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For a variety of geothermal engineering applications, the only indirectly determinable matrix thermal conductivity (λ m) is frequently used to convert the measured bulk rock thermal conductivity (λ b) of air-saturated sandstones to water-saturated conditions. However, the necessary assumption that the absolute value of λ m remains constant irrespective of the pore fluid present turns out to be not valid in practice and the explicit control factors on λ m have not been demonstrated yet for different pore fluids. A pore fluid-controlled change in the λ m value also questions the transferability of empirical proxy models for the estimation of water-saturated λ b when they were calibrated on air-saturated samples. This study applies a multiple regression analysis to quantitative mineralogical composition data and porosity (Φ) to identify the controls of the λ m for different types of pore fluids (water-vs. air-saturation). In addition, the differences in the calculated λ m values resulting from different calculation methods or input data (theoretical geometric mean model or mineralogical composition data) are examined. We further test the suitability of different sandstone properties as potential proxies for the estimation of the λ b , with respect to the pore fluid type. Differences in the absolute value of λ m of sandstones from different measurement conditions (air-or water-saturated) are most probably related to the formation of authigenic kaolinite in the pore space, originating from the alteration of alkali feldspar. In addition, the thermal properties of the rock matrix are mainly controlled by the volume fractions of the high thermally conductive mineral fractions quartz and dolomite. Empirical models that have solely Φ or P-wave velocity (v p) as variables are not suitable for the prediction of water-saturated λ b of sandstones. Instead, quantitative mineralogical data of high thermally conductive mineral phases such as quartz and dolomite have to be included. The easily measureable v p is proposed as a promising proxy for both pore fluid types tested: combined with the total quartz volume/porosity ratio for air-saturated sandstones, and combined with the quartz plus dolomite volume fractions for water-saturated sandstones.

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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 m 2 or (2) a heat-producing granite of large cross-sectional area (300 km 2 ) 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.