Occurrence and Variarion of Clay Minerals as Hydrothermal Scale in the Yamagawa Geothermal Wells, Kyusyu, Japan

Abstract

Several different magnesium-rich clay minerals have been deposited in wells and surface lines in the Yamagawa field. The small quantity of those clay minerals has not caused a severe problem for fluid production. However, the occurrence of magnesium-rich clay minerals is important to characterize a near-coast geothermal system with hydrothermal fluid that is likely to be seawater origin. Examination by X-ray diffraction, scanning electron microprobe, and energy dispersive X-ray analyses revealed that those clay minerals are divided into two groups by their structures; the 2:1 layered structure of saponite and the 1:1 layered structure of chrysotile. Saponite is common, and mostly has very poor crystallinity. Whereas, chrysotile is rare in comparison with saponite, and was not indicated in earlier stage of the production. The variation of 1:1 or 1:2 layered structures is probably derived from the Mg/Si molecular ratios of the hydrothermal fluid. The occurrences of chrysotile might denote a change in the hydrothermal system within the field.