Geological Model and Permeability Framework of Bukit Daun Geothermal Field, Indonesia

Abstract

This thesis investigates controls on permeability in the Bukit Daun geothermal field, Indonesia. A geological model is constructed using published and company (PT Pertamina Geothermal Energy) data that are integrated from regional to wellbore scale. A hydrological model is then developed by synthesising the geological model with fluid flow data acquired from drilling activity during the exploration stage of the field. Pressure and temperature measurement and analysis from well data confirm the potential geothermal resource in this area. Based on the data synthesised in this study, the dimension of the reservoir is at least 15 km². The Bukit Daun geothermal field occurs on the flanks of a stratovolcano that is located in a right-stepping interaction zone between two right-lateral segments of the Sumatra Fault System. It is a type of neutral vapour-core geothermal system where the upflow zone rises in the Mt. Bukit Daun area and manifests as Kawah Belerang steam surface manifestation. The hydrothermal fluid flows laterally toward north through the permeable N-S-striking structural zone and is bounded by NW-SE-striking Hulusimpang 2 fault in the outflow zone, which localises vertical flow as demonstrated by the distribution of the Tujuh Warna steam-heated surface manifestations. The geology of the study area is dominated by Quaternary volcanic products, which overlie and largely obscure Neogene, Paleogene, and Cretaceous volcanic, volcaniclastic, and metasedimentary formations. Accordingly, the geological architecture of the Bukit Daun geothermal system is heterogeneous, with sharp lateral and vertical changes in lithology and a complex structural geometry arising from the current and past tectonic regimes. Hydrothermal fluid flow in the Bukit Daun field is influenced by N-S, NNW-SSE, and NE-SW-striking fractures, which are more common in older formations. Such structures likely formed during forearc basin development in the Paleogene and, in the current tectonic regime, reactivate within a Riedel shear system. Formation permeability also exerts an important influence on fluid flow, as demonstrated by the occurrence and character of high-flux feed zones in the volcanic breccia within the Hulusimpang formation.