Updated Geologic Structures and Stratigraphy of the Darajat Geothermal Field in Indonesia

Abstract

The main objectives of the evaluation of the geologic structures and stratigraphy of the Darajat geothermal field are to refine and update both the Darajat conceptual and geologic1 models. Re-interpretation of the borehole image logs, additional petrographic analysis of rock cuttings and cores, integration of subsurface data from the wells drilled during 2009-2011, and recent resource observations allowed definition of at least seven (7) lithologic units based on the dominant rock type. The updated stratigraphy of the reservoir was correlated with the surface rocks to develop a volcano-stratigraphy and geochronology of the Darajat geothermal system. The andesite-intrusive complex, which comprises the Darajat geothermal reservoir and the hypothesized sub-volcanic portion of an earlier liquid-dominated geothermal system, belongs to the Kendang Volcanics and makes up subsurface Units A and B. The interpretation of the major volcano-tectonic features and structures at Darajat was refined through the analysis of the Light Detection and Ranging (LIDAR) data/imagery that was acquired in 2014. The LIDAR Digital Elevation Model (DEM) hill-shade map shows several conspicuous volcanic vents in Puncak Cae, Gagak and Kiamis. The prominent Kendang Fault, which extends to the Kamojang Field in the northeast, might be a section of the ring structure of an earlier volcano, herein, called Kendang. Wellbore image data from several wells suggest that the Kendang Fault may dip at 70° eastward. Although needing further substantiation, we hypothesize that the decompression event that preceded the development of the steam-dominated geothermal reservoir at present might be related to the eruption of the Kendang volcano. The Gagak Fault is another prominent surface structure and believed to form during the eruption of the resurgent Gagak volcano after the eruption of the older Kendang volcano.