Abstract: Thin reservoirs in the XJ Oilfield are characterized by minor thicknesses, uncertainty in distribution, fast changes in lithologic features and a high risk of drilling out of the target formation. Accordingly, 3D geosteering horizontal drilling technique has been developed. Through in-depth study over lithologic features of the reservoir formations, a detailed geological model has been constructed. At the same time, by taking full advantage of a formation boundary detector and other logging and well logging data, positions of borehole trajectory in target formation could be predicted accurately to predict changes in formation properties. Research results showed that turbidite sand bodies were mostly distributed in strips with thicknesses higher than the frontal sheet sand. Consequently, these formations were favorable for deployment of horizontal wells. Since the distribution of sedimentary microfacies might greatly affect lithologic features of reservoir formations, the lithologic model constructed under the constraint of sedimentary microfacies could effectively predict lithologic features penetrated by horizontal drilling. At the same time, the formation boundary detector could be used to acquire geologic features over large areas around the borehole trajectory, and accordingly, could effectively guide the drilling of horizontal wells in pay zones. The newly developed technology was deployed in three horizontal wells drilled in thin reservoirs with thickness no more than 5.00 m in the XJ Oilfield. The average penetration rate achieved was 90%。Research results and application performances showed the 3D geosteering horizontal drilling technology of in thin reservoirs of XJ Oilfield had outstanding performances and might provide valuable guidance for geosteering drilling operations in thin reservoirs of other areas.