Structural characteristics and implications for hydrocarbon exploration of F Subsag in the Xihu Sag, East China Sea

To explore the structural characteristics and exploration potential of F Subsag in the western slope zone of the Xihu Sag, using available drilling and seismic data, and previous research results, the fault system, subsag structure, evolution process and oil and gas exploration of F Subsag were analyzed via seismic data interpretation, fault activity qualitative assessment, and structural evolutional reconstruction. Results show that: (1) Two groups of NW and NE strike faults were developed in the subsag, and further could be divided into the third-, fourth-, and fifth-order faults, they controlled structural zonation or large fault block distribution, regulated small fault block formation, and complicated the small fault block generation, respectively. Under the constraint of fault system and structural style, the present structure of the subsag is characterized by south-north division and east-west zonation, in which the NW-trending transition zone separated the north-south area of the subsag, forming deep large division in the north and shallow small one in the south, while the NE-trending fault and its structural style controlled the east-west zonation of the sag. (2) The NW-trending faults in the subsag were mainly active in the Mesozoic, while the NE-trending faults were mainly active in the Cenozoic, and from the third-order to the fifth-order faults, the initial activity period and the strongest activity period became later gradually. Under the control of the faults, the subsag has undergone the multi-stage evolution of "rift, post-rift, compression, subsidence" and arranged in a "parallel" pattern relative to the main sag, the multi-stage evolution was caused by the stress transformation of "tension-transtension-compression" formed by the subduction of the paleo-Pacific Plate, the Pacific Plate and the Philippine Sea Plate to the Eurasian Plate. (3) In terms of area, depth of basement and thickness of source rock in the northern and southern parts of the subsag, the northern zone had better conditions for the development of source rock. Under the control of the third-order faults and their associated flower-like structural faults, the northern subsag faults exhibit vertical hydrocarbon migration and along-strike-ridge generation, showing potential for the development of the "along ridge and fault" migration pattern, making it a favorable exploration zone.