Detailed assessment of geothermal potential by integration of a wide range of geological data: Preliminary results of a case study from a Lower Triassic low-enthalpy reservoir in the Tønder area in southern Denmark

Abstract

In the Tønder area in southern Denmark, the Lower Triassic Bunter Sandstone Formation constitutes a potential source of low-enthalpy geothermal energy. The formation is situated in the northern part of the North German Basin and is present at depths of c. 1600–2100 m in the Tønder area. The data coverage is excellent as it includes 7 wells plus 2D and 3D seismic surveys. A multidisciplinary study including interpretation of petrophysical logs, core analysis data, seismic reflection data, diagenesis, pore-water chemistry, burial history, sediment provenance and temperature data is used to estimate the geothermal potential in the Tønder area. The formation contains two primary reservoir units of porous permeable sandstone, of which the lower unit represents the Volpriehausen Member and the upper unit belongs to the Solling Member. The characteristics of the two reservoir units are dissimilar since they were deposited during different climatic conditions in different depositional settings, and the subsequent diagenesis has enhanced the differences. A high-saline brine and the presence of halite cement and nitrogen gas in the upper reservoir unit control reservoir continuity and performance. The lower reservoir unit is widely distributed and displays excellent reservoir properties, whereas the upper reservoir unit has variable reservoir properties due to large variations in reservoir architecture. An integrated understanding of the properties that control the reservoir quality is invaluable in assessing the geothermal potential through a detailed interpretation of the geological parameters. A state-of-the-art geological model has been established by combining data from various disciplines, and the model is used as input to a 3D simulation study of reservoir performance.