Hydrothermal alteration study of the Kamojang geothermal field, West Java

Abstract

The vapor-dominated Kamojang geothermal field is located in West Java (Indonesia). The field lies in high terrain, 1500 m above sea level, and is hosted by volcanic rocks. Seven wells were available for this study, namely KMJ-7 (monitoring well, 536 m deep), KMJ-10 (dry hole, 763 m), KMJ-11 (production well, 1029 m), KMJ-12 (production well, 1500 m), KMJ-53 (exploration/production well, 1300 m ), KMJ-59 (exploration/production well, 1298 m), and CHR-1 (exploration/production well, 1804 m). The reservoir is hosted by interbedded pyroclastics and lavas of andesitic composition with some basaltic andesites. The permeable features are dominated by fractures, but some primary permeable features e.g. intergranular pores, and contacts between different lithologies survive. Fluid produced from the field is now vapor but the hydrothermal minerals present in the reservoir show that the rock altering fluid was liquid. The main steam zone is capped by a steam-heated layer. The water in the upper part of this layer is steam condensate of acid sulphate composition, and the water in the lower part is of near neutral pH. Isotope evidence shows that the recharge water is meteoric. No deep neutral pH water discharges at the surface. There are 2 hydrothermal mineral assemblages present at Kamojang, namely those produced by the "acid" and "neutral" pH fluids. The "acid" mineral assemblage, which occupies shallow levels (down to 100 - 300 m), consists of kaolin, smectite, alunite, quartz, cristobalite, and pyrite. This altering fluid was of acid sulphate type which formed due to the oxidation of H2S. The "neutral" mineral assemblage occupies the deeper levels, and comprises varying proportions of quartz, adularia, albite epidote, titanite, wairakite, laumontite, calcite, siderite, hematite, pyrite, anhydrite, smectite, chlorite, illite, and interlayered clays. The altering fluid was liquid of near neutral pH, and chloride-sulphate type. Mass transfer was dominated by removal of some major constituents, but only small portions of them taken from the rocks during alteration were incorporated into secondary minerals in cavities; the rest was removed in solution. Si, Al, Ca, Na, and K were depleted; Fe 3+ and Mg were mostly lost; Ti, Mn, and P were mostly gained, while the behaviour of Fe 2+ varied. Trace elements were variably gained or lost. In general, the present-day measured downwell temperatures are lower than those indicated by the mineralogy (e.g. epidote, wairakite, laumontite, and clays) by about 10 - 50 °C and fluid inclusion homogenisation temperatures (by about 60 °C). This suggest cooling, although the downwell measured temperatures may be too low. Permeability has decreased due to mineral deposition but tectonic activity has helped maintained it by reopening vertically permeable features. Hydrothermal mineral parageneses and vein cross-cutting relationships in the deeper levels, for example, veins of quartz+wairakite-calcite-anhydrite cut by veins of quartz-pyrite, suggest that although this field has undergone at least 3 hydrothermal episodes, the composition of the altering fluid remained constant, i.e., of near neutral pH. An early episode is characterised by simple quartz vein and a middle one by many mineralisation with more diverse mineralogy, including quartz, calcite, siderite, hematite, chlorite, illite, wairakite, epidote, and pyrite. Anhydrite, which formed due to the presence of acid sulphate-rich water, marks the last stage of the middle episode. The last episode was characterised by the deposition of quartz and pyrite. Like Larderello (Italy), and The Geysers (California), other vapor-dominated geothermal fields, Kamojang may have evolved from an earlier hot water or liquid-dominated system. With the reduced permeability resulting from the intensive water-rock interaction, heated by a potent heat supply (possibly magmatic), the hot water boiled off more water than recharge could replace, and the reservoir fluid became dominated by vapor. The liquid phase contained within pore spaces has lower mobility than vapor so that only small amount of liquid enters the producing wells.