东非鲁武马盆地渐新统富砂深水朵体复合体特征及影响因素

Sedimentary characteristics and influential factors of Oligocene deep water sand-rich lobe complex in the Rovuma basin,East Africa

近年来的天然气发现证实地处东非被动大陆边缘的鲁武马盆地渐新统朵体复合体具有巨大的资源潜力和良好的勘探前景。朵体复合体具有粗粒、巨厚、宽厚比低、向单侧偏转的特征,明显异于常规席状砂。利用油气勘探过程中获得的岩芯、测井及地震信息,分析深水沉积朵体复合体不同级别的岩性特征、电性响应和地震反射特征;借助地震剖面解剖和沿层相干切片识别了不同级别的朵体复合体内部组成,分析朵体复合体的沉积过程,同时探讨了影响朵体复合体沉积的主要因素。研究表明,鲁武马盆地渐新统朵体复合体具有"无根"的外部形态及复杂的地震内部反射结构,可以分为朵体复合体、朵体、朵体单元及层-层组等四个级别。朵体复合体形成于块体搬运沉积之后和水道复合体沉积之前,为下降期体系域晚期的产物,推测海平面变化速率影响朵体复合体储层垂向分布,基准面下降速率越大,单层砂岩厚度越大,两者呈现出正相关的特征。朵体复合体沉积经历了多期朵体的沉积过程:早期填平补齐的限制性沉积,中期多期侧向迁移的非限制性沉积,以及末期边部高弯曲迁移的富泥水道沉积;从补给水道口向朵体复合体远端,伴随浊流强度逐渐衰减,携带粗粒物质的能力逐渐降低,近端及主体部位厚度大,砂泥比高,边缘及远端沉积厚度减薄,砂泥比降低;主体部位的砂泥比是补给水道口及较远端的2～3倍;陆架形态、地形坡度及与基准面之间的关系影响朵体复合体的沉积位置、外部形态及迁移方式:"窄陆架、陡陆坡"促使粗粒、富砂的朵体复合体近源堆积;地形坡度与基准面一致时浊流在陆坡区的"过路不留",导致朵体复合体形成"无根"的外部形态;沿陆坡走向的坡度变化导致朵体的侧向迁移。

In recent years, the natural gas discovery has proved that the Oligocene lobe complex in the Rovuma Basin, located at the edge of the passive continent of East Africa, has great potential for resources and a good prospect for exploration. The lobe complex is characterized by coarse grain, huge thickness, a low ratio of width to thickness and unilateral migration, which is obviously different from conventional sheet sand. The lithology characteristics, electrical response and seismic reflection characteristics of a deep-water sedimentary lobe complex were analyzed by using the core, logging and seismic information obtained during oil and gas exploration. The internal composition of the different hierarchies of the lobe complex was identified by means of seismic profile and coherence slices along horizon, and the deposition process of the lobe complex was analyzed. Meanwhile, the main factors affecting the deposition of the lobe complex were also discussed. The study shows that the Oligocene lobe complex in the Rovuma Basin has "rootless" external morphology and complex internal reflection configuration. It can be divided into four hierarchies: the lobe complex, the lobe, the lobe element, layer and the layer group. The lobe complex formed after the mass transport deposition and before the channel complex deposition was the product of the late falling-stage systems tract. It is inferred that the sea level fluctuate rate affects the vertical distribution of the lobe complex reservoir. The greater the base level descending rate, the greater the thickness of monolayer sandstone. There is a positive correlation between the two. The deposition of the lobe complex has undergone multiple stages of lobe deposition. In the early stage, the restricted deposition in the way of backfilling, the non restrictive deposition of mid-term multiphase lateral migration, and the mud-rich channels with high bending and migration at the end of the boundary were last phase. From the feeder channel to the distal end of the lobe complex, the ability to carry coarse particles gradually decreases with reduction gravity flow intensity. The sedimentary thickness of the near and proximate part is large, the ratio of sand to mud is high, the sedimentary thickness of the edge and the distal side is thinner, and the ratio of sand to mud decreases. The sand to mud ratio of the proximate body is 2~3 times that of the feeder mouth and the far side. The shape of the continental shelf, the relationship between the gradient of the slope and the sea level affect the location, the external form and the way of migration of the lobe complex. The narrow continental shelf and steep continental slope promote the near source accumulation of coarse sand-rich lobe complexes. When the terrain gradient is consistent with the base level, the turbidity currents in the slope do not deposit, leading to the formation of the "rootless" of the lobe complex. The slope changes along the slope strike to cause lateral migration of the lobe.