Geothermal play typing in Germany, case study Molasse Basin: a modern concept to categorise geothermal resources related to crustal permeability

Abstract

The majority of running geothermal plants worldwide are located in geological settings with convection- or advection-dominant heat transport. In Germany as in most regions in Europe, conduction is the dominating heat transport mechanism, with a resulting average geothermal gradient. The geothermal play type concept is a modern methodology to group geothermal resources according to their geological setting, and characteristic heat transport mechanisms. In particular, the quantity of heat transport is related to fluid flow in natural or engineered geothermal reservoirs. Hence, the permeability structure is a key element for geothermal play typing. Following the existing geothermal play type catalogue, four major geothermal play types can be identified for Germany: intracratonic basins, foreland basins and basement/crystalline rock provinces as conduction-dominated play types, and extensional terrains as the convection-dominated play type. The installed capacity of geothermal facilities sums up to 397.1 MWth by the end of 2018. District heating plants accounted for the largest portion, with about 337.0 MWth. The majority of these installations are located in the play type ‘foreland basin’, namely the Molasse Basin in southern Germany. The stratigraphic unit for geothermal use is the Upper Jurassic, also known as ‘Malm’ formation, a carbonate reservoir with high variability in porosity and permeability. Recently drilled wells in the southernmost Molasse Basin indicate the Upper Jurassic as a tight, fracture-controlled reservoir, not usable for conventional hydrothermal well doublets. Our new data compilation including the recently drilled deep geothermal well Geretsried reveals the relation of porosity and permeability to depth. The results suggest that obviously diagenetic processes control permeability with depth in carbonate rock, diminishing the predictability of reservoir porosity and permeability. The play type concept helps to delineate these property variations in play type levels because it is based on geological constraints, common for exploration geology. Following the general idea of play typing, the results from this play analysis can be transferred to geological analogues as carbonate rock play levels in varying depth.

Porosity estimation of a geothermal carbonate reservoir in the German Molasse Basin based on seismic amplitude inversion

“…As a result, in most cases no general trends describing the porositypermeability relationship in carbonates based on stratigraphic units or lithology can be determined. This circumstance has already been described by Ehrenberg and Wadas and von Hartmann Geothermal Energy (2022) 10:13 Nadeau (2005); Homuth (2014); Bohnsack et al (2020) and Moeck et al (2020) for the carbonate reservoir in the Upper Jurassic formations of the GMB, which is a mainly fracture-and karst-controlled reservoir. As a consequence, a simplified classification of a carbonate reservoir based on small samples, e.g., from cores (Homuth 2014), is often not sufficient, because they are unable to depict the entire range of reservoir heterogeneity.…”

“…The main acquifer for hydrothermal exploitation in the German Molasse Basin (GMB) is located within the Upper Jurassic Malm carbonates (Stober and Villinger 1997;Jodocy and Stober 2009;Stober et al 2013;Steiner et al 2014;Stober 2014). The preferential targets in carbonate deposits of the GMB are fault zones, reef facies and karstified areas, since they are expected to act as hydraulically permeable zones due to high porosity and high permeability (Lüschen et al 2011;Birner et al 2012;Böhm et al 2013;Homuth et al 2015;Moeck et al 2020). One of the most ambitious projects in the GMB is the creation of a CO 2 emission-free district heating network for the city of Munich by 2040.…”

The rock mass as the governing factor for successfully developing deep geothermal systems in Southern Germany

“…Hydrothermal plays, e.g., geothermal wells Munich have either high permeability or high porosity, whereas petrothermal EGS have a low permeability and porosity (Geretsried) [3] Fig. 2 The drill trajectory of the Geretsried wells GEN-1 and GEN-1ST-A1, with the associated geology and location of the coring section with the seven core runs [4] G. Stockinger, K.…”

3DHIP-Calculator—A New Tool to Stochastically Assess Deep Geothermal Potential Using the Heat-In-Place Method from Voxel-Based 3D Geological Models

“…Currently, their deep geothermal energy potential is still untapped. It is well-known that these aquifers are geothermal reservoirs that have been exploited for a long time in other places of central and western Europe, such as the Malm limestones of the Molasse Basin in Germany [51], the Dogger limestones of the Paris Basin [52], and the Buntsandstein sandstones in northern Germany [53] and the Upper Rhine Graben [54].…”