南海东北部陆坡冷泉系统的浅地层剖面特征与分析

本文通过对南海东北部陆坡区浅地层剖面资料的处理,结合横跨陆坡区的长地震测线,总结了该区浅层气聚集、海底流体运移、泥火山和冷泉活动在浅地层剖面中的特征,并对剖面中出现的声学空白带现象做了分类.研究认为浊反射、增强反射等现象,反映了该区域存在广泛的浅层气聚集.对声学空白带成因的分析,认为窄空白带很可能对应海底流体运移的通道,而宽空白带则可能由沿地层分布的浅层气引起,同时提出了自生碳酸盐岩体散落分布造成声学空白的可能性.我们在南海东北部陆坡区发现了泥火山,并讨论了其运移通道及对BSR的破坏.同时发现研究区水体中存在声学异常体,推断为冷泉羽状流,指示该区存在活动的冷泉.本文依托大量的浅地层剖面数据,对南海东北部陆坡区的冷泉系统有了较全面的认识,对冷泉系统剖面特征的成因进行了初步探讨,可为南海冷泉系统研究的进一步开展提供有益参考.

Characteristics and formation mechanism of cold seep system in the northeastern continental slope of South China Sea from sub-bottom profiler data

Gas Hydrate reservoir in the northeastern continental slope of South China Sea(SCS) have been confirmed by investigations and well drillings. Also, large amounts of methane-derived authigenic carbonates (MADC) were discovered in 2004 among this area. Based on sub-bottom profiles and seismic profiles, attributes and features associated with cold seepage system in the northeastern continental slope of SCS were analyzed. A seismic line cross the northeastern continental slope of SCS was used to show local sedimentary environment. Sub-bottom data were pieced together and denoised. Shallow gas, fluid migration and morphology features associated with cold seepage in processed sub-bottom profile were identified and discussed, with assistance of corresponding seismic lines and bathymetric charts. From observation of sub-bottom profiles and comparison with seismic and bathymetry data, cold seepage related features were listed as follow: (1) Acoustic plume (suspected of being cold seep) was found in a sub-bottom profile, with height of about 30 meters and width of 50 meters. (2) Mud volcanoes were found in this area, and caused discontinuity of Bottom Simulating Reflectors in seismic profiles. (3) Acoustic voids, the most frequent features in this region, had two types: "narrow" acoustic void and "broad" acoustic void. "narrow" acoustic void had width of 80~400 m and no layers information, "broad" acoustic void had width over 1000m and weak layers information. (4) The area of "narrow" acoustic void overlaid with mud volcanoes concentrated area, and the acoustic plume was located around "broad" acoustic voids. Cold seepage activities exist on the northeastern continental slope of SCS, both in history and in present. The results suggest that "narrow" acoustic voids on sub-bottom profiler correspond to fluid migration path, while "broad" acoustic voids possibly related with shallow gas accumulation along rock layers. The relationship between cold seepage system and gas hydrate in the northeastern continental slope of SCS worth further investigation.