Evolution of the transtensional structure in Huimin sag of Bohai Bay basin, eastern China

Transtensional basins containing petroleum reserves are common in eastern China. To better understand the evolution of the oil reservoirs, we investigated the generation mechanism of the Huimin sag transtensional structure, eastern China. We analyzed three-dimensional seismic surveys and drill-core data and concluded that most faults in the Huimin sag are planar or listric with negative flower-structured vertical profiles. The main faults are ENE- and E-striking: the Linshang fault belt to the north and the Xiakou fault belt to the south. Seismic profiles indicate three sets of basement faults in the Huimin sag, striking NNE, ENE, and NNW. The NNW-striking basement faults originated in the middle Triassic; the ENE and NNE faults formed during the late Jurassic. During the Paleogene, the Huimin sag was subjected to N–S extensional forces that reactivated the ENE faults, leading to dextral strike-slip displacement in the Linshang and Xiakou faults, which were oblique to the extensional direction. This generated the conjugate brushed-shaped Linshang and Xiakou fault system, producing a typical oblique extensional basin with characteristic transtensional structure. To reconstruct the evolution of the transtensional structure in Huimin sag, we used Lagrangian analysis software to model the main faults in the western Huimin sag. The simulation verified the re-activation of the NE-, ENE-, and NNW-striking faults under the N–S extension. The NNW fault faded, while the other two strengthened, producing brush-shaped fault systems and E–W- and ENE-striking normal faults. The simulation results show good agreement with the field-data analysis.