The Awibengkok (Salak) geothermal system is a liquid-dominated, fracture-controlled reservoir with benign chemistry and low-to-moderate non-condensable gas content. The geothermal system is hosted mainly by andesitic-to-rhyodacitic rocks, and floored by Miocene marine sedimentary rocks cut by igneous intrusions. The volcanic sequence is capped by an 8400-year-old phreatic explosion breccia, rhyolite fallout tuff (>8400 years and <40,000 years), rhyolite lavas, domes and related tuffs (>= 40-120 ka), and dacite-to-rhyodate lavas and domes (185-280 ka) that were erupted across the eastern part of the field from NNE-trending vents controlled by a major fault. More regionally extensive basaltic-andesite to andesite volcanic centers are mostly between 180 and 1610 ka old. Surface and subsurface fault patterns, formation image logs and tracer studies indicate strongly anisotropic permeability aligned with the dominant N to NE fracture trend, dividing the field into a number of subcompartments that are locally connected by fractured aquifers and NW- and E-W-trending fractures. Shallow argillic alteration gives way with increasing depth and temperature to argillic-phyllic and propylitic zones, with the latter accounting for the bulk of the fluid produced from the geothermal system. The commercial Awibengkok reservoir is a moderate-to-high temperature (240-312 degrees C) geothermal resource with high fracture permeability, moderate porosity (mean = 10.6%) and moderate-to-low matrix permeability (geometric mean = 0.026 md). The principal deep upflow zone, with fluid temperatures in the 275-312 degrees C range, is located in the western part of the field. The ascending fluids move up along N- or NNE-trending structures that breach low-permeability tuff layers in the central and east-central parts of the field. Fluids in the central part of the reservoir are uniform in composition and temperature, representing the mixing of upflow and convective reflux. Fluids ascend and flow laterally to the shallow top of the reservoir in the eastern area near drilling pads Awi 1 and 13 (fluid production temperatures: 240 to 270 degrees C. The eastern reservoir is shallower and has been more affected by volcanic eruptions during the period of hydrothermal activity. This has led to steam loss, condensation, and local ingress of shallow steam-heated water that has resulted in lower temperatures in this area. Delineation of potential fluid production or injection zones adjoining northern and western margins of the proven reservoir has been accomplished by integration of new geophysical, geologic and geochemical studies, followed by drilling. A facies model for small calderas was developed and used during the assessment of the western area. (C) 2008 Published by Elsevier Ltd.