3-D MAM inversion in Sibayak geothermal field, Indonesia

Sibayak geothermal field is located about 65 km to the southwest of Medan in the North Sumatra Province, Indonesia. Starting from 1998, 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 Energy plans to increase the capacity to 20 MWe. Accordingly, detailed knowledge of the reservoir structure and its extension must be determined for a new production target. 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. A conventional one-dimensional MAM data processing has been done to obtain lateral variation of subsurface resistivity. However, the result is not satisfied to image real condition of the geology on the subsurface. Therefore, we carried out an advanced 3-D MAM inversion based on the smoothness-constrained least-squares method using a homogeneous earth as the simplest assumption of the starting model. Interpretation of the 3-D MAM model was done by combining the results with previous geo-electrical data and borehole information to image a promising reservoir zone. The resistivity model obtained from this study is characterized by a very low resistivity cap rock beneath a resistive layer and rather high resistivity layer of the reservoir. The very low resistivity layer is due to clay minerals such as montmorillonite. This interpretation result of the 3-D MAM model indicates that reservoir zones trend to the north-northeast direction of the study area, between Mt. Sibayak and Mt. Pratektekan, and shows a good correlation with the formation temperature and the lost circulation zone. This geophysical information is useful for the task of selecting sites for the promising zone in the Sibayak geothermal field.