RESERVOIR REGIME AND CONTROLS IN THE KARAHA-TELAGA BODAS GEOTHERMAL FIELD, INDONESIA

Karaha - Telaga Bodas is a partially vapor-dominated, fracture controlled geothermal system located on the flank of Galunggung Volcano in western Java, Indonesia. The geothermal system consists of: 1) a cap rock, ranging from several hundred meters to 1600 m thick that is characterized by steep, conductive temperature gradients and low permeabilities; 2) an underlying vapor-dominated zone that extends below sea level; and 3) a deep liquid-dominated zone with measured temperatures up to 350oC. Heat is provided by a tabular granodiorite stock encountered at depths of ~3 km. A structural analysis of this geothermal system was conducted using the results of core logging, downhole image logs, production data and petrologic studies. This analysis shows that the effective base of the reservoir is controlled by the stress regime's effect on fractures within volcanic rocks located above the brittle/ductile deformation boundary. The base of the caprock is controlled by the distribution of initially low-permeability lithologies above the reservoir; the extent of pervasive clay alteration that has significantly reduced primary rock permeabilities; the distribution of secondary minerals deposited by descending waters; and by a downward change from a strike-slip to an extensional stress regime. Producing zones are controlled by both matrix and fracture permeabilities. High matrix permeabilities are associated with lacustrine, pyroclastic and epiclastic deposits. Productive fractures are those that display the greatest tendency to slip and dilate under the present-day stress conditions. Although the reservoir appears to be in pressure communication across its length, fluid and gas chemistries vary laterally, suggesting the presence of isolated convection cells.