Fluid Inclusions from Deep Borehole SN-7D, Sumikawa Geothermal System, Sengan Area, Northeast Honshu, Japan

Abstract

The deep well SN-7D is located in the Sumikawa geothermal System, Sengan area, Northeast Honshu, and is one of the most productive wells in Japan. It produced 161 t/h of steam and 318 t/h of hot water during the flow test in the fall of 1988. The geothermal fluids originate mainly from fracture zones in the deepest part of the well, where many secondary inclusions are observed in healed fracture planes in the tonalitic intrusives. The Sumikawa geothermal system is located on the northern flank of the andesitic stratovolcano of Akita-Yakeyama which has had several historical eruptions. Lacustrine deposits of siltstone and tuff alternations of Pleistocene ages are distributed beneath Akita-Yakeyama. Tertiary formations are composed of mainly dacite, andesite and black shale, underlying the lacustrine deposits. They are intruded by the tonalite at the deepest level of the well. The fluid inclusions occur mainly along fracture trails, clearly denoting their secondary origin in igneous quartz crystals of the tonalite and volcanic rocks as well as in late hydrothermal veins of calcite, anhydrite and wairakite. The calcite and wairakite also host a few primary inclusions. Four core samples from 1505, 1921, 2292, and 2484 m depth and two samples of cuttings, with wairakite from 1045 m and anhydrite from 1435 m, have fluid inclusions suitable for microthermometric measurements. The widespread coexistence of liquid-rich and vapor-rich inclusions indicates extensive boiling in the host-water dominated system. The Th and Tm characteristics of fluid inclusions in wairakite and some calcite in veins is consistent with the measured temperature and salinity of geothermal fluids from SN-7D. High and intermediate salinity inclusions are found exclusively in healed fracture planes in the igneous quartz of the tonalite. Their higher temperature range suggests that they might be related to previous thermal events.