Characteristics of Hydrothermal Alteration in Part of the Northern Vapor-Dominated Reservoir of the Wayang Windu Geothermal Field, West Java

Abstract

The Wayang Windu geothermal field is transitional between liquid-dominated and vapor-dominated conditions. A petrologic study of 522 core and cutting samples from four wells was undertaken. Three of the wells intersect the northern vapor-dominated reservoir at Wayang Windu. The fourth does not and has been used to make a comparison. The aim of the study is to characterize the alteration of the vapor-dominated reservoir in order to see if this yields useful information regarding its formation and how it may be best utilized. The samples were subjected to petrography, X-ray diffraction and fluid inclusion analyses. The results of this study show that the hydrothermal alteration above and within part of the northern vapor-dominated zone of Wayang Windu can be grouped into four main zones consisting of: (1) smectite zone (occurring at the surface down to ~700m depth); (2) transition zone (present at depths between 700m – 900m); (3) illite zone (~900m to ~1390m); and (4) amphibole zone (>1390m depths), although this zone may be restricted to the vicinity of a dyke. There are also zones of advanced argillic alteration. The presence of amphibole, epidote and wairakite, along with high temperature advanced argillic alteration, within a lower temperature vapor-dominated zone reflects earlier alteration episodes formed when this part of the system was more magmatically-related and water-dominated. Alteration associated with the current vapor-dominated conditions in the reservoir could not be clearly identified. However, there is overprinting above the reservoir where a condensate zone has formed.