Assessment of High Temperature Aquifer Storage Potential in Depleted Oil-Reservoirs from the South German Molasse Basin

In the discussion about the future role of geothermal in the energy transition policy, the topic of underground heat storage became recently more and more prominent. High Temperature Aquifer Storage (HTAS) may make geothermal more efficient by extending it beyond its traditional usage as a base load with also covering middle and even peak load. Depleted oil reservoirs can provide this underground storage capacity and Stricker et. al (2021) have numerically described the thermal storage potential in depleted oil fields from examples of the Upper Rhine Graben. Hydrocarbon exploration and production in the Northern Alpine Foreland Basin accelerated after 1950. It reached its peak in the 1980s, and then decreased mainly due to the low oil prices. Numerous separated reservoir units were successfully developed and exploited. The related extensive exploration campaigns provide exhaustive seismic profiles and borehole data for delineation of geometric underground features and reservoir properties. Since the outgoing 1990, parts of this data were already applied for the successful hydro-geothermal exploration of the Upper Jurassic Malm, especially in the greater Area of Munich and the Eastern part of the Molasse. The present study focusses on the geological and hydrogeological potential of high temperature storage in the surrounding of the existing oil fields in the South German Molasse basin. Reservoir information and data as e.g. thickness, porosity and depth of the reservoir rock as well as overlying barrier properties are compiled from two meta-studies, the Geothermal Atlas of Bavaria (STMWi, 2004) and Storage Catalogue of Germany (BGR, 2011). As a result, about one third of the area of the Bavarian Molasse shows a potential underground storage with a reservoir thickness of 10 m and more in depths between about 500 and 1700 m. In the Western part, the potential storage units are the “Bausteinschichten” of the Lower Oligocene with a porosity ranging from 5 – 31 %, and the Middle Jurassic Dogger “Eisensandstein” with an average of 15%. In the Eastern part, Chattian sandstones of the Upper Oligocene with porosities of 20% are present. In a next step, oil field information with the borehole data and its exploitation history has to be investigated, to gather more details on local reservoir characteristics as e.g. temperature, pore pressure and to develop an exploration and exploitation strategy to better determine the uncertainties and risks.