南海北部神狐海域天然气水合物钻探区第四纪以来的沉积演化特征

Sedimentary evolution since Quaternary in the Shenhu hydrate drilling area,northern South China Sea

南海北部神狐海域是我国首次获取海洋天然气水合物实物样品的海域.然而,陆坡区深水水道和海底峡谷的侵蚀以及频发的沉积物失稳,将会加剧地层对比和沉积相识别的难度,导致目前该区域典型地震相-沉积相特征、沉积体类型、成因机制和空间匹配关系等方面还缺少精细的研究,特别是第四纪以来的沉积演化涉及较少,区域内水合物形成和分布的沉积地质条件尚不清晰.基于海底地形特征的描述、层序地层格架的对比和地震资料的综合解释,本次研究在第四纪以来的沉积充填序列中识别出5种典型的地震相类型,并分析了对应的沉积体类型:进积型的陆坡、第四纪早期发育的小型浊积水道、沉积物失稳(滑移和滑塌)、海底峡谷和伴生的沉积物变形、以及深海沉积-块体流沉积的复合体.通过沉积单元的空间匹配关系,将沉积演化划分为3个阶段:浊积水道侵蚀-沉积物再沉积阶段、陆坡进积-沉积物失稳阶段、海底峡谷的侵蚀-充填阶段.研究结果表明,受第四纪早期小型浊积水道的侵蚀,再沉积的沉积物将在中-下陆坡以"近源"的方式堆积下来,可能具有相对较好的物性条件,从而可被视为适于水合物赋存的有利沉积体.进积型陆坡带来的沉积物易于发生失稳,在研究区内广泛分布,因其具有较小的沉积物颗粒粒度和较好的垂向连续性,可被认为是水合物的区域盖层.大量发育的海底峡谷及伴生的沉积物变形,将会侵蚀和破坏先前沉积的有利沉积体,使其呈现为"斑状/补丁状"的平面展布特征,进而影响了神狐海域水合物的分布.因此,神狐海域第四纪以来的沉积演化是钻探区水合物不均匀性分布的关键控制因素之一.

The marine gas hydrate samples in China were for the first time acquired in the Shenhu area,northern continental slope of the South China Sea.However,due to erosion by the submarine canyons,the stratigraphic comparison and identification of sedimentary facies were difficult,leading to the lack of description of seismic-sedimentary facies,analysis of forming mechanisms and distribution of the sedimentary units.Moreover,few researches focused on the sedimentary evolution since Quaternary,and the crucial controlling factors on the heterogeneous distribution of gas hydrate are not clearly understood.Using high-resolution 2D/3Dseismic data,this study attempts to classify the seismic facies and discuss the sedimentary evolution in this area since Quaternary.This work is based on the high-resolution 2Dand 3Dseismic data obtained by Guangzhou Marine Geological Survey in 2006,with a trace interval of 12.5mand vertical resolution of 10~20 m.Firstly,we make an integrated analysis of seafloor topographic features,sequence stratigraphic framework and high-resolution seismic data,perform a classification of seismic facies have been classified,and describe morphological features and internal architectures accurately.Secondly,in terms of sediment supplies from the north and seafloor topography,deep-water sedimentary process is analyzed.Also,the sedimentary evolutions since Quaternary are re-built.Subsequently,the relationship between deep-water sediments since Quaternary and distribution of gas hydrates are discussed.Five types of seismic facies were identified,involving prograding slopes,small-scale turbidite channels in the Early Quaternary,sediment failures（including slides and slumps）,submarine canyons and concomitant transformation of deposits,and the mixture of abysmal deposits and mass transport complex.The three phases of sedimentary evolution since Quaternary in the Shenhu area were re-built,which are turbidite channels erosion and re-deposited sediments,prograding slopes and sediments failures,and erosion and sedimentation of submarine canyons.Eroded by small-scale turbidites,re-deposited sediments would be near-source stacked with relatively good property conditions,which might be regarded as the potential suitable reservoirs for gas hydrates.The deposits transported from prograding slopes could lose the balance easily and result in sediments failures widespread in the Shenhu area.Because of the fine-grain particle and vertical continuity,sediment failures could be thought as the regional cap rock.Abundant submarine canyons and concomitant transformation of deposits could erode and destroy underlying strata,which modified suitable deposits as patch distribution.Hence,the sedimentary evolution since Quaternary was considered as one of the crucial controlling factors on the heterogeneous distribution of gas hydrate in the Shenhu hydrate drilling area,northern South China Sea.