The fracture system related with geothermal fluid flows

Examples in the Yuzawa-Ogachi geothermal field, Akita, Japan

Abstract

Recently geothermal fluid flows have become to be regarded as mainly controled by fracture permeability rather than formation permeability, especially on a reservoir scale. In order to clarify the role of the fracture system for geothermal fluid flow and reservoir behaviour, the properties of fractures and their associated hydrothermal veins were analyzed on surface outcrops and drill core samples in the Yuzawa-Ogachi geothermal field, Akita, Japan. This field can be divided structurally into the Okumaemori-Oyasudake upheaval belt and the Kijiyama subsidence belt by the Doroyu and associated faults. The former is characterized by high-angle-fractures in NW-SE trend, mid-angle-fractures in N-S trend, and hydrothermal veins in W-E trend, and the latter is characterized by high-angle-fractures in WSW-ENE and WNW-ESE trends, low-angle-fractures, and hydrothermal veins which are oblique to dominant fracture trends. Bore hole temperature logging data have revealed the detailed subsurface thermal structure in this field, and the isothermal contours suggested the coupling of two convection cells around the Doroyu and the Oyasu, and two geothermal heat source regions around the Takamatsu-dake and the Oyasu-dake. Based on these fracture and subsurface isothermal contour data, the deep seated geothermal fluids around the Doroyu can be inferred to ascend obliquely north-northwestward from the geothermal heat source region to geothermal reservoir zones along N-S and NW-SE trending, high-dipping fractures at deeper part, and preserved and/or deposited as hydrothermal veins in W-E trend fractures at shallow part. It is assumed that the essential geothermal fluids obliquely ascend by transferring several fractures rather than passing through a single fracture beneath the reservoir. Whereas around the Oyasu, the Sanzugawa formation consists of impermeable siltstone, which can be inferred to play an important role on fluid flow control as a cap rock to the geothermal reservoirand a lateral geothermal fluid flow. Based on the stress analysis of conjugate faults, the low angle fractures around the Oyasu can be regarded to be formed as the reverse faults under NE-SW trending compressive stress, and other high angle fractures around the Doroyu and the Oyasu can be regarded to be formed as the normal or lateral faults.