Depositional Sequences and Prediction of Favorable Reservoir Sand of Shahejie Formation of Liaoxi Depression

The existence of a complete stratigraphic sequence in Paleogene Shahejie Formation in northern Liaoxi depression is confirmed and five sequences are identified in this paper by comprehensive analysis of seismic profiles,well logs and cores data.It is found that there is not any unconformity surface between Member 1 and Member 2 of Shahejie Formation,so they form the highstand system tract and lowstand-transgressive system tract of the sequence respectively.SQs3-1(the upper submember of Member 3)is nearly denudated in uplift area and widely distributed in the depression zone.All kinds of sedimentary facies have been recognized in the study area,including the fan delta system,braided delta system,nearshore subaqueous fan system and beach-bar system.The sedimentary system distribution is controlled by syndepositional fault,paleo-uplift and depression.The favorable sand bodies in the study area consist of those developed in the lowstand system tract at the edge of the paleouplift,deep water gravity flow sand in the center of the sub-sag and carbonate in the upper part of the paleouplift.With the underlying massive dark mudstone and shale in the third sequence of Shahejie Formation and the overlying pelolithic layer of the first sequence of Shahejie Formation,they can form good reservoir-cap assemblages and form litho-stratigraphic traps in the uplift,sub-sag,fault slope break and the gentle slope belt.