Sibayak geothermal field is located about 65 km to the southwest of Medan in the North Sumatra Province, Indonesia. Recently, a small-scale geothermal power plant (2 MWe) has been installed in this area. Since electricity demand increases in the North Sumatra Province, Pertamina (Geothermal Division) plans to increase the capacity to 20 MWe in the year 2005. Accordingly, detailed knowledge of the reservoir structure and its extension must be determined. The mise-a-la-masse (MAM) surveys were carried out in this field using the exploration well SBY-1 and the production well SBY-4 to delineate a new production target for further field development. Interpretation of the MAM data was done to correlate the results with formation temperatures and lost circulation zones and finally to image a promising reservoir zone. This interpretation result of MAM data indicates that reservoir zones trends to the north-northeast direction of the study area and shows a good correlation with formation temperature and lost circulation zone. This fact leads us to propose that the best production target for the development of the Sibayak area is characterized by the high temperature, high permeability and high well productivity region marked by the dome- shaped resistivity zone below the conductive cap rocks.

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