Chemical compositions of natural gases in Japan.

Abstract

Chemical compositions of 96 natural gas samples from various locations and gas sources at 67 sampling sites in Japan were analyzed. A gas chromatograph system was developed for precise analyses of concentra tions of N2, CH4, C02, 02, Ar, He, Ne, H2, C2H6 and H2S. Overall analytical errors were estimated to be less than 5% for most samples. A sampling reservoir was newly designed in this work. Main components of natural gas samples were C02, N2 and CH4. Whereas N2 showed positive correlations with 4He and 3He concentrations, CO2 and CH4 showed negative correlations with them. Either a mechanism of dilution of the N2 gas enriched in magmatic He by CO2 and/or CH4, or of dissipation of CO2 from magmatic fluid was suggested. Concentrations of Ar and Ne were high in N,2-rich gas samples. Ratios Ne/N2 and Ar/N2 imply the presence of N2 of non-atmospheric origin. The close relationship between CO2 discharge and volcanic activities was pointed out. A possible source of CO2 is carbonates of sedimentary rocks from which CO2 may be released either by pyrolysis or by decomposition due to magmatic activity. Nitrogen and CO2 derived from oceanic sediments may be included in Japanese natural gases associated with subduction of a down-going slab. INTRODUCTIONThe JapaneseIslands are located in the subduction zone of the circum-Pacific orogenic belt. Geophysical features of NE Japan represent a typical island arc system. More than 2,000 hot springs and mineral springs are distributed in the islands and from some of them bubble gases are released.Gases are released also from gas and oil fields.Natural gases studied in this work include various types of occurrence, such as gases emerged from above sources. Chemical composition data on natural gases provide useful information regarding their origin, primordial volatiles of the Earth, and the evolution of the atmosphere.Chemical studies of natural gases started in 1900's (Moureu, 1904(Moureu, , 1923Yamada, 1922;Pencheff, 1929). Since 1960's, a number of data have been reported with the progress in gas chromatography (Gunter and Musgrave, 1966; Maki et al., 1966;Inai et al., 1967) It is difficult to determine Ne with ordinary gas chromatographic technique owing to its retention time close to that of He and low sen sitivity. Kawabe et al. (1981) reported Ne values in terms of Ne/Ar or Ne/N2 ratio using a 11 12 A. Urabe et al.TCD (thermal conductivity detector) gas chromatograph with He carrier gas. Sensitivity of Ne on the TCD, however, is not high enough to analyze samples with low Ne concentrations. Sugisaki et al. (1982) separated He from Ne in atmospheric samples using an 8-m MS 5A column.The method, however, is not ap propriate for natural gas analyses, because He/ Ne ratios in natural gases are generally 2 to 3 orders of magnitude higher than those of the atmosphere, which would result in insufficient separation of Ne from He.In this study a USD (ultrasonic detector)gas chromatograph with He carrier gas was used for the Ne determination.The USD is particu larly effective to det...

Volatile element isotope systematics at Ontake volcano, Japan.

“…(Sano et al, 1998b), and summarized as follows: a negative correlation between N 2 and CO 2 , a positive correlation between N 2 and Ar, minor contents of CH 4 , and relatively lower He contents in CO 2 -rich gases than in N 2 -rich. This is consistent with those found in hydrothermal systems in subduction zones (Urabe et al, 1985;Giggenbach et al, 1993). Note that the distribution of CO 2 discharges generally coincides with major zones of seismicity throughout the world (Irwin and Barnes, 1980).…”

“…Since He contents are larger in N 2 -rich gases than in CO 2 -rich gases of the Ontake samples and a positive correlation was often found in hot spring gases from subduction zones (Urabe et al, 1985;Giggenbach et al, 1993), He may be first accompanied with N 2 and then diluted by CO 2 . Therefore it may be useful to couple the 3 He/ 4 He ratios and δ 15 N values.…”

Hydrogen and carbon isotopic compositions of methane in N2- and CO2-rich gases from Central Japan.

“…In both N2 and C02-rich gas samples, methane concentrations range from 23.4 to 0.8% and no C2 hydrocarbon gases have been detected. According to Urabe et al (1985), the Yuya(Gifu), Shikanoyu, Byakko and Yuya 2(Aichi) bubble gases contain less than 0.01% C2H6. The concentrations of hydrogen gas detected are less than 74 ppm.…”

“…Recent studies of the helium isotopes of the gases have indicated the presence of mantle-derived helium in addi tion to radiogenic crustal helium in the gases (Sano and Wakita, 1985) and suggested that the gases may be mixtures of gas components deriv ed from deep-seated sources, such as the mantle, subducting slab and lower crust (e.g. ; Urabe et al, 1985). This study, therefore, examines whether the bubble gas methane has characteris tic hydrogen and carbon isotopic compositions as compared with methanes of various genetic types which have already been documented.…”

Origin and fate of deep-sea seeping methane bubbles at Kuroshima Knoll, Ryukyu forearc region, Japan

“…Thus, if we persist with the contribution of methane hydrate, we must assume additional He-enriched gases for the source of the bubbles, which seems unrealistic for the area. Because the 4 He/CH 4 ratios in the Kuroshima bubbles can be classified into the representative 4 He/CH 4 ratios of thermogenic natural gases in and around Japan islands (Wakita and Sano, 1983;Urabe et al, 1985;Poreda et al, 1988), direct leakage of hydrocarbons from sub-seafloor gas reservoirs to seafloor is the most probable process to explain the origin of the bubbles in Kuroshima Knoll.…”