HYDROTHERMAL ALTERATION AND RESERVOIR FLUIDS EVOLUTION OF THE PATUHA GEOTHERMAL FIELD, WEST JAVA

• Main Author: Anthyanto, Indika
• Format: Final Project
• Language: Indonesia
• Subjects: Geologi, hidrologi & meteorologi
• Online Access: https://digilib.itb.ac.id/gdl/view/47244
• Institution: Institut Teknologi Bandung

Patuha Geothermal Field is a vapor-dominated geothermal system, associated with Quaternary Patuha volcano. However, the occurrence of hydrothermal minerals, such as epidote, garnet, and wairakite indicates that Patuha Geothermal Field was once liquiddominated system. The goal of this research is to understand the evolution of reservoir fluids (from liquid-dominated to vapor-dominated) by comparing the hydrothermal alteration mineralogy with present geothermal system conditions. This research uses secondary data of four wells (IA2, IA4, IA6, and IA8). This secondary data consists of 36 well cutting data, pressure-temperature measurement data, and drilling mud log report. This research uses few methods i.e. megascopic and microscopic observation of cutting samples especially from reservoir zone, X-ray powder diffraction (XRD) analysis of cutting samples, and pressure-temperature measurement curve analysis. The lithology of Patuha Geothermal Field consists of andesite lava and pyroclastic of late Pliocene to Holocene age. An unexposed intrusion of porphyritic andesite is discovered on wells. The intensity of alteration from samples is moderate to intense. Alteration zones consists of [1] Smectite-Hematite Zone (Paleocaprock Zone) which occurs at 2,030-1,950 masl, [2] Smectite-Chlorite Zone (Paleocaprock Zone) which occurs at 1,950-1,770 masl, [3] Smectite-Chlorite-Calcite Zone (Transition Zone) which occurs at 1,770-1,470 masl, [4] Illite-Chlorite-Epidote-Calcite Zone (Paleoreservoir Zone) which occurs below 1,470 masl, and [5] Sulfur Zone (Acid Zone) which occurs at 2,150-1,420 masl. Alteration parageneses from Paleoreservoir Zone indicates that paleoreservoir fluids were meteoric water with paleotemperature between 280 and 310°C, neutral pH, and low CO2 content. Paleoreservoir fluids interacted with surrounding rocks, had a slightly cooling, and boiled. Boiling happened continuously and caused paleoreservoir fluids to become denser. Denser paleoreservoir fluids caused liquid reservoir to decline with average decline of 1,020 m. Continuous boiling also caused vapor reservoir with average thickness of 720 m to form above the liquid reservoir and caused system to evolve to a vapor-dominated system.