Ternary porosity systems: New perspectives for Buntsandstein geothermal reservoirs in the Upper Rhine Graben, SW Germany
The clastic Lower Triassic Buntsandstein Formation in the Upper Rhine Graben of SW Germany and NE France has been identified as an attractive geothermal reservoir due to its fracture density and exceptionally high matrix porosity at depth levels of economic geothermal energy extraction. New petrophysical data from deep exploration wells reveal the existence of ternary porosity systems evolved during a multi-phase subsidence history and diagenesis especially at intra-graben structural highs. Primary elements of these porosity systems are high-permeability faults and fractures which can be utilized as technical fluid conduits connecting geothermal injectors and producers. Second component is the primary matrix porosity, controlled by pure mechanical compaction. Third component is an interconnected system of secondary pores and micropores. Secondary porosity originates from diagenetic dissolution of chemically and mechanically unstable framework grains like feldspars and rock fragments. At depths of about 2.300 m secondary porosity and microporosity can exceed the compaction-controlled primary porosity of around 7 %, causing high total pore volumes of up to 21 %. All matrix pore types are linked to form an interconnected pore network hosting significant connate brine volumes. These brine volumes don´t contribute to technical hydrogeothermal fluid cycling but increase the thermal capacity of the reservoir and favour heat conduction. Although this phenomenon has been described from hydrocarbon pools, their quantitative significance in geothermal reservoirs is still poorly understood. Micro-scale reservoir simulations may help to upgrade geothermal prospects.