珠江口盆地白云凹陷中新统珠江组潮流改造的砂质海底扇沉积

Tide Current-Reworked Sandy Submarine Fan Deposits in Miocene Zhujiang Formation,Baiyun Sag of Pearl River Mouth Basin

近年来,浅海扇和底流改造作用的较多发现对深水扇术语、鲍马序列等传统认识带来了挑战,有待进一步研究.珠江口盆地白云凹陷是我国海底扇油气勘探的重要场所之一,中新统珠江组SQ21层序(层序底界年龄21 Ma)是该凹陷砂质海底扇发育和油气发现的主力层位.采取岩心观察与测井相、地震反射结构、地震地貌分析相结合,揭示了该层序下降期体系域和低位体系域发育为数众多的海底扇,由重力流水道、天然堤和席状朵叶体构成,在地震属性图上分别表现为均方根振幅高值条带和朵状形态.上陆坡带重力流水道由块状-递变层理中-细砂岩、波状层理粉-细砂岩组成,具有正旋回、箱型-钟型测井相、水道充填地震相等识别标志.其中,波状层理粉-细砂岩夹较多泥岩纹层,局部表现为单粘土层和双粘土层构造,且富含潜穴、生物扰动构造和菱铁质结核,指示该时期海平面大幅下降直至陆架坡折以下,上陆坡带逐渐演化为浅海环境,导致研究区砂质海底扇发育及其重力流水道沉积在上陆坡带遭受明显的潮流改造.地震地貌分析方法的应用,保证了潮流改造型海底扇沉积相的准确识别.此外,本文的研究还表明,鲍马序列可以是底流改造成因;“深水扇”术语并非广泛适用.

Recently increasingly-identified shallow marine fans and bottom current-reworking processes,which bought challenge on traditional knowledge such as the item of deep-water fan and Bouma Sequence,are needed for further research.The Baiyun Sag of Pearl River Mouth Basin is an important area for oil and gas exploration of submarine fans.The Miocene sequence SQ21 in this sag,whose base was dated as 21 Ma,is the main stratum hosting sandy submarine fans and petroleum discoveries,so was taken as the object in this study.Integrating core observation with analyses of well logging motifs,seismic reflection configurations and seismic geomorphology,this study revealed a large number of submarine fans in the falling stage and lowstand systems tracts of SQ21.The fans are constructed by gravity flow channels,levees and sheet lobes,corresponding to high-amplitude strips and lobate shapes,respectively,in seismic root mean square amplitude attribute maps.At the upper continental slope,the channels were deposited with mainly massive medium-to fine-grained sandstone and ripple-laminated fine-grained sandstone to siltstone,presenting upward-fining successions.They are characterized by cylindrical-to bell-shaped logging motifs,channel-infilling seismic reflections.Further,ripple-laminated sandstone to siltstone is intercalated with a lot of mudstone laminae partly illustrating single-clay and double-clay structures.This,together with abundant biological burrows,disturbance structure and siderite nodule,indicates that sea level greatly fell below shelf break.Consequently,the sandy submarine fan deposits,which were generated with their gravity flow channels at the upper continental slope,had been severely reworked by tide current,while shallow sea environment gradually occurred at the upper continental slope along with sea level greatly falling till below the shelf break.In this study,the application of seismic geomorphological analyses guarantees correct identification of the tidal current-reworked submarine fans.Moreover,it is suggested that the Bouma Sequence can be resulted from bottom current-reworking processes,and term of deep-water fan should not be overused.