南海北部湾盆地协调-非协调供源样式与沉积交互作用耦合响应:以涠西南凹陷C洼为例

Coupled Response of Concordant-Discordant Input Systems and Depositional Interactions within Beibuwan Basin,South China Sea:A Case Study from C Sag,Weixinan Depression

断陷湖盆具有多物源注入和地貌复杂的特征,开展从源到汇的分析有利于深化对有利砂体展布的认识.以涠西南凹陷C洼及邻近凸起为例,基于测井和地震资料刻画了源-汇系统的要素特征,区分出协调和非协调两类供源样式,明确了多物源汇聚下沉积体系的时空展布规律.研究表明,涠西南凹陷C洼流三段接受北部、南部短轴体系和西部长轴体系的联合供源,南部体系可进一步划分为S-Ⅰ、S-Ⅱ、S-Ⅲ三个次级体系.其中,长轴体系具有低缓的源区和宽缓的沟谷,表现为常年稳定和牵引流主导的协调型供源样式,在沉积区发育粒度和含砂率相关变化的辫状河体系;短轴体系具有相对高陡的源区、深切的沟谷,如S-Ⅱ单元,代表着脉冲式和重力流主导的非协调供源样式,在沉积区发育厚层泥岩夹薄层砾岩的盆底扇和扇三角洲体系.在流三下亚段沉积时期,在南部短轴非协调体系的强烈供源下,长轴协调体系主体被推离C洼南部而分布在中北部;在流三上亚段沉积时期,湖平面上升后短轴源区面积减小使得非协调体系供源减弱,长轴协调体系则保持稳定的供源强度并向南部偏转以填充余出的可容空间.通过对涠西南C洼及邻近凸起开展源-汇系统分析,强调供源样式和沉积交互作用耦合控制了有利砂体的时空展布,对涠西南凹陷C洼及相似断陷湖盆的油气勘探具有一定指导意义.

Lacustrine rift basin is characterized by multiple input systems and complex paleo-geomorphology,and a source-to-sink analysis can help improve sandstone prediction.Based on logging and 3D seismic data,this study distinguishes concordant and discordant input systems and establishes the spatio-temporal distribution pattern of depositional systems within the C Sag,Weixinan Depression.The results indicate that the C Sag was supplied by the northern and southern transverse input systems as well as the western axial system during deposition of the third member of Liushagang Formation,and the southern system can be further divided into three second-order subsystems(i.e.,subsystems S-Ⅰ、S-Ⅱ、S-Ⅲ).The axial system with gentle catchment and wide valley was characterized as a concordant input system with steady flux and tractive flow,as manifested by a braided delta with correlative grain size and sand ratio.In contrast,the discordant transverse input system,with a steep catchment and incised valleys(e.g.,sub-system S-Ⅱ) was characterized by sediment pulse and gravity flow,as manifested by basin floor fan and fan delta with thick mudstone intercalated with thin conglomerate-bearing layer.The axial braided delta developed in the lower part of third member of Liushagang Formation was distributed mostly in the northern C Sag,as a result of intensive transverse sediment contributions from southern Weixinan Low Uplift.The reduced sediment flux from transverse input system,however,resulted in southward shifted axial delta in the upper part of third member of Liushagang Formation.Through a source-to-sink analysis of the C Sag,this study demonstrates that different input systems and interactions between depositional systems have exerted important controls on the spatio-temporal distribution patterns of favorable sandstone,with implications for hydrocarbon explorations in C Sag and other lacustrine rift basins with similar characteristics.