Study On Sedimentary System Of Shahejie Formation In Cangdong Sag

Candong Depression is located in the southern part of Huanghua Depression in Bohai Bay Basin,and it is the second largest oil and gas depression in Huanghua Depression.after drilling wells,low mature oil and gas shows,and the reservoir effect is good,and there are still differences in the source system,sedimentary phase type and spreading characteristics of Shahejie Formation.In view of the above-mentioned research significance and problems,this paper is guided by the theories of stratigraphic stratigraphy,sedimentology and logging geology,and based on cores,logging data,seismic data and analytical and chemical data,we study the spreading characteristics of sedimentary system and depositional pattern of the Shahejie Formation in the Candong Depression in depth.Based on the previous studies,the Shahejie Formation was divided into five tertiary sequences,namely SQ1(Sha San Lower Sub-section),SQ2(Sha San Sub-section),SQ3(Sha San Upper Sub-section),SQ4(Sha 2 Section)and SQ5(Sha 1 Section).The gravity anomaly,seismic profile,light minerals,rock chip type,heavy minerals and sand body dispersion system combination features were used to analyze the source of the Shahejie Formation in the Candong Depression,and the source was found to be from the Cangxian Uplift,Xuhe Uplift,Dongguang Uplift and Kongdian Uplift.Based on the analysis of seismic,logging and lithological phases,the study area identifies the fan delta,curved river delta,braided river delta and lake sedimentary phases,with the fan delta developing plains and leading edges,the curved river delta developing only leading edges,the braided river delta developing plains and leading edges,and the lake phase developing shallow coastal lake,semi-deep lake-deep lake,sandy beach dam and carbonate beach dam deposits.The study area is divided into four major sedimentary system groups,namely,the eastern sedimentary system group,the southern sedimentary system group,the western sedimentary system group and the Kongdian sedimentary system group,through comprehensive analysis of the sedimentary system with well profiles and planes.The eastern steep slope belt of SQ1～SQ3develops a fan delta,the southern part develops a meandering river delta,the southern part of the western part develops a braided river delta,and the northern part of the western part develops a fan delta.SQ4 is affected by faulting,and the delta shows obvious progressive features,the fan delta on the north side of the west disappears,only lake deposits and a few sandy beach dams are developed,and the water body is shallow,the delta of the Braided River is developed in the south,and the fan delta is developed in the east.SQ5 is affected by the combined effect of faulting and sea invasion,the source area is drastically reduced,the sand body of the delta shows obvious regression,the lake expands rapidly,and only the western part of the Cangdong depression is developed Braided river deltas and sandy beach dams are only developed in the western part of the Cangdong depression,and bioclastic beach dams appear around the perimeter of the Shennvsi,Cangdong sub-concavity and Kongdian Uplift.