The central inversion zone in the Xihu Depression, East China Sea, was formed during the Late Miocene. The entire Tertiary succession, nearly 10,000 m thick, has undergone inversion here. The succession is composed mainly of sandstones and mudstones with minor coals and non-marine limestones. Hydrocarbons are present in a series of structural traps which formed as a result of inversion-related deformation. Oil and gas exploration in the Xihu Depressions has demonstrated that at least 90% of the commercial reserves so far found occur in inversion-related structural traps in the central inversion zone, mainly in Eocene (Pinghu Formation) and Oligocene (Huagang Formation) sandstone reservoirs. Previous studies have shown that structural traps are controlled by the intensity and style of inversion tectonics. However, the relationship between the geometry, kinematics and size of the inversion structures and the oil and gas accumulations is not well understood.
In this paper, the factors controlling trap formation (and hydrocarbon acumulations) in the study area are considered. Data came from 2D reflection seismic and electic logs from a number of wells. Controlling factors include inversion fault activity rate, inversion rate and the thickness of sediments eroded as a result of inversion-related uplift of the Tertiary succession. The results show that the best exploration targets are located in the southern part of the central inversion zone. This area has a relatively low inversion fault activity rate (average 3.0 m/Ma) and a low inversion rate (average 0.4); a relatively small thickness of sediments was eroded from the Miocene Longjing and Liulang Formations (< 400 m) and the Eocene Pinghu Formation. By contrast, the northern part of the central inversion zone has a relatively high inversion fault activity rate (average 6.8 m/Ma) and a high inversion rate (0.8), and greater thicknesses of sediments were erosively removed (up to 1600 m). This may have resulted in the less favourable preservation of traps and the large-scale leakage of oil and gas. Most oil and gas accumulations occur in the southern part of the central inversion zone, especially in reservoirs in the Longjing, Liulang and Pinghu Formations.