petrographic and isotopic evidence for diagenetic processes in

Clay Minerals (1994) 29, 637--650

P E T R O G R A P H I C A N D I S O T O P I C E V I D E N C E F O R D I A G E N E T I C P R O C E S S E S IN M I D D L E J U R A S S I C

S A N D S T O N E S A N D M U D R O C K S F R O M T H E B R A E A R E A , N O R T H S E A

P. J . G R E E N W O O D * , H . F . S H A W AND A . E . F A L L I C K t

Department of Geology, Imperial College, London, UK and ~Isotope Geosciences Unit SURRC, East Kilbride, Glasgow, UK

(Received 18 June 1993; revised 15 February 1994)

A B S T R A C T : The diagenetic history of an interbedded sequence of Middle Jurassic Sleipner (or Pentland), Hugin and Heather Formation sandstones and mudrocks from the Brae Area has been investigated using petrographic, stable isotope, fluid inclusion and K-Ar dating techniques. Generally, similar diagenetic processes affected both the mudrocks and sandstones resulting in the formation of carbonate, quartz and clay cements and evidence of dissolution and secondary porosity except for the absence of kaolinite in the Hugin Formation. The mudrocks were possible sources of some components involved in sandstone diagenesis but were not passive exporters of such reactants as similar reactions also occurred in the mudrocks. The stable isotope data indicate that most of the diagenetic processes occurred in the presence of marine or evolved marine pore-waters.

The Middle Jurassic sequence encountered by wells in the North Brae-East Miller Area (UK Blocks 16/7 and 16/8) on the western margin of the South Viking Graben (Fig. 1A) is summarized in Fig. lB. The Bathonian Sleipner Format ion (or Pentland Format ion in UKCS) comprises a thick sequence of interbedded sandstones, mudrocks and coals, representing mainly non-marine fluvio- deltaic sediments, but with a change to delta plain marginal marine conditions, in the late Batho- nian. In the North Brae Area , prodel ta sands of the Callovian Hugin Format ion interdigitate with contemporaneous open marine mudrocks of the Heather Formation. In the East Miller Area , the Middle to Uppe r Callovian Hea ther Format ion sandstones, interbedded with the open marine mudrocks, have been interpreted as turbidite deposits in submarine channels (Cockings et al., 1992). These interbedded sandstones and mudrocks provide suitable sequences to examine the inter-relationships between sandstone and mudrock diagenesis, which was a major aim of this research study.

Mudrocks have frequently been implicated as possible sources of reactants for sandstone

*Current address: Geochem Group Ltd., Chester, UK.

cements, particularly via the transformation of smectite to illite (e.g. Boles & Franks, 1979) and pressure solution of detrital grains (Evans, 1990). Organic maturat ion reactions in mudrocks have also been invoked as sources of aggressive fluids creating secondary porosity in sandstones (Schmidt & McDonald , 1979; Surdam & Crossey, 1985), although their significance is disputed (Giles & Marshall, 1986).

There have been very few published studies involving detailed petrological analyses of both sandstones and mudrocks in the same sequences to establish lines of evidence linking diagenetic processes in sandstones and mudrocks.

M A T E R I A L S A N D M E T H O D O L O G Y

Core material f rom three wells was made available for this study, comprising --430 ft of core from which 51 sandstone samples and 26 mudrock samples were selected for analysis, specifically to investigate the diagenetic reactions in both lithologies and possible links between them.

A range of analytical techniques was used to study the selected samples. Mineralogy and

�9 1994 The Mineralogical Society

638

(A) P. J. Greenwood et al.

(B)

KIMMERIDGI~ : __-- ~ 7-- : : l

I , : T - - _ T - - T - - - - - I OXF. ======================= :io

- . . . . . .

~ . . - = ~ z ~ ,i.~ CALL 1 B A T H . - . . . . . . . . . . . . . . . ~ - -~ o

. . . . . . . . . . . . o1,," . . . . . . . . . . . . . . . . . . . . . n , - ( . ~

- - ; _ - : , - - _ ~ - - - - ~1=~

B A J . ? - - T , ~ : - - : ~ >

Ft6.1. (A) Map of the North Sea to illustrate the general location of the study area, adjacent to the Fladen Ground Spur in the South Viking Graben (after Brown, 1990). (B) Lithostratigraphy of the South Viking Graben Middle Jurassic sequence.

petrography were investigated using standard optical microscopy of sandstone thin-sections, plus electron microscopy of sandstone and mudrock samples incorporating secondary electron imaging (SEI) and backscattered electron imaging (BSEI) techniques. Whole rock and clay fraction X-ray diffraction (XRD) analysis con- firmed the sample mineralogy. Separated carbonate and illite cements were analysed for stable carbon and oxygen or oxygen and hydrogen isotopes, respectively. Selected illites from the sandstones and mudrocks were dated using the K-At method. Stable and radiogenic isotope analyses used the conventional procedures employed by SURRC (Hamilton etal. , 1989; Greenwood, 1992). Analyses of fluid inclusions hosted in quartz and carbonate cements were undertaken using a Linkham THM600 system (Greenwood, 1992).

R E S U L T S

Petrography and diagenes&

Sandstones from the three formations are essentially quartz arenites with some examples of subarkoses and sublitharenites (Pettijohn et al., 1973). The paragenetic sequence of the diagenetic processes in the sandstones (Fig. 2A) has been established using textural relationships observed during thin-section and scanning electron microscopy (SEM) analyses (Fig. 3).

The mudrocks are dark grey to black, clay-rich but with a variable silt-grade component. Their mineralogy has been established from XRD analyses of selected samples and BSEI was used to differentiate detrital and authigenic minerals. The clay mineralogy of the <2 ~tm fraction of the mudrocks is dominated by illite (95-75%), and

'(A) EOGENESISi

PYRITE 123

APATITE ~ 23 I

QUARTZ 123 ? ?

Diagenetic processes in Jurassic sandstones and mudrocks

(B) MESOGENESIS EOGENESIS i

123 SECONDARY

POROSITY

KAOLtNITE 23 .,i,.~,,~. .~,.,.~

ILLITE 123 ?

CALCITE 123 ~ ? 12 3

ALBITE t 23 ; ?

.qPHALERITE 1 ~

BARIrE 23 ?

~ SIDERITE 3

ANKERITE 123

COMPACTION ?

EARLY no time scale implied ~ LATE

Key: ' Precipitation 1 Hugin Formation

. . . . . Dissolution 2 Sleipner Formation

Possible continued 3 Heather Formation precipitation

pyRITE 123

APATITE 12 ~ - -

SIDERITE 12

KAOLINITE 23

DOLOMITE z

CALCITE 2

AN KERITE 23

! FELDSPAR T 23

QUARTZ 123

ILLITE 1 z 3

COMPACTION

EARLY

MESOGENESIS

- - ?

1 2

inten~owth= verm~orm

Illclura~ill

?

3

alb~le

? . . . . . . . ?

no time scale implied

Key:

639

LATE

Precipitation 1 Hugln Formation

. . . . . Dissolution ~z Sleipner Formation

- - Possible continued 3 Heather Forma~on precipitation

FiG. 2. Paragenetic sequence of diagenetic minerals in (A) the sandstones; (B) the mudrocks.

kaolinite (5-25%). 'lllite' here includes ordered illite-smectite with >90% illite.

The paragenesis of the anthigenic components in the mudrocks (Fig. 2B) has been established from textural relationships observed during BSE1 and SEI analyses (Fig. 4). Due to the fine grain size of these samples the sequence established is more tentative than that for the sandstones.

Isotopic and K-Ar data

The oxygen and carbon stable isotopic data for the carbonates, and oxygen and hydrogen data for the silicate cements are presented in Tables 1 and 2. Table 3 summarizes the K-Ar dates for the illites separated from the sandstones and mudrocks.

I N T E R P R E T A T I O N

Petrography

The present study of mudrocks from the Sleipner, Hugin and Heather Formations has shown a complex diagenetic history, with many similarities to the interbedded sandstones. Cavi- ties, arising from dissolution of bioclasts and feldspar grains, have provided sites for the precipitation of authigenic minerals in micro- environments surrounded by a fine-grained matrix (cf. Shaw & Primmer, 1991).

Early diagenesis in the mudrocks reflects the dominantly marine depositional environments with abundant framboidal aggregates of pyrite forming via bacterial sulphate reduction in the first few metres of burial (Curtis, 1980) (Fig. 4A). Within the sandstones, minor pyrite cements

640 P.J. Greenwood et al.

A

C D

E

Fl6.3 (A) SEI of a quartz and ankerite cemented sandstone. (B) SEM-CL image of (A) illustrating the degree of quartz cementation. Detrital quartz grains exhibiting varying luminescence intensities are cemented by generally non-luminescent overgrowths. The overgrowths display faint luminescent zonation (arrows) reflecting their multi-stage precipitation history. Quartz grains show little evidence of strong compaction, but are locally fractured and cemented by non-luminescent quartz. Note highly luminescent resin in pore-space (P). Heather Formation, 16/8a-4, 15705.0 feet. (C) SEI of authigenic kaolinite illustrating the euhedral morphology of the pseudo-hexagonal crystals, locally enclosed by quartz overgrowths (lower magnified image). Heather Formation, 16/8a-9Z, 15000.0 feet. (D) SEI of fibrous authigenic illite infilling an oversized secondary pore. Hugin Formation, 16/7a-30Z, 14464.0 ft. (E) SEI of euhedral authigenic albite crystal enclosing autbigenic illite fibres in a secondary pore. Heather Formation 16/8a-4, 15656.3 feet. (F) BSEI of rhombic ankerite cement (A) enclosing barite cement, with a bright backscatter contrast, in turn post-dating quartz overgrowths (arrowed). Sleipner

Formation, 16/7a-30Z, 15371.0 feet.

A

Diagenetic processes in Jurassic sandstones and mudrocks 641

C D

E F

FIG. 4. (A) BSEI of framboidal pyrite (P) within an illite-rich clay matrix, containing detrital quartz (Q) and mica. Sleipner Formation, 16/7a-30Z, 15378.0 feet. (B) BSEI of corroded and flattened quartz grains (arrowed), together with kaolinite- cemented muscovite (M), surrounded by an illitic clay matrix, early dolomite cement (D) displays marginal alteration to ankerite. Sleipner Formation, 16/7a-30Z, 15384.0 feet. (C) BSEI of a rhomb-shaped cavity containing corroded dolomite cement (D), compactional fabrics suggest that carbonate dissolution was a late diagenetic event. Heather Formation 16/8a- 4, 15643.6 ft. (D) BSEI of authigenic vermiform kaolinite (K) infilling a secondary pore resulting from feldspar dissolution; an adjacent rhomb-shaped cavity contains corroded dolomite remnants (D). Heather Formation, 16/8a-4, 15540.1 feet. (E) SEI of small authigenic euhedral crystals of authigenic quartz (Q) within the clay matrix of a mudrock, enclosing illite plates. Heather Formation, 16/8a-4, 15550.0 feet. (F) SEI showing delicate fibres of authigenic illite (arrowed) projecting

from platy illite clays comprisin~ the mudrock matrix. Hu~in Formation, 16/7a-30Z, 14411.0 feet.

642 P.J. Greenwood et al.

TABLE 1. Stable isotope compositions of carbonate cements and bioclasts from sandstones and mudrocks of the studied formations.

Formation/Well

Sandstones: Hugin Formation 16/7a-30Z

Sleipner Formation 16/7a-30Z

Heather Formation 16/8a-4

Mudrocks: Hugin Formation 16/7a-30Z

Sleipner Formation 16/7a-30Z

Depth (ft) Mineral 613CpDB~ 6180pDB%O ~180SMOW

14464.0 Calcite -0 .99 - 13.84 + 16.65 14464.0* Calcite - 1.11 - 13.81 + 16.68

15375.0 Calcite l -4 .92 - 13.98 + 16.50 15375.0* Calcite 1 -4.65 - 14.05 + 16.43 15375.0 Ankerite 1 -5 .86 - 12.67 + 17.85 15375.0" Ankerite ~ -7.03 - 13.70 + 16.78

15689.0 Ankerite -3 .32 - 13.81 + 16.67 15689.0" Ankerite -3 .48 - 13.89 + 16.60 15707.0 Ankerite -6 .40 - 11.90 + 18.65 15707.0' Ankerite 6.40 - 11.90 + 18.65 15689.0 Siderite -2 .59 12.40 + 18.13 15689.0" Siderite -2 .90 - 12.70 + 17.82

14409.0 Siderite -4 .08 -6.35 +24.36 14409.0* Siderite -4 .14 -6 .46 +24.25

15382.0 Calcite 2 -2 .58 - 11.19 + 19.38 15389.0 Calcite 2 -2 .96 - 11.30 + 19.26 15389.0 Calcite 2 -2 .79 11.22 + 19.34 15376.5 Calcite 3 +0.46 -4 .90 +25.86 15376.5 Calcite 3 +0.62 -4 .82 +25.94 15389.0 Calcite 3 +0.65 5.43 +25.31

Key: * = duplicate samples; 1 = calcite and ankerite separated by differential acid attack; 2 calcite cement from fractures; 3 - calcite from bivalve shells.

ref lec t s imi l a r p r o c e s s e s l i m i t ed by a l ower d e g r e e

o f a n o x i a a n d less r eac t ive de t r i t a l i ron . Ea r ly

ca lc i te a n d d o l o m i t e c e m e n t s in t he m u d r o c k s o f

t h e S l e i p n e r a n d H e a t h e r F o r m a t i o n s prec ip i -

t a t e d in re la t ive ly u n c o n s o l i d a t e d s e d i m e n t s wi th

m a r i n e p o r e - w a t e r s , u t i l i z ing b i c a r b o n a t e d e r i v e d

f r o m p r i m a r y m a r i n e c a r b o n a t e o r as a by-

p r o d u c t o f s u l p h a t e r e d u c t i o n r e a c t i o n s (C ur t i s &

C o l e m a n , 1986). Ea r ly s ide r i t e c e m e n t s w e r e on ly

o b s e r v e d in t h e H u g i n F o r m a t i o n a n d s u g g e s t t he

i n f luence o f m e t e o r i c w a t e r s in t h e f an de l t a

s y s t e m , a h y p o t h e s i s s u p p o r t e d by t h e i so top ic

d a t a ( see be low) . K a o l i n i t e i n t e r g r o w t h s wi th

m i c a s a r e c o m m o n in b o t h t h e s a n d s t o n e s a n d

m u d r o c k s o f t h e S l e i p n e r a n d H e a t h e r F o r m a -

t i ons a n d m a y a lso be i nd i ca t i ve o f t h e i n f l uence o f

m e t e o r i c f o r m a t i o n w a t e r s .

Q u a r t z o v e r g r o w t h s a re t h e m o s t v o l u m e t r i -

cal ly s ign i f i can t c e m e n t s in t h e s a n d s t o n e s .

O b s e r v e d C L z o n a t i o n s a n d r e l a t i o n s h i p s wi th

o t h e r c e m e n t s s u g g e s t t h a t q u a r t z c e m e n t a t i o n

was a m u l t i - s t a g e , t h o u g h p e r h a p s a s e m i - c o n -

t i n u o u s , p r o c e s s (Figs . 3 A , B ) . T h e o b s e r v a t i o n

o f c o r r o d e d q u a r t z in t h e m u d r o c k s s u g g e s t s t h a t

th is cou ld be a p o t e n t i a l s o u r c e o f q u a r t z c e m e n t s

in t he s a n d s t o n e s (Fig. 4B) . T h e p r e s e n c e o f

q u a r t z c e m e n t s in t h e s e m u d r o c k s s h o w s t h a t n o t

all t h e si l ica f r o m q u a r t z d i s s o l u t i o n is ava i l ab l e

for e x p o r t to t he a d j a c e n t s a n d s t o n e s (Fig. 4E) .

H o w e v e r , qua l i t a t i ve ly , it a p p e a r s t ha t t h e

a m o u n t o f si l ica l i b e r a t e d d u r i n g d i a g e n e s i s

e x c e e d s t he a m o u n t o f a u t h i g e n i c si l ica in t h e

m u d r o c k s , t h u s i m p l y i n g a n e t e x p o r t o f silica.

S e c o n d a r y p o r o s i t y r e s u l t i n g f r o m t h e d i s so lu -

t i on o f f e l d s p a r s in t h e s a n d s t o n e s , a n d o f

f e l d s p a r s a n d c a r b o n a t e s in t h e m u d r o c k s , is

c o m m o n (Fig. 4C) a n d t ha t d i s s o l u t i o n p o s t - d a t e s

s ign i f i can t bur ia l in b o t h l i tho log ies . It is un l i ke ly

t h a t e x t e n s i v e m e t e o r i c f l u sh ing cou ld h a v e

o c c u r r e d t h r o u g h t h e low p e r m e a b i l i t y m u d r o c k s

n o r in t h e H e a t h e r F o r m a t i o n t u r b i d i t e s a n d -

s t o n e s w h i c h a r e e n c l o s e d in m a r i n e m u d r o c k s ,

t h u s i so la t ing t h e m f r o m p o s s i b l e s u r f a c e p o r e -

w a t e r r e c h a r g e . T h e r e f o r e , it s e e m s m o r e l ikely

t ha t t he s e c o n d a r y p o r o s i t y is r e l a t e d to t h e

p r e s e n c e o f c o r r o s i v e d e e p b a s i n a l f luids. O r g a n i c

Diagenetic processes in Jurassic sandstones and mudrocks 643

TABLE 2. Oxygen and hydrogen stable isotope compositions of illite separates from the sandstones and mudrocks of the Formations studied.

Formation/Well

6180 6D Yield

Depth (ft) Fraction (gin) (8MOW) (%) Fraction (gin) (SMOW) (%0) (~_tmol/mg)

Mudrocks: Hugin Fmn 14411.0 <0.2 + 14.4 <0.2 41 3.13 16/7a-30Z 0.243.5 + 14.4

0.5-1.0 +12.0 +15.4"

1.0-2.0 +13.5 Sleipner Finn 15386.0 <0.2 + 13.4 <0.2 -43 4.02 16/7a-30Z + 15.4"

0.243.5 +16.8 0.5-1.0 +15.7

+15.2' 1.0-2.0 +16.5

Heather Finn 15557.0 <0.2 + 18.4 16/8a-4 0.243.5 + 16.1 0.243.5 -47 4.33

0.5-1.11 +14.1" +12.8

1.0-2.0 +14.9 Heather Fmn 15605.6 <0.2 + 15.1 <0.2 -49 3.48 16/8a-4 0.243.5 +15.2 (t.243.5 39 3.87

0.5-1.0 +14.2 0.5-1.0 -36 3.2(t +15.3"

1.0-2.0 + 15.5 1.0-2.0 -44 3.32 +14.3"

Heather Finn 15685.0 <0.2 +15. I 16/8a-4 + 14.7"

0,243.5 + 15.4 0.5-1.0 +14.7 1.0-2.0 +15.2

Heather Finn 15727.1 <0.2 + 13.3 <0.2 -37 3.14 16/8a-4 0.2-(t. 5 + 17.8

+14.1" 0.5-1.0 +13.1

+15.0" 1.0-2.0 + 13.5

Sandstones: 15634.6 <0.2 + 16.9 Heather Fmn + 13.9" 16/8a-4 0.243.5 + 15.5

0.5-1.0 +12.9 +14.7"

1.0-2.0 + 15.3 1.02.0 -55 2.43 -51 2.39

Heather Fmn 15751.4 <0.2 + 13.9 16/8a-4 + 14.0"

0.243.5 + 13.1 0.5-1.0 + 14.6 1.0-2.0 + 15.2 1.0-2.0 -53 1.96

Key: * - preferred value from duplicate analyses, based upon sample yield as compared to theoretical value of 15.22 gmol/mg for illite.

644 P.J . Greenwood et al.

TABLE 3. Results of K-Ar age determinations on illite separates from mudrock and sandstone samples of the Formations studied.

Sample

Sandstone: Heather Formation 16/8a-4 15634.6 feet(<l.0 gm)

Mudrocks: Heather Formation 16/8a-4 15605.6 feet (<0.2 gm)

Hugin Formation 16/7a-30Z 14411.0 feet (<0.5 Ixm)

Sleipner Formation 16/7a-30Z 15386.0 feet (0.2-2.0 ~tm)

Radiogenic 4~ K (wt%) (x 10-l0 moles/g) % Radiogenic 4~ Age (Ma)

6.91 5.89 51.46 48.45 _+ 1.3

5.97 10.31 45.92 96.95 + 2.9

6.61 9.81 63.13 83.63 _+ 2.0

4.06 8.34 49.79 114.65 _+ 3.2

maturat ion reactions in the mudrocks could provide the source of aggressive fluids, e.g. from decarboxylation of organic matter (Schmidt & McDonald , 1979), or generat ion of organic acids (Surdam & Crossey, 1985). The dissolution of silicates and carbonates in the mudrocks supports the view of Giles & Marshall (1986) that such processes consume some of the acidity created during organic maturat ion, thus restricting the amount of acid released from the mudrocks into adjacent sandstones.

The pore-filling vermiform kaolinite cements, occluding secondary pores resulting from feldspar dissolution in both the sandstones and mudrocks (Figs. 3C, 4D) and infilling bioclast cavities in the mudrocks, are consistent with precipitation following feldspar dissolution. The finely crystal- line morphology of the vermiform kaolinite is typical of that associated with slow diffusion controlled growth in stagnant pore-waters (Hurst & Irwin, 1982).

The absence of detectable kaolinite (detrital or diagenetic) in the Hugin Format ion sandstones and mudrocks is significant but difficult to explain. Studies by various workers (see Morton e t a l . , 1992) have also noted the absence of kaolinite in the deeper parts of the Brent Forma- tion which has been interpreted as the result of the transformation of kaolinite to illite at temperatures in excess of 100~ However , this explana- tion cannot be applied here as kaolinite is observed in the more deeply buried Sleipner Format ion but absent in the overlying Hugin Formation. In the Heather and Sleipner Forma- tions the formation of diagenetic kaolinite and

illite is, in part, associated with the breakdown of K-feldspars but in the Hugin Format ion only authigenic illite occurs within the corroded K feldspars (Figs. 4F, 3D). It must, therefore, be concluded that in the Hugin Format ion the formation fluids reacting with the K-feldspars allowed the formation of illite but not kaolinite and it is this rather than depth of burial and temperature that has inhibited the formation of diagenetic kaolinite. Provenance or facies must be an important factor in controlling the absence of detrital kaolinite in the Hugin Format ion lithologies.

lllite is an important authigenic mineral in the sandstones and has also been identified in the mudrocks of the three formations from SEI and BSEI analyses (Figs. 3D, 4F). The iflite in the mudrocks is both detrital and authigenic in origin. The petrographic evidence suggests that the authigenic illite in the mudrocks could be neo- formed by direct precipitation (Shaw & Primmer, 1991) rather than being the end-point of an illitization of illite-smectite transformation reac- tion (e.g. Pearson & Small, 1988).

Other diagenetically late cements which are common to the sandstones and mudrocks are calcite, ankerite and albite (Figs. 3E,F).

In the sandstones, barite and sphalerite cements require a source of sulphate (or a source of sulphide), which is unlikely to have been derived from either the sandstones or the interbedded mudrocks. A possible source of sulphate is from the underlying Permian/Triassic evapor- ites (Burley et al . , 1989) supplied through mass transfer of fluids via faults. The mudrocks could

Diagenetic processes in Jurassic sandstones and rnudrocks

be the source of metal cat ions (e.g. Zn) for these cements (Bains et al . , 1993).

The b road similarity in the paragenes is of authigenic minerals in bo th the sands tones and mudrocks of the Middle Jurassic sequences in Brae reflects the i r shared burial history, of ten resul t ing in a c o m m o n evolut ion of the mineral assemblages , though not necessarily synchro- nously, in the different l i thologies.

Stable isotope and K-Ar data

Stable isotope data f rom ca rbona te and illite cements have enab led const ra in ts to be placed on the pore -wate r composi t ions dur ing the i r precipi- ta t ion, and also provided significant da ta with regard to the fo rmat ion of illites in the mudrocks (Tables 1 and 2). The oxygen isotope composit ions of potent ia l Jurassic fo rmat ion waters (i.e. mar ine or meteor ic) were t aken to be 6180 =

645

- - 1.2%o for mar ine waters, and 61~O = -6.0%0 for meteor ic waters relat ive to S M O W , based upon values used by previous authors (e.g. Haszeldine et al . , 1992).

Sideri te in tire Hugin Format ion mudrocks is an early, low- tempera tu re cement . It is appa ren t f rom Fig. 5A that meteor ic waters are necessary for siderite fo rmat ion at low tempera tu res . The mean 61~C value of -4.11%o (PDB) for this siderite is not indicative of any single ca rbon source (Irwin et al . , 1977), but p robab ly a mixture of b i ca rbona te der ived from pr imary mar ine ca rbona te (61~C ~ 0%0) and organically der ived ca rbon ( - 1 0 to 25%0). Calcite cements infilling fractures in the Sleipner Format ion mudrocks prec ip i ta ted at t empera tu re s a round 70~ accord- ing to thei r mean fluid inclusion homogeniza t ion t empe ra tu r e ( G r e e n w o o d , 1992). Using this tempera tu re to derive the isotope compos i t ion of the precipi ta t ing fluid (Fig. 5A) , a value close to

(A) (B) +6

~ / ~<~:./"

�9 ,- 2 0-

- i - ____Lu_"e__ss~c.~a~.o.~!er___ ~ -I..__J_U__W~p, C MA.~!Ne._W__A~. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . " ~

( :~ - 3 -

�9 O ', I " ~-o - 4 - 7, -4- , ', i - 5 - s - ==== i i

Jurassic i -6 - leteonc 6 JURASSIC METEORIC WATER

-7- .7- i I , ' : / ~ i i i i ! -e- .8- / / ~ ' i i i i i i

- 1 0 i i i i i i i i i i i i , i , , i i

20 40 60 80 100 120 140 20 40 60 80 1 0 0 1 2 0 140

T e m p e r a t u r e ( ~ T e m p e r a t u r e (*C)

FIG. 5. (A) 8180 (SMOW) values of pore-water vs. temperature for siderite cements from Hugin Formation mudrock, and for calcite cements from fractures in mudrocks of the Sleipner Formation. See text for discussion. Curves calculated using

the following fractionation equations:

1 0 0 0 l n e g s i d e r i t e _ w a t e r = 3.13 • 106 x T 2 _ 3.5 (Carothers etal. 1988) 1 0 0 0 1 n a : c a l c i t e _ w a t e r = 2.78 x 106 x T -2 - 2.89 (Friedman & O'Neil, 1977)

(B) ~5180 (SMOW) values of pore-water vs. temperature for carbonate cements from sandstones of the Formations studied. See text for discussion. Curves for siderite and calcite calculated using the equations given in Fig. 5(A). Curves for ankerite

calculated using the equation:

1 0 0 0 l n ~ a n k e r i t e _ w a t e r : 2.78 x 106 • T -~ + 0.32 (Dutton & Land, 1985).

646 P. J. Greenwood et al.

Jurassic marine water is obtained. The 613C composition of this calcite (mean = -2.78%0) reflects a dominantly marine carbonate source, with a minor contribution from organically derived carbon. Dissolution of bivalve shells within the Sleipner Format ion mudrocks (mean 613C = +0.58%0) may have supplied the marine carbonate signature.

Carbonate cements from the sandstones show a narrow range of 8180 (PDB) values of between - 1 2 and -14%o (Table 1). Petrographically, these carbonate cements, particularly the ankerites, appear to be relatively late, indicating elevated precipitation temperatures. Figure 5B shows that such temperatures are more consistent with precipitation from marine waters (c. 95-125~ than from meteoric waters (c. 55-80~ Carbon isotope values for these cements range from c. - 1 to -7%0 (PDB) (Table 1), reflecting a predomi- nantly marine bicarbonate source, with a variable but minor organic carbon contribution.

The oxygen isotope data for illites from the sandstones and mudrocks show a range of 6~'~O (SMOW) between c. +13 and +17%o (Table 2). The mean values from the two lithologies are +14.5%o for the sandstones and +15.0%o for the mudrocks, and are thus effectively identical within the analytical error. Petrographic analyses proved inconclusive in establishing whether the mudrock illite was wholly authigenic. However , the restricted range of 61sO values, coinciding with the values for the authigenic illite from the sandstones, suggests that the illite extracted from the mudrocks for the isotope analyses is dominantly authigenic. There are very few published data on the oxygen isotope compositions of North Sea illites, and even less for mudrock illites. Repor ted 6180 values for illites from the Brent Sandstone are comparable with this study and range between +13 and +17%o (SMOW) (Glas- mann e t a l . , 1989a,c) and + 1 l and +14.5%o (SMOW) (Hogg, 1989). Illite from a single mudrock sample was also analysed by Hogg (1989) and gave a similar composit ion of +12.3 to + 13.2%o for two size-fractions.

Hydrogen isotope data for the illites shows a range of 6D values between - 3 6 and -49%0 for the mudrocks, and from - 5 1 to -55%0 for the sandstones (Table 2). We are not aware of any published 6D data for mudrock illites from the North Sea, but a limited amount of data from illites in sandstones is available. Glasmann et al.

(1989c) reported comparable 6D values of - 4 9 . 8 to -59.6%o for illites from the Brent Sandstones of the Heather Field; Hogg (1989) reported 6D values of - 3 3 to -50%o for Brent Sandstones from Alwyn South. A crossplot of the 6180 vs. 6 D

(Fig. 6A) shows that the data cluster around a line representing the range of illite O and H stable isotope compositions in equilibrium with Jurassic seawater, over a range of temperatures. The wide range of 6D values, observed in this study and others of authigenic clays from North Sea Jurassic sequences, may be a function of the susceptibility of the hydrogen isotopes in clays for exchange, which occcurs independently of oxygen isotopes (Longstaffe & Ayalon, 1990). It has also been suggested that in the North Sea the hydrocarbon accumulations in the sandstones represent a potential reservoir of hydrogen with which clays could exchange (Fallick et al . , 1993).

The K-Ar dating of illite from one sandstone and three mudrock samples has given a wide range of ages (Table 3). The single sandstone illite sample has an age of 48.5 Ma, typical of those reported from other North Sea Jurassic sandstones (mostly 55 to 30 Ma, see review in Burley & MacQuaker , 1992). The three mudrock samples show illite ages in the range from c. 115 to 84 Ma, and are thus significantly older than those in the sandstones but younger than the depositional age of the sediments (>163 Ma). These generally represent maximum ages, as the illite separates from mudrocks are likely to contain a mixture of authigenic and detrital illite due to their similar sizes (Glasmann e t a l . , 1989b). However , it is probable that the majority of the analysed illite is authigenic, as only minor conta- minants are required to significantly increase the K-Ar age (Hamil ton et al . , 1989). The oldest age of 115 Ma for the illites from the Sleipner Formation was measured using material up to 2 gm and may thus reflect a greater proport ion of coarser detrital illite or mica contamination. Two previous studies have analysed the K-Ar ages of mudrock illites in the North Sea and obtained comparable results, of 106-31 Ma (Glasmann et al . , 1989b) and 153~8 Ma (Burley & Flisch, 1989) after correction for est imated contamination.

Assuming the illite age of 48.5 Ma, derived from the sandstone sample, represents an effec- tive diagenetic illite formation age, then precipitation temperatures around 100~ would be

~A)

+ 1 0 '

5D%,, SMOW

-6 -4 +2

Diagenetic processes in Jurassic sandstones and m u d r o c k s

o / F o n , n l l ~ i m e l l ~ i:l i r-on,r, l i ~ mudr=~k I rl~6"?.O II. �9 i l l I I I ~ �9 i Foar, ad~ nludm=k I Irtl#.l II.

~ lell, n ",W~ ol nludll=a~l.

81s0 ~ S M O W

4-4 +6 +8 +10 +12 +14 +16 +18 +20

�9 150(;

12sc �9

100c �9

a'

(B)

0 s =o 7o

+6-

+5-

+ 4 -

+ 3 -

+ 2 -

+ 1 -

0 -

- 1 -

- 2 -

- 3 -

- 4 -

- 5 -

- 6 -

- 7 -

- 8 -

- 9 -

-10 -

4

Jurassic marine water _ . _ /

/ i II~_1-~, ,,

.SO '~ 0 20 40 60 80

/ T e m p e r a t u r e (~

647

f

100 120 140

Fro. 6. (A) 6tSo (SMOW) vs. 6D (SMOW) plot showing the compositional range of illites analysed in this study. Line a-a' represents the range of illite O and H stable isotope compositions in equilibrium with Jurassic seawater over a range of

temperatures. See text for discussion. Line a-a' was calculated using the following fractionation equations:

Oxygen isotope illite-water fractionation:

10001n0qllite_wate r = -2.87 + 1.83 x 106 T 2 + 0.0614 x (106 T 2)2 _ 0.00115 x (106 T 2)3 (Savin & Lee, 1988)

Hydrogen isotope iflite-water fractionation:

10001n0qllite . . . . . . rite-water = - 1 9 . 6 x 103 T -1 + 25 (Yeh, 1980)

(B) ~5t80 (SMOW) values of pore-water vs. temperature for illite having the mean composition of 15.0%o found in the mudrocks. The possible isotopic composition of formation fluids precipitating the illite were calculated using temperatures derived from illite K-Ar ages and the burial history of the sequence studied, shown in Fig. 7. The temperature derived from the sandstone age is contrasted with the much lower temperatures derived from the mudrock ages, which are probably maximum ages due to sample contamination. Curve for illite calculated using the fractionation equation of Savin & Lee

(1988).

impl ied f rom the buria l history model for the sequence (Fig. 7). This again is similar to data f rom o the r s tudies (Burley & M a c Q u a k e r , 1992). Using this t e m p e r a t u r e to der ive the oxygen isotope compos i t ion of the precipi ta t ing fluid (Fig. 6B), gives a 61SO ( S M O W ) value of +2%o suggesting tha t the illites prec ip i ta ted f rom pore- wate r with an evolved mar ine s ignature. This is consis tent with the pore -wate r composi t ions de- r ived for the fo rmat ion of the late c a rbona t e cements .

D I S C U S S I O N

The pa t te rns of diagenesis in the Middle Jurassic mudrocks and sands tones f rom the Brae A r e a reflect similari t ies in the diagenet ic processes and the reactivity of the fo rmat ion fluids with the detr i tal mineralogy. It is difficult to be cer ta in as to w h e t h e r similar d iagenet ic events in the different l i thologies were synchronous . Intui- tively, it could be cons idered tha t the diagenet ic react ions in the mudrocks are likely to be early,


1 6 / 7 A - - 3 O Z

G E O H I S T O R Y D I A G R A M

JURASSIC

o o o

2 0 0 0 2

3 0 0 0 :

40002 50C

5 0 0 0 i ' 6 0 C

- =

-ooo:!----~__ .oooii'~

, , o o o , oc r- , . . . . , o . _ - - _ _ ~ _ _ ~ - - =. ~.,.,~.c~.y

14000 - 140C

. . . . ~ _ _ ~ " / . . . . . Hugin Focma~n

1 ~OC ~ ~ ~ - - ~ - = - - - - - - Sleipnef Formalion

- - - ~ i . . . .

T I M E B. P . ( M Y )

Fl~. 7. Decompacted burial history model of the sequence encountered in wells 16/7a-30 and -30Z. The position of the oil window is shown by the vertically hatched pattern, above a vitrinite reflectance value of 0.6%Ro.

before the mudrocks are r ende red essentially impe rmeab le by compact ion . However , the similarity in the pa t te rns of diagenesis in bo th the mudrocks and sands tones and the evidence for late-stage diagenesis in the sands tones might suggest tha t some of the diagenesis in the mudrocks was also a relatively late-stage event . This in te rpre ta t ion then raises fundamen ta l quest ions abou t mechan isms for such diagenet ic react ions in ' impe rmeab le ' mudrocks .

It is likely tha t the mudrocks provided components for diagenet ic react ions in the adjacent sands tones , e.g. silica for quar tz cements , acidic solutions for the deve l opm en t of secondary porosity and meta l cat ions for su lphide/sulphate cements . However , this study has shown tha t mudrocks are not simply gross expor ters of such reactants and many mobi l ized componen t s are involved in equiva lent diagenet ic react ions in the mudrocks r a the r than passively migra t ing out into the sandstones . This concept is of great importance in calculat ing the mass ba lance of individual componen t s for diagenet ic react ions and empha- sizes the benefi ts f rom careful examina t ion of

pe t rography of the associated mudrocks in any study of sands tone diagenesis.

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

We would like to thank Marathon Oil (UK) Ltd for funding and the Brae Group of companies (Marathon, BP, Bow Valley, Kerr McGee, British Gas, LL & E, Sovereign and British Borneo) for allowing access to the data for the project with specific thanks to Colin Turner for his help throughout. Additional material was also supplied by Shell Petroleum for which we are grateful. The initial draft of the paper was improved by the suggestions and comments of Jim Marshall and an anonymous reviewer and we would like to acknowledge their help.

R E F E R E N C E S

BAINS S.J., BURLEY S.D. & G1ZE A.P. (1993) Base metal sulphide mineralisation in North Sea hydrocarbon reservoirs: evidence for mass solute transfer during burial. Geofluids 93 Abstract Volume 435-438.

BOLES J.R. & FRANKS S.G. (1979) Clay diagenesis in Wilcox sandstones of southwest Texas: implications of smectite diagenesis on sandstone cementation. J. Sed. Pet. 49, 55-70.

Diagenetic processes in Jurassic sandstones and mudrocks

BROWN S. (1990) The Jurassic. Pp. 219-254 in: Introduction to the Petroleum Geology of the North Sea (K.W. Glennie, editor) (3rd edition). Blackwell Scientific Publi- cations, Oxford.

BURLEY S.D. & FLISCH M. (1989) K-At geochronology and the timing of detrital I/S clay illitization and authigenic illite precipitation in the Piper and Tartan Fields, Outer Moray Firth, UK North Sea. Clay Miner. 24, 285-315.

BURLEY S.D., MULLIS J. & MATTER A. (1989) Timing diagenesis in the Tartan reservoir (UK North Sea): constraints from combined cathodoluminescence microscopy and fluid inclusion studies. Mar. Petrol. Geol. 6, 98-120.

BURLEY S.D. & MACQUAKER J.H.S. (1992) Authigenic clays, diagenetic sequences and conceptual diagenetic models in contrasting basin-margin and basin-centre North Sea Jurassic sandstones and mudstones. Pp. 81- 110 in: Origin, Diagenesis and Petrophysics of Clay Minerals in Sandstones. SEPM Spec. Publ. No. 47.

CAROTHERS W.W., ADAMI L.J. & ROSENBAUER R.J. (1988) Experimental oxygen isotope fractionation between siderite-water and phosphoric acid liberated CO2- siderite. Geochim. Cosmochim. Acta 52, 2445 2450.

COCKINGS J.H., KESSLER L.G. I1, MAZZA T.A. & RILEY L.A. (1992) Bathonian to mid-Oxfordian sequence stratigraphy of the South Viking Graben, North Sea. Pp. 65-105 in: Exploration Britain: Geological Insights for the Next Decade. (R.F.P. Hardman, editor) Geol. Soc. Spec. PuN. No. 67.

CURTIS C.D. (1980) Diagenetic alteration in black shales. Y. Geol. Soc. Lond. 137, 189-194.

CURTIS C.D. & COLEMAN M.L. (1986) Controls on the precipitation of early diagenetic calcite, dolomite and siderite concretions in complex depositional sequences. Pp. 23-33 in: Roles of Organic Matter in Sediment Diagenes&. (D.L. Gautier, editor) SEPM Spec. Publ. 38.

DuTroN S.P. & LAND L.S. (1985) Meteoric burial diagenesis of Pennsylvaniaa arkosic sandstones, S.W. Anadarko Basin, Texas. Am. Assoc. Petrol. Geol. Bull., 69, 22-38.

EVANS J. (1990) The importance of detrital clay mineralogy in controlling geochemical fluxes during shale diagenesis. Proc. Int. Clay Conf. Strasbourg, 163-172.

FALLICK A.E., HASZELDINE R.S. & PEARSON M.J. (1993) Overview of clay mineral stable isotope (6180,(5D) systematics in the Northern North Sea. Conference on Diagenesis, Overpressure and Reservoir Quality, Cam- bridge, 1993. Abstracts volume, no. 26.

FRIEDMAN I. & O'NEIL J.R. (1977) Compilation of stable isotope fractionation factors of geochemical interest. In: Data of Geochemistry 6th Edition. (M. Fleischer, editor) U,S. Geol. Surv. Prof. Paper 440-KK. 12pp.

GILES M.R. & MARSHALL J.D. (1986) Constraints on the development of secondary porosity in the subsurface: re- evaluation of processes. Mar. Petrol. Geol. 3, 243-255.

GLASMANN J.R., CLARK R.A., LARTER S., BRIEDIS N.A. & LUNDEGARD P.D. (1989a) Diagenesis and hydrocarbon

649

accumulation, Brent Sandstone (Jurassic), Bergen High Area, North Sea. Am. Ass. Petrol. Geol. Bull. 73, 1341-1360.

GLASMANN J.R., LARTER S., BRIEDIS N.A. & LUNDEGARD P.D. (1989b) Shale diagenesis in the Bergen High Area, North Sea. Clays Clay Miner. 37, 97-112.

GLASMANN J.R., LUNDEBARD P.D., CLARK R.A., PENNY BK. & COLLINS I.D. (1989c) Geochemical evidence for the history of diagenesis and fluid migration: Brent Sandstone, Heather Field, North Sea. Clay Miner. 24, 255-284.

GREENWOOD P.J. (1992) Diagenesis of Middle Jurassic sandstones and mudrocks, Brae Area, North Sea. PhD thesis, Univ. London, UK.

HAMILI-ON P.J., KELLEY S. & FAt, LICK A.E. (1989) K-Ar dating of illite in hydrocarbon reservoirs. Clay Miner. 24, 215-231.

HASZELDINE R.S., BRINY J.F., FALLICK A.E., HAMILTON P.J. & BROWN S. (1992) Open and restricted hydrologies in Brent Group diagenesis, North Sea. Pp. 401-419 in: Geology of the Brent Group. (A.C. Morton, R.S. Haszetdine, M.R. Giles and S. Brown, editors) Geol. Soc. Spec. Publ., No. 61.

HOGG A.J.C. (1989) Petrographic and isotopic constraints on the diagenesis and reservoir properties of the Brent Group Sandstones, Alwyn South, Northern U.K. North Sea. PhD thesis, Univ. Aberdeen, UK.

HURST A. & IRWIN H. (1982) Geological modelling of clay diagenesis in sandstones. Clay Miner. 17, 5-22.

IRWIN H., CURTIS C. & COLEMAN M. (1977) Isotopic evidence for source of diagenetic carbonates formed during burial of organic-rich sediments. Nature, 269, 209-213.

LONGSrAFFE F.J. & AYALON A. (1990) Hydrogen-isotope geochemistry of diagenetic clay minerals from Creta- ceous sandstones, Alberta, Canada: evidence for exchange. Appl, Geochem. 5, 657~i68,

MORTON A.C., HASZELD1NE R.S., GILES M.R. & BROWN S. (1992) Geology of the Brent Group. Geol. Soc. Special PUN., No. 61,506 pp.

PEARSON M.J. & SMALL J.S. (1988) Illite-smectite diagenesis and palaeotemperatures in Northern North Sea Quaternary to Mesozoic shale sequences. Clay Miner. 23, 10%132.

PEITIJOHN F.J., POTTER P.E. & SIEVER R. (1973) Sand and Sandstone. Springer-Verlag, New York.

SaviN S.M. & LEE M. (1988) Isotopic studies of phyllosilicates. Pp. 189-223 in: Hydrous Phyllosilicates (Ex- clusive of Micas). (S.W. Bailey, editor) Reviews in Mineralogy 19, Mineralogical Society of America, Washington, DC.

SCHMIDT V. & McDONALD D.A. (1979) The role of secondary porosity in the course of sandstone diagenesis. Pp. 175~07 in: Aspects of Diagenesis. (P.A. Scholle & P.R. Schluger, editors) SEPM Spec. PuN. 26.


Shaw H.F. & PRIMMER T.J. (1991) Diagenesis of mudrocks from the Kimmeridge Clay Formation of the Brae Area, U.K. North Sea. Mar. Petrol. Geol. 8, pp. 270-277.

StJROAM R.C. & CROSSEr L.J, (1985) Organic-inorganic reactions during progressive burial: key to porosity and

permeability enhancement and preservation. Phil. Trans. Roy. Soc. Lond. A315, 135-156.

YEn H.W. (1980) D/H ratios and late stage dehydration of shales during burial. Geochim. Cosmochim. Acta 44, 341-352.

petrographic and isotopic evidence for diagenetic processes in

Documents