Mitigation of geothermal energy drilling risks in the presence of hydrocarbons: A basin scale approach applied to the eastern Swiss Plateau area (Schlattingen-1, Herdern-1 and St. Gallen-GT-1 geothermal transect)

Geothermal exploration can be complicated by the presence of hydrocarbons in the subsurface. This is a well-known issue in Switzerland, where geothermal projects were affected by the consequences of an unexpected finding of hydrocarbons during drilling operations. This was the case of the Schlattingen-1 well in 2011 and the St. Gallen-GT-1 well in 2013, both located in the north-eastern part of the Swiss Plateau. To confidently predict the potential of hydrocarbon occurrence in the subsurface, a 2D basin and petroleum system modelling (BPSM) was performed along a regional geological section of the eastern sector of the Swiss Plateau, including the Schlattingen-1, Herdern-1 and St. Gallen-GT-1 wells.

The BPSM reconstructs the temperature history of the basin by considering the geodynamic, structural and stratigraphic evolution of the basin. The modelling results were calibrated with present-day borehole temperature, porosity and vitrinite reflectance data. To calculate the hydrocarbon generation and accumulation potential in the studied area, the source rocks were geochemically and petrographically characterized. In the southern part of the section, gas accumulations are predicted by the model at the top of the Permo-Carboniferous graben, just beneath the St. Gallen GT-1 well. In the northern part, around the Schlattingen-1 and Herdern-1 well sites, the model indicates the migration and accumulation of hydrocarbons within the Mesozoic units. Migration to the uppermost Molasse units also occurs along faults, thrust planes and permeable units, when present.

The main uncertainties of the modelling results include: i) distribution and characteristics of faults; ii) thickness and hydrocarbon potential of the source rocks; iii) lithological and petrophysical properties of potential reservoir and seal units; iv) calibration data with big ranges and not uniformly measured. To refine the assumptions and enhance the understanding of the basin's evolution, further data should be collected and existing data reanalysed. Building risk analysis and probabilistic models would also help to plan drilling mitigation measures aimed at reducing these uncertainties. The workflow presented in this study can be highly valuable to geothermal projects. It allows more effective planning of the drilling procedure, thus reducing the operational risks and contributing to the overall project success.