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