The Franconian Basin thermal anomaly: testing its origin by conceptual 2-D models of deep-seated heat sources covered by low thermal conductivity sediments

Abstract

This study presents conceptual 2-D models for coupled fluid flow and heat transport simulations of the Franconian Basin in SE Germany to verify the plausibility of different hypothesis on the origin of a local temperature anomaly. The simulated geothermal systems consist of a deep-seated heat source within an impermeable basement (heat-producing granite or enhanced background heat flow), covered by low thermal conductivity sediments. Solely conductive or additional convective heat transport including the presence of a permeable fault was applied. We found that heat transfer in the model setups is strongly controlled by (1) the volume of the heat-producing granite, (2) the amount of the background heat flow, (3) the permeability of the basement rocks, (4) the thermal conductivity contrasts between the sedimentary cover and the basement, and (5) the type of heat transport. If there is no reliable information on these model parameters, a high degree of uncertainty with regard to quantitative statements on the heat transfer in the specific geothermal system can be expected. An equilibrium temperature log from the study area could only be reproduced by (1) an enhanced background heat flow of 0.115 W m−2, in combination with a permeable fault zone of permeability 1.0 × 10−13 m2 or (2) a heat-producing granite of large cross-sectional area (300 km2) in combination with an average background heat flow of 0.070 W m−2. Moreover, high temperatures were only achieved in the presence of a low conductive, insulating cover above the heat source.