Bulletin of the Geological Survey of Japan Top Page

Bulletin of the Geological Survey of Japan Vol.52 No.10 (2001)

Cover photograph | Table of Contents | Abstract

Cover photograph

Orbicular granite of the Mt. Sanage, Aichi Prefecture.

   An orbicular granite occurs in the Mt. Sanage of Toyota City, and designated as National Monument since 1931. The granite belongs to the most fractionated phase of the late Cretaceous ilmenite-series biotite granite of the Ryoke Youndder type, and its orbicular textures are caused by variation of the grain size and amounts of K-feldspar, quartz and biotite. Unlike the famed orbicular granite in the Mineyama, Mt. Tsukuba, of similar tectonic setting, no cordierite is observed in this locality.

(Shunso ISHIHARA)

Table of Contents

Title	Author	PDF
Article
Organic geochemical study of natural gases from major gas fields in Japan.	Shunichiro IGARI (445-470)	52_10_01.pdf [1,858 KB]
Genesis of Late Cretaceous-Paleogene Granitoids with Contrasting Chemical Trends in the Chubu District, Central Japan.	Shunso ISHIHARA and Chengyu Wu (471-491)	52_10_02.pdf [1,501 KB]

 

Abstract

Organic geochemical study of natural gases from major gas fields in Japan.

Shunichiro IGARI

   Light hydrocarbons from several natural gas fields in Japan wre measured, including neopentane which has been rarely measured. Logarithms of ethane/propane and neopentane/isopentane, athane/propene and eopentane/isobutene, and neopentane/isopentane andneopentane/isobutene rations 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 fraction 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 rocks and mineral samples were measured. Hydrocarbons used for this experiment were CH₄, C₂H₆, C₃H₈, iso-C₄H₁₀, n-C₄H₁₀, iso-C₅H₁₂ and n-C₅H₁₂. The fractionation became larger with wxpendable cly minerals or zeolites. The older of retention times depends on the species of clay minerals or zeolrtes. When montmorillonite, halloysite, vermiculite and mordenite were used, the older was CH₄ < C₂H₆ < C₃H₈ < iso-C₄H₁₀ < n-C₄H₁₀ < iso-C₅H₁₂ and n-C₅H₁₂. The fractionation 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 older was CH₄ < C₂H₆ < C₃H₈ < iso-C₄H₁₀ < n-C₄H₁₀ < iso-C₅H₁₂ < n-C₅H₁₂. When clinoptilolite was used, the older was CH₄ < iso-C₄H₁₀ < C₃H₆ < iso-C₅H₁₂ < C₃H₈ < n-C₄H₁₀. The fractionation became lager with aging Of the mineral samples. These results show that the dehydrated state of the interlayer spaces of clay minerals or zeolites plays an important role in fractionation. Furthermore, these results suggest that the large fractionation by clay minerals or zeolites dose not occur under normal subsurface conditions where these minerals are hydrated.
   Carbon isotopic rations 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 rations of methane, ethane and propane (δ¹³C(CH₄), δ¹³C(C₂H₆) and δ¹³C(C₃H₈),respectively) were analyzed for oil field gases from Niigata and Akita in Japan. A strong correlation between δ¹³C(C₂H₆) andδ¹³C(C₃H₈) was observed.This relation is well explained by a kinetic theory proposed by Chung et al. (1998). 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 rations 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.

Genesis of Late Cretaceous-Paleogene Granitoids with Contrasting Chemical Trends in the Chubu District, Central Japan.

Shunso ISHIHARA and Chengyu Wu

   Late Creataceous to Paleogene rhyolite (11 samples) (Shirakawa 29, Toki 7, Naegi 9, Ryoke 10; total 55 samples), which have high intial Sr rations, 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 onzodiorite 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-Palepgene 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 igneous 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 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 studies granitoids have chemical compositions reflecting basically their basement characteristics, and also partly their degree of magmatic fractionation.