Abstract: The H3 sand group of the upper section of the Oligocene Huagang Formation in the western sub-sag of the Xihu Sag of the East China Sea Continental Shelf Basin is a tight sandstone reservoir with low porosity and low or ultra-low permeability. The distribution of high-quality reservoirs has become a key factor restricting productivity. Based on coring, well logging and physical property test data, the sedimentary microfacies and physical properties of the H3 sand group were studied, and a fine grading evaluation and "sweet spot" prediction were carried out. The reservoir distribution in the H3 sand group was mainly affected by four factors: structural characteristics, sedimentary microfacies, sand thickness and conditions of porosity and permeability. Moreover, on the basis of comprehensive analysis of the relationship among these factors and gas saturation, a four-factor reservoir fine grading evaluation method was proposed. The H3 sand group reservoirs were divided into three types: "sweet spot" reservoirs (type Ⅰ), medium reservoirs (type Ⅱ) and ineffective reservoirs (type Ⅲ).The H3-3 single sand layer has a great exploration potential, followed by the H3-1, H3-2 and H3-4 single sand layers.