FRACTURE SYSTEM RELATED TO GEOTHERMAL RESERVOIR BASED ON CORE SAMPLESOF SLIM HOLES - EXAMPLE FROM THE UENOTAI GEOTHERMAL-FIELD, NORTHERN HONSHU, JAPAN

In order to clarify the role of fracture permeability in a reservoir, the properties and formation mechanism of fractures were investigated on core samples from the Uenotai geothermal field, Japan. The fracture s were located and identified from the records of lost circulation whi le drilling. Productive fractures are classified into a dominant high- angle (average: 77 degrees) group, and a less common low-angle (averag e: 28 degrees) group. The high-angle fractures were formed as extensio nal hydrothermal fractures in response to fault-valve behavior trigger ed by fluid pressure activation under lithostatic conditions. The low- angle fractures formed as extensional and/or extensional shear fractur es under hydrostatic-lithostatic transitional conditions, with subhori zontal sigma(1) stress. Both kinds of fractures were accompanied by hy drothermal mineral deposition.