三角洲型被动陆缘盆地陆坡地貌演化研究——以尼日尔三角洲盆地某深水研究区为例

Geomorphic Evolution of a Continental Slope in a Delta Reformed Passive Continental Margin Basin:A case study of a deep-water zone in the Niger Delta Basin

【目的】陆坡因其巨大的沉积物总量和丰富的油气资源而成为勘探的热点领域。明确陆坡地貌演化特征能够助力于深水沉积学研究和油气勘探。研究旨在针对三角洲型被动陆缘盆地陆坡具有强物源供给条件的特征,探索一种基于沉积构型原理的古陆坡地貌演化研究新方法。【方法】以尼日尔三角洲盆地某陆坡研究区为例,综合应用地震、测井和岩心资料,根据13个层序的沉积构型时—空演化规律反演古陆坡地貌演化特征。【结果】研究区中新世至今的地貌演化过程经历了两个阶段。阶段1中,地貌演化受大型三角洲进积驱动,研究区由深海平原演化为陆坡,地貌始终为非限制型。阶段2中,在重力滑动作用下先后经历了由逆冲断层、泥底辟、填平补齐主导的演化过程,地貌由限制型、半限制型演变为现今的非限制型。逆冲断层期,泥底辟强度有限,地貌形态受控于断层活动,研究区处于重力滑动体系的远缘逆冲构造区。泥底辟期,底辟强度大幅增强,具备了独立改造地貌格局的能力,研究区由逆冲构造区向泥底辟构造区转化。【结论】重力滑动体系由沉积作用驱动,表现出与沉积相类似的空间组合与演化特征。平面上相邻的伸展、泥底辟、逆冲构造区存在成因关联,与具有排序特征的沉积相类似。因此,研究区两期构造活动的叠合关系指示了陆坡向海推进的过程。综上,典型深水研究区沉积构型的时—空演化特征能够为陆坡古地貌演化过程的恢复提供重要依据。

[Objective]Continental slopes have become a major focus in petroleum exploration because of their enormous sediment and hydrocarbon potential.Research on the geomorphic evolution of continental slopes contributes to deep-water sedimentology and petroleum exploration.This paper focused on the abundant terrigenous supply of a continental slope in a delta reformed passive continental margin basin.A new methodology for the restoration of ancient slope geomorphic evolution based on the principle of depositional architecture was discussed.[Methods]Taking a study area on a continental slope in Niger Delta Basin as an example,seismic,logging,and core data were comprehensively applied to reveal the geomorphic evolution characteristics of the continental slope based on the spatiotemporal evolution of depositional architecture in 13 sequences.[Results]The geomorphic evolution of the study area can be divided into 2 stages.In stage 1,study area transformed from an abyssal plain to continental slope,and the landform was unrestricted.The geomorphic evolution in this stage was driven by the progradation of large deltas.The geomorphic evolution in stage 2 included 3 periods dominated by thrust faults,mud-diapirs,and sedimentation.The landform evolved gradually from restricted to semi-restricted and then to the present non-restricted type.The geomorphic evolution in stage 2 was controlled by gravity sliding.During the thrust faulting,the activity intensity of the underlying flowing mudstone was limited,and the landform was controlled by thrust faults.The study area was located in the remote thrust province of a gravity sliding system.During the mud-diapir period,the intensity of diaper activity increased significantly and was able to change the geomorphic characteristics independently.As a result,the topography of study area began to transform from a thrust to mud-diapir province.[Conclusions]The gravity sliding structural system was driven by sedimentary processes,and its spatial combination and evolution characteristics were similar to sedimentary facies.The adjacent extensional,mud-diapir and thrust province were closely related in genesis,which was similar to a sequential sedimentary facies.Therefore,the vertical superposition relationship of different structural activities indicated the macro trend of the continental slope advancing seaward contemporaneously.In conclusion,the spatio-temporal evolution characteristics of the gravity flow depositional architecture in the typical deep-water study area can provide important evidence for the restoration of paleogeomorphic evolution of a deep-water continental slope.