Geologic Structure and Fracture System of HDR Reservoir on NEDO Hijiori Field in Yamagata Prefecture, Northeast Japan

Abstract

The geologic structure in Hijiori field and the fracture characteristics of HDR reservoir were revealed by the areal geologic survey an well logging, which contained the information of orientedcores, BHTV images and PTS logs. There are seven geologic units from A to G in this area. Uppermost unit A is related to Holocene Hijiori Volcano that erupted about 10, 000 years ago with the caldera collapse. Unit G is the geologic basement of Cretaceous granodiorite whose fracture system is the base of the reservoir. In the HDR test site, near southern rim of the Hijiori caldera, top of the basement lies at 1, 445m through 1, 480m in vertical depth. Fractures striking N50°W to N90°W with steep dipping angle more than 70°dominate in the basement granodiorite. The predominant fractures in the granodiorite are main fluid flowing paths of the injected circulating water. The growth direction of the HDR reservoir is strongly controlled by the distribution of favorably oriented pre-existing fractures and their interaction with the stress field.