Abstract: The Enping 21−4 Oilfield, located in the eastern part of the South China Sea, has limited reserves. In order to meet the needs of economic and efficient development, two ultra-deep extended reach wells have been deployed.To address the challenges such as high risk of lost circulation, difficult trajectory control, poor wellbore cleaning efficiency, narrow safety density window, and high friction and torque in the ultra-deep extended reach wells during drilling and completion, a series of technologies were developed and applied. These technologies include wellbore trajectory design and precise control, drilling fluid and cement slurry system optimization, safe casing running, dynamic monitoring auxiliary diagnosis technology, and anti-wear friction-reducing completion strings solution. These innovations establish the key drilling and completion technologies for ultra-deep extended reach wells, ensuring the smooth drilling and production of these two wells. Notably, Well Enping 21−4−A1H achieved a total depth of 9 508.0 m with a horizontal displacement of 8 689.0 m, and the ratio of horizontal displacement to vertical depth is 4.43, which sets a new record for the deepest offshore oil and gas well in China. The key technologies for drilling and completion of ultra-deep extended reach wells applied in Enping 21−4 Oilfield not only enrich and improve the technical system of China’s extended reach wells but also provide robust technical support for the efficient development of 10 000-meter-class marginal oilfields.