Structure of hydrothermal convection in the Upper Rhine Graben as inferred from corrected temperature data and basin-scale numerical models

Abstract

International audienceGeothermal anomalies in sedimentary basins are strongly controlled by fluid circulation within permeable zones. This study presents a new compilation of newly corrected bottom-hole temperature data in the French part of the Upper Rhine Graben, where the Soultz-sous-Forêts temperature anomaly is locked beneath a local horst structure. After a geostatistically constrained interpolation procedure, maps and cross-sections are extracted from the 3D thermal block, together with the associated standard deviations. Thermal anomalies are preferentially associated with the thickest zones of the permeable fractured Buntsandstein (sandstones) formation, in apparent contradiction with previous models where two major fault zones were suggested to control fluid flow. The underlying fractured granitic basement hosts fluid circulation patterns which are apparently controlled at large-scale by the inclined basement-sediments interface. Based on these observations, numerical models of hydrothermal convection including an inclined basement-sediments interface, a local horst structure, and realistic petrophysical properties have been carried out. The depth-decrease of permeability, the inclination of the interface and the fixed heat flow condition at the base of the model, explain why only a few upwellings can be triggered. Thermal anomalies and a measured temperature profile can be reproduced when fault permeability equals 10-14 m². Interestingly, structure of convective patterns also exhibits a second hotter upwelling, 8 km east of the Soultz-sous-Forêts upwelling zone, where another geothermal exploration project is now underway. The understanding of hydrothermal convection with realistic fluid and rock properties clearly appears as a predictive tool for geothermal exploration strategie

Numerical Modelling of Thermal Convection Related to Fracture Permeability - Implications for Geothermal Exploration and Basin Modelling

“…In general, topographic relief, climatic changes, or sedimentation and uplift can affect the temperature gradient in a given region (Saar, 2011;Guillou--Frottier et al, 2013). Anisotropy and heterogeneity in thermal conductivity between rock layers can also cause non--linear geothermal heat patterns.…”

“…Temperature dataset analyses in Europe have revealed significant thermal anomalies located in graben systems and sedimentary basins, which have been linked to convective fluid flow and neighbouring brittle deformational structures. In fact, several studies have demonstrated that geothermal anomalies in sedimentary basins are strongly controlled by fluid flow in permeable layers (Pasquale et al, 2013;Guillou--Frottier et al, 2013, Garibaldi et al, 2010. The study of groundwater circulation has revealed that convective fluid flow leaves a distinct pattern on the local geothermal gradient.…”

“…A low geothermal gradient characterizes the interval from 1500 -4000m, which is believed to be the direct result of hydrothermal convection along faults in the horst and graben system (Baillieux et al, 2013;Guillou--Frottier et al, 2013). This thermal signature appears in many localities where a layer of increased permeability, possibly due to the presence of faults and fracture networks, is linked to fluid flow.…”

“…Several different approaches and techniques have been implemented in studies, most of which aim to link local structural features with thermal convection in order to explain the observed deep temperature variations (Guillou--Frottier et al, 2013). Petrophysical properties obtained from boreholes have been used to compare theoretical and measured temperature gradients in regions where fluid flow could be localized in fault zones Siffert et al, 2013).…”

“…Petrophysical properties obtained from boreholes have been used to compare theoretical and measured temperature gradients in regions where fluid flow could be localized in fault zones Siffert et al, 2013). The role of depth--dependent permeability on convective fluid flow patterns in permeable fault zones has been intensely investigated within the Provence basin (Garibaldi et al, 2010) as well as the Upper Rhine Graben (Guillou--Frottier et al, 2013). Both reservoir and basin scale numerical modelling are used to address signature thermal features that have been linked to convective fluid flow as deep as 5km.…”