| Abstract |
Kamojang, the first Indonesian operating geothermal field, has been producing electricity since 1983. The estimated field area is about 21 km2, and its potential is about 300 MWe. Seventy wells had been drilled by the end of 1996. The field is currently producing 140 MWe from 24 production wells, with 4 make-up and 3 reinjection wells (Sudarman, et al, 1995). The reservoir rocks encountered by drilling comprise interbedded pyroclastics and lavas of andesitic composition with some basaltic andesite and lacustrine tuffs. There are two hydrothermal mineral assemblages present, namely those produced by ìacidî and ìneutralî pH fluids. The ìacidî assemblage occupies shallow levels (from surface down to 100 - 300 m) and comprises kaolin, smectite, alunite, quartz, cristobalite, and pyrite. The ìneutralî assemblage, which occupies deeper levels, consists of varying proportions of quartz, adularia, albite, epidote, titanite, wairakite, laumontite, calcite, siderite, hematite, pyrite, anhydrite, smectite, chlorite, illite, and interlayered clays. The present-day measured downwell temperatures are lower than those indicated by the alteration mineralogy by about 10 - 50oC and fluid inclusion homogenisation temperatures by about 60oC. This may record a previous hotter regime, but alternatively the measured temperatures may be too low. The field is now vapor-dominated but the hydrothermal minerals show that the rock-altering fluid was dominantly a liquid. At the time the observed mineral assemblages were formed, the depth of the boundary between the steamcondensate layer and the deep reservoir fluid may have been located at about 200 - 300 m depth, but this has now dropped to depths varying from 544 to 1700 m. The depths of the sulfate-rich water penetration in the past varied irregularly from 200 down to 1800 m. Vein assemblages mark the presence of high channel permeability in the past. The domination of conduit flow type is supported by the absence of well-defined interlayered clay sequences. Although the permeability has decreased due to mineral deposition, petrographic evidence, such as sheared cores and secondary minerals with deformed cleavages indicate a deformation events after their deposition. This may have created new permeability that helped sustain the system . |