Clusters of cataclastic deformation bands in porous sandstones

Abstract

Clusters of cataclastic deformation bands represent potential barriers or baffles to reservoir fluid flow, and their processes of formation remain debated. In this work, we rely on an integrated field study at seven sites to describe the extent of clusters, their morphology and their density of deformation as a function of several parameters: the tectonic loading, the burial depth of deformation, the Andersonian stress regime and the lithology of the sandstone. We perform porosity, sorting and grain shape analyses of the deformed material to improve the understanding of microscopic process of cluster development. In agreement with previous works on cataclastic deformation bands, our results reveal that the tectonic loading constrains the extent and the morphology of the clusters. Extensional tectonics favors the formation in normal-fault Andersonian regime of series of hundreds of meter long, rather thin and dense clusters, forming kilometer long networks associated with faults. We find that the formation of major slip-surfaces (faulting) can occur at any stage of cluster thickening. Contractional tectonics favors the formation of strike-slip clusters of medium-thickness, and more rarely the formation of tens of meter long, low-density, thick thrust clusters. The clusters formed in contractional tectonic settings are sparse. Since 15%–79% of the bands/slip-surface of the clusters have permeabilities close to or lower than 1 × 101 mD, and because clusters can have significant dimensions, they may be significant baffles, particularly in extensional tectonics. Our analysis of the porosity and shape evolution of the clasts with increasing deformation corroborates the hypothesis of strain hardening of the band by combined increase of cohesion and friction induced by the cataclasis. We propose that the variability of hardening associated with the variable packing and related cementation in specific parts of the deformed material controls the development and the morphology of the clusters as a function of the tectonic loading.