The microcosmic characteristics of the Siegenian Formation reservoir in X block are analyzed by studying the effect of lithology, diagenesis, pore types and clay minerals on pore structure based on cores, well-logs, and formation test data including the rock thin sections and cast thin sections, the observation of scanning electron microscopy, pressured-mercury testing and X-ray diffraction analysis of clay minerals. Reservoir lithology is mainly quartz fine sandstone with high compositional maturity. In the opposite, dissolution is important in improving the quality of reservoir. This study of the reservoir will guide the future exploration of this area.