As the world’s population continues to increase, the demand for energy also increases. However, the use of fossil fuel energy has resulted in disadvantageous impacts for humans and the Earth. This condition becomes a good momentum to find a clean and more sustainable energy resources, given the fact that fossil fuel energy is a non-renewable resource that someday, in the future, its availability becomes scarce. Additionally, environmental awareness concerning energy-mix use and combating climate change also increases globally. Geothermal energy is one of the better alternatives for energy sources, as it is renewable as well as clean and green. A study from the Netherlands Organization for Applied Scientific Research (TNO) says that deeper Triassic sandstones, with possibly higher temperatures, could also potentially contain geothermal reservoirs. Therefore, this condition has paved the way for the exploration of the deep reservoir. The assessment of geothermal production usually faces considerable uncertainty due to, among other things, lack a comprehensive geomechanical model. Therefore, knowledge of the current state of stress is essential to address a wide range of problems that might arise during geothermal exploration and production—those problems such as wellbore stability, fault reactivation, induced seismicity, and deformation in depleting reservoirs. This study aims to construct a 3D geomechanical model in the Lower Germanic Triassic group, in De Lier field. By using the effective stress ratio concept, a 3D geomechanical model is constructed to describe the principal stresses distribution. The principal stresses distribution determines how the faulting stress regime will be formed. The vertical stress, as one of the principal stresses, is controlled by depth and density. On the other hand, the minimum horizontal stress is controlled by Poisson’s ratio. Four models are constructed based on several assumptions. In the model where gravity is the only source of stress, the maximum principal stress σ1 is always vertical. Whereas, in the models where tectonic stress is included, three depth intervals related to the faulting stress regime are observed. Imposing greater tectonic stress to the model will shift the depth of transition downward. Cross-section analysis shows that the local principal stresses variation due to the presence of different stratigraphic units and geological structures (faults and fractures). Fractures and faults at particular depth are inactive under the current stress field. Furthermore, pore pressure and friction coefficient have a significant impact on faults and fractures stability.