Subsurface fluid flow of reservoirs in active tectonic regions is mainly controlled by permeability structures, like faults and fractures, in the subsurface. Therefore, the location and characterization of permeability structures is an important step towards estimating the ultimate productivity of a reservoir. Moreover, subsurface fluid flow controls pressure and temperature conditions in the reservoir. In this study, the influence of fault zones on subsurface fluid flow in geothermal reservoirs is investigated using advanced exploration methods including fluid-rock interaction and numerical simulation. The results show, that the Lahendong, Indonesia, geothermal field consists of two geochemically distinct reservoir sections of which one is characterized by acidic water, considerable gas discharge and high productivity, while the other is characterized by neutral water and lower productivity. The two reservoir sections are separated by faults, which are less permeable across strike than along strike. Hydrochemical studies show, that increased fluid flow in these highly fractured areas enhance, in an alteration stage, chemical reactions resulting in strong induced hydrothermal alteration of surrounding rocks. Numerical simulations result in a detailed permeability and fluid flow pattern for the Lahendong geothermal reservoir. Adjusting model permeability values reveals the location of fractured zones, which have not been traceable in former surface studies. A further result is the subsurface temperature distribution, which suggests convective heat flow driven by fluid buoyancy. This hydraulic gradient causes a pressure drop along the reservoir. High pressure occurs in recharge areas at a foot of a volcano, while discharge is through permeable zones towards hot springs at the surface. Detailed investigation of subsurface fluid flow in geothermal reservoirs is crucial for sustainable exploitation avoiding drilling into less productive areas. Although the target area is Lahendong, our approaches are applicable for other geothermal sites consisting of similar boundary conditions.

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