PETROLEUM ACCUMULATION OF INVERSION STRUCTURES IN THE XIHU DEPRESSION, EAST CHINA SEA BASIN
Feng-li Yang1, Xi Xu, Wen-Fang Zhao and Zhuan Sun
School of Ocean and Earth Science, Tongji University, Shanghai, 200092, China.
1 Corresponding author, email: firstname.lastname@example.org
Present address: 5F, Ocean Building, School of Ocean & Earth Science, Tongji University, 1239 Siping Road, Shanghai 200092, P.R. China.
The central inversion zone in the Xihu Depression, East China Sea, was formed during the Late Miocene and inversion here involves the entire Tertiary succession, nearly 10,000 m thick. Lithologically, the succession is composed mainly of sandstones and mudstones with minor coals and limestones. Hydrocarbons are present in a series of structural traps which formed as a result of the inversion process. Oil and gas exploration in the Xihu Depression has demonstrated that at least 90% of the commercial reserves so far found occur in inversion-related structural traps in the Eocene Pinghu Formation and Oligocene Huagang Formation (with minor reserves in Miocene strata) in the central inversion zone. The main reservoir lithology is sandstone. Studies have also confirmed that effective structural traps are controlled by the strength and style of inversion tectonics. However, the relationship between the geometry, kinematics and magnitude of the inversion structures and the oil and gas accumulations is not well understood.
Based on 2D reflection seismic results and data from several wells, factors influencing the accumulation of hydrocarbons in the study area are considered, and include inversion fault activity rate, inversion ratio and the thickness of sediments eroded. 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), a low inversion ratio (average 0.4), and a relatively small thickness of sediments were eroded from the Miocene Longjing and Liulang Formation (≤400 m) and Eocene Pinghu Formation. By contrast, the northern part of the inversion zone has a relatively high inversion fault activity rate (average 6.8 m/Ma), a high inversion ratio (0.8), and greater thicknesses were eroded (400 m to 1200 m). This may have resulted in the less favourable preservation conditions and the large-scale leakage of oil and gas. An analysis of the relationship between the inversion-related structural traps and hydrocarbon accumulations shows that most oil and gas accumulations occur in the southern part of the central inversion zone, especially in the Longjing, Liulang and Pinghu Formations.
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