Abstract: K oilfield of Kazakhstan is a medium-high porosity and medium-high permeability layered sandstone oilfield with complex reservoir lithology and strong Interlayer heterogeneity.It has been exploited by inverted nine spot water flooding pattern for 37 years.By the end of September 2015,the comprehensive water content of K oilfield has reached 90.1% and been in high water stage.The drilling of K2357 C well that is the first sidetrack horizontal well of K oilfield was finished at the end of 2010,which was the prelude for large scale implementation of sidetracking horizontal well.To study and evaluate the potential,by applying the theory of geology,well logging and reservoir engineering,the low productive and inefficient wells were selected,the layers were optimized,a reasonable length of horizontal section was demonstrated,the vertical position of the horizontal segment and a reasonable amount of fluid production were analyzed.After deploying 45 sidetrack horizontal wells,the daily oil production of single well and water cut were superior to that of straight wells at the same period,which proved the feasibility of sidetracking horizontal wells in extra-high water sandstone mature oilfield.