| Abstract |
Most of the geothermal anomalies located in the Upper Rhine Graben (URG) are related to natural brine advection inside a nearly vertical multi-scale fracture system cross-cutting both deep-seated Triassic sediments and Paleozoic crystalline basement. In northern Alsace, France, this sediment/basement fractured interface is characterized by a convective regime evidenced by a low geothermal gradient (~10°/km) and localized negative thermal anomalies that match fracture zones. A comprehensive analysis of geothermal data available from the Soultz deep wells (GPK-2, GPK-3, GPK-4) has been carried out. Natural permeability evidenced from partial or total drilling mud losses, natural outflow, occurrences of Helium gas and drilling mud temperature variations were spatially correlated with geological and geophysical data. Thus, a series of nine permeable fracture zones has been outlined in the Triassic sediments of GPK-2, GPK-3 and GPK-4 wells. Within the Muschelkalk limestones, three fracture zones are located in GPK-2, 2 in GPK-3 and one in GPK-4 respectively. In the Buntsandstein sandstones, a total of 6 zones were detected in GPK-2, GPK-3 and GPK-4. Based on indication of total or partial mud losses, those fracture zones have been classified as permeable or not. However, the thermal impact of those fracture zones in GPK-4 is not clearly visible on temperature profiles. In Northern Alsace, the uppermost sedimentary section extending from late Trias to Tertiary is rather impermeable and governed by a conductive thermal regime and therefore corresponds to a horizontal permeable barrier. Below this sedimentary impermeable section, hydrothermal circulations percolate within the vertical fracture system from the top of the Paleozoic granite basement, through the fractured Buntsandstein and finally to the Middle Triassic formation (Muschelkalk). Late Triassic unit (Keuper) is assumed to be a horizontal barrier to the vertical per ascensum convective flow and behaves as a geothermal cap rock which trapped the uppermost structure of the convective cells. |