Lithofacies and depth dependency of thermo- and petrophysical rock parameters of the Upper Jurassic geothermal carbonate reservoirs of the Molasse Basin

Abstract

In the early stages of hydrothermal reservoir exploration, the thermo-physical characterization of the reservoir is accomplished by evaluating drilling data and seismic surveys. Especially in carbonate reservoirs the distinction of different facies zones and heterogeneities in general is very complex. For economic reasons a sufficiently high flow rate toward the production well and an according high fluid temperature is necessary. For reservoir predictions and modelling, geothermal parameters such as permeability, thermal conductivity/diffusivity, and specific heat capacity have to be quantified. The thermo-physical parameters are facies related. The application of a thermofacies classification to Upper Jurassic limestones serves to understand the heterogeneities and to identify production zones. Outcrop analogue studies enable the determination and correlation of facies related thermo-physical parameters and structural geology data and thus the geothermal exploration concept becomes more precise and quantitative. The analogue outcrops of the Swabian and Franconian Alb represent the target formations of Upper Jurassic carbonate reservoirs in the adjacent Molasse Basin. These limestone formations contain the main flow paths through fractures, faults, and characteristic of limestone formations also through karstification. The type and grade of karstification is also facies related. In general the matrix permeability has only a minor effect on the reservoir's sustainability except for some grainstones and dolomitized zones with higher porosities and permeabilities. Permeabilities range from 10-18 to 10-13 m² (0.001 mD to 100 mD). The permeability range of mudand wackestones is about the same. A high variation of thermo-physical parameters is recognized within individual facies zones or stratigraphic units. Mud-and wackestones show thermal conductivities around 2 W/(mK), while mudstones have lower thermal conductivities than wackestones. The thermal conductivities of massive reefal limestones show values of 1.8 to 3.9 W/(mK). Secondarily silicified reefal limestones and dolomites show the highest values of thermal conductivity. These parameters determined on oven dried samples have to be corrected for water saturated rocks under the according temperature and pressure conditions using transfer models. A comparison of calculated reservoir properties with measurements from deep drill cores confirms a good correlation. Based on the investigation of the matrix parameters in combination with reservoir transfer models, the reservoir prognosis and numerical simulation can be improved. The facies related characterization and prediction of reservoir formations is a powerful tool for the exploration, operation, extension and quality management of geothermal reservoirs in the Molasse Basin. Kurzfassung In der Planungsphase einer hydrothermalen Reservoirerkundung erfolgt die thermophysikalische Charakterisierung des Reservoirs durch die Auswertung von Bohrungsdaten und seismischen Erkundungen. Im Falle von Karbonatreservoiren ist...

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