Abstract:

Hydrocarbon exploration in the Xihu Sag of the East China Sea Shelf Basin is concentrated primarily in the Eocene Pinghu Formation and the Oligocene Huagang Formation. In recent years， it has shifted to the Baoshi Formation in the western slope of the sag. However， fundamental research on the Baoshi Formation remains limited， leading to an unclear understanding of its sedimentary environment and characteristics. In this study， we investigate the sedimentary filling characteristics of the Baoshi Formation in the Xihu Sag using seismic， borehole， log， and paleontological data， as well as data from elemental geochemical analysis. The results indicate that the Baoshi Formation occurs between the T{}_{\mathrm{4}}^{\mathrm{0}} and T{}_{\mathrm{8}}^{\mathrm{0}} seismic reflection interfaces. This formation consists of a series of clastic layers， with its top and bottom boundaries characterized by pronounced onlapping seismic reflection and truncation. It has a thickness of up to 2 000 m at the sag center， providing a material basis for the formation of source rocks. The Baoshi Formation is deposited under the control of segmented fault activities， resulting in the formation of multiple independent depocenters. The western slope exhibits varying structural styles from north to south. Specifically， the northern Hangzhou slope zone occurs as a reversely harmonic tectonic ramp， the central Pinghu slope zone consists of a steep slope structure shaped by multi-stage fault superimposition， while the southern Tiantai slope zone represents a normally harmonic tectonic ramp. The Baoshi Formation is formed in a brackish， weakly oxidizing to weakly reducing sedimentary environment， with paleosalinity increasing from north to south. This formation consists of the deposit of the marine-continental transitional facies in shallow water with frequent lake/sea level fluctuations. It exhibits distinct sedimentary systems in its northern and southern portions. Specifically， the northern Hangzhou slope zone is characterized by a braided river delta-bay sedimentary system， whereas the southern Tiantai and Pinghu slope zones （excluding the Pingbei area） are dominated by a tidal flat-tidal delta-littoral neritic sedimentary system in a restricted marine sedimentary environment. The northern and southern sedimentary systems along the western slope are dissected by a low salient in the central Pinghu slope zone， which serves to shelter lagoon and bay sedimentary systems in a restricted setting. The thickly laminated mudstones in these sedimentary systems act as the major source rocks of the Baoshi Formation in the Xihu Sag.

Key words: sedimentary system, sedimentary environment, Baoshi Formation, western slope zone, Xihu Sag, East China Sea Shelf Basin