LAUSR

Abstract Orogeny-driven fluids that circulate in foreland basins can have strong impacts on petroleum systems and reservoir properties. This applies to the Upper Cretaceous Lacq reservoir of the Aquitaine Basin in southwestern France located north of the Pyrenean Mountains. We study a drillcore from a 650 m deep oil reservoir to document how the evolution of a foreland basin, which formed after a hyperextension phase, affected fluid circulation and eventually reservoir diagenesis. Using drillcore samples, petrographic observations, fluid inclusion studies coupled with thermodynamic modeling, isotopic and rare earth element geochemistry, as well as basin modeling were performed to investigate and describe the fluid types and sources, the pressure-temperature evolution and the timing of diagenetic fluid flow. Early diagenesis involves some bacterial activity represented by micrite rims and framboidal pyrites, as well as early dolomitization involving a mixture of meteoric and formation fluids. During burial, dolomite overgrowth, saddle dolomites, coarse blocky calcites and anhydrites precipitate. The last products consist of iron oxide precipitation and meteoric recharge of the reservoir with groundwater. Our results indicate that after the deposition of the Paleocene flysch formation, Mg-rich, low salinity, hot, and relatively deep clay-released fluids migrated along thrust faults from deeper parts of the basin during the climax of the compressional Pyrenean phase (Eocene). They acted as dolomitizing hydrothermal fluids with a thermal disequilibrium in the order of 30–40 °C hotter than the ambient host rocks. Another orogeny-driven, hydrothermal and Mg-poor fluid mixed with oil generated by Jurassic-Barremian source rocks and precipitated calcites and anhydrites. Finally, during uplift, oxidizing fluids were laterally introduced as the present-day groundwater. Hence, by combining different petrographic, geochemical and modeling proxies, we document how the Eocene orogenic phase played a crucial role in basin-scale fluid flow and carbonate diagenesis.