Hypothesis on the karstification/dolomitization of the Barremian reservoir in Parentis Basin

The Parentis sub-Basin, located southwest of Bordeaux (France), has been known since the 1950s as a prolific oil reservoir. It lies at the eastern end of the Bay of Biscay. Several fields are exploited in the onshore part of the basin, the largest being at Parentis-en-Born, discovered by ESSOREP in 1954 and exploited by VERMILION REP since 2006. The field is an east-west anticlinal structure, formed during various geodynamic episodes in connection with halokinesis of the Triassic and Hettangian series. One of the reservoirs exploited is a limestone/dolomite dating from the Barremian to the Upper Jurassic characterized by a matrix porosity of 14% and a permeability of 100 mD over an average thickness of 200 metres. Numerous studies and acquisitions (3D seismic, 122 boreholes) have been carried out by ESSOREP and VERMILION REP exploration geologists to characterise the limestone/dolomite reservoir, particularly regarding the origin of the dolomitization and porosity. Models of emplacement suggest post-sedimentary dolomitization by reflux and epigenic karstification linked to early tectonic bulges. Our study aims to use modern methods and concepts to gain a better understanding of the major stages in the development of this reservoir in relation to the geodynamic events. It is based on a detailed study of cores and thin sections. A total of 160 samples were taken from 7 boreholes to conduct a diagenetic study (petrography, cathodoluminescence and initial results on strontium isotopes). The macroscopic study of the cores showed the presence of numerous mud losses (no-recovery zones), often correlated with faults (striations observed), which were previously interpreted as epigenic karstification. Numerous veins were observed in the fracture zones, composed of calcite, saddle dolomite and anhydrite. Microscopically, the paragenesis shows primary dolomitization prior to burial (euhedral to subhedral dolomite), followed by phases of circulation of magnesium-rich hydrothermal fluids (saddle dolomite - ‘non-planar’) and sulphates (anhydrite). Initial interpretations suggest that the void zones encountered in the boreholes are linked to the upwelling of deep-seated fluids sourced by Triassic/Lower Jurassic evaporites and dolomites, resulting in hypogenic karstification. These episodes would be linked to the resumption of the anticline during the Pyrenean phase.