Differential fracturing pattern in clay/limestone alternations at Tournemire (Aveyron, France) and in the Maltese Islands
Titre du congrès :Fault Zones: Structure, Geomechanics and Fluid Flow
Ville du congrès :London
Date du congrès :16/09/2008
IRSN has reviewed ANDRA’s 2005 clay dossier on the feasibility of deep geological disposal of high level, long-lived radioactive waste in the Callovo-Oxfordian clay formation (COX) located in the Meuse/Haute-Marne (MHM) area, Eastern Paris Basin, and investigated by ANDRA using the Bure Underground Research Laboratory (URL). An important task was to deal with the influence of tectonic structures, which is a key question of the safety evaluation as they may have potential for locally leading to a channelling of flows through the sedimentary beds and thus for affecting transfer times, dilution factors and outlet positions.
Using available data from ANDRA’s geological survey, IRSN suspected the existence of faults belonging to the same fault family, but with a different expression within the Mesozoic sedimentary pile: near the Bure URL, faults suspected under the Lower Bathonian (using a 3D seismic survey) do not seem to have propagated through the COX (according to oblique boreholes), whereas a few kilometres to the SW, structural and hydrogeological data (from boreholes) allow to postulate that neighbouring faults hydraulically impact the entire sedimentary pile, including the COX. Therefore, given these possible differences in fault propagation, the fracturing pattern in the COX can not be straightforwardly extrapolated to the whole MHM area.
The propagation of fractures from limestones to clays is related to the so-called “differential fracturing phenomenon”, which means that ductile layers (clays) show a less intense and concentrate fracturing than do breaking layers (such as limestones). This is commonly observed at a decimetre scale and has been studied for joints, but it is still insufficiently understood for faulting at the scale of tens to hundreds metre thick formations. This poorly understood phenomenon is being addressed by IRSN on the basis of field observations at analog sites at various scales, so as to elaborate a model of differential fracturing in clay/limestone alternations. First fracturing observations were carried out in two analog sites, Tournemire (Aveyron, France) and the Maltese Islands.
The present paper intends to describe the results obtained from these observations and summarized below, as well as IRSN’s perspectives to continue this work.
Differential fracturing was revealed at a plurihectometric scale in IRSN’s Tournemire experimental station, an ancient railway tunnel crossing a 150-m thick Toarcian clay formation at 250 m depth. The following features were observed (3D seismic survey, tunnel, boreholes): (i) in the limestones underlying the Toarcian clays, a normal fault re-activated as strike-slip is associated with a narrow fractured zone, (ii) this structure continues into the clays as a wide diffuse zone of thin, strike-slip faults, (iii) the structure is once more expressed within a narrow fracture zone in the overlying limestones and then widens as it fades toward the surface.
In the Maltese Islands, along the seashore, the 2 to 20 m thick blue clays outcrop between two tens of metres thick limestone formations of Oligo-Miocene age affected by slight extensional tectonics. The underlying limestone layers show numerous joints and faults, whereas few of them affect (partially or fully) the clay layer, usually with a smaller dip. Some fractures have conducted fluids from limestones to clays; others seem to have been generated by fluid overpressure.