A BSTRACT The “X” geothermal area is located in West Java Province, which is a geothermal area with high terrain geothermal system that has all the elements of a complete geothermal system, one of which is the cap rock. Cap rock is an impermeable rock formations that can block the geothermal fluids to pass vertically to the surface and is one of the important elements in a high terrain geothermal system. The cap rock of this particular geothermal system is consist of Quaternary volcanic rocks and materials, which is lapillic tephra, tuff, tuffaceous breccia, andesite breccia, and andesite. Based on the zoning of the hydrothermal alteration, the “X” cap rock consist of smectite-kaolinite-halloysite-silica alteration zone comparable with argilic alteration zone and illite-smectite-chlorite-quartz alteration zone, which is comparable to outer propylitic alteration zone. The smectite-kaolinite-halloysite-silica alteration zone is formed at temperatures of 120-200°C with chemically acidic forming environment, while the illite-smectite-chlorite-quartz alteration zone is formed at temperatures of 180-250°C with chemically neutral forming environment (Corbett and Leach, 1997). The “X” cap rock is not completely impermeable because of the abundance of hematite as an alteration product of the rock matrix, ferromagnesian, and volcanic glass, it is an indicator of ground water influx in the cap rock formation. There is also an anomaly on the cap rock’s geometry, which is the thinning of the clay cap trending east to northeast.