琼东南盆地典型渗漏型天然气水合物成藏系统的特征与控藏机制

为了深入探究渗漏型天然气水合物（以下简称水合物）运聚成藏的机制，基于高分辨率地震、海底观测、水合物钻探、随钻测井、岩心资料及钻后地球化学分析结果，从气源、运聚、储集、封盖等成藏条件入手，剖析了南海北部琼东南盆地渗漏型水合物成藏系统的特征与控藏机制。研究结果表明：①渗漏型水合物富集成藏受控于松南低凸起构造及其上发育的气烟囱，气烟囱构成的天然气运聚通道与水合物稳定域空间匹配决定了水合物的主要分布范围；②海底冷泉系统及地震剖面上出现的天然气渗漏地震反射异常，是渗漏型水合物分布与赋存的有力指示标志；③渗漏型水合物差异分布特征明显，横向上水合物聚集在具有明显渗漏通道构成的储集空间内部，而没有通道沟通水合物稳定域的区域仅聚集薄层水合物，垂向上水合物的分布厚度、产状及饱和度等也表现出差异性；④水合物气源具有生物气及热解气双重供给特征，热解气与深部中央水道岩性气藏及古近纪烃源岩有着密切的成因联系；⑤充足的天然气通过气烟囱及水道边界断层等通道运移聚集，并在海底浅层块体流沉积（MTDs）细粒黏土质粉砂封盖条件下富集水合物，MTDs内部的裂缝构成了渗漏型水合物主要的储集空间。结论认为，在低凸起及富生烃凹陷成藏的背景下，充足的天然气通过低凸起上气烟囱高效运聚成藏，气烟囱上覆MTDs内部是否出现渗漏通道是能否形成和富集水合物的关键，也是渗漏型水合物差异运聚成藏的主要控制因素。

Characteristics and controlling mechanism of typical leakage gas hydrate reservoir forming system in the Qiongdongnan Basin,northern South China Sea

In order to explore the hydrocarbon migration and accumulation mechanism of leakage gas hydrates, we analyzed the typical leakage gas hydrate system and its controlling factors in the Qiongdongnan Basin based on high-resolution seismic data, submarine observation, drilling and logging, coring, and post-drilling geochemical testing data. Besides, gas source, migration pathway, reservoiring as well as sealing conditions were studied. The following research results were obtained. First, leakage hydrate enrichment and accumulation is controlled by the Songnan Low Uplift and a gas chimney root on it. The coupling between the gas chimney and gas hydrate stability zone (GHSZ) determines the main distribution of hydrates. Second, the submarine cold seep system and the anomalous seismic reflections of seepage on the seismic profile are powerful indicators for the occurrence of leakage hydrates. Third, the differential distribution of leakage hydrates is obvious. Laterally, hydrates accumulate in the storage space with obvious leakage pathways, and hydrates of a certain thickness are difficult to gather without pathways connecting the GHSZ. Vertically, hydrate thickness, occurrence and saturation also show differences. Forth, both biogenic gas and thermogenic gas contribute to the accumulation of gas hydrates, and thermogenic gas is closely related to deep central channel sand gas reservoirs and Paleogene source rocks. Finally, sufficient gas migrates and accumulates through gas chimneys and accumulates hydrates under the storage space formed by the cracks and capping conditions in the fine-grained clay-silt sediments of shallow mass transport deposits (MTDs). In conclusion, under the background of low uplift and hydrocarbon-rich sag, sufficient gas is efficiently transported and accumulated through the gas chimney. Whether or not the leakage pathways appear inside the MTDs over the gas chimney is the key to the formation and enrichment of hydrates, and it is also the main controlling factor for differential accumulation of hydrates.