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Abstract

Characterizing the permeability structure in geothermal greenfield exploration requires collection and integration of hydrochemical and structural geological data related to background geology and present-day stress field. A methodological combination of field based structural geology with hydrochemistry plays an important role in identifying the geothermal system and geologic controls on permeable zones, temperature distribution, fluid origin and the overall flow pattern. A case study in the Tarutung Basin in North Central Sumatra 170 km SW from the provincial capital city Medan, is addressed to this research topic in geothermal greenfield exploration of fault controlled geothermal plays in tropical environments. The Tarutung Basin is located at a right sided step over region of the prominent right lateral active 1650 km long Sumatra Fault Zone oriented in NW-SE. Satellite imagery and field mapping of the Tarutung Basin evidenced dominating NW-SE faults with secondary fractures striking N-S (extensional fractures), NNE-SSW (synthetic Riedel shears), and NE-SW (antithetic Riedel shears). Recently active extensional fractures and extensional shear fractures striking N-S & NW-SE serve as preferential fluid path ways. From these fractures, hot springs continuously discharge predominately bicarbonate water of 38 – 65.6° C and suggest that the permeability structure is controlled by the regional current stress regime in N-S direction. Fault slip data in on Miocene andesite and Quaternary travertine reflect the present-day stress field with the maximum principal stress as horizontal in N-S and a strike-slip stress regime. Clockwise rotation of secondary fractures indicates simple shear deformation in the Tarutung Basin, which is obviously a pull-apart basin. A conceptual model is developed to accommodate this rotation with the formation of dilational jogs between NNE-SSW striking secondary faults and the NW-SE striking major fault. These dilational jogs are associated with hot springs and a systematic pattern of geothermal surface manifestations can be explained that occurs at the eastern margin of the Tarutung Basin. This rotation also generated impermeable zones along ENE-WSW oriented faults that are at a high angle to the maximum principal stress direction. These faults act as barriers and might control the separation of two spring complexes of different chemical composition (acidic versus pH neutral bicarbonate water) at the northern basin margin. Hydrochemistry data show that calcium-bicarbonate water with significant chloride content dominates the composition of spring water at the eastern margin of Tarutung Basin. The surface temperature of these springs is up to 65.5°C. A non-equilibrium state of fluid-rock as shown with a four-cations-diagram (Na-K-Mg-Ca) suggests that a dissolution process in a low temperature condition is dominant in the Tarutung geothermal system. There are no evidences in spring water hydrochemistry for fluid-rock interaction processes at high temperatures. For this reason a silica geothermometer is selected rather than a cation based geothermometer and results in a maximum temperature of 115°C which could occur underneath the eastern spring area of Tarutung Basin. Only up to 10% of thermal water in the Tarutung Basin is of magmatic origin revealed from oxygen and deuterium isotopes. This isotope data imply that meteoric water is more dominant in this field. A permeable zone at the eastern side of the Tarutung Basin, as interpreted from structural geological data, provides a favorable pathway for meteoric water to infiltrate the area and to mix with the deeper fluids. Summarizing the Tarutung Basin is not associated with active magmatism. Instead a deep intrusion might generate the heat source for the hot spring waters that are recharged by predominately meteoric water. Fluid pathways are controlled by dilational and shear fractures striking N-S, NW-SE and NE-SW while ENE-WSW faults act as barriers.