HYDROTHERMAL SYSTEM IN MAGMATIC AREAS : FORMATION OF GEOTHERMAL SYSTEM AND ALTERATION IN CUBADAK REGION, PASAMAN REGENCY, WEST SUMATERA

• Main Author: NUR HADI, MOCHAMAD
• Format: Theses
• Language: Indonesia
• Subjects: Geologi, hidrologi & meteorologi
• Online Access: https://digilib.itb.ac.id/gdl/view/14318
• Institution: Institut Teknologi Bandung

Cubadak geothermal field is located in Pasaman regency, West Sumatra, and it lies on western part of Panti – Rao graben. Research of geothermal and alteration was conducted to prove the geothermal potency besides recognizing the evolution of hydrothermal fluid. Research compiled geology, geochemistry and geophysics methods. Petrography and XRD analysis were conducted to determine the rock alteration type and chemical analysis held to determine the fluid type and reservoir characterizations. Interpretations of geophysics are to describe the subsurface model. The occurrence of sericite - quartz minerals alteration represented in phyllic zone and chlorite – epidote - quartz formed in propylitic altered zone indicated the hydrothermal fluid was neutral pH alkaline Cl water having temperature < 340°C. Cubadak hotspring temperature was measured 75 °C, pH 6,4 and plotted of Cl and HCO3 are placed in diluted chloride bicarbonate water. Isotope and Gas ratio of CO2 / CH4 and N2 / Ar has shown fluid source were mixed in shallow meteoric water. Geothermometer calculation of Na/K is 220 °C (medium enthalpy). The fluid evolutions in Cubadak geothermal field begin from phyllic zone overprinted by propylitic and last were argillic, indicated of hydrothermal temperature were cooling. The measured of gravity shown contrast density value (2,5 gr/cm3) is diorite body placed more than 900 m beneath Cubadak hotspring as heat sources. The measured of sounding and resistivity interpreted that low resistivities layer (< 20 ohmm) are alteration zone as cap iii rock have thick 400 m began in 50 m in depth and resistivities (> 30 ohmm) assumed as shallow reservoir in old volcanic and metamorphic rock, top reservoir at 450 m. The result of research in Cubadak geothermal field affirmed that recent fluid Cubadak geothermal system has related to volcanic geothermal type in medium terrain. Cubadak prospecting areas has 2.5 km2 with potency of possible reserve are 30 MW.