Geochemical modelling of diageneticreactions in a sub-arkosic sandstone
S. A. BARCLAY {AND R. H . WORDEN*, {
School of Geosciences, The Queen's University of Belfast, Belfast BT7 1NN, UK
(Received 1 May 1998; revised 21 July 1999)
ABSTRACT: A reaction path model was constructed in a bid to simulate diagenesis in the Magnus
Sandstone, an Upper Jurassic turbidite reservoir in the Northern North Sea, UKCS. The model,
involving a flux of source rock-derived CO2 into an arkosic sandstone, successfully reproduced
simultaneous dissolution of detrital K-feldspar and growth of authigenic quartz, ankerite and illite.
Generation of CO2 occurred before and during the main phase of oil generation linking source rock
maturation with patterns of diagenesis in arkosic sandstones and limiting this type of diagenesis to
the earlier stages of oil charging. Independent corroborative evidence for the model is provided by
formation water geochemical data, carbon isotope data from ankerite and produced gas phase CO2
and the presence of petroleum inclusions within the mineral cements. The model involves a closed
system with respect to relatively insoluble species such as SiO2 and Al2O3 but is an open system with
respect to CO2. There are up to seven possible rate-controlling steps including: influx of CO2,
dissolution of K-feldspar, precipitation of quartz, ankerite and illite, diffusive transport of SiO2 and
Al2O3 from the site of dissolution to the site of precipitation and possibly the rate of influx of Mg2+
and Ca2+. Given the large number of possible controls, and contrary to modern popular belief, the
rate of quartz precipitation is thus not always rate limiting.
KEYWORDS: quartz cementation, diagenesis, geochemical modelling, K-feldspar reaction, carbon dioxide.
Numerous sources of quartz cement in sandstones
have been mooted in the literature (e.g. McBride,
1989; Worden & Morad, 2000). The specific source
of silica for quartz cementation has important
ramifications for any predictive scheme for quartz
cementation. It is becoming current practice to
assume that the only source of quartz that need be
considered is stylolites. However, alternative ways
of supplying quartz for cementation include:
sources external to the sandstone (typically
thought to require mass convection; e.g. Burley,
1993), clay mineral transformations (e.g. Hartmann
et al., 2000), biogenic sources from siliceous
bioclasts (e.g. Hendry & Trewin, 1995) and
feldspar-related reactions (e.g. Glasmann, 1992).
In this paper, we consider the systematics of a
common potential source of quartz cement. We
assess the main influences on the feasibility and
extent of feldspar-sourced quartz in a sandstone
reservoir. A thermodynamic model has been
developed to account for petrographic observations
and has been validated with independent data
(water chemistry, gas phase and core stable
isotopes). Whilst we have not developed an
integrated kinetic description of the component
processes that we reveal, we can show that it is
insufficient to assume that any one step during
quartz cementation will always control the overall
rate of the process.
In this paper we use geochemical modelling to
test the hypothesis that quartz cement (and possibly
other authigenic minerals) in the Magnus Field,
ClayMinerals (2000) 35, 57±67
* E-mail: [email protected]{ Present address: Department of Geology &Geophysics, University of Edinburgh, West MainsRoad, Edinburgh EH9 3JW, UK{ Present address: Department of Earth Sciences,University of Liverpool, Brownlow Street, LiverpoolL69 3BX, UK
# 2000 The Mineralogical Society
Northern North Sea, was sourced from the reaction
of detrital K-feldspar (KAlSi3O8) with source rock-
d e r i v e d CO2 p r o du c i n g q u a r t z , i l l i t e
(KAl3Si3O10(OH)2), kaolinite (Al2Si2O5(OH)4) and
ankerite ((Mg,Ca,Fe)(CO3)) via the following series
of reactions (Garrels & Howard, 1959; Smith &
Ehrenberg, 1989; Bjùrlykke & Aagaard, 1992):
CO2 + H2O ? HCO3ÿ+ H+ (R1)
3KAlSi3O8 + 2H+ ?KAl3Si3O10(OH)2 + 6SiO2 + 2K+ (R2)
2KAlSi3O8 + 2H+ + H2O ?Al2Si2O5(OH)4 + 4SiO2 + 2K+ (R3)
Al2Si2O5(OH)4 + KAlSi3O8 ?KAl3Si3O10(OH)2 + 2SiO2 + H2O (R4)
(Mg2+ + Ca2+ + Fe2+) + CO3ÿ ?
(Mg,Ca,Fe)(CO3) (R5)
The decision to assess the impact of CO2 on
feldspar decay reactions in the model was based
upon: (1) the present-day Magnus Field associated
gas that contains up to 2 vol.% CO2, (2) the d13Cof this CO2 is in the range: ÿ13 to ÿ16% PDB,
indicating an organic source (Magnus Field gas
data made available by BP Production Co. Ltd.);
and (3) the arkosic Magnus sandstone reservoir
displays signs of K-feldspar dissolution and
replacement.
THE MAGNUS F IELD
The Magnus Field lies 160 km northeast of the
Shetland Islands and is within UKCS blocks
211/12a and 211/7a (Fig. 1a). The field occurs in
Cretaceous
Triassic
2
3
4
WNW ESE
Upper Kimmeridge Clay Fm.
Magnus Sandstone Member
Lower Kimmeridge Clay Fm.
Depth (km)
Middle JurassicLower JurassicHorizontal scale = Vertical scale
SHETLANDISLANDS
60
61
62N
02W 0 02E
Statfjord
Brent
Frigg
Ninian
MAGNUS
FIELD
UKCS NOCS
Alwyn
Columba
N. Alwyn
Heather
S. Cormorant
Tern
N. Cormorant
Dunlin
Thistle Murchison
50km0
Shetland Islands
200km0
N
Scotland
a
b
Line ofsection
FIG. 1. (a) Magnus Field location. (b) Cross-section through the Magnus Field.
58 S. A. Barclay and R. H. Worden
the Magnus province at the southern margin of the
North Shetland Basin. The Upper Jurassic reser-
voir, the Magnus Sandstone Member (MSM),
occurs stratigraphically between the Lower and
Upper Kimmeridge Clay Formations (LKCF and
UKCF respectively; Fig. 1b). The MSM was
deposited as a submarine fan complex incorpor-
ating thickly- and thinly-bedded turbidite sands and
various fine-grained facies including a small
amount of sensu stricto mudrock (De'Ath &
Schuyleman, 1981).
In the late Jurassic to early Cretaceous, the
reservoir was tilted and eroded in the crestal area of
the field prior to the deposition of the Cretaceous
Cromer Knoll Group. A second break in deposition
occurred during the late Cretaceous followed by
burial during the rest of Cretaceous and Tertiary.
The reservoir is a combined structural and
stratigraphic trap; the seal is the unconformable
mudrock of the Cromer Knoll Group. The source
rock was the off-structure Upper Kimmeridge Clay
Formation (though deeper and hotter than the
stratigraphically equivalent reservoir). Oil filling
occurred at ~75 Ma (Shepherd et al., 1990).
The MSM is a fine- to coarse-grained and
generally poorly-sorted sandstone, classified as
sub-arkosic to arkosic (De'Ath & Schuyleman,
1981). Significant quantities (~14% of the rock
volume) of the detrital feldspars have been
dissolved. The most volumetrically significant
diagenetic cements present in the MSM are:
quartz, illite, kaolinite and ankerite (Shepherd et
al., 1990).
The carbonate cements within the MSM have
d13C values of ÿ7.7 to ÿ12.8% PDB (Macaulay et
al., 1992). The CO2 is present in the associated gas
in Magnus at a concentration of ~2% by volume.
The CO2 has d13C values of between ÿ13 and
ÿ16% PDB strongly suggesting an organic source
for the gas. The mineral and gas isotope data
suggest that carbonate cement and CO2 gas may
share a common carbon source.
SOURCES AND T IMING OFQUARTZ CEMENT IN THE MSM
Macaulay et al. (1992) used stable isotope data
from authigenic minerals within the MSM to
demonstrate that large-scale movement of water
through the reservoir was unlikely. They suggested
that most of the quartz cement in Magnus was
sourced internally from the dissolution of detrital
K-feldspar grains with a minor contribution of
externally sourced silica from clay reactions in
adjacent mudrocks. Other internal sources can
effectively be ruled out due to: (1) absence of any
well developed quartz grain-grain (pressure solu-
tion) textures in the MSM (Macaulay et al., 1992;
Emery et al., 1993); (2) lack of any biogenic silica;
and (3) lack of macroscopic or microscopic
stylolites. Emery et al. (1993), using a combination
of fluid inclusion data from quartz cements,
authigenic ankerite stable isotope data, K-Ar ages
of authigenic illite and thermal history modelling of
the hydrocarbon source rock and the MSM,
suggested that quartz cementation occurred
synchronously with oil generation and migration
into the MSM. The presence of primary petroleum
inclusions in quartz cement shows unequivocally
that quartz cement grew in the presence of some oil
in the pore system.
THERMODYNAMIC MODELL INGINPUT
Thermodynamic modelling can be used effectively
to show how a system should evolve geochemically
in terms of mineral, water and gas chemistry. The
resulting output is exact although it is clearly
dependent on the quality and internal consistency of
the thermodynamic properties and the chemistry
and proportions of solution, gas and minerals. It
should be noted that this approach cannot predict
whether a reaction will happen, rather it assesses
the way a system should evolve to reach
equilibrium. The thermodynamic modelling route
is a vital step towards prediction as it leads to an
understanding of the significance of the component
processes (allowing these to be assessed, ultimately,
in terms of rate).
The dissolution and reaction of K-feldspar was
modelled as a function of CO2 fugacity (fCO2: where
fugacity of a gas can be approximated by the partial
pressure) and the number of pore volumes of fluid
reactants to establish whether paired reactions R1
and R2 or R1 and R3 were viable sources of quartz
cement. The CO2 under consideration may have
been transported into the reservoir dissolved as a
separate gas phase or entrained in either water or
oil, depending on the pressure, temperature and
fluid composition.
The models were run using the computer
program React2 (Bethke, 1994), and a set of
thermodynamic data compiled and distributed by
Geochemical modelling of diagenesis 59
Lawrence Livermore National Laboratory (Delaney
& Lundeen, 1990). This program follows a
reaction-path model (i.e. thermodynamic model)
that traces the chemical evolution of either open or
closed fluid-rock systems. The data presented here
are from calculations based on version R46 of a
thermodynamic database compiled at the Lawrence
Livermore National Laboratory (Delaney &
Lundeen, 1990; Johnson et al., 1991), and employ
the extended Debye-HuÈckel method (Helgeson,
1969) for calculating activity coefficients. Activity
diagrams were prepared using the computer
program Act2 and the same set of thermodynamic
data.
Boundary conditions, essential to constrain the
model, include temperature, water chemistry, gas
chemistry and gas migration effects. The boundary
conditions of the models are given in Table 1.
Temperature
Measured homogenization temperatures from
primary aqueous fluid inclusions in quartz cement
(Emery et al., 1993) show that quartz cement was
precipitated in the range 90ÿ1208C, with the
majority of inclusions yielding temperatures in the
range 110 to 1208C. The mid-point of this range
(1158C) was taken as an estimate of the temperature
during quartz cementation.
Pre-diagenesis rock composition
The MSM is classified as sub-arkosic to arkosic
and therefore contains predominantly quartz, with
significant quantities of feldspar. Before any
diagenetic reactions had taken place, it is likely
that the quantity of K-feldspar in the MSM was
higher than the present quantity. It is thus estimated
that the pre-diagenesis mineralogy had quartz and
K-feldspar in a ratio of 3:1, whilst the present ratio
is ~5:1. The absence of other minerals is not
considered to be of importance for the model
because other minerals which could affect mineral
precipitation (e.g. grain-coating clays), were prob-
ably precipitated during diagenesis. It was assumed
that all Ca2+ and Mg2+ in authigenic ankerite were
derived from the MSM pore-waters (i.e. internally
sourced). However, this may not be the case and is
the subject of further work.
Pre-diagenesis water chemistry
To obtain a water composition before any
diagenetic reactions had taken place in MSM, the
end-products of these reactions (diagenetic
minerals) and the present-day formation water
chemistry (Warren & Smalley, 1994) were exam-
ined. The following assumptions were then made:
(1) Both SiO2 and Al2O3 were re-precipitated
locally within the MSM (as they have very low
aqueous solubility; e.g. Bjùrlykke, 1995) thus
involving no export of these species from the
MSM. (2) Ankerite co-precipitated with quartz (and
clays) removing Ca2+, Mg2+ and Fe2+ from solution.
The pre-diagenesis concentrations of Ca2+, Mg2+
and Fe2+ may thus have been higher than present-
day concentrations due to the precipitation of
ankerite. (3) The pre-diagenesis concentration of
K+ was probably lower than present day concentra-
tion. Before reaction of K-feldspar, the MSM
contained no significant source of aqueous K+.
(4) The pre-diagenesis volume of CO2 in the
reservoir was probably very low. However, dissolu-
tion of syndepositional calcite may have controlled
the pre-existing HCO3ÿ concentration in pre-burial
diagenesis formation water.
TABLE 1. Physical and chemical input parameters used in React2 model.
Parameter Value
Temperature 1158CVolume of reacting fluid 50ÿ500 pore volumesfCO2
0.25ÿ2.00 atmospheresPre-diagenesis minerals present Quartz and K-feldsparPre-diagenesis quartz/K-feldspar ratio 3Pre-diagenesis water chemistry (anions, mg lÿ1) Clÿ (16250), HCOÿ3 (1400)Pre-diagenesis water chemistry (cations, mg lÿ1) Na+ (10100), K+ (30)
Ca2+ (1000), Mg2+ (1000)
60 S. A. Barclay and R. H. Worden
Source and amount of CO2
The source of CO2 was assumed to be from:
(1) the UKCF and LKCF mudrocks surrounding the
reservoir; and (2) the mature oil LKFC source rock
down dip of the reservoir. Previous workers have
demonstrated that the Kimmeridge Clay Formation
generates CO2 upon heating (Andresen et al., 1994).
Although both the LKCF and UKCF surrounding the
reservoir were immature with respect to oil
generation at the time that diagenesis took place, it
is likely that they generated CO2 during burial as this
gas is typically expelled prior to the main phase of
oil generation (Fig. 2a). Additional input of CO2
would also have come from the more deeply buried,
mature source rocks before the main phase of oil
generation. The method of calculating the fCO2
available for reaction in the MSM is illustrated in
Fig. 2b and the input parameters used for the
calculation are given in Table 2. The method relies
on calculating the mass of source rock required to
generate the volume of oil observed in the reservoir,
and relating this source rock mass to the experimen-
tally-determined CO2-generating potential of the
source rock. Using the figures given in Table 2, the
LKCF, UKCF and KCF can generate fCO2of 1.2 atm
at a temperature of 1158C.
Migration losses and pore volumes of
reactants
If oil migration losses (from the secondary
migration pathway and tertiary migration out of
the reservoir) of 96% are assumed (after Jones,
1981), then the fCO2calculated above represents the
minimum amount of CO2 that has entered the
reservoir available for reaction (equal to one pore
volume [pv] of CO2-bearing fluid). The oil volume
in place thus represents the absolute minimum
volume of CO2-bearing fluid that has passed
through the reservoir. To allow for this, the model
was run: (1) at different pore volumes of
CO2-bearing fluid passing through the reservoir;
and (2) at fCO2values >1.2 atm.
THERMODYNAMIC MODELL INGOUTPUT
Effect of input of CO2 into the reservoir
Figure 3a shows that the input of CO2 into the
reservoir causes simultaneous dissolution of
K-feldspar and precipitation of quartz, illite and
ankerite. If we assume that at least some of the CO2
was transported into the reservoir at the same time
Decarboxylationof kerogen
Generation ofliquid hydrocarbons
Volume produced
0
1
2
3
4
35 Co
90 Co
CO2CO2
Oil
Depth
(km
)
Oil volume (m ) and density (kg m )3 -3
Source TOC (wt%)
Source genetic potential (kg ton )-1
Pyrolysis CO yield of source
(mmol CO per g TOC)2
2
Mass of source requiredto generate oil in place
(Oil volume x density)Source genetic potential
=
(Source TOC100
x Mass of source) x CO yield
of source2Moles of CO =2
Use pv = RT to convert molesof CO to pressure
n
2
Convert pressure to fugacity
a b
FIG. 2. (a) Production of CO2 from organic-rich rocks occurs prior to oil generation, typically between
temperatures of 35 and 908C (equivalent burial depth of 1ÿ2.5 km, assuming a geothermal gradient of 358Ckmÿ1). Modified after Curtis (1978). (b) Steps in calculating the minimum fCO2
generated by an organic-rich
rock.
Geochemical modelling of diagenesis 61
as liquid hydrocarbons (a reasonable assumption as
the temperatures of CO2 and oil generation overlap,
Fig. 2a) then the model output substantially agrees
with the synchronous cementation and migration
model of Emery et al. (1993). In fact, it seems that
an early source rock maturation product of the off-
structure KCF (i.e. CO2) has initiated diagenesis in
the reservoir by creating the conditions necessary
for K-feldspar dissolution, i.e. low pH (Garrels &
Howard, 1959) via reactions R1ÿ3, (Fig. 3b). By
using stable carbon isotope data, Macaulay et al.
(1993) concluded that the ankerite in the MSM was
derived from an organic source of HCOÿ3 (with the
d13C of ankerite being in the range ÿ7.7 to
TABLE 2. Input parameters used to calculate minimum CO2 available for
reaction in the Magnus Field.
Parameter Value
Oil in place in Magnus (1) 26109 m3
Magnus oil density (1) 0.826103 kg mÿ3
Source TOC (1) 5.4 wt.%Source genetic potential (1) 30 Kg tonÿ1
Pyrolysis CO2(2) yield of source 2.53 mmol CO2 gÿ1 TOC
(1) Shepherd et al. (1990)(2) Andreson et al. (1994)
Mineralprecipitation
Mineraldissolution
Reaction progress Reaction progress
Quartz
K-feldspar
6.50
6.25
6.00
5.75
5.50
pH
0.0 0.0
0.5 0.5
1.0 1.0
1.5 1.5
2.0 2.0
0 5 10 15 20
K-feldspar % dissolved (in terms of total rock volume) Quartz % precipitated (in terms of total rock volume)
2 4 6 80
Initial pH
Influx of CO reduces pH2
Dissolution of K-feldsparbuffers pH of system
a b
c d
IlliteAnkerite
disequilibrium equilibrium
MinimumfCO2
50 pv
100 pv
150 pv
200 pv
500 pv
Line A
50 pv
100 pv
150 pv
200 pv
500 pv
Line B
Line C
MinimumfCO2
f CO
2
f CO
2
FIG. 3. (a) General diagram produced using React2 showing that as K-feldspar is dissolved, there is simultaneous
precipitation of quartz, illite and ankerite. (b) Example showing evolution in pore-water pH as a model runs
(50 pv, fCO2= 1.2 atm.), the same trend is observed in all model runs. (c) Volume of K-feldspar dissolved as a
function of fCO2and pore volume (pv) of reactants. (d) Volume of quartz precipitated as a function of fCO2
and
pore volume (pv) of reactants.
62 S. A. Barclay and R. H. Worden
ÿ12.8% PDB). The d13C of gaseous CO2 from the
reservoir, reported earlier, overlaps with these data
and suggests that the ankerite is linked genetically
to the CO2. This conclusion agrees with the import
of source rock-derived CO2 as initially assumed in
the model.
Sensitivity of model to fCO2
Assuming that fCO2is the main control on pH in
the model, then according to modelled reactions R2
and R3, the amounts of feldspar dissolution and
quartz, illite and ankerite precipitation are propor-
tional to the fCO2in the reservoir (Fig. 3c,d). Smith
& Ehrenberg (1989) stated that the amount of
carbonate mineral precipitation is likely to be
controlled by the concentrations of Ca2+ and
Mg2+, even when there is an excess of CO2-
derived HCOÿ3 in solution. This is almost certainly
the case in Magnus where the initial concentrations
of both Ca2+ and Mg2+ decrease to an equilibrium
value during reaction (Fig. 4a). This is reflected by
the small amount of ankerite precipitated by the
model (Fig. 3a). As the MSM contains up to
4 vol.% of authigenic ankerite (i.e. more than
predicted by the model) this implies that an external
source of Mg2+ and Ca2+ may have been important.
Sensitivity of model to fluid volumes
Increasing the volume of fluid passing through
the system is a method of exposing the minerals in
the reservoir to greater volumes of CO2 at a given
fugacity value during the reaction. This is shown in
Fig. 3c for the K-feldspar reaction where the data
show that the amount of K-feldspar dissolved
increases with increasing pore-fluid volume, and
hence increasing quantities of CO2.
Formation of illite vs. kaolinite
The model predicts the formation of illite. In a
previous section, it was stated that the MSM
contains both illite and kaolinite. Possible explana-
tions for the lack of modelled kaolinite precipitation
include: (1) the model initially precipitates some
kaolinite which then is either partially or totally
illitized via reaction R4; or (2) the model does not
precipitate kaolinite, as both aK+ and pH are too
high. De'Ath & Schuyleman (1981) concluded from
0
-1
-2
-3
-4
-5
-6
log
activity
Reaction progress
Mg2+
Ca2+
K+
equilibriumdisequilibrium
-4 -3.5 -3 -2.5 -2-2
-1
0
1
2
3
4
5
6
7
8
Clinoptilolite
Gibbsite
Pyrophyllite
115oC
log
K/H
a+
+
log SiO (aq)a 2
Kaolinite3
Muscovite/Illite*2
K-feldspar1
*high crystallinity = muscovite, low crystallinity = illite
a b
FIG. 4. (a) Example showing evolution of bulk pore-water composition as a model runs (50 pv, fCO2= 1.2 atm).
K+ activity initially increases due to K-feldspar dissolution then decreases to an equilibrium value as muscovite/
illite precipitation takes place. The activities of Ca2+ and Mg2+ decrease gradually to an equilibrium value as
ankerite precipitation takes place. (b) Activity diagram for modelled system at 1158C. Before influx of CO2,
pore-water lies in K-feldspar stability field (1); as CO2 dissolves, K-feldspar pore-water moves into the
muscovite/illite stability field (2). If conditions allow, the water can also move into the kaolinite stability
field (3).
Geochemical modelling of diagenesis 63
petrographic data that illite in the MSM was (at least
partly) derived from the illitization of authigenic
kaolinite showing that a combination of explanations
(1) and (2) may be correct. Influx of CO2 into the
modelled system lowers the pH of the pore-fluid via
reaction R1 causing dissolution of K-feldspar and
precipitation of kaolinite via reaction R3. Kaolinite
is stable typically in pore-fluids with low pH and
low aK+ (Fig. 4b). The dissolution of K-feldspar via
reaction R3 releases K+ into the pore-fluid thus
increasing the aK+ in the pore-water. Another effect
of reaction R3 is to increase the pH of the pore-fluid
by consuming H+ from the pore-fluid. Kaolinite is
not stable under these pore-fluid conditions.
However, illite is stable in high pH, high aK+
pore-fluids (Fig. 4b). Therefore in the modelled
system, all of the kaolinite precipitated is trans-
formed into illite via reaction R4.
The fact that authigenic kaolinite is observed in
the MSM means that the model does not accurately
predict the illitization of kaolinite, since the model
predicts that all of the kaolinite is transformed into
illite.
Where did all the K go?
The model predicts the formation of illite from
the dissolution of K-feldspar and this is a potential
problem as illite and K-feldspar contain ~4.5% and
~10% K respectively (K contents from Deer et al.,
1966). There has to be a `sink' for the remaining K
in solution after the precipitation of illite. Analysis
of the adjacent LKCF and UKCF using quantitative
XRD (Barclay, 1999) demonstrated that the major
clay mineral in the LKCF and UKCF is illite
(where the illite is broadly the same as muscovite in
composition, McHardy et al., 1982). It has been
suggested that one of the major pre-burial clay
minerals present in the LKCF and UKCF mudrocks
was mixed-layer smectite-illite (Scotchman, 1987;
Shaw & Primmer, 1991). Upon burial and
diagenesis, this mixed smectite-illite can transform
to illite, although this transformation requires the
input of K (Lynch et al., 1997). Therefore, it is
tentatively suggested that the majority of the excess
K derived from K-feldspar dissolution in the MSM
was exported to the adjacent LKCF and UKCF
formations. This K was then consumed by the
smectite to illite transformation reaction. A minor
amount of K remains in the pore-fluids thus
producing the slight increase in aK+ observed in
Fig. 4a after the model has been run.
Volume of K feldspar dissolved and quartz
precipitated
If it is assumed that all authigenic phases (i.e.
quartz, illite, kaolinite and ankerite) precipitated as
a result of the K-feldspar reactions R2 and R3; then
the vol.% of K-feldspar dissolved (KFD) can be
computed using:
KFD = [AQ + AI + AK + AA] (R6)
where AQ, AI, AK and AA are the vol.% of
authigenic quartz, illite, kaolinite and ankerite,
respectively. These data have been estimated from
modal analyses of thin sections (Barclay, 1999).
This calculation produced an average KFD of
14 vol.% K-feldspar (in terms of total rock
volume) in the MSM. If we assume that 500 pv
of CO2-rich fluid passed through the MSM, then to
dissolve 14 vol.% of K-feldspar would require an
fCO2of 1.5 atm (line A in Fig. 3c). This would
precipitate ~5.5 vol.% of quartz cement (line B in
Fig. 3d). The average amount of quartz cement in
the Magnus Field is ~4.5 (+1.5) vol.%. Again,
assuming that 500 pv of CO2-rich fluid passed
through the MSM, then to precipitate 4.5 (+1.5)%
of quartz cement would require a fluid with fCO2of
1.0 (line C in Fig. 3d). There exists therefore the
possibility of precipitating the average amount of
quartz cement in the Magnus Field by the import of
relatively small volumes of CO2-bearing fluid (i.e.
�108 pore volumes per rock volume required for
external import of silica, discussed previously by
Bjùrlykke & Egeberg (1993). The amount of
K-feldspar reacted, and therefore the amount of
various diagenetic cements that were precipitated, is
thus a function of the interplay between the number
of pore volumes of fluid and its fCO2.
Present-day formation water evidence
Analyses of the present-day formation waters in
the Magnus Field (Warren & Smalley, 1994) report
an average K+/(Ca2+ + Mg2+) ratio of 1. The North
Sea Piper Field is of comparable age, sedimentary
facies, burial depth, formation water salinity and is
also in intimate contact with the KCF (the probable
CO2 source), but contained little or no detrital
K-feldspar (Piper Field reservoir lithofacies being
classified as lithic to sublithic arenites; Burley,
1986). Published present-day formation water
analyses (Warren & Smalley, 1994) for the Piper
Field report an average K+/(Ca2+ + Mg2+) ratio of
64 S. A. Barclay and R. H. Worden
0.1. This order of magnitude difference in K+/(Ca2+
+ Mg2+) ratio supports reactions R1, R2, R3 and R5
as being important in the MSM. The presence and
reaction of K-feldspar in Magnus has led to
elevated K+ and depleted Ca2+ and Mg2+ as a
consequence of quartz, illite and ankerite cementa-
tion. The absence of K-feldspar in Piper has led to a
different water geochemistry through a different
diagenetic history.
IMPL ICAT IONS OF THEMODELL ING WORK
This work has shown that synchronous growth of
quartz, illite and ankerite cements can be explained
simply by an influx of CO2-laden fluids into arkosic
sandstones. The association of these three minerals
during burial diagenesis at temperatures of
80ÿ1208C is very common (especially in arkosic
or sub-arkosic sandstones; Primmer et al., 1997).
The association of quartz, illite and ankerite is
potentially the result of the early expulsion of CO2
from petroleum source rocks and subsequent
migration into sandstones. It is noteworthy that
the model involves a closed system for Al and
silica, but an open system for CO2 (and potentially
Ca and Mg). The combination of open and closed
systems is essential to make the model work and
may be fundamental for burial diagenesis in
feldspar-bearing sandstones.
We have identified at least six steps involved in
quartz cementation in the MSM: (1) CO2 influx;
(2) feldspar dissolution; (3) quartz precipitation;
(4) illite precipitation; (5) ankerite precipitation;
and (6) local transport of silica/alumina from the
site of dissolution to the point of precipitation
(likely to be controlled by the oil content of the
pore network and wettability in oil fields, Worden
et al., 1998). A seventh possible step is the rate of
influx of Ca2+ and Mg2+ from neighbouring
mudrocks since these are required to remove
bicarbonate and thus allow the generation of more
protons (for reaction with K-feldspar) via reaction
R1. The slowest of these steps will control the
overall rate of quartz cementation (Fig. 5).
The rate of quartz cementation and the
achievement of a predictive capability has been
the subject of much work (e.g. Bjùrkum et al.,
1998). Key assumptions in currently fashionable
predictive schemes are that: (1) quartz cement is
sourced from stylolites; (2) quartz cementation is
effectively continuous once it has overcome
temperature related kinetic barriers; and (3)
quartz cementation is only limited by the rate of
influx
of
CO2
K-feldspar
reaction
influx
of Ca
& Mg
SiO2
diffusion
quartz
precipitation
growth
of
dolomite
If dolomite fails to grow = less acidity
available for K-feldspar attack
growth
of
clay
If clay growth is slow, this results in
inhibition of K-feldspar dissolution
FIG. 5. Schematic diagram showing how all the processes (1ÿ6) are related. There is a seventh possible step
involving the inward flux of Ca2+ and Mg2+ required to account for the actual amount of ankerite found in the
MSM. The slowest step in the series will be rate limiting. Despite recent assertions, it is not necessarily safe to
assume that quartz cementation will be controlled by the rate of quartz precipitation.
Geochemical modelling of diagenesis 65
quartz precipitation. Our modelling, observations
and analyses on the Magnus Sandstone Member
appear to cast doubts over such assumptions.
Quartz cementation in the MSM is not sourced
from stylolites and models involving this assump-
tion would be inappropriate. The rate of quartz
cementation in the MSM is potentially limited by
any of the six (or seven) component processes
listed above (Fig. 5) involved in quartz cementa-
tion. Modelling quartz cementation in the MSM
assuming that it is controlled by the rate of quartz
precipitation is potentially inappropriate since it is
an arkosic sandstone undergoing CO2-influx and
has six (or seven) possible rate-controlling steps.
Whilst modelling work based on assumptions
about the control of the precipitation rate is
capable of producing precise output, such output
is essentially useless in cases where the rock
volume in question does not conform to the
fundamental assumptions.
CONCLUS IONS
(1) Moderate volumes of K-feldspar reacting with
CO2-bearing fluids under geologically reasonable
conditions can account for the volume of quartz
cement observed in the MSM.
(2) Increasing (a) the fCO2or (b) the pore-
volumes of fluid that have flowed through the MSM
increases the amount of K-feldspar reaction and the
amounts of authigenic phases (quartz, illite and
ankerite) which are consequently precipitated.
(3) The reaction path model presented here is a
mixture of open- and closed-system diagenesis, but
does not require very large volumes of water to
cause quartz cementation (as suggested for exter-
nally-sourced silica in quartz cementation, e.g. 108
pore-volumes, Bjùrlykke, 1995).
(4) The unrealistically small volume of ankerite
precipitation predicted by a model closed to the
import of Ca2+ and Mg2 implies that an external
source of Ca2+ and Mg2+ may be required to
precipitate the higher than expected volume of
ankerite actually observed in the MSM.
(5) The model overpredicts the volume of illite
precipitated in the MSM and does not predict the
amount of kaolinite precipitated.
(6) The model integrates petrographic, miner-
alogical and water compositional data derived from
the MSM and the results are largely corroborated
by published formation water geochemistry and
mineral and produced gas stable isotope data.
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