The diagenetic history of the giant Lacq gas field, witness to the apto-albian rifting and the Pyrenean orogeny, revealed by fluid and basin modeling

Abstract

The link between fluid circulation schemes and basin histories remain one of the most valuable tools to understand the diagenetic evolution of petroleum reservoirs. This study proposes a diagenetic model for the Deep Lacq reservoir constrained by petrography, geochemistry, fluid inclusions studies and basin modeling analyses. Drill cores penetrating the 3000 m deep Late Jurassic to Early Cretaceous “Deep Lacq” reservoirs, in the Aquitaine basin SW France, were used to investigate how the geodynamic evolution of the basin influenced the diagenesis of petroleum reservoirs. This basin experienced a phase of rifting during the Lower Cretaceous followed by inversion and compression during the Paleogene. Petrographic and geochemical results indicate that early diagenesis involved bacterial activity and early dolomitization. Fluid thermodynamic modeling, coupled with the reconstruction of the basin history provided insight into the timing of diagenetic fluid circulations. Burial diagenesis involved a rift-related dolomitization episode linked mainly to the recrystallization of earlier dolomites and the circulation of deep hot fluids connected to the Triassic evaporites. The dolomitizing fluids circulated at temperatures close to 136–144 °C and pressures of 338–386 bars, during the Aptian (120-116 Ma). During the post-rift uplift and its subsequent thermal re-equilibration, multiple fluid pulses precipitated coarse blocky calcites. These Ca-rich fluids circulated between 109 and 97 Ma, at high-pressure conditions at around of 704–766 bars and with temperatures of 146–156 °C. The circulation of another fluid pulse that resulted in the precipitation of anhydrite cements is linked to the onset of the Pyrenean orogeny. No evidence that support in-situ TSR reactions were found, which suggests migration of TSR-related H2S from a kitchen in the deeper parts of the basin. This multi-modeling approach shows how the detailed thermodynamic analyses of fluid inclusions can add valuable constraints on a diagenetic model and eventually link it to the larger-scale basin's history.