Abstract: The Huagang Formation is a shallow-water delta depositional system in the middle and south of Xihu sag, East China Sea shelf basin. It is difficult to predict the reservoir because of the large buried depth of the main exploration target layer, the similar impedance of sandstone and mudstone, complex relationship between channel stages and overlaying. To solve this problem, through detailed dissection of the Shuey approximate gradient term of Zoeppritz equation, combined with seismic forward analysis, it is clear that AVO gradient is related to Poisson’s ratio, and it is the most sensitive and stable to reservoir response. Using AVO gradient to finely delineate channel boundary, a total of four distributary channels are identified in H5 layer. Channel 1 and 4 are NW-SE trending, with the average width, width-to-thickness ratio and curvature are 0.7km and 0.8km, 29 and 28, 1.04 and 1.06, respectively. Channel 2 and 3 are NE-SW trending, with average width, widthto- thickness ratio and curvature are 0.85km and 3.1km, 26 and 70, 1.40 and 1.09, respectively. The channel stages are divided into channel 1, channel 2, channel 3 and channel 4 from early to late by the sequence stratigraphy and relative isochronous surface flattening method. It is considered that the four channels of H5 layer are not connected and have the conditions for independent hydrocarbon accumulation according to cross-validation analysis of seismic response characteristics of different channels and possible gas bearing flat points and other multi information. Establish a “fault-sand coupling” hydrocarbon accumulation mode for anticlinal wings lithologic traps. It is considered that the anticlinal wings have better hydrocarbon accumulation conditions than the core. Based on the above understanding, exploration wells were drilled in the channel 4 of structure A and the channel 3 of structure B, both obtained good lithology exploration results for the first time in the study area, which proved the practicability and reliability of the research technique and method, and pointed out the next favorable exploration area.

Key words: Xihu Sag, Huagang Formation, Shallow water delta, Channel sand body, Reservoir prediction, Lithologic trap, Hydrocarbon accumulation mode