The Investigation Of Depositional Environment From The Third Member Of Shahejie Formation In Chenjia Sag

In the paper, the water body of the continental lake is categorized into three kinds of system, ie., closed, semi-opened and opened. Through analysis on the factors affecting the lacustrine sequence formation, it is found out that the structures and climate changes are the major elements for the forming of lacustrine sequence formation, and the lake level change is not the major element for forming of lacustrine facies sequence formation. The Third Member of Shahejie Formation of Chenjia Sag is divided into 3 three-level-sequences based on seismic, integrative well completion, palaeobiology and lithology data. Based on intermittent balanced movement pattern, the dichotomizing method is used to divide the system tract. As a result, the three-level sequence is divided into lake expanding system tract and lake retracting system tract. Regionally, the three-level sequences are correlated and E2S3 isochronism framework is established based on sedimentation cycles and palaeontological marker, combined with seismic features in some local areas.The deposit system of Chenjia Sag in E2S3 is fan delta-sub-lacustrine fan-lake deposit. In the paper, the deposit system is divided into 8 sedimentary subfacies and more than 10 microfacies with the sedimentation features described. Based on the sequence framework, we have addressed the lateral distribution feature of the sedimentation facies. For sequence I to sequence III, the lake basin expanding system tract is characterized by development of fan delta and sub-water fan. In the south of Chenjia Sag, the lake expanding system tract of sequence II to sequence III is characterized by development of sub-water fan gravity current channel in long axis direction and extend further in southeast direction. The lake convergence system tract of sequence III is characterized by development of braided river and sub water distributary channel.Based on analysis on the corresponding relationship of structure and function of ostracode in modern oceans and lakes, it is found out that the structure, shape, size, convexity and abdomen plane of ostracode are related to the depth of the water body. Based on the above view, the ostracode palaeoecology of Liaohe basin in Paleogene has been established. In view of the palaeoecology of ostracode, Gastropoda, algae fossil, combined with sedimentation facies features, the biological facies of Chenjia Sag in E2S3 period has been established, including five biological facies, i.e., facies A, B, C, D and E, which respectively represents different water body environments.Based on tectonic movement and climate changes features, combined with sedimentation facies and palaeocology, the sedimentation environments of Chenjia Sag in E2S3 are addressed under the isochron framework. For Chenjia Sag during E2S3 the center of water body, depo-center, deep water biota moved regularly with the migration of depo-center. During Sequence I deposition period, they migrated from the western slope belt to the eastern slope belt, in well area of Lei 60 -Leng 175. During Sequence II and III deposition period, they gradually migrated from well Lei 60 in the north to west of well Leng 618 and south of well Leng 172. By the end of sequence III period, because of the change of water body environment, ie., change of water quality and getting shallow of the water body, Huabeinia–Bohaidinia biota extinguished extensively.