Geological Structure And Evolution Of Peripheral Sub-sags In Zhanhua Sag

Zhanhua Sag has been an important hydrocarbon target in the Jiyang Depression of the Bohai Bay Basin.This sag entered in a deep exploration stage,thus,its peripheral sub-sags gradually become an important replacement area for further oil and gas exploration.In this paper,three peripheral sub-sags on the southern margin of Zhanhua Sag were studied,including Shaojia sub-sag,Sanhecun sub-sag and Fulin sub-sag.Intensive interpretation of3D-seismic,well-drilling,and well-logging data were used to reflect the stratigraphic structure,sedimentary characteristics,and fracture patterns.On the fault activation and balanced-section restoration and burial history analysis,the geological structure characteristics and tectonic evolution of these sags were clarified.Combined with the regional geological background analysis and structural physics simulation experiments,the genetic relationship and separation mechanism between peripheral sub-sags and main sag are discussed.There are obvious differences in geological structure between peripheral sub-sags and main sag.The Shaojia sub-sag is mainly controlled by the NE-trending Yinan Fault.Mesozoic sediment thickness is large,Es₂ and Es₃^S formation are missing.The Sanhecun sub-sag is mainly controlled by the EW-trending Gunan Fault,Es₂,Es₃^S and Ek formation are missing.The Fulin sub-sag is mainly controlled by the EW-trending Kenli Fault,and the NNE-trending Kendong Fault and the NW-trending Guxi Fault has secondary affection on it,and Es₂^S,Es₄^x and Ek formation are missing.The difference of basin-controlling fault activation mainly causes the difference of geological structure between peripheral sub-sags and main sag.Compared to main sag,the controlling faults of peripheral sub-sags start activating lately,with short activation time and weak activation intensity.The peripheral sub-sags and main sag are mostly separated by faults,whereas the formation period of the separation zones between different peripheral sub-sags and main sag are different.The settlement and settlement velocity of peripheral sub-sags are small.They are experienced three stages of evolution,during the first stage,the peripheral sub-sags developed as part of the Zhanhua sag,and during the secondary stage,the peripheral sub-sags are separated from the main sag,and during the third stage,the peripheral sub-sags are connected to the main sag again.The peripheral sub-sags and the main sag are formed under the same regional structure background,thus,the local stress field is the main reason for the differential separation between peripheral sub-sags and main sag.The reduction of the extension along the long axis made Shaojia sub-sag and Bonan sub-sag separated from the main sag.The uplifting of the short axis（gentle slope tilt）leads to the separation of Sanhecun sub-sag and Bonan sub-sag from the main sag,as well as the Fulin sub-sag and the Gunan sub-sag.The differential separation between Sanhecun sub-sag and Gunan sub-sag is mainly controlled by structural transformation.