Middle-late Miocene fault activity and its petroleum exploration significance of middle-shallow layers in the Xihu Sag, East China Sea

The Xihu Sag experienced strong compression inversion in the middle-late Miocene, and developed a large number of late active faults. However, the lack of systematic understanding of the type, distribution and properties of active fault in the middle-late Miocene has restricted the progress of oil and gas exploration in shallow and new strata. The type, distribution and property of the middle-late Miocene active fault in the central and western part of the Xihu Sag were systematically analyzed, based on which the fault activity significance of petroleum exploration in the middle-shallow layers was discussed. Results show that the fault is characteristic of concentrated zonation in this period, the fault properties and combination patterns in different zones show obvious diversity due to the change of local stress field caused by the distribution of rigid basement rock. Among them, the central inversion belt is dominated by compressional oil-source fracture, and the associated anticline core is mostly dominated by tension and torsion hanging fracture. The high belt of the western slope is dominated by tensional oil-source fracture in the northern section, and the southern section of the slope is dominated by compressional oil-source fracture. The western sub-sag is dominated by local hanging fracture, which has weak strike slip characteristics. Based on the above understanding, we considered that the middle-shallow layers in the northern section of the Xihu Sag are the replacement areas worthy of future exploration. In the northern section of the Central Inversion Belt, under the background of development of compressional oil-source fracture, driven by deep overpressure and "relay-transfer" through the hanging fracture, which is favorable for oil and gas accumulation in the middle-shallow layers. In the northern section of the western slope belt, under the condition of limited lateral sealing ability of tensional oil-source fracture, the buried hill and overlying strata in the convex area of the western margin have the conditions to capture hydrocarbon.