深水层序地层划分——以孟加拉扇某三维区块为例

Sequence Stratigraphy in Deep-water Deposition——A Case Study of 3D Exploration Block in Bengal Fan

深水层序地层是国际沉积学研究的热点,深水扇是了解深水沉积过程的最佳场所。深水沉积地层由于缺乏削截、上超等典型的层序地层界面特征,难以进行层序划分利用孟加拉扇深水区高分辨率三维地震及钻测井资料,探讨深水层序地层划分。利用地震反射振幅、连续性及构型,在孟加拉扇研究区内识别出了U形相、丘状相、强振幅平行相、中弱振幅平行相及透明杂乱反射相等5种典型地震相。结合前人研究成果,认为这5种地震相分别对应水道、水道一堤岸、浊积朵体、深海披覆沉积及块状搬运复合体等深水重力流沉积单元。深水层序多起始于相对海平面较低时,大型重力流沉积体发育;终止于海平面迅速上升期,以反映高海平面的披覆凝缩层沉积为标志。最大海泛面通常位于重力流沉积的底部和深海披覆沉积的顶部,依据沉积体分布特征可识别深水层序界面。识别出的深水层序界面在测井曲线及地震均方根属性图上都具有明显响应特征。

Deepwater sequence stratigraphy is a hot spot in international sedimentary research, and submarine fan is the best place for studying the depositional process. It is very difficult to conduct sequence division in deepwater strata because there are no typi cal sequence stratigraphic interfaces, such as truncation, and onlapping. The sequence stratigraphy division in deepwater settings is discussed with 3D highresolution seismic and logging data in Bengal fan. Combined with the seismic reflection amplitude, continu ity and configuration, five typical seismic facies have been idenfied in the study area, including Ushaped facies, domeshaped facies, parallel seismic facies with high amplitude, parallel seismic facies with midlow amplitude and transparent and chaotic reflecting facies. Integrated with the previous study results, these five kinds of seismic facies are deep water gravity sedimentation units, corre sponding to incised channel, channellevee complex, turbidity lobes, deepwater drape sediment and masstransport deposits, respec tively. Most of deepwater sequences started at a relatively low sea level, with huge gravity units well developed in the deepwater setting. The deepwater sequences terminate when the sea level rised rapidly, marked by the sedimentation of condensed section. The maximum flooding surfaces are usually located at the bottom of deep water gravity flow deposits and on the top of deepsea draped sediment. The sequence boundaries can be recognized by the distribution characteristics of the sedimentary units, which have appar ent response features in the logs and RMS attribute map.