摘要： 西湖凹陷位于东海陆架盆地东部坳陷带,是该盆地规模最大的富油气凹陷。然而,西湖凹陷渐新世沉积环境与沉积体系类型一直存在较大争议。本文以井、震为基础,岩心为核心,结合地球化学指标,通过泥岩甾烷与自生海绿石的特征,明确了渐新世西湖凹陷南部整体处于海陆交互的过渡环境,且发生5次主要的海侵事件。西湖凹陷南部渐新统以厚层砂岩与薄层泥岩互层为特征,发育典型的双向交错层理、双黏土层、透镜状层理及泥质披覆,共识别出4种主要的岩相类型: 含泥砾块状中粗粒砂岩相、交错层理中细粒砂岩相、沙纹层理粉细砂岩相、纹层状泥岩相。渐新世,西湖凹陷南部主要发育潮控河口湾体系,其中包括潮汐水道、潮汐沙坝、沙质潮坪及泥质潮坪等多个沉积微相,其沉积地形在SW向逐渐变为开阔的展布特征,说明研究区河流供源来自东北部,而潮汐水流来自西南部。渐新世西湖凹陷南部与开阔海连通,受到海侵作用下潮汐水流的强烈改造,且由于地形坡度较缓,无大规模的河流携带碎屑物质注入,易形成潮汐作用为主的河口湾体系。

关键词: 海侵作用, 潮控河口湾, 沉积序列, 西湖凹陷, 东海陆架盆地

Abstract: The Xihu sag is located in the eastern depression of the East China Sea Shelf Basin,which is the largest hydrocarbon-bearing sag. This study was mainly based on well-log,seismic,and core data,combining with geochemical parameters. Characteristics of sterane in mudstone and autogenous glauconites indicate that the southern Xihu sag was in a transitional environment between marine and terrestrial in the Oligocene,which experienced five major marine transgressions. The Oligocene in the Xihu sag was characterized by thick-bedded sandstone and thin-bedded mudstone,developing typical bidirectional cross-beddings,double mud layer,flaser beddings,and mud drapes. Four types of lithofacies were identified: Structureless medium-to-coarse grained sandstone with mudclasts,cross-bedded fine-to-medium grained sandstone,rippled siltstone and fine-grained sandstone,laminated mudstone. During the Oligocene,tide-dominated estuary systems were mainly developed in the southern Xihu sag,which was comprised of sedimentary units of tidal channel,tidal bar,sand flat,and mud flat. The estuary systems widened toward SW,which suggests feeding fluvial systems were from northeast,and the tidal currents were from southwest. In the Oligocene,the southern Xihu sag was connected with the open marine,strongly reworked by tidal currents on the gentle slope gradients without large volume of fluvial sediment flux during marine transgressions,forming tide-dominated estuary systems.

Key words: marine transgressions, tide-dominated estuary, sedimentary sequences, Xihu sag, East China Sea Shelf Basin

中图分类号: 

• P512.32