The Doseo Basin in Chad is currently an important production replacement area for CNPC in Africa after Sudan. The rapid lateral and vertical lithofacies changes in the region, coupled with significant variations in formation water salinity, result in the coexistence of high-resistivity water layers and low-contrast oil layers, making it difficult to distinguish oil, gas, and water. Additionally, the simplicity of logging suites greatly increases the difficulty of logging evaluation. Based on rock physics experiments and the study of reservoir lithofacies and water characteristics, this paper analyzes the formation mechanism of low-contrast oil layers by focusing on factors such as formation water salinity, clay content, bound water saturation, and clay-associated conductivity. It is concluded that factors such as formation water and clay content leading to high bound water saturation are the main causes of low-contrast oil layers in the Doseo Basin. By considering the distribution characteristics of formation water and oil layers, zonation on a planar scale and stratification on a vertical scale are proposed to address the challenge of identifying low-contrast oil–water layers. Furthermore, the application of this technology has guided the identification of oil layers within new stratigraphic units, opening up new reserves growth opportunities.