尼日尔三角洲盆地陆坡逆冲构造区海底扇分布规律

近年来的深水油气勘探揭示了陆坡逆冲构造区复杂的海底扇分布特征。目前，国内外学者重点研究了逆冲相关地貌对海底扇宏观分布的影响，但对于不同逆冲活动阶段的海底扇分布及空间演化规律仍缺乏系统的认识。文章以尼日尔三角洲盆地某深水研究区为例，综合地震和测井资料，开展陆坡逆冲构造区的海底扇分布规律研究。研究表明，研究区目的层段（上中新统—第四系）可依据地层超覆及海平面变化划分为9个三级层序，在其内部共识别出了块体搬运沉积、水道沉积及朵叶沉积3类海底扇沉积单元。垂向上，海底扇主要分布在三级层序的海退域；而在平面上，受控于不同逆冲活动阶段的差异古地貌特征，海底扇可表现出复杂的分布演化规律。综合分析表明，研究区海底扇的沉积演化可分为4个阶段：①均衡陆坡阶段，该阶段逆冲断层未开始活动，陆坡地貌相对平缓，其对海底扇的限制性较弱，此时海底扇体系主要由高弯度条带状水道组成，发育于研究区中部；②逆冲断层阶段，该阶段逆冲断层向深水区强烈挤压，在其上盘处形成微盆地，阻止了重力流的搬运，形成大型连片状的复合朵叶；③泥底辟阶段，该阶段逆冲断层活动减弱，并在核部伴随有向上的泥底辟作用，此时在研究区中部发育泥底辟背斜，而在其东西两侧发育顺陆坡倾向的微盆地，大量的重力流沉积物沿这些纵向微盆地沉积过路，形成东西分异的海底扇水道-朵叶复合体；④沉积填平补齐阶段，该阶段逆冲活动基本停止，陆坡地貌逐渐趋于均衡，此时研究区内均匀发育低弯度条带状的水道沉积。

Distribution of submarine fans in the thrust fault zone of continental slope,Niger Delta Basin

The recent exploration for oil and gas in deep water has revealed that the distribution of submarine fans is complex in thrust fault zone on continental slope.While the influences of thrusting related topography on macroscopic distribution of submarine fans have been studied both at home and abroad,few researches have been carried out in terms of its distribution and evolution at different stages of thrusting.This paper studied the distribution pattern of submarine fans in the thrust fault zone on continental slope in a deep-water zone of the Niger Delta Basin,utilizing well-logging and seismic data.Results show that the Upper Miocene-Quaternary, the target in the study area,is divided into nine third-order sequences according to stratigraphic overlap and sea level variation,and three types of submarine fan depositional units(massive transport,channel and lobe deposits) are identified.Vertically,the submarine fans are mainly located in the re-trogressive system tracts(RST) of each third-order sequence,while laterally,they present complex evolution and distribution patterns due to the control of differential palaeogeomorphology at different stages of thrusting.In the study area,the submarine fans have evolved through four stages.The first one is equilibrium slope stage.As few thrust faults were active and the slope was relatively gentle in geomorphology at this stage,thrusting delivered little control to the submarine fans.And the submarine fan system was developed in the central part of the study area and was dominated by sinuous stripe-shaped channels.The second one is thrusting stage.The vigorous activity of thrust faults into the deep water triggered the formation of minibasins on the hanging walls of thrust faults;thus the transportation of gravity flow was prevented,and large composite lobes, linked with each other,occurred.The third one is mud diapire stage.The thrust fault activity slowed down and upward mud diapirism occurred in its core during this stage.Then the mud diapire anticline developed in the central part of the study area,while minibasins along the direction of continental slope grew on the east and west sides of the anticline.Plenty of gravity flow sediments bypassed these longitudinal minibasins,forming the submarine fan channel-lobe complexes that differentiated in east-west direction.The fourth one is sedimentation-dominant stage.As the thrusting in the study area almost stopped,the continental slope gradually turned to equilibrium and stripe-shaped channel deposits with low sinuosity were developed quite even in the whole area.