Abstract: Non-hydrocarbon gases,such as CO₂ (carbon dioxide)and N₂ (nitrogen),have been discovered in the marginal basin of northern South China Sea.The formation and accumulation of these gases require some specific geological conditions and controlling factors.The CO₂ of Crustal type and crust-mantle mixed type and the N₂ of crustal organic type and crustal organic-inorganic hybrid type are mainly enriched in the shallow part of the central diapir area of the Yinggehai basin along the northwest strike-slip extensional edge of the northern South China Sea,under the joint control of marine calcareous shale and the physical and chemical activities caused by the heat flow derived from mud diapiric intrusion;and the mantle-type volcanic CO₂ is mainly distributed around the deep faults on the borders of the depressions in the eastern Qiongdongnan basin and along the salients in the Zhujiangkou basin.The degasification of the volcanic mantle provided the gas sources and the deep faults were the path for gas migration to traps.