Cenozoic Tectonic Characteristics And Evolution And Its Control On Petroleum Accumulation In The Liaodong Bay Depression, East China

The Liaodong Bay Depression is a natural extension of the waters under the Liaohe Depression as an important part of the Bohai Bay Basin. This region is the important area for hydrocarbon production in Bohai Sea, and with a complex tectonic characteristics and evolution resulting into the variety of the hydrocarbon reservoirs. But, most of oil and gas fields or oil and gas structures were found in the midwest so far, the eastern areas were rarely found in large scale reserves of oil and gas fields. The famous Tan-Lu Fault Zone of east China is through this area, lots of tectonic belt themselves is a part of the Tan-Lu Fault Zone in the Liaodong Bay Depression, the differences of eastern and western distribution of oil and gas resources in the Liaodong Bay Depression, whether related to the different activity and reforming and location of Tan-Lu Fault Zone, these factors are worthy of further study. Therefore, the study about Cenozoic tectonic characteristics and evlution has a great significance on distribution geological conditions of oil and gas and regional tectonic evolution.Based on interpretation of 2D/3D seismic refection data, drilling data and wireline logs, this study investigate the structural features of faults in Liaodong Bay sub-basin. In addition, different stages and deformation mechanism of faults in study area were identified and analysised using fault elements statistics and activity velocity. It was concluded that there were four stages of fault activities during the evolution of Liaodong Bay sub-basin:① during the deposition of the Kongdian and Sha3 sequences(65-38Ma) (Large-scale extension stage):the extension strength were more intense in the western and southern sub-basin.② during the deposition of the Sha2 and Dong3 sequences(38-30Ma)(extension together with right lateral stike-slip motion stage):rift tectonism dominated the depositional period of Dong3 sequence and right lateral movement occurred in relatively small part of the area;③ during the deposition of the Dong2 and Dongl sequences(30-24.6Ma)(right lateral slip with extension stage):the right lateral motion were more intense in central and eastern strike-slip zone.④from the deposition stage of Guantao sequence to now (24.6-OMa)(right lateral strike-slip movement with less intense extension stage).Features of faults were mainly controlled by pre-existing basement structure, transfer zones and tectonic stress field.Based on oil and gas exploration and geological data in the Bohai sea, through modeling burial history-subsidence history, recovering balanced cross section, flatting the layer, analyzing apatite fission track dating of the core, studying seismic interpretation profile deeply, interpreting sedimentary-tectonic characteristic and tectonic evolution in Liaodong Bay Depression, the result shows that there are three tectonic uplift and erosion events in Liaodong Bay Depression in Cenozoic:late Miocene-early Pliocene (corresponding peak age of 5-7Ma); the end of Dongying period in Oligocene-early Miocene (corresponding peak age of 18-22Ma and 24-26Ma), the end of the third period of Shahejie in Eocene (corresponding peak age for 36-40Ma). These tectonic uplift events are different in space:the events that happened in the southern part are earlier and stronger than in the northern part. In the Eocene, there was a big depression contained very thick sediment in the east of study area. But after the depositing, Shahejie the third section was reformed by fault-cutting, and Liaodong Uplift was formed and been eroded strongly, so that the "two sags and a rise structure" was formed in the larger depression. The transformation process is relevant with the changes of active feature and intensity of Tan-Lu Fault, it can cause the difference of uplift characteristics and late transformation strength between east and west in Liaodong Bay Depression. The change of tectonic pattern in Liaodong Bay Depression is related to regional dynamic environment and background closely. These three tectonic uplift events have profound impacts on East China. So our study is helpful to further improve and enhance the entire understanding of Cenozoic basins in eastern China and offshore, and provides basic data and scientific basis for the study of regional dynamic environment and background, petroleum exploration and resource evaluation in East China.Structural evolution can be divided into four stages:synrift early stage (from Kongdian period to Sha-4 period), synrift strong stage (Sha-3 period), late-stage deformation postrift stage (from Sha-2 period to Dongying period) and strike-slip deformation stage (from Guantao period to now).Studying the structural features and reforming evolution of Liaodong Bay sub-basin, we reassessed the hydrocarbon accumulation elements characteristics. Results show that Cenozoic structure and evolution of Liaodong Bay sub-basin strongly influenced petroleum accumulation. The later activity of Tan-Lu fault zone reformed the pre-existing tectonic-sedimentary framework, generating new uplifts with high quality reservoirs in hydrocarbon-rich sags, as well as the high angle faults acting as pathways in hydrocarbon migration. These geological features were suitable for petroleum accumulation along uplifts in hydrocarbon-rich sags.