In situ decarboxylation of acetic and formic acids in aqueous inclusions as a possible way to produce excess CH4

Accurate reconstruction of diagenetic P-T conditions in petroleum reservoirs from fluid inclusion data relies on valid measurements of methane concentration in aqueous inclusions. Techniques have been developed (Raman spectrometry) to provide sufficiently accurate data, assuming measured methane concentration has not been modified after aqueous inclusion entrapment. This study investigates the likelihood that organic acids derived from petroleum fluids and dissolved in formation water might suffer decarboxylation upon postentrapment heating within the fluid inclusion chamber, thereby generating excess CH4 in the inclusions. Four different experiments were conducted in fused silica capillary capsules (FSCCs), mimicking fluid inclusions. The capsules were loaded with acetic (CH3COOH) or formic (HCOOH) acid solution and were heated to 250°C for short durations (<72 h) in closed-system conditions, with or without applying a fixed PH2. Reaction products were characterized by Raman and FT-IR spectrometry. Results indicate that decarboxylation reactions did take place, at variable degrees of progress, and that measurable excess CH4 was produced in one experiment using acetic acid. This suggests that methane may be produced from dissolved organic acids in natural aqueous inclusions in specific situations, possibly inducing errors in the thermodynamic interpretation.