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Biogenic Gas Generation Process: Application to the Bay of Biscay: Data Integration and Basin Modeling


Sub-seafloor sediments near continental margins are known to be enriched in biogenic CH4. The microbial production of methane is the result of complex digestion processes of the Organic Matter (OM) that take place at different diagenesis stages. The efficiency of methane production depending on OM degradability potential, is largely determined by the biogeochemical processes at the sea bottom linked to the quality and quantity of the OM. In offshore environment, the accumulation and production of biogenic gas is likely and of commercial interest since it starts under the oxygenation zone of organic compounds from the reduction of sulphate in sea water. The rates of OM degradation and burial play a crucial role in the production of microbial gas. Therefore, the knowledge of OM spatial distribution in sediments and the temperature is a prerequisite for estimating the biogenic CH4-generating potential of marine sediments. The objective of this study is a better understanding of the microbial gas generation/migration process by a sensitivity analysis on the key controlling factors, using a 3D basin modelling approach. The numerical approach to describe biogenic CH4 production and implemented into TemisFlow™ takes into account the TOC distribution, the sedimentation rate and degradability law.The macroscopic post processing model of microbial CH4 generation, was applied to the Bay of Biscay. In 2013, fluid emissions were found in the offshore of the Aquitaine Shelf, east of the shelf break, at a water depth around 140 to 220 m. The gas is primarily composed by CH4 and secondarily by CO2. The 13C isotopic signatures conclude to a purely biogenic origin derived from CO2 reduction. However, the isotopic signature of the C1 is slightly heterogeneous as well as the amount of the gas emitted along the different sites of the shelf break. These differences could be explained by several factors such as the quality of the substratum, the temperature at which the methane is produced, the oxidation of the hydrocarbon, the mixing with different gas sources and mixture of hydrocarbon transport/migration process. All of these factors were analyzed and tested in order to obtain a generation/migration biogenic gas model calibrated on real data. The OM potential of the Bay of Biscay was analyzed using a 3D basin modelling approach. After the model calibration, several scenario of C1 generation and migration were tested, compared to observed data and discussed.