Geology and geochemistry of the Isobe gas field, Gunma Prefecture

Abstract

In addition to an analytical result of two gas samples, published geological and geochemical data on the Isobe gas field, which had produced CO₂ commercially during about 30 years, are examined in this paper from the viewpoint of petroleum geology

Chemical and carbon isotopic compositions of gases from the field indicate that CO₂ is magmatic origin and has no relationship with hydrocarbons which are derived from the thermal decomposition of organic matters. Faults are ascending paths of CO₂, which is driving force for hot spring waters.

Chemical and isotopic compositions of associated waters with gases which are used as hot spring waters indicate that the waters are fossil sea waters which were entrapped during deposition and undergone diagenetic alteration. Higher temperatures of the waters estimated by the Mg-Li geothermometer than a bottom temperature at 1,500 m depth suggest that the waters are formed in deeper parts and derived from there.