clay diagenesis in the sandstone reservoir of the ellon field

~Tays and Clay Minerals, Vol. 47, No. 3, 269-285, 1999.

CLAY DIAGENESIS IN THE S A N D S T O N E RESERVOIR OF THE ELLON FIELD ( A L W Y N , NORTH SEA)

LHOUSSAIN HASSOUTA, MARTINE D. BUATIER, JEAN-Luc POTDEVIN, AND NICOLE LIEWIG 1

Universit6 Lille 1, URA 719, Laboratoire de S6dimentologie et Gdodynamique, 59655 Villeneuve d'Ascq, France i Centre de G6ochimie de la Surface, CNRS, 1, rue Blessig, 67084 Strasbourg Cedex, France

Abs t r ac t - -The nature, composition, and relative abundance of clay minerals in the sandstones of the Brent Group reservoir were studied between 3200-3300 m in a well of the Ellon Field (Alwyn area, North Sea). The sandstones have a heterogeneous calcite cement which occurred during early-diagenesis. Clay diagenesis of the cemented and uncemented sandstones was investigated using optical microscopy, scanning electron microscopy (SEM), X ray diffraction analyses (XRD), and infrared spectroscopy (IR). The influence of cementation on clay neoformation is demonstrated in this study. Detrital illite and authigenic kaolinite are present in both the calcite-cemented and uncemented sandstones suggesting that kaolinite precipitated before calcite cementation. In the uncemented sandstones, blocky dickite replaces vermiform kaolinite with increasing depth. At 3205 m, authigenic illite begins to replace kaolinite and shows progressive morphological changes (fibrous to lath-shape transition). At 3260 m, all sandstones are not cemented by calcite. Illite is the only clay mineral and shows a platelet morphology.

In the cemented samples, vermiform kaolinite is preserved at all depths, suggesting that dickite transformation was inhibited by the presence of the calcite cement. This observation suggests that calcite cement would prevent fluid circulation and dissolution-precipitation reactions.

Key Words--Calci te , Diagenesis, Dickite, Illite, Kaolinite, North Sea, Pressure-Solution, Sandstone.

I N T R O D U C T I O N G E O L O G I C A L S E T T I N G

Clay d iagenes i s can great ly change the phys ica l proper t ies of sands tones in pe t ro l eum reservoirs . Pre- c ip i ta t ion of clay minera l s in the pore spaces and the

evo lu t ion of c lays t h r ough bur ia l and t empera tu re were desc r ibed by m a n y authors ; for ins tance, in the Bren t G r o u p rese rvo i r s of the (Midd le Jurass ic ) Nor th Sea, the evo lu t ion of ill i te and kaol in morpho log ie s wi th dep th and t empera tu re is wel l d o c u m e n t e d (Kan to row- icz, 1984; Thomas , 1986; G l a s m a n n et al., 1989; Gi les et al. , 1992; Hasze ld ine et al . , 1992). E h r e n b e r g et al.

(1993) and L a n s o n et al. (1996) showed the re la t ion-

ship b e t w e e n kaol in m o r p h o l o g y (i.e., ve rmicu la r or b locky kaol in) and the nature of the kaol in po ly type (i .e. , kaol in i te or dickite) . In the B r e n t G r o u p reservoirs, i l l i te rep laces kaol in i te and muscov i t e and shows morpho log ica l changes wi th bur ia l depth. The i l l i t izat ion o f kaol in i te was desc r ibed by B j c r l y k k e (1983) by the react ion: Kaol in i te + K- fe ldspar ---) Il l i te

+ Quartz . A c c o r d i n g to E h r e n b e r g et al. (1993) and L a n s o n et al. (1996), the kao l in -po ly type evo lu t ion and the i l l i t izat ion reac t ion are t empera tu re dependan t .

The pu rpose o f this paper is to descr ibe c lay d iagenes i s in a sands tone r e se rvo i r of the Bren t G r o u p in the

Nor th Sea that is he t e rogeneous ly c e m e n t e d by calcite. We c o m p a r e clay d iagenes i s in ear ly ca l c i t e - cemen ted

sands tones wi th sands tones in w h i c h the calc i te ce- m e n t was absen t dur ing bur ia l d iagenes is . These com-

par i sons a l low d i scuss ion of the m e c h a n i s m s of i l l i te prec ip i ta t ion and the kaol in i te to dicki te t rans i t ion wi th

re fe rence to depth , l i thology, and cementa t ion .

The s tudied sands tones were co l lec ted f rom a wel l o f the E l lon Fie ld (Nor th Sea), loca ted in the Grea t A l w y n area (Figure 1). The A l w y n area is a in t e rme- d ia te - fau l ted terrace be tween the Eas t She t l and Plat- f o rm and the Vik ing Graben . The s t ructure of the E l lon Fie ld is ma in ly the resul t o f r i f t ing o f the Vik ing Gra- ben in two stages (Faure, 1990). T h e first s tage occur red f rom P e r m i a n to Triassic. The second stage is Jurass ic and resul ted in t i l ted b lock faul t ing. Then , an impor t an t e ros ion and a me teor i c f luid f low ep i sode occur red dur ing the C i m m e r i a n phase (Sommer , 1978). A new stage of tec tonic ex tens ion occur red dur ing the late Cre taceous wi th a h igh s ed imen ta t i on rate wh ich sealed the bas in s tructure. The s tud ied sands tones be- long to the Bren t G r o u p w h i c h con ta ins the mos t pro- lific p e t r o l e u m rese rvo i r s in the no r the rn N o r t h Sea B a s i n (B lanche and Whi taker , 1978, H a n c o c k and Tay- lor, 1978, Gi les et al. , 1992). The Bren t G r o u p is composed of five Jurass ic fo rma t ions (late B a j o c i a n to early Ba thon ian) : B r o o m , Rannoch , Et ive, Ness , and Tar- ber t (Bowen , 1975; D e e g a n and Scull , 1977). F r o m a sed imen to log ica l v iew, the Bren t sequence records the p rogres s ion f rom the coasta l s h a l l o w - m a r i n e env i ron- m e n t s o f the L o w e r Bren t Group , to the a l luvial or del ta ic i n t e rbedded sands, shales, and coals of tile M i d d l e Brent , .and ends wi th the t rans i t iona l to m a r i n e sands of the U p p e r Bren t G r o u p ( Jourdan et al . , 1987). In the s tudied well , on ly the Ness B and Tarber t For- ma t i ons were co red (Figure 2). The Ness B is d iv ided in to a lower t i de -domina t ed sec t ion and an uppe r sec- t ion o f three shoreface- t ida l complexes . These two

Copyright �9 1999, The Clay Minerals Society 269

270 Hassouta, Buatier, Potdevin, and Liewig Clays and Clay Minerals

\/ PLAVFORM

Figure 1. Location map of the studied well.

par ts are separa ted by th in l agoona l or coa ly -mar sh facies. The lower Tarbert F o r m a t i o n compr i se s two uppe r shorefaces separa ted by a th in t r ansgress ive lag. The upper Tarbert Fo rma t ion cons is t s of a th ick t rans- gress ive lag (Figure 2). The sands tones f rom the Ness and Tarber t f o rma t ions are he t e rogeneous ly c e m e n t e d by calc i te f rom 3200 to 3260 m. B e l o w 3260 m, the calci te c e m e n t is absent .

S A M P L I N G A P P R O A C H

Sampl ing has focused on c o m p a r i s o n of calci te-ce- m e n t e d sands tones and sands tones where the calci te c e m e n t is absen t ( " u n c e m e n t e d " sands tones) . Bo th sands tones were sampled at d i f ferent dep ths in the Ness and Tarber t Fo rma t ions of the Bren t G r o u p (Fig- ure 2; Table 1). Ca lcu la ted and m e a s u r e d poros i t ies of the s tudied sands tones are - 2 0 % in the u n c e m e n t e d sands tones and < 3 % in the c e m e n t e d sands tones (Pot- dev in and Hassouta , 1997). Cons ide r ing the calc i te con ten t o f the c e m e n t e d sands tones , the ini t ial poros i ty before c e m e n t a t i o n was - 4 0 % . Th i s h i g h ini t ia l porosi ty and the spari t ic texture o f the c e m e n t sugges t tha t calci te cemen ta t i on was an early d iagene t i c process occur r ing at sha l low depth, as sugges ted by Zieg-

Figure 2. Schematic log of the sedimentary facies within the studied well. Symbols: IT = Transgressive lag, SS = Up- per shoreface, M = Marsh, S Shoreface, CT = Tidal complex, BE = Mouth bar. Coal and organic shale are in black. The cemented zones are shown in grey. The filled circles are cemented samples. The open circles represent uncemented samples.

lar and Spot ts (1978). T h e calc i te c emen t a t i o n is he t - e rogeneous . T h e con tac t b e t w e e n c e m e n t e d and un- c e m e n t e d sands tones is a lways ve ry sharp (Figure 3) and does not s eem to c o r r e s p o n d to an ini t ia l he te ro- gene i ty but m a y be a d i sso lu t ion or a p rec ip i t a t ion front. T h e early calc i te c e m e n t a t i o n and the v e r y low poros i ty of the c e m e n t e d sands tones p r e s e r v e d the sands tone gra ins f rom wate r - rock in te rac t ions and fluid flow dur ing burial . S tudy ing the c e m e n t e d and unce -

Vol. 47, No. 3, 1999 Clay diagenesis in a sandstone reservoir 271

Table 1. S u m m a r y of major physical properties of the studied samples.

Medium Opti- Cal- grain cal

Sample depth cite size poro- (m) TVD Formation Facies cement ( tzm) Sorting sity

3200 nc T A R B E R T SS 24 3200 c T A R B E R T SS X - - - - - - 3222 nc NESS S - - 20 3222 c NESS S X - - - - - - 3229 nc NESS BE - - 180 P M 22 3229 c NESS BE X - - - - - - 3234 nc NESS S 15 3234 c NESS S X - - - - - - 3238 nc NESS S - - 155 W 20 3238 c NESS S X 160 W - - 3244 nc NESS S - - 170 W 22 3244 c NESS S X 162 W - - 3245 nc NESS S - - 145 W 20 3245 c NESS S X 142 W - - 3246 nc NESS S 130 W 16 3246 c NESS S X - - - - - - 3254 nc NESS S - - 155 W 17 3254 c NESS S X 160 W - - 3256 nc NESS S - - 160 W 15 3256 c NESS S X 150 W - - 3257 nc NESS S - - - - 13 3257 c NESS S X - - - - 3272 NESS CT - - - - 19 3275 NESS CT - - 185 W 12 3277 NESS CT - - - - - - 16

X = present, SS = Upper shoreface, S = Shoreface, BE = Mouth bar, CT = Tidal complex, PM = poor to moderate, W = well, nc = uncemented sandstone, c = cemented sandstone.

m e n t e d s a n d s t o n e s a l l o w s u s to c o m p a r e c l a y bu r i a l

d i a g e n e s i s in w a t e r - p r e s e n t a n d w a t e r - a b s e n t s a n d -

s t o n e s o f t he s a m e in i t ia l c o m p o s i t i o n . T w e n t y f o u r

pa i r s o f a d j a c e n t c e m e n t e d a n d u n c e m e n t e d s a n d s t o n e s

w e r e s a m p l e d to c o m p a r e c l a y d i a g e n e s i s as a f u n c t i o n

o f d e p t h , l i t h o l o g y , a n d c e m e n t a t i o n (T ab l e 1).

M E T H O D S A N D A N A L Y T I C A L T E C H N I Q U E S

T h i n s e c t i o n s w e r e e x a m i n e d u s i n g p e t r o g r a p h i c

a n d c a t h o d o l u m i n e s c e n c e m i c r o s c o p e s . A T e c h n o s y n

C o l d C a t h o d o l u m i n e s c e n c e M o d e l 8 2 0 0 M k l I o p e r a t -

i n g at 1 5 - 2 0 k v a n d 3 5 0 txA w a s u s e d . B u l k s a m p l e s

w e r e a l so o b s e r v e d b y s c a n n i n g e l e c t r o n m i c r o s c o p y

( S E M ) at 15 k v w i t h a C a m b r i d g e S t e r e o s c a n 240 .

S a m p l e s l o c a t e d in t h e oil z o n e w e r e w a s h e d at t h e

T o t a l - B e a u p l a n C e n t r e to r e m o v e o r g a n i c m a t t e r ( h y -

d r o c a r b o n s ) f r o m the p o r e s p a c e s . T h e c l a y s i ze f r ac -

t i o n s ( < 2 p~m a n d < 5 txm) w e r e s e p a r a t e d f r o m b u l k

s a m p l e s b y s e t t l i ng in a w a t e r c o l u m n . P r i o r to s e p a -

r a t i on , e a c h s a m p l e w a s d i s p e r s e d in d e i o n i z e d wa te r ,

d i s a g g r e g a t e d a n d d e c a r b o n a t e d w i t h a 0 . 2 M HC1 so-

l u t i o n a n d w a s h e d s e v e r a l t i m e s . C l a y m i n e r a l a s s e m -

b l a g e s w e r e d e t e r m i n e d b y X - r a y d i f f r a c t i o n ( X R D )

o n o r i e n t e d a n d p o w d e r e d s a m p l e s w i t h a P h i l i p s P W

1729 d i f f r a c t o m e t e r u s i n g CuKc~ r a d i a t i o n a n d a N i

f i l ter w i t h a s c a n s p e e d o f 2 ~ T h e m o d e l i n g

Figure 3. Section o f sample (3244) showing the sharp transition between (A) uncemented sandstone and (B) a calcite cemented sandstone.

p r o g r a m o f L a n s o n a n d B e s s o n ( 1 9 9 2 ) w a s u s e d to

a n a l y z e X R D p a t t e r n s o f c l a y m i n e r a l s in t he r a n g e 5 -

12.5 ~ T h i s p r o g r a m a l l o w s t h e d e t e r m i n a t i o n o f t h e

c o m p o s i t i o n o f t h e c l a y m i n e r a l f r a c t i o n b y f i t t ing t h e

X R D p a t t e r n to t h e s u m o f t he t h e o r e t i c a l i n d i v i d u a l

p a t t e r n s o f d i f f e r e n t c l a y m i n e r a l s . I n f r a r e d s p e c t r a

w e r e r e c o r d e d in t h e 3 8 0 0 - 2 0 0 c m -~ r a n g e o n a N i -

co l l e t 5 1 0 i n f r a r e d s p e c t r o p h o t o m e t e r u s i n g p r e s s e d

d i s k s p r e p a r e d b y m i x i n g 4 m g o f s a m p l e w i t h 3 0 0

m g o f KBr . C h e m i c a l a n a l y s e s o f s e l e c t e d m i n e r a l s

w e r e p e r f o r m e d u s i n g t h e e l e c t r o n m i c r o p r o b e C a -

m e b a x , S X 50 m o d e l , o f t he C a m p a r i s C e n t e r o f P a r i s

V I I U n i v e r s i t y . S t a n d a r d s w e r e n a t u r a l s i l i c a t e s a n d

o x i d e s , t h e a c c e l e r a t i n g v o l t a g e w a s 15 kv , t he c o u n t -

i n g t i m e 1 0 - 2 0 s, t he c u r r e n t i n t e n s i t y 15 n A a n d t he

s p o t s i ze 1 Ixm.

R E S U L T S

Petrography

T h e b u l k m i n e r a l o g y is v e r y s i m i l a r in al l s a n d -

s t o n e s (Tab le 2). T h e de t r i t a l g r a i n s a re q u a r t z , f e l d -

s p a r s (a lb i te a n d m i c r o c l i n e ) , m i c a , a n d v a r i o u s t y p e s

o f l i th ic f r a g m e n t s . M i n e r a l o g i c v a r i a t i o n s c o n c e r n

p r i n c i p a l l y t he d i a g e n e t i c p h a s e s w h i c h a re c a l c i t e c e -

m e n t , q u a r t z o v e r g r o w t h , py r i t e , kao l i n , a n d i l l i te . M i -

n o r a n k e r i t e w a s a l so d e t e c t e d b y X R D . C a l c i t e a n d

a n k e r i t e a r e o n l y p r e s e n t i n c e m e n t e d s a n d s t o n e s a n d

are a b s e n t b e l o w 3 2 6 0 m e t e r s . K a o l i n m i n e r a l s o c c u r

in c e m e n t e d a n d u n c e m e n t e d s a n d s t o n e s a b o v e 3 2 6 0

m . Q u a r t z o v e r g r o w t h s a n d i l l i te a re v e r y r a re in c e -

m e n t e d s a n d s t o n e s b u t a b u n d a n t in u n c e m e n t e d s a n d -

s t o n e s l o c a t e d b e l o w 3 2 0 5 m .

Quartz. Q u a r t z r e p r e s e n t s 5 0 - 7 0 % o f t h e s a n d s t o n e s .

I t i s m o s t l y de t r i t a l b u t q u a r t z o v e r g r o w t h s a re c o m -

m o n in t he u n c e m e n t e d s a n d s t o n e s . T h e de t r i t a l c o r e

is r i m m e d b y o n e o r t w o s t a g e s o f d i a g e n e t i c qua r t z ,

r e d u c i n g t h e p o r e s p a c e b e t w e e n g r a in s . O v e r g r o w t h s

g i v e a n e u h e d r a l s h a p e to q u a r t z g r a i n s as s h o w n o n


Table 2. Mineralogical data on the studied samples. Relative abundances of each mineral were calculated from modal analyses calculated from point counting on thin section. XRD data are also presented in this table.

Sample depth (ra)

T V D Cal R f O m Py Q K-F Ab M Qo ill Kln K I D I ilP

3200 nc 0.0 0.0 0.8 0.4 65.6 4.4 0.0 0.2 3.2 0.2 0.6 XX - - - - 3200 c 40.0 0.6 0.0 0.0 55.2 4.2 0.0 0.0 - - 0.0 0.0 XX - - - - 3222 nc 0.0 0.4 2.5 0.7 57.3 3.6 0.2 2.0 6.8 3.6 2.2 XX - - X 3222 c 42.9 1.7 5.3 0.6 39.2 3.8 0.3 4.1 - - 0.0 1.9 XX 3229 nc 0.0 0.1 1.8 0.0 51.8 3.2 0.2 1.7 6.0 6.3 6.0 XX - - X 3229 c 47.9 0.5 0.0 1.9 41.1 4.0 0.8 0.7 - - 0.4 2.7 XX - - - - 3234 nc 0.0 0.4 5.4 1.5 49.8 3.6 0.8 5.1 6.4 6.7 5.3 XX X X 3234 c 43.3 1.3 4.5 1.5 40.2 4.2 0.1 3.2 - - 0.0 1.7 XX - - - - 3238 nc 0.0 2.5 0.0 2.9 54.9 2.5 0.2 4.1 3.8 5.1 3.8 XX X X 3238 c 42.3 3.6 0.0 3.7 38.1 5.3 0.0 3.2 0.9 2.8 XX - - 3244 nc 0.0 0.6 2.0 1.1 55.5 1.5 0.5 3.7 4.2 4.8 3.8 XX X X 3244 c 47.4 1.0 3.2 1.2 40.6 2.2 0.4 2.2 - - 0.8 1.0 XX - - - - 3245 nc 0.0 1.2 3.5 1.6 52.1 2.9 0.2 3.3 3.7 6.2 4.6 X X X 3245 c 43.6 0.0 3.4 1.6 42.8 3.0 0.0 3.0 - - 0.0 2.6 XX - - - - 3246 nc 0.0 1.2 2.6 1.0 49.4 2.2 0.4 4.2 8.5 7.8 6.2 X X X 3246 c 46.5 1.0 2.1 0.5 43.4 2.6 0.0 2.6 0.0 1.3 XX - - - - 3254 nc 0.0 1.8 2.0 2.2 47.1 2.6 0.0 5.0 6.6 11.1 4.2 X XX X 3254 c 45.0 6.6 0.0 2.8 39.6 2.2 0.0 2.8 - - 0.0 1.0 XX - - - - 3256 nc 0.0 0.5 2.4 2.2 51.4 2.5 0.2 5.0 5.2 11.1 4.5 - - XX X 3256 c 49.1 0.7 1.9 5.1 36.5 2.8 0.2 1.9 - - 0.9 0.9 XX - - - - 3257 nc 0.0 1.6 2.9 5.5 50.4 2.8 0.4 6.3 4.9 8.7 2.9 - - XX XX 3257 c 46.4 1.0 2.3 4.9 37.0 2.5 0.2 4.2 0.2 1.3 XX - - - - 3272 0.0 2.8 0.0 1.7 63.5 0.3 0.0 4.8 3.0 4.9 0.0 - - - - XX 3275 0.0 2.0 0.2 2.6 67.8 1.6 0.0 2.8 4.6 6.2 0.0 - - - - XX 3277 0.0 0.6 1.8 2.6 70.8 1.0 0.0 2.0 2.4 2.8 0.0 - - - - XX

Data from XRD, X present, XX dominant. Cal = Calcite, Rf = Rock fragments, Om = Organic matter, Py = Pyrite, Q = Quartz, K-F - K-Feldspar, Ab = Albite,

M Mica, Qo = Quartz Overgrowths, ill illite, Kln Kaolin, K ~ = Kaolinite; D ~ = Dickite, ilP = illite, nc = uncemented sandstone, c = cemented sandstone.

opt ical m i c r o s c o p e i m a g e s (F igu re 4A). I n d e n t e d and

s ty lol i t ized con tac t s b e t w e e n qua r t z g ra ins indica te

that p r e s s u r e - s o l u t i o n w a s an eff ic ient m e c h a n i s m de-

c r ea s ing r o c k poros i ty . Q u a r t z o v e r g r o w t h s are a b s e n t

in the calci te c e m e n t e d s ands tones .

Feldspar. K - f e l d s p a r is the m o s t a b u n d a n t f e ld spa r bu t

its c o n c e n t r a t i o n d e p e n d s on the s e d i m e n t a r y facies .

A c c o r d i n g to m o d a l ana lyses , K - f e l d s p a r r e p r e s e n t s 4 -

5% o f the rock v o l u m e in the u p p e r s h o r e f a c e bu t 1 -

7% in the l o w e r s h o r e f a c e facies . T h e a m o u n t o f K-

f e ld spa r a l so va r ies w i th i n c r e a s i n g dep th and c e m e n -

tation. W h e n dep th inc reases , m o s t f e ldspa r s d i sp l ay

d i s so lu t i on m a r k s in u n c e m e n t e d s a n d s t o n e s (F igu re

4B). In c a l c i t e - c e m e n t e d s a n d s t o n e s , K - f e l d s p a r s are

wel l p r e s e r v e d (F igu re 4C) and can d i sp lay an euhed -

ral s h a p e w h i c h resu l t s in th in K - f e l s p a r o v e r g r o w t h s .

The m i c r o p r o b e ana ly se s s h o w a v e r y pu re K - f e l d s p a r

c o m p o s i t i o n o f these o v e r g r o w t h s . T h e y are b l ack

( n o n l u m i n e s c e n t ) in c a t h o d o l u m i n e s c e n c e , ind ica t ing a

v e r y l o w t r a c e - e l e m e n t con t en t r e su l t ing f r o m a l o w

t e m p e r a t u r e o f c rys t a l l i za t ion (i.e., < 1 0 0 ~ Marsha l l ,

1988); this c o n f i r m s the i r au th igen ic origin. K - f e l d s p a r

o v e r g r o w t h s are no t o b s e r v e d in u n c e m e n t e d sand-

s tones . T h e lack o f o v e r g r o w t h s cou ld resu l t in an im-

po r t an t d i s s o l u t i o n o f the K - f e l d s p a r g ra ins in these

sands tones .

Detrital mica. Detr i ta l m u s c o v i t e is p r e s e n t in all s a m -

pies . T h e a m o u n t is no t re la ted to c e m e n t a t i o n , bu t it

is d e p e n d a n t on the s e d i m e n t a r y facies . U n d u l a t o r y

ex t inc t ion and f lexed m i c a c r y s t a l s in the u n c e m e n t e d

s a n d s t o n e s s u g g e s t that d e f o r m a t i o n o c c u r r e d du r ing

c o m p a c t i o n . In c a l c i t e - c e m e n t e d s a n d s t o n e s , m i c a

g ra ins are we l l p r e s e r v e d wi th m i n o r kao l in i t i za t ion

d e v e l o p e d on ly at the g ra in b o u n d a r i e s ( F igu r e 4D) ,

w h e r e a s in u n c e m e n t e d s a n d s t o n e s the m i c a s d i sp l ay

o p e n layers filled b y kao l i n and illite. In s o m e c rys t a l s ,

these t w o m i n e r a l s en t i re ly r ep l ace the mica .

Calcite. In c e m e n t e d s a n d s t o n e s , a calci te c e m e n t fills

the pores . T h e calci te o c c u r s as la rge c r y s t a l s in w h i c h

detri tal g ra ins are enc losed . D e s p i t e e v i d e n c e o f in-

t ense r ec rys ta l l i za t ion , c a t h o d o l u m i n e s c e n c e r evea l s

g r o w t h z o n e s w h i c h s h o w the spar i t ic na tu re o f the

ear ly c eme n t . T h e m i c r o p r o b e ana ly se s s h o w that the

calci te c rys t a l s h a v e the s t ruc tu ra l f o r mu la : Ca0.948- Mg0.047Mn0.004Sr0.00jBa0.001CO 3. S o m e rel ics o f the cal-

cite c e m e n t w e r e f o u n d ins ide qua r t z o v e r g r o w t h s sug-

ges t ing a d i s s o l u t i o n e p i s o d e o f the calci te c e m e n t pri-

or to qua r t z cemen ta t i on .

Kaolin minerals. T h r e e m o r p h o l o g i e s o f kao l in m i n -

era ls w e r e o b s e r v e d b y S E M . (1) V e r m i f o r m kao l in


Figure 4. Optical microscope image of studied sandstones. (A) Quartz grains showing one or two phases of overgrowth (O1 and O2), indented contacts between detrital grains of quartz are shown by arrows, (B) dissolved K-feldspar in uncemented sandstone, (C) preserved microcline (Kfs) in cemented sandstone, and (D) alteration of muscovite to kaolinite in a cemented sandstone. The kaolinite crystals appear at the edge of the muscovite crystal. Plane polarized light. Symbols: Qtz Quartz, Kfs = K-Feldspar, Mus = Muscovite, Kln = Kaolinite, ill = illite, Cal = Calcite.

fills the pores (Figure 5A). (2) Clus ter kao l in (Figure 5B) forms ad jacent to, or be tween , e x p a n d e d layers of detr i tal mica. This m o r p h o l o g y is c o m m o n in the Ness Format ion , par t icular ly in the shoreface facies, and is p robab ly the resul t of the h igher concen t r a t ion of mus- covi te in these sands tones . V e r m i f o r m and c lus te r morpho log ie s are p resen t b o t h in c e m e n t e d and un-

c e m e n t e d sands tones sugges t ing that kao l in i t e prec ip- i ta t ion p receded calci te cementa t ion . (3) B l o c k y kao l in (Figure 5C) is p resen t in the deeper u n c e m e n t e d samples (->3232 m) but absen t in the c o r r e s p o n d i n g ce- m e n t e d sands tones and ve ry rare in i m p e r m e a b l e coa- ly -marsh facies. These c rys ta l s are p s e u d o - h e x a g o n a l in shape, 5 - 1 0 ~tm in th ickness wi th a d i ame te r o f 1 0 -


Figure 5. SEM micrographs of the kaolin polytype morphologies. (A) Vermiform kaolin crystals (3204 m), (B) Cluster kaolin crystals (3224 m), (C) Blocky kaolin crystals (3254 m), (D) Replacement of vermiform kaolin by blocky crystals (3204 m). (A), (C), and (D) are secondary electron images. (B) is a backscattered electron image. Symbols: Mus = Muscovite, Kin = Kaolinite.

20 p~m. This morphology is absent in all calcite-cemented sandstones.

SEM observations of the kaolin morphologies in the uncemented sandstones show a progressive change with depth. In the most shallow samples (3200 m), aggregates of vermiform kaolin crystals were observed, - -80-100 txm, and the diameter of the vermi- cules is <10 Ixm. Deeper in the core (--3204 m), the vermiform crystals are progressively replaced by larg-

er crystals (diameter >10 ~xm) (Figure 5D). Between 3254-3260 m, the blocky morphology is dominant. In the cemented sandstones, only vermiform and cluster kaolinite crystals are observed. Below 3260 m, no kaolin was observed by SEM.

Diagenetic illite. Diagenetic illite is only present in the uncemented sandstones. It occurs as aggregates of small crystals filling the pore spaces. Illite can replace


Figure 6. SEM micrographs of illite morphology changes with depth. (A) Filamentous illite (3204 m), (B) lath illite (3244 m), (C) platelet illite (>3260 m).

the kaolin minerals and detrital muscovite. In SEM images, kaolinite aggregates are replaced by fibrous illite. The SEM images show that illite morphology varies with depth. Filamentous illite (Figure 6A) is observed in the shallowest samples (3204 m), and it is followed by lath morphology (3238 m) (Figure 6B).

Both morphologies occur with kaolin minerals to a depth of 3260 m. Deeper, illite is the only clay mineral identified. It appears as platelet crystals (Figure 6C) which fill the pores and replace detrital mica.

Clay mineralogy (XRD)

The clay fraction (<5 p,m) consists mainly of illite and kaolin (Figure 7). Chlorite is rare and is absent in the upper part of the Tarbert Formation. Kaolins are characterized by the (001) and (002) reflections, respectively, at 7.14 or 7.22 and 3.58 ,~. Illite is characterized by the presence of three peaks at 10, 5, and 3.3 ,~ corresponding respectively to the (001), (002), and (003) reflections. In all of the cemented samples, kaolinite is the major phase in the clay fraction, whereas in the uncemented samples, the intensity of the illite peaks increases with depth. Below 3260 meters, illite is the only clay present in the clay fraction (Table 2).

Kaolin polytype characterization

X-ray diffraction data. The random mounts of the 5- Ixm fraction of the samples were analyzed to characterize the kaolin polytype present in the sandstones. Polytype identification can be difficult because of the superposition of the quartz and feldspar reflections with those of the kaolin minerals (Ehrenberg et al., 1993). However, some characteristic reflections were used to distinguish the various polytypes. In the cemented samples, kaolinite is the only kaolin polytype and is characterized by reflections at 7.21, 3.58, 3.136, 3,32, and 2.34 ,~ (Figure 8A). In the uncemented samples, the XRD patterns display mineralogical changes with depth (Figure 9; Table 2). Samples located between 3200-3223 m contain kaolinite. Below 3223 m, the occurrence of a reflection at 4.13 A indicates the presence of dickite. In samples where dickite is the major kaolin polytype, reflections at 2.32 and 4.13 are clearly visible (Figures 8B and 9).

IR spectroscopy. More accurate distinction between kaolinite and dickite can be determined by the position and relative intensity of the OH-stretching bands in the 3600-3700 cm ~ region of IR patterns. Well-crystallized kaolinite shows a strong absorption at 3697 cm 1, a band of medium-strong intensity at 3620 cm 1 and two bands with relatively weak intensity at 3669 and 3652 cm -1. In contrast, dickite shows a strong absorption band at 3621 cm -~ and two medium-strong bands at 3704 and 3654 cm -~ (Ehrenberg et al., 1993). IR spectra on the <5-1xm fraction of samples from different depths are shown in Figure 10. They confirm the XRD data, i.e., the presence of kaolinite (well- crystallized) in the cemented samples and the presence of dickite in the uncemented samples located at 3254 m (Figure 10A and 10B). The uncemented samples between 3230-3244 m contain both kaolin polytypes. The increase of dickite proportion with depth is clearly


3200

IT

SS , IT

SS

M

tCT

3222 ~ , S

3 2 2 9 u ~

3234 ~ . d 3238 S

3244 ~-~ 3245 W 3246 b M 3247 ~

CT 3254 r~ 3256 ~ W : S 3257 L

~ M CT

~ M

K

K

I K c~ K

I . I I

3272 B5 3275 CT , ~ , . , , , , , 3277 " m 17.6 8. 5.9 4.4 3.5

Q u a r t z

2.9 (A)

Figure 7. XRD patterns of a pair of cemented (a) and uncemented (b) sandstones from 3238 m. The kaolinite peaks are more intense than illite in the cemented sandstones. (c) XRD patterns of a deep uncemented sandstone (3275 m). The kaolinite peaks are absent.

shown by the IR spectra (Figure 10B). In the sample at 3257 m, a shoulder at 3668 cm -~, characterizing the presence of kaolinite, is still visible in the IR spectra. The higher proportion of kaolinite in this clay fraction could be related to the relatively low porosity of this sample (--13%) compared with the other samples (porosity of - 1 8 % ) where kaolinite is in smaller proportion based on IR spectra. In the cemented samples, kaolinite is the only kaolin polytype detected at all depths (<3260 In) (Figure 10A).

Illite characterization

X R D data. The diffraction patterns of the oriented clay fraction of both cemented and uncemented samples display a large and asymmetric peak between 5-11 ~ (Figure 11). In the air-dried samples located above 3260 m, the most intense peak is at 10 ,~; below 3260 m, the illite reflections are the most intense, and a double reflection is visible with a peak at 10.5 .~ and another one at 10 .~. After glycolation, the 10.5-A peak disappears, whereas the peak at 10 .~ becomes more intense, but the reflection is asymmetric at the

lower ~ side. A comparison of the XRD data with data generated by the Newmod program (Reynolds, 1985) indicates the presence of illite rich I-S mixed layers ( - 9 0 % illite layers). With the decomposition method of Lanson and Besson (1992), the simulated XRD pattern reproduces the air-dried experimental pattern in the range 5-11 ~ when they are generated by three gaussian curves respectively at - 1 1 (8.03 ~ 10.5 (8.5 ~ and 10 A (8.8 ~ (Figures 11 and 12). Experimental patterns of the glycolated sam- pies are reproduced correctly with four gaussian curves respectively at --7.3, 8.3, 8.7, and 8.8 ~ These decompositions are very similar to those obtained by Lanson et al. (1996) for diagenetic illite from the sandstones of the Rotliegend Formation. Ac- cording to these authors, these reflections indicate that the illite particles are composed of well crystallized illite (8.8 ~ in air-dried and glycolated samples), poorly crystallized illite (8.6 ~ in air-dried samples) and illite-srnectite mixed layers with ~80% illite layers (--8 ~ in air-dried samples). All the samples can be decomposed with similar gaussian-fit curves (e.g.,


7.17 tl I I 3.58 II I I

] 1[ 4 4 8 7 1 A II I 2.34.1 (~) I Jl " ~l] I ~ . J IF I ~ "13 2.49 /1

I / 3254m I

| ,A413 /I, 232 I 7.17 II II , I. | ~ F [ ~ / Unemented

I I 3254m I ] i I

i , i , t l , i : i i i [ , , , n i i , ~ i p i i i i i i i i i i i i i i i , i ~ , l i i

5 10 15 20 25 30 35 40 45 50

~ Cu Ket

Figure 8. XRD pattern of a powder clay fraction from a calcite cemented sandstone (A) and an uncemented sandstone (B). Peak labels: I = Illite, Q = Quartz, F = Feldspar, D = Dickite, K Kaolinite.

7.21

J Q Q

3 .58

448 t3.,4 L

] . h t4.13 F I

I 7 17 I ' I , I ~ t2.32

, J,I i I , I I . . . . - " I I [ t I I I I t t I I r I I I I P I t I I I I i I I I I F I I I I I I I t I I I

5 10 15 20 25 30 35 40 45 50

~ Cukct

Figure 9. XRD pattern of a random oriented powder clay fraction (<5 Ixm) of uncemented sandstones showing the evolution of the crystallographic structure of kaolin (polytype) with depth. Peak labels: I = Illite, F - Feldspar, Q = Quartz.


I 3668

I I

3800 3700 3600

WAVE NUMBER (cm-1)

Uncemented sandstone

1 3204 m

3234 m

3238 m

3244 m

o r

O q,j c~

<

3246 m

3254 m

3257 m

3652

3668 / 3620 \

i 3696

I

3702

I I

3700 3600

WAVE NUMBER(cm-1)

Figure 10. IR spectra of clay fractions (<5 p~m) from a calcite-cemented sandstone (A) and an uncemented sandstone (B). The four characteristic bands of kaolinite are present in the calcite-cemented sandstone. Only three bands are present in uncemented sandstone from the same depth, characterizing the dickite polytype. The evolution of the IR reflectance spectra as a function of depth in the uncemented sandstone is shown in B whereas in calcite cemented sandstones (A) there is no change with depth.

L a n s o n et al., 1996; L a n s o n and Besson , 1992). How- ever, c o m p a r i s o n of samples f rom dif ferent dep ths shows var ia t ions of the re la t ive in tens i ty of each gaus- s ian cu rve (Figures 11 and 12). PCI (poor ly crysta l - l ized illi te) is more in tense in the deepes t samples whereas W C I (well c rys ta l l ized illite) decreases and " I S " ( = o rdered mixed layers) p resen ts the same in- tensi ty (Table 3).

Relation to illite morphology. X R D pat te rns of the c lay f rac t ion of samples wi th f i l amentous and la th i l l i te par- t icles ( 3 2 0 4 - 3 2 6 0 m depth) have a symmet r i ca l 10 ref lect ions. X R D pa t te rns o f samples be low 3260 m wi th ill i te c rys ta ls h a v i n g a p la te le t m o r p h o l o g y dis- p lay a doub le peak ref lec t ion at 10 and 10.5 A. The 10.5-A peak is less in tense (Figure 11) or absen t on the X R D pa t te rn o f the samples showing il l i te wi th


400

300

._~ 200

i

100

600

500

400

300

200

100

0

3244 m Air dried /

PC]

_.:,_._,~ . . . . . . . . . . . . . . . .

6 7 8 Position (~ Cu KR )

W C I

9 10 11

500

400

�9 ~ 300 = o

2OO

100

0

@ 3275 m

Air dried f

PCI

6 7 8

WCI

: , U'Z-, 9 10 11

Position (~ Cu Kc~) 6|174 500 ] Glycolated A

2001 A\ we, I-S

WCI 100 I-S

5 6 7 8 9 10 11 Position (~ eu Kct )

Figure 12. Decomposition of both (A) air-dried and (B) gly- 6 7 8 9 ~0 11 colated XRD profile of a sample from 3275 m. Symbols: WCI

= Well crystallized illite, PCI = Poorly crystallized illite, Po~itio, (o20 Cu K~ ) "IS" = Ordered mixed layers.

Figure 11. Decomposition of both (A) air-dried and (B) glycolated XRD profile of a sample from 3244 m. Symbols: WCI = Well crystallized illite, PCI = Poorly crystallized illite, "IS" = Ordered mixed layers.

f i lamentous and lath morphologies . H o w e v e r this peak is more intense in the deeper samples where only platelet illite crystals are present (Figure 12).

D I S C U S S I O N

Chronology of diagenetic processes

Accord ing to the petrographical data the chronology of the main diagenet ic stages is the fo l lowing (Figure 13): (1) K-fe ldspar overgrowths and kaolinite precipitation. Both minerals are enc losed in calcite and therefore fo rmed prior to calcite cementa t ion. (2) Hetero- geneous calcite cementat ion. The calcite cementa t ion is an early diagenet ic process which occurred at depths shal lower than 1000 m (Potdevin and Hassouta, 1997). (3) Kaol ini te-dicki te transition. This t ransi t ion is only visible in uncemen ted sands tones and therefore occurred after cementat ion. (4) Illite precipi tat ion, quartz

overgrowth, and fe ldspar dissolut ion. Illite replaces kaolin in samples be low 3204 m. In the deepes t sandstones, kaolin is comple te ly absent, and the pore spaces are filled with diagenet ic illite. Quartz overgrowths ,

illite authigenesis , and fe ldspar d issolut ion are ve ry rare in the cemen t ed sandstones and therefore occurred after calcite cementa t ion.

Kaolinite precipitation

Petrographic invest igat ions suggest that kaolinite precipi ta ted early in the burial h is tory of these sandstones. Kaol ini te precipi ta t ion is a major diagenet ic

Table 3. Relative intensity of each gaussian curve obtained by decomposition of air-dried XRD pattern between 5-11 ~ PCI is more intense in the deepest samples whereas WCI decreases and "IS" presents the same intensity.

Depth (m) PCI (11 ,~) "IS" (10.51 ,~) WCI (/0 ,~)

3238 nc 13.01 53.77 33.22 3238.2 nc 21.05 48.42 30.52 3244 nc 13.71 50.47 35.82 3254 nc 16.00 45.07 38.93 3270.3 nc 19.94 64.65 15.48 3272 nc 6.20 55.43 38.37 3274.8 nc 18.21 68.99 12.80 3275 nc 17.03 55.16 27.81

WCI = Well crystallized illite, PCI = Poorly crystallized illite, "IS" = Ordered mixed layers, nc = uncemented sandstone, c = cemented sandstone.


Brent sandstone deposition

Meteoric water flushing ?

I r~

~D

Vemficular kaolinite Heterogeneous calcite

cementation ( depth < 1000m)

~D

l

Vermicular kaolinite

Dickite

28

O

i

N

I I I I I

Vermicular kaolinite + K-feldspar

1 Illite + quartz overgrowth (Filamentous illite)

Lath illite

?1 Platelet illite

Meteoric water flux related to Cimmerian orogeny ?

Figure 13. Relationships between the clay diagenetic evolution of the sandstones and the major diagenetic events occurring in the North Sea.


event in North Sea sandstones and was discussed by several authors. Sommer (1978) and Blanche and Whi- taker (1978) suggested that kaolinite precipitated during the later stages of burial diagenesis, whereas an early origin from the precipitation by meteoritic fluids is suggested by other authors (Glasmann, 1992; BjCr- lykke and Aagaard, 1992; Haszeldine et al. , 1992). The present study confirms the latter hypothesis. In- deed, in the studied sandstones, kaolinite is embedded in a calcitic cement which precipitated at <1000 m depth (Potdevin and Hassouta, 1997).

Calc i te c emen ta t i on

Calcite cementation is an important factor limiting the diagenetic reactions (Kantorowicz et al . , 1987; Sai- gal and BjCrlykke, 1987; Walderhaug et al. , 1989). According to Bjcrlykke et al. , (1992), the carbonate cement intervals in the Brent Group are generally thin, usually <1 m, and usually more abundant in the marine part of the Brent Group (i.e., Rannoch, Etive) than in the deltaic environment of the Ness Formation. This is well demonstrated in studies of the Heather Field (Glasmann et al., 1989). Scotchman et al. (1989) showed that in the NW Hutton Field, the sub-littoral sheet sand and the wave-dominated delta-front sand contain more carbonate cement than distributary mouth bar and crevasse splay lobe facies. In the Ellon Field, no relationships are observed between the amount of carbonate cement and the depositional facies. According to Potdevin and Hassouta (1997), the cement fills 34-42% of the sandstone volume, suggesting a porosity before cementation of 37-44% (present porosity in cemented sandstones is --2-3%, see Table 1). These high-porosity values indicate that calcite cementation is an early diagenetic episode occurring at shallow depth (<1000 m) (Zieglar and Spotts, 1978).

The absence of carbonate fossils in the studied sandstones and the presence of pyrite inclusions inside calcite crystals indicate that the supply of CO2 could re- sult from organic matter degradation. A similar explanation is given by Walderhaug and Bjcrkum (1992) for the calcite origin of the Oseberg Formation. Cal- cium might be derived from plagioclase, potassium feldspar, and/or heavy minerals dissolution.

Kaol in i t e -d i ck i t e t rans fo rma t ion

Changes in the kaolin polytype morphology in sedimentary basins were initially described for a coal basin in Russia (Kossovskaya and Shutov, 1963; Shutov et al . , 1970), and more recently in many reservoir sandstones from the North Sea. Ehrenberg et al. (1993) described a transition from vermicular to blocky kaolin crystals in Triassic and Jurassic reservoirs of the Norway platform. The same morphological changes were described by McAulay e t al. (1993) in Brent sandstones from the Hutton and NW Hutton

Field (North Sea) and by Lanson et al. (1996) in the sandstone reservoirs from the Rotliegend Formation (North Sea). The transition from vermiform to blocky crystals of kaolin is also observed in the present study, but only in uncemented sandstones. IR and XRD data confirm the structural modifications, i.e., the polytype is dependent on the morphological changes. The kaolinite-dickite transition is observed in uncemented sandstone at 3240 m and is characterized by the progressive replacement of vermiform kaolinite by blocky dickite crystals (Figure 14). The depth of the transition is similar to the depth interval (2.8-3.6 km) proposed by Ehrenberg et al. (1993) and Beaufort et al. (1998) in the sandstone reservoirs from the continental platform of Norway and from the Rotliegend Formation, respectively. Ehrenberg et al. (1993) suggested that the kaolinite-dickite transition is temperature dependant and that it occurs at -120~

The morphological changes observed suggest that the kaolinite-dickite transition is a dissolution-precipitation mechanism. However, Lanson et al. (1996) suggested that this could be a two step process, consisting of a solid-state transformation without any morphological changes, followed by a dissolution-precipitation reaction involving replacement of vermiform kaolin by blocky crystals. Further work is needed to confirm this hypothesis.

D i a g e n e t i c illite

SEM observations of the sandstones above 3260 m show the presence of filamentous and lath particles of illite. These particles seem to grow from kaolinite layers, suggesting that an alteration of kaolinite into illite occurred in these sandstones. Illite authigenesis coin- ciding with kaolinite and K-feldspar dissolution was described previously in the Brent sandstones from the North Sea by several authors (Bjcrlykke, 1983; BjCrk- um et al., 1993; Bjcrlykke and Aagaard, 1992). The reaction which results in the formation of illite is ex- pressed as (Bjcrlykke, 1983):

A12Si2Os(OH)4 + 0.75KA1Si30 s

K0.vsAlz(Si3.25A10.75O10)(OH)2 + S i t 2 q- H20

Kaolinite + K-Feldspar

--, Illite + Quartz

A similar reaction can be proposed in the uncemented sandstones. For instance, in thin sections, K- feldspar dissolution coincides with the appearance of filamentous illite. To characterize and quantify the mobility of elements during illite precipitation in the studied sandstones, mass-balance calculations by the Gre- sens method (Potdevin, 1993) were performed, com- paring the chemical composition of cemented and uncemented sandstones (Potdevin and Hassouta, 1997). Gluyas and Coleman (1992) performed a similar com-


Uncemented sandstone

10 pm

-- 3204

--I 3257

I I

I 1 0 ~tm ~ Depth (m)

Figure 14. Schematic diagram of kaolin-morphology changes with depth in the uncemented sandstones.

parison in about ten reservoir sandstones, of Permian to Tertiary ages, from oilfields worldwide. They show that some early carbonate concretions preserve both the sedimentary fabric and the composition of the sandstones before calcite cementation from later diagenetic processes. In our case, the cemented sandstone represents the sandstones before illite authigenesis and the uncemented sandstone containing illite represents the final stage of the rock after illite precipitation. Composition-volume diagrams following Gresens

(1967) show that Si, A1, and K among other elements are immobile during diagenesis (Potdevin and Hassou- ta, 1997; Figure 15). Therefore, these elements are not brought by fluid flow and illite precipitation occurred in a system closed to Si, A1, and K. The K of illite is provided by dissolution of K-feldspar and the silica released by kaolinite and feldspar dissolution results in quartz overgrowths. In the studied uncemented sandstones, both mass-balance computations and observations agree with the reaction proposed by Bjerlykke (1983) to explain illite authigenesis.

Below 3260 m, illite with a platelet morphology is the only clay mineral present in the pore spaces of the uncemented sandstones. The morphological changes of illite particles with depth could be related to an increase in the crystallinity with burial. However, below 3260 m, clays are in smaller proportion according to modal analyses and kaolinite is absent. Platelet illite crystals seem to have a different origin; they could have precipitated directly from fluids without a kaolin precursor. Greenwood et al. (1994) described similar diagenetic phases in the sandstones sampled in the Hingin Formation from the Brae in the North Sea. These authors suggested that diagenetic kaolin was probably inhibited by temperature and depth of burial. Furthermore, the decomposition of the XRD patterns between 5-11 ~ suggests a decrease in illite crystallinity, i.e., an increase of the PCI (10.5 ,~) curve vs. the WCI (10 ,~) with depth. The increase of the 10.5- ,& reflection is correlated with the presence of platelet illite, whereas above 3260 m, where illite particles present a filamentous or lath morphology, the 10-,~ reflection is more intense. Filamentous and lath morphologies are found at depth lower than 3260 m in the shoreface facies where detrital micas are very abundant. Therefore, the contribution of the diagenetic illite (10.5-,~ peak) could be masked by the stronger contribution of the detrital mica (10-A peak) within the clay fraction. In the deeper samples (>3260 m) the sedimentary facies changes (tidal complex), detrital micas are less abundant, and the diagenetic illite reflection becomes relatively more intense. The second explanation for the increase of the 10.5-,~ reflection in the deeper samples is that platelet particles could cor- respond to poorly crystallized illite that directly precipitated from fluids.

Factors promot ing clay diagenesis in sandstone reservoirs

The clay diagenetic evolution described is clearly different in cemented and uncemented sandstones (Figure 13). Mineralogical data (IR, XRD, and SEM) suggest that kaolinite is the unique authigenic clay mineral in all the studied cemented sandstones whereas a kaolinite-dickite transition is observed in the uncemented sandstones with increasing depth.


ce ' 2

Sample 3238 c 3238 nc -o SiO2 47.20 79.00 "-by 1.6

>

AI203 5.40 8.80 u_ o 1.2 Fe203 1.30 1.20 "

~1 0.8 MnO 0.07 0.01

MgO 0.88 0.19 0.4

TiO2 0.62 0.80 " CaO 22.90 0.20 <El :~

0

Na20 0.45 0.59 ~ -04

K20 1.48 2.40 >" -0.8 S 0.80 1.14 25

O

LOI 18.61 4.77 E -1.2

Total 99.71 99.10 = -~ .s .... O

Density 2.615 2.025 ,-, -2

S TiO 2 Fe2 03 Na20

MnO

MgO

0.77 1 Volume factor Fv

I

CaO

2

Figure 15. An example of mass-balance calculations of illite authigenesis and quartz overgrowths using the Gresens method (1967). (A) The chemical analysis of the calcite cemented sandstone (sample 3238 c) gives the rock composition before illite authigenesis and quartz overgrowths. The chemical analysis of an uncemented sandstone sampled from the same lithology (3238 nc) gives the rock composition after illite authigenesis and quartz overgrowths. (B) Composition-volume diagram of absolute mobility of major oxides Am/M0 vs. Fv. 2xm/M 0 the gain or loss of an oxide (in mass percentage of initial rock) is given by the general relation derived by Gresens (1967). In this diagram, the absolute mobilities of the oxides are graphically shown vs. the possible volume changes accompanying the transformation from a cemented sandstones to an uncemented one. Symbols m~ and m are the mass of the oxide in the initial and final rock (i.e., the cemented and uncemented sandstones), c 0 and c are the weight percentages in this oxide, d o and d the densities and Fv the volume factor or ratio of final volume to initial volume. In the composition-volume diagram, the three major elements Si, AI, and K are simultaneously immobile for a Fv value of 0.77 (i.e., a volume change of 23%). It cannot be a coincidence (Gresens, 1967) and these elements are really immobile during illite authigenesis and quartz overgrowths. CaO, MgO, MnO, and FeO mobilities are related to carbonate cementation (see Potdevin and Hassouta, 1997 for a more comprehensive study of mass balance calculations).

These data sugges t that the kaol in i te -d icki te t ransi- t ion can be inh ib i t ed by the calci te c e m e n t and that t empera tu re and buria l are not the only pa rame te r s con t ro l l ing this t ransi t ion. Z i m m e r l e and R 6 s c h (1991) sugges ted that the d icki te occur rence could be restr ict- ed to s ed imen ta ry rocks of h igh poros i ty and pe rme- ability. This is also conf i rmed in the present study: in cemen ted sands tones ( 0 - 3 % poros i ty) kao l in i te is the only kao l in phase f o r m e d be fo re calc i te c em en t a t i on and in u n c e m e n t e d sands tones ( 1 3 - 2 0 % poros i ty) bo th kaol in i te and dicki te were identif ied. By compar i son , Eh renbe rg et al . (1993), M c A u l a y et al. (1993), and M c A u l a y et al. (1994) ind ica ted that t empera tu re is the m a j o r factor con t ro l l ing the kaol in i te -d ick i te transition. A recent s tudy by Bua t ie r e t al . (1997) of clay- mine ra l evo lu t ion in thrus t faul ts f rom the sou th Pyr- enean bas in demons t r a t ed that de fo rma t ion could be an impor t an t fac tor p r o m o t i n g dicki te crys ta l l iza t ion. The present s tudy demons t r a t e s that porosi ty and per- meab i l i ty are impor t an t pa ramete r s also. The calci te cemen t wou ld p reven t fluid flow in c e m e n t e d sandstones. By fil l ing the pores , ca lc i te c e m e n t wil l p r even t

f lu id-rock in te rac t ions and chemica l reac t ions by dis- so lu t ion-prec ip i ta t ion , such as the kao l in i t e -d ick i te t ransi t ion.

C O N C L U S I O N S

The sands tones o f the E l lon Fie ld r e se rvo i r (Nor th Sea) show a c o m p l e x d iagene t ic evo lu t ion ; the first au th igen ic c lay mine ra l is kao l in i te w h i c h p rec ip i t a ted dur ing early d iagenes is . Then , the sands tones were he t e rogeneous ly c e m e n t e d by calcite. This calc i te ce- m e n t p r e s e r v e d kaol ini te , whereas in u n c e m e n t e d sands tones , kao l in i te was p rogress ive ly rep laced by d icki te wi th inc reas ing depth, Later, i l l i te p rec ip i ta ted in u n c e m e n t e d sandstones . Its m o r p h o l o g y changes wi th depth: f i lamentous , lath, and pla te le t i l l i tes are success ive ly found at 3204, 3244, and 3275 m. This s tudy shows that t empera tu re and dep th are no t the on ly pa ramete r s wh ich cont ro l c lay reac t ions (kaol in- i te-dicki te t ransi t ion, i l l i te precipi ta t ion) . Pore space

avai labi l i ty , fluid compos i t ion , or fluid flow cou ld be also i m p o r t a n t factors. Our data sugges t tha t the kao-


l ini te-dickite transit ion cannot be used as a s imple geotherrnometer .

A C K N O W L E D G M E N T S

We are indebted to S.N. Erhenberg and M. Batchelder for their constructive review of the manuscript and to TOTAL- CST France for permission to release this study. C. Demars and E Sommer (TOTAL-CST) are acknowledged for provid- ing the samples studied and S. Petit (University of Poitiers) for helping us with the IR spectra study. The comments and discussions with E. Brosse, C. Durand (IFP), and B. Lanson (University of Grenoble) were also greatly appreciated.

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(Received 1 October 1997; accepted 30 September 1998; Ms. 97-092)

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clay diagenesis in the sandstone reservoir of the ellon field