Online ISSN : 2186-490X
Print ISSN : 1346-4272
ISSN-L : 1346-4272
Volume 52 , Issue 10
Bulletin of the Geological Survey of Japan
Showing 1-2 articles out of 2 articles from the selected issue
  • Shunichiro lGARI
    2001 Volume 52 Issue 10 Pages 445-469
    Published: December 25, 2001
    Released: March 21, 2015
    Light hydrocarbons from several natural gas fields in Japan were measured, including neopentane which has been rarely measured. Logarithms of ethane/propane and neopentane/isopentane, ethane/propane and neopentane/isobutane, and neopentane/isopentane and neopentane/isobutane ratios show good straight line correlations, respectively. These correlations can be explained by decomposition of these hydrocarbons due to hydrogen abstraction. It is concluded that the decomposition process of the hydrocarbons is the major determining factor for the hydrocarbon composition of natural gases. Effect of rock types on the fractionation of light hydrocarbons in natural gases during migration through rocks was investigated. For this purpose the retention times of hydrocarbons on gas chromatographic columns packed with various rock and mineral samples were measured. Hydrocarbons used for this experiment were CH4, C2H6, C3H8’iso-C4H10, n-C4H10, iso-C5H12 and n-C5H12・Thefractionation occurred with expandable clay minerals or zeolites. The order of retention times depends on the species of clay minerals or zeolites. When montmorillonite, halloysite, vermiculite and mordenite were used, the order was CH4くC2H6<C3H8くiso-C4H10<n-C4H10くiso-C5H12くn-C5H12・Whencli noptilolite was used, the order was CH4 < iso-C4H10 < C2H6くiso-C5H12< C3H8 < n-C4H10・Thefrac tionation became larger with aging of the mineral samples. When the samples were not aged, only a small fractionation was observed. These results show that the dehydrated state of the interlayer spaces of clay minerals or pore spaces of zeolites plays an important role in fractionation. Furthermore‘these results suggest that the large fractionation by clay minerals or zeolites does not occur under normal subsurface conditions where these minerals are hydrated. Carbon isotopic ratios of methane (δ13C(CH4)) were measured for dissolved-in-water type natural gases from several gas fields in Japan. Some samples have relatively low δ13C(CH4) values. This indicates that they are of biogenic origin as has been suggested by previous researchers. However, several samples from Fukushima, Shizuoka, Miyazaki and Kagoshima prefectures have relatively high δ13C(CH4) values, which cannot be of biogenic origin, but of thermogenic origin. This is the first evidence to confirm the occurrence of dissolved-in-water type gas of thermogenic origin in Japan. Carbon isotopic ratios of methane, ethane and propane (δ13C(CH4),δ13C(C2H6) and δ13C(C3Hg), respectively) were analyzed for oil field gases from Niigata and Akita in Japan. A strong correlation between δ13C( C2H6) and δ13C(C3H8) was observed. This relation is well explained by a kinetic theory proposed by Chung et al. (1988). On the other hand, only a weak correlation was observed between δ13C(CH4) and δ13C(C2H6). The weak correlation is inferred to be due to a mixing of biogenic gas that has low δ13C(CH4). Furthermore, the mixing ratios of thermogenic gas and biogenic gas can be calculated from the values of δ13C(CH4),δ13C( C2H6) and δ13C(C3H8). It is found that almost all the oil field gases in Akita and Niigata are mixtures of thermogenic and biogenic gases.
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  • Shunso ISHIHARA, Chengyu Wu
    2001 Volume 52 Issue 10 Pages 471-491
    Published: December 25, 2001
    Released: March 21, 2015
    Late Cretaceous to Paleogene rhyolites (11 samples) and granitoids (Shirakawa 29, Toki 7, Naegi 9, Ryoke 10; total 55 samples), which have high intial Sr ratios, were analyzed by XRF and ICP-MS for 11 major elements and 32 trace elements. Granitoids of the Shirakawa area intruding the Nohi Rhyolites in the Hida metamorphic terrane, belong to I-type magnetite series and contain commonly mafic enclaves. They are composed of a high Na2O group of monzodiorite to granodiorite and a low-Na2O group of biotite granites. The former is rich in mafic and calcic components, and was generated in mafic igneous source rocks of the Hida metamorphic terrane. The latter is felsic and leucogranitic, yet its Rb/Sr is not high enough to be a fractionated magma. The leucogranite, typically of the Hirase body, could be a minimum melt generated from an intermediate igneous source. The Shirakawa granitoids are depleted in Y and HREE, implying the existence of garnet and hornblende in the source region. Late Cretaceous-Paleogene granitoids occurring in the Mino sedimentary terrane belong to I -type ilmenite-series composed of high-level plutons of the Toki and Naegi areas and intermediate-level plutons of the Ryoke granitoids. These granitoids are reduced and per-aluminous implying genetic connection with sedimentary and less igenous sources. The Toki and Naegi bodies are composed mostly of biotite granite, intruding discordantly, and are rich in lithophile elements. The Naegi granite is especially high in Rb, Y, Th, and U and its Rb/Sr ratio is the highest among the studied rocks. Its REE pattern is flat with high HREE and a strong Eu anomaly, which could be considered as fractionated I type. The Ryoke granitoids are mostly biotite granite but horblende-biotite granodiorite is also common. They are less fractionated and have own geochemical characters compared with the Toki and Naegi granites. The studied granitoids have chemical compositions reflecting basically their basement characteristics, and also partly their degree of the magmatic fractionation.
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