Sedimentary and diagenetic processes at the origin of the 3D architecture of heterogeneous lacustrine and palustrine limestones.

Lacustrine and palustrine carbonates form in lakes and swamps of various shapes and sizes. They can accumulate through chemical or biological processes and are constantly under the influence of external alteration sources such as meteoric waters, vegetation or fauna. Thus, there are the result of a complex primary fabric later modified by early and late diagenesis processes which makes them highly heterogeneous.

Their study is mainly motivated by their ability to act as a reservoir for underground water or for Carbon Capture & Storage (CCS), which both require a precise understanding of their structure and variability of permeable bodies.

The 3D architecture and the scale of heterogeneity of such carbonates remains a question as depositional and alteration processes can occur on centimetre scales or less (e.g., roots), to metre and kilometre scale (e.g., subaerial exposure). This variability cannot be assessed without any large, pluri-directional outcrop or without a high density of cored boreholes.

The O-ZNS platform (“Observatoire des transferts dans la Zone Non-Saturée”) located near Orleans, France, is an observatory of the vadose zone of the Beauce aquifer. The host rocks are aquitanian lacustrine and palustrine limestones (the Beauce Limestones formation).

The observatory offers an exceptional 20 m deep and 6 m diameter well surrounded by eight cored boreholes (20-25 m deep) within a radius of 30 m, which were described at a centimetric scale (1:6).

Our study focuses on the control of the 3D architecture of the sedimentary facies by understanding the chronology of the successive depositional environments. Also, it aims to consider the diagenetic overprint of the Beauce Limestones to decipher what is the impact of the primary fabric on secondary processes that finally lead to the heterogeneities we observe today.

This contribution will present: (1) How the 16 sedimentary facies distributed in 4 depositional environments (lake, lake margins, external palustrine, internal palustrine) vary at a decametric scale, (2) the paleo-environmental evolution of the site, located at the transition between lacustrine and palustrine settings, built thanks to the correlation of 8 transgressive-regressive cycles, and (3) the link between sedimentary facies, diagenesis and petrophysical properties (porosity, permeability).