Polygonal fault systems (PFS) are widely developed along many continental margin basins and some interior cratonic and foreland basins. They are networks of small normal faults that are usually found in tiers which are crudely layer-bound successions consisting mainly of fine-grained sediments. Their origin has been widely debated with explanations ranging from dewatering of overpressured layers to gravitational loading under fixed lateral boundary conditions. Their distribution in fine-grained intervals suggests that their genesis is connected to their initial lithology. Recent experimental work suggests that diagenesis of clay-rich sediments can lead to shear failure under low confining stresses. This explanation accords well with all subsurface observations of PFS made to date, and seems the most likely mechanism to explain their genesis. This diagenetic mode of shear fracturing in fine-grained sediments could be much more prevalent than in polygonal fault systems alone, and this has significant implications for shale gas exploration, CO2 sequestration and exploration for methane hydrates. |
