Directory UMM :Data Elmu:jurnal:O:Organic Geochemistry:Vol32.Issue1.2001:
Organic Geochemistry 32 (2001) 127±141
www.elsevier.nl/locate/orggeochem
Brightness of pollen as an indicator of thermal alteration
by means of a computer-driven image processor:
statistical thermal alteration index (stTAI)
Yoshihiro Ujiie *
Department of Earth and Environmental Sciences, Faculty of Science and Technology, Hirosaki University, Hirosaki 036-8561, Japan
Received 11 May 1999; accepted 20 September 2000
(returned to author for revision 11 November 1999)
Abstract
The brightness, or gray level, of pollen of Pinus, Podocarpus, Abies, Picea and Tsuga from the Neogene sediments in
northern Japan was measured using a transmitted-light microscope with a computer-driven digital image processor.
The mean value of the modes for the complete array of the indigenous pollen in a rock sample was called here ``the
statistical thermal alteration index'' (stTAI). In the present investigation an inverse relationship between stTAI and
vitrinite re¯ectance (RO) in sediments was found. The application of stTAI to samples from three boreholes indicated a
decreasing trend with depth. By using this trend, threshold values of intense oil generation can be evaluated (145±110).
Therefore, stTAI can be a useful parameter for determining organic maturation and for identifying the threshold zone
of intense oil generation. # 2001 Elsevier Science Ltd. All rights reserved.
Keywords: Thermal alteration index (TAI); Statistical thermal alteration index (stTAI); Brightness of pollen; Vitrinite re¯ectance;
Japanese Neogene; Organic maturation; Oil generation
1. Introduction
Organic matter in sediments changes diagenetically
during burial. These changes are re¯ected by progressive
alteration of physical characteristics such as color,
re¯ectance, ¯uorescence properties, etc. Measurements
of these characteristics of organic matter, especially
palynomorphs, are widely used techniques for the
assessment of coal rank and the extent of petroleum
generation.
The color variations of pollen and spores in coal beds
resulting from diagenesis have been recognized since the
1920s (Gutjahr, 1966). Staplin (1969) ®rst used the
thermal alteration index (TAI), the variation in color of
organic material as measured under a microscope, to
determine the relative opacity of organic matter. Now
* Tel.: +81-172-39-3952; fax: +81-172-39-3952.
E-mail address: [email protected] u.ac.jp
TAI is widely used to measure organic maturity in
sedimentary rocks, especially in petroleum source
rocks.
There are two microscopic methods to measure TAI
or organic maturity of palynomorphs, especially pollen
and spores. One is to distinguish their morphology and
color with transmitted light using the operator's own
eyes (Schopf, 1948; Wilson, 1961; Correia, 1967; Staplin,
1969; Burgess, 1974; Gray and Boucot, 1975; Peters et
al., 1977; Shimazaki, 1986). All these studies have color
scales from 7 to 23 points. This method has the merits of
simplicity and economy (Hunt, 1979; Tissot and Welte,
1984; Akiba et al., 1992). It also has the disadvantages
that its scales are subjective and qualitative and operatordependant because it is very dicult to visually distinguish
subtle changes in color of organic material under a
microscope. The results of this method are hard to
apply to interlaboratory comparison (Taguchi, 1978;
Robert, 1985; Shimazaki, 1986).
The other method devised for overcoming these disadvantages is to determine the translucency of organic
0146-6380/01/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved.
PII: S0146-6380(00)00146-7
Y. Ujiie / Organic Geochemistry 32 (2001) 127±141
128
matter by photoelectric measurements. Gutjahr (1966)
measured the translucency, or light absorption, of pollen
grains and spores with a photocell attached to a
transmitted-light microscope. Lo (1988) measured the
transmittance of palynomorphs on strew-mounted
kerogen slides at a wavelength of 546 nm and converted
the values to an equivalent TAI scale. Marshall (1991)
applied the Commission Internationale de l'Eclairage
(CIE) color system to the measurement of spore color.
He measured the transmittance of spores in the range of
400±750 nm with a microspectrophotometer. Van Gijzel
et al. (1992) measured the transmittance colour index
(TCI) on amorphous organic material and applied TCI
and RO to a one-dimension basin analysis model. Yule
et al. (1998), using a color video camera attached to a
microscope and linked to an image analyzer, presented
the colour image analysis (CIA) as a system for the
quanti®cation of spore color. Yule et al. (1999) recorded
spore color as the amount of light transmitted at dierent
wavelengths in the visible spectrum using a spectral
scanning microphotometer. Staplin (1977) and Robert
(1985) summarized studies regarding color changes in
organic matters in sediments with diagenesis.
This study is aimed at developing an objective and
quantitative TAI scale with simple measuring equipment that can be used as a common laboratory tool. The
brightness of bisaccate pollen of Pinus, Podocarpus, Abies
and Picea, and monosaccate pollen of Tsuga from the
Neogene sediments in northern Japan was measured using
a transmitted-light microscope, by means of a computerdriven digital image processor. The mean value of
modes in brightness of the indigenous pollen in each rock
sample was called here ``the statistical thermal alteration
index'' (stTAI) as an organic maturation indicator.
2. Experimental
2.1. Samples
The brightness of pollen in 30 Neogene mudstone
samples from northern Japan were measured as standard
Table 1
Vitrinite re¯ectance (RO) and statisticalc thermal alteration index (stTAI) data of mudstone samples used in making a stTAI scale
stTAI
RO (%)
(95% con®dence limit)
Core/cuttings/outcrop
Formation/group
Epoch
Locality
0.290.03
0.290.02
0.300.01
0.310.01
0.340.04
0.340.04
0.370.01
0.380.01
0.400.03
0.440.04
0.450.02
0.450.02
0.460.03
0.500.02
0.500.01
0.530.03
0.530.02
0.540.03
0.560.02
0.570.02
0.600.04
0.620.01
0.630.02
0.640.02
0.680.02
0.710.02
0.780.02
0.850.04
0.880.06
0.900.02
Cuttings
Cuttings
Outcrop
Outcrop
Cuttings
Cuttings
Cuttings
Outcrop
Cuttings
Cuttings
Core
Cuttings
Core
Core
Cuttings
Core
Cuttings
Cuttings
Core
Cuttings
Cuttings
Core
Core
Core
Cuttings
Core
Core
Cuttings
Cuttings
Core
Shibikawa F.
Shibikawa F.
Taiaki F.
Taiaki F.
Sasaoka F.
Uonuma G.
Upper Tentokuji F.
Akaishi F.
Lower Tentokuji F.
Funakawa F.
Lower Tentokuji F.
Lower Tentokuji F.
Nishiyama F.
Lower Teradomari F.
Funakawa F.
Wakkanai G.
Funakawa F.
Funakawa F.
Lower Teradomari F.
Nishiyama F.
Onnagawa F.
Ishikari G.
Nishikurosawa F.
Lower Teradomari F.
``Green Tu F.''
Ishikari G.
Nanbayama F.
Nishikurosawa F.
``Green Tu F.''
Hiuchiyama F.
Pleistocene
Pleistocene
Miocene
Miocene
Pleistocene
Pleistocene
Pleistocene
Miocene
Pliocene
Pliocene
Miocene
Pliocene
Pleistocene
Miocene
Pliocene
Miocene
Miocene
Miocene
Miocene
Pliocene
Miocene
Eocene
Miocene
Miocene
Miocene
Eocene
Miocene
Miocene
Miocene
Miocene
Akita-shi, Akita Pref.
Akita-shi, Akita Pref.
Nakatsugaru-gun, Aomori Pref.
Nakatsugaru-gun, Aomori Pref.
Akita-shi, Akita Pref.
Nishikanbara-gun, Niigata Pref.
Honjo-shi, Akita Pref.
Nakatsugaru-gun, Aomori Pref.
Akita-shi, Akita Pref.
Akita-shi, Akita Pref.
Nakakubiki-gun, Niigata Pref.
Honjo-shi, Akita Pref.
Nishikanbara-gun, Niigata Pref.
Nakakubiki-gun, Niigata Pref.
Honjo-shi, Akita Pref.
Wakkanai-shi, Hokkaido
Akita-shi, Akita Pref.
Honjo-shi, Akita Pref.
Nakakubiki-gun, Niigata Pref.
Nishikanbara-gun, Niigata Pref.
Akita-shi, Akita Pref.
Wakkanai-shi, Hokkaido
Honjo-shi, Akita Pref.
Nakakubiki-gun, Niigata Pref.
Honjo-shi, Akita Pref.
Wakkanai-shi, Hokkaido
Nakakubiki-gun, Niigata Pref.
Akita-shi, Akita Pref.
Akita-shi, Akita Pref.
Nakakubiki-gun, Niigata Pref.
1502.9
1571.4
1392.5
1473.3
1514.7
1442.3
1412.7
1365.4
1343.1
1363.1
1342.1
1442.8
1433.1
1353.4
1422.2
1243.2
1111.9
1094.1
1123.2
952.4
871.8
841.7
844.0
813.4
725.4
722.9
624.7
562.6
502.4
440.0
Y. Ujiie / Organic Geochemistry 32 (2001) 127±141
Fig. 1. Schematic ¯ow chart for the measurement of brightness.
Fig. 2. Site preference of brightness measurement in bisaccate
pollen (Pinus, Podocarpus, Abies and Picea).
129
calibration samples. Except for three outcrop samples
they were all cores and cuttings samples from six boreholes.
The RO values ranged from 0.29 to 0.90% (Table 1).
Three series of samples from the MITI Yuri-OkiChubu borehole, the MITI Honjo-Oki borehole and the
MITI Shin-Takenomachi borehole were measured for
their brightness in order to study its change with depth
for the application to petroleum exploration.
The MITI Yuri-Oki-Chubu borehole was drilled to a
depth of 5000 m through an interval of Pleistocene to
the lower Miocene sediments at N39 360 41.84000 ,
E139 560 43.76600 in the Sea of Japan o the west coast
of Honshu Island. There is only one sedimentary gap
of some 700,000 years at the Pleistocene±Pliocene
boundary at 1492 m depth (Japan National Oil
Corporation, 1993b). The 16 samples from this borehole
were all cuttings.
The MITI Honjo-Oki borehole was drilled to a depth
of 4800 m through a Pleistocene to the lower Miocene
interval at N39 250 10.26400 , E139 510 31.94400 in the Sea
of Japan o the west coast of Honshu Island. This
sedimentation was continuous during deposition (Japan
National Oil Corporation, 1994). Six cuttings samples
and one core sample (3628.25 m deep) were measured.
The MITI Shin-Takenomachi borehole was drilled to
a depth of 6310 m through a Pleistocene to the lower
Miocene interval at N37 460 05.11300 , E138 520 43.28600 in
the Niigata Prefecture. These sediments also continuously deposited without any gaps (Japan National
Fig. 3. Example of a brightness distribution in a single measurement of pollen. Minimum=76, maximum=223, mode=168, frequency at the mode=531, mean=162, standard deviation=23.9, standard error=0.15 and sum total=4,095,787.
Fig. 4. Example of a brightness distribution in a single measurement of a living pollen, Pinus thunbergii. Minimum=133, maximum=223, mode=196, frequency at the mode=471, mean=190, standard deviation=13.4, standard error=0.12 and sum
total=2,469,280.
130
Y. Ujiie / Organic Geochemistry 32 (2001) 127±141
Oil Corporation, 1993a). Seven were cuttings samples
and two were core samples (3000.3 and 4255.9 m deep).
The former two boreholes are located in the Akita
basin and the latter borehole in the Niigata basin.
Although oil and gas are being recovered from the
Neogene sediments in these basins, neither economic oil
nor gas was found in these three boreholes.
2.2. Measurements
Crushed mudstone samples (from 0.3 to 5 mm in
diameter) were treated with 15% hydrochloric acid
overnight at room temperature and with 46% hydro¯uoric
acid in a 70 C water-bath for 6 h to remove carbonates
and silicates. Residual organic matter, ranging from 32
Fig. 5. Histograms showing brightness of pollen at dierent maturity (RO). An arrow with double heads indicates a range of indigenous pollen. ``stTAI ''=statistical thermal alteration index.
Y. Ujiie / Organic Geochemistry 32 (2001) 127±141
131
Fig. 5. (continued)
to 100 mm in diameter, was concentrated and ``Entellan
neu'' polymer (Merck Company) was used to mount
this organic matter on glass slides.
Color has three attributes: luminance, hue and
saturation. It is very dicult to physically measure all
three attributes of color, especially with a microspectrophotometer. In this study, the dierence in pollen
brightness as measured using a transmitted-light microscope, was utilized to indicate color change. The brightness
of pollen was measured as follows. Using an Olympus
BHS-323 microscope at 400 (an objective lens of 40
and an eye piece lens of 10) with a 100 W halogen bulb at
a color temperature of 3000±3100 K, images of individual
slide-mounted pollen-grains were obtained with an Ikegami
IF-8500 camera and displayed on a TV monitor. These
images were then transferred to a Nippon Avionics TV
IP-4100 image processor (Fig. 1).
When fossil pollen contained a spherical opaque
inclusion, most likely pyrite, it was excluded from the
measured area of that pollen grain. In the case of bisaccate
132
Y. Ujiie / Organic Geochemistry 32 (2001) 127±141
Fig. 6. Three origins of pollen in a single mudstone sample: (a) indigenous pollen, (b) reworked pollen from older sediments and (c)
contaminated pollen from cavings or drilling mud.
pollen, dierent parts of a single pollen grain had different values of brightness because of dierent thickness
of the walls, thus only sites on the saccus were measured
(Fig. 2). The selection of these images was done on the
IP-4100 image processor.
The analog images of the site of the pollen to be
measured were then converted to digital data by an
image processor connected to a NEC PC-9801RA computer with Ratoc System Engineering Image Command
4198 software. The full image size is 480 pixels in height
and 512 pixels in width, where one pixel corresponds to
0.36 0.36 mm in actual size at 400. This system can
distinguish 256 stages of brightness ranging from 0 at
the darkest to 255 at the lightest. The brightness of the
halogen illuminator in the absence of any slide was set
by adjusting the diaphragm of the illuminator light such
that the 43rd gray level corresponded to the illuminator
light o state, and the 223rd to the illuminator light on
state after focusing. Checking up the brightness of the
illuminator was done both just before and just after
measurements of samples.
One brightness measurement of a pollen grain can
provide the minimum and the maximum value, the
mode and its frequency, the mean value, the standard
deviation, the standard error and the sum total (Fig. 3).
The replicability of the measurement results was
checked by duplicate measurements of about 30 pollen
grains in one sample.
3. Results and discussion
3.1. Scale of thermal alteration index by means of pollen
brightness
Fig. 7. Example of interpretation of a histogram of the
mode from various pollen grains from a single sample:
``reworked''=reworked pollen derived from older sediments,
``contamination''=contaminated pollen from cavings and/or
drilling mud, and ``stTAI ''=statistical thermal alteration
index.
It has been proven by microscopical observation of
over 400 Neogene mudstone samples, that the pollen
which are most widely distributed stratigraphically and
geographically around Japan, and are most easily identi®ed
by virtue of their speci®c shape, are bisaccate pollen of
Pinus, Podocarpus, Abies and Picea, and monosaccate
pollen of Tsuga. Each of these genera of pollen have the
same range of brightness at speci®c maturities as measured
by RO (UjiieÂ, 1996b).
The histogram of brightness of a single pollen grain
from a sediment has a broad distribution (Fig. 3), however,
that of a living pollen grain does not have a narrow
distribution (Fig. 4; UjiieÂ, 1996a). The range of brightness
Y. Ujiie / Organic Geochemistry 32 (2001) 127±141
of the former is about 1.6 times as broad as that of the
latter.
Among the eight parameters listed above for gray
scale levels, namely the minimum and the maximum
133
value, the mode and its frequency, the mean value, the
standard deviation, the standard error and the sum total
of brightness of one pollen grain, the parameter least
in¯uenced by pollen grain size, contaminating inclusions,
Fig. 8. Relationship between statistical thermal alteration index (stTAI) and vitrinite re¯ectance (RO). Crossed bars indicate 95%
con®dence limits of stTAI and RO values.
134
Y. Ujiie / Organic Geochemistry 32 (2001) 127±141
Fig. 9. Histogram showing brightness of living pollen, Pinus
thunbergii. ``stTAI ''=statistical thermal alteration index.
Fig. 10. Histogram showing brightness of pollen from surface
sediments in the Sea of Japan. An arrow with double heads
indicates a range of indigenous pollen. ``stTAI ''=statistical
thermal alteration index.
partial alterations and deformations, is the mode. The
minimum, the maximum and the mean value are likely
to be aected by alterations and deformation of small
parts of pollen grain and minute contaminating inclusions.
The sum total of brightness, or the integration of the
histogram, depends on the size of a pollen grain.
Therefore, the mode of brightness was used as a maturity
indicator for thermal alteration in this study.
Fig. 5 shows the results of brightness measurements
of pollen grains and RO of 30 mudstone samples. These
histograms are grouped into classes of ®ve stages of
brightness. Almost every sample had a wide distribution
of brightness. Gutjahr (1966), Staplin (1969), Peters et al.
(1977) and Robert (1985) ®rst recognized that sediments
are contaminated by recycled organic materials derived
from older deposits, and/or by dierent-aged organic
materials from downhole cavings and drilling mud of
boreholes. The phenomena like this are observed in RO
measurements in sedimentary rocks (Robert, 1985). The
vitrinite with a higher re¯ectance than the average re¯ectance characteristic of the diagenesis of the sedimentary
series is assigned to be ``reworked'', and the vitrinite
with a lower re¯ectance to be ``cavings''. The statistical
rejection of ``reworked'' and ``cavings'' from the autochthonous vitrinite is impossible, so the determination
of these three origins of vitrinite was made on the basis
of the distribution pattern on the re¯ectance diagram of
each sample and the increasing trend of re¯ectance with
burial depth in the sedimentary series (Robert, 1985). In
this study, the determination of the origin of pollen was
treated in a similar way. Therefore, pollen with the
relatively lower brightness in a sample was assumed to
be ``reworked'', and pollen with relatively higher
brightness to be younger ``contamination'' (Fig. 6). The
remaining pollen were interpreted as autochthonous, or
indigenous. The reworked palynomorphs including pollen
and spores from older sediments, and younger contaminations from cavings and drilling mud have been
recognized in the Tertiary samples from the Japanese
boreholes (e.g. Atake, 1973; Shimazaki, 1983). The
brightness of the indigenous pollen grains should plot as
a normal distribution on the histogram. The value of
arithmetical mean of brightness of all these indigenous
pollen grains in a sample is called here ``the statistical
thermal alteration index'' (stTAI) for that sample
(Fig. 7).
The value of stTAI determined in 30 samples (Table 1)
shows an inverse relation to RO (Fig. 8). The maturation
pathway has two cusps at 0.5 and 0.6% RO. On the
maturation pathway lower than 0.5% RO the pathway
has a slightly broader linear trend with a lower gradient,
so RO is more sensitive than stTAI. The gradient of the
trend line between 0.5 and 0.6% RO is higher and thus
stTAI is more sensitive to maturation than RO. This
rapid change in stTAI at about 0.5±0.6% RO is indicative
of the generation of hydrocarbons from the breakdown
of the pollen wall (Yule et al., 1999). Pollen and spores
are similar in chemical composition to oil generating
material (type II) so that this rapid color change is an
ideal indicator for the major phase of oil generation
(Marshall, 1991). The trend line greater than 0.6% RO
has an intermediate gradient. This relationship was
recognized between the luminance of spore and RO
(Marshall 1991), and between red±green intensity of
spore color and RO (Yule et al. 1998). As RO is usually
used as a standard parameter for organic maturation,
this relationship indicates that stTAI is a useful indicator for maturation.
The highest measurable limit of stTAI is at a maturity
level of about 1.0% RO, because pollen above this
maturity level can not be identi®ed in the slides. The
lowest measurable limit of RO in organic maturation is
about 0.30% (Hirai, 1979). The stTAI value of living
Y. Ujiie / Organic Geochemistry 32 (2001) 127±141
pollen of Pinus thunbergii is 204 (Fig. 9; UjiieÂ, 1996a), so
an stTAI dierence between living pollen and fossilized
pollen (160±135) of about 0.30% RO (Fig. 7) is more
than 50. The stTAI values of the surface sediments from
the Sea of Japan are 182±189 (Fig. 10), namely intermediate values between living pollen and fossilized pollen
of about 0.30% RO (UjiieÂ, 1998). Therefore, stTAI
values can be a new indicator for very early stages of
organic maturation beyond the lowest limit of RO measurement.
3.2. Application to petroleum exploration
The new parameter, stTAI, established above, was
applied to core and cuttings samples from three
135
boreholes and the index of petroleum generation in
these boreholes.
The value of RO at the threshold of intense oil generation is in general 0.5% (Tissot and Welte, 1984).
Therefore, the value of stTAI at this threshold was
determined to be 145±110, as estimated from the relationship in Fig. 8, considering 95% con®dence limit of
RO and stTAI values.
The values of stTAI were determined for the 16 samples
from the MITI Yuri-Oki-Chubu borehole (Fig. 11). The
values decrease sub-linearly with increasing depth from
156 at 200 m to 50 at 4800 m. The threshold zone of
intense oil generation as estimated from stTAI is determined to be at 1100±3000 m in depth. The RO values of
the MITI Yuri-Oki-Chubu borehole (Japan National
Fig. 11. Depth plots of statistical thermal alteration index (stTAI) and vitrinite re¯ectance (RO) in the MITI Yuri-Oki-Chubu borehole. A horizontal bar in the stTAI diagram indicates 95% con®dence limits. The RO data were obtained from the Japan National Oil
Corporation (1993b).
136
Y. Ujiie / Organic Geochemistry 32 (2001) 127±141
Oil Corporation, 1993b) show an increasing trend with
depth from around 0.3% at 200 m to 0.81% at 3900 m
(Fig. 11). The depth at which the RO value crosses 0.5%
of the threshold of intense oil generation is indicated as
2800 m. There are six samples outside the maturation
pathway in a RO vs. stTAI diagram (Fig. 12). Among
them, three samples plotted under the maturation pathway, have RO values from 0.42 to 0.45%, and the other
three samples plotted above the maturation pathway have
RO values higher than 0.69%.
Fig. 12. Relationship between statistical thermal alteration index (stTAI) and vitrinite re¯ectance (RO) in the MITI Yuri-Oki-Chubu
borehole. Crossed bars indicate 95% con®dence limits of stTAI and RO values.
Y. Ujiie / Organic Geochemistry 32 (2001) 127±141
The values of stTAI were determined for seven
samples from the MITI Honjo-Oki borehole (Fig. 13).
Those values decrease with increasing depth from
around 160 at 500 m to 70 at 4000 m. The threshold
zone of intense oil generation as estimated from stTAI is
determined to be at about 1000±3000 m in depth. The
RO values of the MITI Honjo-Oki borehole (Japan
National Oil Corporation, 1994) also show an increasing
trend with depth from around 0.3% at 700 m to 0.76%
137
at 4500 m (Fig. 13). The depth at which the RO value
crosses 0.5% of the threshold of intense oil generation is
indicated as 2400 m. There is only one sample outside
the maturation pathway in a RO vs. stTAI diagram
(Fig. 14).
The values of stTAI were determined for the nine
samples from the MITI Shin-Takenomachi borehole
(Fig. 15). Those values do not show a decreasing trend
from 500 to 2000 m but then have a decreasing trend
Fig. 13. Depth plots of statistical thermal alteration index (stTAI) and vitrinite re¯ectance (RO) in the MITI Honjo-Oki borehole. A
horizontal bar in the stTAI diagram indicates 95% con®dence limits. The RO data were from the Japan National Oil Corporation
(1994).
138
Y. Ujiie / Organic Geochemistry 32 (2001) 127±141
with increasing depth from about 150 at 2000 m to 90 at
4256 m. The threshold zone of intense oil generation as
estimated from stTAI is determined to be at 2900±3900
m. The RO values of the MITI Shin-Takenomachi
borehole samples (Japan National Oil Corporation,
1993a) show an overall increase with increasing depth
albeit with local reversals (Fig. 15). The depth at which
the RO value crosses 0.5% of the threshold of intense oil
generation is indicated as 3300 m. All ®ve samples,
except for one sample which plotted just under the
maturation pathway, are located on the maturation
pathway (Fig. 16).
Fig. 14. Relationship between statistical thermal alteration index (stTAI) and vitrinite re¯ectance (RO) in the MITI Honjo-Oki borehole. Crossed bars indicate 95% con®dence limits of stTAI and RO values.
Y. Ujiie / Organic Geochemistry 32 (2001) 127±141
Although RO can indicate the threshold of intense oil
generation with only one value (0.5%), stTAI can indicate the threshold with a range of values (110±140) as
shown in Fig. 8. The actual depths of the threshold of
intense oil generation, estimated from RO, could be
determined at just one value, namely at 2800 m in the
MITI Yuri-Oki-Chubu borehole, at 2400 m in the MITI
Honjo-Oki borehole and at 3300 m in the MITI ShinTakenomachi borehole. However, the actual depths of
the threshold zone of intense oil generation estimated
139
from stTAI were estimated to be 1100±3000 m in the
MITI Yuri-Oki-Chubu borehole, 1000±3000 m in the
MITI Honjo-Oki borehole and 2900±3900 m in the MITI
Shin-Takenomachi borehole. The depths of the threshold
of intense oil generation estimated from RO were in the
range of depth of the threshold zone estimated from
stTAI in these three boreholes. Therefore, it is concluded that stTAI, like RO, can be a useful parameter
for determining organic maturation and for identifying
oil generation in source rocks.
Fig. 15. Depth plots of statistical thermal alteration index (stTAI) and vitrinite re¯ectance (RO) in the MITI Shin-Takenomachi
borehole. A horizontal bar in stTAI diagram indicates 95% con®dence limits. The RO data were from the Japan National Oil Corporation (1993a).
140
Y. Ujiie / Organic Geochemistry 32 (2001) 127±141
Fig. 16. Relationship between statistical thermal alteration index (stTAI) and vitrinite re¯ectance (RO) in the MITI Shin-Takenomachi borehole. Crossed bars indicate 95% con®dence limits of stTAI and RO values.
4. Conclusions
This paper has described a novel method for the measurement of the brightness of pollen as organic maturity by
means of a computer-driven image processor. The mean
value of the mode in brightness for the complete array of
the indigenous pollen in a rock sample was called ``the
statistical thermal alteration index'' (stTAI). The value of
stTAI showed an inverse relation to RO in 30 standard
samples and was determined to be 110±145 at the threshold zone of intense oil generation as estimated from their
relationship. The application of stTAI to samples from
three boreholes supported these observations. Therefore,
stTAI, like RO, can be a useful parameter for determining
organic maturation and for identifying the threshold zone
of intense oil generation in petroleum source rocks.
Further work to more fully establish stTAI should
resolve problems concerning the uniformity of mechanical
Y. Ujiie / Organic Geochemistry 32 (2001) 127±141
performance of a microscope, a TV camera and an
image processor and should be applied to more samples
other than those from Japanese Neogene sediments.
Acknowledgements
The author is indebted to A. Hirai and H. Kurita for
the suggestion to improve the procedure for the isolation
of organic matter from mudstone. He appreciates comments by R. Ishiwatari and S. Nakashima. He is grateful
to the Ministry of International Trade and Industry of
Japan, Japan National Oil Corporation, Japan Petroleum
Exploration Company Ltd., Teikoku Oil Company
Ltd., Idemitsu Oil Development Company Ltd. for
providing samples and data, and permission to publish
this paper. This research was supported by grant-in-aid
for General Scienti®c Research (No. 05640543) from the
Ministry of Education, Science and Culture of Japan. J.
Marshall, W. Pickel and L. Schwark are thanked for
constructive and useful reviews.
Associate EditorÐL. Schwark
References
Akiba, F., Kato, S., Sato, T., Sato, M., 1992. Exploration data of
the MITI boreholes. In: Japan Natural Gas Association (Ed.),
Petroleum and Natural Gas Resources in Japan (revised edition).
Japan Natural Gas Association and Japan Oshore Petroleum
Development Association, Tokyo, pp. 467±499 (In Japanese).
Atake, M., 1973. Basic survey of MITI boreholes. In: Japanese
Association for Petroleum Technology (Ed.), Petroleum Mining
Industries and Technology in Japan. Japanese Association for
Petroleum Technology, Tokyo, pp. 128±176 (in Japanese).
Burgess, J.D., 1974. Microscopic examination of kerogen (dispersed organic matter) in petroleum exploration. In: Dutcher,
R.R., Hacquebard, P.A., Schopf, J.M., Simon, J.A. (Eds.),
Carbonaceous Materials as Indicators of Metamorphism
(Geological Society of America, Special Paper, No. 153).
Geological Society of America, CO, pp. 19±30.
Correia, M., 1967. Relations possibles entre l'eÂat de conserbation
des eÂleÂments ®gureÂs de la matieÂre organique (microfossiles palynoplanctologiques) et l'existence de gisemints d'hydrocarbures.
Revue de L'institut FrancËais du PeÂtrole 22, 1285±1306.
Gray, J., Boucot, A.J., 1975. Color changes in pollen and spores: a
review. Geological Society of America Bulletin 86, 1019±1033.
Gutjahr, C.C.M., 1966. Carbonization measurements of pollengrains and spores and their application. Leidse Geologische
Mededelingen 38, 1±29.
Hirai, A., 1979. Vitrinite re¯ectance. Journal of Japanese Association for Petroleum Technology 44, 190±195 (in Japanese).
Hunt, J.M., 1979. Petroleum geochemistry and geology. W. H.
Freeman and Company, San Francisco (pp. 321-327).
Japan National Oil Corporation, 1993a. Data Book of the
MITI Shin-Takenomachi Borehole. Japan National Oil
Corporation, Tokyo (in Japanese).
Japan National Oil Corporation, 1993b. Data Book of the
MITI Yuri-Oki-Chubu Borehole. Japan National Oil Corporation, Tokyo (in Japanese).
141
Japan National Oil Corporation, 1994. Data Book of the MITI
Honjo-Oki Borehole. Japan National Oil Corporation,
Tokyo (in Japanese).
Lo, H.B., 1988. Photometric methods for measuring the thermal maturity on strew-mounted kerogen slides. Organic
Geochemistry 12, 303±307.
Marshall, J.E.A., 1991. Quantitative spore colour. Journal of
the Geological Society, London 148, 223±233.
Peters, K.E., Ishiwatari, R., Kaplan, I.R., 1977. Color of
kerogen as index of organic maturity. American Association
of Petroleum Geologists Bulletin 61, 504±510.
Robert, P., 1985. Organic Metamorphism and Geothermal
History. D. Reidel Publishing Company, Dordrecht, The
Netherlands pp. 108±111.
Schopf, J.M., 1948. Variable coali®cation: the processes
involved in coal formation. Economic Geology 43, 207±225.
Shimazaki, T., 1983. Paleontological survey of pollen and
spores. In: Japanese Association for Petroleum Technology
(Ed.), A Handbook of Petroleum Mining Industries in
Japan. Japanese Association for Petroleum Technology,
Tokyo, pp. 157±161 (in Japanese).
Shimazaki, T., 1986. Method of visual kerogen analysis for
petroleum exploration and its application. In: Taguchi Taikan Kinenkai (Ed.), Contribution to Petroleum Geoscience.
Taguchi Taikan Kinenkai, Sendai, Japan, pp. 269±302 (in
Japanese with English abstract).
Staplin, F.L., 1969. Sedimentary organic matter, organic metamorphism, and oil and gas occurrence. Bulletin of Canadian
Petroleum Geology 17, 47±66.
Staplin, F.L., 1977. Interpretation of thermal history from color of
particulate organic matter Ð a review. Palynology 1, 9±18.
Taguchi, K., 1978. Organic maturation Ð a review, with the special
reference to its signi®cance in the inorganic process of diagenesis
and incipient metamorphism. Memoir of Geological Society of
Japan 15, 165±190 (in Japanese with English abstract).
Tissot, B., Welte, D.H., 1984. Petroleum Formation and
Occurrence, 2nd Ed. Springer-Verlag, Berlin, pp. 515±547.
UjiieÂ, Y., 1996a. Diagenetic change of Pinus pollen and its application of evaluation of organic maturation (in Japanese with
English abstract). Researches in Organic Geochemistry 11, 1±4.
UjiieÂ, Y., 1996b. A new quantitative parameter for thermal
alteration Ð statistical thermal alteration index. Journal of
Japanese Association for Petroleum Technology 61, 434±442
(in Japanese with English abstract).
UjiieÂ, Y., 1998. Organic maturation of recent marine sediments
evaluated by stTAI method. Researches in Organic Geochemistry 13, 1±4 (in Japanese with English abstract).
Van Gijzel, P., Robinson, C.R., Smith, M.A., Bissada, K.K.,
Lerche, I., Liu, J., 1992. Thermal history modeling of the
Gerges Bank, USA: thermal inversion of transmittance color
index (tci) and vitrinite re¯ectance (vr) data. Applied Geochemistry 7, 135±143.
Wilson, L.R., 1961. Palynological fossil response to low-grade
metamorphism in the Arkoma Basin. Tulsa Geological
Society Digest 29, 131±140.
Yule, B., Carr, A.D., Marshall, J.E.A., Roberts, S., 1999. Spore
transmittance (% St): a quantitative method for spore colour analysis. Organic Geochemistry 30, 567±581.
Yule, B., Roberts, S., Marshall, J.E.A., Milton, J.A., 1998.
Quantitative spore colour measurement using colour image
analysis. Organic Geochemistry 28, 139±149.
www.elsevier.nl/locate/orggeochem
Brightness of pollen as an indicator of thermal alteration
by means of a computer-driven image processor:
statistical thermal alteration index (stTAI)
Yoshihiro Ujiie *
Department of Earth and Environmental Sciences, Faculty of Science and Technology, Hirosaki University, Hirosaki 036-8561, Japan
Received 11 May 1999; accepted 20 September 2000
(returned to author for revision 11 November 1999)
Abstract
The brightness, or gray level, of pollen of Pinus, Podocarpus, Abies, Picea and Tsuga from the Neogene sediments in
northern Japan was measured using a transmitted-light microscope with a computer-driven digital image processor.
The mean value of the modes for the complete array of the indigenous pollen in a rock sample was called here ``the
statistical thermal alteration index'' (stTAI). In the present investigation an inverse relationship between stTAI and
vitrinite re¯ectance (RO) in sediments was found. The application of stTAI to samples from three boreholes indicated a
decreasing trend with depth. By using this trend, threshold values of intense oil generation can be evaluated (145±110).
Therefore, stTAI can be a useful parameter for determining organic maturation and for identifying the threshold zone
of intense oil generation. # 2001 Elsevier Science Ltd. All rights reserved.
Keywords: Thermal alteration index (TAI); Statistical thermal alteration index (stTAI); Brightness of pollen; Vitrinite re¯ectance;
Japanese Neogene; Organic maturation; Oil generation
1. Introduction
Organic matter in sediments changes diagenetically
during burial. These changes are re¯ected by progressive
alteration of physical characteristics such as color,
re¯ectance, ¯uorescence properties, etc. Measurements
of these characteristics of organic matter, especially
palynomorphs, are widely used techniques for the
assessment of coal rank and the extent of petroleum
generation.
The color variations of pollen and spores in coal beds
resulting from diagenesis have been recognized since the
1920s (Gutjahr, 1966). Staplin (1969) ®rst used the
thermal alteration index (TAI), the variation in color of
organic material as measured under a microscope, to
determine the relative opacity of organic matter. Now
* Tel.: +81-172-39-3952; fax: +81-172-39-3952.
E-mail address: [email protected] u.ac.jp
TAI is widely used to measure organic maturity in
sedimentary rocks, especially in petroleum source
rocks.
There are two microscopic methods to measure TAI
or organic maturity of palynomorphs, especially pollen
and spores. One is to distinguish their morphology and
color with transmitted light using the operator's own
eyes (Schopf, 1948; Wilson, 1961; Correia, 1967; Staplin,
1969; Burgess, 1974; Gray and Boucot, 1975; Peters et
al., 1977; Shimazaki, 1986). All these studies have color
scales from 7 to 23 points. This method has the merits of
simplicity and economy (Hunt, 1979; Tissot and Welte,
1984; Akiba et al., 1992). It also has the disadvantages
that its scales are subjective and qualitative and operatordependant because it is very dicult to visually distinguish
subtle changes in color of organic material under a
microscope. The results of this method are hard to
apply to interlaboratory comparison (Taguchi, 1978;
Robert, 1985; Shimazaki, 1986).
The other method devised for overcoming these disadvantages is to determine the translucency of organic
0146-6380/01/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved.
PII: S0146-6380(00)00146-7
Y. Ujiie / Organic Geochemistry 32 (2001) 127±141
128
matter by photoelectric measurements. Gutjahr (1966)
measured the translucency, or light absorption, of pollen
grains and spores with a photocell attached to a
transmitted-light microscope. Lo (1988) measured the
transmittance of palynomorphs on strew-mounted
kerogen slides at a wavelength of 546 nm and converted
the values to an equivalent TAI scale. Marshall (1991)
applied the Commission Internationale de l'Eclairage
(CIE) color system to the measurement of spore color.
He measured the transmittance of spores in the range of
400±750 nm with a microspectrophotometer. Van Gijzel
et al. (1992) measured the transmittance colour index
(TCI) on amorphous organic material and applied TCI
and RO to a one-dimension basin analysis model. Yule
et al. (1998), using a color video camera attached to a
microscope and linked to an image analyzer, presented
the colour image analysis (CIA) as a system for the
quanti®cation of spore color. Yule et al. (1999) recorded
spore color as the amount of light transmitted at dierent
wavelengths in the visible spectrum using a spectral
scanning microphotometer. Staplin (1977) and Robert
(1985) summarized studies regarding color changes in
organic matters in sediments with diagenesis.
This study is aimed at developing an objective and
quantitative TAI scale with simple measuring equipment that can be used as a common laboratory tool. The
brightness of bisaccate pollen of Pinus, Podocarpus, Abies
and Picea, and monosaccate pollen of Tsuga from the
Neogene sediments in northern Japan was measured using
a transmitted-light microscope, by means of a computerdriven digital image processor. The mean value of
modes in brightness of the indigenous pollen in each rock
sample was called here ``the statistical thermal alteration
index'' (stTAI) as an organic maturation indicator.
2. Experimental
2.1. Samples
The brightness of pollen in 30 Neogene mudstone
samples from northern Japan were measured as standard
Table 1
Vitrinite re¯ectance (RO) and statisticalc thermal alteration index (stTAI) data of mudstone samples used in making a stTAI scale
stTAI
RO (%)
(95% con®dence limit)
Core/cuttings/outcrop
Formation/group
Epoch
Locality
0.290.03
0.290.02
0.300.01
0.310.01
0.340.04
0.340.04
0.370.01
0.380.01
0.400.03
0.440.04
0.450.02
0.450.02
0.460.03
0.500.02
0.500.01
0.530.03
0.530.02
0.540.03
0.560.02
0.570.02
0.600.04
0.620.01
0.630.02
0.640.02
0.680.02
0.710.02
0.780.02
0.850.04
0.880.06
0.900.02
Cuttings
Cuttings
Outcrop
Outcrop
Cuttings
Cuttings
Cuttings
Outcrop
Cuttings
Cuttings
Core
Cuttings
Core
Core
Cuttings
Core
Cuttings
Cuttings
Core
Cuttings
Cuttings
Core
Core
Core
Cuttings
Core
Core
Cuttings
Cuttings
Core
Shibikawa F.
Shibikawa F.
Taiaki F.
Taiaki F.
Sasaoka F.
Uonuma G.
Upper Tentokuji F.
Akaishi F.
Lower Tentokuji F.
Funakawa F.
Lower Tentokuji F.
Lower Tentokuji F.
Nishiyama F.
Lower Teradomari F.
Funakawa F.
Wakkanai G.
Funakawa F.
Funakawa F.
Lower Teradomari F.
Nishiyama F.
Onnagawa F.
Ishikari G.
Nishikurosawa F.
Lower Teradomari F.
``Green Tu F.''
Ishikari G.
Nanbayama F.
Nishikurosawa F.
``Green Tu F.''
Hiuchiyama F.
Pleistocene
Pleistocene
Miocene
Miocene
Pleistocene
Pleistocene
Pleistocene
Miocene
Pliocene
Pliocene
Miocene
Pliocene
Pleistocene
Miocene
Pliocene
Miocene
Miocene
Miocene
Miocene
Pliocene
Miocene
Eocene
Miocene
Miocene
Miocene
Eocene
Miocene
Miocene
Miocene
Miocene
Akita-shi, Akita Pref.
Akita-shi, Akita Pref.
Nakatsugaru-gun, Aomori Pref.
Nakatsugaru-gun, Aomori Pref.
Akita-shi, Akita Pref.
Nishikanbara-gun, Niigata Pref.
Honjo-shi, Akita Pref.
Nakatsugaru-gun, Aomori Pref.
Akita-shi, Akita Pref.
Akita-shi, Akita Pref.
Nakakubiki-gun, Niigata Pref.
Honjo-shi, Akita Pref.
Nishikanbara-gun, Niigata Pref.
Nakakubiki-gun, Niigata Pref.
Honjo-shi, Akita Pref.
Wakkanai-shi, Hokkaido
Akita-shi, Akita Pref.
Honjo-shi, Akita Pref.
Nakakubiki-gun, Niigata Pref.
Nishikanbara-gun, Niigata Pref.
Akita-shi, Akita Pref.
Wakkanai-shi, Hokkaido
Honjo-shi, Akita Pref.
Nakakubiki-gun, Niigata Pref.
Honjo-shi, Akita Pref.
Wakkanai-shi, Hokkaido
Nakakubiki-gun, Niigata Pref.
Akita-shi, Akita Pref.
Akita-shi, Akita Pref.
Nakakubiki-gun, Niigata Pref.
1502.9
1571.4
1392.5
1473.3
1514.7
1442.3
1412.7
1365.4
1343.1
1363.1
1342.1
1442.8
1433.1
1353.4
1422.2
1243.2
1111.9
1094.1
1123.2
952.4
871.8
841.7
844.0
813.4
725.4
722.9
624.7
562.6
502.4
440.0
Y. Ujiie / Organic Geochemistry 32 (2001) 127±141
Fig. 1. Schematic ¯ow chart for the measurement of brightness.
Fig. 2. Site preference of brightness measurement in bisaccate
pollen (Pinus, Podocarpus, Abies and Picea).
129
calibration samples. Except for three outcrop samples
they were all cores and cuttings samples from six boreholes.
The RO values ranged from 0.29 to 0.90% (Table 1).
Three series of samples from the MITI Yuri-OkiChubu borehole, the MITI Honjo-Oki borehole and the
MITI Shin-Takenomachi borehole were measured for
their brightness in order to study its change with depth
for the application to petroleum exploration.
The MITI Yuri-Oki-Chubu borehole was drilled to a
depth of 5000 m through an interval of Pleistocene to
the lower Miocene sediments at N39 360 41.84000 ,
E139 560 43.76600 in the Sea of Japan o the west coast
of Honshu Island. There is only one sedimentary gap
of some 700,000 years at the Pleistocene±Pliocene
boundary at 1492 m depth (Japan National Oil
Corporation, 1993b). The 16 samples from this borehole
were all cuttings.
The MITI Honjo-Oki borehole was drilled to a depth
of 4800 m through a Pleistocene to the lower Miocene
interval at N39 250 10.26400 , E139 510 31.94400 in the Sea
of Japan o the west coast of Honshu Island. This
sedimentation was continuous during deposition (Japan
National Oil Corporation, 1994). Six cuttings samples
and one core sample (3628.25 m deep) were measured.
The MITI Shin-Takenomachi borehole was drilled to
a depth of 6310 m through a Pleistocene to the lower
Miocene interval at N37 460 05.11300 , E138 520 43.28600 in
the Niigata Prefecture. These sediments also continuously deposited without any gaps (Japan National
Fig. 3. Example of a brightness distribution in a single measurement of pollen. Minimum=76, maximum=223, mode=168, frequency at the mode=531, mean=162, standard deviation=23.9, standard error=0.15 and sum total=4,095,787.
Fig. 4. Example of a brightness distribution in a single measurement of a living pollen, Pinus thunbergii. Minimum=133, maximum=223, mode=196, frequency at the mode=471, mean=190, standard deviation=13.4, standard error=0.12 and sum
total=2,469,280.
130
Y. Ujiie / Organic Geochemistry 32 (2001) 127±141
Oil Corporation, 1993a). Seven were cuttings samples
and two were core samples (3000.3 and 4255.9 m deep).
The former two boreholes are located in the Akita
basin and the latter borehole in the Niigata basin.
Although oil and gas are being recovered from the
Neogene sediments in these basins, neither economic oil
nor gas was found in these three boreholes.
2.2. Measurements
Crushed mudstone samples (from 0.3 to 5 mm in
diameter) were treated with 15% hydrochloric acid
overnight at room temperature and with 46% hydro¯uoric
acid in a 70 C water-bath for 6 h to remove carbonates
and silicates. Residual organic matter, ranging from 32
Fig. 5. Histograms showing brightness of pollen at dierent maturity (RO). An arrow with double heads indicates a range of indigenous pollen. ``stTAI ''=statistical thermal alteration index.
Y. Ujiie / Organic Geochemistry 32 (2001) 127±141
131
Fig. 5. (continued)
to 100 mm in diameter, was concentrated and ``Entellan
neu'' polymer (Merck Company) was used to mount
this organic matter on glass slides.
Color has three attributes: luminance, hue and
saturation. It is very dicult to physically measure all
three attributes of color, especially with a microspectrophotometer. In this study, the dierence in pollen
brightness as measured using a transmitted-light microscope, was utilized to indicate color change. The brightness
of pollen was measured as follows. Using an Olympus
BHS-323 microscope at 400 (an objective lens of 40
and an eye piece lens of 10) with a 100 W halogen bulb at
a color temperature of 3000±3100 K, images of individual
slide-mounted pollen-grains were obtained with an Ikegami
IF-8500 camera and displayed on a TV monitor. These
images were then transferred to a Nippon Avionics TV
IP-4100 image processor (Fig. 1).
When fossil pollen contained a spherical opaque
inclusion, most likely pyrite, it was excluded from the
measured area of that pollen grain. In the case of bisaccate
132
Y. Ujiie / Organic Geochemistry 32 (2001) 127±141
Fig. 6. Three origins of pollen in a single mudstone sample: (a) indigenous pollen, (b) reworked pollen from older sediments and (c)
contaminated pollen from cavings or drilling mud.
pollen, dierent parts of a single pollen grain had different values of brightness because of dierent thickness
of the walls, thus only sites on the saccus were measured
(Fig. 2). The selection of these images was done on the
IP-4100 image processor.
The analog images of the site of the pollen to be
measured were then converted to digital data by an
image processor connected to a NEC PC-9801RA computer with Ratoc System Engineering Image Command
4198 software. The full image size is 480 pixels in height
and 512 pixels in width, where one pixel corresponds to
0.36 0.36 mm in actual size at 400. This system can
distinguish 256 stages of brightness ranging from 0 at
the darkest to 255 at the lightest. The brightness of the
halogen illuminator in the absence of any slide was set
by adjusting the diaphragm of the illuminator light such
that the 43rd gray level corresponded to the illuminator
light o state, and the 223rd to the illuminator light on
state after focusing. Checking up the brightness of the
illuminator was done both just before and just after
measurements of samples.
One brightness measurement of a pollen grain can
provide the minimum and the maximum value, the
mode and its frequency, the mean value, the standard
deviation, the standard error and the sum total (Fig. 3).
The replicability of the measurement results was
checked by duplicate measurements of about 30 pollen
grains in one sample.
3. Results and discussion
3.1. Scale of thermal alteration index by means of pollen
brightness
Fig. 7. Example of interpretation of a histogram of the
mode from various pollen grains from a single sample:
``reworked''=reworked pollen derived from older sediments,
``contamination''=contaminated pollen from cavings and/or
drilling mud, and ``stTAI ''=statistical thermal alteration
index.
It has been proven by microscopical observation of
over 400 Neogene mudstone samples, that the pollen
which are most widely distributed stratigraphically and
geographically around Japan, and are most easily identi®ed
by virtue of their speci®c shape, are bisaccate pollen of
Pinus, Podocarpus, Abies and Picea, and monosaccate
pollen of Tsuga. Each of these genera of pollen have the
same range of brightness at speci®c maturities as measured
by RO (UjiieÂ, 1996b).
The histogram of brightness of a single pollen grain
from a sediment has a broad distribution (Fig. 3), however,
that of a living pollen grain does not have a narrow
distribution (Fig. 4; UjiieÂ, 1996a). The range of brightness
Y. Ujiie / Organic Geochemistry 32 (2001) 127±141
of the former is about 1.6 times as broad as that of the
latter.
Among the eight parameters listed above for gray
scale levels, namely the minimum and the maximum
133
value, the mode and its frequency, the mean value, the
standard deviation, the standard error and the sum total
of brightness of one pollen grain, the parameter least
in¯uenced by pollen grain size, contaminating inclusions,
Fig. 8. Relationship between statistical thermal alteration index (stTAI) and vitrinite re¯ectance (RO). Crossed bars indicate 95%
con®dence limits of stTAI and RO values.
134
Y. Ujiie / Organic Geochemistry 32 (2001) 127±141
Fig. 9. Histogram showing brightness of living pollen, Pinus
thunbergii. ``stTAI ''=statistical thermal alteration index.
Fig. 10. Histogram showing brightness of pollen from surface
sediments in the Sea of Japan. An arrow with double heads
indicates a range of indigenous pollen. ``stTAI ''=statistical
thermal alteration index.
partial alterations and deformations, is the mode. The
minimum, the maximum and the mean value are likely
to be aected by alterations and deformation of small
parts of pollen grain and minute contaminating inclusions.
The sum total of brightness, or the integration of the
histogram, depends on the size of a pollen grain.
Therefore, the mode of brightness was used as a maturity
indicator for thermal alteration in this study.
Fig. 5 shows the results of brightness measurements
of pollen grains and RO of 30 mudstone samples. These
histograms are grouped into classes of ®ve stages of
brightness. Almost every sample had a wide distribution
of brightness. Gutjahr (1966), Staplin (1969), Peters et al.
(1977) and Robert (1985) ®rst recognized that sediments
are contaminated by recycled organic materials derived
from older deposits, and/or by dierent-aged organic
materials from downhole cavings and drilling mud of
boreholes. The phenomena like this are observed in RO
measurements in sedimentary rocks (Robert, 1985). The
vitrinite with a higher re¯ectance than the average re¯ectance characteristic of the diagenesis of the sedimentary
series is assigned to be ``reworked'', and the vitrinite
with a lower re¯ectance to be ``cavings''. The statistical
rejection of ``reworked'' and ``cavings'' from the autochthonous vitrinite is impossible, so the determination
of these three origins of vitrinite was made on the basis
of the distribution pattern on the re¯ectance diagram of
each sample and the increasing trend of re¯ectance with
burial depth in the sedimentary series (Robert, 1985). In
this study, the determination of the origin of pollen was
treated in a similar way. Therefore, pollen with the
relatively lower brightness in a sample was assumed to
be ``reworked'', and pollen with relatively higher
brightness to be younger ``contamination'' (Fig. 6). The
remaining pollen were interpreted as autochthonous, or
indigenous. The reworked palynomorphs including pollen
and spores from older sediments, and younger contaminations from cavings and drilling mud have been
recognized in the Tertiary samples from the Japanese
boreholes (e.g. Atake, 1973; Shimazaki, 1983). The
brightness of the indigenous pollen grains should plot as
a normal distribution on the histogram. The value of
arithmetical mean of brightness of all these indigenous
pollen grains in a sample is called here ``the statistical
thermal alteration index'' (stTAI) for that sample
(Fig. 7).
The value of stTAI determined in 30 samples (Table 1)
shows an inverse relation to RO (Fig. 8). The maturation
pathway has two cusps at 0.5 and 0.6% RO. On the
maturation pathway lower than 0.5% RO the pathway
has a slightly broader linear trend with a lower gradient,
so RO is more sensitive than stTAI. The gradient of the
trend line between 0.5 and 0.6% RO is higher and thus
stTAI is more sensitive to maturation than RO. This
rapid change in stTAI at about 0.5±0.6% RO is indicative
of the generation of hydrocarbons from the breakdown
of the pollen wall (Yule et al., 1999). Pollen and spores
are similar in chemical composition to oil generating
material (type II) so that this rapid color change is an
ideal indicator for the major phase of oil generation
(Marshall, 1991). The trend line greater than 0.6% RO
has an intermediate gradient. This relationship was
recognized between the luminance of spore and RO
(Marshall 1991), and between red±green intensity of
spore color and RO (Yule et al. 1998). As RO is usually
used as a standard parameter for organic maturation,
this relationship indicates that stTAI is a useful indicator for maturation.
The highest measurable limit of stTAI is at a maturity
level of about 1.0% RO, because pollen above this
maturity level can not be identi®ed in the slides. The
lowest measurable limit of RO in organic maturation is
about 0.30% (Hirai, 1979). The stTAI value of living
Y. Ujiie / Organic Geochemistry 32 (2001) 127±141
pollen of Pinus thunbergii is 204 (Fig. 9; UjiieÂ, 1996a), so
an stTAI dierence between living pollen and fossilized
pollen (160±135) of about 0.30% RO (Fig. 7) is more
than 50. The stTAI values of the surface sediments from
the Sea of Japan are 182±189 (Fig. 10), namely intermediate values between living pollen and fossilized pollen
of about 0.30% RO (UjiieÂ, 1998). Therefore, stTAI
values can be a new indicator for very early stages of
organic maturation beyond the lowest limit of RO measurement.
3.2. Application to petroleum exploration
The new parameter, stTAI, established above, was
applied to core and cuttings samples from three
135
boreholes and the index of petroleum generation in
these boreholes.
The value of RO at the threshold of intense oil generation is in general 0.5% (Tissot and Welte, 1984).
Therefore, the value of stTAI at this threshold was
determined to be 145±110, as estimated from the relationship in Fig. 8, considering 95% con®dence limit of
RO and stTAI values.
The values of stTAI were determined for the 16 samples
from the MITI Yuri-Oki-Chubu borehole (Fig. 11). The
values decrease sub-linearly with increasing depth from
156 at 200 m to 50 at 4800 m. The threshold zone of
intense oil generation as estimated from stTAI is determined to be at 1100±3000 m in depth. The RO values of
the MITI Yuri-Oki-Chubu borehole (Japan National
Fig. 11. Depth plots of statistical thermal alteration index (stTAI) and vitrinite re¯ectance (RO) in the MITI Yuri-Oki-Chubu borehole. A horizontal bar in the stTAI diagram indicates 95% con®dence limits. The RO data were obtained from the Japan National Oil
Corporation (1993b).
136
Y. Ujiie / Organic Geochemistry 32 (2001) 127±141
Oil Corporation, 1993b) show an increasing trend with
depth from around 0.3% at 200 m to 0.81% at 3900 m
(Fig. 11). The depth at which the RO value crosses 0.5%
of the threshold of intense oil generation is indicated as
2800 m. There are six samples outside the maturation
pathway in a RO vs. stTAI diagram (Fig. 12). Among
them, three samples plotted under the maturation pathway, have RO values from 0.42 to 0.45%, and the other
three samples plotted above the maturation pathway have
RO values higher than 0.69%.
Fig. 12. Relationship between statistical thermal alteration index (stTAI) and vitrinite re¯ectance (RO) in the MITI Yuri-Oki-Chubu
borehole. Crossed bars indicate 95% con®dence limits of stTAI and RO values.
Y. Ujiie / Organic Geochemistry 32 (2001) 127±141
The values of stTAI were determined for seven
samples from the MITI Honjo-Oki borehole (Fig. 13).
Those values decrease with increasing depth from
around 160 at 500 m to 70 at 4000 m. The threshold
zone of intense oil generation as estimated from stTAI is
determined to be at about 1000±3000 m in depth. The
RO values of the MITI Honjo-Oki borehole (Japan
National Oil Corporation, 1994) also show an increasing
trend with depth from around 0.3% at 700 m to 0.76%
137
at 4500 m (Fig. 13). The depth at which the RO value
crosses 0.5% of the threshold of intense oil generation is
indicated as 2400 m. There is only one sample outside
the maturation pathway in a RO vs. stTAI diagram
(Fig. 14).
The values of stTAI were determined for the nine
samples from the MITI Shin-Takenomachi borehole
(Fig. 15). Those values do not show a decreasing trend
from 500 to 2000 m but then have a decreasing trend
Fig. 13. Depth plots of statistical thermal alteration index (stTAI) and vitrinite re¯ectance (RO) in the MITI Honjo-Oki borehole. A
horizontal bar in the stTAI diagram indicates 95% con®dence limits. The RO data were from the Japan National Oil Corporation
(1994).
138
Y. Ujiie / Organic Geochemistry 32 (2001) 127±141
with increasing depth from about 150 at 2000 m to 90 at
4256 m. The threshold zone of intense oil generation as
estimated from stTAI is determined to be at 2900±3900
m. The RO values of the MITI Shin-Takenomachi
borehole samples (Japan National Oil Corporation,
1993a) show an overall increase with increasing depth
albeit with local reversals (Fig. 15). The depth at which
the RO value crosses 0.5% of the threshold of intense oil
generation is indicated as 3300 m. All ®ve samples,
except for one sample which plotted just under the
maturation pathway, are located on the maturation
pathway (Fig. 16).
Fig. 14. Relationship between statistical thermal alteration index (stTAI) and vitrinite re¯ectance (RO) in the MITI Honjo-Oki borehole. Crossed bars indicate 95% con®dence limits of stTAI and RO values.
Y. Ujiie / Organic Geochemistry 32 (2001) 127±141
Although RO can indicate the threshold of intense oil
generation with only one value (0.5%), stTAI can indicate the threshold with a range of values (110±140) as
shown in Fig. 8. The actual depths of the threshold of
intense oil generation, estimated from RO, could be
determined at just one value, namely at 2800 m in the
MITI Yuri-Oki-Chubu borehole, at 2400 m in the MITI
Honjo-Oki borehole and at 3300 m in the MITI ShinTakenomachi borehole. However, the actual depths of
the threshold zone of intense oil generation estimated
139
from stTAI were estimated to be 1100±3000 m in the
MITI Yuri-Oki-Chubu borehole, 1000±3000 m in the
MITI Honjo-Oki borehole and 2900±3900 m in the MITI
Shin-Takenomachi borehole. The depths of the threshold
of intense oil generation estimated from RO were in the
range of depth of the threshold zone estimated from
stTAI in these three boreholes. Therefore, it is concluded that stTAI, like RO, can be a useful parameter
for determining organic maturation and for identifying
oil generation in source rocks.
Fig. 15. Depth plots of statistical thermal alteration index (stTAI) and vitrinite re¯ectance (RO) in the MITI Shin-Takenomachi
borehole. A horizontal bar in stTAI diagram indicates 95% con®dence limits. The RO data were from the Japan National Oil Corporation (1993a).
140
Y. Ujiie / Organic Geochemistry 32 (2001) 127±141
Fig. 16. Relationship between statistical thermal alteration index (stTAI) and vitrinite re¯ectance (RO) in the MITI Shin-Takenomachi borehole. Crossed bars indicate 95% con®dence limits of stTAI and RO values.
4. Conclusions
This paper has described a novel method for the measurement of the brightness of pollen as organic maturity by
means of a computer-driven image processor. The mean
value of the mode in brightness for the complete array of
the indigenous pollen in a rock sample was called ``the
statistical thermal alteration index'' (stTAI). The value of
stTAI showed an inverse relation to RO in 30 standard
samples and was determined to be 110±145 at the threshold zone of intense oil generation as estimated from their
relationship. The application of stTAI to samples from
three boreholes supported these observations. Therefore,
stTAI, like RO, can be a useful parameter for determining
organic maturation and for identifying the threshold zone
of intense oil generation in petroleum source rocks.
Further work to more fully establish stTAI should
resolve problems concerning the uniformity of mechanical
Y. Ujiie / Organic Geochemistry 32 (2001) 127±141
performance of a microscope, a TV camera and an
image processor and should be applied to more samples
other than those from Japanese Neogene sediments.
Acknowledgements
The author is indebted to A. Hirai and H. Kurita for
the suggestion to improve the procedure for the isolation
of organic matter from mudstone. He appreciates comments by R. Ishiwatari and S. Nakashima. He is grateful
to the Ministry of International Trade and Industry of
Japan, Japan National Oil Corporation, Japan Petroleum
Exploration Company Ltd., Teikoku Oil Company
Ltd., Idemitsu Oil Development Company Ltd. for
providing samples and data, and permission to publish
this paper. This research was supported by grant-in-aid
for General Scienti®c Research (No. 05640543) from the
Ministry of Education, Science and Culture of Japan. J.
Marshall, W. Pickel and L. Schwark are thanked for
constructive and useful reviews.
Associate EditorÐL. Schwark
References
Akiba, F., Kato, S., Sato, T., Sato, M., 1992. Exploration data of
the MITI boreholes. In: Japan Natural Gas Association (Ed.),
Petroleum and Natural Gas Resources in Japan (revised edition).
Japan Natural Gas Association and Japan Oshore Petroleum
Development Association, Tokyo, pp. 467±499 (In Japanese).
Atake, M., 1973. Basic survey of MITI boreholes. In: Japanese
Association for Petroleum Technology (Ed.), Petroleum Mining
Industries and Technology in Japan. Japanese Association for
Petroleum Technology, Tokyo, pp. 128±176 (in Japanese).
Burgess, J.D., 1974. Microscopic examination of kerogen (dispersed organic matter) in petroleum exploration. In: Dutcher,
R.R., Hacquebard, P.A., Schopf, J.M., Simon, J.A. (Eds.),
Carbonaceous Materials as Indicators of Metamorphism
(Geological Society of America, Special Paper, No. 153).
Geological Society of America, CO, pp. 19±30.
Correia, M., 1967. Relations possibles entre l'eÂat de conserbation
des eÂleÂments ®gureÂs de la matieÂre organique (microfossiles palynoplanctologiques) et l'existence de gisemints d'hydrocarbures.
Revue de L'institut FrancËais du PeÂtrole 22, 1285±1306.
Gray, J., Boucot, A.J., 1975. Color changes in pollen and spores: a
review. Geological Society of America Bulletin 86, 1019±1033.
Gutjahr, C.C.M., 1966. Carbonization measurements of pollengrains and spores and their application. Leidse Geologische
Mededelingen 38, 1±29.
Hirai, A., 1979. Vitrinite re¯ectance. Journal of Japanese Association for Petroleum Technology 44, 190±195 (in Japanese).
Hunt, J.M., 1979. Petroleum geochemistry and geology. W. H.
Freeman and Company, San Francisco (pp. 321-327).
Japan National Oil Corporation, 1993a. Data Book of the
MITI Shin-Takenomachi Borehole. Japan National Oil
Corporation, Tokyo (in Japanese).
Japan National Oil Corporation, 1993b. Data Book of the
MITI Yuri-Oki-Chubu Borehole. Japan National Oil Corporation, Tokyo (in Japanese).
141
Japan National Oil Corporation, 1994. Data Book of the MITI
Honjo-Oki Borehole. Japan National Oil Corporation,
Tokyo (in Japanese).
Lo, H.B., 1988. Photometric methods for measuring the thermal maturity on strew-mounted kerogen slides. Organic
Geochemistry 12, 303±307.
Marshall, J.E.A., 1991. Quantitative spore colour. Journal of
the Geological Society, London 148, 223±233.
Peters, K.E., Ishiwatari, R., Kaplan, I.R., 1977. Color of
kerogen as index of organic maturity. American Association
of Petroleum Geologists Bulletin 61, 504±510.
Robert, P., 1985. Organic Metamorphism and Geothermal
History. D. Reidel Publishing Company, Dordrecht, The
Netherlands pp. 108±111.
Schopf, J.M., 1948. Variable coali®cation: the processes
involved in coal formation. Economic Geology 43, 207±225.
Shimazaki, T., 1983. Paleontological survey of pollen and
spores. In: Japanese Association for Petroleum Technology
(Ed.), A Handbook of Petroleum Mining Industries in
Japan. Japanese Association for Petroleum Technology,
Tokyo, pp. 157±161 (in Japanese).
Shimazaki, T., 1986. Method of visual kerogen analysis for
petroleum exploration and its application. In: Taguchi Taikan Kinenkai (Ed.), Contribution to Petroleum Geoscience.
Taguchi Taikan Kinenkai, Sendai, Japan, pp. 269±302 (in
Japanese with English abstract).
Staplin, F.L., 1969. Sedimentary organic matter, organic metamorphism, and oil and gas occurrence. Bulletin of Canadian
Petroleum Geology 17, 47±66.
Staplin, F.L., 1977. Interpretation of thermal history from color of
particulate organic matter Ð a review. Palynology 1, 9±18.
Taguchi, K., 1978. Organic maturation Ð a review, with the special
reference to its signi®cance in the inorganic process of diagenesis
and incipient metamorphism. Memoir of Geological Society of
Japan 15, 165±190 (in Japanese with English abstract).
Tissot, B., Welte, D.H., 1984. Petroleum Formation and
Occurrence, 2nd Ed. Springer-Verlag, Berlin, pp. 515±547.
UjiieÂ, Y., 1996a. Diagenetic change of Pinus pollen and its application of evaluation of organic maturation (in Japanese with
English abstract). Researches in Organic Geochemistry 11, 1±4.
UjiieÂ, Y., 1996b. A new quantitative parameter for thermal
alteration Ð statistical thermal alteration index. Journal of
Japanese Association for Petroleum Technology 61, 434±442
(in Japanese with English abstract).
UjiieÂ, Y., 1998. Organic maturation of recent marine sediments
evaluated by stTAI method. Researches in Organic Geochemistry 13, 1±4 (in Japanese with English abstract).
Van Gijzel, P., Robinson, C.R., Smith, M.A., Bissada, K.K.,
Lerche, I., Liu, J., 1992. Thermal history modeling of the
Gerges Bank, USA: thermal inversion of transmittance color
index (tci) and vitrinite re¯ectance (vr) data. Applied Geochemistry 7, 135±143.
Wilson, L.R., 1961. Palynological fossil response to low-grade
metamorphism in the Arkoma Basin. Tulsa Geological
Society Digest 29, 131±140.
Yule, B., Carr, A.D., Marshall, J.E.A., Roberts, S., 1999. Spore
transmittance (% St): a quantitative method for spore colour analysis. Organic Geochemistry 30, 567±581.
Yule, B., Roberts, S., Marshall, J.E.A., Milton, J.A., 1998.
Quantitative spore colour measurement using colour image
analysis. Organic Geochemistry 28, 139±149.