Patrick Gaviglio

Matrix strains along normal fault planes in the Campanian White Chalk of Belgium: structural consequences

Abstract Significant matrix strains were associated with normal faulting in the Campanian chalk of the Mons basin. These transformations, mainly due to pressure solution, located against the fault planes, brought about systematic changes in the porous network: a reduction in volume against the fault planes, a change in access diameter and an increase in heterogeneity of the material. The measurements of other physical properties (elastic waves velocity, capillary rise, permeability) of the chalk provide good evidence for estimating the width of the transformed zone: it ranges between 100 and 150 mm. These transformations cannot be detected when the medium is observed at the grain scale: the most visible evidences of transformation are limited in a 50 mm thick fringe. The transformation may have occurred either in a closed or in an open system. If we follow the first assumption we can estimate the change in volume, and therefore the change in horizontal dimension of the faulted blocks: it is small compared to the horizontal extension. The second assumption is supported by a few evidences: tilted and striated rostra of belemnites along fault planes. In that case the geometrical change can be estimated too and reveals that the horizontal shortening is in the same range as the horizontal extension. It means that the extensional deformation of the Mons basin might have been partially hindered by chemically induced matrix strains.

Mass transfer controlled by fracturing in micritic carbonate rocks

Abstract The fractured Coniacian chalk from the Omey area (Paris Basin, France) displays strong evidence of modifications controlled by brittle deformation. Fracturing is associated with important changes in pore space (decrease in total porosity and pore interconnection, change in distribution of pore access diameters and capillary characteristics), nannofacies (gradual evolution from a point-contact fabric to a welded, interlocked or coalescent fabric) and chemical composition (Sr concentration decrease). These modifications result from fluid–rock interaction that control significant mass transfer (percentage of secondary calcite >50%). Sr is a remarkable indicator of these mass transfers. Sr analyses allowed us to prove that the deformed zone (26.7 m) is wider than the fractured zone (11.3 m). They also indicate that the footwall block is less affected than the hanging wall block. A physicochemical model of the deformation mechanism is proposed. It shows that a cyclic process of fracturing controls the temporal evolution of the fluid saturation and fluid pressure and, consequently, the mass transfer.

Measurement of natural stresses in a Provence mine (Southern France)

Abstract Stress measurements were carried out in the Arc syncline using drifs in a lignite mine. Eleven sites were investigated using the flat jack and hydraulic fracturing (or stimulating) methods. Two stress states were found to coexist, one isotropic, the other highly anisotropic. The orientations of the principal stresses are not homogeneous and an orientation ranging from E-W to NE-SW predominates locally. This does not accord with the regional stress field. The vertical stresses are systematically underestimated.

Reconstitution du réseau poreux d'un échantillon décomprimé. Mise en évidence d'une anisotropie de connectivité

The porosity network of samples cored in highly stressed granitic material is investigated by mercury injection. In addition to threshold and porous volume classically measured with these techniques, we obtained information on heterogeneity and anisotropy of the porous network. Anisotropy of the crack network connectivity is inferred as the relationship between porous volume measured on classic sample and porous volume measured on a sample partly covered by resin. Mercury injection into samples partly covered by resin is possible only along the core axis.