强制湖退期湖底扇沉积构型的多样性与砂体分布特征——以渤海湾盆地东营凹陷沙三段中亚段为例

Diversity of depositional architecture and sandbody distribution of sublacustrine fans during forced regression:A case study of Paleogene Middle Sha 3 Member in Dongying Sag,Bohai Bay Basin,East China

针对目前关于陆相湖盆不同体系域内部重力流沉积的差异性研究薄弱的情况,以渤海湾盆地东营凹陷史深100地区古近系沙河街组三段中亚段为例,综合利用岩心、测井和三维地震等资料,分析强制湖退期湖底扇沉积构型特征及四级基准面变化对其发育的影响。研究表明,四级基准面上升期湖底扇以异重流成因为主,下降晚期滑塌成因比例逐渐上升。四级基准面从上升到下降,异重流成因湖底扇内部水道延伸距离持续减小,导致湖底扇的平面形态从高度沟道化的扇形向裙带状的扇形转变;此外湖底扇内部的分支水道复合体构型样式由垂向加积转为侧向迁移,单一水道的横向规模逐渐减小;朵叶体复合体的构型样式从侧向摆动型补偿叠置演变为加积型叠置,单一朵叶体的横向规模逐渐增大。该研究深化了高频层序地层中重力流沉积规律的认识,并为湖底扇油气藏精细开发提供了地质依据。

Currently,the differences in gravity flow deposits within different systems tracts in continental lacustrine basins are not clear.Taking the middle submember of the third member of Paleogene Shahejie Formation(Sha 3 Member)in the Shishen 100 area of the Dongying Sag in the Bohai Bay Basin as an example,the depositional architecture of sublacustrine fans during forced regression and the impact of the fourth-order base-level changes on their growth were investigated using cores,well logs and 3D seismic data.Sublacustrine fans were mainly caused by hyperpycnal flow during the fourth-order base-level rise,while the proportion of slump-induced sublacustrine fans gradually increased during the late fourth-order base-level fall.From rising to falling of the fourth-order base-level,the extension distance of channels inside hyperpycnal-fed sublacustrine fans reduced progressively,resulting in the transformation in their morphology from a significantly channelized fan to a skirt-like fan.Furthermore,the depositional architecture of distributary channel complexes in sublacustrine fans changed from vertical aggradation to lateral migration,and the lateral size of individual channel steadily decreased.The lobe complex’s architectural patterns evolved from compensational stacking of lateral migration to aggradational stacking,and the lateral size of individual lobe steadily grew.This study deepens the understanding of depositional features of gravity flow in high-frequency sequence stratigraphy and provides a geological foundation for the fine development of sublacustrine fan reservoirs.