Abstract: The reservoirs of the Z oilfield in the western South China Sea generally have complex conditions such as poor physical properties, low structural accuracy, and sand body pinching out. Boundary detection while drilling was introduced to effectively improve Drilling Ratio and Reserve Utilization. This technology can measure three-layer model formation resistivity, distance from drill bit to upper/lower surrounding rock and reservoir boundary. It can also perform boundary strata characterization and provide Geosteering services. Boundary detection while drilling has been widely used in complex offshore oil and gas fields development, ecpecially in fields with conditions of low proven structure, strong reservoir heterogeneity, and edge/bottom water reservoirs. This paper briefly describes the development of Geosteering technology, sorting out the principle and technical characteristics of boundary detection while drilling, and demonstrates the methods to adjust the wellbore trajectory through identified reservoir boundary and reservoir change results. Boundary detection technology increases the travel distance of the trajectory of the horizontal section of Well H in the "sweet spot" reservoir, achieving a good steering application effect. The actual production exceeds the pre-drilling allocation by about 60 m³/d. At the same time, it avoids geological and engineering risks and can be used as guidance for subsequent drilling of horizontal wells in similar oil and gas reservoirs.