The Xushui Sag

located in the western Bohai Bay Basin

is separated from the Taihang Mountain uplift by the Taihang Mountain piedmont fault to its west.In order to improve the understanding of the formation of the sag and facilitate petroleum exploration in the region

we systematically studied the geologic architecture of Xushui Sag through the drilling and seismic data.Characteristics of the major faults in the study area were revealed based on fine interpretation of the seismic profiles within the sag.The post-Paleogene tectonic evolution process of Xushui Sag was reconstructed via the balanced cross-section technique.The results show that there are two regional unconformities in the Cenozoic in the Xu-shui Sag

one being at the base of the Paleogene and the other being at the base of the Neogene.According to these two unconformities

three tectonic layers

namely the basement

the Paleogene and the Neogene-Quaternary layers

are subdivided.The structural framework of the study area with faults occurring in its west and north and overlapping occurring in its east and south

is controlled by the NE-trending Taihang Mountain piedmont fault and the NWW-trending Laishui fault.This study demonstrates that the post-Cenozoic tectonic evolution of the study area can be divided into four stages:intense rifting stage

continuous rifting stage

transitional stage from rifting to depression and depression stage.Rifting in the Xushui Sag

characterized by episodic activities

began to grow in the Early Paleocene and proceeded until the Late Oligocene.