A geothermal bonanza bare of geomanifestations
Distinct local expressions of ongoing or past geological processes are referred to as geomanifestations. Many of them result from anomalies that may be an indicator of a “geopotential” in terms of subsurface resources capable for human use and valorisation. This specifically applies to geothermal manifestations like hot springs, vapour and gas emanations, sinter and tufa formations which are evidence for geothermal anomalies (increased heat flux) or faults tapping a deep-seated geothermal reservoir. However – nature is not always cooperative – such geothermal geomanifestations are rather the exception than the rule as they evolve at marked anomalies in bedrock terrains and areas with a thin cover of younger sediments only. Thick overburden blurs, displaces or completely averts the emergence of geothermal manifestations as field evidence. Prime examples are foreland basins, filled with thick sedimentary successions, these commonly feature a considerable geothermal potential but only very rarely any geomanifestation related to this subsurface potential.
Foreland basins develop along the forefront of emerging orogenic mountain belts due to the large-scale downwarping of the Earth’s crust that results from the enormous load produced by the crustal thickening caused by compressional tectonic forces. Due to the deep subsidence of the aquifers in the downgoing underthrusted slab and the basal basin fill, foreland basins are among the most promising hydrothermal plays although these basins are considered hypothermal (cooler than normal), with decreased geothermal gradient and heat flux as a consequence of the crustal thickening.
One striking example is the North Alpine Foreland Basin (Molasse Basin) that stretches for more than 1.000 km along the northern margin of the Alps, from Lake Geneva to almost Vienna. In its central part, the Bavarian Molasse Basin, bare of geomanifestations pointing to geothermal prospectivity, the conversion of an oil exploration drilling that encountered 65°C thermal water at 2.300 m depth into a geothermal well north of Munich in the mid 1990’s first proved the viability of geothermal production.
Since then, the Bavarian Molasse Basin turned out a true geothermal bonanza, especially in the Greater Munich area. To date, 23 geothermal installations have been completed successfully for thermal spas, district heating and power generation, two more are under construction. Thermal water of up to 150°C is tapped in depths down to more than 5.000 m. Despite some resent setbacks at the up to now deepest and hottest (160°C) exploration drillings the success story continues.
Beyond its outstanding hydrothermal potential the Molasse Basin features further natural resources. It is an important reservoir for drinking water production and bears considerable hydrocarbon deposits, albeit, after decades of investigation, considered mature in terms of oil and gas exploration. Most oil and gas fields are regarded economically depleted many larger ones, however, are after-used for gas storage to balance the seasonal swing of demand, or are envisaged for future grid energy storage of solar fuels.
Because the subsurface is a complex and limited resource this multiple use requires sound fundamentals for the unbiased and holistic subsurface management – to avoid use conflicts and to unlock potential synergies of developments in close proximity. The Bavarian Molasse Basin will be one of GeoConnect³d’s case studies to draw generic lessons from for formulating rules and proposals on the sustainable and optimized utilisation of the subsurface in order to support decision makers and regulators across Europe.
None of the resources and the deep geopotential of the Bavarian Molasse Basin find expression in a surface or subcrop manifestation. All the visible geomanifestations of the Molasse Basin are the testimony of glacial and interglacial processes of the Pleistocene ice ages.
Nevertheless, hydrothermal anomalies may form exceptionally impressive geomanifestations. See next week’s blog for a geothermal terrain of outstanding geomanifestations, but in an instable and inhospitable environment making its resources hard to utilize.
Gerold Diepolder
Bavarian Environment Agency (LfU) – Geological Survey, Germany
References / Further Reading:
Bayerisches Staatsministerium für Wirtschaft, Infrastruktur, Verkehr und Technologie (ed.) 2010: Bayerischer Geothermieatlas, 2nd ed., München. 104 pp., map-CD.
GeoMol Team (2015): GeoMol – Assessing subsurface potentials of the Alpine Foreland Basins for sustainable planning and use of natural resources – Project Report, LfU, Augsburg. 188 pp. http://www.geomol.eu/report/, last assessed February 2019.
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