An aqueous speciation code using a personal computer is developed and applied to some Japanese geothermal well discharges. The calculated activities of component ions are used to plot the compositions of discharges on activity diagrams and to check the saturations of some alteration minerals. Many minerals are saturated at temperatures close to the reservoir temperature estimated by silica -and alkali- geothermometers. The temperature and mineral assemblage of a reservoir can be estimated by plotting temperature vs. saturation index. Factors introducing the uncertainties for the calculations are also discussed.
Almost all hydrothermal areas of eastern China distribute in the epicontinental region to the east of the Daxinganling-Taihangshan-Wulingshan gravity gradient zone, and the temperature of hot springs is intermediate or low. The underground temperature gradient in the sedimentary blanket is decreasing with the increase of the basement depth. The temperature profile corresponds to the relief of basement, that is, the higher geothermal anomalies appear in the uplifted region of the upper mantle, and the strike of the deep underground temperature anomaly corresponds to that of the tectonic structure. As a whole, the underground temperature field in eastern China has an increasing trend from the north to the south and from the west to the east. This trend corresponds to the decrease of the crustal thickness. The relation between the heat flow profile and the tectonic structures of the crust and upper mantle is very close. These characteristics conclude that the thermal structure in the epicontinental region of eastern China is connected directly with the motions of the Pacific and Eurasian Plates.
This paper describes effects of boundary conditions on a geothermal reservoir analysis, using a porous model and data from Takenoyu geothermal field, in Kumamoto prefecture, Japan. The conclusions are as follows :(1) Under the assumption that the vertical length is 1.0km and there is a horizontal heat source on the bottom, the estimation error of temperature distribution in a case of the horizontal length of 2.5km almost agrees with the case of 5.0km. In the both cases, the heat source length of 1.0km gives the minimum estimation error between the calculation and the observation data.(2) When the heat source is assumed to be on the bottom or the center axis, the calculation agrees with the temperature data in the range from ground surface to 400m depth level. However, the latter calculation disagrees with the data in the range deeper than 400m.(3) Performance index evaluating differences between field data and model calculations should be defined not only by temperature, but also mass and heat discharges at the surface, in order to identify the size and position of the heat source.
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