CHARACTERISTIC OF EPIDOTE IN SUBSURFACE ALTERATION IN THE WAYANG WINDU GEOTHERMAL FIELD, PANGALENGAN, WEST JAVA, INDONESIA
Mira Meirawaty (a*), Dwiyogarani Malik (b)
a) Lecturer of Geological Engineering Dept, Universitas Trisakti
Jl. Kyai Tapa No.1, Jakarta Barat-11440
b) PT. Star Energy Geothermal
Abstract
Epidote is a Calc-silicate mineral which is important in geothermal system because it shows high temperature reservoir when the reservoir in neutral pH fluids condition. The presence of epidote in the Wayang Windu geothermal field is very interesting because it comes in two forms, the epidote that replacing primary felsic minerals and the epidote that have been replaced by low temperature calcite-anhydrite minerals. This research is carried out by microscopic analysis (petrographic observation and XRD) of cutting samples from wells in southern area that represent liquid dominated system, and one in the northern that represent the vapor dominated system. The calcite and anhydrite overprinting epidote occur in all three reservoir-s wells, their occurance show abundance of CO2 and H2S gas in the reservoir. These type of epidotes occur right above the heat loss zone in reservoirs well with liquid dominated system, its being intrepeted associate with the boiling zone in the past. Whereas its occurrence in vapour dominated reservoir that present continuesly from certain depth was intrepreted associate with water condensate that move downward to the reservoir. The Calc-silicate minerals in vapour dominated reservoir reflects earlier alteration episodes when the system was more liquid-dominated. The vapor dominated system in the northern area is well indicated by a thick cap rock consist of smectite-cristobalite, whereas this zone is become thinner to the liquid dominated system in the southern. The reservoir of vapor dominated system are composed of acidic pyrophyllite and sericite-illite alteration, meanwhile the reservoir of liquid dominated composed of Calc-alkali mineral that show neutral pH fluids. This research is supported by fluid inclusion analysis to see further fluid evolution in the study area.
Keywords: Epidote, Geothermal, Wayang Windu, Vapour dominated, Liquid dominated
Topic: Geology