Characterization of organic matter in the Miocene turbidites and hemipelagic mudstones in the Niigata oil field, central Japan

Org. Geochem.Vol. 29, No. 1-3, pp. 605-611, 1998 Pergamon PII: S0146-6380(98)00082-5

@) 1998ElsevierScienceLtd. All rights reserved Printed in Great Britain 0146-6380/98/$- see front matter

Characterization of organic matter in the Miocene turbidites and hemipelagic mudstones in the Niigata oil field, central Japan HARUMI WATANABE* and MASAHIKO A K I Y A M A t Department of Geology, Shinshu University, 3-1-1 Asahi, Matsumoto 390-8621, Japan Abstract--The Teradomari Formation, a principal petroleum source rock in Japan, consists of turbidite

(Et) and hemipelagic (Ep) mudstones of a Bouma sequence at the type section in Niigata Prefecture, central Japan. Maceral composition of immature kerogen shows characteristic differences between Et and Ep mudstones. Et kerogen consists mainly of vitrinite and non-fluorescent amorphous organic matter (NFA) both of terrestrial origin, while Ep kerogen mainly consists of weakly fluorescent amorphous organic matter (WFA) of marine plankton origin. Elemental analysis, Rock-Eval pyrolysis, FTIR spectra and stable carbon isotope ratios support the differences in maceral composition between Et and Ep kerogens. The carbon isotopic ratios of WFA from Ep kerogen, separated by centrifugation specific gravity method, were almost the same as for bulk Ep kerogen. This supports the case for their origin from marine plankton indicated by maceral analysis. © 1998 Elsevier Science Ltd. All rights reserved

Key words--turbidite, hemipelagic mudstone, kerogen, maceral composition, Rock-Eval, FTIR, 613C, oil-source rock correlation, Niigata oil field, Teradomari Formation

INTRODUCTION

The Middle to Upper Miocene Teradomari Formation is one of the main petroleum source rocks in the Niigata oil field, central Japan (Kikuchi et al., 1991). This formation consists mainly of turbidite sediments. The uppermost unit consists of alternating layers of turbidite mudstones (Et) and hemipelagic mudstones (Ep). The turbidite mudstones are gray and silty, whereas the overlying hemipelagic mudstones are brownish clayey mud and as such easily distinguished in outcrop exposures. These two mudstone units have different maceral compositions and foraminiferal associations (Shimazaki, 1986). The purpose of this paper is to characterize and to examine the differences in chemical characteristics of kerogens in the turbidite (Et) and the hemipelagic (Ep) mudstones of the Teradomari Formation.

site descriptions have been published in Watanabe and Akiyama (1996) following the route map published by Miyashita and Mitsunashi (1974). Thirteen samples belong to the Et category (dark brown to brownish gray and bioturbated silty mudstone) and 11 belong to the Ep category (dark brown clayey mudstone), as shown in the stratigraphic column in Fig. 2.

Analytical procedures

Kerogen separation. Bitumen was extracted from mudstone samples crushed to less than 30 to 40 mesh using a Soxhlet extractor with a mixture of benzene and methanol (3:1 v/v). Carbonate and silicate minerals were dissolved by 5 M HC1 followed by a mixture solution of 10 M HC1 and 46% HF (1:1). Then framboidal pyrites were removed by a solution of NaBH4 (McIver, 1967). The remaining material was composed mostly of kerogen. Visual kerogen analysis. Kerogen samples were placed with a small amount of Entellan neu SAMPLES AND ANALYTICAL PROCEDURE (Merck) on a non-fluorescent slide glass, then covered with a micro cover glass (Hirai, 1980; Sawada Samples and Akiyama, 1994). Maceral composition of keroTwenty-four mudstone samples were collected gen was determined by counting 500 points at each from the type section of the Teradomari Formation 100 #m interval under a reflected light fluorescence along the Okozu diversion channel, Niigata microscope (Olympus BX50, with a mercury lamp, Prefecture (Fig. 1). Sampling locations and some 330-385nm of excitation filter, and 420nm of absorption filter). Maceral composition shown in *Present address: Institute for Hydrospheric-Atmospheric Table 1 is an average value of two measurements as Sciences, Nagoya University, Chikusa-ku, Nagoya 464- mentioned above. There are almost negligible differ01, Japan. tTo whom correspondence should be addressed. Tel.: ences between the two measurements. Classification +81-263-37-2487; Fax: +81-263-37-2506; E-mail: of macerals refers to Senftle et al. (1993). [email protected]. phous organic matter is divided into the following 605

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Fig. 1. Index map of the study area in Niigata Prefecture, Japan.

three categories by its fluorescent characteristics; non-fluorescent (NFA), weakly fluorescent (WFA) and rather strong fluorescent (FA) (Sawada and Akiyama, 1994; Akiyama et al., 1995; Hunt, 1995). FTIR measurements. A mixture of kerogen and powdered KBr (1:I) was measured by a diffusionreflection method using FTIR-300E (JASCO) in the 4000 to 650 cm -~ at a resolution of 4 cm -j. CentriJugation specific gravity separation. Each maceral was separated by centrifugation specific gravity method (Kinghorn and Rahman, 1983) for chemical characterization. Each specific gravity solution was adjusted with an amount of ZnBr2 in the following six stepwise divisions: < 1.20, 1.20-1.30, 1.30-1.33, 1.33-1.40, 1.40-1.53, 1.53-1.70 g/cm 3 (Sawada and Akiyama, 1994). Total organic" carbon (TOC). TOC of the powdered carbonate-free samples treated with 2 M HC1 was measured by a Y A N A C O CHN analyzer MT-

5, after drying at 100°C in vacuum for an hour. Elemental analysis of kerogen was conducted by the same procedure as the TOC analysis, using antipyrine (C:70.19%, 0:8.50%, N:14.88%, H:6.43%) as the standard. Rock-Eval analysis. 10-15 mg of kerogen pulverized to less than 200 mesh were used for analysis using a Rock-Eval plus 2 at the Technical Research Center, Japan Oil Corporation. Stable carbon isotope measurement. A few mg of each kerogen sample were heated in a furnace at 450°C for 30 min and then at 850°C for 3 h. After contaminating gases were removed, purified CO2 was analyzed for carbon isotope ratios on a Finnigan MAT 252 mass spectrometer at the Institute for Hydrospheric-Atmospheric Sciences, Nagoya University. The values of ,513C are presented on the PDB scale, using a working standard that has been compared to the PDB standard. The uncertainty in the 6~3C measurement is estimated to be +0.2%0.

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H. Watanabe and M. Akiyama Table 1. Analytical data of turbidite (Et) and hemipelagic (Ep) mudstones of the Teradomari Formation

Sample TOC No. Lithofacies (%)

HI

Ol

24 21 18 16 13 12 5 4 3 2 1 40 38 37 33 32 60 59 58 57 55 54 52 51

507 184 320 223 196 265 356 237 181 222 343 233 445 478 472 278 487 275 467 253 371 181

13.5 17.6 15.1 16.9 28.5 22.2 14.5 21.7 52.0 44.2 12.3 18.4 9.2 12.4 20.2 26.1 24.0 28.8 21.7 18.8 15.4 16.6

Ep Et Ep Et Et Et Ep Et Et Et Ep Ep Et Ep Ep Et Ep Et Ep Et Ep Et Ep Et

1.67 1.00 1.46 0.93 0.99 1.12 1.80 1.06 0.83 1.01 1.16 1.54 1.03 1.87 1.92 0.63 2.35 1.09 2.08 1.11 2.53 1.14 1.93 1.09

6 13C (%0)

FA (%)

WFA (%)

NFA (%)

-23.18 -24.58 -23.33 -25.28 -24.87 -25.05 -23.17 -25.20 -25.18 -25.05 -23.71 -22.92 -23.91 -22.69 -22.77 -24.43 -22.88 -24.32 -22.79 -24.36 -22.76 -24.40 -23.15 -24.40

4.4 0 2.7 0.1 2.7 0.8 5.4 3.0 1.6 2.0 5.3 4.3 0.6 2.6 3.9 0 1.7 0.5 2.2 2.3 0 1.5 2.7 0.6

90.2 6.3 87.8 2.2 3.8 6.7 75.2 6.6 3 0.8 72.3 63.4 8.5 83.2 89.1 3.2 84.9 0 84.3 0 91.5 0 84.9 3.6

0 62.9 1.6 72.1 81.9 60.4 1.2 29.5 14.4 14.7 1.3 5.0 38.8 7.5 0 45.9 0 54.0 0 46.1 0 32.9 0 28.6

Vitrinite Sporinite Alginite Cutinite Resinite Sclerotinite (%) (%) (%) (%) (%) (%) 1.5 27.2 3.8 24.5 7.2 29.2 14.6 58.9 79.2 74.3 15.9 22.4 48.4 3.5 0.6 47.4 5.8 35.8 6.0 43.0 2.2 61.3 4.8 60.1

1.8 3.3 1.3 1.1 3.4 2.6 2.7 1.8 1.6 3.1 4.3 3.6 2.7 1.7 1.9 2.9 5.8 7.0 3.7 5.5 2.2 3.7 4.8 3.6

2.0 0.1 2.8 0 1.0 0.1 0.8 0.1 0 0 0.8 1.3 0 1.4 4.4 0.3 1.7 2.7 3.0 2.3 3.7 0.7 2.7 3.6

0.1 0.2 0 0 0 0.2 0.1 0.1 0.2 0.1 0.1 0 0.9 0.1 0 0.1 0 0 0.8 0.8 0 0 0 0

0 0 0 0 0 0 0 0 0 0 0 0 0.1 0 0 0 0 0 0 0 0 0 0 0

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0.1 0.2 0 0 0 0 0 0 0 0

FA: fluorescent amorphous kerogen, WFA: weakly fluorescent amorphous kerogen, NFA: non-fluorescent amorphous kerogen, HI: hydrogen index (mg hydrocarbon/g TOC), OI: oxygen index (mg CO2/g TOC), TOC = total organic carbon. ANALYTICAL RESULTS AND DISCUSSION

Characteristics of bulk kerogens (Table 1 and Fig. 2) Visual kerogen. Kerogen maceral composition is completely different between turbidite (Et) and hemipelagic (Ep) mudstones of the Teradomari Formation. Et kerogen is composed of 25% to 80% vitrinite in general, 15% to 75% of N F A and 1% to 7% sporinite, which are considered to be of terrestrial origin. On the other hand, Ep kerogen consists of 63% to 90% of W F A which is considered to be of marine plankton origin (Sawada and Akiyama, 1994). TOC. Most Et kerogens contain less than 1% TOC while Ep kerogens have a higher concentration in the range 1.2% to 2.5%. Rock-Eval analysis. Hydrogen index (HI) and oxygen index (OI) are shown in Table 1. An HI vs OI diagram suggests that Ep and Et kerogens belong mostly to Type II and Type I I - I I I kerogens (Tissot and Welte, 1984; Espitali~ et al., 1985), respectively (Fig. 3(A)). Carbon isotope composition. Et kerogens show 613C values between -24%0 to -25%0, while Ep kerogens lie between -22%0 to -23%0. Terrestrial plants (C3) have 613C values between -25%0 to -28%0 and marine planktons between -19%o to -21%o (Nissenbaum et al., 1972; Newman et al., 1973; Hedges and Parker, 1976; Jasper and Gagosian, 1990; Whelan and Thompson-Rizer, 1993). Chung et al. (1992) stated that the carbon isotope value of Miocene marine origin oils is -23.5%0, showing heavier than that before 25 Ma due to decreased atmospheric CO2 concentration, whereas the oils of terrestrial plant origin show -25%0

regardless of geologic ages. This result is consistent with the isotopic data of Et and Ep kerogens in the Teradomari Formation. FTIR analysis. The main infrared absorption bands of interest are; at 2930 and 2860 cm -1 due to C - H stretching, at 1710 cm -I due to C--O stretching and at 1605 cm -~ due to C=C stretching of aromatic rings (Rouxhet et al., 1980; Landais and Rochdi, 1990). The relative intensities of the CH stretching and C--O stretching bands divided by the relative intensity of the C--C stretching band in their height of each IR spectrum were compared for each kerogen sample (Fig. 3(B)). The analysis suggests that the aliphatic hydrocarbon content of Ep kerogen is higher than that of Et kerogen. Ep kerogens are plotted mostly between Type I and II, whereas Et kerogens are widely dispersed around Type II on an IR maturation path diagram (Takemura and Akiyama, 1994; Akiyama et al., 1995).

Characteristics of each kerogen maceral Elemental composition (Table 2). As shown in Table 1, the kerogen samples in this study are composed of heterogeneous macerals. Maceral separation was conducted to examine the chemical characteristics of each maceral. W F A fractions were obtained from Ep kerogens in the specific gravity range 1.33-1.40 g/cm 3 in samples No. 5 and No. 7, and the range 1.40-1.53 g/cm 3 in sample No. 40. It is not clear whether these differences come from the original organic matter or from the heavy fine pyrite contamination. These fractions have relatively high H/C ratios in the range 1.1 to 1.3 and N/C ratios spanning 0.036 to 0.038. Vitrinite with or without N F A fractions (specific gravity > 1.40)

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Fig. 3. (A) Hydrogen index (nag hydrocarbon/g TOC) vs oxygen index (mg CO2/g TOC) and (B) IR absorption of C H vs C=O, TOC = total organic carbon. Refer to the captions in Fig. 1 of Et and Ep kerogens from the Teradomari Formation. have lower H/C values (0.9 to 1.0) and lower N/C values (0.030 to 0.036). Carbon isotope composition. W F A in the Ep mudstones have a carbon isotopic composition in the range -22.7 to -23.2%o. Since Ep kerogen is composed mainly of W F A organic matter, the bulk kerogen isotopic composition ranges from -22.8 to -23.4%0, this range corresponds to that of W F A values. A small amount of W F A in the Et mudstones shows almost the same value as in Ep kerogens (-23.5%o). Vitrinite fractions with N F A show lighter carbon isotopic values between -24.4 and -25.7%0, which also correspond to 613C values of Et bulk kerogens. (See Fig. 4)

Origin of Ep and Et kerogens In the Miocene Teradomari Formation, turbidite (Et) kerogen consists mainly of vitrinite and other organic matter of terrestrial origin (Type III kerogen) and hemipelagic (Ep) kerogen is dominated by W F A of marine plankton origin (Type I I - I I I ) (Sawada and Akiyama, 1994). Carbon isotope com-

position is consistent with maceral composition, since 613C values in Et and Ep kerogens support terrestrial and marine plankton in origin, respectively. High molecular weight n-alkanes, oleanane and C29 sterane dominate in the extracted bitumen fractions from the Et mudstones, though less amounts are present in the Ep of the Teradomari Formation (Fujita et al,, 1997). This evidence supports that Et mudstones contain more terrestrial organic matter than Ep, as was stated above. Characteristic differences between Et and Ep kerogens in maceral composition and some chemical properties analyzed in this study are summarized in Table 3. It is suggested that Ep kerogen has higher hydrocarbon generating potential than Et kerogen because the former contains more W F A organic matter. In general, the bulk carbon isotope composition of kerogen has often been available to the oil-source rock correlation (Omokawa, 1982, 1985; Hunt, 1995). Usually, petroleum source argillaceous rocks are analysed without being differentiated into

Table 2. Analytical data of macerals separated by centrifugation specific gravity separation (refer to the captions in Table 1 and Fig. 2) Sample No. 5(Ep) 4(Et) 4(Et) 3(Et) 40(Ep) 40(Ep) 38(Et) 38(Et) 38(Et) 37(Ep) 37(Ep) 37(Ep)

Organic matter composition

Specific gravity

H/C

N/C

613C (%0)

WFA sporinite + vitrinite vitrinite + NFA vitrinite + NFA sporinite + WFA WFA sporinite + vitrinite vitrinite WFA + vitrinite sporinite + WFA WFA WFA

1.37-1.40 1.40-1.53 1.53-1.70 1.40-1.85 1.33-1.40 1.40-1.53 1.40-1.43 1.43-I .53 1.53-1.70 1.30-1.33 1.33-1.37 1.37-1.40

1.14 0.96 0.93 0.94 1.45 1.12 1.18 1.00 0.99 1.42 1.14 1.28

0.037 0.023 0.032 0.031 0.026 0.036 0.043 0.036 0.034. 0.038 0.037 0.038

-23.38 -24.88 -25.44 -25.67 -23.32 -23.05 -24.4 -24.42 -23.55 -23.07 -22.84 -22.71

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6 130(96o) Fig. 4. Relation between H/C and 613C (%0) of several organic matter from Et and Ep kerogens of the Teradomari Formation. Ep and Et mudstones, then carbon isotope values of the bulk organic matter are given by the mixture of Ep and Et kerogens. The carbon isotope value of oils is slightly heavier than that of total kerogen in the mudstones, since the former value is mainly controlled by W F A organic matter of rather heavier carbon isotope value in Ep mudstones with less contribution of Et kerogen mainly of terrestrial origin with lighter carbon isotope value. Our study suggests that the investigation on carbon isotopic composition of W F A may be a better choice than isotopic values of the bulk kerogen for the oilsource rock correlation, since W F A is considered to be the main organic matter from which oil is generated.

CONCLUSION (1) Kerogens in turbidite (Et) and hemipelagic (Ep) mudstones in the Teradomari F o r m a t i o n are composed mainly of organic matter of terrestrial and marine plankton in origin, respectively, and have very different characteristics in various chemical parameters such as TOC, Rock-Eval, F T I R and stable carbon isotope composition.

(2) Carbon isotope composition supports that W F A organic matter is the main component of Ep kerogen of marine plankton origin and vitrinites including N F A are main components of Et kerogen and are of terrestrial origin. (3) Ep mudstones are considered more important as petroleum source rocks than Et mudstones in the Bouma sequence, and it is recommended to use carbon isotopic composition of W F A for oil-source rock correlation. Acknowledgements--We are grateful to Mr K. Yokoi and Ms K. Hatano of Technology Research Center, Japan National Oil Corporation for Rock-Eval analysis, and to Professor I. Kobayashi of Niigata University and Dr K. Hoyanagi of Shinshu University for their help at collecting the rock samples. Thanks are also due to Dr K. Ohta, Dr T. Hama and Dr K. Sawada of the Institute for Hydrospheric-Atmospheric Sciences for carbon isotope analysis and to Dr R. K. Kotra of U.S. Geological Survey, Reston for his critical reading of this manuscript. A part of this study was supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Science, Culture and Sports, Japan (No. 06453002). Our work has benefitted from the constructive comments of R. G. Schaefer, K. Chandra and W. Kalkreuth.

REFERENCES

Table 3. Characteristic differences between turbidite (Et) and hemipelagic (Ep) mudstones (refer to the captions in Table 1 and Fig. 2)

TOC (%) HI OI 613C (%0) Dominant organic matter

Et

Ep

0.6 l.l 180-280 17-52 -25 to -24 vitrinite+ NFA

1.2-2.5 220 510 12 44 -22 to -23 WFA

HI = hydrogen index (mg HC/g TOC). OI = oxygen index (mg CO2/g TOC).

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