The geological environment of Matsukawa geothermal area, Japan

Abstract

Listen

Translate article

Matsukawa geothermal area is covered by some lava of Marumori Volcano and Matsukawa andesite of Quaternary age. So, difficulty of geothermal exploitation was suggested because the geological sequence and structure of the underlying formations had been hidden under the above-mentioned younger volcanic materials. This area has such a characteristic feature that there are only a few thermal manifestations though a wide distribution of altered rocks. Geothermal exploitation has been carried out by the Nihon Metals and Chemicals Co. Ltd. (older name, Azuma Kako Co. Ltd.) with the co-operation of the Geological Survey of Japan since 1958. Based on the data obtained from geological mapping and logging, study of rock alteration, electric and seismic prospecting and drilling of four test holes 214–570 m deep and six productive wells 945–1500 m deep, the subsurface structure of this area is considered as follows. The basement rocks covered by younger volcanic rocks are composed of dacitic welded tuff named Tamagawa Welded Tuff Formation, marine sediments named Yamatsuda Formation and the so-called Green Tuff Formation of Neogene from upper to lower. Along the Matsukawa river, there may be a fractured zone with step faults trending from NE to SW. This has been assumed by stratigraphical correlation of core samples. Contrary to the physical property of rocks with low permeability, in Matsukawa andesite and in the upper part of Tamagawa Welded Formation, the lower part of the Welded Tuff Formation and the Yamatsuda Formation are rich in cracks. In the Yamatsuda Formation, especially, remarkable development of cracks was detected. From these data, it is summarized that hydrothermal water in Matsukawa area is reserved in cracks along welding planes of hard welded tuff formation and fractured marine sediments which were formed by crustal movement relating to the extrusion of Marumori Volcano. To know the detailed subsurface structure, seismic prospecting by reflection method is carried on in the altered rock area.