SOURCES AND THERMAL EVOLUTION OF PALEO-FLUIDS IN THE BARRA VELHA FORMATION (PRE-SALT, SANTOS BASIN) INDICATED BY HYDROGEN AND OXYGEN ISOTOPES, AND FLUID INCLUSIONS MICROTHERMOMETRY

The genesis of the Pre-Salt rocks of the Brazilian marginal basins involves an intricate set of depositional and diagenetic processes, resulting in complex relationships among carbonate, silica and Mg-clay minerals. Aiming to provide a better understanding of the formation and alteration of these deposits, this study integrates δD and δ18O isotopic data and fluid inclusions microthermometry to constrain the composition and role of paleo-fluids during sedimentation and diagenesis. The samples correspond to partially silicified muddy spherulstones constituted by calcite spherulites and Mg-clays, and to totally silicified rocks (cherts) of the Barra Velha Formation from one well located ≈200 km offshore. The whole-rock isotopic data for clay minerals in the muddy spherulstones presents δD values between -19.2 and -36.7‰ and δ18O from 17.2 to 22.7‰. These values are comparable with typical isotopic trends of interlayer water from clay minerals formed in highly saline conditions (Li et al., 2019). The cherts present δD values between 13.5 and -79.0‰ and δ18O of 28.5 to 35.6‰ with a scattered distribution. Petrographic evidence show that the silica replaced a clay precursor during eodiagenesis. The δ18O values indicate that silica occurred under stronger evaporitic conditions than those of the clays. The dispersion of δD values suggests recrystallization and/or post-depositional alteration by deuterium-depleted juvenile fluids. Macrocrystalline quartz cement in silicified muddy spherulstones was precipitated at 50-60 °C during the end of eodiagenesis, from evaporitic fluids with salinity of 13-22 weight percent NaCl-equivalent (wt%), as indicated by fluid inclusions microthermometry. The different fluorescence colours of oil fluid inclusion suggest the multiple petroleum charge, without a clear sequence of entrapment. The occurrence of light oil entrapped as liquid and vapour (gas-condensate) may indicate different source rock maturity stages or more than one oil kitchen at different maturity stages. The coexistence of light and heavy oil could be explained by differences in organic matter in the source rocks, or by degradation by bacterial activity prior to the entrapment. Quartz was also precipitated at higher temperatures during mesodiagenesis, as recorded by the coexistence of aqueous and light to medium oil primary fluid inclusions entrapped at 105 °C from fluids with salinity of 21-20 wt% and evaporitic influence. This indicates the presence of undersaturated liquid petroleum (no free gas phase) during burial. The aqueous fluid inclusions indicate the predominance of basinal brines during quartz precipitation. The post-trapping reequilibrated temperatures of fluid inclusions indicate maximum burial temperatures of 130-140 °C. The isotopic and microthermometric data allowed to unravel the genetic and thermal evolution of the mineral phases. The isotopes of the clay minerals possibly record the brine composition of evaporitic lake water. The chert δ18O values show similarity with evaporitic environments of the Kwanza Basin and of East African Rift alkaline lakes (Teboul et al., 2019). The fluid inclusions data allowed to constrain a petroleum system with a complex fluid percolation history. The initial percolation of diverse fluids ranging from gas-condensate to heavy oils during early diagenesis was possibly related to long distance migration of an atypical hydrothermal petroleum system (Guzzo et al., 2018), followed by fluids with light to medium oil percolating during mesodiagenesis, possibly linked to the first pulses of the main oil accumulation. - Guzzo et al., 2018. Hydrothermal petroleums and solid reservoir bitumens: recognition of an atypical petroleum system in the Brazilian Pre-salt deposits (Santos Basin); in XV Latin American Congress on Organic Geochemistry - Li et al., 2019. DOI: 10.5772/intechopen.88532 - Teboul et al., 2019. DOI:10.1016/j.apgeochem.2018.10.019