It is of crucial importance to map and evaluate properties of fractures (for example permeability) in geothermal areas, because it has been recognized that fractures act as dominant fluid flow paths. However, there is little information on the permeability of fractures and fluid flow paths. In this paper, we report an estimate of in-situ permeability from hypocenter migration of microearthquakes that were observed during the build-up tests in the Takinoue geothermal area. If we assume that microearthquakes are triggered when pore pressure reaches a certain threshold level, we may expect hypocenters to migrate as pore pressure changes during the build-up tests. Then we might be able to estimate permeability from the migration rate of hypocenters, and fluid flow paths from distribution of hypocenters. From simple calculation, we found that it is possible to estimate permeability if we fit the space-time relation of hypocenters by t∼x2 curve, even if pressure history is not known, as in the case of the buildup tests in the Takinoue geothermal area.
More than 2, 000 fluid inclusions in 34 samples of calcite, quartz and wairakite collected from drill cores of the Doroyu geothermal area, Southern Akita, were studied in order to know the relation between the homogenization temperatures of fluid inclusions and measured well temperatures. Geology of the Doroyu area is mainly composed of the Pre-Tertiary basement, Miocene andesitic and dacitic formations, late Miocene lacustrine sediments and Quaternary volcanic formations. Geothermal fluids are found near the boundaries between the basement and Tertiary formations and along the NW-SE step-faults which separate the southern Oyasudake upheaval zone and northern Kijiyama subsided zone. The studied drill holes were selected to represent two vertical sections, the one of which cuts through the western low-temperature zone of the area, and the other runs through the eastern hightemperature zone. Calcite veinlets are abundant in the high-temperature zone and the homogenization temperatures of fluid inclusions in the calcites change concordantly with the present well temperatures. In the low temperature zone, secondary fluid inclusions in quartz grains of dacitic tuffs were mainly used for temperature determination owing to the scarcity of calcite veinlets. In general, ranges of homogenization temperatures in the lowtemperature zone are much higher than the present well temperatures. The present fluid inclusion study revealed that high temperature hydrothermal fluids were once active in the western zone but the activity is waning at present, and that the activity of the eastern zone is at the apex or on the way to increase.
There are many methods for estimating subsurface temperature for the geothermalassessment of a wide area of several tens kilometers by several tens kilometers, but the best way is to obtain data from boreholes directly. Ninety-nine borehole temperature log data in the Hakkoda geothermal area were gathered and a temperature gradient map was drawn. The data were obtained from many kinds of boreholes of geothermal and mineral exploration and hot spring prospecting. Background temperature gradient of the map ranges from 40°C/km to 60°C/km, and high temperature gradient regions over 100°C/km occur over the northern Hakkoda volcanoes and the Okiura and Aoni hot springs. The value of the background corresponds to the average temperature gradient of the backarc side of Japanese Islands. The map was compared with volcanic activities, geochemical and geophysical prospecting including magnetotelluric, gravity, and magnetic surveys. The map of temperature gradient reveals the subsurface temperature between the ground surface and several kilometers depth and is appropriate to assess geothermal resources and choose promising areas in a reconnaissance stage. Borehole data of hot springs without any description of temperature logs are useful only if their measured depths are sufficiently deep.
Based on the structural contour maps drawn by the well lithological and circulation loss data during drilling, following characteristics of fractures in the Matsukawa geothermal reservoir can be pointed out: 1) The main reservoir is located in the three vertical fractures, Akagawa Fault (NE-SW direction). Yunomori Fault (ENE-WSW direction) and Sumikawa Fault (NW-SE direction). 2) The above fractures correspond to the distribution of hydrothermal alteration mineral zones, distribution of low frequency waves and low signal noise ratio obtained from the reflective seismic survey. 3) A large quantity of geothermal fluids seem to be flowing up through the fractures. 4) Geochemically, geothermal fluids collected from wells are divided into two fracture groups, Akagawa and Yunomori Faults, and Sumikawa Fault.