compositional variations in diagenetic chlorites and illites, and ...

Clay Minerals (1989) 24, 157-170

C O M P O S I T I O N A L V A R I A T I O N S IN D I A G E N E T I C C H L O R I T E S A N D ILLITES, A N D R E L A T I O N S H I P S

WITH F O R M A T I O N - W A T E R C H E M I S T R Y

J. S. J A H R E N AND P. A A G A A R D

Department of Geology, University of Oslo, PO Box 1047, Blindern, N-0316 Oslo 3, Norway

(Received 23 January 1989)

ABSTRACT: Authigenic illites and chlorites from elastic reservoirs, offshore Norway, have been studied by analytical transmission electron microscopy (ATEM). Present-day reservoir conditions range from 1600 m burial depth and 70~ to 4400 m and 160~ The reservoir units have experienced continuous subsidence since early Cretaceous, and are presently at maximum burial. Textural and morphological evidence indicates that the authigenic illites and chlorites investigated are late diagenetic products, except for an early-formed berthierine in the shallowest reservoir. While the illites show very limited chemical variability at temperatures between 140 ~ and 160~ the chlorites exhibit definite compositional trends with increasing temperatures. Tetrahedral Ai increases substantially from 100~176 and the octahedral vacancy decreases correspondingly. These observations indicate that continuous recrystallization of authigenic clays has taken place. This is also supported by the chemical composition of present-day porewaters, which are close to equilibrium with the clay mineral assemblage.

Illites and chlorites are diagenetic minerals of considerable importance both because of their general geochemical reorganization during diagenesis and also their influence on the production of hydrocarbons from elastic reservoirs. A better understanding of the control and mechanisms of their formation is needed in order to model their distribution in sedimentary basins. This paper focuses on the formation of iUites and chlorites from kaolinites in sandstones.

Illites forming from kaolinite precursors are normally characterized by low octahedral Mg and Fe (Srodon & Eberl, 1984), and the same is true for hydrothermal illites. No compositional trends have been demonstrated for these illites in the diagenetic regime, although Cathelineau (1987) has used the compositions of hydrothermal illites coexisting with chlorites as geothermometers.

Velde & Medhioub (1988) have recently discussed the compositional range of diagenetic chlorites within both sandstones and mudstones from the equilibrium standpoint. This study was based on microprobe analyses of chlorites from a series of samples taken from two wells covering temperature ranges of respectively 30~ ~ and 80~176 They concluded that these diagenetic chlorites appear to form under equilibrium conditions, and that they repeatedly recrystallized during increasing diagenetic grade.

We have adopted a similar approach in this study. Samples from Jurassic clastic reservoirs offshore Norway, representing a wide range of burial depths and temperatures, have been examined to establish both the occurrence of diagenetic chlorites and illites, and their compositional variation.

Any thermodynamic discussion of diagenetic clay minerals must refer to the activity- composition relations of these minerals (Aagaard & Helgeson, 1983; Tardy & Fritz, 1981;

158 J. S. Jahren and P. Aagaard

Stoessel, 1979, 1981, 1984; Cathelineau & Nieva, 1985), and also have available accurate and reliable chemical compositional data. No method can match the analytical electron microscope in obtaining chemical analyses of individual clay-sized grains. The purpose of the present contribution, therefore, is to report the compositional variation of chlorites and illites of clastic reservoirs, offshore Norway, obtained from detailed analytical transmission electron microscope (ATEM) analyses, to apply these data in the evaluation of their equilibrium relations with present-day porewater, and to compare these results with the actual formation-water chemistry.

S A M P L E S A N D G E O L O G I C A L S E T T I N G

The samples used in this study were provided by STATOIL through the VISTA program. All samples are core samples from Jurassic hydrocarbon reservoirs, offshore Norway. They are all taken from feldspathic sandstones ranging in burial depth from 1600 m to 4400 m. Formation-water data were available for all samples. Sample depths, formations and formation P - T conditions, lithology and locations are given in Table 1.

Samples I, II and III define a section from the east flank of the Viking Graben (Fig. 1) off

t 66*-

64:

62"_..

t4

60_*

58"

56*_

�9 o 4) o o o o

2 0 2 4 6 8 10 I I I I I I I

FIG. 1. Location of wells from which core and water samples were taken. Samples I, II and Ill are from the east flank of the Viking Graben, while samples VI and V are from the Halten-

banken area.

Compositions of diagenetic chlorite and illite

TABLE 1. Sample details.

159

Depth Porewater Sample (m) Locat ion Formation Stage P(b)/T(~ Lithology

I 1595 T r o l l Fens f j o rd Caiiovian 160/70 Feidspathic sandstone II 2852 Statf jord Stat f jord Het tangian 400/100 Feldspathic sandstone III 4258 Hild Tarbert Bathonian 775/150 Feldspathic sandstone IV 3803 Tyrihans Garn Bajocian 380/140 Feldspathic sandstone V 4370 Heidrun Garn Bajocian 420/160 Feidspathic sandstone

the coast of southwest Norway towards the middle of the graben structure; they show an increase in burial depth from 1600 to 4300 m. This variation in burial depth is reflected in the authigenic mineral assemblages present, as identified by optical microscopy and X-ray diffraction (XRD). Samples IV and V were taken for comparison from a different region off the coast of mid-Norway, the Haltenbanken area. Both samples belong to the same Jurassic sandstone formation (Garn) but were at different burial depths. This depth range (600 m) makes an assessment of trends in crystallinity with depth for illites possible.

Detailed description of the locations mentioned above can be found in Spencer et al. (1987).

M E T H O D S

Thin-sections were prepared using Crystalbond, a glue that melts around 60~ and is soluble in acetone. After conventional optical microscopy and scanning electron microscopy (SEM), selected areas of the thin-sections were chosen for ATEM and mounted on copper grids after coring by an ultrasonic drill. The ATEM samples were ion thinned and carbon coated in order to minimize specimen heating and charging.

The microscope used was a JEOL 2000 Fx 200-kV instrument with side-entry stage having tilting angles of up to 30 ~ C~ (spherical aberration coefficient) of 2.3 mm and a point-to-point resolution of 2-8 A. Analytical electron microscopy was performed in the same microscope using a Tracor Northern TN 2000 multichannel analyser for energy-dispersive X-ray analysis. The analyses were performed using the Cliff-Lorimer approximations for thin films (Cliff & Lorimer, 1975).

All samples were studied at an accelerating voltage of 200 kV. Lattice-fringe spacings, selected-area diffraction (SAD) and chemical analysis were used to differentiate between the various clay minerals encountered in the specimens. Lattice fringes were diagnostic in most cases since chlorites and illites have different basal spacings (7 and 14 A in chlorites and 10 and sometimes 20 A in illites). Tilting of small crystals in a conventional TEM in order to obtain proper crystal orientations for lattice imaging is not easy even at eucentric height in the SAD-mode. Convergent-beam electron diffraction (CBED) which maps out more of the reciprocal space is a better method generally, but it is difficult to use CBED on clay minerals without a cold stage, and even then the rapid rate of beam damage associated with clay minerals is a severe limitation on the usefulness of the method.

O B S E R V A T I O N S

Sample I, the shallowest sample studied, was interesting mainly due to a phase of early- formed diagenetic chlorite. The specimens taken from this sample contained substantial


0<

Q

r

=o ~ "

6 ~

Compositions o f diagenetic chlorite and illite 161

<

4

3

~J

2 .(7.

I '-..-

0

I I I

2- - - . .

f_lb - c h l o r i t e s i I _ \ - - - - - - - - = ~

I " III '~, I ~?e \ , k c h l o r t t e ~ A �9 \~, I J , ~ - - " -V IV" / rine / / V TM ~ j

/ I II ] - - - / I

Swe l l i ng ch lo r i tes /I I t I I I I t l I I I

.1 .2 .3 .4 .5 .6 .7 .8 .9 1.0

O c t a h e d r a l F e / ( F e + M g )

FIG. 3. Compositional variation of chlorites plotted as tetrahedral A1 vs. Fe/(Fe + Mg) (Hayes, 1970). Classification of authigenic chlorites according to Curtis et al. (1985), with the compositional range exhibited by metamorphic IIb chlorites (Foster, 1960) shown for

comparison.

amounts of pore-filling authigenic 7 A chlorite of the berthierine type. The chlorite crystals found were large and relatively well crystalline as can be seen from Fig. 2a. In some of the crystals a transformation from 7 to 14 A was clearly visible. This transformation is generally thought to occur at temperatures of ~ 100~ for diagenetic environments (Velde, 1985), and in this sample was initiated at ~ ?0~ The transformation of berthierine to chamosite will be described in a later paper. Chemically, the 7 A chlorite phase was similar to berthierines studied by Curtis et al. (1985) and falls within the berthierine formation field (Fig. 3). A change in octahedral occupancy as a result of the transformation from a 7 A structure to a 14.4, structure was indicated by X-ray analysis of the sample.

Sample II contained an authigenic phase of what Curtis et al. (1984) described as a swelling chlorite, i.e. a chlorite intergrown with a smectite and/or a proto-illitic phase. The identifi- cation of 10 A lattice fringes sporadically intercalated in the chlorite aggregates supports this assumption. The rather poorly crystalline chlorite crystal(s) shown in Fig. 2b also support the idea of a mixed phase. The fringe spacing visible includes both 7 A and 14 A but, as can be seen from the SAD pattern (001-row) taken from the crystal imaged, this is a 14 A chlorite (i.e. the 7 A appearance of the crystal is not due to a 7 A (berthierine-type) chlorite but a product of experimental and specimen conditions). The phenomenon occurs because the chlorite structure contains two different octahedral sheets with different scattering power. At certain crystal thicknesses and experimental conditions these scattering powers are equal or nearly equal, and the structure based on lattice fringes alone will look like a 7 A structure in a high-resolution electron microscopy (HREM) image. This causes problems if it is necessary to discriminate between 7 A and 14 A chlorite structures from HREM images only. Interpretations based on lattice images alone are therefore not recommended if based solely on an intuitive conception of the crystal image. The reason for this is that intuitive concepts do not take into account the strong dynamic interactions between the emitted electrons and


the specimen in the microscope. Computer calculations of HREM images can help in avoiding problems of this sort in the interpretations of images and is therefore extensively used in our laboratory. The general results of this approach will be presented in a subsequent paper by the first author.

Structurally, the chlorite in sample II is a Ib-fl = 90 ~ polytype. It contains potassium, which also indicates that an illitic phase is present in the assemblage. The phase plots (Fig. 3) within the swelling chlorite field outlined by Curtis et al. (1985). Despite the fact that the subdivision of authigenic chlorites by Curtis et al. (1985) was made rather arbitrarily, it is interesting to note that all the chlorite samples studied in this work fall inside their appropriate fields. The origin of swelling chlorites found in diagenetic environments is at the moment somewhat obscure but they are most probably metastable with respect to the pure chlorite (and illite). The phase found in sample II seems partly to have formed as an alteration product of kaolinite.

Sample III, taken from the Viking Graben in the North Sea, has a diagenetic layer-silicate assemblage consisting of kaolinite, illite and a minor chlorite phase. Illite is found mainly as the 1M polytype, but occasionally the 2M polytype is also present intercalated in 1M illites. This coexistence indicates that either one can form under low-pressure and -temperature conditions. The 1M polytype appears to form more readily, but is not the most stable polytype of the two (Velde, 1985; Helgeson et al., 1978). The composition of the phase is given in Fig. 4. Chlorite was not found in the ion-thinned specimens studied, but some additional TEM work using crushed samples revealed that this phase was present. The composition of the phase is given in Fig. 3 and is consistent with an Fe- and Al-rich chlorite of the lb polytype.

Sample IV was taken from the same sandstone formation as sample V, but at a 600 m shallower burial depth. This burial depth (3800 m) corresponds to the kaolinite-to-iUite transformation depth found by Bjorlykke et al. (1986) for the Haltenbanken area. That the specimens investigated were close in depth to the onset of illite formation was dearly reflected in the illite crystals. Those from sample IV were characterized by diffuse scattering and small crystal sizes, while'the illite crystals in sample V were well-ordered and found as large aggregates. Chemically the two illites are identical (Fig. 4), and the variation in crystallinity must therefore be attributed to the difference in residence time in the illite formation field which the two samples have experienced. The chlorite phase encountered in this sample was similar to the one found in samples V and III, i.e. an overgrowth of the/b- type on quartz. Fig. 2c shows a well-crystalline example of the phase where the crystal exhibits a regular fringe spacing of 14 A with some intercalated fringes at ~9 /~ and ~6 A apart. These layers correspond to single talc-like and single brucite-like layers in the structure (Veblen, 1983). The chlorite composition of this sample is given in Fig. 4b.

Sample V is the deepest sample studied in this investigation. It contains both chlorite and illite together with kaolinite. The presence of unreacted kaolinite in the sample together with K-feldspar indicates that total iUitization, in a geochemical sense, is incomplete. In the Haltenbanken area the onset of illitization occurs at ~ 3800 m burial depth according to Bjorlykke et al. (1986), which is 600 m shallower than the burial depth of sample V. A phenomenon which was observed in several instances in sample V is shown in Fig. 2d, where an intergrowth of 20 A layers in an assemblage of 10 A layers can be clearly observed. The reason for this is the kinetically-forbidden 20/~ reflection in 2M illites due to a glide plane (space group C2/c), which appears because of the strong dynamic scattering in the crystal in special projections. This is interpreted as layers o f 2 M illite in a matrix of 1M illite, the latter being the common illite polytype formed in low-temperature environments. Chemically, all

(a)


Mu

A +Ill + '*

Ce py

163

(b) Si AI

AI Fe Mg Fe

FIG. 4. Chemical composition (molar basis) of diagenetic illites and chlorites from clastic reservoirs on the Norwegian shelf. (a) Illite composition in terms of octahedral and tetrahedral substitution (Srodon & Eberl, 1984). (b) Chlorite composition in ternary diagrams. Each point

represents the composition of a separate mineral grain.

the illites studied are K-deficient muscovites with a phengitic component, so the deficiency is compensated through a higher tetrahedral Si occupancy (Fig. 4a). The chlorite content of the sample is rather low; the chemical signature of the mineral is a relatively high Fe/Mg ratio, the phase plotting in the lb chlorite field (Fig. 3).

C O M P O S I T I O N A L V A R I A T I O N S

The spread in analysis results from individual illite crystals taken from the same sample (Fig. 4a) is attributed mainly to the mobility of the alkali ions under the electron beam, and indicates essentially no compositional variation within the sample. The results from all


MR 3

Sample I Chl Well Depth f s p / / ~ - - J / %

I I v 33 /9 -9 2852 Ce

I V * I z ] 640613-1 3803

. 1 650717-1 &370

III + a 2916-1 /.258

k a o l n-~ 2R 3 c h l o r i t e 3R z

FxG. 5. Chemical compositions ofdiagenetic illites and chlorites from Jurassic elastic reservoirs, offshore Norway, plotted on a Velde (1985) diagram. The data represent coexisting iliites and

chlorites (except for sample II where illite is missing).

samples show a great deal of overlap and indicate that illites have a fairly restricted compositional field of formation in the diagenetic environments encountered in this study. Chemical compositions are summarized in a Velde diagram (Fig. 5), this showing that no significant compositional variations were detectable in the illite phases investigated. This reflects the similar formation conditions (pressure, temperature, formation-water chemistry) experienced by the different iUite phases studied. To del!neate any compositional P - T trend, illites formed at either lower or higher temperatures need to be examined.

Chlorites show much greater compositional variations than the illites (Fig. 4b). The spread in analysis results from individual chlorite crystals taken from the same sample are very small, and the compositional shift from one sample to another is clearly depicted in the ternary diagram in Fig. 4b. To analyse the compositional variations of these chlorites, we have adopted the approach taken by Cathelineau & Nivea (1985). They found an increase in tetrahedral Al as a function of increased temperature in hydrothermally-formed chlorites from Los Azufres, Mexico. The same type of increase is found in this study of diagenetically- formed chlorites (Fig. 6a). The increase in tetrahedral Al is substantial over a relatively narrow temperature interval (50~ Sample I is omitted from the plot because of the different chlorite structure (7 A) found in this sample. Another plot which supports the assumption that temperature is a significant control in the formation of chlorite crystals is presented in Fig. 6b, where the proportion of vacant octahedral sites is plotted against temperature. It is evident from this plot that the octahedral vacancy is also highly dependent on temperature. Also, the data points follow the same trend as proposed by Cathelineau & Nivea (1985), although plotting slightly above their line.

S O L I D - S O L U T I O N M O D E L S A N D E Q U I L I B R I U M C A L C U L A T I O N S

The activity-composition relationships of illites and chlorites have received considerable attention over the last few years (Aagaard & Helgeson, 1983; Tardy & Fritz, 1981 ; Stoessel,

(a)

>., u t - -

u

>

(11

r r u

O


lOt t 0.5

0.0 l i 50 100 150 200

TEMPERATURE (*C)

(b) I I

_ 1 . 0 - I-i <

L -

"10 r

,= 0.5

l .=-

Cat )

0.0 I J 50 100 150 200

TEMPERATURE ("C) F[6. 6. Compositional variations of diagenetie chlorites with reservoir temperature. (a) Calculated octahedral vacancy based on the ideal trioctahedral formula (six octahedral cations). (b) Calculated tetrahedral A1. The temperature dependence of hydrothermal chlorites proposed

by Cathelineau & Nieva (1985) is shown on both (a) and (b) for comparison.

165

1979, 1981, 1984; Ransom & Helgeson, 1988), and a series of solid-solution models has been proposed. However, because the accuracy of thermodynamic data of end-member components is closely interrelated with the choice of model parameters and the reliability of the model chosen, we feel that a simple model such as the ideal site-mixing one (Helgeson & Aagaard, 1985) is sufficiently precise at the present stage.

As we use the thermodynamic database of Helgeson and coworkers (1978, 1981), we are representing illite and chlorite by muscovite and clinochlore components respectively. The


- r" O

0-=-2 =

~ - 3 - dr ~ : - 4 -

O -5 -

-6 50

! I

Berthier ine r l

1 1 I I I I Q I I i i t I

Illites

ChLorites O\ ~(lb-p=90*)

\ O O TM

O"

] ] I ] ] I ] ] ] t t

100 150

TEMPERATURE (*C)

, 0

- -1

- - 2

- - 3

--- /4

- 5

-6 200

J o

t~

7g

t j3

O

O :I:

FIG. 7. Calculated activities of the clinochlore component in chlorites and the muscovite component in illites. Activities are plotted against the present reservoir temperatures at which

the samples were collected.

component activities, which have been calculated from the formulae given by Aagaard & Helgeson (1983) and Helgeson & Aagaard (1985), are shown in Fig. 7. The nearly identical chemical compositions of the illites are reflected in activities of KAI3Si3010(OH)2 ranging from 0.59 to 0.70. The clinochlore activities in the chlorites on the other hand range from 4.1 x 10 -3 (sample II) to 5.2 x 10 -5 for the deepest samples. The trend of decreasing clinochlore activity with increasing temperature depicted by Fig. 7 is not expected to continue, as the clinochlore end-member tends to be stabilized with higher temperature as the Mg content increases (Cathelineau & Nieva, 1985).

However, we will use the trends of component activities delineated in Fig. 7 to evaluate the equilibrium status of illites and chlorites in these reservoirs. The equilibrium relationships between authigenic minerals and formation-water chemistry can conveniently be presented as aqueous activity diagrams, on to which compositional water data can be plotted and compared with mineral stability fields and boundaries. Based on thermodynamic data by Helgeson et al. (1978, 1981), and choosing pressures corresponding to average North Sea gradients, aqueous activity diagrams for 100 ~ (Fig. 8) and 150~ were constructed. Activities of Mg 2§ K + and H + were calculated by the ion-association model WATSPEC (Wigley, 1977), modified to handle temperatures up to 200~ (Egeberg & Aagaard, 1989).

The formation-water data plot either close to the boundary between iUite and chlorite or within the chlorite stability field, indicating that chlorite alone, or in coexistence with illite, is stable. This observation also adds support to the view of Velde & Medhioub (1988) of an equilibrium chlorite formation and steady recrystallization during increasing diagenetic grade. It is also interesting to note that the whole formation-water data set from the Norwegian shelf (Egeberg & Aagaard, 1989) delineates the same illite-chlorite equilibrium relation. The activity ratio in our water samples is depicted i~ Fig. 9; variations in the clinochlore activity are not able to shift the univariant curve to any significant extent within the scale of this figure.

The homogeneous equilibrium in chlorite solid-solutions can be used to evaluate the activity ratio of Fe z+ to Mg 2+ ions in the porewater from which the chlorite precipitated

Compositions of diagenetic chlorite and Elite

100~ /~00 bar, Otz sat

41- -1-

a~

+ xr

o 3 o

' ' '-' ' / ; '

K - f e l d s p a r !l I

I l l i t e

K a o l i n i t e

0 J ' l

2 4 6 8 16

Ch lo r i t e I I I I I

kll I i

10 12 I/,

I o g ( a M g § 2 4 7 )

FIO. 8. Aqueous activity diagram for I00~ and 400 bar representing stability relationships between K-feldspar, illite, kaolinite and chlorite. The diagram is based on data from Helgeson et al. (1978), and the calculated activities of the clinochlore and the muscovite components in the chlorite and illite observed in sample If. The filled circle represents the corresponding formation

water (see Egeberg & Aagaard, 1989).

167

(Stoessel, 1984). The intracrystalline exchange reaction between Fe 2+ and Mg 2+ in chlorites has been correlated with the coexisting aqueous solution through the equilibrium reaction between the amesite and chamosite components:

Fe4A12(AI2Si2)Oto(OH) a + 4Mg 2+ = Mg4AI2(AI2Si2)O10(OH)8 + 4Fe 2+ (1)

The equilibrium constant for this reaction is given by

logK = 4 log (aFe~,/aMg2.) + log (aaraes./acham.) (2)

By rearranging this expression and applying the ideal site-mixing model to calculate the end- member component activities, the following equation explicitly relates the Fe/Mg ratio in chlorites with the activity ratio of Fe 2+ and Mg 2+ ions in coexisting porewater:

log (aFe2+/aMg2§ 0"25 log K + log (YFe/,~rMg)chlorite The equilibrium constant for reaction (1) was calculated by SUPCRT along an average Norwegian offshore gradient. Standard Gibbs' free energies of amesite and chamosite at 25~ and 1 bar were taken from Stoessel (1984, data set no 1) and heat capacities and third- law entropies were estimated according to procedures given by Helgeson et al. (1978). The results of these calculations are depicted in Fig. 10, where it can be seen that authigenic chlorites in the whole diagenetic regime would be in equilibrium with very low Fe(II) concentrations in the porewater. This implies that growing chlorites would try to incorporate all available Fe from the porewater. Thus the Fe/Mg ratio of diagenetic chlorites will simply reflect the local Fe/Mg ratio of the sediments. This feature has also been observed for hydrothermal chlorites (Cathelineau & Nieva, 1985).

168

0

- 2 0

- , o

-80 O~ 0 -100

t.C3 I ~ 1 2 0

"1- - 1 4 0

-r y - 1 6 0

r

o C~ - 2 0 0

- 2 2 0

- 2 4 0

J. S . Jahren and P . A a g a a r d

.u cov

I I I I I I I I I I I I I I I I I I

20 40 60 80 100 120 140 160 180 200

TEMPERATURE (~

FIG. 9. The equilibrium boundary between muscovite and clinochlore as a function of reservoir temperature along an average North Sea gradient. Quartz saturation is assumed. Filled circles represent the activity product (ag+/aa+) 2 (aMg2+/aa§ Is calculated from formation-water analyses

(Egeberg & Aagaard, 1989).

- 8

- 10

§

§ -12 g u_

0 ~ -1/.

I I

Fe/Mg ra t ios _ in ch lor i tes / /

\ /oO

I I _1,,50r,I 100 150 200

TEMPERATURE (~ FIG. 10. Calculated aqueous activity ratio of Fe 2+ to Mg 2+ ions in equilibrium with chlorites of varying Fe/Mg composition. Filled circles correspond to actual compositions of authigenic

chlorites and present reservoir temperature.

Compositions of diagenetic chlorite and illite 169

S U M M A R Y A N D C O N C L U S I O N S

The range in chemical composition of diagenetic chlorites and illites observed in this study is similar to ranges found in previous studies (Curtis et al., 1985; Velde, 1985). However, the temperature-dependence of the chlorite compositions documented here is new, and indicates that these minerals crystallized at near equilibrium compositions The uniformity of the chemical data within individual samples indicates that 'old' authigenic chlorite phases are replaced by 'new' authigenic chlorite phases with slightly different compositions as a response to small increases in temperature.

No structural variations were detected among the four 14 A phases studied, except for a difference in appearance of the swelling chlorite compared to the other types. The formation of swelling chlorite is attributed to a high content of reactive Mg-containing minerals and/or coatings which will produce a high Mg porewater concentration from which both illite and chlorite are readily formed. In addition, kaolinite may always also act as a source of AI.

Tetrahedral A1 and octahedral occupancy in the chlorites reflect the temperature- dependence of chlorite formation to the extent that these parameters can be used as indicators of the chlorite crystallization conditions. The Fe(II)/Mg ratio in the chlorites on the other hand is not sensitive to P - T conditions, but merely reflects the porewater composition.

Berthierine, which was detected in the shallowest sample studied, is believed to have formed under the influence of organic activity very early in the diagenetic history. The berthierine is unstable both chemically and structurally relative to chamosites and is, at its present temperature (,,~ 70~ in the process of recrystallization to chamosite.

The illites studied cover a limited range of P - T conditions and show no significant variation in compositions from sample to sample. The variations within samples indicates uniform formation conditions for the iUites encountered. Structurally, a definite increase in crystallinity with depth is apparent for the Haltenbanken samples (IV and V). This is interpreted as a recrystallization of authigenic illite crystals with depth. Another structurally important phenomenon is the appearance of packets of 2M illites intercalated in 1M illites. The 1M polytype is the main illite structure observed among diagenetically-formed illites, indicating that the formation of this polytype is favoured kinetically. However, the 2M polytype is thermodynamically more stable and, with sufficient time and/or temperature, the 1M illite will be transformed to the 2M polytype.

ACKNOWLEDGMENTS The research reported above was supported by VISTA, a research cooperation between the Norwegian Academy of Science and Letters, Statoil, and the Norwegian Research Council for Science and the Humanities, and constitutes partly the principal author's MSc thesis at The University of Oslo. Samples were provided by Statoil and the Norwegian Petroleum Directorate. The ATEM work was carried out in The Institute of Physics at The University of Oslo, where we are especially grateful to Erik Sorbroden and Arne Olsen. We acknowledge the help and assistance of Per Kr. Egeberg on formation-water data, and reviews by Knut Bj6rlykke, Jim Boles and Henning Dypvik.

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