Geodynamic control on carbonate diagenesis: Petrographic and isotopic investigation of the Upper Jurassic formations of the Paris Basin (France).
Résumé
The Oxfordian and Kimmeridgian carbonates of the eastern edge of the Paris Basin display poor reservoir properties (Φb15% and Kb0.1 md). The petrographic investigation and the resultant paragenetic sequence show that this is due to extensive precipitation of 2 types of almost synchronous blocky LMC (Low Magnesian Calcite) cements during mesogenesis (burial). Stable isotope investigation of these non-recrystallized cements, through both microdrilling sampling and in-situ SIMS (Secondary IonMass Spectrometry) analysis, shows that they originated from slightly buffered meteoric fluids (−3‰SMOW to −8‰SMOW) flowing through the Malm carbonate aquifers of the eastern edge of the Paris Basin. After a review of the post-Upper Jurassic history of the Paris Basin, it appears that meteoric water supplies leading to such cementations were associated with the evolution of the basin's geodynamic setting, and notably with the LCU (Late Cimmerian Unconformity) major event which corresponds to the uplift of the London–Brabant massif. Denudation of the Oxfordian and Kimmeridgian carbonates on the southern flank of the London–Brabant massif, to the north of the study area, then lead to lateral recharging of the associated aquifers with meteoric fluids. This scenario is similar to the one proposed to explain cementations of the Jurassic carbonates in the Wessex–Weald Basins which emphasizes the main role played by the Cretaceous unconformities over carbonate diagenesis at the scale of the northwestern Tethys margin.