Geological research focused on high density "T₂" fault system in the past, and its forming mechanism is of great controversy, whereas "T₁₁" also as a high density fault system has been ignored. Systematic researches on geometry and geological setting of "T₁₁"fault system in Zhaozhou area within Sanzhao sag were conducted, and the theory that "T₁₁"is a typical polygonal fault was firstly proposed, formation of which can not be explained easily only by tectonic evolution history. Based on the fact that fault is in accordance with the distribution of mudstone, which bears abnormal pore-fluid pressure in K₁qn Formation, it is considered that pervasivedeveloped fault activated in the reverse period caused density inversion of K₁qn mudstone formation, which lead to the formation of polygonal fault, and periodically active fault that formed by the accumulation of overpressure is the migration pathway of oil and gas in tbe Putaohua reservoir.