The Structural Process Of Cenozoic Basin Forming And Oil Accumulation In The Chezhen Sub-depression Of The Bohai Bay Basin And Destruction Of The North China Craton

The Chezhen Sub-depression,one of seven sub-depressions of the Jiyang Depression, is located in a unique E-W striking structural zone of the Bohai Bay Basin. Researches on Chezhen sub-depression play a pivotal role in understanding evolution of the Bohai Bay Basin. On the basis of seismic data interpretation and log data analysis, researches on the fault system have been carried out. Roles which faults play in the process of oil formation and accumulation are analyzed and origin of the Chezhen Sag is discussed integrated with sedimentary unit distribution and finite element modeling. With further reference to other geological study on the whole Bohai Bay Basin, relationship between the Bohai Bay Basin's Cenozoic evolution and surrounding tectonic movements is on discussion alongside how and why the North China Craton ceased destruction. The major conclusions are presented as follows:The boundary fault of the Chezhen sub-depression falls apart into three segments of E-W, NE and NW-striking respectively, with the NE segment the sharpest. Based on fault geometry, the fault surface can be subdivided into three kinds of geometry:platy, shovel-shape, chair and domino-like. According to number of faults and combination on the gentle slope, it is divided into three types:scarp slope one, domino-like faulted one and half garben one. Most parts of Guojuzi Sag and Chexi Sag have the scarp slope one while the gentle half graben slope is mainly located in the south of Dawangbei Sag and Dawangbei western subsag. The domino-like faulted slope is characteristic of south slope of the Chexi Sag and the Che-3 nose-like structure.T6 is a substantial time surface in the structural evolution of Cenozoic Chezhen Sub-depression. There are fewer faults under the surface and most dip opposite the central sag in the comparison with the fact that more normal faults are above the surface and dip to the central sag. In other words, faults forming during Kongdian to Es4 period dip same with the Chengnan Fault while the others forming during Es3 to Dongying period dip to opposite derection. It is showed that more and long term active faults in Chexi Sag than in Guojizi Sag. Therefore, these differences illustrate the Jiyang Movement is probably an event which changes structural stress field of the Chezhen Sub-depression. NE striking normal faults are dextral and E-W striking Da 1 fault is sinistral. Combined with finite element modeling, it is proposed that dextral strike slip and pull apart is a reasonable model for Chezhen sub depression after Es4.The distribution of Es3 glutenite in the sharp slope is controlled by faults on the Chengzikou uplift. In the sags, the major area of semi-deep lake and deep lake facies transforms from the Sag Guojuzi and the Dawangbei Sag during late Es4 period to the Chexi Sag during early Es3,which show that the Jiyang Movement causes large subsidence and changes lithofacies and paleogeography in the sub-depression. Before early Es2,fault activity was strong and strata distribute like wedge. Deposition center was near Chengnan fault. The period from late Es2 to Dongying period is a transitional period from traphrogeny to depression. Layers are platy and become thinner to the slopes and deposition center moves southwards.Most of Cenozoic oil reservoirs in the scarp slope are dominated by sedimentary facies. However, besides lithological reservoirs, there are nose-like structure oil reservoirs and fault-related reservoirs, which illustrate a control of faulting. Most faults dipping to the central sags help oil migration toward the gentle slope and therefore they are not in favor of oil reservoirs. On the other hand, Paleogene faults dipping opposite to the central sags are beneficial to formation of oil reservoirs.According to the rule, it is deduced that the domino-like faulted slope is not good for oil-trap formation, while the gentle slope between the Taoerhe and the Dongfengang oilfields has a large exploration potential.Cennozoic taphrogeny of the Bohai Bay Basin can be divided into two stages.During the first stage from Kongdian period to Es4, the tectonic framework is NW striking narrow horst and graben systems in between NE-N striking Tanlu and Lanliao Strike Silp fault systems. However, from Es3 to Dongying period the basin changes to be under regional NW-SE extension interacted with dextral strike slip-pull apart force, which results from dextral movements of Tanlu fault system and LanLiao fault system. In terms of force sources, both the Pacific plate subduction and India-Eurasia collision controlled the Bohai Bay Basin structural evolution.The Dongyin movement marks the termination of destruction of the North China Craton. One reason is probably mantle loss caused by massive volcanic activity in the Japan Sea. Accordingly, the asthenosphere under the Bohai Bay Basin started to flow eastwards and thus the lithosphere began thermal subsidence.