陆坡海底水道类型与沉积构型模式——以尼日尔三角洲盆地的深水研究区为例

海底水道是深水油气地质研究的热点,其复杂多变的内部结构制约了该类油气藏的勘探与开发。以尼日尔三角洲盆地陆坡区为例,利用三维地震、测井和岩心资料,分析了典型水道与同期陆坡间的相对位置关系以弥补研究区面积有限的不足,建立了陆坡海底水道沉积构型分布与演化模式。研究结果表明,研究区内主要发育两种典型海底水道。侵蚀型水道以大型下切谷为边界,两侧不易形成天然堤,具有相对顺直的平面形态,内部可出现由碎屑流向浊流的过渡。沉积型水道以多个水道复合体的包络线为边界,两侧天然堤相对发育,弯曲度通常较高,内部主要为浊流沉积。地形坡度和搬运距离是海底水道沉积构型的两点主控因素。地形坡度越大,重力流能量越强,导致海底水道对下伏地层的侵蚀能力越强,细粒物质不易溢出水道形成天然堤。搬运距离较近处,水道内部可先后发育碎屑流和浊流沉积;随着搬运距离的增加,水道中碎屑流沉积物减少。故侵蚀型水道多出现于地形坡度较陡的陆坡近端,而沉积型水道常发育于地形平缓的陆坡远端。由于搬运距离较近,理想的侵蚀型水道体系内部依次发育碎屑流水道复合体(强制海退早期)、富砂质浊积水道复合体(强制海退晚期)和浊积水道-天然堤复合体(低位期),反映了一次完整的海平面升降旋回。沉积型水道由于远离物源,较少受到强制海退早期碎屑流沉积的影响,水道体系主要由搬运能力较强的浊积水道-天然堤复合体(强制海退晚期+低位期)组成。

Types and sedimentary structure model of submarine channels on continental slope:a case study of a deepwater study area in Niger Delta Basin

Submarine channel is the hotspot of deepwater petroleum geological research,whose complex and diversified internal structure restricts the exploration and exploitation of this kind of oil-gas reservoir.This paper is a case study of the continental slope area in Niger Delta Basin.Using 3D seismic survey,logging and core data,the relative position relationship between typical channels and the continental slope in the same period was analyzed to make up for the limited study area,and the sedimentary architecture distribution and evolution model of submarine channels on the continental slope was also established.The results show that two typical submarine channels are mainly developed in the study area.The eroded channel is bounded by a large incised valley,and it is difficult to form natural levees on its both sides.It shows a relatively smooth and straight plane shape,and there can be a transition from debris flow to turbidity current inside.The sedimentary channel is bounded by the envelopes of multiple channel complexes;natural levees on both sides are relatively developed,usually with high curvature;there are mainly turbidity current deposits inside.Terrain slope and transport distance are two main controlling factors for the sedimentary architecture of submarine channel.The larger the terrain slope is,the stronger the gravity flow energy is,resulting in the stronger erosion of the submarine channel to the underlying strata and that the fine-grained material is not easy to overflow the channel to form natural levee.Debris flow and turbidity current deposits can develop successively in the channel where the transport distance is short;with the increase of transport distance,the debris flow deposits in the channel decrease.Therefore,most of eroded channels appear in the near end of the continental slope with steep terrain slope,while sedimentary channels usually develop in the far end of the continental slope with gentle terrain.Due to the short transport distance,the debris flow channel complex (early forced regression),sandy turbidite channel complex (late forced regression) and turbidite channel-natural levee complex (lowstand period) are developed in the ideal eroded channel system,reflecting a complete sea-level rise and fall cycle.The sedimentary channel is less affected by the debris flow deposits in the early forced regression because it is far away from the source.The channel system is mainly composed of turbidite channel-natural levee complex (late forced regression and lowstand period) with strong transport capacity.