Recently, the reporter learned from CNOOC that Enping 21-4 Oilfield, the first offshore ultra deep extended reach well project independently developed by China, has been put into full production in the the Pearl River Mouth Basin, with a daily output of light crude oil up to 740 tons. The Enping 21-4 oilfield is located in the sea area about 200 kilometers southwest of Shenzhen, with an average depth of about 89 meters. It is a small sandstone marginal oilfield. China National Offshore Oil Corporation (CNOOC) is innovating its technology and management models, relying on the Enping 20-5 unmanned platform located 8 kilometers away to implement two ultra deep and large displacement wells, and economically and efficiently develop oil and gas resources. The maximum depth of the well reached 9508 meters, setting a record for the deepest offshore drilling in China. It is understood that the trajectory of the extended reach well is not vertically downward, but gradually "turns" and "laterally" drills into the designated oil area after reaching a certain depth. The water to vertical ratio of the ultra deep and large displacement well in Enping 21-4 oilfield reaches 4.43, which is equivalent to taking nearly 4.5 meters forward for every 1 meter step. The drilling tools almost traverse more than 8000 meters in a "lying" posture, with extremely high technical difficulty and operational risks. There are hundreds of oil and gas marginal structures in the eastern waters of the South China Sea, with abundant oil and gas reserves. Large displacement well technology is a powerful tool for effectively developing and utilizing the reserves of offshore marginal oil fields. The use of large displacement well technology can save about 40% of investment costs, which makes some oil fields regain their value for exploitation. This technology can expand the reserves of marginal oil fields in the eastern South China Sea by over 50 million tons. (Lai Xin She)