Imaging Reservoir Structure of the Sibayak Geothermal Field (Indonesia) Based on Magnetotelluric Observations

Sibayak geothermal field is located 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 this area, Pertamina (Geothermal Division) plan to expand the capacity to 20 MWe in the year 2005. Accordingly, detailed knowledge of the reservoir structure and its extension must be determined. Magnetotellurics is thought to be one of the geophysical tools that is powerful for delineating reservoir structure. This paper describes the 2-D inversion of MT data and its correlation to the reservoir structure and extension in the Sibayak field. Borehole data and other previous geophysical work conducted by the first author are also used for constraining the MT data interpretation. It is revealed from this study that the reservoir structure in the Sibayak field is characterized by an up-domed shape of a resistive zone (resistivity = 50-200 ohm-m). The reservoir is mainly located in the central area between Mt Sibayak and Mt Pratektekan, where the highest subsurface temperature (300°C), the highest production (30 to >50 ton/hr steam) as well as the highest permeability (kh = 2-4 darcy-m) zone were recorded from the existing wells at depths of 1500 to 2000 meters. The reservoir is found within the sedimentary formation and also along the contact between the volcanic and sedimentary formations. This fact leads us to propose that future development of the Sibayak area should take place in the high temperature, high permeability and high well productivity region marked by the up-domed shape resisitive zone below the conductive cap rock.