The origin of gas and mud-volcano-like sediments in the Horonobe area of northern Hokkaido, Japan 1
1
Miyakawa K. , Tokiwa T. , Murakami H. 1
1
Horonobe Underground Research Unit, Japan Atomic Energy Agency, Hokushin 432-2, Horonobe-cho, Hokkaido 098-3224 Japan (
[email protected])
The understanding of groundwater and gas migration pathways is important with respect to the development of relevant techniques for geological disposal of high-level radioactive wastes (HLW). With respect to the utilization of underground facilities for geological disposal of HLW, consideration of onshore mud volcanoes (MV) is important because presence of a MV is possibly an indication of significant flow paths in the subsurface. It is important to reveal the depth of source material because this gives an indication of possible path lengths and transport mechanism. Therefore, studying the ejecta and determining the possible source is potentially important. In the Horonobe area, mud-volcano-like sediments have been reported by Sakai et al. (2010), and they reported the possibility of the occurrence of MV (the Kamihoronobe MV, KMV). This study was intended as an investigation of the origin of the mud and gas in the KMV. Accordingly, we determined the mineralogy and chemical composition of the mud and measured isotope and chemical compositions of the gas. wa et al., Figure 1 The Horonobe area is located on the eastern margin of a Neogene to Quaternary sedimentary basin in northwestern Hokkaido (the Tempoku basin). In the Tempoku basin, a deep drilling survey had been conducted to obtain information for petroleum exploration (the Tempoku drilling; JNOC, 1995). According to the report on the Tempoku drilling, the sediments in the basin consist of, with 50 N LEGEND increasing depth, the Koetoi Formation, the Wakkani F., Alluvium Anticline Sarabetsu Fm. the Masuporo F., the Magaribuchi F., the Haboro F., the Amurian Plate Syncline Horonobe Yuchi Fm. Hakobuchi F., and the Ezo F. The KMV is located on Fault Okhotsk Plate Koetoi Fm. Concealed fault Neogene sedimentary rocks of the Masuporo F. (Fig. 1), Wakkanai Fm. 40 N Masuporo Fm. and on the axis of the Matsuo anticline (Fig. 1) (Nagao, Soya coal-bearing Fm. 1960). Besides extruded mud, chloride-rich groundwater Pacific Plate Fault (or buried Fault) inferred from subsurface data (Cl ≈ 6900 mg/L) is also emanating from the KMV along 130 E 140 E 150 E 200 km with gas bubbles (Sakai et al., 2010). Six mud samples were collected from the surface Horonobe URL Kamihoronobe where the groundwater and the gas emanate for X-ray Mud Volcano (N45°2'3'' E141°56'50'') diffraction analysis (XRD) and X-ray fluorescence analysis (XRF). Gas samples were collected using a N water displacement method. Chemical compositions (He, Matsuo Anticline H2, Ar, N2, O2, CH4, CO2, C2H6, C3H8) and isotope ratios 13 13 3 4 20 22 40 36 (δ CCH4, δ CCO2, He/ He, Ne/ Ne, Ar/ Ar ratios) of gases were determined (Table 1). Fig. 1: (a) Location map of the study area. The gas samples are rich in methane (90.7 ± 1.2 (b) Geological map around the %v/v), and have a high concentration of He (39 ± 3 ppmv) Kamihoronobe MV. The geological map is 3 4 (Table 1a). Although the helium isotope ratio ( He/ He) of Direction of plate migration
Direction of plate migration
no Horo
be Fa
ult
Om
ari ag
u Fa
lt
modified from Nagao (1960).
1
4
20
3
40
36
the gas sample is close to that of air, the He/ Ne (2.96 × 10 ) and Ar/ Ar (511) ratios are significantly higher than those for air. We estimated the elapsed time which is needed for the value of 40 36 Ar/ Ar ratio in air-saturated water to become that of the KMV assuming that the groundwater and Hakobuchi F. in closed system. The result was of the same order of magnitude as the geologic time of Haboro or Hakobuchi Fs. These results indicate that the gas samples originated from depth in the subsurface. Table 1a. Elemental compositions of bubbling gas (n=6) emanating from the Kamihoronobe MV. He
H2
Ar
O2
N2
CH4
CO2
C2H6
C3H8
(ppmv)
(ppmv)
(ppmv)
(ppmv)
(%v/v)
(%v/v)
(%v/v)
(ppmv)
(ppmv)
39 ± 3
31 ± 31
62 ± 78
0.01 ± 0.05
4.5 ± 0.7
90.7 ± 1.2
3.4 ± 0.5
155 ± 5
nd
Note: Fluctuation is one standard deviation; nd = not detected.
Table 1b. Isotopic compositions of bubbling gas emanating from the Kamihoronobe MV. 4
He
20
Ne
40
Ar
(ppmv)
(ppmv)
(ppmv)
34.6
0.0117
55.0
3
4
He/ He -6
(10 )
4 20
22
40
Ne/ Ne
36
Ar/ Ar
20
He/ Ne 3
(10 )
1.065 ±
9.862 ±
511.13 ±
2.96 ±
0.023
0.024
2.13
0.33
13
13
δ CCH4
δ CCO2
(‰)
(‰)
- 41.0
7.6
TiO2 (%)
SiO2 (%)
To identify the origin of the mud, we compared mineral compositions of mud samples from the Figure 3 KMV with those of the core samples from the Tempoku drilling. The mud samples are rich in quartz. Other characteristic minerals in the mud samples are heulandite, plagioclase, and calcite. From the data from the report on Tempoku drilling, we can see that 100 : Wk heulandite was only detected in the Hakobuchi F., which is also : Mp : Hb 80 rich in quartz and plagioclase. Judging from the comparison, it can : Hk : MV be seen that mineral composition of the mud samples is almost 60 identical with that of Hakobuchi F. The chemical composition of the mud samples is also closer 40 to the chemical composition of the Hakobuchi F. than to the 20 overlying formations. Figure 2 shows comparisons of SiO2, Al2O3, 5 10 0 15 20 1.0 and TiO2 contents in the mud samples and in the core samples from the Tempoku drilling. There are excellent matches between the SiO2, Al2O3 and TiO2 contents of the mud samples with those of the Hakobuchi F. (Fig. 2). Therefore, we conclude that the origin of 0.5 the mud samples is the Hakobuchi F. Judging from the Tempoku drilling, a total thickness of the Magaribuchi F. and the Haboro F. is about 1500 m, and thickness 0.0 0 5 10 15 20 of the Masuporo F. is about 1000 m. Therefore, the mud samples Al O (%) had ascended to the surface from approximately 1500-2500 m deep. This study provides an example of an onshore MV that Fig. 2: Comparison of SiO2, Al2O3 releases thermogenic gas and mud from the deep sedimentary and TiO2 contents of the mud layers. samples and in core samples. 2
3
Japan National Oil Corporation (JNOC), 1995. Report on the MITI Tempoku drilling (in Japanese). Nagao S., 1960. Explanatory text of the geological map of Japan: scale 1:50,000: Toyotomi. Geological Survey of Hokkaido (in Japanese with English abstract). Sakai T., Ioka S., Ishijima, Y. and Ito N., 2010. The mud-volcano discovered in Horonobe district, northern part of Hokkaido, Japan. Chishitsu News 676, 63-67 (in Japanese).
2
