SUBSURFACE ALTERATION AND HYDROTHERMAL FLUID CHARACTERISTICS IN SIKIDANG BLOCK OF DIENG GEOTHERMAL FIELD, CENTRAL JAVA

• Main Author: Saumi Juta, Alyssa
• Format: Final Project
• Language: Indonesia
• Subjects: Geologi, hidrologi & meteorologi
• Online Access: https://digilib.itb.ac.id/gdl/view/77700
• Institution: Institut Teknologi Bandung

Dieng Geothermal Field is a volcanic hydrothermal system located in Central Java, Indonesia. It consists of three prospect areas, i.e. Sileri, Sikidang, and Pakuwaja blocks. The Sikidang block is known to have serious problems of silica scaling and corrosion in the production facility. The problems are related to the subsurface geology and hydrothermal fluid characteristics of the system. This study aims to characterize the subsurface alteration and hydrothermal fluid in Sikidang reservoir. The methods carried out are literature study, petrographic analysis of cutting samples, clay treated X-Ray Diffraction, and fluid geochemistry analysis from three selected wells in Sikidang. The object of this study is three wells, i.e. ASJ-1, ASJ-2, and ASJ-3 (not the real name). The results show that the subsurface lithology consists of andesite lava, andesitic pyroclastic rocks, and microdiorite. All of these lithologies have undergone hydrothermal alteration with intensities ranging from low to intense. At well ASJ-1 there are two subsurface alteration zones, i.e. from shallow to greater depths, smectite and sericite zones. Well ASJ-2 has four alteration zones, i.e. from shallow to greater depths, smectite, chlorite-smectite, chlorite, and illite-epidote zones with the presence of kaolinite that are not continuous. Well ASJ-3 has similar subsurface alteration zones as ASJ-2, only in the deepest part epidote appears with chlorite, kaolinite, and pyrophyllite. The results of fluid geochemistry analysis show that well ASJ-1 discharge SO4-type water with acid pH, water from well ASJ-2 has a neutral Cl-type water, and water from well ASJ-3 has a neutral SO4-type water. A comparison between the results yielded from study of the hydrothermal alteration and the condition at the present well discharge suggests that the reservoir fluid in Sikidang has a relatively stable temperature of 280–300°C. The presence of alteration minerals in wells ASJ-1 shows consistency with the current acidity of the well discharges. A Change in the water acidity is well shown at ASJ-2 and ASJ-3; it is indicated by the presence of acid alteration minerals, e.g. kaolinite and pyrophyllite, that are overprinted by neutral alteration minerals, e.g. epidote and chlorite. The results also indicate that the top of reservoir in Sikidang has deepened.