Numerical Simulation And Structural Analysis On The Formation And Evolution Of The Dongpu Sag, Bohai Bay Basin

The Dongpu Sag,located in the Bohai Bay basin,NE China,is a Cenozoic continental rift basin.As one of the representative basin,nature and tectonic evolution of the Dongpu Sag is the key to understand the Cenozoic Bohai Bay basin,the Cenozoic basin evolution along the eastern margin of Eurasia,and to guide the exploration practice of oil and gas.To determine the nature and origin of basin evolution of the Dongpu Sag,the modelling methods on syn-rift and the post rift period of basin have been improved.We present the basin numerical models refer to as stratigraphic geometry method and oblique shear model.For modelling normal fault-layer geometry,these two models establish equations predicting normal fault form from the hanging wall layer strike and predicting hanging wall layer strike from normal fault form,respectively.We then present total heat flow extension model to describe the post-rift evolution of basins.Synthesizing extension model,backstripping and stretch factor inversion,a method used to distinguish post-rift anormalous subsidence by seismic and logging data is proposed and programed by MATLAB.Based on fault-layer geometric model,refined seismic profiles,along-strike throw profiles and syn-rift layers isopachous maps,conclusions are summarized as following:Lanliao fault could be divided into five segments,i.e.,Pucheng segment?PFS?,Fan-Mao fault segment?FMFS?,Qianliyuan fault segment?QFS?,Bai-Ge fault segment?BGFS?and Mafang fault segment?MFS?.MFS connected with BGFS during the Es₄ period,and QFS attached to MFS and BGFS during the Es₃² period.Lanliao fault finally linked during the Es₃¹ period by the successive progress of these five segmens.Twice changes of the depocenter during syn-rift stage are distinguished.Depocenter relocated from Gegangji subsag controlled by Lanliao fault to Bagongqiao subsag controlled by Changyuan fault,then moved to Menggangji subsag controlled by Huanghe fault.Formation of“uplift within two subsags pattern”in the middle and south Dongpu sag was finally done during Ed period.Anormalous subsidence is identified in post-rift stage of Dongpu sag.During the Miocene,the observed subsidence was smaller than the predicted subsidence,leaving negative discrepancies referred to here as“insufficient subsidence”ranging from–343 to–96 m.In contrast,during the Pliocene–Quaternary the observed subsidence was greater than the predicted subsidence by+123 to+407 m,which left positive discrepancies referred to as“over-sufficient subsidence”.Therefore,we infer a transition from insufficient to over-sufficient subsidence during the post-rift stage.We propose a new tectonic model in which variations in the conditions associated with subduction of the Pacific plate resulted in a change of heat flow decreasing from a linear to a curvilinear pattern,leading to a transition from insufficient to over-sufficient subsidence.