Abstract:

Based on seismic and log data,we used back-stripping technology and trend analysis method to restore structure-paleogeomorphology of the Qingdong sag during the sedimentation of the lower part of third member of the Paleogene Shahejie Formation (E3s³).We also explained the control of structure-paleogeomorphology on sedimentary systems.Controlled by the Tan-Lu fault and its induced faults,the Qingdong sag subsided as a whole during the sedimentation of the lower part of E3s³.It was a dustpan-shaped rift which was faulted in its eastern part and overlapped in its western part.Within the sag,there were three near east-west-trending sub-uplifts which separated the Qingdong sag into four sub-sags.From south to north,the size of these sub-sags became larger gradually.Controlled by structure-geomorphology,fan deltas were developed in the east of the Qingdong sag,while braided deltas were developed in the west and in the south during the deposition of the lower part of E3s³.The major part of the delta sedimentary system was obviously controlled by the sub-sags.Arenaceous sediments of fan delta and braided delta facies overlapped,in various degrees,the sub-uplifts because of the small size of the sag and sufficient supply of deposits.The shallow water and strong energy resulted in the arenaceous beach-bar facies of different scales on the sub-uplifts.Favorable reservoir conditions and traps are found in delta sandbodies developed on the eastern ramp,braided delta sandbodies developed on the western gentle slope and arenaceous bench bar deposits developed on the sub-uplifts.These sandbodies should be the major exploration targets.

Key words: lower part of E3s³, structure-paleogeomorphology, Paleogene, Qingdong Sag, Bozhong Depression