Syn- to post-rift thermal evolution of the Pechelbronn sub-basin (Upper Rhine Graben)

Abstract

The Upper Rhine Graben (URG) is a key target for geothermal projects in Western Europe, requiring an understanding of the development and spatial evolution of geothermal fluids. Its thermal and geological history was impacted during the Neogene period by a major change in rift kinematics, evidenced by a regional-scale and gradual erosive unconformity and coeval volcanic activity. This study investigates the interplay between URG syn- and post-rift burial history and heat flow evolution in the westerly Pechelbronn sub-basin - hosting petroleum and geothermal fields – in comparison to the more central Rastatt Trough. The method uses thermal maturity assessment by vitrinite reflectance and 2D burial coupled to conductive thermal modelling. Results reveal that the maximum burial occurs during the Aquitanian (post-rift), and that the maximum cumulative sediment thickness related to the Neogene erosion was approximately 1200 m in the Pechelbronn sub-basin, from the top of the Schistes à Poissons (Rupelian shale) to the Aquitanian sediments. Toarcian source rock (Schistes carton) reached the oil window at 29–28 Ma within the Pechelbronn sub-basin and at 43 Ma for the Rastatt Trough at Roeschwoog well. The comparison of current adjacent local heat anomalies reveals variations in thermal history of the Pechelbronn sub-basin, highlighting the dominant influence of syn-rift burial in some areas, e.g. in Soultz-sous-Forêts or additional heat input attributed to geothermal fluids in others, e.g. in Rittershoffen. Moreover, it appears that the heat flow must have been considerably higher (around 180 mW/m², under the pure conduction hypothesis) during faults activation around 15 Ma in the Rittershoffen area, compared to Soultz-sous-Forêts (150 mW/m²). Changes in local heat flows are attributed to east-west migration of hydrothermal cells positions through time in relationship to fluid circulation at basin scale. These results provide a broad geological time frame for further thermal modeling in the URG and provide new insights into the thermal evolution of the URG and its implications for sub-surface resources exploration.