埕岛低凸起深水斜坡扇与滑塌扇砂体成因类型、沉积模式及分布规律

Genetic Types,Sedimentation Models and Distribution Regularities of Sandbodies of Subaqueous Fan and Slopping Subaqueous Fan in Chengdao Low-uplift Slope Area

埕岛低凸起东斜坡东营组主要油藏的沉积成因为深水沉积。综合岩心、测井、地震资料,从储层发育背景和沉积过程及响应,探讨砂体成因类型、分布规律及连通关系。研究认为:东营组深水储层为砂质碎屑流、泥质碎屑流、浊流等多种重力流沉积,并受牵引底流改造。层序发育的不同阶段,发育斜坡水下扇和滑塌水下扇。低位体系域时期,洪水携带坡上风化剥蚀及垮塌的陆源碎屑物,以点源供给,因坡度获得加速度,泥石流在下倾、渐宽、底平的水下洼槽内,侧向搬动和长流程迁移,受湖水稀释和砂泥分异作用,析出纯净砂岩,形成砂质碎屑流辫状沟道,此类砂体多层叠置,呈拼合状储层结构。高位体系域时期,沉积坡折处进积型扇三角洲沉积失稳而滑塌,以线源供给,块体流渗入湖水,颠簸滑行、离解、重力分异而密度分层,形成以砂质碎屑流为主的水道、浊流为主的浊积舌和受牵引底流改造过的浊积席状砂,这种砂体孤立于泥岩中呈馅饼状储层结构。斜坡水下扇砂体有序分布,砂体规模大且连片;而滑塌水下扇砂体随机分布,砂体数量多、厚度变化大且不连片。

The main reservoir type of Dongying Formation in the eastern slope of Chengdao low-uplift belongs to deep-water sediment.Based on core-logging and seismic data,we study the distribution and connection of sandbodies from the developmental background,sedimentation process and response characteristics of the reservoirs.It is revealed that the deep-water reservoir of Dongying Formation is composed of sandy debris flow,argillaceous debris flow,turbidity current,and other gravity flow deposits that were modified by traction underflow.In the various sequence development stages,slope/slump subaqueous fans were developed.During the low systems tract sedimentation,floods carried terrestrial debris from weathering,denudation and collapse on slopes,and formed debris flow with point source supply.The debris flow was accelerated along the slope,and migrated laterally in an inclined,wide,flat,and long-distance underwater trough.Meanwhile,pure sandstone was precipitated by lake-water dilution and sand-mud differentiation,forming braided channels of sandy debris flow.Sandbodies are superimposed in many layers and show a mosaic reservoir structure.During the high systems tract sedimentation,unstable prograding fan delta at sedimentary slope break may have collapsed,forming block flows with line source supply.The block flows seeped into lake water and moved bumpily.Channels dominated by sandy debris flow,turbidity tongues dominated by turbidity and turbidity sheet sand modified by traction underflow were developed by dissociation,gravity variation and density stratification of the block flow in lake water.Sheet sandbodies are isolated from mudstone,forming pie-like reservoir structure.The sandbodies of slope subaqueous fans are distributed regularly,and are large and continuous,whereas those of slump subaqueous fans are distributed randomly,non-fragmented,in large number and with varying thicknesses.