Simulation Study On The History Of The Cenozoic Subsidence In Dongpu Sag

The study area is located in the Dongpu Sag of the Bohai Bay Basin.In order to study the tectonic evolution and sedimentary-subsidence characteristics of the sag,well logging,drilling,seismic profiles,stratigraphic residual thickness maps,sandstone and mudstone percentage maps,different lithology porosity,compaction coefficients,etc.were collected.Through the stripping of 92 wells?including artificial wells?in the area,The total subsidence,tectonic subsidence and subsidence rate of different sedimentary periods of seven major sub-structural units in the whole district?the Liutun-Guancheng Depression,the Pucheng East Depression,the Haitongji Depression,the Qianliyuan Depression,the Changyuan East-Nanhejia depression,the Dongming-Gegangji depression,and the Xuji-Sanchunji protrusion?.were obtained.Based on the observed structural subsidence,a simulation study of tectonic subsidence history was carried out.The finite-time uniform extension model with radioactive heat production is determined as the best model by simulating and comparing three forward models,namely instantaneous uniform extension model,finite-time uniform extension model and finite-time uniform extension model with radioactive heat production.And determine the initial lithosphere and crust initial state under paleo-geological conditions.Based on the forward modeling,the strain rate inversion was performed on the four sections to obtain the variation of the extension coefficient and strain rate.Comparing the simulated structural subsidence with the observed structural subsidence,the characteristics of abnormal subsidence in the post-rift period of this area are analyzed,and the formation mechanism is discussed.The stripping analysis obtained the distribution characteristics of the observed structural subsidence in this area.The largest total subsidence of the Cenozoic is the Qianliyuan depression,which can reach 3015 m;the largest tectonic subsidence in the syn-rift period is also in the Qianliyuan depression,reaching 2750 m;and the largest tectonic subsidence in the post-rift period is Changyuan East-Nanhejia depression,up to 399 m.The tectonic subsidence at different stages of the Cenozoic has similar characteristics to the total subsidence;the subsidence center of the fourth member of the Shahejie Formation is located in the Dongming-Gegangji depression;the sedimentary period of the third member of the Shahejie Formation migrated to the Qianliyuan depression;The sedimentary period of the second member of the Shahejie Formation-the Dongying Formation migrated to the Changyuan East-Nanhejia depression;the Neogene-Quaternary subsidence center remained in the Changyuan East-Nanhejia depression.The corresponding subsidence rate of the depression with large subsidence in different stages is also faster.Forward modeling shows that the initial results are 110 km and the crust thickness is 32 km can gets the best results.The strain rate inversion simulations of four representative sections show that the extension coefficient and strain rate in the middle of the depression are larger in the syn-rift period,and the edge of the depression is smaller.The extension coefficient in the northern part is smaller than that in the middle and the south,which is 1.1-1.41.The extension coefficient of the central part is 1.12-1.84,and the south is 1.14-1.91;the largest strain rate of each zone is in the sedimentary period of the third member of the Shahejie Formation,reaching1.5×10^-15s^-14×10^-15s^-1.There is abnormal subsidence in the post-rift period of this area.Among them,the insufficient subsidence occurred during the sedimentary period of the Guantao Formation,and the insufficient subsidence of the southern part of the depression was larger than 200 m;the sedimentary period of the Minghuazhen Formation-Plane group was an abnormal over-sufficient subsidence stage,and the anomalous over-sufficient subsidence in the southern part of the depression was greater than In the central and northern parts,the largest part of the southwestern part of the depression can reach more than 600 m.