Origin of polygonal fault systems: A case from the Sanzhao sag in the Songliao Basin, East China

Polygonal fault systems are mainly found in fine-grained sediments. They are controlled by layers vertically and characterized by polygonal planform geometry. They are also characterized by small fault throw, high fault density, various strikes, and small lateral extension. Their genetic mechanisms, which are usually recognized as non-tectonic, include density inversion, dewatering contraction, gravitational loading, and dissolution-induced shear failure, with the last one being dominant. Many small extensional faults have been found in the Cretaceous in the Sanzhao sag of the Songliao Basin. Having the characteristics of polygonal faults vertically, horizontally and in shape, they are defined as typical polygonal fault systems. There are more polygonal fault systems in the formation with heavier dissolution, these polygonal fault systems may be formed by the mechanism of dissolution-induced shear failure. Polygonal fault systems are migration pathway for the Putaohua and Fuyang oil layers in the Sanzhao sag and they significantly control on oil-water distribution. The areas where sandstones and polygonal fault systems are developed together are favorable exploration areas.