore-filling cement.
described calcrete and/or dolocrete
ripup clasts common at the channel bases and provide
excellent nuclei for subsequent groundwater carbonate
cementation shortly after deposition. Pore-filling carbonate
cements generally show a rhombic crystal structure with a
5 10 15
Porosity, % (point count)
0 5 10 15 20 25 30 35 40 45
Fig. 10 a Intergranular cement and porosity (point count); b
intergranular cement and intergranular volume (IGV); c compactional
porosity loss (COPL) and cementational porosity loss (CEPL)
prominent cleavage pattern and tend to infill the pore space
completely where present (Fig. 8a). Fluid inclusion
analysis on pore-filling carbonate cements in the Seagull
sample [22/29-2—4174.84 m (13,697 ft)] shows high
aqueous fluid inclusions homogenization temperatures of
85–135 C (Table 5).
Authigenic clay mineral cements and grain coatings, i.e.
chlorite or illite, are widely reported for the Skagerrak
(e.g. Humphreys et al. 1989; McKie and
Audretsch 2005; Nguyen et al. 2013; Grant et al. 2014;
Stricker and Jones 2016; Stricker et al. 2016b)
, and their
presence has been linked to low authigenic quartz cement
(e.g. Taylor et al. 2015)
. Authigenic clay mineral
cements in the Seagull sample sets are common as grain
coatings (Fig. 8b, c) or are present as intergranular and
pore-filling clay cements (Fig. 8f). Detailed SEM, SEM–
EDS and XRD analysis shows that clay mineral cements
and grain coatings consist generally of authigenic chlorite
and illite (Fig. 8b, c, Tables 6, 7) or present as a mixture of
authigenic chlorite and illite (Fig. 8d). SEM–EDS analysis
indicates that the authigenic clay coatings are fully
transformed at the present day and no-precursor clay minerals
are detected. Chlorite coatings were analysed according to
the Fe/Mg-ratio by
Hillier and Velde (1992)
classified as intermediate to Fe-rich according to
with a Fe/Mg-ratio of 0.43–0.49. Authigenic clay
coatings are generally widespread in both Seagull sample
sets and coat an average of 72.1% (22/29-2) and 66.4%
(22/29-3) grains per sample (Tables 1, 2). The grain
coatings are generally continuously developed with thicknesses
from 5 to 20 lm, (average: 10 lm; Fig. 8e). The thickness
is correlated to the illite content and increases with
increasing illite content. The clay coatings commonly show
a two-layered structure, with an inner layer or root zone
(sensus Pittman et al. 1992)
and an outer layer. The root
calculated according to
, for the confined fluvial
channel sandstone samples (CF) of the 22/29-2 and 22/29-3 wells
zone consists of densely packed and laminated crystal
sheets, oriented parallel or sub-parallel to the detrital quartz
grain surface. The outer coating layer commonly consists
of well-defined chlorite crystals (Fig. 8b, d) or illite fibres
(Fig. 8c, d).
Pore-filling clay cements in both Seagull sample sets are
identified as authigenic chlorite and illite, with an increased
and wider occurrence of pore-filling clay cement within the
intergranular volume of the confined fluvial channel
sandstones of the 22/29-3 sample set (Figs. 9, 10b). SEM
analysis showed mainly fibrous illite (Fig. 8f) in
association with smaller chlorite platelets in densely packed
arrangements as the main pore-filling clay mineral
assemblage, which can represent over 30% of the bulk rock
volume and infill up to 90% of the remaining intergranular
volume (Fig. 9).
Quartz cement is common as thick and blocky
macroquartz overgrowths (Fig. 11a, b), present at non-coated
grain surfaces (Fig. 11c) and at breaks within the clay
mineral coatings, or as very thin microcrystalline quartz
overgrowths (Fig. 11d). The amount of quartz cement is
generally below 10% bulk volume, but can exceed 10%
bulk volume in single samples. Fluid inclusion analyses on
the quartz overgrowths have been undertaken on selected
samples and show high homogenization temperatures. The
aqueous homogenization temperatures range from 129.5 to
144 C and from 128.5 to 145.5 C for the 22/29-2 and
22/29-3 sample sets, respectively (Table 5). Oil inclusions
are generally less common, and homogenization
temperatures are between 72.3 and 108.6 C (22/29-2) and at 65 C
for one (22/29-3) (Table 5).
K-feldspar dissolution and alteration is common in the
Seagull sample sets
(Taylor et al. 2015)
generally during late burial, after the clay mineral
transformation and grain coating (Fig. 11e). Original grain shapes
Fig. 11 CL images, micrographs and SEM images of a, b a detrital
quartz grain with macroquartz overgrowth and grain-coating clay
minerals (a SEM, b CL) [22/29-2; 4201.97 m (13,786.0 ft)];
c macroquartz overgrowth on a detrital quartz grain [22/29-2;
4119.17 m (13,514.33 ft)]; d illite grain coating with microquartz
intergrowth [22/29-3; 4206.09 m (13,799.5 ft)]; e partly dissolved
K-feldspar (22/29-2; 13,538’4’’); f partly dissolved K-feldspar with
K-feldspar overgrowth [22/29-3; 4216.64 m (13,834.1 ft)]
of partly or fully dissolved grains are commonly preserved
by clay mineral coatings, and late-stage blocky authigenic
K-feldspar overgrowth (\ 5% bulk rock) can be observed
on uncoated and partly dissolved K-feldspar grains
(Fig. 11f). Aqueous fluid inclusions in feldspar
overgrowths (22/29-2) indicate cementation at high
temperatures above 130 C (Table 5). Oil inclusions
encountered in feldspars show significantly lower temperatures
between 67 and 138.5 C across the two sample sets
(Table 5). Highly altered and dissolved K-feldspar grains
Table 8 Intergranular volume
(IGV), total cement (C) and
porosity (PPC) values for
channel sandstone samples of
the 22/29-2 and 22/29-3 sample
All data are point count derived
can coexist in close proximity to unaltered grains,
consistent with the understanding that variations in feldspar
microtextures and feldspar composition exert control on
their reactivity as demonstrated by
Trevena and Nash
Parsons et al. (2005)
4.4 Intergranular volume (IGV) and porosity loss
by compaction versus cementation
Intergranular volume (IGV) values
Houseknecht 1987, 1988; Paxton et al. 2002)
confined fluvial channel sandstones of the two sample sets
show similar values, but comprise wide internal variations
of 18.3% to 40% (22/29-2) and 23.6% to 40.3% (22/29-3)
(Table 8). IGV values exclude samples with dilatation
bands and are generally grain-size dependent, with the very
fine-grained samples showing higher values than the fine or
medium-grained samples. The comparison of the total
cement (C) values highlights increased cementation in the
22/29-3 (average 29.2%) sample set compared to the
22/29-2 sample set (average 19.9%) (Figs. 9, 10a, b). The
increased pore-filling clay mineral cementation led to
significant lower effective porosities in the 22/29-3 sample
set (Fig. 9, Fig. 10a).
Porosity loss by compaction (COPL) and by
cementation (CEPL) is calculated by the
using total cement volume and intragranular volume
(Table 8) with the initial or depositional porosity assumed
to be 45%
(Beard and Weyl 1973; Chuhan et al. 2002)
calculated COPL–CEPL results indicate compaction as the
main driver for porosity loss within both sample sets, with
a strong addition of cementational porosity loss in the
22/29-3 sample set (Fig. 10c).
4.5 Fractures and dilatant disaggregation bands
Diagenetic events in the Judy Sandstone Member of the
Seagull field are generally linked to the tectonic setting,
structural evolution of the mini-basin and the location
within the salt-walled mini-basin. The structural evolution
of the Skagerrak Formation is generally controlled by the
overall extensional regime within the Central Graben and
the underlying Zechstein salt, even though each mini-basin
has been affected differently due to the timing and
magnitude of the salt movement (McKie and Audretsch 2005;
b Fig. 12 Fractures and disaggregation band distribution in core
sections from the Seagull core material a 22/29-3; 4177.09 m TVDSS
to 4177.70 m TVDSS (13,704.36–13,706.36 ft); b 22/29-3;
4192.90 m TVDSS to 4193.43 m TVDSS (13,756.23–13,757.97 ft);
c 22/29-3; 4208.72 m TVDSS to 4209.02 m TVDSS
(13,808.14–13,809.12 ft); d 22/29-3; 4209.36 m TVDSS to
4209.70 m TVDSS (13,810.24–13,811.35 ft); e 22/29-3; 4186.12 m
TVDSS to 4187.04 m TVDSS (13,733.99–13,737.01 ft); f
disaggregation bands in thin section [22/29-3; 4186.37 m (13,734.81 ft)
TVDSS]; g the grain arrangement of the host rock [22/29-3;
4186.37 m (13,734.81 ft) TVDSS]; h the grain arrangement of the
disaggregation band with minor grain alignment and fracture porosity
(blue) [22/29-3; 4186.37 m (13,734.81 ft) TVDSS]
22/29-2 (Av.: 72.1%)
22/29-3 (Av.: 66.4%)
Egret (Av.: 86.1%)
Heron (Av.: 79.3%)
Skua (Av.: 70.1%)
Fraction of coated grains, %
Fig. 13 Fraction of clay mineral coated grains ([ 80% coated) per
300 counts for the Seagull sample sets, in comparison with the Egret,
Heron and Skua field, from Stricker et al. (2016b)
McKie et al. 2010). The Seagull mini-basin or pod is
grounded on an edge of one of the underlying Rotliegend
basement blocks (Fig. 2a, b) and is tilted as a result of the
ongoing evacuation of salt from under the subsiding
(Helgeson 1999; McKie and Audretsch 2005)
grounding and ongoing evacuation of salt from below the
mini-basin led to increased fracturing and faulting of the
semi-consolidated Skagerrak Formation sediments at the
mini-basin flanks in comparison with the mini-basin centre
and the reshaping of the sediment body and internal stress
and strain changes. The movement of the large salt walls
influenced deposition throughout the Smith Bank
Formation of the Early Triassic and continued influencing
sedimentation of the Skagerrak Formation until the late
Triassic (end Norian, Fig. 3).
Core material of the 22/29-3 well shows dense patterns
of fractures (e.g. Fig. 12). Orientation and aperture of the
infilled fractures varies throughout the core material, with
fracture apertures from 1 to 50 mm (Fig. 12a–e). Thin
sections of infilled fractures and adjacent host rock material
were prepared for further analysis of the fractures and
fracture infill (4177.44 m TVDSS (13,705.5 ft); 4186.37 m
TVDSS (13,734.8 ft); 4191.18 m TVDSS (13,750.6 ft);
4193.26 m TVDSS (13,757.4 ft); 4202.43 m TVDSS
[13,787.5 ft); 4208.92 m TVDSS (13,808.8 ft)]. The
fractures are generally characterized by small shear offsets and
show no indicators of cataclastic deformation (fractured
grains or reduced grain size) or grain dissolution (Fig. 12f,
h). Fracture infills contain arkosic sands with similar
grainsized to the host rock material and intergranular clay
mineral cements identical to the host rock material
(Fig. 12g, h). Minor shear-related grain alignment can be
observed within the fractures, but no carbonate or quartz
cementation could be observed (Fig. 12f, h).
Fractures within the 22/29-3 core material are classified
as dilational shear bands (shear-related, dilatant,
(Du Bernard et al. 2002; Fossen et al. 2007;
Fossen and Bale 2007)
. Dilatant disaggregation bands are
commonly reported for poorly consolidated sandstones at
surface or near-surface conditions (\ 1000 m), where
minor vertical effective stress is applied to the grain
(Mandl et al. 1977; Du Bernard et al. 2002;
Fossen et al. 2007; McKie et al. 2010)
disaggregation bands are commonly characterized by small shear
displacement, a reduced grain framework density, larger
pores and a small porosity increase of up to 8% with
respect to the host rock
(Antonellini et al. 1994; Du
Bernard et al. 2002; Fossen and Bale 2007)
. Increased porosity
and decreased grain framework density make dilatant
disaggregation bands a suitable pathway for meteoric water
influx (Du Bernard et al. 2002), which is commonly
associated by clay mineral infiltration and diagenetic
(Matlack et al. 1989)
5.1 Clay mineral cementation and reservoir
The amount, distribution and morphology of clay minerals
can have significant effects on the reservoir quality of
sandstones. Infiltrated clay minerals and authigenic clay
mineral cements can enhance or reduce effective porosity
and permeability. The development of clay mineral
coatings in sandstones is for example, often reported to be
closely linked with the absence of extensive quartz
cementation and porosity preservation
(e.g. Matlack et al.
1989; Billault et al. 2003; Ajdukiewicz and Larese 2012;
Taylor et al. 2015)
. Pore-filling clay mineral cements,
however, are known to decrease effective porosity and to
reduce the reservoir permeability significantly
Worden and Morad 2003; Wilson et al. 2014)
. Authigenic clay
mineral cements are widely reported for the Skagerrak
Na+ & K+ infiltration
Formation, especially grain-coating chlorite which have
been linked to the absence of extensive quartz cementation
in the Skagerrak Formation sandstones
et al. 1989; Nguyen et al. 2013; Taylor et al. 2015; Stricker
et al. 2016b; Stricker and Jones 2016)
The general clay mineral assemblage in the 22/29-2 and
22/29-3 sample sets consists of authigenic chlorite and
authigenic illite. The ratio between chlorite and illite varies
from 83% and 17% to 40% and 60% with an average 54%
to 46%, respectively. Kaolinite, as reported by
Taylor et al.
, has not been observed in the sample sets (Table 6).
The authigenic clay mineral cements are common as clay
mineral grain coatings (Fig. 8b, c) and pore-filling clay
aggregates (Fig. 8f). Clay mineral coatings are commonly
well developed and tend to cover around 72.1% (22/29-2)
and 66.4% (22/29-3) of the detrital grains (Fig. 13). The
authigenic clay mineral coatings tend to enhance the
reservoir quality, where they are well developed and fully
coating (Fig. 8b, c, d), inhibition of quartz cement is noted.
However, where the authigenic coatings are poorly
developed, quartz cement nucleated and the pore space has been
infilled by macroquartz cements (Fig. 11a, b, c). A
comparison of the authigenic clay mineral coatings between the
sample sets of the Heron, Egret, Seagull and Skua fields
(Fig. 1b) shows a lower fraction of coated grains and lower
surface coverage rates for the Seagull sample sets
(Fig. 13). This is reflected by an increase of pore-filling
macroquartz cement in comparison with the Heron, Egret
or Skua fields
(Taylor et al. 2015)
. Pore-filling clay mineral
cements within the confined fluvial channel sandstones of
the Seagull sample sets are either authigenic illite (Fig. 8f)
or a mixture of authigenic illite and chlorite. The
porefilling clay mineral cements can occupy up to 90% of the
remaining intergranular volume (Fig. 9) and reduce
effective porosity and permeability significantly within the
channel sandstones of the Judy Sandstone Member.
The authigenic clay mineral cements are postulated to
neo-formed from post-depositional, infiltrated allogenic
clay minerals. Post-depositional clay infiltration by muddy
water has been described as an effective mechanism to
emplace allogenic clay mineral aggregates and allogenic
clay mineral coats post-depositional into sand in arid and
(Matlack et al. 1989; Moraes and de Ros
1990; Worden and Morad 2003; Ajdukiewicz et al. 2010)
The allogenic clay minerals are infiltrated by meteoric
water during rain or flood events into semi-consolidated
(Worden and Morad 2003; McKie et al. 2010)
fractures and disaggregation bands increase the infiltration
depth (Fig. 12). High amounts of infiltrated allogenic clay
minerals resulted in thick allogenic clay mineral coatings,
allogenic clay mineral bridges between clay mineral
coatings and pore-filling allogenic clay mineral aggregates
(Matlack et al. 1989; Moraes and de Ros 1990)
clay minerals tend to transform with ongoing burial and
increasing temperatures along existing structures into
authigenic chlorite and illite which led to present-day
authigenic clay coatings (Fig. 8b, c, d), pore-bridging clay
structures and pore-filling clay aggregates (Fig. 8f).
However, alternative formation scenarios for the
authigenic pore-filling clay mineral cements are possible, such
as illitization of kaolinite in combination with K-feldspars
(Bjørkum and Gjelsvik 1988; Chuhan et al. 2000; Worden
Fig. 15 Fractures and disaggregation band distribution in core
sections from Huntington (22/14b-4) and Fiddich (22/19-1) core
material a 22/14b-4; 3726.79–3727.09 m MD (12,227–12,228 ft);
b 22/14b-4; 37,270.09–3727.40 m MD (12,228–12,229 ft); c
22/14b4; 3754.53–3754.83 m MD (12,318–12,319 ft); d 22/19-1;
3506.11–3506.42 m MD (11,503–11,504 ft)
and Morad 2003; Franks and Zwingmann 2010). Kaolinite
and K-feldspar are known to react to fibrous illite at low
temperatures (* 50 C)
(Bjørkum and Gjelsvik 1988)
increase their reactivity with increasing temperatures
([ 70 C) with illitization being pervasive at temperatures
greater than 130 C
(Worden and Morad 2003)
and K-feldspar could have transformed to pore-filling
fibrous illite and quartz in the Judy Sandstone Member,
until one of the reactants was completely consumed
(Chuhan et al. 2000; Franks and Zwingmann 2010)
Although, no remnants of kaolinite were observed in the
Seagull sample sets, the eogenetic formation of kaolinite
from feldspar dissolution cannot be excluded. Kaolinite is
not found in the Skagerrak Formation of the area
Humphreys et al. 1989; Nguyen et al. 2013; Stricker et al.
, and no chemical or structural remnants of kaolinite
could be detected during XRD, SEM or SEM–EDS
analysis (Table 7), so it is either completely illitised or was not
present initially. Traces of illite/smectite have been
detected in the sample sets (Table 7), which points towards
mechanically infiltrated smectite clay minerals as a main
precursor material for the authigenic clay mineral
assemblage of the Seagull sample sets.
Variations in the amount and distribution of the
authigenic clay minerals between the 22/29-2 and 22/29-3
sample sets are proposed to relate to variable amounts of
infiltrated clay minerals through fractures and
disaggregation bands (Fig. 14). Intergranular and pore-filling clay
mineral aggregates are common in both sample sets, but
are particularly abundant in the 22/29-3 sample set
(Table 2, Fig. 10b). Even though the authigenic clay
mineral aggregates comprise significant microporosity
(Fig. 5), they reduce effective porosity by infilling the open
pore space, and tend to decrease fluid flow significantly by
blocking pore throats.
McKie and Audretsch (2005)
reported significant permeability reductions (hundreds of
mD) in the channel sandstones of the Seagull field, in
comparison with the Heron field ([ 10,000 mD), where the
clay minerals tend to line the pore space and pore-filling
clay aggregates are less common
(Stricker et al. 2016b)
5.2 Spatial variations of reservoir quality
within salt-walled mini-basins
The Seagull mini-basin formed due to differential loading
and withdrawal of Late Permian Zechstein salt. The
Permian salt has negligible shear strength on long timescales,
and hence, it deforms by differential sedimentary or
tectonic loading. Coeval halokinesis during mini-basin infill
and post-depositional halokinesis affected reservoir quality
of the confined channel sandstones due to spatially
variability of mineral alterations, induced by spatial variations
in the pore fluid chemistry, depending on the proximity to
the Permian salt (meteoric to hypersaline pore fluids)
(Fig. 14) and halokinesis-induced faults and fractures
(syndepositional and/or post-depositional) which enhance or
reduce fluid flow locally (Fig. 12).
The Judy Sandstone Member within the Seagull
minibasin shows strong reservoir quality variations between the
22/29-2 and 22/29-3 wells. The 22/29-3 sample set shows
generally lower porosities and poorer reservoir quality than
the 22/29-2 sample set (Fig. 5). Both sample sets have
undergone a similar burial history, reflecting similar VES
and temperature histories (Fig. 6) and show comparable
IGV values (Table 8), with similar average values of
30.3% (22/29-2) and 32.5% (22/29-3). The intergranular
cement volumes, however, show significant variations
between the two sample sets (Figs. 9, 10 and Table 8) and
indicating a strong relationship between reservoir quality,
diagenesis and the degree to which clay infiltration has
influenced early fabric. Detailed petrographic analysis of
the confined fluvial channel sandstone samples shows
significant ([ 10%) more authigenic clay mineral cements
within the 22/29-3 samples (Figs. 9, 10a, b). Variations in
the distribution of the clay mineral cement correlate to the
increased occurrence of fractures and disaggregation bands
towards the salt wall at the mini-basin margin (Fig. 4).
Disaggregation bands and fractures can be encountered
in both cores, but are significantly more common in the
22/29-3 core proximal towards the salt wall (Figs. 4, 12).
Faulting of mini-basin infill can commonly be encountered
proximal to mini-basin margins or to tectonically active
salt walls, e.g. the Huntington field (22/14b-4) or the
Fiddich field (22/19-1) (Figs. 4, 15). The contemporaneous
salt movement and post-depositional salt withdrawal at the
mini-basin flanks lead to changes in the internal stresses of
the sediment bodies and to higher densities of fractures and
disaggregation bands in the sedimentary sequence (Fig. 14)
(e.g. Fox 1998; Mark and Rowan 1999)
. Fractures and
disaggregation bands represent ideal pathways for meteoric
fluids and generate local fluid chemistry variations within
the sediment body and the mini-basin. The higher fracture
densities at the marginal areas proximal to the salt walls
hold the potential for increased meteoric influx, hypersaline
pore fluid chemistry due to the proximity to salt bodies,
increased clay mineral infiltration and localized pore fluid
changes during shallow burial. This leads to diagenetic
variations in the mineral and especially clay mineral
assemblage during shallow and deep burial. The higher
density of disaggregation bands at the mini-basin margin
can also complicate fluid flow and reservoir quality
prediction as fractures, faults and disaggregation bands create
Hydrocarbon exploration in salt basins can be challenging
due to the wide range of potential tectonostratigraphic
controls on the temporal and spatial facies distribution and
the diagenetic interplay between salt and sediments during
(e.g. Smith et al. 1993; Barde et al. 2002; Matthews
et al. 2007)
. The tectonic effects of halokinesis on
saltwalled mini-basins are significant and can influence the
shape and orientation of the mini-basin, as well as the
sediment deposition and diagenetic alteration within it.
Salt-walled mini-basins are commonly faulted and tilted in
the Central North Sea
(McKie et al. 2010)
, which creates
structural highs at the marginal areas of the mini-basin.
These structural highs can form structural traps with up-dip
closure against impermeable salt and provide excellent
hydrocarbon plays. However, this research has highlighted
strong intra-basin reservoir quality variations within the
mini-basins, due halokinesis and diagenetic alterations.
Banham and Mountney (2013b) have highlighted
depositional variations within salt-walled mini-basins and have
shown higher reservoir potential with more stacked
channel sandstones for the mini-basin centres. This research has
taken spatial reservoir quality analysis even further and
provides valuable insights into the diagenesis of the
channel sandstones. It has been shown that fluvial channel
sandstones in the central areas of mini-basins exhibit
higher reservoir quality due to less clay mineral
cementation and lower density of fracture and disaggregation
Central areas of salt-walled mini-basins exhibit better
reservoir quality due to higher ratios of stacked channel
sandstones, less clay infiltration and less diagenetic
alteration, whereas marginal areas provide excellent traps but
poorer reservoir quality.
Excellent reservoir quality with anomalously high
porosities of up to 20% at burial depths of [ 4000 m
([ 13,123 ft) is preserved in Skagerrak Formation of
the Seagull field (UK Quadrant 22).
Reservoir quality varies significantly, with
maximum porosity from 8% to 20% (PPC), within
confined channel sandstones in the salt-walled
mini-basin despite similar burial and diagenetic
An increase in fracture and disaggregation band
density proximal to salt walls has led to increase clay
mineral infiltration and subsequent porosity
reduction due to higher fractions of pore-filling authigenic
chlorite and illite within fluvial channel sandstones
proximal to the salt walls.
Reservoir quality appears to be influenced by spatial
positioning within salt-walled mini-basins. Within
the fluvial channel facies, reservoir quality can be
shown to vary in relation to distance from salt walls,
with lower porosity adjacent to the salt walls.
Acknowledgements The research consortium GeoPOP sponsored by
BG, BP, Chevron, ConocoPhillips, DONG Energy, E. ON, ENI,
Petrobras, Petronas, Statoil and Tullow Oil at Durham University is
thanked for funding this research. We acknowledge support from the
BGS for access to core material from the Seagull wells and X-Ray
Mineral Services Ltd for XRD analysis. Norman Oxtoby is thanked
for conducting the fluid inclusion analysis. We thank Andra´s Fall,
Philip D. Heppard, Ben Kilhams, Mark Osborne, Andrew R. Thomas
and Guanghui Yuan for their suggestions and constructive reviews to
help improve the manuscript. The results presented have been
improved through collaborative discussions with many colleagues
including Peter Andras, Andy Aplin, Mark Brodie, Jon Gluyas, Neil
Goulty, Neil T. Grant, Sean O’Neil and Shanvas Sathar.
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