Structural Evolution And Basin Prototype Of The Qukou Sag. Bohaui Bay Basin

The Qikou Sag is at the north-central of the Huanghua Depression of the Bohai Bay Basin, being a Cenozoic basin superimposed on top of the Mesozoic basin, far away the plate margins up to 1000 km and rich region in oil and gas resources. After fifty years of exploration and research, structures, faults, evolution, and dynamic mechanism of the basin have been made great progress. Many experts and scholars published their different views, identifying several genetic types. Based on a large number of two-dimensional, three-dimensional seismic profiles and drilling data, researches focus on structural framework, basement strike-slip faults, main faults, typical structure belts, migration of the depocenter and subsidence centers of the Qikou Sag, and balanced cross sections across the sag. Major conlusion is summarized as follows:1. Half graben with faulting in the north and overlapping in the south commonly developed in the Qikou Sag, primarily in a tandem and an en echelon of structural pattern, they mainly include extensional, strike-slip and decolloment structures.2. There are four basement-involved strike-slip fault zones in the sag. They are the old Cangdong basement-involved strike-slip zone, the western basement-involved strike-slip zone, the coastal strike-slip zone and the Yang'erzhuang basement-involved strike-slip zone. As the accomodation of the basement-involved strike-slip zones and the transfer structures, the strain distribution within the basin is more and easily balanced, which plays an important role in structural accomodation.3. The major faults are characterized by multiple stages and spatial migration in the size and activity. Since the local Sha-3 marginal lithofacies exists in the Cangxian Uplift, the most other region of the Cangxian uplift has not been observed. Therefore,"the Great Qikou Sag"is significant. The activity of major tectonic belts is migrated from north to south.4. The depocenter and subsidence centers study reveals that there are many differences between the western and the eastern. The depocenter of the Banqiao Sag shows slow and northward migration, the Qibei Sag basically remains the same position. The migration of the depocenter in the Qinan Sag and the Qikou main Sag is southward. Although the depocenter and the subsidence centers is located at the eastern area-the main Qikou Sag, the whole basin shows a trend of southern migration. Formations of the sedimentary system are controlled mainly by the Cangdong fault, the Haihe fault, the Gangxi and the Gangdong faults, the Nandagang fault and other secondary faults, but the Yangerzhaung fault has not obvious control on the sedimentation. 5. The Qikou Sag undergoes many tectonic evens, including the Kongdian Uplifting, the Jiyang Event and the Dongying Event. The tectonic evolution is divided into four stages: Eocene propagating rifting, Early Oligocene pull-apart graben, Late Oligocene half-graben, and Miocence-Pleistocene thermal subsidence under dish-like bukling. During the evolution stage, rift stage is very important. Strike-slip pull-apart exists in the later Minghua stage. Research of basement-involved and cover-invovled faults confirmed that the cover faults is obviously under a control of the basement-involved faults.6. There are 3 steps of detachment fault in the Qikou Sag which is characterized by strike-slip pull-apart faulting. According to research of the evolution and 3D structural numerical modelling, the strike-slip pull-apart faulting is the most possible mode for the Qikou Sag. The dynamic mechanism of secondary structural units is controlled by the principal tectonic units. The dynamic mechanism of the Bohai Bay Basin is affected by the deep upwelling of the asthenosphere and collision of the surrounding plates, particularly the angle changes and the velocity of the subduction of Pacific plate and the creep in eastern China impacted by the Indian plate subduction and collision. All these elements are combined with the destruction of the North China Craton, the Qikou Sag is considered to be an "intraplate Pull-apart basin ".