Abstract: The Oligocene Huagang Formation is mainly composed of a shallow-water delta sedimentary system in the central–southern Xihu
Sag, East China Sea shelf basin. It is difficult to predict the reservoir due to the large burial depth of the main target layer, similar impedance of sandstone and mudstone, complex channel stages and superposition relationship. In response to this problem, the Shuey approximate gradient term of Zoeppritz equation has been studied in detail, and seismic forward analysis has been conducted, which clarify that AVO gradient is related to Poisson’s ratio, and it is the most sensitive and consistent to reservoir response. Using AVO gradient to finely delineate the channel boundary, a total of four distributary channels have been identified in H5 layer. Channels No.1 and No.4 are characterized by NW–SE trending, with the average width, width-to-thickness ratio and curvature of 0.7 km and 0.8 km, 29 and 28, 1.04 and 1.06, respectively. Channels No.2 and No.3 are characterized by NE–SW trending, with average width, width-to-thickness ratio and curvature of 0.85 km and 3.1 km, 26 and 70, 1.40 and 1.09, respectively. Based on sequence stratigraphy and relative isochronous surface flattening method, the stages of channels have been identified, including channels No.1, No.2, No.3 and No.4 from early to late stages. After multi-information cross-validation analysis such as seismic response characteristics of various channels and possible gas bearing flat points, it is regarded that the four channels in H5 layer are disconnected and have the conditions for independent hydrocarbon accumulation. In addition, a “fault-sand coupling” hydrocarbon accumulation mode for lithologic traps in anticlinal wings has been established, which indicates that the anticlinal wings have better hydrocarbon accumulation conditions than the core. Based on the above understanding, one exploration well has been drilled each for channel No.4 in structure A and channel No.3 in structure B, and good exploration results have been obtained in lithologic oil and gas reservoirs for the first time in the study area, confirming the applicability and reliability of the technique and method, and putting forward the favorable exploration zone in the near future.

Key words: Xihu Sag, Huagang Formation, shallow-water delta, channel sand body, reservoir prediction, lithologic trap, hydrocarbon accumulation mode