Clay Mineralogy: A Signature of Granitic Geothermal Reservoirs of the Central Upper Rhine Graben

Clay minerals are the signature of hydrothermal alterations related to fluid circulation in volcanic and crystalline rocks. In French part Upper Rhine Graben, deep-seated granites, illitic (illite I/S mixed layers (ml)) typical products structurally-controlled argillic alteration Paleozoic granitic basement. new Illkirch geothermal well, GIL-1, drill-cuttings were studied with various petrographic methods determine characteristics illite paleo- present-permeable zones, compare mineralogy that Soultz-sous-Forêts Rittershoffen sites. Alteration petrography, crystal structure as well chemical composition altered bulk rocks performed all along well. This complete characterization, combined geophysical logs structural results, highlighted at Illkirch, Soultz-sous-Forêts, composed illite-rich illite-smectite (I/S ml) (<10% smectite). Two mineralogical assemblages distinguished: chlorite + resulting from propylitic after emplacement basement under temperatures higher than 350 °C, ml carbonates quartz due fractures between 130 160 °C. Fracture zones characterized by occurrence ml), specifically, quantities present-day permeable paleo-permeable zones. A conceptual model fracture interface overlying sedimentary is proposed. The permeability distribution controlled fault network, which consists sealed unsealed Fluid convection URG implies paleo present fluids circulating both fractured reservoirs. Such circulations develop could be considered exploration guides for future sites URG. At repartition (KFZs) GIL-1 indicates moderately argillically granite distally situated Eschau more intensely close fault.