EVOLUTION OF SEDIMENTARY SYSTEM AND ANALYSIS OF SEDIMENTARY SOURCE IN PALEOGENE OF BOZHONG SAG, BOHAI BAY

Comprehensive research is applied to sedimentary system of Paleogene in Bozhong sag, Bohai Bay through making use of data on geology, well logging and sample testing synthetically, and the results show that sedimentary system of Paleogene is mainly controlled by structural movement and change of lake surface, and has experienced 3 phases of rift and intrusion of lake.Lower Tertiary(Paleogene) is divided into Shahejie Formation and Dongying Formation; Shahejie Formation can be divided into 4 members, namely Sha 1,2,3,and 4; Dongying Formation can be divided into upper Dongying member and lower Dongying member, or into 3 members, that is, Dong 1,2,and 3. The main sedimentary facies of lower Tertiary is sublacustrine fan, fan-delta, delta, turbidite fan, beach bar and carbonate beach bar.
In Sha 4-Kongdian period, that is the beginning of rift, sedimentary facies are mainly shallow lacustrine facies; In Sha 3 period, an intensive rifting period, sedimentary facies are fan delta and turbidite fan; In Sha 2-1 period, lacustrine basin began expanding and sedimentary facies are fan delta, sublacustrine fan and clastic beach bar and shallow water platform; during Dong 3 period, that is the beginning of lacustrine basin expansion, fan, fan delta and sublacustrine fan are the main facies; during Dong 2 period, that is the expansion period of lacustrine basin, gigantic lacustrine deltas are prosperous; Dong 1 period was a shrinking period of lacustrine fan, sediments delta plain and braided river were formed, and Bozhong sag was changing from rift to depression.
Analysis of structure and sedimentary system indicated that sedimentary source of Sha 3 period was very abundant, and mainly came from Shijiutuo, Bonan and Miaoxi lobes. Sedimentary source became limited during Sha 1 and Sha 2 periods. The main sedimentary source came from Shijiutuo and Bodongnan lobes. During Dong 2 period, Bozhong sag was in the environment of shallow lake-half-deep lake because of intrusion of lake, when rich sediments were carried into the sag by ancient water systems to have large-scale fan-delta developed.