GNGTS 2013 - Atti del 32° Convegno Nazionale

The geochemical fingerprints of thermal fluids contribute significantly in planning resources assessment, sustainable exploitation, and in mitigating possible environmental threats. In particular, the trace element geochemistry, the application of conservative and non- conservative isotopic systematics and the geochemistry of dissolved gas species can be used to investigate the fluid origin, the water-rock and water-gas interaction processes and in the reconstruction of the flowpaths at depth. The present study is focused on a conceptual model for the geothermal water reservoirs hosted in the Mesozoic and Early Paleogene carbonates of the Friuli Venezia Giulia carbonatic platform buried beneath the north-Adriatic coastal plain, to understand formation hypothesis, evolutionary patterns through time and hydrothermal circulation paths, possibly enabling an improved assessment of the geothermal resources. Geology and water sampling. The carbonatic reservoirs in the study area (Cesarolo-1 well) consist of Upper Jurassic-Lower Cretaceous well-stratified limestones with breccias, overlain by karstified limestones with abundant rudists and dolostones dated Upper Cretaceous and locally by grey bioclastic limestones dated Upper Cretaceous – lower Eocene. These sequences are buried beneath the Quaternary and Late Cenozoic (flysch and molasses) sediments which blanket the Adriatic Apulian foreland. Multi-channel seismic profiles and high-resolution single-channel profiles acquired by OGS and University of Trieste across the Grado-Marano lagoon and the Gulf of Trieste, in addition to the regional information obtained from ENI exploration wells and seismic profiles inland, indicate the occurrence of SW-verging Dinaric compressive structures, segmented by NE-SW-oriented faults with normal and transcurrent components (Cimolino et al. , 2010). During Neogene times the inherited Alpine NW-SE ramp- anticline of the thrust underwent a northward tilting and deformation, connected with the evolution of the South Alpine foreland. The paleogenic Dinaric thrusts (often reactivated in Neogene times) involve the carbonatic platform which is fractured, karstified and affected by intense deformational regimes, mainly in its upper portion. Waters were collected from natural thermal springs, thermal baths and wells near the Monfalcone thermal springs (Petrini et al. , 2013); from the Grado-1 deep well (1100 m: Della Vedova, 2009) and from a deep well in Lignano Sabbiadoro (1505 m) (Fig. 1). Experimental methods. Water physical-chemical parameters were measured in the field. Major ion chemistry was determined by conventional ion chromatography. Trace elements were determined by ICP-OES and ICP-MS spectroscopy. Oxygen and hydrogen isotopic data were determined by IRMS using a Delta Plus spectrometer: The isotopic data were expressed as per mil deviation from the V-SMOW standard (Vienna Standard Mean Ocean Water) using the conventional 18 O and D notation, where = [(R sample /R standard ) − 1] x 1000 (‰) and R represent the 18 O/ 16 O or D/H isotopic ratios. The strontium isotopic composition was determined by thermal ionization mass-spectrometry (TIMS), after chemical extraction of the element by the matrix using ion-exchange chromatography. The measures of the 87 Sr/ 86 Sr ratio were fractionation-corrected using 86 Sr/ 88 Sr=0.1194. Chemical analyses were carried out on the gas phase extracted after the attainment of equilibrium (at constant temperature) between the water sample and a known volume of host, high purity gas (argon), injected inside the sampling bottle. The analytical determinations were carried out by gas-chromatography. Results and discussion. It is observed that waters with the highest electrical conductivity are characterized by the highest temperature, suggesting that the geothermal reservoir feeding all the sampling sites is constituted by saline waters. In the Monfalcone area, waters from springs, thermal baths and wells define a dilution series, indicating that in this site the uprising thermal waters mix with fresh and colder waters. The molar ratios between major and trace constituents which behave conservatively, such as chloride and bromide, indicate that the saline thermal end-member is seawater. Using chloride as a marker of the marine component, it is noted that most major and trace ions are distributed along a simple binary mixing line between the seawater end-member, best 270 GNGTS 2013 S essione 1.3