Influence of basement morphology on hydrothermal convection in the Upper Rhine Graben 

Abstract

The Upper Rhine Graben (URG) is the central segment of the European Cenozoic Rift System which is known for hosting some of Europe’s major geothermal anomalies. Historically, the region has been explored for its hydrocarbon resources and more recently the area has been targeted for deep geothermal energy. Since then, several heat and/or power plants have been commissioned and are currently in operation. However, despite the gradual development of the sector, the technical potential of the URG remains under-exploited. While the first deep geothermal projects benefited from thermal anomalies known at surface, new projects require costly exploration techniques to ensure a right combination of elevated temperature and sufficient permeability. Several numerical modelling studies have attempted to reproduce thermal anomalies by integrating a complex three-dimensional geometry of the URG and assuming a topography-induced forced convection largely dominating free convection. As a result, the authors observe a basin-wide graben-perpendicular flow from the graben shoulders towards its center, with an upflow axis approximately below the Rhine River. These conclusions are in contrast to previous geochemical studies which suggest that deep brines discharged from the granitic basement are rather homogeneous on a large scale and have a common origine in deep Triassic sedimentary formations with temperatures close to 225 ± 25 °C. The brines would then migrate through sedimentary layers and permeable fault zones in the basement, from the center of the graben to its western flank, where they would flow up into horst structures such as Soultz or Landau. Moreover, this deep brine circulation in the central part of the graben is thought to be almost completely decoupled both from the circulation of less saline fluids in its upper part, in the Tertiary layers, and from flows along bordering faults, which would be characterized by a rapid recycling of meteoric water via deep circulation loops. Here, we suggest that thermal anomalies in the French western border of the graben result from deep convective cells developing in the basement along the inclined basement-sediments interface without any help from external pressure forces. Therefore, we used the GeORG public database to build a simplified three-dimensional numerical model of the central part of the URG. Results are obtained using the OpenGeoSys software. Conceptual numerical experiments of thermo-hydraulically coupled simulations were carried out, assuming density-driven convective heat transport with thermal dependence of density and viscosity parameters. The first series of models were constructed without any faults, and we show that an integration of basement morphology, a depth-decreasing basement permeability and a fixed heat flow condition at the base of the model is sufficient to trigger multiple upwellings in the basement within a few million years. Current on-going work is to further calibrate the model to reproduce known existing temperature records and to observe how the integration of permeability heterogeneity or one or more fault zones can reorganize the convective system, thus allowing to trace an effective permeable pattern at larger scale.