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OCR for page 3440
Proc. Natl. Acad. Sci. USA
Vol. 96, pp. 3440-3446, March 1999
Colloquium Paper
This paper was presented at the National Academy of Sciences colloquium "Geology, Mineralogy, and Human Welfare, "
held November 8-9, 1998 at the Arnold and Mabel Beckman Center in Irvine, CA.
Illite and hydrocarbon exploration
DAVID R. PEVEAR
Exxon PrOdUCtiOn Research CO., P.O. BOX 2189, HOUStOn' TX 77252-2189
ABSTRACT Illite is a general term for the dioctahedral
mica-like clay mineral common in sedimentary rocks, espe-
cially shales. Illite is of interest to the petroleum industry
because it can provide a K-Ar isotope date that constrains the
timing of basin heating events. It is critical to establish that
hydrocarbon formation and migration occurred after the
formation of the trap (anticline, etc.) that is to hold the oil.
Illite also may precipitate in the pores of sandstone reservoirs,
impeding fluid flow. Illite in shales is a mixture of detrital
mica and its weathering products with diagenetic illite formed
by reaction with pore fluids during burial. K-Ar ages are
apparent ages of mixtures of detrital and diagenetic end
members, and what we need are the ages of the end members
themselves. This paper describes a methodology, based on
mineralogy and crystallography, for interpreting the K-Ar
ages from illites in sedimentary rocks and for estimating the
ages of the end members.
-
Illite is a general term for the dioctahedral mica-like clay
mineral common in sedimentary rocks, especially shales (1, 2~.
Although it has a strict mineralogical definition (3), the name
illite is often loosely used for any clay mineral with a 1-nary
repeat in the x-ray powder diffraction data (4~. Because shale
is abundant at the earth's surface, its typical clay mineral, illite,
impacts human welfare in several ways. In the petroleum
industry, illite is of interest for two reasons: (i) It can provide
an isotope date constraining basin heating events, and (ii) it
may precipitate in the pores of sandstone reservoirs, impeding
fluid flow. Because it is a potassium aluminum phyllosilicate,
its time of formation can be determined by using K-Ar isotope
dating. Illite holds Ar tightly because of the difficulty of
migration (diffusion) through the crystal structure layers (5) at
low temperatures.
Of particular concern in resource exploration is the timing
of hydrocarbon (HC) generation. When were the organic-rich
source rock shales heated to ~100°C, cracking the solid
organic matter to oil and gas? It is critical to establish that HC
formation and migration occurred after the formation of the
trap (anticline, etc.) that is to hold the oil. We have long been
able to find traps by using seismic methods, but we seldom are
able to predict the presence of HC without expensive drilling.
If integrated geologic evaluation of outcrops or nearby wells
can show HC generation after trap formation, the risk of
drilling a dry hole is reduced. Because illite forms in shales in
response to heating in the same temperature range as oil
formation (6), its K-Ar age is useful indeed.
It has been recognized for some time (7) that illite in shales
is a mixture of detrital mica and its weathering products with
diagenetic illite precipitated from pore fluids during burial.
Two important lines of evidence support this conclusion. First,
grain size vs. mineralogy relations show a mixture of 2M~ and
1M (including lMd) polytypes, with 1M increasingly abundant
in the finer size fractions (7~. Polytypes are a variety of
PNAS is available online at www.pnas.org.
polymorph distinguished by various repeating stacking ar-
rangements of identical layers (3~. 1M means one layer,
monoclinic, etc. The 2M~ polytype certainly is expected (8) for
the large detrital micas eroded from slates, schists, and phyl-
lites. As we shall see, diagenetic illite that grows in bentonites
and sandstones is exclusively 1M, which suggests that similar
material mixed with 2M~ muscovite in shales is also diagenetic.
Secondly, grain size vs. K-Ar age relations in shales invariably
show age decreasing with grain size: The coarse fractions are
typically older than the depositional (stratigraphic) age of the
shale whereas the fine fractions are younger (9~. The foregoing
shows that illite in shales is a mixture of detrital and diagenetic
components, with the latter more abundant in the fine frac-
lions. But it also identifies the principal problem with practical
use of K-Ar dating of illite in shales: The ages of bulk mixtures
of detrital and diagenetic end members are rather meaning-
less, and what we need are the separate ages of the end
members themselves. I describe a methodology, based on
mineralogy and crystallography, for interpreting the K-Ar ages
from illites in sedimentary rocks and for estimating the ages of
the end members.
Illite in Sedimentary Rocks
v~~ ~~~ ~ ~~~ ~-he r
One cannot discuss illite without touching the subject of
mixed-layer illite/smectite (I/S), a mineral in which unit cell
scale layers of illite and smectite are shuffled like a deck of
cards. Clay mineralogists typically disaggregate a sample and
prepare one or more grain size fractions as oriented aggregates
(1()) on ~ slide for x-rav nowder diffraction (XRD) with a
focusing diffractometer. Because the particles orient with 001
parallel to the slide, only the 001 reflections appear in the data.
Illite has a series of 001 reflections based on a 1-nary periodicity;
smectite, with interlayer water, has a 1.4-nm periodicity that
can vary with humidity or treatment with organics. XRD
patterns (001 series) for I/S typically are nonperiodic (nonin-
tegral; they do not obey Bragg's Law) and do not look like a
physical mixture of illite and smectite. They are interpreted (6)
to result from a single diffraction from a faulted layer structure
composed of two types of unit cells. There is a mature
technology (10) for quantifying and modeling XRD data from
mixed-layer clay minerals.
I/S is common in shales; indeed, much of the illite in shales
may be in the form of I/S. The percent of illite in I/S typically
increases with depth and temperature in most of the world's
sedimentary basins and with geologic age (6~. This has been
interpreted (or inferred) to indicate a progressive solid state or
layer-by-layer transformation of smectite to illite in which the
initial structure of the smectite is inherited by the illite (11~.
More recently, Nadeau (6, 10, 12) has introduced the dual
concepts of fundamental particles and interparticle diffraction
to explain mixed-layer clays. In this view, thin (2- to 10-unit
Abbreviations: HC, hydrocarbon; I/S, illite/smectite; XRD, x-ray
powder diffraction; AFM, atomic force microscopy; IAA, Illite Age
Analysis; my, million years.
3440
OCR for page 3441
Colloquium Paper: Pevear
cells) illite crystals precipitate in shales whereas smectite,
feldspars, and other minerals dissolve. The diffraction effects
of I/S result from coherent (in OOl) scattering amongst thin
face-to-face illite crystals with hydrated interfaces that behave
like smectite (are turbostratic). As crystals grow thicker, the
number of interfaces decreases, which is seen in the XRD data
as a decrease in smectite component of I/S. The observation
of thin ideomorphic crystals of 1M illite with 1-nary surface
growth steps in sandstones and shales (13) supports Nadeau's
ideas. The subject of I/S remains controversial, but here I
assume that increase in illite content of I/S with burial depth
simply represents the growth of progressively thicker illite
crystals.
To extract useful chronologic information from K-Ar dating
of illite, I have found the concept of grain-size vs. age spectra
(size-age spectra) useful (Fig. la). A sample is routinely
divided into three clay-size fractions: coarse (C = 0.2-2.0 lam),
medium (M = 0.02-0.2 am), and fine (F = <0.02 ,um), and,
for each, a routine K-Ar age is obtained. Using the c2-,um
fraction generally excludes feldspar, so that the only K-bearing
phases are illite and micas. Plotting these as simple bar graphs
has revealed three major spectra shapes for sedimentary rocks:
inclined, flat, and benched. These are typical of shales, K-
bentonites, and sandstones, respectively.
An inclined spectrum (Fig. la) is typical for shales, which are
deposited with a wide initial size range of detrital micas.
Usually the C fraction is older than the depositional age, but
this depends on the proportion of diagenetic illite. The F
fraction is typically younger than the depositional age because
of the dominance of diagenetic illite. Importantly, as pointed
out 35 years ago by Hower et al. (9), there is no way to use these
dates, except as crude limits. All fractions appear to be physical
mixtures, and we do not know the proportions. The mixture of
old and young illite in shales can for some samples give K-Ar
ages fortuitously close to depositional age (9~. Note that K-Ar
data from shales cannot be successfully interpreted by using
the isochron method because shales are mixtures of things that
formed at different times. They do, however, often give
nice-looking, linear, but useless, "mixochrons."
Bentonites (stratigraphic definition) are an uncommon class
of shale bed consisting of air-fall glassy volcanic ash altered to
smectite (3~. K-bentonites (3) are those that have undergone
subsequent diagenesis to illite or I/S. They are of great value
to illite studies because they do not contain detrital dioctahe-
dral micas, only diagenetic illite. The size-age spectrum of a
K-bentonite is typically flat (Fig. lb); i.e., all size fractions have
the same K-Ar age, younger than depositional age. Bentonites
SHALE BENTONITE
I b
F M C
FrG. 1. (a) Size-age spectrum for shale. The sample is divided into
three clay-size fractions: coarse (C = 0.2-2.0 lam), medium (M =
0.02-0.2 lam), and fine (F = <0.02 lam). An inclined spectrum is
typical for shales, which are deposited with a wide initial size range of
detrital micas. Usually, the C *action is older than the depositional
age, but this depends on the proportion of detrital mica. The F fraction
is typically younger than the depositional age because of the domi-
nance of diagenetic illite. (b) Size-age spectrum for a K-Bentonite is
flat; i.e., all size fractions have the same K-Ar age, younger than
depositional age. Bentonites give the diagenetic age directly because
they do not contain detrital illite.
Proc. Natl. Acad. Sci. USA 96 (1999J 3441
give the mean diagenetic age directly. If Bentonites were
common in the stratigraphic record, we could forget about
trying to get meaningful ages from ordinary shales. They are
useful for our dating problem because they give us an idea of
what the pristine diagenetic illite is like. Mineralogic studies of
K-bentonites are numerous, and XRD shows the illite and I/S
to be entirely 1M polytype with moderate amounts of 120°
rotational disorder (14, 15~. 2MI muscovite is never found as
a diagenetic phase in K-bentonites of sedimentary basins. This
is good news because it gives us a possible way to differentiate
and quantify the diagenetic and detrital components in shales.
Atomic force microscopy (AFM) shows the K-bentonite
illite crystals to be only a few nanometers thick (Fig. 2), with
a predominance of 1-nary growth steps. The former is con-
firmed by XRD studies of the 001 reflections (16~; the latter
agrees with their 1M polytype. The extraordinary thinness
likely explains the abundance of diagenetic illite in the fine
fractions of shales.
Sandstones with a shale-like depositional matrix or abun-
dant lithic grains have size-age spectra similar to shales and
will not be discussed further. Clean sandstones consist only of
sand-sized grains of quartz, feldspars, mica, etc., and lack
depositional clay. They are deposited in a high-energy envi-
ronment (like a beach) in which the fines are winnowed away.
During diagenesis, feldspars and other rock constituents may
react with pore fluids to precipitate illite or other diagenetic
clays; hence, the fine material in these sandstones tends to be
mostly diagenetic, and more so than for shales. A typical
sandstone size-age spectrum (Fig. 3) is bench-shaped; i.e., the
C fraction is older than depositional age whereas the M and F
fractions have the same age, younger than depositional age.
This flattening out in the finer fractions permits us to conclude
that fine detrital mica is absent in these fractions and that we
have measured the mean age of illite formation. Unfortu-
nately, diagenetic illite is not so universally abundant in
sandstones as it is in shales, and not all sandstones are clean
sandstones.
There are many studies of pore filling illites, both mineral-
ogic and K-Ar dating (2, 6, 10~. The abundant literature is
primarily due to the negative effect illite has on permeability
FIG. 2. AFM deflection image of illite crystals from the Tioga
K-bentonite. Scale is in nanometers. Individual growth steps are 1 rim
high; the largest crystal is 7 nm thick. The image was made in air,
contact mode, on a Digital Instruments (Santa Barbara, CA) Multi-
Mode Nannoscope IIIa.
OCR for page 3442
3442 Colloquium Paper: Pevear
SANDSTONE
of:
5, .; ;
_~ ~
FIG. 3. Size-age spectrum of sandstone. The spectrum is typically
bench-shaped; i.e., the C fraction is older than depositional age
whereas the M and F fractions have the same age, younger than
depositional age. The flattening out in the finer fractions indicates that
fine detrital mica is absent in these fractions and that we have
measured the mean age of illite formation. Symbols are same as in
Fig. 1.
of sandstone petroleum reservoirs. The illites are typically
ideomorphic with a pronounced fibrous (lath) habit (long axis
is crystallographic a axis) making them interesting subjects for
microscopy (Fig. 4~. They are often called "hairy illite" in the
petroleum industry. The crystals are ideomorphic because they
precipitate unconstrained from fluid in a relatively large pore.
They are all 1M polytype, with a minor 120° rotational
disorder. As in K-bentonites, they are thin (2-10 nary), with
1-nary growth steps and some evidence of spiral growth.
Samples composed of especially thin crystals are I/S by XRD.
There is no evidence for a smectite precursor. Individual laths
may be intergrown at 120° to produce star-like aggregates or
twins (Fig. 5~. The twinning (a rotation of 120° with respect to
the mirror plane containing the empty octahedral site) is after
the "common mica twin law" (8) and likely accounts for much
of the rotational disorder seen in the XRD data.
The preceding has established that thin diagenetic illite
crystals grow in sedimentary rocks and that they have distinct
mineralogical features, such as I/S XRD effects and 1M
polytype, that distinguish them from 2M~ muscovite. Much of
our knowledge of disordered illite polytypes and I/S comes
from the use of the programs NEWMOD (10) and WILDFIRE
(14), which permit easy calculation of the complete powder
XRD patterns of clay minerals. These programs form the basis
for "unmixing" the mixtures we have been discussing. In the
process of matching calculated to experimental data on poly
.
of_
FIG. 4. Scanning electron micrograph of pore-filling fibrous illite
in a sandstone.
Proc. Natl. Acad. Sci. USA 96 (1999)
A'
1 ~
as
R
3598
2598
2988
I see
588
~.2 .4 .6 .8 1
8 588 1888 15 - ~8 25 - 38eG 3568
FIG. 5. (A) AFM deflection image of sandstone illite. Laths are
intergrown at 120° in a star-like aggregate or twin after the common
mica twin law (a rotation of 120° with respect to the mirror plane
containing the empty octahedral site) (8~. Granular materials adhering
to illite (especially on the right) are salts precipitated during sample
preparation. The scale is in micrometers; the crystal is ~1 ,um long.
This and subsequent images were made in air, contact mode, on a
Universal AFM (ThermoMicroscopes, Sunnyvale, CA). (B) Close-up
of the center in A. Lines show measurements of step height made on
the height image (not shown). Note interlaced growth of 1-nary (10-~)
growth steps. Individual laths have a thickness of 6-8 nm. By powder
XRD, this sample is 1M, with a minor 120° rotational disorder. Only
the center will contribute to the disorder; the projecting laths (A) will
not. The scale is in angstroms.
types and disorder in illite, some generalizations have
emerged. Bentonites and fibrous (sandstone) illites are similar
in many respects (1M with some 120° rotational disorder) but
differ in that the cis-vacant form (15, 17) is more common in
bentonites and the trans-vacant form (the traditional 1M
structure) is more typical of fibers "discussion of nomenclature
(14~.
Shales are different in that most shale illites (excluding the
2M~ component) show nearly maximum rotational disorder,
including both 120° and 60° rotations (14) and are therefore the
lMd polytype (8~. This means that each successive 1-nary layer
is unrelated to the layer below it except that the hexagonal
oxygen rings align to accommodate K. On the basis of AFM
morphological observations, bentonite and sandstone illites
OCR for page 3443
Colloquium Paper: Pevear
grow primarily by spiral or step mechanisms whereas shale
illites grow by nucleation (birth and spreading). Illites in shales
(Fig. 6) show many small 1-nm-thick nuclei on the 001 of a
larger substrate that may be detrital mica. These appear to be
randomly placed epitaxial growths. Continued similar growth
would create a lMd illite. Bentonite and fiberous illites have
nearly featureless 001 faces with one or more parallel growth
steps. The contrasting mechanisms (growth vs. nucleation) are
roughly in accord with the early discussion on the origin of
polytypes (8~.
Transmission electron microscopy paints an apparently
somewhat different view of shale illite (18), but it is not clear
to me how much of that difference is related to the method of
investigation (transmission electron microscopy vs. XRD). For
example, the requirements for coherency are likely more
stringent for XRD than for transmission electron microscopy.
The predominance of 2M~ polytype in ion-milled whole-rock
samples (18) is possibly due to detrital muscovite; at least, that
is what shale K-Ar data (older than depositional age) suggest.
Further discussion is beyond the scope of this review, but the
questions raised by the transmission electron microscopy work
on illite offer exciting directions for future research.
Illite Age Analysis
Returning to the shale size-age spectrum (Fig. la), it is
obvious that a simple way to estimate the ages of the detrital
and diagenetic end members is to quantitatively determine (by
XRD) the proportions of the end members in each of the three
size fractions, plot the points (normalized to 100%) as appar-
ent K-Ar age vs. percent of detrital illite, and linearly extra-
polate to O and 100% detrital to get the end member ages (Fig.
7~. I call this Illite Age Analysis (IAA), and it is the subject of
an Exxon patent (19~. The extrapolated "diagenetic age" is the
mean (integrated) age of the time interval over which illite
grew. This could be a nearly instantaneous event in the case of
illite formed in response to an igneous intrusion, or a 50-
million-year (my) interval of burial in a sedimentary basin.
Similarly, the "detrital age" is the mean age of the coarse
micas, which may themselves be a mixture. Ideally, the detrital
age corresponds to the mean time of uplift and cooling of the
source terrain below the so-called blocking temperature for
sage
3008
2094
l O8Q
~ 1 888 2088 3900 4008 5008
FIG. 6. AFM deflection image of a shale illite crystal. The surface
is covered with small, 1-nm-thick growths or nuclei, possibly on the 001
of a larger substrate that may be detrital mica. These appear to be
randomly placed epitaxial growths. Continued similar growth would
create a lMd illite. XRD shows 60% lMd, with the rest 2M~. The XRD
pattern for this sample is in Fig. 9b (C). The scale is in angstroms.
Proc. Natl. Acad. Sci. USA 96 (1999' 3443
4oo t.
300
~s
~s
200
100
Diagenetic 0`
End-Member
Age 0 20
(Modeled Age)
500 .
I AA P I ot 383 Ma 1
90°/0 conf.)
. M
F ~
- ~33 Ma (0~8 Ma ~ 90% conf.,
40 60 80 100
% Detrital Illite
) ~\ ~Illite
2 4 6 8 10 12 14 16 18 20
°2 ~
Detrital
End
Member
Aqe
FIG. 7. IAA plot of a shale sample. To estimate the ages of the
detrital and diagenetic end members, we quantitatively determine
(XRD) the proportions of the end members in each of three size
fractions, plot the points (normalized to lOO~o) as apparent K-Ar age
vs. percent of detrital illite, and linearly extrapolate to O and 100%
detrital to get the end member ages. Lower diagram is XRD pattern
(oriented aggregate, Cu radiation) showing discrete illite (detrital) and
diagenetic I/S.
muscovite (250-300°C), below which Ar no longer diffuses out
of the structure (204.
Some distinctly questionable assumptions are made in using
this method. First, can we treat the complex mixture that is
shale as a two-component system with respect to illite? For
example, what if there is detrital (recycled) 1M illite? Where
we have had an independent test, such as a convenient
bentonite interbedded with shale (21), or a date on large micas
physically separated from the rock, the method works. Diage-
netic illite is likely more easily weathered because of fine grain
size; it may not survive as detritus. What if illite grew during
two heating events 50 my apart? As we will see, for calibrating
the thermal history of basins, only the integrated age is
important. Certainly, two separate ages could not be extrap-
olated from IAA data alone. Could Ar leak out of the tiny illite
crystals so the age would be too young? Illite formed by contact
metamorphism gives the same age as the pluton, showing illite
to be retentive of Ar (22~. Small crystals often have fewer
defects than large ones, and defects may control Ar loss (atom
hopping vs. migration down tubes and cracks). Also, if a crystal
is disrupted so it loses Ar, it will likely also lose K from the
same region because it is in contact with a Na-rich pore fluid,
in which case the K-Ar age will be unaffected. As long as the
samples have not been heated above the generally accepted
250°C muscovite-blocking temperature, thermal argon diffu-
sion is unlikely, but we really have few data on illite itself.
Fortunately, drill holes in most sedimentary basins seldom get
close to 200°C. How do we know that the relation in Fig. 7 is
linear? It is not, really, but if the K content of both end
members is similar, it is close enough. This is suggested by the
observation that most of our many data sets fit a straight line
rather well.
OCR for page 3444
3444 Colloquium Paper: Pevear
In practice, there are two approaches to quantify the end
members using XRD. The first uses the 001 peaks and assumes
the diagenetic illite is in I/S, and the detrital end member is
discrete mica. These two can be distinguished on an XRD
pattern (Fig. 8a). Quantification is the critical step and the
source of most of the uncertainty in the IAA method. We
calculate, from first principles, XRD patterns to match the
experimental pattern (Fig. 8b). The basic method is that of
NEWMOD (10), but the actual calculation and matching are
controlled by a genetic algorithm (234. From the range of
calculations that have a good fit, we estimate an uncertainty for
each point on the IAA plot and use a Monte Carlo method to
project these uncertainties into the extrapolated end member
ages. For samples that have data points mostly at one end or
the other of the IAA plot, the uncertainty in estimating the age
at the opposite end can be quite large.
The second method uses polytypes (lMd and EMU; see Fig.
9a). Because shales contain large amounts of rotationally
disordered illite with a few, broad XRD peaks (Fig. 9b),
anything resembling a real quantitative analysis was not easily
done until WILDFIRE became available (14~. Using an approach
similar to the first, for each fraction, a calculated XRD pattern
is optimized to the experimental data (Fig. 10~. The polytype
method is especially useful for samples lacking I/S, in which
the peaks for illite and mica are superimposed. A similar
routine was applied to a Paleozoic shale from Illinois (24~.
L ILL,TE I
| ILLITE |
ILLITE I
ILLITE I
~ ILLITE |
I SMECTITE I
I ILLITE I
SMECTITE
| SMECTITE I
I ILLITE |
L
Proc. Natl. Acad. Sci. USA 96 (1999J
a.
a--- ~ Of
Detrital
(2M1)
Diagenetic
(1 M)
rm.-r,.~,,~.~ v- ~
15 20 25 30 35 40 45
Polytypes Degrees 28
b
' ~ /_ - ~ | · ~F
W ~A~
~ J ~C
16 22 27 33
Degrees 28
J ;~
a.
Detrital
Jo
~11 Diagenetic
/. Jo It (I/S)
Wit _
............................
0 5 10 15 20 25 30 35 40
Degrees 2~3
, Illite 40°/O
38 44
FIG. 9. (a) XRD patterns (assuming random powder sample, Cu
radiation) calculated with WILDFIRE of nondisordered 1M and 2M~
polytypes of dioctahedral mica. Note the distinguishing peaks in the
central part of the patterns. At the left is the a-b projection of the unit
cell showing the stacking sequences that characterize each polytype.
(b ) XRD patterns for the shale shown in Fig. 6. Size fractions are same
as Fig. 1. K-Ar ages are C = 151, M = 110, and F = 78 my. The F
pattern is typical for maximum disordered lMd illite. The C pattern
shows modulations for 2M~, and the model indicates 40% 2M~.
Random powder mount, Cu radiation.
The IAA technique permits only estimation of the compo-
nent ages. Precision, calculated as above, averages ~ + 15~o of
the estimated value (e.g., 20 + 3 my) based on our experience,
and can be larger where the diagenetic age is <10 my.
Accuracy is unknown, but where tested (21) is almost as good
as precision. Certainly, the diagenetic age from IAA is a much
better choice for calibrating basin thermal history than a whole
rock K-Ar age from shale or the age of an arbitrary fine
fraction. The 40/39Ar dating technique has been used as an
~ ~ lL
, ~ ~
1 1
0 5 10 15 20 25 30 35 40
Degrees 2 ~
FIG. 8. (a) Calculated XRD patterns (oriented aggregate, Cu
radiation) for discrete illite and I/S made with NEWMOD. Patterns like
these are added to match an experimental pattern. Blocks on the left
show basic structural 2:1 layers. (b) This illustrates how an experi-
mental XRD pattern (oriented aggregate, Cu radiation) is matched,
and thus quantified, by a calculated mixture of discrete illite and I/S.
Calculated pattern at the top is for 40% discrete (detrital) illite.
/\
~ ~71 <~ at, a/,
~... ~ I ~ ~ ~ ~ 1 r ~ ~ ~ 1 r ~ ~ ~ 1 ~ ~ ·
15 20 25 30 35
Degrees 28
40 45
FIG. 10. Experimental XRD pattern (Upper) and a calculated
match (Lower) of a sample containing 15% 2M~ and the rest moder-
ately disordered 1M.
OCR for page 3445
Colloquium Paper: Pevear
alternative for separating diagenetic from detrital ages in
mixtures (25~. Although at present this is not as effective as
IAA, continued progress on methodology and diffusion mod-
els may ultimately make this the method of choice.
Applications
Models are the key to using illite in basin thermal history
calibration. The petroleum industry typically uses burial his-
tory, based on the stratigraphy (age and depth) in a well, to
estimate thermal history (264. Other geologic data are used to
estimate the amount of sediment eroded from unconformities,
timing of uplift events, and basal heat flow. A computational
model, which includes compaction, estimated rock thermal
conductivity, and radiogenic heat generation, computes the
temperature of each sedimentary layer through time as it is
buried. The modeled results, such as present-day thermal
gradient, are then compared with measured well temperatures,
which define the real thermal gradient, and the model is
adjusted to fit the measured data. Once the model is calibrated
to data, kinetic expressions for thermal generation of oil and
gas can be applied to the thermal history to give timing of HC
generation.
Unfortunately, present-day conditions may not tell us much
about when a particular shale bed generated oil tens of millions
of years ago. Present thermal gradient may not be a guide to
past conditions. We need paleothermometers, rock properties
that tell us about past thermal events, to properly constrain the
model. The most widely used paleothermometer is based on
vitrinite reflectance (%R), the increase in reflectivity of a
coaly material found in rocks as a function of time and
especially temperature (26~. The thermal history is applied to
a kinetic expression for %R, and model %R values are
obtained; these are compared with measured values from
rocks obtained from the well, and the model is adjusted to give
a reasonable fit. But there is a problem: %R really gives only
the maximum temperature; it tells us nothing about when that
temperature was reached, and that is when the HCs were
generated.
A downhole increase in shale diagenetic illite (%I in I/S) is
observed in many basins of the world (6, 2O, 26), and this
relation has been used as a paleothermometer in the same way
as ~oR (27~. We use experimental kinetics developed by Exxon
(27~; see ref. 28 for a comparison of several published kinetic
expressions. The measured values of HI in I/S are compared
with the modeled curve (Fig. 11), and the model is adjusted to
optimize the fit. This alone does not give us much more than
%R, but the age of the diagenetic illite can also be easily
modeled by using the kinetic expression and can be calibrated
5,000
_ 1 0,000
8 1 5,000
- I/S Model ·
· Observed
20,000 . . . . . . . . .
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
Illite Fraction in US
FIG. 11. Plot of decimal fraction of illite in I/S from shales in a
typical well. Individual points are sample measurements; the line is
calculated from a burial history by using an experimental kinetic
expression (23~.
Proc. Natl. Acad. Sci. USA 96 (1999) 3445
with the measured diagenetic age from IAA. This gives us a
powerful piece of chronologic information that is independent
of assumptions about burial history. Note that a modeled age,
like the IAA diagenetic age, will be an integrated age the
mean age of an illite-forming time interval.
The integration of paleothermometers is shown on a sche-
matic thermal history in Fig. 12. Illite data constrain the burial
or heating phase of a basin's thermal history, %R records
maximum temperature, and apatite fission track analysis con-
strains timing of uplift and cooling. Discussion of the last
technique is beyond the scope of this review.
A diagrammatic example application is given in Fig. 13. The
cross section shows petroleum source rocks separated from a
structural trap by an unconformity at A. Did the source rocks
mature (get heated) before or after deposition of the upper
units containing the trap? If they produced oil before time A,
then it is much less likely that they will be able to act as source
for the trap. The question is one of amount of missing (eroded)
section at A. If there was a large amount of uplift and erosion,
then the source rocks could have been deep (hot) enough
before A to produce oil. To solve the problem, samples are
obtained from the source shales from outcrops or nearby wells
(Fig. 13A, 1) and IAA (Fig. 13B, 2) is done to get the diagenetic
age (Fig. 13B, 3~. The thermal history plot (Fig. 13B, 4) is now
anchored by a real date (IAA), which constrains the source
heating and HC yields to post A time (Fig. 13B, 5~. This
indicates that HC supply to the trap will not be a risk factor for
this prospect. IAA is especially useful in areas of complex
structure, like fold and thrust belts, in which thermal history
is not just a function of simple burial.
Variants of the methods described have been used to
successfully date normal and thrust faults (time of trap for-
mation) and to predict growth of permeability-reducing illite
in reservoir sandstones (29, 13~. Growth of illite in shales, as
in sandstones, appears to be a pore-filling process, but the
pores are smaller and flatter. Shale permeability, like that of
sandstones, is likely reduced by illite growth. This could
improve the quality of a shale seal above a trap or could
otherwise effect the mechanical properties of the shale.
Clay-rich fault gouge typically has a flat size-age spectrum
or an inclined spectrum with all ages younger than depositional
age (294. It appears that upper crustal faulting (low temper-
ature) can reset the illite K-Ar clock, but the mechanism is
unclear. Heating seems unlikely, as ~oR indicates low temper-
atures. Crystal growth under conditions of deformation and
unique fluid chemistry are likely involved. It is not clear that
deformation alone can cause total Ar loss from illite. Fault
gouge illites are an area of evolving research.
Ficcion Track~ Erasec
Max. ' ' E '
TemP. E
~ r ~ ~
Illite Age Fission-Track Age Time - ~
FIG. 12. Thermal history schematic showing integration of paleo-
thermometers. Illite data constrain the burial or heating phase of a
basin's thermal history, ~oR records maximum temperature, and
apatite fission track analysis constrains timing of uplift and cooling.
OCR for page 3446
3446 Colloquium Paper: Pevear
(Samples O
B
-
Y ~
|IAAPIOt 1@ ~ E
Detrital =) ~ ~
D agen~ ~ ~ ~i O
% Id ~
Improved
Thermal History
a, 200 Time °
Improved
Yield Model
FIG. 13. (A) Cross section showing petroleum source rocks sepa-
rated from a structural trap by an unconformity at A. Did the source
rocks get heated before or after deposition of the upper units
containing the trap? If they produced oil before time A, then it is much
less likely that they will be able to act as source for the trap. The
question is one of amount of missing (eroded) section at A. If there was
a large amount of uplift and erosion, then the source rocks could have
been deep (hot) enough before A to produce oil. (B) To solve the
problem in A, samples are obtained from the source shales (1), and
IAA (2) is done to get the diagenetic age (3~. The thermal history plot
(4) is now anchored by a real date (IAA), which constrains the source
heating, and HC yields to post-A time (5~. Sharp peaks on 5 show
model generation of oil and gas, respectively. HC supply to the trap will
not be a risk factor for this prospect.
Conclusions
Illite is a common mineral in sedimentary rocks, especially
shales. Careful mineralogical analysis using new techniques
developed by the clay mineral research community permits the
extraction of quantitative information on the time and tem-
perature of diagenetic illite formation. In hydrocarbon explo-
ration, these data are used to calibrate the heating history of
sedimentary basins to ascertain that oil or gas generation from
source shales postdated trap formation. If generation preceded
trap formation, the oil or gas would presumably have leaked
off, and the well should not be drilled. Application of the
mineralogical work reported here will decrease the risk of
drilling a dry hole, reducing not only the expense but also any
disturbance that might be caused by drilling. Further, because
thermal conditions partly control the likelihood of the trap
being filled with gas vs. oil, the illite work helps us find the
particular type of HC we are looking for. Application to fault
dating is useful not only to estimate HC trap timing but also
may have potential in evaluating earthquake hazards.
The price of oil and gas has remained low because of the
combined effects of open competition and applied technology.
Proc. Natl. Acad. Sci. USA 96 (1999J
Although three-dimensional seismic is often featured by the
media, many less dramatic advances also contribute to improv
ing the efficiency of exploration and production. Because the
earth is made of minerals, it is not surprising that mineralogy
plays an essential role.
I thank Exxon Production Research Co. for a productive research
environment and thank several Exxon people: P. J. Houser for the
AFM work, D. W. Webb and T. C. Phillips for XRD, and R. F. Ylagan
for the polytype work and useful advice. Without the work and
friendship of R. C. Reynolds, none of this would have been possible.
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Representative terms from entire chapter:
thermal history
