Clay Minerals (1989) 24, 137-156
T H E C H E M I C A L C O M P O S I T I O N OF A U T H I G E N I C
ILLITE WITHIN TWO SANDSTONE
RESERVOIRS
AS A N A L Y S E D BY A T E M
E. A . W A R R E N *
AND C. D . C U R T I S 1 "
Department of Geology, University of Sheffield, Mappin Street, Sheffield $3 7HF
(Received 12 April 1988; revised 6 February 1989)
ABSTRACT: Analytical transmission electron microscopy (ATEM) was used to obtain
chemical analyses of single illite crystals from Upper Carboniferous reservoir sandstones of the
Bothamsall Oilfield, East Midlands, UK, and from Rotliegendes (Permian) sandstones of the
North Sea. All samples were found to be highly aluminous, with only minor Mg and Fe, and to
have near-ideal dioctahedral sheet totals. No significant var!ation in chemical composition of
illite was found within the Bothamsall reservoir rocks, irrespective of paragenesis or stratigraphic horizon. Similar results were obtained from the Rotliegendes samples. Variation was
found, however, between Bothamsall and Rotliegendes analyses populations. The Rotliegendes
illites were distinctly more K-rich, which is the result of greater charge deficiency in the
octahedral sheet. These results indicate that all the illite in both reservoirs precipitated in
equilibrium with the reservoir pore-fluid. Furthermore, they imply that the physico-chemical
composition of the pore-fluid did not evolve significantly between the different iUite generations
in the Bothamsall samples. These data, when compared with published analyses of authigenic
illite, indicate that the compositionalfield for illite in sandstones is restricted in comparison with
that of illite in mudrocks.
Illite is a common authigenic mineral in many sandstones and, as with all minerals, an
accurate knowledge of its composition and chemical variation is an important prerequisite to
understanding its precipitation and occurrence. However, until recently its fine grain size has
prevented accurate analysis at the particle scale. This paper describes the use of a highresolution technique, analytical transmission electron microscopy (ATEM), to investigate
intra-formation compositional variations of authigenic illite in two geologically unrelated
reservoir rocks.
Most published chemical d a t a of illite have been obtained from mudrocks using indirect
analysis methods of disaggregated bulk samples (Newman, 1987; W e a v e r & Pollard, 1973,
Deer et al., 1962). These indicate that illite is a mineral with an extremely variable chemical
composition. Direct analyses of illite in sandstones using more m o d e m techniques such as the
electron microprobe (e.g. Merino & Ransom, 1982; Velde, 1984) suggest that the
compositional variation for illite in sandstones m a y be somewhat smaller. In all these cases,
however, the fine particle size of illite is beyond the resolution of the standard analytical
techniques used so that these analyses inevitably represent averages of m a n y thousands of
millions of single crystals. The homogeneity of compositions within formations cannot be
Present addresses:
* BP Research PLC, Sunbury Research Centre, Chertsey Road, Sunbury on Thames, Middlesex TW16
7LN.
1"Dept. of Geology, University of Manchester, Oxford Road, Manchester M13 9PL.
9 1989 The Mineralogical Society
138
E. A. Warren and C. D. Curtis
readily resolved as sample contamination from fine particles (e.g. hematite platelets) or
detrital minerals is almost impossible to quantify. These problems can be largely overcome
with the ATEM, which features one major advantage over other analytical techniques - - a
high resolution (up to 0-25 nm) image. This makes it the best readily available technique for
the study of clay compositions, as recent studies of authigenic clay minerals in sandstones
(Ireland et al., 1983; Huggett, 1986) and mudrocks (Duplay, 1984; Lee & Peacor, 1986)
demonstrate.
SAMPLE LOCATIONS
Samples were selected from two geologically unrelated sandstone reservoirs within the
United Kingdom Continental Shelf. The first, from the Bothamsall oilfield, Nottinghamshire (Fig. 1), comprises two major fluviatile-channel sandstone members of Westphalian A
(Upper Carboniferous) age which have been correlated with the Crawshaw and sub-Alton
sandstones at outcrop (Hawkins, 1972, 1978). Both members are principally sub-arkosic,
fining-upwards units up to 30 m thick, separated vertically by 40 m of mudrocks.
Substantial post-burial modification of the sandstones has occurred, in which detrital
feldspar and mica have largely been dissolved or altered, and authigenic cements of quartz,
kaolinite, illite and ankerite have precipitated (Hawkins, 1978; Warren, 1987a). Two
generations of illite have been recognized: one preceding kaolinite formation, the second
postdating kaolinite and quartz precipitation (Warren, 1987a, b). These modifications are
considered to reflect the complexity of the burial history (Warren, 1987b); rapid burial in the
late Carboniferous was followed by Permian uplift and substantial erosion (Smith et al.,
1973). Maximum burial and temperature were attained in the late Jurassic, followed by a
R
Fie. 1. Location map of the Bothamsalloilfield(B) and Rotliegendes(R) samples.
Chemistry of authigenic illite
139
second inversion in the early Tertiary (Kent, 1980) resulting in present burial depths of
1000 m for the Westphalian. Thermal maturity indicators (Smith et al., 1973) suggest that
maximum temperatures of 100~176
were attained in the late Jurassic.
Nine samples from the BothamsaU oilfield were selected for ATEM analysis from five well
cores spread across the oilfield. These samples are located at the top, the middle and the base
of the two sandstone members.
The second suite of samples is of Rotliegendes sandstone (Lower Permian) from the
southern North Sea. Three samples, A-I, A-2, and A-3, are from different depths within one
well, whilst sample B is from a second well. All are very well-sorted, medium-sand-size
arenites. Grains are well rounded and contain red hematitic dust rims. These samples are
typical of the aeolian facies rocks of the Rotliegendes sandstone.
In contrast with the Carboniferous samples, these have had a relatively simple diagenetic
history, with illite and dolomite the only major cements. The burial history is also simpler
(Glennie, 1986): continuous subsidence, punctuated by a period of inversion in the late
Cretaceous, to the present depths of approximately 2700 m.
EXPERIMENTAL
METHODS
The principal methods used in this study were optical petrography of standard thin-sections,
scanning electron microscopy (SEM), transmission electron microscopy (TEM), ATEM, Xray fluorescence spectroscopy (XRF) and X-ray diffractometry (XRD). Generally, routine
preparation and analytical procedures similar to those described in Wilson (1987) were
followed.
Secondary electron images (SEI) were obtained of gold-coated frature surfaces of air-dried
specimens using the SEM. Critical-point drying was not used.
Ultra-thin ( < 300 nm) rock sections were prepared by argon-ion bombardment of standard
thin-sections according to the method of Phakey et al. (1972).
Clay-size fractions were separated from bulk disaggregated rock samples for XRD and
TEM. Samples were first disaggregated by gentle rolling between pestle and mortar in a
dilute solution of sodium hexametaphosphate. The siphoned solution was then repeatedly
washed with distilled water and centrifuged to collect the > 2 #m, 2-0.5 #m and < 0.5/zm size
fractions. Duplicate preferentially-oriented smears of each size fraction were prepared for
XRD. Samples were run after air drying and after glycolation.
The < 0-5/~m size fractions of two samples, Bothamsall 5-3276 and 5-3319, were dispersed
on to carbon films for ATEM analysis and later Pt-shadowed for TEM examination.
A 50 mg sample of the <0.5 #m size fraction of Bothamsall 5-3349 was also analysed by
XRF to permit comparison of bulk analysis with the single-crystal ATEM analyses.
All ATEM analyses were obtained using a Philips 400 TEM/STEM fitted with an EDAX
9200 Energy Dispersive Spectral (EDS) X-ray detector operated at 100 kV with a spot size
~ 100 nm in diameter. Analyses were collected at between 1000-2000 cps for 100 live-time
seconds. After background stripping, element intensities were converted, using 'K-values'
(Table 1), to mica formulae on the basis of 22 oxygen equivalents and four assumed hydroxyls
per unit-cell. Perfect tetrahedral totals of 8 atoms per formula unit were assumed, but the
octahedral total was left unfixed. A ferrous/ferric iron ratio of 1:4 was used for all samples, as
determined from wet chemical analysis of illites by Ireland (1982). However, a ratio of 2:3
was measured by M6ssbauer spectroscopy for a nearly pure monomineralic illite sample
(Bothamsall 5-3349). Importantly, no iron oxide contaminants were discovered in this
140
E. A. Warren and C. D. Curtis
Chemistry o f authigenic illite
141
TABLE 1. Experimentally determined
proportionality constants (K-values)
used in this study. From Hughes et al.
(1989).
Element
kXSi
Mean
a
Mg
A1
Si
K
Ti
Mn
Fe
1.60
1"14
1.00
1"19
1"10
1.24
1.28
0.10
0.05
0.00
0-13
0"10
0.16
0.11
analysis (Davey, personal communication). Use of this value would only alter the calculated
octahedral totals slightly because of the small amounts of iron generally present.
RESULTS
Textural observations
The illite in both reservoirs was observed to consist of thin, elongate, fibrous crystals,
commonly described as 'hairs' (Figs 2 and 3). Individual fibres were considerably longer and
wider in the Rotliegendes samples (Fig. 3) than in the Bothamsall samples (Fig. 2). In the
Bothamsall samples the fibres often appeared as coalesced aggregates (Figs 4 and 5),
probably the result of air-drying. These coalesced fibres were either partially enclosed by
quartz overgrowths (Fig. 4), clearly pre-dating the quartz, or entirely enveloped the quartz
grain surface (Fig. 5).
In T E M of ultra-thin sections from the Bothamsall samples, illite was seen as small
( < 0.5 #m long) packets of crystals usually 5-10 nm wide (Fig. 6). These were observed either
interlocking with each other or rimming quartz grains and kaolinite plates. These packets are
considered to be the fibres observed in SEM; the mats may represent sections through the
coalesced aggregates observed in Figs 4 and 5. Individual crystal packets of Rotliegendes
illite were generally found to be both longer and wider (10-20 nm) than the illite in the
Bothamsall samples.
FIG. 2. Authigenic i11ite of fibrous morphology from the Upper Carboniferous sandstones,
Bothamsall. The iilitc fibres arc partially enclosed by authigenic quartz overgrowth, and hence
the illite predates this generation of quartz. Bothamsall 4-3394.
FIG. 3. Fibrous illite from the Rotliegendes sandstone (Permian). This illite is distinctly coarser
grained than the Bothamsall material of Fig. 2. Some fibres are wide and almost resemble plates
(arrowed). Rotliegendes B.
FIG. 4. Authigenic iUite extending perpendicular to the grain surfaces is partially enclosed by
quartz overgrowths (Q), and kaolinite (K). The platy appearance of the iUite is the result of
coalescence of illitc hairs, most probably the product of air-drying. BothamsaU 5-3276.
FIO. 5. Authigenic iUitc on quartz grain. The illitc almost completely envelops the quartz
surface as a film or mat of coalesced fibres from which a few fibres extend. BothamsaU 2-3659.
142
E. A. Warren and C. D. Curtis
FIG. 6. Ultra-thin section of authigenic illite in TEM. Individual crystals of 10-20 nm thickness
are arranged into sub-parallel packets which fn turn coalesce to form a mat. This texture may
well be equivalent to the coalesced fibres observed in SEM (Figs 4 and 5). Bothamsall 5-3276.
FIG. 7. A Pt-shadowed, illite fibre dispersed on a carbon film illustrates the acicular nature of
individual illite crystals, elongated parallel to the a-axis. Bothamsall 5-3319.
The only particle type observed in the two dispersed samples consisted of long, narrow
acicular crystals (Fig. 7). Thicknesses were found to vary between 2 and 12 nm, with a mode
of 5 nm.
XRD
The < 0.5 #m size fractions of five Bothamsall samples (Bothamsall 4-3223, 4-3256, 5-3276,
5-3319, 5-3349) produced traces of a clay mineral with a consistent 10.4 A basal spacing (Fig.
8a,b). This basal spacing shifted to 9.95 A after glycolation and heating. A small amount of
kaolinite was identified in the sample (Bothamsall 5-3319) used for X R F analysis. The
Rotliegendes illite yielded sharp reflections at 10-05 A.
Chemical data obtained with A T E M
Two-hundred and fifty single-crystal analyses were obtained from the twelve samples
studied and are listed by Warren (1987a). Calculated means with one sample standard
deviation of analyses from individual rock samples are presented in Table 2.
143
Chemistry of authigenic illite
~a
,s
d-specl~g
,0
?
;
u
i
(ooa}
1002)
J
,Z.
"~
,~
,'3
,'6
,;
i2
DEGREES 21)
,p,4.,
,2
~:
Cu K~
d-spacing '~
.
,o
- -
Air dried
--- .........
Glyco|ated
Heated350eC
......
Heated550*C
ill
-~.
/S/
o.
\
K
c4
~
~
~
~
~
~b
1'1
DEGREES 21) Cu K~
t'a
13
14
FIG. 8. (a) Air-dried XRD trace for preferentially oriented, <0-5 #m clay size fraction of
Bothamsall 5-3349. The major peaks are at 10.40 .~ (001), 5-02 A (002) and 3.30 A (003).
Kaolinite (K) is also present. (b) Various treatments illustrating the collapse of the (001) peak on
glycolation and heating.
Whilst most analyses were obtained from ultra-thin sections, some of the analyses obtained
for samples Bothamsall 5-3276 and 5-3319 are of individual crystallites of the fine-particle
separations used for X R D (Table 3).
DISCUSSION
XRD characterization of Bothamsall illite
The shift of the (001) peak from 10-40 A to 9.95 A after glycolation and heating
observed in the X R D traces (Fig. 8a,b) suggests that this illite is a mixture of illite and a small
144
E. A. Warren and C. D. Curtis
TABLE 2. Averages of ATEM analyses from single crystals.
Bothamsall
4-3394
Cbpf
Bothamsall
4-3253
Abpf
Bothamsall
5-3319
Cmpf
Bothamsall
5-3276
Ctpf
Mean
(n = 7)
a
Mean
(n = 36)
a
Mean
(n = 16)
a
Mean
(n = 94)
Si
A1
6.77
1.23
0.17
0.17
6.61
1.39
0.24
0-24
6.82
1.18
0. l0
0.10
6-78
1.22
0.18
0.18
Al
Ti
Fe(III)
Fe(II)
Mn
Mg
3-54
0.00
0.10
0.02
0.00
0.25
0-06
0.00
0.01
0.00
0.00
0.04
3.58
0.00
0.22
0.06
0-00
0.27
0.22
0.01
0-11
0.03
0-00
0.13
3-69
0.00
0-11
0.03
0-00
0.26
0.08
0.00
0.04
0-01
0.00
0.03
3-66
0-00
0.11
0.03
0.00
0.20
0-21
0.01
0.06
0.02
0.00
0.14
Ca
Na
K
0.09
0-00
1.58
0.03
0.00
0.18
0.05
0.01
1.20
0.06
0.03
0.35
0.04
0-00
1-14
0.04
0.00
0.16
0-09
0-05
1.21
0.14
0-15
0.30
Octahedral
Interlayer
OH
3.91
1.67
4.00
0.07
0.19
0.00
4.13
1.26
4.00
0.14
0.36
0.00
4.09
1.18
4.00
0.06
0.16
0.00
4.00
1.35
4-00
0.16
0.37
0.00
Bothamsall
6-3357
Ctplpf
Bothamsaii
6-3406
Cbpf
Bothamsall
16-3319
Abpf
Mean
Mean
(n = 16)
tr
(n = 8)
(n = 5)
Si
A1
6"93
1-07
0"21
0"21
6.79
l'21
0' 15
0.15
6"71
1-29
0"05
0-05
A1
Ti
Fe(III)
Fe(II)
Mn
Mg
3.44
0.00
0.18
0.04
0.00
0.24
0.16
0-02
0.11
0.03
0-00
0.07
3' 56
0.02
0.15
0.04
0.00
0.35
0.15
0.04
0.07
0.02
0.00
0.09
3"56
0.00
0-10
0-03
0.00
0.23
0"06
0"01
0.04
0.01
0.00
0.04
Ca
Na
K
0.07
0.05
1.43
0.03
0.12
0.38
0.04
0.00
1.15
0.02
0.00
0.25
0.10
0.01
1.56
0.08
0.03
0.15
Octahedral
Interlayer
OH
3.91
1"55
4.00
0.10
0-35
0"00
4-12
1.19
4"00
0.07
0-24
0.00
4.92
1.68
4"00
0.06
0.11
0.00
Mean
Chemistry of authigenic illite
TABLE 2
145
continued
Bothamsall
20-3448
Atpl
Bothamsall
20-3472
Abpl
Mean
(n = 6)
Mean
(n = 15)
Si
AI
7"02
0"98
0"16
0"16
6"95
1"05
0.05
0.05
A1
Ti
Fe(III)
Fe(II)
Mn
Mg
3"63
0"00
0.07
0.02
0.00
0.33
0.05
0.01
0.01
0.00
0.00
0.04
3.51
0.00
0.12
0.03
0-00
0.37
0"06
0"01
0.04
0.01
0.00
0.02
Ca
Na
K
0.05
0.02
1.09
0.02
0.05
0-12
0.08
0.02
1.16
0.02
0.04
0.13
Octahedral
Interlayer
OH
4.04
1.16
4.00
0.02
0.08
0.00
4.03
1.26
4.00
0.06
0.15
0.00
Rotliegendes A-1
pl
Rotliegendes A-2
pl
Rotliegendes A-3
pfpl
Mean
(n = 9)
Mean
(n = 26)
Mean
(n = 13)
Rotliegendes B
pl
Mean
(n= 11)
Si
A1
6"60
1'40
0-09
0-09
6' 55
1"45
0"09
0.09
6-63
1"37
0.22
0"22
6-71
1"29
0"13
0-13
A!
Ti
Fe(III)
Fe(II)
Mn
Mg
3.46
0"00
0.18
0.04
0.00
0.25
0.08
0-00
0.03
0.01
0.00
0.03
3"40
0.00
0.20
0.05
0.00
0.30
0"15
0"01
0.09
0.02
0.00
0.07
3"28
0-02
0.22
0.06
0.00
0.32
0"17
0-05
0-05
0.01
0.00
0.10
3"36
0"00
0.19
0.05
0.00
0.32
0-09
0.00
0-04
0.01
0.00
0.06
Ca
Na
K
0.01
0.00
1.89
0.01
0.00
0-14
0.01
0.03
1.91
0.01
0.07
0.16
0.04
0.01
1"91
0.06
0.02
0.15
0-05
0-01
1.79
0-09
0.02
0.24
Octahedral
Interlayer
OH
3"93
1.90
4.00
0.05
0.14
0.00
3.95
1.95
4-00
0.04
0.12
0-00
3.90
1.96
4-00
0.06
0.17
0.00
3.92
1.85
4.00
0.08
0.23
0.00
Codes for Bothamsall samples:
A: sub-Alton sandstone
C: Crawshaw sandstone
t: top, m: middle, b: base
pf: pore filling, pi: pore lining.
E. A. Warren and C. D. Curtis
146
TABLE3. Analyses from dispersed particles.
Bothamsall
5-3319
suspension
Si
AI
AI
Ti
Fe(III)
Fe(II)
Mn
Mg
Ca
Na
K
Octahedral
Interlayer
OH
Bothamsall
5-3276
suspension
Mean
(n = 16)
a
Mean
(n = 25)
6-82
1.18
3.69
0.00
0.11
0.03
0-00
0.26
0.04
0.00
1.14
4.09
1.18
4-00
0.10
0.10
0.08
0-00
0.04
0.00
0-00
0.03
0.04
0.00
0.16
0.06
0-16
0.00
6.72
1.28
3.60
0-00
0.09
0.02
0.00
0.33
0.05
0.12
1.28
4.04
1.45
4.00
0.16
0.16
0.21
0.01
0.05
0.01
0.00
0-15
0-11
0.24
0.24
0.16
0.40
0-00
amount ( < 10%) of a regularly interstratified ISII-type mixed-layer illite/smectite (Reynolds,
1980). Accordingly, the intensity ratio (Ir) was calculated to determine the extent of mixing
using the equation of Srodon & Eberl (1984):
1(001)/1(003) [air dried]
Ir = I(001)/I(003) [glycolated]
(1)
which for the Bothamsall illite is 1-07. No significant amount of mixing is indicated.
The extreme thinness of the illite particles of this <0-5/tm size fraction revealed by TEM
(mean thickness six unit-layers) suggests that the mixed-layering indicated by XRD is a
phenomenon of the particle interaction and not a property of individual particles themselves.
Chemical analyses of the same particles (Fig. 9) show random variation between the major
elements, and these are clearly illites (Table 3). Importantly, no systematic variations in the
data are observed between the illite and smectite poles. These data do not therefore appear to
be mixtures.
It appears that the expandability noted in the (001) peak is not the result of mixtures but an
intrinsic property of the sample. Perhaps it results from adsorbed water between the surfaces
of the tiny illite particles, a form of interparticle effect (cf. Nadeau et al., 1984).
Classification of illite from A T E M data
The illite samples (Table 2) all show octahedral totals of 4.0 +0-1 per 22 oxygen
equivalents and are, therefore, dioctahedral. Aluminium is the most abundant cation in the
octahedral sheet, lying between 3.6 and 3.8 for all samples. Iron and Mg are minor; virtually
no Ti and no Mn were detected. These chemical data are all within the AIPEA criteria (1986)
(also Bailey, 1980) required to classify the analysed clay minerals as illite species.
Chemistry of authigenic illite
147
BOTHAMSALL ILLITE ( 39 Analyses)
$i
AI
K
FIG.9. Single-crystalanalysesof dispersed particles plotted on a major-elementtriangle (Si-AIK) on which the positions of kaolinite (K), muscovite (M), K-feldspar (F), quartz (Q) and
pyrophyllite(P) are shown. No trend towards smectitic (pyrophyUite)compositionsis apparent.
These particles do not appear to be mineral mixtures.
Tetrahedral A1 totals range between 1.0 and 1.4, much less than those of muscovite. The
total interlayer charge varies from 1.3 in the Bothamsall samples to 1.9 in the Rotliegendes
samples and counterbalances charge deficiencies resulting from substituion of divalent Fe
and Mg, and vacancies for AI in the octahedral sheet, and of AI for Si in the tetrahedral sheet.
The latter is clearly the most significant contributor to the charge deficiency in both reservoir
illites.
Principles of variation assessment for A TEM data
Variation within a suite of chemical analyses obtained with ATEM may be due to one
or more of five factors: (i) true compositional variation (solid-solution) of the mineral
considered, (ii) variation due to admixtures of other discrete mineral phases, (iii) mineral
instability or element volatility in the electron beam, (iv) X-ray absorption due to specimen
thickness, and (v) general analytical errors associated with the machine and operator.
Clearly, before any conclusions can be made about true chemical variation within a mineral,
the possibility of variations introduced through any combination of the other four factors
must be eliminated. This is not necessarily straightforward as the analytical error is not
readily quantifiable (q.v. Hughes et al., 1989). For the most part these errors result in
systematic variations in one or more elements and hence are expressed as trends in element
diagrams, with the exception of general machine and operator errors which are assumed to be
random and constant.
The accuracy limit in any analysis will be that for the experimentally determined
proportionality constants, the 'K values', used in the calculation routine for any element x:
Cx = gxs i &
Csi
Isi
148
E. A. Warren and C. D. Curtis
which, for this study, is + 5% (Hughes et al., 1989) for all elements except K ( + 10%). The
actual analytical error will therefore be somewhat greater. The effect of these errors on clay
compositions has been calculated and will be described elsewhere (Warren & Ransom, in
preparation).
Comparison of A T E M and X R F data
The mean of all 203 single-crystal analyses from the Bothamsall samples is compared with
the XRF analysis of a bulk sample converted to a mica formula (Table 4). Significant
differences are apparent in the amounts of Si and Al. This represents kaolinite contamination, calculated to be some 30~o by weight, although such quantities were not obvious from
the XRD trace (Fig. 8). Evidently contamination is far more easily avoided with the ATEM.
TABLE4. Comparison of ATEM and XRF data for
Bothamsali.
Bothamsall
all analyses
Si
A1
AI
Ti
Fe(III)
Fe(II)
Mn
Mg
Ca
Na
K
Oetahedral
Interlayer
OH
Mean
(n = 203)
a
5-3349 Cb
XRF
6"78
1"22
3"61
0-00
0.14
0.03
0.00
0.25
0.07
0.03
1.23
4.03
1.33
4.00
0"20
0"20
0.19
0.01
0-08
0.02
0.00
0.12
0.09
0.09
0.30
0.14
0.33
0.00
6"54
1"46
3'79
0.03
0.11
0.03
0.00
0.15
0.00
0.00
1-31
4.09
1.31
4.00
Variation within a sample suite
Variation within a thin-section sample. Mean analyses from two adjacent pores within a
single thin-section of a Rotliegendes sandstone sample are compared in Table 5. Despite the
small number of analyses for each suite, little variation is apparent between the two suites and
the standard deviations are very small ( < 4% for A1 and Si). These variations are well within
those of the K-values used, and hence represent analytical error. Therefore no true variation
in chemical composition is apparent within adjacent pores in this sample.
Analyses from different areas of a thin-section are presented from one sample, Bothamsall
5-3276, in Table 6. As with the Rotliegendes analyses discussed above, nearly all variations
apparent between the two means are within one standard deviation, and are small. One
notable exception is the Ca content which, for 5-3276 LI0, is large and has very large
Chemistry of authigenic illite
149
TABLI~ 5. Variation within a single 3 mm grid.
Rotliegendes B
pore A
Rotliegendes B
pore B
Mean
(n = 4)
a
Mean
(n = 3)
Si
AI
6.75
1.25
0.10
0.10
6.80
1.20
0.11
0.11
A1
Ti
Fe(III)
Fe(II)
Mn
Mg
3.43
0.00
0.15
0.04
0.00
0.33
0.05
0.00
0.03
0.01
0.00
0.06
3.29
0.00
0-21
0.05
0.00
0.36
0.13
0.00
0.05
0.01
0.00
0.07
Ca
Na
K
0.01
0.01
1.75
0.02
0.02
0.12
0.13
0.01
1.60
0.17
0.02
0.16
Octahedral
Interlayer
OH
3.95
1.77
4-00
0-02
0.12
0.00
3.92
1.74
4.00
0.13
0.16
0.00
TABLE 6. Variation within a thin-section sample
Bothamsall
5-3276
grid LI0
Bothamsall
5-3276
grid N10
Mean
(n = 18)
tr
Mean
(n = 18)
Si
A1
6.89
1.11
0.21
0.21
6.83
1.17
0.20
0.20
A1
Ti
Fe(III)
Fe(II)
Mn
Mg
3.51
0"00
0.16
0-04
0.00
0.17
0.25
0.00
0.10
0.03
0.00
0.08
3.72
0.00
0.11
0.03
0-00
0.15
0.01
0.01
0.10
0.03
0.00
0.07
Ca
Na
K
0.25
0.00
1.18
0.23
0.00
0.15
0.04
0.00
1.26
0.03
0.00
0.27
Octahedral
Interlayer
OH
3.88
1.43
4.00
0.19
0.31
0.00
4.00
1.29
4.00
0.13
0.27
0-00
s t a n d a r d d e v i a t i o n . T h i s is n o t c o n s i d e r e d t o r e p r e s e n t a t r u e v a r i a t i o n b u t c o n t a m i n a t i o n b y
adjacent ankerite cement.
O v e r a l l , n o c h e m i c a l v a r i a t i o n w i t h i n a suite o f a n a l y s e s , o t h e r t h a n t h a t r e p r e s e n t e d b y
a n a l y t i c a l e r r o r o r c o n t a m i n a t i o n , is a p p a r e n t w i t h i n a t h i n - s e c t i o n i n e i t h e r t h e R o t l i e g e n d e s
o r t h e B o t h a m s a l l s a n d s t o n e s a m p l e s . E a c h p o p u l a t i o n a p p e a r s to b e h o m o g e n e o u s ,
150
E. A. Warren and C. D. Curtis
BOTHAMSALL ILLITE (203 Analyses)
Fe
Si
AI
K+Na Mg
K+Na
FIG. 10. Analysesofillite fromthe Bothamsalloilfieldcluster on the major-elementdiagram and
show no obvious trend. This variation is most consistent with that produced by the effect of
random analytical error on a single composition. Rather more scatter is observed in the minorelement diagram and probably reflectsthe lower analytical accuracy for K and Fe (see Table 1),
combined with the very low contents of Fe and Mg.
Variation between different samples of the same formation. The data sets collected from
various locations both laterally and vertically within both the Crawshaw and the sub-Alton
sandstone members (Table 2) do not show any variation beyond that expected to result from
analytical error or obvious contamination. Comparatively high standard deviations are
apparent for all elements of some suites, 6-3357 for example, but these are considered to
reflect the statistically small sample size.
The data, when plotted on a major element diagram (Fig. 10). form a single cluster of
apparently random distribution. No identifiable chemical differences between the
petrographically distinct generations of illite are indicated as the data do not fall within two
sets; this is also suggested from inspection of the sample means of Table 2. The illite in these
Carboniferous sandstones therefore appears to be essentially homogeneous.
The variation between the data sets of the Rotliegendes sandstones reflects that of the
Bothamsall samples and is even smaller. These data again indicate that the illite is chemically
homogeneous.
It is, perhaps, remarkable that the illite composition appears homogeneous within the
Bothamsall reservoir, which not only contains substantial detrital inhomogeneity (Warren,
1987a,b) but two petrographically distinct generations. This homogeneity strongly suggests
that the illite precipitated in equilibrium with the concomitant pore-fluid. Otherwise, local
perturbations to pore-fluid chemistry, resulting perhaps from the presence of unstable grains,
and fluctuations in temperature would be expected to cause variations in illite composition.
Variation between different sample suites
The 203 analyses obtained from the Bothamsall samples are compared with 58 from the
Rotliegendes sandstones in Table 7. The relatively large size of these two data suites suggests
Chemistry of authigenic illite
151
TABLE7. Comparison of Bothamsall and Rotliegendes means.
Bothamsall
all analyses
Rotliegendes
all analyses
Mean
(n = 203)
a
Mean
(n = 58)
a
Si
A1
6.78
1.22
0.20
0.20
6.61
1.39
0.15
0.10
A1
Ti
Fe(III)
Fe(II)
Mn
Mg
3.61
0.00
0.14
0.03
0.00
0-25
0.19
0-01
0.08
0.02
0-00
0-12
3-38
0-01
0.20
0.05
0.00
0-31
0.15
0.02
0.07
0-02
0.00
0.07
Ca
Na
K
0.07
0.03
1.23
0.09
0.09
0.30
0.02
0.02
1.88
0.05
0.05
0.16
Octahedral
Interlayer
OH
A1 (total)
4.03
1.33
4.00
4-83
0.14
0.33
0.00
3.93
1.92
4.00
4.77
0.05
0.15
0.00
that any differences between them are likely to be real rather than the result of statistical
artefacts produced by small sample populations.
The magnitude of the sample standard deviations of the Rotliegendes suite are all less than
those of the Bothamsall suite, despite the far smaller sample population. On major-element
plots (Fig. 11) the analyses of each sample cluster, those of the Rotliegendes suite rather more
R O T L I E G E N D E S ILLITE (56 Analyses)
Fe
Si
AI
K§
Mg
K+Na
FIG. 11. Analysesof Rotlicgendes illite also cluster on the major-element diagram. Some tailing
with respect to K is observed in the minor-element plot. This may be an experimental artefact
produced by volatile loss during analysis or X-ray absorption due to specimen thickness.
152
E. A. Warren and C. D. Curtis
tightly than the BothamsaU suite. No obvious trend is apparent in either data set; there are no
systematic chemical variations, thereby ruling out mixtures. Rather, the data of each set form
single populations of apparently random variation.
Whilst the relative magnitude of the standard deviations would be expected to decrease
with increasing sample population for a consistent analytical error, the errors observed here
for each population are both small ( < 3%) and within those of the K-values used. The random
variation in the analyses presented here is thus considered to be the product of analytical
error only, and not to represent any real chemical variation in either suite.
However, it is obvious that the two populations are chemically different (Fig. 11 ; Table 7).
The most striking difference between the two suites is in the interlayer content. Potassium is
the only significant cation in both suites. The Rotliegendes illite has a K content of
1.88 +_0.16 which is close to that of muscovite, 2.00. The Bothamsall illite contains much less
K, 1.23 _+0.3.
This large difference is due primarily to increased Al-for-Si substitution in the tetrahedral
sheet and deficiency in the octahedral total for the Rotliegendes suite. Differences in
octahedral Fe and Mg is not an important contribution to the overall charge deficiency.
The data presented here show that authigenic illite has an homogeneous chemical
composition in both localities, but from the differences between the two suites it is clear that
this mineral does have a truly variable chemical composition. Unfortunately the ancient ages
of these illites prevents any further conclusions as to the cause of this chemical variation. It
could be the result of differences in pore-fluid chemistry at precipitation, or thermal history.
Considering the huge chemical variations in pore-fluids indicated by the diagenetic histories
of both reservoirs, it is unlikely that these subtle variations in illite composition result from
pore-fluid chemistry alone. Rather, the difference in thermal history appears the more
probable cause.
Comparison with literature data
The application of ATEM to diagenetic studies of authigenic clay minerals is a
comparatively recent development. Consequently the available data for illite are still more
limited. Duplay (1984) analysed dispersed particles of illite from a variety of type-clay and
-mudrock localities. Other authors have chiefly analysed authigenic illite in situ within ultrathin sections; among these are: Huggett (1984, 1986) of various Westphalian samples of
Great Britain, Ireland et aL (1983) from Lower Cretaceous mudrocks of the Alberta Basin,
Canada, and Lee & Peacor (1986) from Salton Sea mudrocks and sandstones. Other
important data sets are contained in several unpublished PhD theses including: Duplay
(1982), Huggett (1982), Ireland (1982) and Hughes (1987). Despite their paucity, these data
do cover a remarkably wide range of ages and thermal histories, thus making a comparison
with data obtained by standard techniques meaningful.
These data sets all plot within a small area delimited by muscovite, K-feldspar, kaolinite
and pyrophyllite on a Si-AI-K diagram (Fig. 12a), and many fields overlap. The analyses of
Hughes (1987) straddle the small region of overlap between the Bothamsall and Rotliegendes
fields (this study). The analyses from mudrocks (Ireland et al., 1983; Deer et al., 1962;
Newman, 1987) all plot further towards the Si pole, reflecting lower A1 content due to greater
phengitic substitution.
These differences are emphasized on the Fe-Mg-(K + Na) diagram (Fig. 12b). The
mudrock analyses generally have higher Mg and Fe ratios relative to alkali than the
Chemistry of authigenic illite
(a)
Si
153
(b)
~ 9
".:
_0 ~ :.:'-I
'
AI
1
\
K+Na
Bothamsall
Rotliegendes
.....
.......
Hughes (1987)
Ireland (1982)
Huggett (1982)
~- \ % \
-
,
\
Mg ~
.......
---
K+Na
Merino & Ransom (1982)
Bulk analyses
FT~. 12. Bothamsall and Rotliegendes data of this study compared with published data on
diagrams of major elements (a) and minor elements (b). All major-elementdata are contained in
a field within a triangle demarcated by kaolinite-K-feldspar-muscovite and largely overlap.
Someseparation of analysesfrom mudrocks (Ireland, 1982; various bulk analyses)from those of
sandstones is apparent in the minor-element diagram.
authigenic illites in sandstones of this study and of Hughes (1987). Exceptions are the
analyses of Huggett (1982) and Merino & Ransom (1982), which spread over the two fields.
However, because this diagram principally plots minor, rather than major, elements, larger
scatter is to be expected.
Comparison with data of Velde (1984)
The same two data sets have been plotted on the diagram constructed by Velde (1977) in
which illite compositions have been identified as lying within a distinct field (Velde, 1984).
The populations are considerably scattered (Fig. 13), and many of the analyses plot well
outside the limits of the illite field. The sample suites have a linear trend, parallel to the MR32R3 side of the triangle. This could indicate that the analyses are of mixtures, those plotting
towards the MR3 pole containing feldspars, and those towards the 2R3 pole containing
expandable clays such as beidellite. However as discussed above, such mixing trends were
not observed on the more conventional major- and minor-element diagrams presented above.
Instead, the analyses formed clusters which, as indicated above, merely represent the spread
produced by random analytical error. The trends on these diagrams are therefore thought to
represent the same statistical variation within the individual populations which is
exaggerated by the choice of apex parameters. That the result is linear is due to one axis (3R2)
being composed of Mg and Fe - - two minor elements in the illites.
Interestingly, plots of the analytical means for these two populations both lie within the
illite field (Fig. 14). The suites of authigenic illite in sandstones generally plot very close to the
MR3-2R3 join, similar to the results of Velde (1984). The Rotliegendes mean plots on the
E. A. Warren and C. D. Curtis
154
BOTHAMSALL ILLITE (203 Analyses)
ROTLIEGENDES ILLITE (56 Analyses)
MR 3
MR 3
2R 3
3R 2 2R 3
3R 2
FIG. 13. The same data sets of Fig, 13 are plotted according to the coordinates of Velde (1977),
i.e. MR3 = K + Na-~-2(Ca), 2R3 ~-~[ -{A1"1-Fel~21)- MR3.]/2, 3R2 = (Mg--~-Fell -~ MR)/3. The
muscovite (m) - - celadonite (c) tieline and the proposed illite compositional field are also
illustrated. Rather than cluster, as in Fig. 13, the analyses of each data set form a linear spread
through the illite field.
o
9
Bothamsall
Rotliegendes
w
f
i
h
k
Weaver & Pollard 1973
Fithlan
Ireland 1982
Hughes 1987
Pye & Krinsley1986
illite field
/
diagenetic
illite
FIG. 14. Plots of the statistical means of the ATEM data sets, together with those of the current
literature on the same diagram as Fig. 13. All means lie within the illite field delineated by Velde
(1984). The authigenic illite from sandstone samples all plot parallel to the MR3-2R3 axis in a
similar position to those of Velde (1984). Those of mudrocks plot further towards the 3R2 pole
reflecting higher ferro-magnesian content.
m u s c o v i t e - c e l a d o n i t e join, in the p h e n g i t e field, ostensibly at the top of a t r e n d t h r o u g h the
authigenic illite samples parallel to the M R 3 - 2 R 3 join. T h i s plots in the position of highert e m p e r a t u r e diagenetic, or m e t a m o r p h i c illites as f o u n d by Velde (1984) a n d hence m a y
reflect crystallization from higher t e m p e r a t u r e t h a n the Bothamsall illite.
Chemistry o f authigenic illite
155
CONCLUSIONS
1. Results from the two large data sets failed to reveal any intra-formational chemical
variation of authigenic illite beyond the experimental error, even where petrographic
evidence demonstrates multiple precipitation events. These indicate that the pore-fluid
precipitating the iUite was not influenced by local chemical perturbations, such as different
detrital grains, but was essentially homogeneous within the reservoirs. Clearly the internal
reservoir systems were both open during illite precipitation.
2. Both data sets form distinct populations. These variations de n e t represent systematic
compositional trends but random analytical errors. These are often as small as the accuracy in
the proportionality constants used, 5-10~o. However, chemical differences between the two
populations show that authigenic illite does have a variable chemical composition.
3. These populations compare favourably with published A T E M data which indicate that
authigenic illite in sandstones has a relatively restricted compositional range.
ACKNOWLEDGEMENTS
A British Petroleum Research Studentship received by one of us (EAW) is gratefully acknowledged and we
thank Dr Jenny Huggett of BP for her assistance and advice in coordinating this project. Core material for
Bothamsall was made available by the British Geological Surveyand for Rotliegendes from British Petroleum.
We are indebted to Dr John Whiteman of the Dept. of Metallurgy for permission for, and assistance in, using
the ATEM facility at Sheffield. Lastly, this manuscript has benefited greatly from copious and perceptive
comments of an earlier draft by two anonymous reviewers.
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