Fluids evolutions from rift to post-orogenic stages: example of the Northern Pyrenees (France)

International audienceFluids evolutions from rift to post-orogenic stages: example of the Northern Pyrenees (France)Michels R.1*, Gaucher E.C. 2, Pironon J. 1, Elias-Bahnan A. 1, Rallakis D. 1, Barré G. 2,3, Carpentier C. 1, Calassou S. 21 Université de Lorraine, CNRS, GeoRessources lab, Nancy, France; 2 TOTAL-CSTJF, Pau, France, 3 Université de Pau et des Pays de l’Adour, CNRS, TOTAL, LFCR, Pau, France.*Raymond.michels@univ-lorraine.frKeywords: geodynamics, oil&gas, fluid inclusions The evolution and genesis of the Aquitaine basin is closely linked to the geodynamics of the Pyrenees and the interaction between the Iberian and European plates. Recently, the geodynamic model of the Pyrenees and the South Aquitaine Basin were revisited and updated to include the involvement of an Early Cretaceous rifting phase that evolved to extreme crustal thinning coupled with mantle exhumation. This event was followed by the main Pyrenean convergence leading to collision and thrusting during Late Cretaceous and Paleogene. This revised model highlights the possible impact of hydrothermal fluids on the diagenesis of petroleum reservoirs in the southern Aquitaine Basin. Several research programs involving many teams were supported over the last decade by Total E&P, BRGM, CNRS and the French National Research Agency to set the Pyrenees as a reference analogue to understand the complex interplays between structural evolution, deformation, metamorphism, volcanism, hydrothermalism, fluid evolution, diagenesis and transfer over a complete Wilson cycle. Fluid inclusions, vitrinite reflectance, Raman spectroscopy on graphite (Clerc et al, 2015), have been used to determine paleotemperatures. U-Pb of carbonates helped to determine absolute dating of carbonates for the first time (Motte et al., 2021). Fluid pressure was determined in gas and oil reservoirs by the double isochore technique. A thermal gradient from South to North is recorded over time. The highest temperatures are related to rifting and hyperextension phases of the lower Cretaceous (350-450°C in the Pyrenees Central Domain, up to 550°C near mantle cores) with temperatures varying from 350°C in the foothills (Salardon et al., 2017) to 150°C in the foreland (Renard et al., 2019). Intense thermal gradients are associated with fracturing of Jurassic carbonate deposits allowing fluid circulation and dolomite precipitation. A mantle contribution is suspected for the CO2 and magnesium rich fluids observed, while high salinity is inherited from dissolution of Triassic evaporites. Jurassic petroleum source-rocks were rapidly brought to gas stage and ultimately to graphite. Interactions with sulfate rich fluids allowed intense Thermochemical Sulfate Reduction and hence H2S formation and hydrocarbons destruction.The study of the Pyrenees Wilson cycle thus documents the fluid formation and migration in relation to the structural evolution of the mountain range and foreland basin systems. It highlights especially how earlier stages of evolution set their successive imprints in space and time and which constitute an inheritance strongly influencing later stages of fluid geochemistry and migration.References:Clerc et al. (2015): Solid Earth, 6, 643–668,Motte G. et al. (2021): Marine and Petroleum Geology, 126, 104932.Renard S. et al., (2019): Chemical Geology, 508, 30-46.Salardon R. et al. (2017): Marine and Petroleum Geology, 80, 563-586