南海北部陆坡海底火山活动对天然气水合物成藏的影响

Influence on Gas Hydrates Formation Produced by Volcanic Activity on Northern South China Sea Slope

南海北部陆坡发育海底火山。海底火山的发育是否有利于天然气水合物的成藏是该区进行水合物勘探首先要考虑的一个关键问题。通过地震剖面解释、建立地质模型,并通过模型温度场计算,认为发生岩浆底侵时,高温岩浆使水合物稳定域的厚度变薄。随着热量的散失,水合物稳定域的厚度逐渐恢复。底侵的深度较浅时,水合物稳定域减薄的幅度增大,恢复周期变短。对海底火山喷发的情况,水合物稳定域将完全不存在,但其恢复的时间更短。岩浆底侵深度为6 km时,其对水合物稳定域的影响周期为1 Ma左右。南海北部东沙运动和流花运动都伴随着规模性的火山活动,两次构造运动期间岩浆侵入、侵出引起的热效应到目前为止已对该区水合物稳定域的发育没有影响,但海底火山活动对增大地层的孔隙与通道,从而对甲烷气体的运聚和水合物的成藏都能起到积极的作用。火山活动引起的高地温梯度,对生物甲烷的产生也将起到一定的帮助,从而有利于水合物的形成。南海北部陆坡火山区仍是天然气水合物勘探的有利靶区。

There are volcanoes developed on the northern South China Sea slope.The influence on gas hydrates formation produced by those volcanic activities is the key problem which should be considered first before field exploration conducted.Based on seismic line interpretation and geological thermal model calculation,this paper concluded that the thickness of gas hydrates stability zone would be influenced by magma intrusion.The shallower the magma intrusion depth is,the bigger the influence to the thickness of the gas hydrates stability zone is,but the shorter the thickness of gas hydrates stability zone recover period time is.When volcanic eruption occurred on the seafloor,the gas hydrate stability zone would be destroyed totally at first,however,it would also recovered after a shortest period of time as comparing to the case of magma intrusion.As to the case of magma intrusion depth of 6 km,the total recover time for the gas hydrates stability zone is about 1Ma.There had been volcanic activities during both Dongsha movement and Liuhua movement.Up to now the influences on gas hydrates stability zone produced by those volcanic activities during those two tectonic movements have been disappeared.Finer sediment grain and lower porosity usually developed on slope area are unfavorable factors for the formation of gas hydrates.As the volcanic activity can both enlarge the sediment porosity and accelerate the migration and accumulation of methane in the slope environment,it can help for the formation of gas hydrates on the slope environment.On the other hand,the higher temperature introduced by magma intrusion will help for the formation of gas hydrates by stimulating the activity of methanogens and producing more methane.The volcanic mountain area of northern South China Sea slope is the favorable target area for gas hydrates exploration.