Characterisation of fluid flow conditions and paths in the Buntsandstein Gp. sandstones reservoirs, Upper Rhine Graben

Abstract

Deeply buried sandstone reservoirs are targeted in the Upper Rhine Graben (URG) for geothermal and hydrocarbon resources. These reservoirs, which are located at the top of the geothermal convective cells, have a complex diagenetic and structural history recorded by paragenesis. Here the focus is made on the characterization of carbonates and barite cementations which trace paleo geothermal circulations within the fracture network affecting the sandstones. These mineralizations are studied with a double approach on geochemistry and structural, faults and associated fracture network, to characterize fluid-flow episodes on different structural positions in the rift basin and its shoulders. Barite sulphur isotopic ratios suggest a common signature and source for all the locations. REE patterns, oxygen isotopic ratios, and fluid inclusion study suggest though two regimes of fluid flow forming barite, depending on their location. On the graben shoulders the barite have a higher content in total REE and contain non-saline fluids inclusions, suggesting that fluid circulations at the graben border faults interact with sulphate rich layers, and precipitate at high temperatures .In -deep-seated sandstones, fluid inclusions in barites show a wide range of salinities, suggesting a higher contribution of sedimentary brines, and precipitation at lower temperatures. These barite mineralizations are associated with carbonates and apatite with a diagenetic origin, according to their REE signature. These data are used to build a model for fluids circulation within the graben: Fast and deep down- and up-flows are taking place along the major border faults, which are leaching evaporitic horizons, and precipitates from geothermal fluid during fault activity. A part of these deep-down meteoric waters is reaching the centre of the basin. In this central part of the basin, fluid circulation is slower and restricted to the bottom of the basin, where fluid-mixing with sedimentary brines occurs. This new understanding of fluid pathways in the targeted reservoir brings new insights on the compartmentalization of geothermal circulations at the basin scale.