Abstract: This thesis has researched the reason why Chang 2 oil-bearing formation has high water cut (approximately 90%)but has the main producing layers for more than 30 years in Xiasiwan,which cause our curiosity and attention. On the basis of the theory of the petroleum geology, the reasons for the Chang 2 has high water cut and low stable production have been discovered by using the theory and method of the advanced petroleum system and system science method, carrying out the related geology investigations, analyzing and studying the conventional thin sheet and casting thin sheet technology. The results show that the Chang 2 high water cut reservoir in Xiasiwan Oilfield has five geological causes, including the natural fracture system cause, porosity cause, permeability causes, hydrocarbon closure cause and formation water cause.According to faces-potential-source coupling, the source of Chang 2 still comes from Chang 7.The Xiasiwan oilfield is near the heart of the lake, which caused the stable mechanism of oil supply and accompanied by the factors of sedimentary facies and structural characteristics.Thus, the special reservoir system of high water cut and low stable production in Chang 2 reservoir has been brought up, which has been controlled by the faces-potential-source coupling.