深海弯曲水道内部一种特殊的沉积单元:凹岸坝

在深海水道研究过程中,识别出深海弯曲水道内部存在一种特殊的沉积单元--凹岸坝,基于尼日尔三角洲陆坡区浅层高频三维地震资料,利用地震相分析技术,探究了凹岸坝的沉积结构特征和形成机制,讨论了其与水道弯曲丘(nested mounds)、曲流河凹岸滩坝之间的差异。研究结果表明:凹岸坝是分布于曲率较大、以垂向加积为主的末期水道弯曲凹岸处的坝体沉积单元,该沉积单元在地震剖面上表现为强振幅、连续性较好的反射特征,其内部沉积界面倾向于水道弯曲凸岸处,倾角约1°~20°,且在凹岸弧顶处达到最大值。凹岸坝形成的关键在于惯性作用,其造成水道内部重力流流体在弯曲处发生溢岸,导致流量减少,流体动能相应降低,流体携砂能力小于沉积物负载,造成沉积物快速沉降,从而形成凹岸坝。由于凹岸坝是一种连续性较好的砂体沉积,所以其可成为潜在的、储集性能较好的油气储集体。

A Distinctive Sedimentary Element within the Sinuous Submarine Channel: Outer bank bar

The present study aims to introduce a distinctive depositional element of submarine channels: the outer bank bar. Based on high-resolution three dimensional (3D) seismic data from the shallow subsurface of the Niger Delta continental slope and utilizing the approach of seismic facies analysis, this work investigated the sedimentary characteristics, as well as the occurrence mechanism, of outer bank bars and probed the characteristic differences between them and channel nested mounds or fluvial point bars. The outer bank bar is seismically expressed as high-amplitude, good-continuity reflections in cross sections and commonly occurs at the sharp (high sinuosity) and narrow bend of the late-stage channel. The outer bank bar has interfaces that are inclined to the outer bank, with dip angles of 1 to 20 degrees, reaching the maximum at the apex of the bend. The occurrence of outer bank bars relies on the inertial force, which accounts for the spilling of channel-forming flows and the resultant decrease of flow discharge and kinetic energy. Then the transporting capacity of turbidity currents declines abruptly and the sediments in turn are deposited rapidly, giving rise to the formation of outer bank bars. Due to the good continuity of outer-bank-bar sands, they have great potential to act as hydrocarbon reservoirs.