琼东南盆地块体搬运沉积体系成因及其对水合物成藏的影响

Genesis of mass transport deposits and their effect on gas hydrate accumulation in the Qiongdongnan Basin

琼东南盆地深水区赋存丰富的天然气水合物资源并发育多期块体搬运沉积体系(MTDs),但目前MTDs对水合物成藏的影响机理仍不明确。为此,利用岩心、钻测井、地震及区域地质资料,在精细刻画MTDs主要特征的基础上,综合分析影响水合物成藏的多种因素,以不同背景的MTDs耦合关系为切入点,深入探讨琼东南盆地MTDs成因及其对水合物成藏的影响。结果表明:(1)MTDs与海底原位沉积物的差异显著,MTDs更致密,孔隙度明显偏低,测井曲线具有低声波时差、高电阻率的特征;MTDs段可见生物碎屑、砾石颗粒等;MTDs段地震剖面呈杂乱反射结构。(2)琼东南盆地自5.5Ma以来,快速沉降导致可容纳空间增大、物源进积以及陆坡坡度加大等,为MTDs形成提供了先决条件。北部陆坡MTDs主要由海平面变化、物源、陆坡坡度主控,由岩浆活动触发;南部隆起海底火山翼部MTDs则直接受控于岩浆活动。岩浆活动、南部隆起MTDs、陆坡MTDs三者存在等时耦合关系。(3)深部气源沿着底辟及其衍生的通道运移至稳定域内,并且由浅部的MTDs封盖,使其在适宜的温压条件下形成天然气水合物藏。

The deep-water area of the Qiongdongnan Basin has considerable gas hydrate resources and has developed multi-stage mass transport deposits(MTDs).However,the influence mechanism of MTDs on hydrate accumulation remains to be fully elucidated.In view of this,this study comprehensively analyzes the various factors that affect hydrate accumulation with core,drilling,logging,seismic and regional geological data,taking into account the main characteristics of MTDs.Further,with the coupling relationship of MTDs having different backgrounds as the starting point,the genesis of MTDs and their effect on gas hydrate accumulation in the Qiongdongnan Basin are explored.The following results are obtained.(1) The difference between MTDs and in-situ sediments is significant.MTDs are denser and have much lower porosity.The log curves have low interval transit time and high resistivity.Biological detritus,gravel particles,etc.can be observed in the MTDs section.The seismic profiles of the MTDs section show a chaotic reflection structure.(2) Since 5.5 Ma,the rapid subsidence of the Qiongdongnan Basin has led to the proliferation of provenance and the increases in the accommodating space and the gradient of continental slope,which provide prerequisites for the formation of MTDs.The MTDs developed on the northern continental slope are mainly controlled by sea level change,provenance,and slope gradient and triggered by magmatic activity.Those developed on the flanks of the submarine volcano in the southern uplift are directly controlled by magmatic activity.There is an isochronous coupling relationship among magmatic activity,MTDs of the southern uplift,and MTDs of continental slopes.(3) The deep gas source migrates to the stable zone for gas hydrate along the diapir and its derived channels and is covered by shallow MTDs to form a gas hydrate reservoir under suitable temperature and pressure conditions.