S. Tamanyu et al., FRACTURE SYSTEM RELATED TO GEOTHERMAL RESERVOIR BASED ON CORE SAMPLESOF SLIM HOLES - EXAMPLE FROM THE UENOTAI GEOTHERMAL-FIELD, NORTHERN HONSHU, JAPAN, Geothermics, 27(2), 1998, pp. 143-166
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.