Abstract: Through study on structural evolution and analysis of sequence growth characteristics,Napo for-mation in South Block of Oriente Basin,Ecuador was divided into 4 sequences.On the basis of tidal bed-ding,herringbone cross-bedding,flush cross bedding and plenty of shells,it has been identified that the sedimentary environment of the study area is of tidal flat environment.In view of gentle slopes and interbe-ded sedimentation of siliciclastic rocks and carbonates,the sedimentary facies model was built that is mixed sedimentation of siliciclastic and carbonate on the gentle slope shelf margin.It's believed that the sedimentary facies of coastal plain,tidal flat,restricted platform and continental shelf are developed in Napo Formation in the South Block.Vertical array and plane migration of phase sequences were controlled by sea level fluctuation.The author pointed out that tidal flat sandstone mainly existed in shelf margin system in every sequence,and subaqueous shoal sandstone existed in transgression system.Through the analysis of sedimen-tation,the sand body of tidal channel was considered as the favorite reservoir.