A combination of recent well data from development drilling and exploration wells in Rantau Dedap were used to update the subsurface geology and alteration assemblages. Stratigraphy and alteration analyses were done on individual wells utilizing cuttings, core and log data. Faults were interpreted based on offset lithologies supported by fractures interpreted from borehole image logs. The Rantau Dedap stratigraphy mainly consists of sequences of Quaternary silicic welded tuffs and re-sedimented volcano-sedimentary deposits derived in part from older Tertiary formations, probably forming a complex caldera-fill sequence. The reservoir has a stratigraphic (rather than argillic) cap of Subaqueous Debris Flows, overlain by Quaternary andesitic and silicic volcanics. Hypabyssal intrusions were found at the bottom of deep wells and mainly intruded the lower silicic unit or underlying units, probably as stocks and dike/sill swarms. There are two main fault and fracture orientations, NE-SW and NNW-SSE. These faults exist at depth below ~1,000 masl and are buried by sequences of debris flows and younger stratigraphic units, except a NE-SW collapse structure that offset the lithology up to the surface. Hydrothermal alteration is divided into the following classic geothermal system zones (i.e. Argillic-Transition-Propylitic) with the addition of Phyllic, Potassic/Deuteric and Advanced Argillic zones identified in some wells. Transitional alteration strongly overlaps with a thick volcaniclastic sequence dominated by debris flows and containing a variety of marine sedimentary and igneous clasts in a mud to silt matrix. The Transition top corresponds with the 10 to 20 ohm-m isosurface based on resistivity models. Both rock type and fluid composition appear to influence the relative abundance of alteration assemblages. Potassic-deuteric alteration is found in and near granitic intrusions, whereas Phyllic alteration overlies intrusions. Advanced argillic alteration was found in the upper-part of dike swarms. The currently productive reservoir in Rantau Dedap, which is dominated by a phyllic overprint of propylitic alteration is interpreted to be controlled by a combination of structures and stratigraphy. Late prehnite, wairakite, calcite and anhydrite partially fill open space in veins.

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