Reservoir identification at Dieng geothermal field using 3D inversion modeling of gravity data

It was well known, the reservoir is one of the important parameters in a geothermal system. In general, a geothermal reservoir can be characterized as a graben structure with low to moderate rock density relative to its surroundings. Dieng is a geothermal field that is located in Banjarnegara and Wonosobo regency, Central Java province. The existence of a geothermal system in Dieng is identified by the appearance of surface manifestations such as crater and hot spring. This study was conducted to identify reservoir zones in the Dieng geothermal field using gravity data. To justify the location of reservoir rock, subsurface structure analysis using derivatives analysis which is the First Horizontal Derivatives (FHD) and Second Vertical Derivatives (SVD) method is done. The subsurface structure has an important role as the recharge and discharge zone of the geothermal system that aims towards and leaves the reservoir rock. Three-dimension inversion modeling is also applied because it is more objective and appropriate with the real condition rather than the forward method. The result of this study shows that ten structures are identified by the derivative analysis. The structures are dominated by normal faults. There are identified two grabens that are indicated as the two separated reservoirs in the South and center of the Dieng geothermal field. The results are verified by both geology and magnetotelluric data which have different characteristics and both reservoirs are separated by a low resistivity igneous rock that has been altered with the thermal effect of the surrounding reservoir.