Reservoir Structure of the Sengan Geothermal Field Interpreted from the Resistivity Data

Abstract

In order to study relationship between resistivity structure and reservoir structure of the Sengan geothermal field, especially beneath the Yakeyama and Hachimantai area, two-dimensional analysis using the finite-element method has been applied to the data of Schlumberger soundings of several survey lines obtained by NEDO (New Energy and Industrial Technology Development Organization) and other agencies since 1974. I examined correlation between the resistivity models and other survey results, such as logging data of test wells. Although most of the sounding curves essentially show threelayer structure of which resistivity variation is resistive-conductive-resistive from the surface, four-layer models with resistivities of resistive-conductive-medium-resistive are applied in the 2-D analysis referring to other survey results. Deep test well, SN-7D, drilled at the final stage of the NEDO's projeet in 1987, encountered several large lost-circulations, Succeeding well tests have concluded that they are potential fractures which can produce great amount of geothermal fluid. According to the geologic column of the well, fracture dominated reservoir is located in tuffaceous layers of the Miocene Aniai Formation and Tertiary intrusive rocks. Average resistivity values of those layers are 100Ω·m and 1, 000Ω·m, respectively. They correspond to the third layer and the resistive basement of the resistivity model of the survey line SGN-85B. I Cap rocks are Quaternary lake deposits and the Upper Aniai Formation. They are extremely conductive with the resistivity of less than 5Ω· m, and are obviously analyzed as the second layer of the 2-D model. Dominated clay alteration minerals are montmorillonite for the cap rocks, and chlorite and sericite for the reservoir formation. Since montmorillonite is formed at low temperature environment at borders of geothermal water circulation and it has very low resistivity and low permeability when it contains plenty of water, those layers themselves form electrically conductive cap rocks. On the other hand, high temperature-type clay minerals such as chlorite and sericite seem not to decrease resistivity of host rocks and not to seal fractures. SN-7D is located where the lower boundary of the conductive layer becomes shallow and the temperature distribution shows high anomaly. This is also consistent with distribution of alteration minerals, such as montmorillonite, chlorite and wairakite.