西湖凹陷渐新统花港组大型辫状河沉积体系特征

Characteristics of the large braided river depositional system of the Oligocene Huagang Formation in the Xihu sag

东海盆地西湖凹陷中央反转带渐新统花港组发育多套厚度逾百米的大型砂体,其来源及发育特征不清。在原型盆地恢复的基础上,通过剥蚀量和压实量恢复等技术对西湖凹陷花港组沉积期的古地貌进行恢复,发现西湖凹陷可分为中心宽缓带、东部转换陡坡带和西部缓坡带多级坡折的特征,确定中央反转带花港组限制性河谷中发育以辫状河体系为主的沉积演化模式。综合研究认为,在渐新世大型挤压收缩拗陷背景下,平缓的沉积地形为西湖凹陷中央反转带辫状河砂体大面积分布奠定了基础,低位期北部充足的物源供给和辫状河道的强水动力为厚砂岩的形成创造了条件,多级坡折限定了中北部的河谷形态,控制了大型辫状河沉积体系的继承性发育和稳定分布的巨厚砂体。本文研究成果对西湖凹陷花港组砂体预测及储层研究具有重要的指导意义。

The Oligocene Huagang Formation in the Central Reversal belt of the Xihu sag in the East China Sea basin developed multiple sets of large sandbodies,each over 100 meters thickness,and their source and characteristics are not clear until now.On the basis of the prototype basin restoration,the palaeogomorphology of the sedimentary period of the Huagang Formation of the Xihu sag was restored by means of the denudation and compaction recovery.It is found that the Xihu sag can be divided into the central broad and gentle zone,the eastern transitional steep slope zone,and the western gentle slope zone with multiple slope breaks in terms of palaeogomorphology.The evolution model of the braided river sedimentary system in the restricted valley of the Huagang Formation in the central reverse belt was established.According to the comprehensive study,under the background of the large-scale compression and shrinkage depression in the Oligocene,flat sedimentary terrain laid the foundation for the large area distribution of the braided riv-er sand;the abundant supply of sources from the northern part and the strong hydrodynamic force of the braided channel in the low stand system tract created conditions for the formation of thick sandstone;the multistage slope breaks limited the form of river valley in the middle and northern part,and controlled the inheritance development of the large braid water system and the stable distribution of the large thick sandbodies.The research results in this paper are of great significance to the prediction of sandbodies formation and reservoir study of the Huagang Formation in the Xihu sag.