Clay Minerals (1986) 21, 211-224 211

C L A Y M I N E R A L V A R I A T I O N IN T E R T I A R Y S E D I M E N T S F R O M T H E E A S T E R N F L A N K OF

T H E N I G E R D E L T A

S. P . B R A I D E * ArqO W . D. H U F F

Department of Geology, University of Cincinnati, Cincinnati, Ohio 45221, USA

(Received 26 February 1985; revised 17 January 1986)

A B S T R A C T: Detailed clay mineralogical and chemical analyses of well cuttings of Tertiary sediments from two wells, Uruan- 1 and Uda- 1, on the eastern flank of the Niger delta, have been made in an attempt to investigate clay mineral burial diagenesis. The clay mineralogy indicates a transformation of smectite to an interstratitled iUite-smectite (I/S) phase. The relationship between ordered and random interlayering, however, is nonsystematic. The chemistry of the <0-1/zm size fraction shows some tendency towards a net gain in K20 and A1203 and a net loss in SiO 2 with depth, but the relationship does not correlate well with the thermal gradient. The distribution of kaolinite and chlorite in both wells appears to be unrelated in any regular way to smectite transformation and these two minerals are considered to be either the products of other diagenetic reactions affecting various stratigraphic levels, or the result of primary sediment deposition. In well Uda-1, kaolinite decreases in relative abundance with depth. This trend has been interpreted elsewhere as indicative of a transition from nonmarine to marine facies. The results of this study indicate that lack of ion mobility, rather than availability, is a significant factor in retarding the formation of ordered I/S with depth, and that lithology, overpressuring, carbonate cement, and original smectite layer charge may be controlling factors in the smectite --, illite transformation.

The Niger delta is the result of a series of t ransgressive-regressive cycles which commenced in early Cretaceous time and continue at present with a southward regressive phase initiated during the Eocene. The principal rock strat igraphic units as described by previous authors (Short & Stauble, 1967) are summarized in Fig. 1. Post-Paleocene deltaic sediments representing continental, t ransi t ional and marine environments are known correspondingly in the subsurface as the Benin, A g b a d a and A k a t a Format ions . These units vary from sandy to clay-rich shales and siltstones with most of the fine-grained sediments confined to the A k a t a and A g b a d a Format ions .

Two wells drilled by Shell Petroleum Development Compa ny of Nigeria penetrated the A k a t a and A g b a d a Format ions in the eastern part of the delta (Fig. 2) to depths of approximately 3700 m and cuttings were collected at regular intervals for petrographic and mineralogic study. This paper deals with the clay mineral composi t ion and variat ion with depth of the Aka ta and A g b a d a sediments. The da ta presented are drawn from depths below 1312 m in order to concentrate on the possible effects of depth-related changes on

* Present address: Dept. of Geology, Federal University of Technology, Minna, Niger State, Nigeria.

�9 1986 The Mineralogical Society

212 S. P. Braide and I4/. D. Huff

CHRONOSTRATIGRAPHY OUTCROPS

RECENT - PLEISTOCENE

PLIOCENE

MIOCENE

OLIGOCENE

EOCENE

PALEOCENE

UPPER SENONIAN

LOWER SENONIAN

TURONIAN

ALBIAN

DELTA SUBSURFACE

i . . �9 ". Benin Formation . ." " . . "

A g b a d a

', i.(i ..].i.Q!'asll! Asaba F . . . . t l o n ~ i i ) i .

.

�9 ~ ~ ~ F r n . equivalents unknown

J ~ Awgu Shale

S ~ ~ Ez?/A?/u S h?/le 7 ~ J ~ e / / I [ t . . . . it' o nal

~/~AAj J J ~ ~ ? u ? i v ; r ? r ? u ~ f ~ ~ marine

FIG. 1. Niger Basin stratigraphy and related depositional environments.

o 5o ~o km ATLANTIC OCEAN "~'~.i

FIG. 2. Index map of the Niger Delta showing location of wells studied.

clay mineral variation and chemistry. Previous work by Lambert-Aikhionbare & Shaw (1982) concerned the relationship between clay mineralogy, sandstone petrology and the occurrence of petroleum in the Niger delta. This investigation deals with the evaluation of evidence for clay mineral burial diagenesis.

Clays of the Niger Delta 213

S T R A T I G R A P H Y

The Tertiary Niger delta is composed predominantly of regressive clastic sequences, and its geometry is controlled by movement of various basement blocks linked to the opening of the South Atlantic. Evamy et al. (1978) vertically subdivided the delta into broad facies units, which consist, at the top, of massive sands and gravels deposited under continental conditions. Underlying these units are sands with some shales, considered to be a transitional series. They grade into an alternation of sandstone and shale deposited under paralic conditions. Below this series are marine shales with some turbidites. Paralic and marine/paralic sediments were restricted to the eastern flank during the Paleocene--Eocene, and are considered to be deposits of the incipient Cross River, whose watershed is in the Oban Massif in close proximity to the volcanic Cameroun Mountains. During the Oligocene and earliest Miocene, the development of successive non-overlapping depocentres in the eastern Niger delta (Evamy et al., 1978) probably reflected a rapid advance of the delta front. Miocene hiatuses are recognized in the eastern flank, and are considered to be the boundaries of large-scale sedimentary units in the delta which can be traced stratigraphically in the subsurface.

Lithologies

Well Uruan-1. The base of the Agbada Formation is recognized at 2100 m, and the lithology to this depth is characterized by an alternation of sand and intermittent shale sequences (Fig. 3). In places, the shale sequences are considerably thicker than the sand units. The Agbada Formation can be subdivided, in this well, into an upper and lower part with the boundary placed at a depth of 1509 m. The upper part consists predominantly of sands which are very coarse to very fine-grained, and are mostly unconsolidated. Sorting is very poor, but improves where the amount of shale increases. Shales predominate in the lower part. They are grey and dense, but those from the upper part tend to become more silty. Short & Stuable (1967) interpreted the prominent shale units in the lower part of the Agbada Formation in the type section as a record of localized non-deltaic conditions at the onset of a transgression, and the coarseness and poor sorting of some of the sand units in the upper part was interpreted as indicating a fluviatile origin. They also suggested that the well-sorted (and in general finer-grained) sands represent beach or coastal barrier sand deposits.

The Akata Formation occupies the interval below 2100 m and has a uniform shale development with no more than 25% sand at certain horizons. The shale is generally dark grey. From 3346 to 3773 m, thick sand units occur with intercalation of shale layers. These units are interpreted as turbidites based on electric log signature and stratigraphic position, and are therefore suggestive of a deeper-water environment.

Well Uda-1. The Agbada Formation consists of an upper part (1312-1854 m) which has a higher sand content than a lower part (1854-2149 m) which is predominantly shaly, but with two distinct sandbodies which are also noticeable on electric logs. The sands in the upper part are coarse, generally unconsolidated, and poorly sorted (Fig. 4).

The Akata Formation is predominantly a uniform dark grey shale, with a prominent sandy horizon at ~2593 m. The bottom of the well is at 3688 m.

214

0

=

e~

= 2 r

_ _ h

I -

~e

S. P. Braide and W. D. H u f f

A B

50 100 %

--~-~6,o m

- - - - - - 2 2 9 7

. . . . 2 6 2 6

~ 2 9 5 3

,,.--, �9 . . . . . 0 . , . . . . . : . d - . . 0 . . . ~ . - _

. . . . . t - - - . . . ~ 0 ~ _

. o �9 r 0 * . - - - �9 ~ g .- _*=1~ ~

- - - ~ - - - , . . o . i o . ~ 1 7 6 1 7 6 ~ 1 7 6 o . ~ . . . . ~ 1 7 6 l l . " g g g ; I - -

- - 3 2 8 2

~ 3 6 0 9

C

S P R e s .

s s t

sh

silty s h

I s h

D

4 0 8 0 1 2 0

T E M P E R A T U R E ~

FIG. 3. Logs for Well Uruan-1, eastern Niger Delta: (A) percentage lithology; (B) stratigraphic section based on Log A; (C) SP and resistivity logs; (D) calculated temperature profile.

Geotemperatures in the Niger delta

Evamy et al. (1978) demonstrated a distinct relationship between temperature gradient and sandstone/shale ratio in the subsurface of the Niger delta. The temperature gradient increased with diminishing sand percentage from < 1.84 o C/100 m in the continental sands, to ~2.73~ m in the paralic section, to ~5.47~ m in the continuous shales of the Niger delta. The equation which best fitted the observed measurements was:

T = a -- bS 2 + cD

b.

<

< ea

<

Ik

<

<

A 0 50 100 %

~ 1 6 4 0 m

V-----7 \ / ,

/ \ - - 2 6 2 6

. . . . i . . . . . i . . . . i

. . . . t m _ _ m . . . . i

~ ' - - - 2953 , . . . . j _ - - . , : - - - . . . . . tw - , . . . r - - - -

- r - - - - - -

- - ~__~ - - 3282

- - - - - - ~

- - - - - - I - - - 3609

Clays of the Niger Delta

B CRe s S P .

I \ / i ', /

�9 , No / \ samp les

/ \

- - . . _ - - _ _ ' - - - "2

__--" __- -__"- - - s h

- - - _ - - -~

- 7 - . . - 7 - .__~

T - - ' " - : ' - - - - s i I ty __ , - - . - -

_ _ m -

. . . . s h _ _ m - - m - -

_ _ F - -

s h

215

D

I

t

t

�9 t ' ~ 1=o

T E M P E R A T U R E ~

FIG. 4. Logs for well Uda-1, eastern Niger Delta: (A) percentage lithology; (B) stratigraphic section based on log A; (C) SP and resistivity logs; (D) calculated temperature profile.

where T is the temperature gradient (originally expressed as ~ ft), S is the sand percentage of the selected interval and D is the average depth of the selected interval. The coefficients a, b and c were calculated from equilibrium considerations.

Temperature profiles for wells Uruan-1 and Uda-1 calculated using the above expression are shown in Figs 3 and 4. These compare favourably with plots of geotemperatures at 2591 m and of subsurface depth to 116~ by Evamy et al. (1978). Bottom-hole temperatures agree with previous reports and there is a tendency for gradients to become steeper in the shale-dominated sections.

216 S. P. Braide and W. D. Huff

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

X-ray powder diffraction

The <0.1 /~m size fraction was investigated in detail by XRD since this size range is generally free of non-clay minerals and is considered the most sensitive to burial diagenetic changes (Hower et al., 1976).

10-20 g samples of well cuttings were disaggregated by ultrasonics after soaking in deionized water for ~12 h. Separation of the <0.1 pm size fraction was then made by high-speed centrifugation. XRD patterns were obtained from the <0.1 /~m material oriented on porous ceramic plates by centrifugation.

Air-dried samples were initially X-rayed, then heated for 1 h at 550~ and examined again. A duplicate set of samples was solvated with ethylene glycol prior to XRD analysis.

Chemical analysis

Atomic absorption spectrophotometric analysis of major elements was carried out on both <0.1 #m size fractions and whole-rock specimens. Samples were prepared by fusion with a lithium metaborate flux (Medlin et al., 1969). USGS rock standards were used to establish quantitative reference curves for seven oxides, SiO 2, A1203, MgO, Fe20 3, CaO, Na20 and K20.

C L A Y M I N E R A L O G Y

Five principal clay minerals were identified in this study: smectite, illite, kaolinite, chlorite and interstratified illite-smectite (I/S). The first four minerals were identified by methods described by Carrol (1970) and by Brown & Brindley (1980). I/S was identified by methods described by Reynolds & Hower (1970), Reynolds (1980), and ~rodofi (1980). Semi-quantitative estimation of the other clay minerals were made using the methods of Johns et al. (1954) and Biscaye (1965).

Well Uruan-1

Smectite occurs down to a depth of 1312 m with its relative abundance decreasing from 71 to 31%. Below 1312 m, peak broadening and background shifts indicate the presence of randomly interstratified I/S.

Mixed-layer I/S was detected between 1312 and 3773 m (Fig. 5), and its relative abundance fluctuates between 100 and 28% (Table 1). Within this depth range, the mixed-layer I/S from samples taken between 1312 and 2626 m is randomly interstratified (R0) with 80 to 20% smectite component.

The I/S between 2626 and 3675 m all had ordered interstratification (R1). Ordered interstratification disappears below 3675 m and the presence of a weakly-developed 17/k reflection indicates random I/S mixing with perhaps some tendency toward I/S segregation (Reynolds, 1980).

Kaolinite occurs sporadically throughout the depths investigated (Fig. 5, Table 1). At certain depths it reaches 62% while at other depths only trace amounts occur or it may be absent. Chlorite occurs in only a few samples and in relatively low abundances.

Clays of the Niger Delta

AKATA FORMATION

217

I~O~OERED!IIIIIIIII I l t l l l l ~ l l l l l , ~ l I { l l l l l l ( O~

e--ram i �9 �9 w 1 �9 /w m 1 �9

~ o ~ g o =I ] " 3 :~

F . . . . . . l . .... , . . . . . . . . . . . . . . . II,lllllllHI 1 IIIllll

AGBADA FORMATION

FIG. 5. Clay mineralogy of the <0.1/zm size fraction in Wells Uruan-1 and Uda-1. S = smectite, I/S = illite/smectite and I = illite.

TABLE 1. Semiquantitative estimates of clay mineral abundances (%) in <0.1/~m fractions from well Uruan-1.

Depth (m) Lithology Smectite I/S* **/%S Illite Kaolinite Chlorite

1312 sandy shale 0 71 Ran/80 0 29 0

1640 0 28 Ran/80 Tr 62 Tr 1969 0 100 Rant20 0 0 Tr 2297 0 28 Ran/40 0 62 0 2626 shale 0 78 Reg/40 0 22 0 2953 0 90 Reg/40 0 7 3 3282 0 87 Reg/35 0 Tr 13

3642 0 94 Reg/40 0 Tr 6 3675 0 96 Reg/45 0 0 4 3707 sandstone 0 89 Ran/45 0 11 0 3740 0 92 Ran/60 0 Tr 8 3773 , 0 90 Ran/60 0 10 0

* Interstratified illite/smectite. ** Ran = random. Reg = regular interstratification. Tr = trae~ amounts.

Well Uda-1

Discre te smect i te and illite occur in the shal lower par ts o f Uda-1 but d i sappear with

depth. Bo th m o s t likely reflect detr i tal sed iment compos i t ion and are unre la ted to

diagenet ic events. I /S occurs f rom 1391 m to the b o t t o m o f the well (3688 m), and appears

to increase in abundance with depth (Fig. 5, Tab le 2). The type o f interstrat i f icat ion varies

218 S. P. Braide and leg. D. Huf f

TASLE 2. Semiquantitative estimates of clay mineral abundances (%) in <0.1 #m fractions from well Uda- 1.

Depth (m) Lithology Smectite I/S* **/%S Illite Kaolinite Chlorite

1391 25 25 Ran/90 15 35 0 1719 sandstone 0 20 Ran/80 32 48 0 2067 0 20 Ran/80 38 42 0

2730 shale 0 30 Ran/40 38 32 0 3031 sandy shale 25 40 Reg/30 0 35 Tr 3189 10 50 Reg/30 0 27 0

3340 0 88 Reg/15 0 22 Tr 3360 0 90 Reg/15 0 10 0 3530 shale 0 93 Reg/20 0 7 0 3661 0 86 Reg/20 0 14 0 3688 0 90 Reg/20 0 10 0

* Interstratified. ** Ran = random. Reg = regular interstratification. Tr = trace amounts.

from random in samples from 13 91-2730 m, to short-range ordered interstratification (R 1) in the rest of the samples investigated from 3031-3688 m. Discrete smectite was tentatively identified between 3031 and 3189 m but this may simply be a region of increase in the smectite component of the I/S. It coincides with the appearance of R 1 ordered I/S and could indicate a gradual rather than an abrupt transition between the two forms.

Kaolinite occured in every sample investigated, but there is a decrease in its abundance with depth (Table 2). Its higher abundance in the sandstones may indicate lithological control. In contrast, chlorite is rare except for trace amounts in two samples from 3031 and 3340 m.

C H E M I C A L D A T A

Analytical data for SiO~, A1203, MgO, Fe203, CaO, Na20 , K20 in both <0.1 gm fractions and whole-rock samples for weUs Uruan-1 and Uda-1 are presented in Figs 6, 7, and 8.

In Uruan-l, SiO 2 decreases with depth in the <0.1 gm fraction while there is a slight increase in the whole-rock sample. The variation of A1203 is constant in both size fractions. There is a net loss in Fe203 with depth in the <0.1 pm fraction but no clear trend for the whole-rock samples. MgO varies nonsystematicaUy with depth with maximum and minimum values occurring below 3500 m for both sample groups. CaO is constant with depth in the <0.1 pm fraction, although there is a large increase in the whole-rock sample at ~3000 m. Na20 is fairly constant in the whole-rock sample to ~3000 m and then increases dramatically. However, there is a contrasting decrease in the <0.1 gm fraction. K20 increases with depth by about 4% in the whole-rock sample and probably reflects a lithologic variation in the K-bearing detrital phases such as illite (mica) and K-feldspar.

In the <0.1 gm fraction, however, K20 remains fairly constant at 1-2% to 3281 m. Beyond that it varies erratically, ending at an overall net increase at 3773 m (Fig. 8). The

A 2500

E

Si 0 2

RP 3500

I ' -

a ,

10 t,a

a 1500

CaO

C ep , _.70

\

( i ) /

:.".. i

0 N

Clays of the Niger Delta

E N T R A T I 1 2

150C

2 5 0 0

3500

,Vtg0

50

15OC

0 N

1500

2 5 0 0

3 5 0 0

219

(~) 2 6 8 10

Fe203

250C

3 5 0 0

A I 2 0 3

~i ' ' ' ' " "

250(3

...i

. . . . . . .? : "

N a 2 0 whole rock --

<0.I pm . . . .

FIG. 6. Depth-related variation of major element oxides in whole rock and <0.1 pm fractions from well Uruan- 1.

clear relationship between depth, temperature and mineralogical transformation reported by previous authors is not seen here and the possibility arises that there are other, more overriding controls on clay mineral formation. There is no evidence of a systematic increase in K20 in the <0.1 #m fraction coincident with the transformation of smectite to random, then ordered, I/S. Nor is there a corresponding decrease in whole-rock K20 as would be expected in a closed, lithogically homogeneous system. Similarly, the MgO, A120 a and SiO 2 data do not suggest progressive transformation with depth.

In well Uruan-1 the lithology changes from mud- to sand-dominated <3200 m. Chlorite is more abundant than at shallower depth and kaolinite less so, and this may be reflected in whole-rock increases in MgO and Fe203. K20 is high in whole-rock samples but is lower and varies erratically in the clay fraction. Thus, facies control on mineralogical composition probably exerts a dominating influence on clay mineralogy. The reappearance of random I/S below 3642 m is unexpected and suggests greater immaturity in the burial diagenesis of clays.

In well Uda-1, SiO 2 increases with depth in the whole-rock sample but decreases with depth in the <0.1 pm fraction. A1203 shows a slight gain in both the <0.1 #m fraction and in the whole-rock sample. The variation of Fe203 with depth is unsystematic, although overall there appears to be an increase in concentration in both the <0.1 pm fraction and in the whole-rock sample. The MgO plot suggests a general increase in the whole-rock sample and a decrease in the <0.1 #m fraction with depth. There is an increase in CaO in

220

50

1500

~ _ 2 5 0 0

E

, , r 3 5 0 c

I--

r

lO W

f% 150c

3500

C 50 7,a

SiO 2

O

S. P. Braide and I41. D. Huf f

N C E N T R A T I 1 2

250(

3 5 0 0

MgO

0 N ( ~ )

3 5 0 0

Fe203

l.a 1,8

~1~o 3

1 2 1 2

1500

CaO

1 5 0 0

. . . . .....

~5oo ~ T L . . . . .

Na20

whole rock < O . l

FiG. 7. Depth-related variation of major element oxides in whole rock and <0-1 /~m fractions from well Uda-1.

0

I""

250C L

3500

C O N C E N T R A T I O N (~)

3 4 5 0 1 2 3 4 5

well Uruan-I

whole rock

<0.I zm

3 5 0 (

well Ud~-i

F~6, 8. Depth-related variation of K :O in wells Uruan-1 and Uda-h

Clays of the Niger Delta 221

both size categories studied. Na20 concentration in the <0.1 gm fraction does not show any net increase, but there are significant increases between 3000 and 3500 m.

K20 is highly variable in both groups of samples but displays an overall increase in the clay fraction and a corresponding decrease in whole-rock content. The lithology remains mud-dominated below 3200 m and random I/S does not reappear. Thus there is some indication that K-bearing phases such as K-feldspar in the bulk rock are providing K + for the burial transformation of smectite to I/S. The overall loss of Si and gain of Al supports this view.

D I S C U S S I O N A N D C O N C L U S I O N

Smectite transformation to a mixed-layer I/S phase has been reported by several investigators to be a consequence of burial diagenesis. Since the first published study which recognized this transformation in subsurface samples from the Gulf Coast Tertiary (Burst, 1959), a hypothesis for clay mineral burial diagenesis (Perry & Hower, 1970) now seems widely accepted. The hypothesis suggests the following chemical mechanism for the transformation:

smectite + AP + + K + --* illite + SP +

Boles & Franks (1979) suggested that loss of Si instead of a gain of A1 has the same effect in the above transformation.

Similar clay mineralogical changes have been observed in Mesozoic shales from Papua New Guinea and in Tertiary shales from Louisiana (van Moort, 1971), in late Triassic to late Cretaceous shales from the Canadian Northwest Territories (Foscolos & Powell, 1978), in Mesozoic shales from Montana (Hoffman & Hower, 1979), and in Tertiary sediments from the Rhinegraben, West Germany (Doebl et al., 1972; Heling, 1974, 1978). This same transformation has also been reported in studies of Mesozoic and Cenozoic rocks in which there is no significant depth of burial (Boles & Coombs, 1975; Schultz, 1978; Nadeau & Reynolds, 1981), and in Paleozoic rocks with a long history of shallow burial at relatively low temperatures (Huff & T~rkmeno~lu, 1981 ; Heling, 1974, 1978).

Lambert-Aikhionbare & Shaw (1982) reported the preservation of smectite in the Akata Formation of the Niger delta at depths of ~3937 m and burial temperatures of 120~ From the results of this study, we believe that this was random I/S as observed in well Uruan-1 at depths in excess of 3675 m. Calculated geothermal temperatures for these depths range between 100-120~ (Fig. 3). The reason for this random I/S not having undergone complete transformation to the ordered phase does not seem to be a lack of potassium, a necessary element in the diagenetic process, as chemical data (Fig. 8) indicate the availability of 3-5% K20 at the depths in question.

What other factors may then control the S to I/S transformation? In the Rhinegraben of southwest Germany, Heling (1978) noted that the inherited layer charge of smectite possibly had some bearing on illite diagenesis. He added that higher-charged smectite with more tetrahedral Si substituted by AI would more readily alter to illite than lower-charged smectites. The role of overpressuring has not been evaluated in this study, but may contribute to the inhibition of I/S development. For example, Koster van Groos & Guggenheim (1984) have shown that Na-montmorillonite is stable under normal pressure distributions in sedimentary basins. Thus the dehydration reactions involved in I/S formation must involve more complex chemical reaction with pore-fluids. Still another

222 S. P. Braide and W. D. H u f f

possible explanation for the existence of smectite at 'prohibitive' depths was offered by Foscolos et aL (1982), who suggested that the formation of calcite prevents clay authigenesis, and may be responsible for the absence of mixed-layer clays in deeper buried sandstone units in the Viking sandstone in Central Alberta, Canada. Whole-rock CaO reaches a maximum around 3000 m in both wells Uruan-1 and Uda-1 and thus carbonate control may be significant.

In well Uda-1 random and ordered I/S occur from 1391 m to total depth at 3688 m. The smectite component in the I /S decreases with depth from 90% to 40% in the randomly interstratified phase. There is a further change from 40% at 2730 m to 30% in the ordered interstratified phase observed at 3031 m. The K20 content of the clay fraction increases further below this point but the X R D patterns show no discernible change in I/S characteristics. Reynolds (1980) pointed out the insensitivity of X R D to compositional shifts near the extremes of I/S proportions. Such a sharp drop in the amount of smectite layers in the I/S, over a relatively short stratigraphic interval, may be an indication of some form of stratigraphic or facies control. The distribution of kaolinite and chlorite in both wells studied appears to be unrelated in any systematic way to smectite --, illite transformation. They are considered to be either the products of other diagenetic reactions affecting various stratigraphic levels, or the result of primary sediment deposition. In well Uda-1, however, kaolinite decreases in abundance with depth (Table 2). This trend is probably indicative of a transition from nonmarine to marine facies (Brown et al., 1977), which supports the interpretation of a detrital orgin for kaolinite.

In both wells the observed depth-related mineralogical and geochemical trends could in part be characterized by the burial diagenesis model as described for the Gulf Coast. However, it would appear that factors other than temperature and K-availability also contribute to the observed mineralogy. These include layer-charge characteristics of the starting smectite, oVerpressuring, carbonate deposition and facies changes. Each needs to be considered separately before a complete understanding of clay diagenetic history can be achieved.

ACKNOWLEDGEMENTS

We thank J. B. Maynard and W. A. Pryor for useful discussions, and D. Joyce for technical assistance. Financial support for this study was provided from the Petroleum Technology Development Fund, administered by the Petroleum Inspectorate, Nigerian National Petroleum Corporation. Samples for this study were provided through the courtesy of Shell Petroleum Development Company of Nigeria.

REFERENCES

BISCAYE P.E. (1965) Mineralogy and sedimention of recent deep-sea clay in the Atlantic Ocean and adjacent seas and oceans. Bull. Geol. Soc. Am. 76, 803-832.

BOLES J.R. & COOMaS D.S. 0975) Mineral reactions in zeolitic Triassic tuff, Hokomui Hills, New Zealand. Bull. Geol. Soc. A m. 86, 163-173.

BOLES J.R. & FRANKS S.G. (1979) Clay diagenesis in Wilcox Sandstones of southwest Texas, implication of smectite diagenesis of sandstone cementation. J. Sed. Petrology 49, 55-70.

BROWN G. & BRINDLEY G.W. (1980) X-ray diffraction procedures for clay mineral identification. Pp. 305-359 in: Crystal Structures of Clay Minerals and their Identification. (G. W. Brindley and G. Brown, editors). Mineralogical Society, London.

BROWN L.F., BAILEY S.W., CLINE L.M. & LISTER J.S. (1977) Clay mineralogy in relation to deltaic sedimentation patterns of Desmoinesian cyclothems in Iowa-Missouri. Clays Clay Miner. 25, 171-186.

Clays o f the Niger Delta 223

BURST J.F. (1959) Postdiagenetic clay mineral environmental relationship in the Gulf Coast Eocene. Clays Clay Miner. 6, 327-341.

BURST J.F. (1969) Diagenesis of Gulf Coast clayey sediments and its possible relation to petroleum migration. Bull. Am. Assoc. Petrol. Geol. 53, 73-93.

DOEBL F., HELING D., HOMANN W., KARWIEL J., TEICHMULLER M. 8,~ WELTED. (1972) Diagenesis of Tertiary clayey sediments and included dispersed organic matter in relationship to geothermics in the Upper Rhinegraben. Pp. 192-207 in: Approaches to Taphrogenesis (J. H. lilies and K. Fuchs, editors). Verlagsbuchandlung, Stuttgart.

DUNOYER DE SEGONZAC G. (1970) The transformation of clay minerals during diagensis and low-grade metamorphism: a review. Sedimentology 15, 281-346.

EMERY K.O., UCHUPI E., PHILLIPS J., BROWN C. & MASCLE J. (1975) Continental margin off western Africa. Angola to Sierra Leone. Bull. Am. Assoc. Petrol. Geol. 59, 2209-2265.

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