Abstract: The resistivity of thin sand and mudstone interbeds is affected by mudstone, so that oil saturation calculated directly by Archie equation is lower. The reservoir can be considered as a layered medium with cross distribution of various thin layers based on internal sedimentary structural characteristics of thin sand and mudstone interbeds. Considering common electric conduction effect of interlayer coupling and a mudstone layer, a horizontal-vertical dual mudstone indicator conductive equivalent model is proposed. Thus, a computation model is built for thin sand and mudstone interbed saturation. According to processing and analysis of actual logging data, the water saturation calculated from the model is 10% to 20% lower than that calculated by the Archie equation which is closer to core test data. Research results show that the new saturation model can reduce effects on water saturation calculation of inter-layer and intra-layer mudstone of thin interbeds. It is applicable logging data interpretation for thin sand and mudstone interbed as well as thick sandstone.