High-Resolution Facies Characterization and Metre-Scale Depositional Organization of Palustrine-Lacustrine Carbonates

Accurate 3D modelling of continental limestone reservoirs is still hindered by the high spatial variability of their flow properties. This variability results from the combination of heterogeneous and laterally discontinuous sedimentary facies with vertically stacked pedogenic overprints, which remain poorly constrained at the multi-meter scale. To address this issue, this study provides a sedimentological and stratigraphic characterization of the Aquitanian Pithiviers Limestones hosting the vadose zone of the Beauce aquifer (central France). This analysis takes advantage of a high-resolution dataset (eight 25-m-deep cored boreholes, spaced 5 to 8 m apart). Eleven facies were identified grouped into lacustrine, lake-margins and palustrine depositional environments. Two cross-sections (22 m long) reveal a heterogeneous architecture structured by three hierarchical orders of cyclicity: (1) medium-term cycles (several metres thick) record alternations between widespread lacustrine and palustrine conditions, (2) short-term cycles that document vertical and lateral facies shifts at the metre scale and (3) very-short-term cycles illustrating sub-metre to metre-scale facies variability driven by autogenic processes. Palaeogeographical reconstructions highlight three depositional organizations: lacustrine-dominated, palustrine-dominated, and mixed configurations. Mixed configurations exhibit pronounced spatial heterogeneity, with lacustrine, lake-margins, and palustrine facies coexisting laterally at the scale of the study site, over distances of a few metres. Lake-margins deposits are restricted in extent and frequently absent from vertical successions, indicating rapid pedogenic overprinting. These results demonstrate that autogenic processes dominate metre-scale facies architecture in lacustrine–palustrine carbonate systems and must be explicitly accounted for when modelling fluid flow and connectivity in continental carbonate aquifers.