Evolution of diagenetic conditions and burial history in Buntsandstein Gp. fractured sandstones (Upper Rhine Graben) from in-situ δ¹⁸O of quartz and ⁴⁰Ar/³⁹Ar geochronology of K-feldspar overgrowths

In-situ δ¹⁸O 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 δ¹⁸O: 1) a monotonous decrease of the δ¹⁸Oovergrowth 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 ¹⁸O from clay illitisation; or allochthonous fluids of meteoric origin with δ¹⁸O below − 5%. These allochthonous fluids are in thermal disequilibrium with the host sandstone. The measured signal of δ¹⁸Oovergrowth measured from samples and calculated curves testing hypothetic δ¹⁸Ofluid 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 ⁴⁰Ar/³⁹Ar 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.