Evolution of diagenetic conditions and burial history in Buntsandstein Gp. fractured sandstones (Upper Rhine Graben) from in-situ delta O-18 of quartz and Ar-40/Ar-39 geochronology of K-feldspar overgrowths

In-situ delta O-18 measured in the quartz overgrowths help identify temperature and fluid origin variations responsible for cementation of the pore network (matrix and fracture) in the Buntsandstein Gp. sandstone reservoirs within the Upper Rhine Graben. The overgrowths record two types of the evolution of delta O-18: 1) a monotonous decrease of the delta O-18(overgrowth) interpreted as linked to an increasing burial temperature and 2) random fluctuations, interpreted as pointing out the injection of allochthonous fluids in faulted areas, on the cementation processes of the pore network (both intergranular and fracture planes). Fluids causing the quartz cementation are either autochthonous buffered in O-18 from clay illitisation; or allochthonous fluids of meteoric origin with delta O-18 below - 5%. These allochthonous fluids are in thermal disequilibrium with the host sandstone. The measured signal of delta O-18(overgrowth) measured from samples and calculated curves testing hypothetic delta O-18(fluid) are compared to T-t evolution during burial. This modelling proposes the initiation of quartz cementation during the Jurassic and is validated by the in-situ Ar-40/Ar-39 dating results obtained on the feldspar overgrowths predating quartz overgrowths. A similar diagenetic history is recorded on the graben shoulders and in the buried parts of the basin. Here, the beginning of the pore network cementation predates the structuration in blocks of the basin before the Cenozoic graben opening.