STRUCTURAL CHARACTERISTICS AND EVOLUTION OF THE SHANAN SAG OF BOHAI GULF BASIN

The Shanan Sag is a difficult and hot spot in oil and gas exploration of the Bohai Sea area, and the distribution of oil and gas in the basin is extremely unbalanced. Complex tectonics is supposed to be the key factors restricting hydrocarbon accumulation. In order to research the tectonic evolution characteristics of the Shanan Sag, this paper meticulously depicted the stratigraphic characteristics and faults distribution of the sag, based on the newly acquired 3D seismic data and drilling data. Newly identified are the Mesozoic thrusts and the evolution of the tectonic characteristics in the Mesozoic-Cenozoic as well as the Cenozoic subsidence and sedimentation center of the Shanan Sag. The results show that the Shanan Sag consists of three parts from west to east, i.e. the western sag, the central ridge and the eastern sag. The faults in the study area are predominantly in NW trending, subparallel in EW and NE trending. There are three stages of fault systems developed in the Mesozoic and Cenozoic, which are the thrust fault system in Paleozoic, the extensional fracture system in Paleogene and the conjugate slip fault series in late Neogene. On the control of these faults, four stages of tectonic evolution can be recognized from Mesozoic to Cenozoic, including the Mesozoic denudation-subsidence stage, the initial strong chasmic stage (E₂s³-E₃s¹), the transitional period from the second strong chasmic to depression (Ed³-Ed¹), and the thermal subsidence and conjugate strike-slip period (Neogene). The change of the Paleogene stress field suggests that the bottom of the Dongying Formation is the essential tectonic boundary. The evolution of the two subsidence and sedimentation centers of the Shanan Sag (the eastern sag and the western sag respectively) is very different, especially in the depositional period of Dongying Formation, which controls the scale and maturity of the source rocks in the two centers mentioned above. Basin structures and tectonic evolution, as well as the regional dynamics background demonstrate that the mantle upwelling may be the direct cause for the basin rifting. The configuration of the periphery plates controlled the evolution of the faulted basin and the directions of fault systems. This study can provide some reference and guidance for further oil and gas exploration in the Shanan Sag.