A hydrotectonic model of a geothermal reservoir – A study in Lahendong, Indonesia

• Field survey based structural geological model for the geothermal system Lahendong.
• Increased fault permeability parallel to strike.
• Stress field from inversion of fault slip data in tropical environments.
• Well data infer structural controls on fluid flow.
• Integration of structural geometry and hydrogeological data.

In this study tectonic structural features and their hydraulic characteristics of fault zones were integrated into a consistent hydrotectonic model of the Lahendong geothermal reservoir, Indonesia. Moreover, these elements were analysed with respect to their relevance for the operation of the geothermal power plant at initial conditions, i.e. before the start of operation. The complex tectonic setting with volcanic activity provides evidence for relevant structural and hydrogeological elements, such as fault zones, surface spring discharge and joints at different spatial scales. The study area is highly variable with respect to hydraulic properties and chemical composition of the fluids. It consists of two types of fluids. Acid brine water with a pH of around 3 and an electrical conductivity ranging between 4620 μS/cm and 9700 μS/cm is characteristic for the reservoir in the North with temperatures up to 274 °C. A moderate pH between 4 and 7, an electrical conductivity in the range of 400–1730 μS/cm and temperatures of up to 340 °C characterise the southern study area. The Lahendong geothermal field is subdivided into two sub-reservoirs. Faults are less permeable perpendicular to the strike of the faults than parallel to the strike. The characteristics of the complex reservoir system could be explained by the combination of hydrotectonics and hydrogeological parameters. Understanding the permeability distribution along fault zones is crucial to investigate subsurface fluid pathways as well as to sustainably use the reservoir. A compartmentalisation of the reservoir was derived from a stress field analysis of the tectonic elements and from hydrogeological observations. The information on underground fluid flow is essential to understand the subsurface flow of geothermal fluids. Here, the permeability of structures is identified as the limiting factor.