Abstract: In order to further develop ultra-high water cut oil reservoirs and to solve the issue of interlayers interacting with each other,interlain such reservoirs,this paper proposed an identification method for declined interlayers.It detailed the water flooding mode of various rhythmic layers with certain declination,studied the distribution of the residual oil,and formulated relevant development strategies.In addition,the calculation method for micro water injection index was obtained,and the interlayer interaction issue could be solved by using the results of the analytics to change waterflooding directions,so as to increase the waterflooding volume and collect the plane residual oil.Applying this new technique achieved good performance in the Huzhuangji Oilfield,proving the declined interlayers’ existence in the fan-delta deposits.Such declined interlayers significantly control formation and distribution of residual oil.The waterflooding calculation results could reflect the range and intensity of waterflooding from a micro perspective.Results showed that residual oil in the declined rhythmic and cyclic layers are mostly distributed in the upper part of both interlayers,and the sweep coefficient and recovery efficiency of reverse water injection is slight over those of the normal water injection.The water flooding index is an important factor on oil displacement efficiency and recovery efficiency,where a higher water flooding index will produce a better water displacement effect.The research on the residual oil in the inclined interlayer and derivation of the water flooding index would provide a new technical method in ultra-high water cut oil reservoirs development.