1993 Volume 15 Issue 3 Pages 231-252
The mass spectral data of soil-gases obtained through a “PETREX Fingerprint” survey (Sakai, 1986, 1987) in the Okuaizu geothermal area of Fukushima Prefecture were subjected to principal components analysis in order to identify the detected gas components. The soil-gas composition at three of his sampling points was also analyzed by using a gas chromatograph-mass spectrometer to help the identification. On the basis of the results of these analyses, some of the detected gas components were interpreted as follows:(1) Components m/z43, 57, 71 and 85 are due to undecane and possibly other paraffins with carbon numbers greater than 6.(2) Component m/z106 corresponds to molecular ion of xylene and/or ethylbenzene.(3) Component m/z136 corresponds to molecular ion of monoterpene. Among the gas components, components m/z43, 57, 71 and 85 and component m/z106 directly reflect a geothermal structure of the Okuaizu geothermal area as follows:(1) An anomalous zone characterized by high emission of components m/z43, 57, 71 and 85 is found on up flow zones of the hot water system.(2) Anomalous zones characterized by high emission of component m/z106 also seem to be predominant on the same zones. The distribution of component m/z136 does not appear to reflect the geothermal structure. These distributions of gas components m/z 43, 57, 71 and 85 and component m/z106 are explained by the following hypothetical process:(1) Paraffin, xylene and ethylbenzene are generated by thermal cracking of organic materials in rocks of recharge zone. (2) Fluids containing the hydrocarbons are conducted into the geothermal reservoir by geothermal convection. (3) High temperature of the geothermal reservoir causes the hydrocarbons to vaporize. (4) The vaporized hydrocarbons form gas bubbles and ascend toward the surface, causing anomalous hydrocarbon concentration in the soil-gas
