Clay Minerals (1999)34, 137-149
Textural and geochemical micro-analysis
in the interpretation of clay mineral
characteristics" lessons from sandstone
hydrocarbon reservoirs
A. H U R S T
University of Aberdeen, Dept. of Geology & Petroleum Geology, King's College, Aberdeen AB24 3UE, UK
(Received 5 June 1997," revised 2 December, 1997)
A B S TRACT: Back-scattered electron images of clay minerals from sandstones are used, together
with complementary micro-analytical methods, to identify and quantify mineral microporosity and
geochemistry. Clay minerals typically have a range of microporosity from 10 to >90% dependent on
texture and paragenesis. Fibrous clays are highly microporous; detrital clays have low microporosity
but specific clay minerals have broad ranges of microporosity. The often quoted mineral-chemical
association between thorium (Th) and kaolinite cannot be substantiated by micro-analysis. The Th
content of clay minerals is associated with micro-inclusions within the kaolinite which form
diagenetically or are derived from precursor minerals.
Clay minerals predominate in fine-grained sedimentary rocks and the fine-grained fractions of coarser
grained rocks. Although relatively straightforward
to identify by X-ray diffraction (XRD), clay
minerals have micro-scale characteristics which
are less easily resolved by conventional mineralogical and petrographic analysis. Experience from
the study of clay minerals in sandstone hydrocarbon
reservoirs demonstrates the significance of resolving micro-scale textural and mineralogical features
in clay minerals as part of reservoir characterization. The observations made on these sandstones are
equally relevant to clay minerals in other geological
environments. Identification and quantification of
clay minerals and their proportions alone are rarely
adequate information if the influence of clay on
pore size distribution and geochemical, particularly
trace element, characteristics is to be interpreted
appropriately.
CLAY MINERALS
HYDROCARBON
IN S A N D S T O N E
RESERVOIRS
Although clay minerals rarely account for a major
proportion by wt% of the mineralogy of a sandstone
reservoir, they often play a key role in determining
the fluid storage capacity and permeability. Good
reservoirs generally contain little clay, perhaps
- 0 - 1 0 wt%, and poor reservoirs tend to contain
more, perhaps />15%. Equally, there are welldocumented trends that show finer grained sandstones tend to contain higher proportions of claysized material and have generally lower permeability than coarser sandstones. However, the interrelationships between grain size, permeability and
porosity are not simple, and clay minerals have an
important complicating influence that is attributable
to their textural characteristics (Pallatt et al. 1984;
de Waal et al. 1988; Hurst & Nadeau 1995).
9 1999 The Mineralogical Society
138
A. Hurst
Fic. 1. Fields of view typical for image analysis of petrographic thin-sectons: A - - typical petrographic analysis;
B and C - - micropore analysis of pore-filling clay minerals (e.g. B: kaolinite; C: a detrital clay clast); D:
micropore analysis of grain-coating/pore-lining clay minerals (e.g. chlorite).
Estimation of saturation (i.e. the relative proportions of hydrocarbons and aqueous fluids present) in
hydrocarbon reservoirs is the ultimate goal of
reservoir appraisal. As saturation cannot be measured
directly in the subsurface, an area of technology has
developed, petrophysics, that acquires subsurface
data (including core) and, by using various data
transforms and empirical correlations, makes estimates of fluid saturation. Measurement of formation
resistivity and conductivity are amenable to saturation determination. If the resistivities (conductivity)
of the rock and fluids are known, and the formation
resistivity measured, the proportions of fluids present
can be estimated for a known porosity. Clay minerals
complicate the interpretation of formation resistivity
measurements and may introduce excess charge into
the formation over and above that associated with
any aqueous phase present (Waxman & Stairs, 1968).
Clearly, acquisition of mineralogical data is requisite
to providing a clear understanding of the relationships between electrial properties of reservoirs and
their saturation.
Two aspects of micro-mineralogy are presented
here, both of which may have a profound effect on
reservoir characteristics and their evaluation;
estimation of clay microporosity and clay mineral
chemistry. Both microporosity estimation and clay
mineral chemistry are recognized as useful characteristics in the estimation of saturation. In order
to understand both, and thus make appropriate
interpretations, detailed microscopic investigation is
necessary.
RATIONALE
AND
METHOD
As the influence of clay minerals on reservoir
characteristics is well known, different forms of
clay mineralogical investigations, e.g. thin-section
petrography, SEM or XRD, have become quite
common components of reservoir description.
Because of this, some data are used to demonstrate
simple relationships between clay mineralogy and
characteristics perceived to be useful in reservoir
characterization. Two examples of this are: (1) the
Interpretation of clay mineral characteristics
139
relationships between clay mineralogy and elec- enigmatic geochemistry, a range of analytical
trical characteristics, Qv (charge per unit area) as techniques are used on undisturbed and disaggredefined by Waxman & Smits (1968); (2) the gated samples (Table 1). Prior to disaggregation, a
positive statistical between kaolinite abundance small portion of each sample was used for thinand Th content (Hassan & Hossin, 1975). Both section and scanning electron microscope (SEM)
micro-textural and micro-chemical characteristics analysis. The remaining material was gently
are investigated using back-scattered electron (BSE) disaggregated and cleaned by reflux extraction
with dichloromethane. Any hydrocarbon present
images.
Micropore distribution is estimated by obtaining was extracted quantitatively and analysed along
BSE images of the clay minerals of interest from with the mineral residues. The cleaned sandstone
which pixel grey-level classifications are derived by material was then sub-sampled for quantitative
digital analysis (Dilks & Graham, 1985). Typically, mineralogical analysis including grain-size separapetrographic image analysis is not undertaken at a tion, heavy mineral separation and XRD of the clay
scale that will resolve the microporous character of fraction. A critical aspect of the procedure is to
clay minerals (Fig. 1). Consequently, any influence pinpoint where, and in which minerals, Th and U
that clay mineral texture has on the pore-size occur.
distribution and surface area characteristics of
reservoir rocks is unlikely to be resolved. Nadeau
RESULTS
& Hurst (1991) demonstrated that BSE images from
petrographic thin-sections are amenable to quantiClay mineral microporosity
fication of clay microporosity. Contrast in BSE
A summary of clay microporosity for kaolinite,
images is determined by variations in atomic
number within the specimen. Silicates, that chlorite and illite shows values typical of reservoir
comprise predominantly Si, AI, O, etc., appear clays (Table 2). Note that wide ranges of microlight relative to the epoxy resin (C, H, etc.) which porosity exist for individual clay minerals. Equally
occupies the pores. Quantification of the micro- important is that in any one sandstone, a single
porosity is done by successive deletion of the mineral, e.g. kaolinite, may have a variety of
darker grey zones (epoxy-filled pores) until the textures and microporous characteristics (Hurst &
analyst is satisfied that only mineral surfaces Nadeau, 1995). The process of sample preparation
remain (Nadeau & Hurst, 1991). The percentage causes collapse of some fibrous crystals and hence
of grey tones deleted equals the microporosity of their microporosity is likely to be underestimated
the area of mineral examined. As shown in Fig. 1, by this method. Critical-point drying of samples
the area of examination varies depending on the followed by examination of stereo-pairs of SEM
clay type and texture, but is typically <4 mm 2. images provides more realistic estimates of fibrous
Analytical precision was discussed by Nadeau & clay microporosity (Hurst & Nadeau, 1995).
Hurst (1991), but it is important to note that the Estimation of effective volume (Vo) and volume
operating conditions of an electron microscope and of clay-bound water (Vobw) from mineral volume
the settings of the image acquisition system may (Vm) and microporosity (Om) are given in the
Appendix.
cause variations in grey tone distribution.
To illustrate the importance of obtaining microAccurate relationships between naturally radioactive isotopes (K, Th and U) and their host porosity estimates, the characteristics of two
minerals can only be obtained by careful petro- idealized sandstones, A (Fig. 2) and B (Fig. 3),
graphic and chemical analysis. Potassium is are compared. The Ve for the clay minerals can be
associated with major mineral phases (feldspars, calculated using eqns. (4) and (1) (Appendix).
micas, etc.), but Th and U have more complex Although sandstone A is more quartzose and has
relationships with a wide variety of minor detrital less clay content than sandstone B, the effective
and authigenic phases, including micro-inclusions volume (Ve) of the clays present is greater. The
(Milodowski & Hurst, 1989; Shirodkar et al., 1996). higher Om in sandstone A (Fig. 2) leads to a higher
The BSE images are used to detect and characterize water saturation and lower permeability. Uncritical
detrital and authigenic clay minerals and their evaluation of wireline logs and XRD analysis would
inclusions. Because clay minerals occur in the almost certainly characterize sandstone B as more
fine grain-size fraction and have a somewhat clay rich because of its higher clay mineral wt%.
140
A. Hurst
TABLE 1. Summary of analytical techniques used to characterize
mineralogy and geochemistry of sandstone radiominerals (after Hurst
& Milodowski 1996). Samples are separated into three sub-samples,
one for petrographic study, one for analyses requiring disaggregation,
and one for XRD.
(1) Petrography
preparation of polished thin-sections
fission track registration analysis
optical petrography
back-scattered scanning electron microscopy (BSEM)
laser ablation microprobe-ICP-MS/microprobe analysis
autoradiography
(2) Disaggregation analyses
gentle disaggregation for mineral separation
dichloromethane wash - sample split to a) and b)
(a) whole-rock chemical analysis
(b) size-fraction separation: >63 ~tm, < 6 3 - > 2 lam and <2 gm
heavy mineral separation: >63 gm, < 6 3 - > 2 ~tm and <2 gm
chemical analysis: ICP-MS
(3) XRD analysis
sandstone A
0 = 20%
quartz
f e l d s p a r (f)
mica (m)
=
=
=
90%
5%
2%
kaolinite (k)
i l l i t e (i)
heavy minerals
=
=
=
<2%
<2%
<2%
Vm
Ve
F1o. 2. Idealized sandstone A - with low wt% clay mineral content. K = kaolinite, I - illite and O - porosity. The
clay minerals are authigenic and have substantial microporosity. Vm and Ve are the solid volume of clay mineral
and effective clay mineral volume, respectively.
Interpretation of clay mineral characteristics
TABLE 2. Typical microporosity (Ore) estimates (%) for
clay minerals from sandstone hydrocarbon reservoirs
using BSE analysis. Data marked with an asterix are
from Hurst & Nadeau (1994). x = average, n = number
of samples.
Range
x
n
Kaolinite
diverse*
high microporosity
low microporosity
15-61
26-52
6-22
43
40
16
52
32
16
Chlorite
intergrown platelets*
fibrous
44-58
28-51
51
41
10
36
Illite*
Illite/chlorite (fibrous)
47-76
20 39
63
28
5
100
Smectite (fibrous)
29 44
36
8
Detrital clasts
mixed mineralogy*
illite
chlorite
7-14
1-21
24-47
10
10
32
8
47
8
141
When brine saturated, conductivity measurements
on core are likely to demonstrate that sandstone A
has more clay-rich character, in terms of enhanced
conductivity and higher irreducible water saturation,
because of the higher clay microporous volume.
In the e x a m p l e s , the v a l u e o f o b t a i n i n g
quantitative textural data is critical to understanding
the effects of the clay mineralogy on saturation
characteristics. Simple identification of clays and
quantification of their wt% can easily lead to an
inappropriate interpretation of their influence on
r e s e r v o i r characteristics. W h e n h y d r o c a r b o n s
migrate into a reservoir, their buoyant force
progressively displaces formation water from
progressively smaller capillaries. Typically in
sandstones, the smallest capillaries present are
those associated with clay mineral micropores.
These are the last capillaries from which water
will be displaced by hydrocarbons and, as such,
they occupy pore volume that is less likely to be
available to hydrocarbons.
Porosity and permeability
Although the effects of highly microporous (e.g.
fibrous) clays on permeability are well-known, their
sandstone B
quartz
feldspar(f)
m i c a (m)
chlorite
illite
smectite
heavy minerals
=
=
=
-=
<5%
=
=
2-5%
<2%
c l a s t s : cl = c l a y ,
0 = 22%
60-75%
10-15%
5-10%
<5%
sh = s h a l e
Vm
FT~. 3. Idealized sandstone B with high wt% clay mineral content. The clay minerals are detrital and have minor
microporosity. K - kaolinite, I - illite, C = chlorite and O - porosity. Vn~ and Ve are the solid volume of clay
mineral and effective clay mineral volume, respectively.
142
A. Hurst
quantification has previously been characterized
only in terms of effect on petrophysical measurements (Pallatt et al., 1984; de Waal et al., 1988).
Examination of clay microtexture elucidates the
relationship between pore-size distribution and
permeability. Porous samples which contain clays
with large values of //re but small wt% values, e.g.
fibrous illite, retain high porosity despite having
low permeability. Diagenetic development of the
clay microporous texture destroys little of the total
porosity (as measured by logs and core analysis)
but traps a substantial volume of clay-bound water
that reduces effective porosity and degrades
permeability. The scale of permeability reduction
varies depending on clay microporosity and clay
distribution but typically may produce two orders of
magnitude reduction in permeability for a given
value of porosity (Fig. 4).
Core analysis data
To validate the assumption that the clay
microporosity data do indeed reflect an actual Vo
that excludes larger pores, it is necessary to
compare the data with core analysis data. A set of
core plugs from a fine- to medium-grained, poorlysorted, fluvial sandstone in which kaolinite is the
dominant clay mineral present (Vm averages 87%
10000-
t
100K
(mD)
//
........
.........
?
10-
'
Q
. . . .
0,1
//
II
I
'
I0
20
310
e (%)
Fro. 4. Porosity (O) - permeability (K) cross plot using
a variety of microporous (Om) values that could be
representative of authigenic fibrous illite. O m l -- 8 5 % ,
Ore2 90% and Ore3 95%. The total porosity (Ot)
25%.
kaolinite) shows that there is: (1) no clear
relationship between permeability and porosity
(Fig. 5a); and (2) better correlation between clay
volume % (Ve) and grain size (Fig. 5b,c). From
these relationships one can infer that the clay
microporosity exerts a strong control over micropore distribution, an inference that is confirmed by
the results of mercury injection porosimetry
(Fig. 5d).
Geochemistry
The combination of estimation of elemental
concentration from wireline log data (including
the naturally radioactive elements K, Th and U with
rapid, accurate bulk analysis of trace and minor
elements on rock samples using XRF or ICP-MS
provides a potentially useful method in reservoir
characterization. As clay minerals are well known
for affecting a variety of reservoir characteristics,
interpretation of geochemical data has focused on
the identification and quantification of clay
minerals. In particular, Th concentration and the
Th/K ratio have been associated with the presence
and abundance of kaolinite (Hassan & Hossin,
1975; Serra, 1984).
Kaolinite and thorium
Following an evaluation of the standard mineralchemical data used for radiomineral interpretation,
it was concluded that the existing limited database
had shortcomings both because of the inadequacy of
data and because the chemical data were not
derived from reservoir minerals (Hurst, 1990).
Although it was generally recognized that Th ions
were unlikely to be accommodated in octahedral
sites within the kaolinite crystal structure, it was
assumed that they could be adsorbed onto the
surfaces and edges of kaolinite crystals (Iterron &
Matteson, 1993). Subsequent analytical and experimental studies on a variety of reservoir sandstones
demonstrated that Th had no affinity for kaolinite
(Hurst & Milodowski, 1995; Shirodkar et al., 1996).
Quite why concentrations of Th and K, neither of
which are likely to substitute for A1 in kaolinite,
persisted as inferred indicators of kaolinite presence
is unclear. Quite possibly the explanation is simply
because chemical analyses of kaolinite had failed to
recognize the presence of mineral inclusions and
their significance, or it was assumed that Th and K
were associated with mineral surfaces. In all of the
Interpretation of clay mineral characteristics
10000
143
10000
a
b
1000
t
$0
ee 9
1000
9
100
B
10
10
0
9I
20
15
I
25
I
30
0
35
I
5
0
I
10
t3 He (%)
I
15
9
I
20
I
25
30
Clay Ve (%)
10000
40
c
d
1000
9
9
30
9
8 9
v
g
100
9
~*~
20
qD 9
10
9
10
0
0
]
0.1
r
0.2
I
0.3
I
0.4
I
0.5
I
0.6
0
0.7
grain size
0
I
0.05
I
0.1
I
0.15
0.2
BV Om (Hg)
Fla. 5. Relationships between core analysis and petrographic data from 1" diameter core plugs from 20 (18
samples for BV analysis) fine to medium grained, poorly sorted, fluvial sandstones (North Sea). (a) air
permeab'ility and porosity; (b) air permeability and clay volume (V~); (c) air permeability and grain size; (d) clay
volume (Ve) and where BV O,1 (Hg) where BV Om (Hg) is the fraction of pore throats with pore-throat radii
<1 um • Or.
sandstones studied, from a variety of North Sea
reservoirs, kaolinite is often observed to contain
mineral micro-inclusions. Often these are diagenetically-formed phosphates (Milodowski & Hurst,
1989) but they have a variety of compositions and
radiomineral concentrations. In common with mica
in other rocks undergoing diagenetic alteration and
leaching, kaolinite is often associated with exfoliation and alteration (Fig. 6~/). Other kaolinite may
have a different paragenesis unassociated with mica
alteration and contain different micro-inclusions
and, sometimes, no micro-inclusions are present.
The chemistry of micro-inclusions from altered
micas from several sandstones (Table 3) is diverse,
144
A. Hurst
F~c,. 6. (a) BSEM image showing the exfoliation of muscovite and alteration to kaolinite with inclusions of
authigenic REE phosphate minerals; (b) BSEM image of a detrital mudstone pellet with finely disseminated
inclusions of a bright U-rich TiO2 phase (possibly brannerite) that is replacing altered Ti-Fe oxide. The dark
holes in the image are laser-ablation pits from LAMP-1CP-MS analysis.
Interpretation of clay mineral characteristics
145
TABLE 3. Quantitative SEM-EDX analyses of five authigenic rare-earth phosphate minerals which occur
within kaolinitized muscovite in reservoir sandstones. All values are normalized % after subtraction of
elemental contributions interpreted to be from the host minerals.
Sample
SiO2
A1203
K20
FeO
CaO
SrO
BaO
Y203
La203
Ce203
Pr203
Nd203
Sm203
Eu203
Gd203
Dy203
ThO2
P205
Total
1
10.11
5.90
1.61
n.d.
2.57
n.d.
n.d.
1.56
11.21
25.72
4.25
15.43
2.63
1.57
2.31
tr
2.58
30.17
100
2
6.60
3.86
1.28
0.91
3.29
0.85
n.d.
4.09
11.11
24.84
4.10
14.04
2.42
n.d.
2.64
tr
1.49
31.12
100
3
4
7.63
5.09
1.26
0.58
3.03
n.d.
n.d.
3.29
11.81
26.52
3.63
14.48
2.10
n.d.
2.31
1.75
1.23
29.84
99.99
in particular with relation to Th and other trace
element components. Recalculation of the analyses
to structural formulae give good approximations to
monazitic ({REE,Th,Y,Ca}P04) or rhabdophanic
({REE,Th,Ca}PO4nH20) compositions. Because of
interference from the mica hosts and alteration
products, and the low totals from the analyses, is is
impossible to differentiate between monazitic and
rhabdophanic phases. Similar to coexisiting detrital
monazites, the authigenic phosphates are enriched
in light REEs but contain more Y and Ca and less
Ce and La. Although the authigenic phosphates may
be Th bearing (Table 3) they are substantially less
thoric (<4% Th) than detrital monazites (>4% Th,
Hurst & Milodowski, 1996).
Because there is no uniformity of micro-inclusion
chemistry from kaolinite in one sandstone to that in
another (for example, some are Th enriched but
others are not), it is not possible to apply global
relationships between kaolinite, its micro-inclusions
and their combined chemical signature. Comparison
with work on trace element (including REEs)
mobility during sandstone diagenesis (Bouch et
al., 1995) makes one wary of using trace element
geochemistry to identify and estimate clay mineralogy. These findings raise doubt over the
4.51
2.19
1.06
0.73
2.50
n.d.
n.d.
6.21
13.55
25.06
2.96
14.39
1.94
n.d,
n.d.
1.28
n.d.
32.11
100
5
8.74
6.9 l
1.44
0.54
3.41
1.48
n.d.
1.96
13.31
25.87
2.36
13.69
1.75
n,d.
0.96
1.40
2.48
31.34
100
6
9.64
8.69
n.d.
n.d.
3.33
1.45
n.d.
2.72
11.93
25.59
2.43
14.87
n.d.
n.d.
n.d.
n.d.
3.56
34.12
100
application of chemical data, without supporting
micro-analytical data, when characterizing and
interpreting mineral paragenesis.
Other parageneses
Similar inclusions to those in Table 3 are
recorded in illitized faecal pellets (Fig. 6b). The
pellets are assumed to have previously had a mixed
clay mineral assemblage that has transformed to
illite during burial (Hurst et al., 1996; Shirodkar et
al., 1996). This demonstrates that the inclusions
survive diagenetic changes in the clay minerals that
enclose them, and that they can occur as inclusions
in clays other than kaolinite.
Broader implications of the observations made on
the micro-inclusions are that in any clay-forming
environment, one must be wary of using chemical data
to 'type' individual clays. Without careful highresolution microscopy and complementary geochemical analysis, the significance, and even the presence,
of the inclusions can be missed. It seems that the trace
geochemistry of clay minerals, at least in sandstones,
has a strong association with the composition of
precursor minerals and the extent to which those
precursors have undergone diagenetic change.
146
A. Hurst
DISCUSSION
with low CECs, e.g. chlorite, may have a
substantial i n t e r l i n k e d microporous v o l u m e
(Table 2) that, if filled with brine, will produce
higher conductivity despite the low CEC of the clay
(Hurst, 1987).
Textural and electrical characteristics are
summarized in Fig. 7. Highly microporous textures
may give high Si~ values in clays that do not have
high CECs, e.g. fibrous illite. Therefore, conductivity associated with fibrous illitic clays is more
dependent on the composition of the aqueous phase
present in the micropores than the CEC of the clay
mineral. In contrast, smectitic clay minerals
associated with clay or shale clasts may contribute
to enhancing formation conductivity but, because of
their low microporosity, are unlikely to trap
substantial volumes of formation water.
Recognition of the deleterious effect of fibrous
clays on reservoir characteristics, particularly
permeability, is well known (Pallatt et aI., 1984;
de Waal et al., 1988). Less recognized is the
mechanism by which fibrous clays reduce permeability. It is not a tangled 'seaweed-like' complex
of mineral particles that impede fluid movement by
forming baffles within the intra-pore volume, but
rather the blocking off of inter-pore volume by
Estimation of microporosity and the role of
microporosity in determining irreducible water
saturation (Sift) in sandstone reservoirs, are
important in determining hydrocarbon storage
volume and the influence of clay minerals on
permeability. In this paper, and in earlier work
(Hurst & Nadeau, 1994, 1995), it is demonstrated
that identification of clay minerals and quantification of minerals by wt% may provide an inaccurate
picture of the effective volume occupied by these
minerals. In turn, this can lead to specific minerals,
that may be abundant in wt%, being assigned
inappropriate relationships with saturation and
conductivity. For example, minerals such as
smectite and illite are associated with high CEC
(cation exchange capacity) and a consequent
positive influence on formation conductivity
(Waxman & Smits, 1968). However, if these high
CEC clays are not associated with a high
microporous volume, e.g. if they occur in shale
clasts or clay pellets (Figs. 2, 6b), they have limited
storage capacity for fluids (approximates to Sirr) and
any conductivity associated with them will not
necessarily be proportional to Sir r, in contrast, clays
CEC (mEq g-1 )
1.0
0.1
0.01
1.o
i
.6
/
i
i
i
i
t
r
I
I
I
I
I
I
I
I
I
/
/
~m
/
.4
/
/
.2
KAOLINITE
.... ::::::::::: ......
.... ::::::::::::::::::::::::
!i~!iiiii!ili!~
~'
J
0
F~G. 7. Microporous and cation exchange characteristics of clay minerals. The CEC and QJm a r e not measured on
the same samples and so relationships between them are inferred. The Om data are from Table 2. The CEC data
are from Ormsby et al. (1962) and Revil et al. (in review). The textural/genetic forms (e.g. detrital clasts, pellets,
platelets, etc.) overlie mineralogical characteristics, do not have specific mineralogy but do have distinct
microporous characteristics.
Interpretation of clay mineral characteristics
microporous clay saturated with formation water
located in pore throats. A series of two-dimensional
pore network models illustrates a clay-free pore
system (Fig. 8a), a pore system where fibrous clays
block pore throats (Fig. 8b) and, a pore system
where fibrous clays create baffles in the pore
throats (Fig. 8c). There is no implied difference in
clay volume (Vm), but the scale at which pore
connectivity is degraded is distinctly different.
lntra-pore growth of fibrous clays is likely to be a
precursor to inter-pore blocking, i.e. intra-pore
features are an earlier stage of fibrous growth. In
terms of Darcy's law,
O
147
it
I,
kA (dP)
. (dL)
where Q = volumetric flow rate (cm3/s), i1 - fluid
viscosity (cp), A = sample cross sectional area
(cm2), dP/dL = pressure gradient across sample
(atm/cm), k
a constant (called permeability,
Darcies), the area (A) through which flow (Q)
occurs is reduced, not in proportion to the mineral
volume (Vm), or mineral wt%, but in proportion to
the effective mineral volume (Ve). Implicit in this
relationship is the importance of irreducible water
saturation in reducing permeability, and a strong
relationship between pore-size distribution and
permeability (Fig. 5).
Quantification of microporosity may be important
in many applications other than hydrocarbon
reservoir characterization, for example, when
characterizing pore-size distribution and storage
capacity in fine-grained sedimentary rocks which
may be expected to impede permeation by toxic
fluids and waste. From the studies conducted on
hydrocarbon reservoirs it is clear that just knowing
which clay minerals are present, and in what
relative quantities, is insufficient information for
estimating microporous character. A specific clay
mineral may have a range of microporosity
(Table 2) that is largely dependent on paragenesis.
Detrital clays, independent of their mineralogy, tend
to have lower microporosity than authigenic clays.
However, specific authigenic clays have wide
ranges of microporosity (e.g. kaolinite and illite,
Table 2) that are directly related to crystal texture
and grain size. These characteristics are, in turn,
related to mineral genesis.
Recognition of micro-inclusions of authigenic,
variably radioactive, grains within clay minerals of
authigenic and detrital origin probably explain the
b
C
q,
FIG. 8. Two-dimensional pore network models showing
pores (nodes) and pore throats (connecting lines);
(a) unrestricted flow throughout the network; (b) six
pore throats are blocked (broken lines) by formation
water-saturated, microporous clay; (c) six pore throats
contain baffles formed by the fibrous clay that reduce,
but do not destroy, permeability.
wide compositional ranges recorded from chemically simple minerals such as kaolinite (Newman &
Brown, 1987, Table 1.7). During diagenesis, clay
minerals precipitate from aqueous solutions or
neoform from precursor silicate minerals. These
reactions occur largely in lower than neutral pH
148
A. Hurst
pore water and in response to changes in mineralfluid equlibria with increased burial temperature.
Petrographic evidence (Boles & Johnson, 1983, and
Fig. 6a) shows that authigenic minerals often retain
included remnants of precursor minerals and
inclusions of co-genetic authigenic minerals. A
similar series of reactions takes place in low pH
weathering environments where continued dissolution of detrital minerals pushes the mineral
assemblage toward a simple A1 silicate residua
system (Chesworth, 1975). There is a substantial
documentation of clay mineral chemistry (Newman
& Brown, 1987) that unfortunately does not account
for, or include, data from non-clay mineral
inclusions. Purity of clay minerals used in mineral
analysis seems largely to be based on XRD
evidence of mono-mineralogy; XRD is likely to
miss trace concentrations of non-sheet silicates
present as inclusions. As argued by Herron &
Matteson (1993), excess elements unlikely to be
incorporated within clay mineral structures may
reasonably be expected to be adsorbed onto
negatively charged clay surfaces. An alternative
explanation for the presence of the excess elements
is that they are concentrated in mineral inclusions
(Hurst & Milodowski, 1996).
Characterization of both clay microporosity and
clay micro-inclusions demonstrates the importance
of examination of clays at a scale appropriate to the
problem. In the context of hydrocarbon reservoir
characterization, failure to recognize the significance of the micro-scale features may lead to a
flawed interpretation that could jeopardize estimation of reserves by over- or under-estimating water
saturation. In particular, 'bulk' analyses, even when
accurate, are perceived as potentially misleading if
interpreted without the support of more micro-scale
variations present. In the examples of microporosity
estimation and Th concentration in kaolinite, microanalytical techniques prove the significance of
micro-analysis when interpreting relationships
between minerals and their associated properties
(e.g. irreducible water saturation). The existence of
a statistical relationship, e.g. a good correlation
coefficient between two characteristics, does not
imply the existence of a deterministic relationship.
CONCLUSIONS
A simple method for quantification of microporosity in clay minerals enhances understanding of
the role of clay micro-texture on irreducible water
saturation and permeability. Quantification of
mineralogy alone does not account for the range
of textural variations observed. Hence, knowledge
of reservoir mineralogy alone is inadequate when
characterizing properties influenced by microporosity.
Simple relations between clay minerals (particularly kaolinite) and chemistry (particulary Th and
K) cannot be substantiated. Any Th present appears
to be concentrated within diagenetic inclusions that
are often associated with alteration of precursor
grains. This relationship appears to exist for clay
minerals in other geological environments. Use of
chemical data to characterize clay minerals without
screening for the presence of micro-inclusions
should be made with circumspection.
ACKNOWLEDGMENTS
My various collaborators, and in particular Paul
Nadeau, Tony Milodowski, Anu Shirodkar and Steve
Kiminnau, are thanked for their continued support.
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APPENDIX
Effective volume (V~) of clays
The main goal of microporosity estimation using BSE
images is to estimate the effective volume (Ve) of clay
minerals and, from that, to infer their influence on
saturation and permeability. As described by Hurst &
Nadeau (1995), if clay-bound water (V~bw) is assumed to
be same as the irreducible water in close proximity to
clays, the proportion of microporosity (Ore) for each clay
can be used to calculate V~bw if the volume of solid clay
mineral (Vm) present is known. Thus,
vn,
v. -
(1 - ~ m )
(1)
and
Vcbw -- Ve
Vm
(2)
where Ve - effective clay mineral volume, such that
V~bw -
(1 - ;3,.)
vn,
(3)
A relationship between Vobw and ~ m for different clay
minerals can be derived from eqn. (3) as long as Vm is
known. To calculate V,~ from estimates of mineral wt%
corrections for density and porosity are required. Actual
solid volume % (the minus microporosity volume) for
each mineral, K~, is calculated from
V~
( m a / p ~ (1 - ,@t)
Ei(mi/Pi)
where m~ wt% of mineral a, p~ - density of mineral a,
~ - porosity (as evaluated from wireline logs and
calibrated with core data), and Zimi/Pi - the sum of the
wt% of each mineral over their respective density.
Estimates of mineral wt% may typically be derived fi:om
XRD analysis, but not from petrographic analysis of thinsections.