东非鲁伍马盆地渐新统深水水道-朵体沉积特征及控制因素

近年来在东非鲁伍马盆地深水区古近系深水沉积砂体中连续发现大型天然气藏,该盆地已成为全球天然气勘探热点地区。但是鲁伍马盆地勘探程度低,深水沉积砂体研究相对滞后,严重制约了下一步油气勘探工作。基于岩心、测井及三维地震资料,建立了东非鲁伍马盆地陆坡深水区渐新统层序地层格架,通过分析水道-朵体沉积特征,并结合区域地质资料,探讨了水道、朵体等富砂深水沉积的控制因素,并建立了相应的沉积模式。鲁伍马盆地渐新统自下而上划分为SQ1、SQ2和SQ3共3个三级层序,水道-朵体沉积发育在每个层序的低位域。研究区SQ1和SQ2层序发育水道、朵体沉积,SQ3层序以水道沉积为主。受始新世末东非大陆抬升及全球"冰室"气候影响,盆地物源供给增加,三角洲向海进积,引发陆坡水道-朵体等重力流沉积。向北流动的南极底流改造水道-朵体沉积,通过淘洗重力流沉积提高水道和朵体的砂地比,并在水道北侧形成侧积砂体和漂积体,限制了后期重力流沉积,导致水道向南侧向迁移、朵体向南延伸。研究结果可以为鲁伍马盆地乃至整个东非陆缘盆地深水储集砂体预测及油气勘探提供一定理论依据。

Sedimentary characteristics and controlling factors of Oligocene deep-water channel-lobe in Rovuma Basin of the East Africa

In recent years, several huge natural gas reservoirs have been discovered consecutively in Paleogene deep-water sedimentary sand bodies of deep-water regions in Rovuma Basin of the East Africa, and this basin has been a hot spot for global natural-gas exploration. However, due to the low exploration degree of Rovuma Basin and the relatively lagging research on deep-water sedimentary sand bodies, the further hydrocarbon exploration is seriously restricted. Based on cores, logging and 3D seismic data, the Oligocene sequence stratigraphic framework of the continental-slope deep-water regions in Rovuma Basin of the East Africa is established. Meanwhile, through the analysis on water channel-lobe characteristics in combination with regional geological data, the controlling factors of the rich-sand deep-water sediments such as water channel and lobe are discussed to establish the corresponding sedimentary model. The Oligocene Rovuma Basin is divided into three third -order sequences from bottom to top, i.e., SQ1, SQ2 and SQ3, and the channel-lobe sediments are developed in the low-stand system tract of each sequence. In SQ1 and SQ2 of the study area, the channel-lobe sediments are developed, while SQ3 is dominated by channel sediment. Affected by the East Africa continent uplift and global "icehouse" climate at the end of Eocene, the basin provenance supply was increased and the deltas prograded seawards, leading to the gravity flow sediments such as continental slope channel-lobe. The channel-lobe sediment is reworked by Antarctic bottom water flowing northwards, and the sand-stratum ratio of channel-lobe is increased by washing gravity flow sediment to form lateral accretion bodies and drift bodies on the north side of channel, while the late gravity flow sediment is restricted, resulting in the southward migration of channel and the southward extension of lobe. The research results can provide a certain theoretical basis for the prediction of deep-water reserving sand bodies and hydrocarbon exploration in Rovuma Basin and even the whole East African continental margin basin.