Evaluation of the Productive Geological Structure in Ulubelu Geothermal System

Abstract

Ulubelu geothermal field is one of geothermal field operated by PT. Pertamina Geothermal Energy (PGE). It is located in Lampung, south end of Sumatra Island. This field has been operated since 2012 with generation capacity of 2 x 55 MWe from eleven (11) production wells. (Sieh and Natawijaya, 2000) divided the Great Sumatera Fault System (SFS) into nineteen (19) segments. Ulubelu field is situated in a volcano-tectonic depression zone which is part of Semangko segment in SFS. There are at least three major fault lineaments identified in the study area. The main structures are the NW-SE trending structures (F.Pagaralam and F. Talangmarsum) and WNW-ESE trending structures (F.Sula and F.Gunung Tiga). Within those faults, NNW-SSE trending structures (F.Muara Dua, F. Karangrejo, F.Rindingan) are also visible. F.Pagaralam and F. Talangmarsum are synthetic faults of the Semangko segment. They were dextral strike slip faults and formed prior to the Pliocene or early Quaternary. Due to kinematic changes, the strike slip faults turned into normal faults that formed the Ulubelu pull apart basin. F.Sula and F.Gunung Tiga are antithetic faults of Semangko segment. The faults are sinistral strike slip fault and controlled the permeability of the geothermal system on the north and south of the Ulubelu area. F.Muara Dua, F. Karangrejo,and F.Rindingan are P Shear or secondary faults of synthetic and antithetic faults in Ulubelu which are connected between the faults. Aside from surface geological data, the study also uses subsurface data such as borehole image logs from wells. The logs were performed in two wells (UBL-3 and UBL-26). From the fracture analysis, it shows that most of the Continuous Conductive Fractures (CCF) and Discontinuous Conductive Fractures (DCF) are trending NW-SE orientation. The faults intersected by wells have NW-SE direction and they dip to the SE (F.Rindingan 1, F.Rindingan 2, F.Rindingan 3). Only small fractions show the opposite direction to the NE-SW and to the N - W. This study reveals that the productive geological structure that provide a good permeability for hydrothermal fluid in Ulubelu geothermal system is trending NW-SE within the despression zone.