Stress Fields of a Fractured-Type Reservoir

Abstract

The strike and dip of fractures indicated by slickensides, hydrothermal veins and joints, in drillcores from the Otake-Hatchobaru geothermal field, Japan, have been determined using an static magnetometer. Slickensides provide information about the stress fields that act or acted on rocks, when both the direction of movement of the hanging well and the angle of shear are assumed. The stress fields, then, are classified into six types based on the dip angle of the maximum and minimum stress axes. It is inferred that the NW trending active faults were originally formed by the "ENE-WSW" lateral stress and have been made permeable by the present "SE" normal stress. In a well, the dip angel of the maximum stress axis decreased with increasing depth, and becomes almost horizontal (lateral stress) at a greater depth where it often corresponds to a strongly silicified zone. Therefore, the depth of a fracture- type reservoir can be estimated from the distribution pattern of dip angle of the stress axis.