| Abstract |
The Upper Rhine Graben area is targeted for geothermal energy production within the fractured basement, but also within the overlaying fractured sandstones reservoirs. The permeability of the sandstones is controlled by the matrix features of the pore network, and by its fracture network. Barite cements are considered as traces of geothermal fluids circulations. Barite cements in fractures are studied at the scale of the basin, on several locations. These cements are analyzed with a double approach on geochemical and structural location, faults and associated fracture network. temperature field and chemical conditions of precipitation are investigated, by REE chemistry, oxygen and sulphur isotope geochemistry, and micro-thermometry; the aim is to assess: the origins of the mineralizing fluids and the temperatures of precipitation. Barite cements are present in the deeply buried sandstones from the Buntsandstein Gp. in the URG basin, but also on the rift shoulders. In the deep-seated reservoirs, the barite cements are mainly precipitated within N000-N010°E and N150-N160°E fractures near main faults oriented N020°E to N030°E. On the shoulders, the orientation of barite cemented fractures are mostly N000-N010°E, N040°E, N140-N170°E, in fault systems oriented N000°E to N030°E. Sulphur isotopic ratios suggest a common signature and source for both settings, with a Triassic evaporitic sulphate signature of δ34S around 15.5 ‰ in average. REE patterns, oxygen isotopic ratios, and fluid inclusion study suggest two regimes of fluid flows depending on the two settings. The higher content in total REE from the cements on the shoulders, and the hot, non-saline fluids inclusions, suggest that these circulations in the graben border faults are interacting with sedimentary evaporitic layers at low level, and precipitates at high temperatures (from 180 to 240 °C). Fracture cements in the currently deep- seated sandstones are characterized with inclusions ranging from low saline to high saline fluids, and colder temperatures, suggesting a higher contribution of sedimentary brines, which increase the salinity, and with precipitation at lower temperatures from 80 to 140 °C. This data is used to build a model of fluid circulation within the graben. Fast and deep circulations are developed in the border faults with a branch flowing deeper in the basement, while in the central part of the basin, fluid circulation is slower and restricted to the bottom floor of the basin, where upward-flow of hot, low saline fluids mix with sedimentary brines. Despite the similar orientations of both fault systems, the conditions of circulation and precipitation are different, and could be transposed to current hydrothermal fluids circulations. |