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: r.worden@liv.co.uk{ 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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