De-Risking Geothermal Play Exploration In Active Petroleum Basins: The Case Of St Gallen (Ne Switzerland)

Abstract

Usually, geothermal exploration activities in green fields such as the one in St. Gallen (North-East Switzerland) are confined to limited areas in the surroundings of the well to be drilled.The chance to move to production operations are strongly dependant on the success of the first exploration well. However, the presence of hydrocarbons in the area is notorious as a potential risk for geothermal exploration. If not properly predicted, the manifestation of overpressured hydrocarbon fluids (oil and gas) can hinder the drilling operations and the whole geothermal project. This was the case of the St Gallen geothermal GT-1 well in 2013. The occurrence of this unexpected event opened several scientific questions, such as: are hydrocarbon hosted in a deep reservoir in the area or do local faults network promote the plumbing of fluids? Are there in the subsurface source rocks mature enough to generate hydrocarbons? What is the realistic estimate of the volume of oil and gas in place in the area? The potential for hydrocarbon generation and accumulation in a sedimentary basin is strictly related to the temperature attained by the basin infill over time and the presence of organic matter-rich deposits. The temperatures recorded in the basin are a direct consequence of the tectono-sedimentary evolution of the basin. The extension rate of the crust controls the heat flow in the basin area, as a consequence of the asthenosphere upraises, and the sedimentation rate determines the burial depth. Therefore, in order to estimate the volume of hydrocarbons stocked in the subsoil, the subsidence history of the area is taken into account. The thermal history of the St. Gallen area was quantitatively simulated using computer-based 3D modelling. In the latter, the petrophysical features of the units forming the basin stratigraphic sequence were integrated, together with the geochemical properties of the potential hydrocarbons source rocks. To validate the modelling results, paleothermal proxies were used. The results performed by this work allow us to estimate the thermal maturation state of the potential source rocks of the area and assess the hypothesis on the hydrocarbons generation, migration timing. Thus, an estimate of the volume of hydrocarbons stored in the area can be provided.

AAPG Datapages/Search and Discovery Article #90346 ©2019 AAPG European Region, 3rd Hydrocarbon Geothermal Cross Over Technology Workshop, Geneva, Switzerland, April 9-10, 2019