Characteristics of the structural transfer zone of northern Tiantai slope in Xihu Sag of the East China Sea Basin and their petroleum geological significances

The northern Tiantai slope of the Xihu Sag, next to the Pinghu oil and gas field, is an important exploration target for future replacement. After a through review of the structural characteristics of the slope, this paper is devoted to the analysis of structural styles and evolutionary process of the transfer zone and their significance to petroleum accumulation. The results show that, in terms of structural characteristics, the structural transfer zone of the northern Tiantai Slope is developed upon the NWW Zhoushan-Guotou basement fault zone under the control of the “North-south Blocking”, where a special reverse fault terrace occurs. The northern Tiantai slope can be further divided into two secondary transfer types: the buffering type in the north and the transferring in the south. The torsional transfer structure, along the NWW basement fault, is strong in the south but weak in the north, which is called the faults of "reticulation type" and "horsetail type" respectively in this paper in view of the fault combination. In terms of genetic evolution, the structural transfer zone of the northern Tiantai Slope is developed in the early period of fault subsiding up to late compression and reversal period, showing a continuous and progressive evolutionary process, which adjusts the difference of extension and compression strength of NE-NNE fault system between the north and the south and by which the original extension direction of some faults is changed. Finally, it is revealed that the fault combination under the control of structural transfer in the northern Tiantai slope area is not only favorable to the forming of various twisted trap, but also favorable to the vertical migration and recharge of oil-gas along the twisted faults. Under the control of structural transfer zone, the northern Tiantai slope is also a favorable area for the deposition of Pre-Pinghu high-quality sandy deposits.