| Title | The Application of Three-Dimensional Gravity Inverse Modelling to Reconstruct the Caldera Setting of Ulubelu Geothermal Field |
|---|---|
| Authors | Sotarduga Sahat Nainggolan, Imam Prasetyo, Mulyanto Mulyanto |
| Year | 2023 |
| Conference | World Geothermal Congress |
| Keywords | Ulubelu, Caldera, Gravity, 3D Modelling |
| Abstract | Ulubelu field, operated by PT Pertamina Geothermal Energy, is situated in the Province of Lampung, Indonesia. This field is considered a volcanic-high terrain geothermal system controlled by the Sumatra Fault System, which provides a high-enthalpy, water-dominated reservoir. Since 2017, the Ulubelu field has been generating 4x55 MWe of geothermal energy, which accounts for 25% of the electricity supply of the Province of Lampung (Pertamina, 2018). Radar and Imagery Landsat study by Siahaan et al. (2015) identified the Ulubelu field lies in the lineaments patterns, fault segments, circular features, and collapse structure, which are strongly influenced by the overlapping NW-SE trends parallel to Sumatera Fault System (Husein at al., 2015). Recent geological mapping was done to clarify the existence of the volcanic structure of the Old Sula Volcano by visiting its caldera wall. In addition, gravity exploration was conducted in 2021 to map the Bouguer anomaly of the system. The gravity data consists of 816 stations, covering 20 x 20 km of the study area. Complete Bouguer Anomaly (CBA) was calculated using 2.2 gr/cm3 of Bouguer density. The CBA anomaly of Ulubelu ranges from 24 mGal to 65 mGal, where the higher anomaly surrounds the lower anomaly in the existing exploitation area. The caldera of the Old Sula Volcano is shown as a circular shape of the high-Bouguer anomaly, which ranges from 54 mGal to 65 mGal and associated with andesitic rocks. The graben structure, situated inside the caldera, is shown by a slightly lower Bouguer anomaly 45 mGal – 50 mGal, which associates with pyroclastic rocks. CGG Geoscience performed a three-dimensional inverse modelling in 2021, which used a joint inversion scheme with Magnetotelluric data (CGG, 2021). The 3D model shows the caldera structure of Old Sula Volcano as a high-density 2.5 gr/cm3 - 2.6 gr/cm3 circular feature. Mount Rendingan, which provides the geothermal system's heat source, has a density of 2.6 gr/cm3, shown as a high-density body that intrudes on its lower-density surrounding rocks, is located at the Northern edge of the old Sula Caldera. The model estimates Sula's depression has a thickness of approximately 2500 m, which shows as a low-density 2.0 gr/cm3 - 2.4 gr/cm3 layers that fill the higher density caldera. |