| Abstract |
The morphological features of feed zones of geothermal fluids have been investigated based on previous studies which described the characteristics of drill core samples collected from the Uenotai, Ohnuma-Sumikawa and Okuaizu geothermal fields, Northern Honshu, Japan. These samples show that druse cavities and hydrothermal veins play an important role as feed zones. The morphology of feed zones are greatly different at the micro and macro scales. The microscopic image of mm-order fractures taken by CT tomography shows that the cracks are three dimensionally connected in a very complicated manner. To the naked eye, core samples are characterized by druses and hydrothermal veins along fractures which have clear degrees of dip and strike. However, the accompanying hydrothermal alteration sometimes makes the dip and strike orientation ambiguous. The macroscopic image of feed zones on a reservoir scale is an assemblage of fractures whose locations are controlled normally by fracture zones, and density-contrasted geologic boundaries which are easily fractured. In general, the shapes of reservoirs tend to be nearly vertical if controlled by fractures, and nearly horizontal if controlled by stratigraphic boundaries. The different features of feed zones on different scales seem to be caused by different formation mechanisms. Macroscopic fracture features are mainly derived by brittle deformation under lithostatic condition, whereas, the microscopic features are caused by water-rock interaction under hydrostatic conditions. |