Tectonic Evolution Characteristics Of Qikou Area And Eocene Tectonic Event

Qikou area is formed in Cenozoic, located in the north centre of Huanghua Depression in central of Bohai Bay Basin, also in the intersections of three different crystalline basements which are Yansha, Taihang and Luxi basement. After experiencing multi-period tectonic movement and late reformation in Cenozoic, the area has the structural characteristics of interchanged uplifts and sags with complex fault system. In Cenozoic there developed two structural layers in Qikou area:Paleogene structural layer and Neogene-Quaternary structural layer, the main hydrocarbon source rocks are Sha3member and Sha1member in Shahejie formation in Paleogene, which help Qikou area to be an important hydrocarbon-rich area in Bohai Bay Basin, the research on the Cenozoic tectonic evolution and characteristics and comparison of regional environmental of Qikou area contribute to enrich the research data and the knowledge on Qikou area and Bohai Bay Basin.According to merging processing3D seismic, drilling and logging data, structural characteristics and evolution of major faults in Qikou area have been studied and summarized, on this basis, using quantitative method, the study has made statistical analysis on fault systems of Cenozoic various structural layers and described distribution and characteristics of faults of each strctural layer systematically. The study considers that during Cenozoic, the fault activity strength migrates from southwest to northeast and the direction of faults changes from northwestward in Sha-3period to nearly east-westward in Sha-1period.Using balanced cross-section method and paleostructure recovery technique, Paleogene sag boundary and the evolution between the uplift and sags in Qikou area have been researched, it is resulted that there exists a relatively vast lake in Sha3period in area, the sedimentary boundary exceeds the the residue strata range, and the major uplifts have not formed yet and all sags have been connected with each other, then the lake has been gradually phase separated and shrinked, tectonic movements in the late Eocene and thereafter transforms of the lake are obvious, the present tectonic framewok has been gradually formed until Guantao period. The tectonic event in the late Eocene is not only happened in Qikou area, but also the Eastern and offshore of China which have been influenced in varying degrees. Combining the date from previous researches, the study summarizes the performance characteristics of various aspects about the tectonic event in Eastern of China and explores the possible reasons to this tectonic event.Basing on the research of tectonic evolution and the Paleogene unconformity surfaces, the main rifting of the area can be divided into2episodes:Sha3period episode and Sha1-Dongying period episode; in Neogene the area enters into a stable thermal subsidence period, which belong to depression period.The paleostructure features of major periods in Palogene in Qikou area are restored in paper, it is considered the tectonic evolution of area has the following characteristics:①there is a tectonic migration from southwest to northeast, which perform significantly in the aspects of intensity of faults, sedimentation and magmatic activity;②tectonic stress field has been shifted from northwest-southeast extension in Sha3period to north-south extension with strike-slip in Sha1-Dongying period;③strong late reformation, the original sedimentary pattern in Sha3period has been transformed obviously to present tectonic framework with the characteristics of interchanged uplifts and sags with complex fault system;④various structure types are formed and associated in the tectonic evolution, such as rollover anticlines,"cabbage" structure, decollement and so on, and transverse structures are widely developed. The "cabbage" structure, decollement and transverse structure are analyzed, and the structural features and evolution of these typical structures are summarized in research, which provide a direction of the exploration on the hydrocarbon occurrence-accumulation effects of these structures.