Abstract: Natural gas hydrate is a hot spot of study at present, which may play a very important role in energy, environment and disasters. According to actual geological conditions in the Qiongdongnan Basin, the influence of nonhydrocarbon gas to the thickness of hydrate stability zone (HSZ) is numerically studied for component gases at various depths, combined with the salinity of ocean waters and geothermal gradient in the deep-water area within the basin. The results show that the thickness of HSZ in the Qiongdongnan Basin increases with water depth, and that CO₂ in mixed gas of CH₄+ CO₂ alters the hydrate stability. The effect of CO₂ is not only related to CO₂ content but also to water depth. The thickness of HSZ increases at a shallow depth and then falls as CO₂ content increases for water depth less than or equal to 1.3 km. When water depth is more than or equal to 1.55 km, input of CO₂ decreases the HSZ thickness, and the influence enhances as the CO₂ content grows. The results also indicate that N₂ in mixed gas of CH₄+N₂ decreases the thickness of the HSZ. A 1% increase of N₂ in the mixture can lead to 4m reduction in the thickness of the HSZ.

Key words: natural gas hydrate, hydrate stability zone, Qiongdongnan Basin, nonhydrocarbon gas