In sedimentary basins, geothermal resources often coexist with hydrocarbon fluids that can represent a risk to geothermal exploration. Unexpected occurrence of substantial amounts of hydrocarbons could delay, or even halt drilling operations and damage the social perception of deep geothermal exploration activities. Geothermal exploration projects therefore need to be supported by a full understanding of the evolution of the sedimentary basin and specifically the generation, migration, accumulation, preservation potential and physical properties of hydrocarbon fluids. Resolving the origin of possible hydrocarbons alongside evaluating the processes controlling the active petroleum system(s) in the area targeted for geothermal exploration is therefore key. The Swiss Plateau, part of the more extensive Northern Alpine foreland basin (NAFB), is characterised by widespread occurrences of hydrocarbon seepages, which attest to a complex active petroleum system characterised by multiple plays. Since the last century, oil and gas indications have been regularly found as seepages in deep and shallow wells drilled throughout the Plateau. Nevertheless, in most cases, hydrocarbons have not been found in economically attractive quantities (the exception being, the only short-lived gas field found in the Entlebuch-1 well). Thus, over time, Switzerland has been excluded as a potential hydrocarbon exploration region. However, the geothermal wells drilled during the last twenty years have encountered some oil and gas (e.g. Schlattingen-1; St Gallen; GEo-01; etc.), which in some cases have had both short and long-term negative economic and social impacts on the geothermal exploration projects, both locally and nationally.
Objective 1 of the UNCONGEO project is to evaluate the presence and potential distribution of hydrocarbons in the subsurface of the Swiss Plateau region. To achieve this objective, the first essential step was to understand the active petroleum system in the Swiss Plateau. This required the identification, characterization and quantification of the elements and processes controlling the generation, expulsion, accumulation and preservation of oil and gas in the subsurface through the study area.
Objective 2 of the UNCONGEO project is to evaluate the risk associated with the presence of hydrocarbons in the subsurface throughout the entire Plateau and graphically represent it in Common Risk Segment (CRS) maps. CRS maps illustrate the spatial distribution of geological risk associated with a "working petroleum system" within a wider active Petroleum Play. These maps aim to define the likelihood of encountering hydrocarbon accumulation in the Swiss Plateau where a future deep (> 0.5 km) geothermal well could be drilled. The definition of “risk”, as it is intended throughout the text of this report, explicitly refers to the likelyhood of encountering hydrocarbons in the subsurface. In the past 80 years of subsurface exploration, in Switzerland, occurrences of hydrocarbon proved to be small. Therefore, the word “risk” in this report does not by any means translate into the overall risk that a geothermal project might have if located in a zone defined “medium/high risk” in the CRS maps, but on the likelihood that hydrocarbons are found during the exploration and exploitation activities. Hydrocarbon presence is manageable by adapting adequate technology such BOP and appropriate mud control at the rig side during the critical phases of drilling operations.
