Multi-factor evaluation for fine grading of tight sandstone reservoirs: a case study from H3 sand group in the upper section of Oligocene Huagang Formation, Xihu Sag, East China Sea Basin

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.