Geology and fluid chemistry of the Fushime geothermal field, Kyushu, Japan

The Fushime geothermal ®eld is located in a depression close to the coast line. The system is characterized by very high reservoir temperature (>3508C), and a high salinity production ¯uid. Geological analysis shows that the main reservoir in this ®eld occurs in a fractured zone developed around a dacite intrusion located in the center of the ®eld. High permeability zones recognized by drilling data are found to be associated with fault zones. One of these zones is clearly associated with a NW±SE trending andesite dike swarm which was encountered in some wells. Alteration in the system can be divided into four zones, in order of increasing temperature, based on calcium±magnesium aluminosilicate mineral assemblages: i.e., the smectite, transition, chlorite and epidote zones. The feed zone is located in the chlorite and epidote zones, which can be further divided into three sub-zones according to their potassium or sodium aluminosilicate mineralogy, from the center of the discharge zone: K- feldspar±quartz, sericite±quartz, and albite±chlorite zones. Chloride concentration of the sea-water is 19,800 mg/l, and Br/Cl mole ratio is 1.55. Based on geochemical information, the reservoir chloride concentration of this ®eld ranges from 11,600 to 22,000 mg/kg. The Clres (Cl in reservoir), Br/Cl ratios and stable isotope data indicate that the Fushime geothermal ¯uid originated from sea-water and is diluted by ground water during its ascent. Some ¯uids produced from geothermal wells show low pH 0375-6505/00/$20.00 7 2000 CNR. Published by Elsevier Science Ltd. All rights reserved. PII: S0 37 5 -6 50 5 (9 9) 00 0 63 -2 Geothermics 29 (2000) 279±311 * Corresponding author. Tel.: +81-3-5461-7334; fax: +81-3-5461-7394. E-mail address: okada@gtm.japex.co.jp (H. Okada). (about 4). It is thought that sul®de mineral (PbS, ZnS) precipitation during production produces this acidic ¯uid. 7 2000 CNR. Published by Elsevier Science Ltd. All rights reserved.The Fushime geothermal ®eld is located in a depression close to the coast line. The system is characterized by very high reservoir temperature (>3508C), and a high salinity production ¯uid. Geological analysis shows that the main reservoir in this ®eld occurs in a fractured zone developed around a dacite intrusion located in the center of the ®eld. High permeability zones recognized by drilling data are found to be associated with fault zones. One of these zones is clearly associated with a NW±SE trending andesite dike swarm which was encountered in some wells. Alteration in the system can be divided into four zones, in order of increasing temperature, based on calcium±magnesium aluminosilicate mineral assemblages: i.e., the smectite, transition, chlorite and epidote zones. The feed zone is located in the chlorite and epidote zones, which can be further divided into three sub-zones according to their potassium or sodium aluminosilicate mineralogy, from the center of the discharge zone: K- feldspar±quartz, sericite±quartz, and albite±chlorite zones. Chloride concentration of the sea-water is 19,800 mg/l, and Br/Cl mole ratio is 1.55. Based on geochemical information, the reservoir chloride concentration of this ®eld ranges from 11,600 to 22,000 mg/kg. The Clres (Cl in reservoir), Br/Cl ratios and stable isotope data indicate that the Fushime geothermal ¯uid originated from sea-water and is diluted by ground water during its ascent. Some ¯uids produced from geothermal wells show low pH 0375-6505/00/$20.00 7 2000 CNR. Published by Elsevier Science Ltd. All rights reserved. PII: S0 37 5 -6 50 5 (9 9) 00 0 63 -2 Geothermics 29 (2000) 279±311 * Corresponding author. Tel.: +81-3-5461-7334; fax: +81-3-5461-7394. E-mail address: okada@gtm.japex.co.jp (H. Okada). (about 4). It is thought that sul®de mineral (PbS, ZnS) precipitation during production produces this acidic ¯uid. 7 2000 CNR. Published by Elsevier Science Ltd. All rights reserved.