The extensive data sets obtained by the KTB drilling project (lithological and structural information, BHT values, temperature logs, rock thermal properties) provide a unique opportunity to construct realistic thermal models and thus to shed light on thermal conditions in the upper crust. Our numerical simulation study, a Swiss contribution to the German KTB drilling project, aims to understand the steady-state thermal and hydraulic field in the surroundings of the KTB. The simulations consider state-of-the-art petrophysical aspects relevant to deep, pressurized, high-temperature structures and were performed on discretized 2-D/3-D finite-element meshes that contain topography, geological structures and hydrogeological features. Our analysis of the KTB temperature field suggests three zones of particular geothermal settings: a low-heat-flow zone in the uppermost layers with a transition to high heat flow at 500 m depth; the underlying region accessed by the borehole with its characteristic uniform gradient; and the mid-lower crust that must be responsible for the high-heat-flow regime at the KTB site. The two first zones are treated in the present paper. A 3-D thermo-hydraulic model was set up in order to evaluate the first 2000 m, including the uppermost 500 m low-heat-flow zone. This model incorporates the complex geological information from the KTB pilot hole and topography-driven fluid flow. The lateral boundaries of the model were carefully chosen by analysing the flow pattern within a large, regional 3-D domain. The drilled section is analysed by a 2-D model using the available structural information. Due to dominating refraction effects, a careful temperature gradient analysis has to be carried out for such steeply dipping, anisotropic structures. Both models indicate a thermal regime dominated by diffusive heat transfer. Hydraulic flow seems to be important only for the uppermost (similar to 400 m) part of the drilled depth section; our simulations do not support significant fluid circulation at greater depths. In the drilled section the rather uniform gradient and the pronounced vertical heat-flow variations can now be explained. Finally, the potential and the limitation of the analysis of heat hows and temperature gradients are demonstrated. Heat-flow interpretations are conclusive only for nearly horizontally layered, isotropic geological units. In steeply dipping and anisotropic formations the heat-flow field is perturbed over a large distance (>1 km) around the point of interest. In such geological units only the temperature gradient interpretation can provide reliable information on the surrounding material.
