Top of Reservoir Assessment of Sorik Marapi Geothermal Field Based On Magnetotelluric and Well Data, Sumatra, Indonesia

Top of reservoir (TOR) is one of the key parameter to understand reservoir characteristic of the geothermal field. On the exploration phase, base of conductor (BOC) from Magnetotelluric (MT) data is often used as the basis to determine the TOR. However, the actual finding after drilling showed numerous ranges of separation between BOC and commercial TOR. It is important to integrate the MT data with actual drilling data such as Methylene Blue (MeB) test, alteration mineralogy and downhole temperature data. The wells that drilled towards hypothetical upflow show significant deeper TOR as compared to BOC while the wells in the productive outflow show good correlation between TOR and BOC. Further to the North – East of Sorik Marapi reservoir, the conductive layer is thicker and well data suggest thick layer of sedimentary rocks. As the result, the conductive layer of Northern area and Eastern area is interpreted to associate with the sediment infill of pull-apart basin rather than the hydrothermal alteration. This is intended to evaluate the correlation of BOC, alteration mineralogy and downhole temperature to determine P-10, P-50 and P-90 TOR and its distribution. The final results are used to get more robust understanding of the fieldwide TOR distribution thus the uncertainty in setting the depth of production casing shoe can be reduced.