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 ofPinus, Podocarpus, Abies, PiceaandTsugafrom 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
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 operator-dependant 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 dis-advantages is to determine the translucency of organic 0146-6380/01/$ - see front matter#2001 Elsevier Science Ltd. All rights reserved.
P I I : S 0 1 4 6 - 6 3 8 0 ( 0 0 ) 0 0 1 4 6 - 7
www.elsevier.nl/locate/orggeochem
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 andROto 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 equip-ment that can be used as a common laboratory tool. The brightness of bisaccate pollen ofPinus, 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 computer-driven 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
RO(%) stTAI Core/cuttings/outcrop Formation/group Epoch Locality (95% con®dence limit)
0.290.03 1502.9 Cuttings Shibikawa F. Pleistocene Akita-shi, Akita Pref. 0.290.02 1571.4 Cuttings Shibikawa F. Pleistocene Akita-shi, Akita Pref.
0.300.01 1392.5 Outcrop Taiaki F. Miocene Nakatsugaru-gun, Aomori Pref. 0.310.01 1473.3 Outcrop Taiaki F. Miocene Nakatsugaru-gun, Aomori Pref. 0.340.04 1514.7 Cuttings Sasaoka F. Pleistocene Akita-shi, Akita Pref.
0.340.04 1442.3 Cuttings Uonuma G. Pleistocene Nishikanbara-gun, Niigata Pref. 0.370.01 1412.7 Cuttings Upper Tentokuji F. Pleistocene Honjo-shi, Akita Pref. 0.380.01 1365.4 Outcrop Akaishi F. Miocene Nakatsugaru-gun, Aomori Pref. 0.400.03 1343.1 Cuttings Lower Tentokuji F. Pliocene Akita-shi, Akita Pref.
0.440.04 1363.1 Cuttings Funakawa F. Pliocene Akita-shi, Akita Pref. 0.450.02 1342.1 Core Lower Tentokuji F. Miocene Nakakubiki-gun, Niigata Pref. 0.450.02 1442.8 Cuttings Lower Tentokuji F. Pliocene Honjo-shi, Akita Pref. 0.460.03 1433.1 Core Nishiyama F. Pleistocene Nishikanbara-gun, Niigata Pref. 0.500.02 1353.4 Core Lower Teradomari F. Miocene Nakakubiki-gun, Niigata Pref. 0.500.01 1422.2 Cuttings Funakawa F. Pliocene Honjo-shi, Akita Pref. 0.530.03 1243.2 Core Wakkanai G. Miocene Wakkanai-shi, Hokkaido 0.530.02 1111.9 Cuttings Funakawa F. Miocene Akita-shi, Akita Pref. 0.540.03 1094.1 Cuttings Funakawa F. Miocene Honjo-shi, Akita Pref. 0.560.02 1123.2 Core Lower Teradomari F. Miocene Nakakubiki-gun, Niigata Pref. 0.570.02 952.4 Cuttings Nishiyama F. Pliocene Nishikanbara-gun, Niigata Pref. 0.600.04 871.8 Cuttings Onnagawa F. Miocene Akita-shi, Akita Pref.
0.620.01 841.7 Core Ishikari G. Eocene Wakkanai-shi, Hokkaido
0.630.02 844.0 Core Nishikurosawa F. Miocene Honjo-shi, Akita Pref. 0.640.02 813.4 Core Lower Teradomari F. Miocene Nakakubiki-gun, Niigata Pref. 0.680.02 725.4 Cuttings ``Green Tu F.'' Miocene Honjo-shi, Akita Pref.
0.710.02 722.9 Core Ishikari G. Eocene Wakkanai-shi, Hokkaido
calibration samples. Except for three outcrop samples they were all cores and cuttings samples from six boreholes. TheROvalues ranged from 0.29 to 0.90% (Table 1).
Three series of samples from the MITI Yuri-Oki-Chubu 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 N3936041.84000, E13956043.76600 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 N3925010.26400, E13951031.94400 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 N3746005.11300, E13852043.28600in the Niigata Prefecture. These sediments also con-tinuously deposited without any gaps (Japan National Fig. 1. Schematic ¯ow chart for the measurement of brightness.
Fig. 2. Site preference of brightness measurement in bisaccate pollen (Pinus, Podocarpus, AbiesandPicea).
Fig. 3. Example of a brightness distribution in a single measurement of pollen. Minimum=76, maximum=223, mode=168, fre-quency at the mode=531, mean=162, standard deviation=23.9, standard error=0.15 and sum total=4,095,787.
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 70C water-bath for 6 h to remove carbonates and silicates. Residual organic matter, ranging from 32
to 100mm 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 microspectro-photometer. In this study, the dierence in pollen brightness as measured using a transmitted-light micro-scope, was utilized to indicate color change. The brightness of pollen was measured as follows. Using an Olympus
BHS-323 microscope at400 (an objective lens of40 and an eye piece lens of10) 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).
pollen, dierent parts of a single pollen grain had dif-ferent 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 com-puter 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 0.360.36mm in actual size at400. 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
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 andPicea, and monosaccate pollen ofTsuga. Each of these genera of pollen have the same range of brightness at speci®c maturities as measured byRO(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 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.
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
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,
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 andROof 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 inRO
measurements in sedimentary rocks (Robert, 1985). The vitrinite with a higher re¯ectance than the average re¯ec-tance 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 auto-chthonous 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 con-taminations 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 toRO(Fig. 8). The maturation
pathway has two cusps at 0.5 and 0.6% RO. On the
maturation pathway lower than 0.5%ROthe pathway
has a slightly broader linear trend with a lower gradient, soROis more sensitive than stTAI. The gradient of the
trend line between 0.5 and 0.6% ROis higher and thus
stTAI is more sensitive to maturation than RO. This
rapid change in stTAI at about 0.5±0.6%ROis 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 andRO(Yule et al. 1998). AsROis usually
used as a standard parameter for organic maturation, this relationship indicates that stTAI is a useful indi-cator 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 ROin organic maturation is
about 0.30% (Hirai, 1979). The stTAI value of living Fig. 9. Histogram showing brightness of living pollen,Pinus
thunbergii.``stTAI ''=statistical thermal alteration index.
pollen ofPinus thunbergiiis 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 inter-mediate 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 ofRO
mea-surement.
3.2. Application to petroleum exploration
The new parameter, stTAI, established above, was applied to core and cuttings samples from three
boreholes and the index of petroleum generation in these boreholes.
The value of ROat the threshold of intense oil
gen-eration 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 rela-tionship in Fig. 8, considering 95% con®dence limit of
ROand 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 deter-mined to be at 1100±3000 m in depth. The ROvalues of
the MITI Yuri-Oki-Chubu borehole (Japan National
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 theROvalue crosses 0.5%
of the threshold of intense oil generation is indicated as 2800 m. There are six samples outside the maturation
pathway in aROvs. stTAI diagram (Fig. 12). Among
them, three samples plotted under the maturation path-way, haveROvalues from 0.42 to 0.45%, and the other
three samples plotted above the maturation pathway have
ROvalues higher than 0.69%.
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%
at 4500 m (Fig. 13). The depth at which theROvalue
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
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 theROvalue 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).
AlthoughROcan indicate the threshold of intense oil
generation with only one value (0.5%), stTAI can indi-cate 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 Shin-Takenomachi borehole. However, the actual depths of the threshold zone of intense oil generation estimated
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 fromROwere in the
range of depth of the threshold zone estimated from stTAI in these three boreholes. Therefore, it is con-cluded that stTAI, likeRO, can be a useful parameter
for determining organic maturation and for identifying oil generation in source rocks.
4. Conclusions
This paper has described a novel method for the mea-surement 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 toROin 30 standard
samples and was determined to be 110±145 at the thresh-old zone of intense oil generation as estimated from their relationship. The application of stTAI to samples from three boreholes supported these observations. Therefore, stTAI, likeRO, can be a useful parameter for determining
organic maturation and for identifying the threshold zone of intense oil generation in petroleum source rocks.
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 com-ments 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
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