28 th International Meeting on Organic Geochemistry 17 – 22 September 2017, Florence, Italy REAL-TIME OBSERVATION OF SULFUR SPECIATION AND PARTITIONING BETWEEN AQUEOUS AND HYDROCARBON PHASES DURING TSR EXPERIMENTS R. Michels 1 , G. Barré 1 , L. Truche 2 , V. Vitzthum 3, R. Bounaceur 3 , C. Lorgeoux 1 1 GeoRessources CNRS-UMR 7359, Université de Lorraine, France 2 ISTerre-UMR 5275, France 3 LRGP CNRS-UMR 7274, ENSIC, Université de Lorraine, France Introduction In organic geochemistry, the vast majority of thermal reactions experiments are conducted in batch reactor conditions. In some cases, reactants may be injected or products retrieved during the experiments. Yet most commonly, reaction products are analysed in post experiment conditions (after dismantlement of the reactor or retrieval during the run). Mass balance is therefore calculated accordingly to sets of experiments at various degrees of transformation ratios, while end products of reaction are characterized at each step. Usually, reaction pathways are proposed as to conceal reactants, mass balance and nature of reaction products. Yet, intermediate reactants appearing during the run of the experiment disappear once temperature is set back to ambient. Their nature, abundance and partitioning between phases may change with experimental conditions. Overlooking them may have consequences on our understanding on reaction mechanisms and hence alter the quality of modelling. Also, rarely do experiments account for phase state changes or chemical species partitioning during runs. It is only when specific formalisms can be used that intermediate reaction species can be proposed and reaction networks tested by simulation. It is for instance the case when radical chemistry formalism is applied (REF). The radical chemistry formalism illustrates well the progress in chemical modelling that can be achieved when intermediate reactional species are considered. The formalism explicitly takes into account radical reaction species which have extremely short life time and can therefore not be easily observed during experiments. Yet, radicals have been identified and described in specific conditions, then introduced within the radical chemistry formalism as true reaction intermediates. The derived computer models allow then to relate reactants to products with details on the nature, distribution, quantity of compounds as well as thermodynamic or kinetic parameters. The recent description of the S 3 - radical ion and its role in the Thermochemical Sulfate Reduction (TSR, the reduction of sulfate into sulphides by reaction with hydrocarbons; Orr, 1974; Goldstein and Aizenshtat, 1994; Worden et al., 1995; Amrani, 2014) reactions is another example of the importance to consider intermediate reaction species (Pokrovski and Dubessy, 2015; Truche et al., 2014). TSR is a reaction that involves the transfer of 8 electrons to reduce sulfates (+6) into sulfides (-2), that implied necessary intermediate valence sulfur species to occur. S 3 - is the major intermediate sulfur species in this process due to its stability and its high reactivity. But it is a species only stable at temperature >100°C, so it is necessary to studying it at relevant temperature. A key aspect in the identification of intermediate reaction species at temperature is the possibility to monitor them in-situ the reaction medium. In case of TSR, at least two if not three reaction media are to be monitored: the water phase, the hydrocarbons phase and if present the gas phase. In our work, we are presenting results of the in-situ monitoring of the fate of sulfur species within each phase.