Sibayak geothermal field is located at about 65 km to the southwest of Medan in the North Sumatera Province, Indonesia. This field has been investigated by various geo-scientific methods since 1989, continued by drilling of 10 wells. The exploration results suggested that the Sibayak area is a high potential geothermal field. Recently, a 2 MW mono-block has been installed. However, considerable problems are still encountered in the Sibayak geothermal field: high acidic (corrosive) hot water in the vicinity of Mt Sibayak and scaling problem in several wells located around the southern Singkut caldera rim. To find the best target for further development, an attempt has been performed to image distribution of reservoir permeability in the Sibayak area, using surface and borehole geophysical data together with well productivity data. Surface geophysical data including magnetotelluric soundings were used to image distribution of subsurface conductivity, while gravity data were optimized to image subsurface structure related to permeability. Borehole geophysical data (mise-a-la-masse data) has been analyzed to image conductivity distribution related to permeability in the vicinity of production wells. The results were then integrated with production rates, permeability indication from lost circulation zones and temperature data to develop a map of reservoir permeability. It is concluded that good and moderate permeability extends to the area between Mt Sibayak and Mt Pratektekan where the NE-SW faults are intersected by the NW-SE ones, while low permeability zone is found to the southern part of the field just inside the southern caldera margin. The permeability distribution map can be used for planning drilling program at the Sibayak geothermal field. In particular, this results support exploratory drillings in the area between Mt Sibayak and Mt Pratektekan.

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