Isotopic Signatures of Deep Fluids on Authigenic Carbonates Formation

Abstract

This PhD project focus is relations among fluid sources, fluid migration within sediments and authigenic carbonate formation, coupled with the assessment of the isotopic signatures recorded in carbonates. Northern Apennines show presently active cold seepages, originating from deep reservoirs. The fluids, previously characterised together with the petroleum system, provide clues on former natural fluid migration testified by authigenic carbonates. These latter were sampled in three different locations along the Northern Apennine foothills: Enza field, Stirone field and Secchia field. The outcrops are exposed along the riverbanks due to fluvial incisions within Plio-Pleistocene Argille Azzurre marine successions. Based on morphology, 73 carbonate subsamples were characterised from the three outcrops. Each sample was divided in subsamples to have a micro-characterisation by petrography, SEM-EDX observation, XRD and XRF analyses. These are composed by microcristalline dolomite, above 50%; quartz, feldspars and clay minerals are the main siliciclastics. Saline waters coming from seven present-day seepages and drilled wells, were selected and analysed for geochemistry, δ13C on DIC, δ18O, 87/86Sr and δ34S. Hydrocarbons were characterised in previous works: the reservoirs contain mixed thermogenic, secondary biogenic methane and CO2, presently seeping in mud volcanoes close to the outcrops. δ13C in carbonates strongly varies among different areas and within the same outcrop due to different processes and carbon sources. Processes leading to precipitation of authigenic carbonates are anaerobic oxidation active on thermogenic and biogenic methane leakages. Furthermore, incorporation of heavy C from CO2, derived from secondary methanogenesis, was ascertained in the Secchia River carbonates. This variability testifies that authigenic carbonates record variation in biogeochemical cycles during formation. δ18O and 87/86Sr in dolomite indicate that marine water was mixed with deep connate waters. The mixing models permitted to quantify the relative proportion of these two components and changes in the fluid feeding system.