Seismic-Reflection Imaging of a Pull-Apart Sag within Tan-Lu Strike-Slip Fault Zone: Cenozoic Structure and Evolution of the Weibei Sag, Southeastern Bohai Bay Basin, China

The Weibei sag is a Cenozoic sag located within the Tan-Lu strike-slip fault zone southeast of the Bohai Bay Basin, China. The sag is controlled by two branches of the Tan-Lu right-lateral strike-slip fault zone (strike N30 degrees E) and the Gucheng-Weihekou normal fault (strike E-W). This sag was thought to be a promising exploration area after several oil fields were found there. However, the most highly productive oil and gas wells are located on traps far from the source rocks, a situation thought to be related to the tectonic evolution of this sag. Thus, a regional study of the Weibei sag, involving three-dimensional seismic data sets and drilling cores, has been undertaken to document the sag's evolution. Five groups of secondary faults are distinguished within the Weibei sag: WNW-, E-W-, and ENE-striking normal faults and NNW- and NNE-striking strike-slip faults. Except for the preexisting WNW-striking faults, the other four groups of faults developed under Cenozoic tectonic movement. The combined analysis, including seismic profiles, drilling cores, and structure-contour and isopach maps, reveals that the Weibei sag is a simple right-lateral pull-apart basin, associated with right-stepping of the main displacement zones of the Tan-Lu fault zone, that experienced three stages of tectonic evolution: a right-lateral pull-apart stage (66-41.2 Ma), an uplift and erosion stage (41.2-23 Ma), and a depression stage (23 Ma-present). Based on the vertical and antithetic shear restorations, the minimum amounts of right-lateral strike-slip displacement along the Tan-Lu fault zone are 5.3 and 9.5 km, respectively. Our results suggest that it is important to take tectonic evolution, such as the dip direction and activity of secondary faults, into consideration to target a potential petroleum reservoir.