Abstract: Seismic sedimentologic analysis is superior to conventional seismic stratigraphic analysis in high-frequency and deposi-tional system study. 90°-phase seismic amplitudes can be correlated to log lithologies, and consequently, after phase shift on seismic data, seismic amplitudes can be used for lithologic analysis. The strata slice technology made form the 3-D seismic volume can be used to reveal the distribution characteristics of deposits deposited at the same geologic time, and combined with lithologic log data, the depositional systems can be analyzed accurately on the relative same geologic time surfaces. For the A oilfield, before drilling the development wells, a seismic sedimentology study has been conducted for the target zones. On the basis of the 90°-phased shift, a dynamic interpretation has been done to the target zone through study on a series of stratal slices combined with the drilling core information. It is concluded through interpretation that distributary channel to the distributary bar deposits are dominated in the target zones. Base on the HC accumulation rules, five favorable sand bodies have been forecasted and confirmed through well drilling, and over 1 200×10⁴ m³ proved OOIP has been obtained, changing the unfavorable development conditions of A oilfield of Bohai Sea. In addition, the experience in A oilfield can provide reference for the exploration & development in the other oil fields of this area.