南海北部陆坡神狐海域浅地层与单道地震剖面联合解释--水合物区沉积地层特征

Associated interpretation of sub-bottom and single-channel seismic profiles from slope of Shenhu Area in the northern South China Sea—characteristics of gas hydrate sediment

本文通过南海北部陆坡神狐海域浅地层、单道地震剖面联合解释,发现了一系列与天然气水合物密切相关的海底异常地貌、地层结构。在精细浅地层剖面上发现了陆坡丘状体、浅部断层以及由连续强反射层、声空白补丁、局部增强反射和声空白带构成的海底浅部含气带。浅部含气带位于海底之下34—82m,通过其空间分布位置判断,认为气体来源于深部天然气水合物的分解。在单道地震剖面上识别出麻坑、气体渗漏柱、褶皱、模拟海底反射(BSR,bottom simulating reflector)等结构。BSR位于我国首次钻取的天然气水合物样品深度之下,判断其为该区水合物稳定带底界。依据ODP1148站深海钻井的地层厚度、沉积速率、测年等资料进行地层划分,识别出渐新世、中新世等地层界面,初步建立了神狐海域水合物区沉积地层年代标尺。地层年代划分结果表明BSR、褶皱、首次钻取的水合物样品位于晚中新世至上新世地层内,以上地层成为南海北部神狐海域天然气水合物勘探重点目标层位。

The study area of this paper is the slope of the Shenhu Area in the northern South China Sea.We interpreted both sub-bottom and single-channel seismic profiles to describe the acoustic characteristics of gas hydrate sediment and to discuss the cause of its formation.We distinguished some abnormal physiognomy and geologic objects that are relevantrelative to gas hydrate in profiles.Protuberance,shallow fault,acoustic blank patch,partial enhanced reflection and acoustic blank zone were discovered in the legible sub-bottom profile.The shallow gassy belt locates under the seabed from 34 to 82m.Contrasting the sub-bottom profile with the data of China’s first gas hydrate expedition,we believed that the gas in the shallow gassy belt came from the decompounding of deeper gas hydrate.Pockmark,seepage,drape and Bottom Simulating Reflector（BSR）were recognized in the single-channel seismic profile.The depth of BSR is slightly deeper than that for the sample of China’s first gas hydrate expedition in the study area.The BSR in the seismic profile might be the bottom of gas hydrate.Based on the time-depth conversion,we plotted out Oligocene,early Miocene epoch,middle Miocene epoch and Pliocene in the seismic profile according to the sediment depth,sediment speed and age of ODP1148 and set up the chronology of the gas hydrate sediment.