Oil and gas accumulation characteristics of Structure G in the igneous rock area south of Xihu Sag, East China Sea Basin

Structure G is a large compressional inversion anticline located in the igneous rock development area of the Tiantai Slope Zone in the southwest corner of the Xihu Sag. Due to the lack of understanding of the volcanic activity and later oil-gas preservation and accumulation evolution conditions in this area, the oil-gas exploration process in the Tiantai Slope Zone is severely restricted. Therefore, we interpreted the seismic sounding data in this area, studied the petrology, chronology, and temporal and spatial distribution characteristics of igneous rocks in Structure G, and reconstructed the interrelationship among volcanic activity, trap formation, and hydrocarbon accumulation. Results show that the igneous rocks developed in Structure G are shallow intrusive rocks, effusive extrusive rocks, and channel igneous rocks in the middle Miocene. The development of the igneous rocks fragmented the strata of the Pinghu Formation and Huagang Formation. Due to the existence of early volcanic channels in the core of the Structure G anticline, the traps formed by later extrusion are less effective. From the Longjing Movement (late Miocene, 5.3 Ma) to the depositional period of the Santan Formation (Pliocene), a series of nearly EW-trending secondary faults developed at the turning end of the anticline, further destroying the integrity of the trap. We believe that since the anticline wing is not affected by igneous rocks, the structural-lithological traps there should be an important direction for further exploration.