The Takigami geothermal field is located in the southwestern part of Oita prefecture, Kyushu. Geothermal exploration in this area was initiated in 1979 by Idemitsu Geothermal Co., Ltd. (IGC) and was further developed by Idemitsu Oita Geothermal Co., Ltd. (IOG), which was established in 1993. Various surveys and several well-drillings in the area have been carried out by both IGC and IOG.
The Takigami field has no geothermal manifestations in the surface. It is covered by thick impermeable layer, largely composed of montmorillonite zone, and a thick highly permeable layer emplacing cold groudwater. The montmorillonite zone was formed by hydrothermal alteration and plays the role as the reservoir's cap rock.
The reservoir system is devided into the northeastern and southwestern area based on the distribution of the fluid chemistry, subsurface temperature, and permeability of the layers with the boundary probably correspondent to the Noine fault zone. The reservoir system in the northeastern area has low Cl content (about 450 ppm), relatively low temperature ranging from 160°C to 210°C, and develops in shallow, high permeable zone with a number of fractures in it. On the other hand, the reservoir system in the southwestern area has higher Cl content (about 600 ppm), high temperatutre above 250°C, and develops in deep, low permeablezone. The subsurface geothermal fluid flows from the southwest to the north and northeast of the reservoir.
Numerical simulations suggest that the reservoir is capable of supplying sufficient steam to generate 25 MW over a long period of time. The Takigami power plant will be fully operated in late 1996.

View full abstract

Geothermal exploration in the Kirishima including geological and geophygical surveys, was conducted by Nittetsu Mining Company Ltd.(NMC) from 1973 and by a joint survey between Nippon Steel Corporation (NSC) and NMC starting in 1979. In 1989, a new geothermal development project was agreed formally upon Kyusyu Electric Power Co., Inc. (KEPCo), NSC, and NMC. Nittetsu Kagoshima Geothermal Company Ltd.(NKG), established in 1990 to produce the geo-thermal steam, carried out discharge tests in ten production wells in 1991. and confirmed the existence of a geothermal reservoir which is enough for generating a 30MW geothermal power plant. In November 1994, NKG started the construc-tion of the Ogiri Power Plant, which was scheduled for completion by March 1996. A total of fifty exploratory wells have been drilled with total composite length of 56, 848m; Of these, 35 wells (42, 692m in total) were drilled by the private companies, and 15 wells (14, 156m in total) were drilled by governmental organization.
The Ginyu Fault Reservoir was delineated by the exploratory drillings: It is a simple fracture-type reservoir which is 800 m in length and -77m to -483m in depth and trends with a strike of N71.4°-74.9°E, dipping of 63.2°-72.3°N. Reservoir temperature measured by well logging and calculated by silica geothermometry is 232°C and 240-245°C, respectively. The value of Kh is variable from 20 to 352dm with an average of 66 dm from ten wells. Discharge rate of steam and hot water per well, calculated to be at 5kg/cm2A of wellhead pressure, is 35.1 to 40.4t/h and 178 to 211t/h, respectively. The hot water is Na-K-Cl-SO4 type with pH from 8.6 to 8.8. Concentration of non-condensible gas in the steam is considerably low below 0.01 vol%.
Simulation experiments indicate that the Ginyu fault reservoir has a potential to generate steam with a discharge rate of 350t/h over the next 30 years. Single-flash system and the separated steam flow transmission system have been adopted in the plant design. In order to avoid the deposition of silica scale, a method which injects hot water into the underground by using line pressure and the difference of water head was adopted. The operation of the power plant is faciliated by the central control system unit.

View full abstract

Metal-rich sludgy sediments were collected in the Seikan Undersea Tunnel between Honshu and Hokkaido, Japan. The sediments can be divided into four types: (1) a black slime precipitated on the wall and in the drainages of the pilot tunnel; (2) a red (or yellowish-red) slime precipitated along with the black slime; (3) a white slime deposited around the abandoned shafts in the pilot tunnel and (4) a brown slime sedimented in the settling tank at Hokkaido-side of the pilot tunnel. The results of X-ray diffraction analysis, elemental analysis and scanning electron microscope (SEM) observation indicate the following characteristics of each slime.
1) The black slime is mostly composed of 10Å manganite (todorokite), which is a major component of oceanic manganese oxide deposits. SEM photographs of the slime show a characteristic morphology: aggregates of manganite microspores, 10-20 gm in diameter. Manganese oxide in the slime is considered to be precipitated from seepage waters of oceanic origine, possibly by a rapid manganese-oxidation reaction associated with microbial activity.
2) The red slime contains Fe and Si as major components, while its X-ray diffractogram exhibits no reflection except for gypsum and brushite. The SEM photographs of the slime show a specific form: flocks of filamentary cells, one by 20-30 gm in size. These data imply that the slime may be a product of a certain sheathed bacteria.
3) The white slime is composed of calcite, aragonite, brucite and an albuminous organic matter. These components are considered to be brought with the seepage waters, which have passed through the abandoned shafts filled up with carbonate-albumin gob.
4) The brown slime is a mixture of dolomite, calcite, quartz, gypsum, hausmannite and some organic matters, containing trace amounts(0.1-0.01%) of heavy metals such as Cu, Zn, Mo and Pb. The sources of these precipitates are estimated to be the three kind of (black, red and white) slimes and plastic rocks carried to the settling tank with drain waters.

View full abstract

Platinum-group elements (PGE) in nonmagnetic panned-out detrital samples from the northern Kamuikotan ophi-olitic complex, Hokkaido, was examined by neutron activation method. Their Cl-chondrite normalized PGE patterns are similar to those of ophiolitic host rocks and chromitite. Positive correlation between the iridium and chromium contents imply that iridium-series-PGE anomalies are probably due to chromite-hosted platinum-group minerals. Averaged PGE concentration in the samples from the central Hokkaido (Do-ou) region is almost eight times higher than that from the northern Hokkaido (Do-hoku) region. Based on the chemical analyses and the estimated reserve of chromite, total amounts of PGE included within placer chromite grains are estimated to be 17 kg in the northern Hokkaido, and 245 kg in the central Hokkaido.

View full abstract