The Quaternary of Sanhu area in Chaidamu Basin develops loose sedimentary gas reservoir with low-saturation and high shale content. For the impact of high shale content, bad result will be obtained by using Archie equation to calculate water saturation. Therefore, the authors utilized Indonesia equation that can reduce the impact of shale content. There are many factors influencing the irreducible water saturation, and the main impact factor in Sanhu area is shale. Based on the principle that the water saturation of pure gas layer is equals to irreducible water saturation, the quantitative relationship between irreducible water saturation and shale content has been established to determine irreducible water saturation. The results indicate that the dual saturation method can well distinguish low-saturation gas layer and water layer.