Kämmlein M, Bauer W, Stollhofen H (2017)
Publication Language: English
Publication Type: Conference contribution, Conference Contribution
Publication year: 2017
Pages Range: online
Conference Proceedings Title: German Geothermal Congress GGC 2017
Event location: Munich
A geothermal anomaly in the Franconian Basin/NE Bavaria was already described in the 1980s and further specified in the 2000s. An undisturbed temperature log in a 1309 m deep well provided a geothermal gradient (gradT) of 4.8 K/100m. The enhanced geothermal gradients led to the development of several balneological facilities in the area. However, heat flux in the Franconian Basin has never been quantified. Only supra-regional heat flux densities for the Rhenohercynian and Saxothuringian of 65 – 85 mW/m2 were published, based on a few temperature data only from 50 – 300 m depths and literature derived thermal conductivity values. Within the framework of a study on the geothermal potential of NE Bavaria, a new dataset of statistically evident thermo-physical parameters of almost all stratigraphic units of the up to >1600 m thick sedimentary cover rocks of NE Bavaria was created.
The newly established data pool comprises measurements of 932 core samples, 661 plug samples and 57 rock fragment samples. Thermal conductivity measurements were performed on dry and water saturated core samples and powder samples from rock fragments. For the determination of gradT, available temperature logs and BHT data of deep wells in NE Bavaria were evaluated. Temperature data were derived from log sections, which are not influenced by artesian upflow. For BHT correction different methods were tested. Heat flux density calculations were performed by the interval method, which is considering the thicknesses of the individual stratigraphic units and their key parameters.
In total, temperature data of ten deep wells >500 m in the Franconian Basin were evaluated. Nine of these wells, which are located within the Basin, provide moderate to significantly enhanced geothermal gradients of 3.4 – 4.8 K/100m. Two of these wells do show enhanced gradT in the basement rocks. In contrast, one well at the outer edge of the basin and near the Franconian Line provides a below-normal gradT of 2.5 K/100m. By applying temperature corrected measured thermal conductivities and corrected underground temperatures, the area reveals significantly enhanced heat flux densities between 92 to 128 mW/m2. The deep well with the highest geothermal gradient of 4.8 K/100m provides heat flux densities of 128 mW/m2. The value distribution of heat flux density and geothermal gradients points to a local positive temperature anomaly within an area of about 80 x 50 km.
The calculated heat flux densities lie in the order of magnitude of heat flux densities known from the Upper Rhine Rift, which provides heat flux densities around 110 mW/m2 and within the range of heat fluxes of other rift zones worldwide. The exceptionally high heat flux density value of 128 mW/m2 in the centre is based on an undisturbed temperature log which was measured 567 d after the shutdown of the drilling. This time after circulation is sufficient for thermal equilibrium in the borehole. The enhanced gradT in basement rocks of two wells points to a convective thermal regime, the heat source of which is most probably located in the deeper crust. Results of gravimetric modelling in the area, suggest the presence of local granite bodies within the basement rocks and calculations of radiogenic heat production of granitic outcrop analogues shows them sufficient to explain the enhanced gradT in the area.
APA:
Kämmlein, M., Bauer, W., & Stollhofen, H. (2017). New thermophysical data pool for NE-Bavaria reveals exceptionally high heat fluxes. In German Geothermal Congress GGC 2017 (pp. online). Munich.
MLA:
Kämmlein, Marion, Wolfgang Bauer, and Harald Stollhofen. "New thermophysical data pool for NE-Bavaria reveals exceptionally high heat fluxes." Proceedings of the German Geothermal Congress GGC 2017 (Der Geothermiekongress 2017), Munich 2017. online.
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