Outcrop Analogue Studies for Reservoir Characterization of Deep Geothermal Systems in Upper Jurassic Limestone Formations (South Germany)

Homuth, Sebastian ¹; Götz, Annette E.¹; Sass, Ingo ¹
(1)Institute of Applied Geosciences, Technische Universität Darmstadt, Darmstadt, Germany.

The utilization of deep geothermal systems is based on a detailed knowledge of their distinct reservoir characteristics. In the early stages of hydrothermal reservoir exploration, the thermo-physical characterization of the reservoir is mainly accomplished by evaluation of already existing drilling data and seismic surveys in the vicinity of the target area. For reservoir predictions, geothermal parameters such as permeability, thermal conductivity, specific heat capacity and reservoir heat flow have to be quantified. In addition to these thermo-physical parameters, in-situ stress field analysis through uniaxial stress testing and structural tectonic data are important to assess. Outcrop analogue studies enable the determination and correlation of the thermo-physical parameters and structural geology data with distinct facies patterns, therefore the geothermal exploration concept becomes more precise and descriptive. For the economic utilization of deep geothermal reservoirs, a sufficient high flow rate of thermal waters throughout the reservoir to the production well is necessary. This flow rate is mainly controlled by the reservoir permeability. The outcrops of the Swabian and Franconian Alb as well as the transition zone of these two facies areas represent the target formations of the adjacent Molasse Basin. In the Molasse Basin, the limestone formations of the Upper Jurassic contain the main flow paths through tectonic elements and typically for limestone formations through karstification.

A high variation of thermo-physical parameters is recognized within one facies zone or stratigraphic unit; variations even occur within one outcrop. However, general trends indicate that the hydraulic flow patterns are related to tectonically created weak zones in the formations and that the matrix permeability has only a minor effect on the reservoirs sustainability. On the one hand these preliminary results show the necessity to gather more data from the target formations for setting up a reliable thermofacies model. On the other hand comparing our data with already existing data confirms that the applied methodology is appropriate and very productive.

The facies related characterization and prediction of geothermal reservoir parameters is a powerful tool for the maintenance, operation and quality management of an existing geothermal reservoir. Thus, the results of this study will also be used for further drilling design plans and reservoir enhancement measures.

AAPG Search and Discovery Article #90135©2011 AAPG International Conference and Exhibition, Milan, Italy, 23-26 October 2011.