珠江口盆地晚渐新世陆架边缘三角洲沉积层序结构及演化

Sequence architecture and evolution of shelf-margin deltaic systems of the Late Oligocene in Pearl River Mouth Basin

南海北部上渐新统珠海组广泛发育外陆架-陆架边缘三角洲和深水斜坡扇体系,它们构成了区内最为重要的油气储层。综合地震、测井、岩心等资料,研究了珠海组陆架边缘三角洲及前三角洲斜坡扇的层序结构、沉积成因类型、沉积演化及控制作用。珠海组为一个复合层序,其内可进一步分为4个层序。通过追踪坡折点变化建立了滨岸迁移轨迹,划分了各层序内的沉积成因单元(水进、略上行正常水退、明显上行正常水退、局部下行强制性水退等)。外陆架三角洲以发育厚约100~150 m的前积层为特征,地震上多表现为切线型反射结构;而陆架边缘三角洲前缘-斜坡复合前积层厚达200~1 000 m,具S型、切线斜交型等地震反射结构,以发育厚层的三角洲前缘砂坝和大规模的滑塌变形堆积为特征,并与前三角洲-斜坡扇共生。研究区可识别出6套三角洲前积体,其面积具有逐渐变大、并沿走向扩展的趋势。区内晚渐新世层序的发育受到海平面变化的控制,区域性沉积旋回还受到构造沉降作用叠加的影响。大量沉积物的供给和区域性海平面下降是导致陆架边缘三角洲和前三角洲斜坡扇广泛发育的主控因素。

Outer-shelf to shelf-margin delta and deep-water slope fan systems,the most important petroleum reservoirs in the Pearl River Mouth Basin(PRMB),were widely developed in the Upper Oligocene of the northern South China Sea.Sequence architecture,sedimentary genetic types,as well as sedimentary evolution and its controlling factors of the shelf-margin deltas and associated slope fans in the Upper Oligocene(Zhuhai Formation) in the southern PRMB were studied based on the integrated analysis of seismic,core and well logging data.The Zhuhai Formation is a composite sequence which can be further divided into four sequences.The shoreline trajectories were mapped by trac-king the changes of shoreline breaks,and the depositional genetic units(transgression,slightly-ascending normal regression,apparently-ascending normal regression and locally-descending forced regression,etc.) were identified.The outer-shelf deltas are characterized by tangential reflections on seismic profiles with the thickness of foresets ranging from100 m to 150 m.In contrast,the shelf-margin deltaic clinoforms,with a thickness of 200-1 000 m,display as S-shaped or sigmoid-tangential seismic reflections,and generally consist of thick beds of delta front bar deposits and large-scale slump and deformed sediments,which are associated with prodelta-slope fan systems.In addition,six deltaic clinoforms were identified in the study area,and their areas tend to extend along their strikes.The composite sequence(CS3) is controlled by both relative sea level changes and tectonic subsidence.The massive sediment supply and regional sea level fall are dominant factors controlling the wide-spread distribution of the shelf-margin delta and prodelta slope fan systems.