Abstract: In order to develop marginal oil reservoirs in an oil field in the eastern South China Sea, an extended reach Well M with a HD/VD (horizontal displacement to vertical depth) ratio of up to 4.90 was designed. During the drilling process, technical problems were encountered, such as shallow reservoir buried depth, long tangent open-hole section, narrow safe density window, difficult borehole cleaning, and strong casing-running friction resistance, etc. The drilling operation was completed owing to research and application of technologies including borehole trajectory control and casing program optimization, ECD (equivalent circulating density) control of drilling fluids in the wellbore, and safe and efficient casing-running technology. The application results showed that the borehole wall stability was greatly enhanced by the five-section casing program. The ECD change rate of the bottomhole was reduced to less than 1.9% by the continuous circulating valve system and cuttings bed destroyers. The ϕ244.5 mm casing was run to the designed depth of 4 200.00 m via the floating casing-running technology, and the ϕ177.8 mm liner was run to the depth of 5 772.00 m by the hollowed-out rotating liner-running technology. The application of the key drilling technologies of extended-reach wells with ultra-high HD/VD ratio in Well M sets a new record for China National Offshore Oil Corporation in the drilling of such wells, and provides valuable experience for the subsequent development of similar extended-reach wells.