Porosity distribution in a clay gouge by image processing of 14C-PolyMethylMethAcrylate (14C-PMMA) autoradiographs:: Case study of the fault of St. Julien (Basin of Lodève, France)

Clay-rich fault gouge is a highly heterogeneous material composed of clasts of host rocks surrounded by a clay matrix. Moreover, this fault rock is characterised by different layers with varying properties: clast content, quantity of shear planes and lithology of rocks. Consequently, the accurate porosity characterisation inside this heterogeneous rock is not possible using usual techniques such as microscopy or bulk physical measurements. A specimen representative of the centimetric thickness of Saint Julien fault gouge has been impregnated by a 14C marked MethylMethAcrylate resin. From the micrometric to the centimetric scale, the spatial distribution of porosity (micro to macropores) is revealed on autoradiographs of different sample sections: clasts (sandstone or pelite) and illite-rich matrix composing the gouge are distinguishable according to their porosity contrast, and the layering is highlighted through the entire gouge thickness. Quantification of porosity by a specific image analysis software shows that clast porosity (ranging from 2.7% to 8.1%) is always lower than the porosity of the clay matrix (ranging from 10.5% to 15.3%). The porosity of these two gouge components always presents the same difference in all gouge layers. A detailed observation of porosity maps reveals the spatial relations linking clast to clay matrix porosities: porosity gradients around the clasts indicate that dissolution of carbonate cements by a recent fluid circulation into the clay part of the gouge is responsible of the actual porous network.