Thermal and Nucler Power Engineering Society (TENPES) has proposed the panel disccution on the commercial value of geothermal energy from the view point of management, at the 2003 annual meeting of Geothermal Research Society of Japan held in Sendai. Eighteen commercial geothermal power plans have been constructed in Japan by 1999. However, after 1999 no new geothermal power plant was constructed. For promoting the development of geothermal fields for energy use, we need to investigate and examine the factors that affect the delay of the development. The subject for disccution was focused on the commercial value of geothermal energy from the view point of view of manageiment, and then opinions and suggestions raised in the discussion were summarized.
Mantle diapirs are a concept led from magma geneses that hydrated mantle above subducting slab comes to be partially fused and lightened, ascending adiabatically and further accelerating the partial fusion. However, it is physically difficult to recognize its presence. The Flores to Alor segment of the Lesser Sunda arc, Indonesia, is characterized by en echelon shaped volcanic islands reflecting the NNW-SSE left-lateral shear stress regime accommodated between north-moving Australian continental accretions in the east and relatively fixed Sundaland in the west. Each element of 'en echelon volcanic islands is a topographically elongated dome that consists of two structures : culmination of anticline of volcanic basement units and clustered young volcanoes. Coexistence of both structures in the same area can be explained when mantle diapirs are assumed : i) Mantle diapir generated clustered young volcanoes and ii) Thinner brittle layer caused by the thermal field above mantle diapirs was deformed by the NNW-SSE left-lateral shear, forming elongated dome.
Fluid Flow Tomography (FFT) method is an improved mise-à-la-masse method, which is used as a reservoir monitoring method. This method is a 4-Dimensional (4-D) electrical method in order to visualize the transient subsurface fluid flow by measuring charged electric potentials due to subsurface resistivity and streaming potentials caused by subsurface fluid flow. This method is utilized to monitor the fluid flow during hydraulic fracturing of Hot Dry Rock project and periodical overhaul of geothermal power plant. This method is also used to monitor the flow of heavy crude oil in oil sand formation during thermal Enhanced Oil Recovery (EOR). The observed data of FFT method can be separated into charged electric potentials derived from subsurface resistivity structure and streaming potentials due to a permeable fluid flow. In this paper we describe the algorithm of three-dimensional resistivity inversion and three-dimensional probability tomography of self-potential data. A case study of FFT method was carried out in the Onuma geothermal area. As the results of 3-D analysis of resistivity data, distinguished low resistivity anomalies were detected and resistivity discontinuities where marked with resistivity changes were detected. As the results of 3-D analysis of self-potential data, the main production zone and reinjection zone were delineated from Charge Occurrence Probability (COP) distribution. We believe that the FFT method is a useful tool to monitor the dynamic change of fluid flow behavior in a reservoir during production and reinjection operations.
Tawau in Sabah is only one geothermal area related to the Quaternary volcano in Malaysia. Many hot springs and alteration halos are recognized in this area. The Quaternary eruptions were proved by two age data of 27000yBP by 14C and 1.62Ma by K-Ar methods. However, the age data are not enough to prove Quaternary volcanic activity. New age data of Quaternary volcanic rocks have been obtained for 12 samples by thermoluminecence (TL) method. The youngest age obtained is 0.09Ma for a monogenetic basaltic cinder cone. Ages of such young basaltic volcanos are O.IOMa and 0.11Ma (same outcrop) and 0.14Ma. Ages of dacitic volcanic rocks widely distributed at the southern foot of Maria volcano rage from 0.34Ma to 0.45Ma (4 samples). The ages of underlying andesite lava range form 0.27Ma to 0.52Ma (4 samples). We obtained preliminary age of 0.7∼1.2Ma for this andesite lava formation by weathering correction. Although the relation between such age data and stratigraphical succession is still unknown, such new age data clarified the existence of volcanic activity in the Middle Pleistocene. Alteration ages of 13 samples were also measured by TL method. The youngest ages obtained are 0.15Ma and 0.19Ma for the Upper Tawau hot spring area. Other ages are widely scatted from 0.27Ma to 0.66Ma. They were used for construction of thermal history of each area. Alteration minerals were identified for 9 locations. Temperature inferred from alteration minerals is roughly estimated around 100∼200°C. Tawau area is expected to be a target for further geothermal exploration for medium temperature (around 200°C) based on the preliminary assessment by the above TL ages and other reported data.
Three main episodes of volcanic activities are recognized in the Mt. Sunda volcanic complex eruptions of the pre-, syn- and post-caldera units named Pre-Sunda Volcanics, Sunda Volcanics and Tangkuban Parahu Volcanics. The first episode, is represented by the Batunyusun Andesite (1.1 Ma) which unconformably overlies sedimentary deposits. The second episode, produced the Sunda Volcanics having ages between 0.56 and 0.180 Ma. They consist of the Sunda Andesite with ages of 0.56-0.205 Ma and the huge volume of Sunda Pyroclastics covering an area of 200 km2, associated with a caldera-forming eruption of Sunda caldera between 0.205-0.18 Ma. The post-caldera Tangkuban Parahu Volcanics at the third episode consist of Tangkuban Parahu Andesite and Tangkuban Parahu Pyroclastics with ages of 0.062 - 0.022 Ma. Carbon-14 dating of younger craters reveals an age of 0.0099-0.0014 Ma. They are assumed to be associated with phreatic or hydrothermal eruptions of the Domas, Ratu, Siluman, and Ciater craters. The thermal discharges are mostly bicarbonate waters with subordinate sulphate and chloride type waters. The sulphate waters of Domas and Kancah are associated with high volcanic terrain (900-1, 700 m a.s.l.) . Bicarbonate waters are located at moderate slopes of the volcanoes, and chloride waters are generally located at lower elevations. The Ciater thermal discharge (1, 000 m a.s.l.) has a low pH, high chloride, high fluoride, and relatively high arsenic concentrations. It is linked with active volcanic gases from the Tangkuban Parahu or the Domas crater. The SiO2 and Na/K geothermometry represents a subsurface temperature as 120-310°C. The chemical composition of fumarolic gases from the Ratu crater consists much more of hydrous gases, H2O, CO2, H2S and small amount of SO2. The calculated subsurface temperatures based on gas geothermometry were approximately 280-400°C, probably reflecting the high temperature of fluids. in the depth.