Assessing Paleo-Fluid Circulation in a Geothermal System: From Structural Analysis to Fracture Diagenesis

Assessing cementation in fractures, which may represent important conduits for fluid migration in tight reservoirs, is a crucial matter in subsurface exploration. Our multidisciplinary study combines outcrop analogues and cores of potential subsurface reservoirs to analyze paleo-fluid properties, origins and pathways, and to provide constraints on the present-day cementation conditions. This project focuses on the fossil geothermal system (Upper Mesozoic) of the currently explored Geneva Basin (Switzerland). Our approach consists in 1) fault and fracture geometric analysis; and 2) paleo-fluid origin characterization. Structural analyses on outcrops highlighted the complex structural framework of the study area. Petrographic analyses of calcite-filled fractures exposed a large spectrum of crystal habitus. Synkinematic precipitation was observed thanks to syntaxial textures. Carbon (C) and oxygen (O) stable isotope analyses showed calcite veins have low to negative δ13C interpreted as meteoric-derived fluids and δ18O values are negative suggesting calcite precipitation during burial. Microthermometry results on calcite fluid inclusions indicate low precipitation temperatures and revealed two end-member fluids: a very low saline and a moderately saline. Fluids with intermediate salinities were also reported possibly derived from fluid mixing. Additional clumped isotope (Δ47) thermometry combined with U-Pb geochronology will better constrain precipitation temperature, fluid origin and absolute cementation/opening timing.