Comprehensive Evaluation Of Sealing Capacity Of C-P Caprock In The Northern Bohai Bay Basin

Deep resources in Bohai Bay oil and gas bearing basin have great potential.The oil and gas exploration and development of the the upper Paleozoic C-P have achieved remarkable results in many places.The basins with deep oil and gas enrichment are mostly superimposed basins with strong alteration,and the sealing ability of caprock is the key factor for hydrocarbon preservation.In this paper,the macroscopic characteristics,microscopic sealing and time validity of the C-P caprock in the northern part of the Bohai Bay Basin are analyzed.Then the sealing ability of the C-P in the northern Bohai Bay basin is synthetically evaluated by using the macro and micro factors that affect the sealing ability of the cover.The macroscopic development characteristics of the caprock are related to the lithology,development thickness and plane distribution area of the caprock.Through logging data and lithological data,it is known that the lithology of the Carboniferous Permian caprock in the northern part of the Bohai Bay basin is dominated by argillaceous rocks,the lithology of Shiqianfeng caprock,upper Shihezi caprock and Lower Shihezi caprock is mudstone and sandy mudstone,and the lithology of Shanxi caprock and Taiyuan caprock is mudstone and carbonaceous mudstone,The lithology of Benxi caprock is mudstone,bauxite and aluminous mudstone.The mudstone caprock of the upper Shihezi Formation and the Lower Shihezi Formation have large thickness,large plane distribution area and the best macroscopic development characteristics.The macroscopic development characteristics of the caprock in the Shiqianfeng and Benxi formation are the worst.The microscopic sealing property of caprock mainly involves physical sealing and overpressure sealing.The sealing property of the caprock is characterized by the displacement pressure value.Through mercury injection experiment and mathematical fitting,the distribution of displacement pressure in caprock of the study area is obtained.In general,the pressure range of the C-P caprock in the northern part of the Bohai Bay basin is about 0-12 Mpa.In the Jizhong depression,the area of high displacement pressure is mainly around the well Su 50 and north of Wen 23 well.The area of high displacement pressure in the study area of the Huanghua depression is around the Chuang Gu 2 well,the Chenghai 24 well in the central part,and the well of Wang Gu 1 and Wu Shen 1 in the south.By means of the relationship between the acoustic transit time and the depth,it is found that there are overpressure in some formations in the Shanxi formation and Taiyuan formation.The overpressure area of caprocks in Shanxi formation is mainly distributed around the well Su 50 and Ge 2 in the northern part of the Jizhong depression.Thepressure coefficient is 1.3-1.6.The overpressure area in the Taiyuan formation is mainly distributed around the Chenghai 24 well and the Hai Gu 1 well in the central part of the Huanghua depression,with a pressure coefficient of 1.3-2.0.The effectiveness of the caprock in time refers to the time when the rock develops into effective caprock earlier than that of large-scale oil and gas discharge.Recovery pressure history of key wells in key areas of the study area was restored.According to the history of hydrocarbon expulsion,it is found that the time of sealing oil and gas in the cover is earlier than that of the main hydrocarbon expulsion period.It can be found that the sealing ability of the caprock in the study area is effective in time.The evaluation index Q is established by using the macro and micro factors that affect the sealing ability of the cap.The sealing ability of the caprock is divided by the evaluation value of Q,Then the sealing ability of caprock is comprehensively evaluated by classification.The best quality caprock is the upper Shihezi Formation and the Lower Shihezi Formation caprocks in the northern part of the Jizhong depression,and the caprocks of the Taiyuan formation in the central part of the Huanghua depression.The quality of the caprock in the Shiqianfeng and Benxi formation is generally not good.