THE CHARACTERISTICS OF BASE OF CONDUCTOR IN X GEOTHERMAL FIELD BASED ON THE MAGNETOTELLURIC AND WELL DATA: KARAKTERISTIK BASE OF CONDUCTOR DI LAPANGAN PANAS BUMI X PULAU FLORES BERDASARKAN DATA MAGNETOTELLURIK DAN SUMUR EKSPLORASI

MAKALAH ILMIAH THE CHARACTERISTICS OF BASE OF CONDUCTOR IN X GEOTHERMAL FIELD BASED ON THE MAGNETOTELLURIC AND WELL DATA KARAKTERISTIK BASE OF CONDUCTOR DI LAPANGAN PANAS BUMI X PULAU FLORES BERDASARKAN DATA MAGNETOTELLURIK DAN SUMUR EKSPLORASI 1 Dizanissa Purnama Sari1, Yunus Daud1, Haris Siagian2 Master Program in Geothermal Exploration, Department of Physics, University of Indonesia 2 KS Orka, Recapital Building, Jakarta ABSTRACT The X Geothermal Field in Flores, East Nusa Tenggara, has surface thermal manifestations such as hot springs, fumaroles, and alteration rocks, which indicate potential geothermal resources. This research aims to model and characterize the base of the conductor in X geothermal field as a supporting data for geothermal exploration studies. The modeling was performed using 3D magnetotelluric inversion analysis that was correlated with well data, i.e, drill cuttings, methylene blue, and Pressure and Temperature Test. According to the results, the subsurface condition of the study area consists of an argillic zone around 400-300 masl, a transition zone around 300 and 0 masl, and a propylitic zone below 0 masl. At an elevation of 400-300 masl, the base of conductor marks the boundary between the reservoir and the clay cap. The BOC distribution map reveals that the average BOC elevation is between 300 and 550 meters above sea level. The exploration potential is in the northern part with a prospect area of 3,4 km2 in research. Keywords: Base of conductor, Geothermal, Magnetotelluric, Well Data, Resistivity, 3D inversion ABSTRAK Lapangan Panas Bumi X di Pulau Flores, Nusa Tenggara Timur, memiliki manifestasi termal permukaan seperti mata air panas, fumarol, dan batuan alterasi, yang mengindikasikan potensi sumber daya panas bumi. Penelitian ini bertujuan untuk memodelkan dan mengkarakterisasi base of conductor di lapangan panas bumi X sebagai data pendukung eksplorasi panas bumi. Pemodelan dilakukan dengan menggunakan analisis inversi 3D magnetotellurik yang dikorelasikan dengan data sumur yaitu serbuk bor, metilen biru, dan Uji Tekanan dan Temperatur. Berdasarkan hasil penelitian, kondisi bawah permukaan daerah penelitian terdiri dari zona argilik sekitar 400-300 mdpl, zona transisi sekitar 300 dan 0 mdpl, dan zona propilitik di bawah 0 mdpl. Pada ketinggian 400-300 mdpl, base of conductor menandai batas antara reservoir dan claycap. Peta sebaran BOC menunjukkan bahwa ratarata elevasi BOC adalah antara 300 dan 550 meter di atas permukaan laut. Potensi eksplorasi tertinggi terdapat di bagian utara daerah penelitian dengan luas prospek 3,4 km2. Kata kunci: Base of conductor, Panas Bumi, Magnetotellurik, Data Sumur, Resistivitas, Inversi 3D INTRODUCTION Geothermal energy is thermal energy stored in the rock below earth surface as an alternative to renewable energy sources and as a source of electrical power. This energy is typically provided in hot fluids or steam with certain geological conditions Buletin Sumber Daya Geologi Volume 18 Nomor 1 - 2023 15 MAKALAH ILMIAH under the earth’s surface. The main components in the geothermal system include the reservoir zone, clay cap, and heat source (Yunus Daud et al., 2019). The reservoir zone is a permeable rock where hot fluids accumulate with a clay cap at the top. This clay cap zone can isolate hot fluid and steam in the system, preventing leakage (Syafitri and Putra, 2018). The hot fluid in the reservoir can be utilized to generate electricity. Therefore, an analysis of the conceptual model is required to describe the subsurface zone’s condition for the guidance of well targeting. One of the geophysical methods for describing the conceptual model of a geothermal system is the magnetotelluric (MT) method. This method is generally applied in geothermal exploration for mapping the subsurface resistivity structure associated with clay alteration cap, reservoir geometry and possible heat source. The MT method utilizes fluctuations in the perpendicular natural electric field and magnetic field at the earth’s surface to determine the conductivity value of rocks under the surface (Simpson and Bahr, 2005). Zones in geothermal systems with high conductivity or low resistivity values of rock are clay cap zones due to hydrothermal alteration of the clay cap. Thus, the MT method can be used to map the clay cap zone, reservoir zone, and the boundary between the two zones (base of conductor) with 3D MT of inversion modelling. Correlating MT analysis with well data is necessary for supporting the geothermal system analysis, including Methylene Blue (MeB) analysis, drill cuttings, and pressure and temperature test (P&T Test). The X geothermal field located on the island of Flores, East Nusa Tenggara (Figure 1) has great potential of geothermal energy. Because the field manifests hot springs and fumaroles, the X geothermal field indicates geothermal prospects (Harvey et al., 1998). The X geothermal field has hot springs with temperatures from 32-61oC and fumaroles from 78-97oC 16 (Sarmiento et al., 2019). This nature indicates that the X geothermal field has the potential to be of geothermal energy. Unfortunately, there are no studies about the characteristic of the base of conductor as identification of geothermal energy potential based on MT analysis. Therefore, the objective of this research is to model and characterize the base of conductor zone of the X geothermal field. METHODOLOGY Identification of the base of conductor from X field is performed using MT analysis supported by well data MeB, drill cuttings, and P&T survey. A total of 65 MT stations are used for data collecting (Figure 2a). The data was then reprocessed using standard MT data processing, including static shift correction using StaticShifter-X software. This correction is supposed to eliminate the static effect of the data portrayed by a vertical shift of the apparent resistivity curve uniformly at all frequencies or periods intervals. The following stage 3D inversion using MT3DInv-X applications. The result of 3D inversion was visualized using GeoSlicer-X software. All the softwares are developed by NewQuest Geotechnology. Supporting data for MT analysis is given in well data, MeB, drill cuttings, and P&T surveys. The results of MT data analysis are correlated with well data to construct a conceptual model supported by geological and geochemical data. The steps are illustrated in the flow chart as shown in Figure 2b. GEOLOGICAL AND BACKGROUND GEOCHEMICAL X geothermal field is located on Flores Island, Ende Regency, East Nusa Tenggara. Tectonically, this region is situated in the Banda Arc, where the IndoAustralian plate is subducting at a rate of about 6 cm per year (Hamilton, 1973). The geothermal volcanic complex in the study area covers about 400 km2 and extends for 23 km in a southwest-southwest (SSW) north-northeast (NNE) direction (Harvey et al., 2000). Buletin Sumber Daya Geologi Volume 18 Nomor 1 - 2023 : 15 - 26 MAKALAH IL (...truncated)