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Fluid circulation and diagenesis of the Lacq Petroleum System : Impact of the geodynamic evolution

Abstract : The geodynamic evolution of basins exerts a major control on the evolution of the pathways, temperature, pressure and chemistry of circulating fluids. This in turn influences the diagenesis and petrophysical properties of petroleum reservoirs. Recently, geodynamic models 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, followed by an inversion and convergence during the Late Cretaceous and the Paleogene. This revised model highlights the possible impact of hydrothermal fluids on the diagenesis of petroleum reservoirs in the southern Aquitaine Basin. This work is based on the study of three drill cores penetrating the Upper Cretaceous (Upper Lacq) reservoirs and the Upper Jurassic to Early Cretaceous (Deep Lacq) reservoirs. Using a multitude of petrographic, geochemical and modeling tools, we document how the Aptian hyperextension was coupled with the circulation of Mg-rich fluids that partially dolomitized Deep Lacq. Mg was provided from the clay-rich formations at depths while salinities were the result of fluid interaction with the Triassic evaporites. Different fracturing episodes during the post-rift stage, when the N-S convergence started, connected Deep Lacq with a pool of fluids oversaturated with respect to calcites. A third fluid pulse, linked to the Eocene compression, precipitated massive anhydrites in Deep Lacq. At the same time in Upper Lacq, diagenesis was marked by orogeny-driven hydrothermal fluid circulations along the major faults and fractures on the crest of Lacq anticline. Hydrothermal dolomites were documented with temperatures nearly 40°C higher than the surrounding host rocks. Different fracturing episodes, related to the Pyrenean orogeny, were later responsible for calcite and anhydrite cementation. The oil and gas infilling of Upper and Deep Lacq respectively are assumed to be post-orogenic. Therefore, it is evident that the overall geodynamic setting and the geologic evolution of the Pyrenean foothills, governed mainly by the Aptian hyperextension and the Eocene convergence, had a significant control on the types, sources and circulation pathways of the diagenetic fluids.
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Alexy Elias Bahnan. Fluid circulation and diagenesis of the Lacq Petroleum System : Impact of the geodynamic evolution. Earth Sciences. Université de Lorraine, 2019. English. ⟨NNT : 2019LORR0221⟩. ⟨tel-02528914⟩

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