西湖凹陷渐新统花港组大型沿岸砂坝沉积特征及主控因素*

西湖凹陷位于东海陆架盆地东部坳陷带,是该盆地最大的富生烃凹陷,其主力储集层渐新统花港组的沉积环境与沉积体系类型一直存在较大争议。以岩心为基础,结合测井响应与地震反射特征,识别出西湖凹陷中东部花港组厚层砂岩主要有块状含砾中砂岩、冲洗层理中细砂岩、沙纹层理粉细砂岩共3种典型岩石类型,并通过统计分析沉积构造参数,明确花港组厚层砂岩主要发育在岸线附近的冲洗带,水动力强,波浪作用明显;该厚层砂岩在伽马测井曲线上整体表现为下部低幅锯齿箱状、顶部逐渐过渡为高幅锯齿状特征,地震上主要为中等振幅与连续性的、底平顶凸的丘状反射,为大型沿岸砂坝沉积。最后结合测井曲线样式、地震属性与含砂率拟合,刻画了沿岸砂坝的叠置样式与空间展布,确定西湖凹陷花港组大型沿岸砂坝主要分布在研究区东部三角洲前缘外侧,是由于基准面下降、水体变浅,早期形成的辫状河三角洲前缘受波浪作用改造而成。

Sedimentary characteristics and controlling factors of large-scale longshore bar in the Oligocene Huagang Formation,Xihu sag

The Xihu sag is located in the eastern depression of the East China Sea Shelf Basin,which is the largest hydrocarbon-bearing sag in this basin. It has been debated on sedimentary environments and sedimentary systems of the Oligocene Huagang Formation as the major reservoir. On the basis of cores,well-logging response,and seismic reflection characteristics,three typical lithofacies types of thick sandstone in the Huagang Formation are identified in the central-eastern Xihu sag,namely as massive gravelly medium sandstone,fine-medium sandstone with swash beddings,and silt-fine sandstone with ripples. Statistics of sedimentary structures parameters indicate the thick-bedded sandstone of the Huagang Formation were developed in washing zone along shoreline with strong hydrodynamic condition and wave action. On the gamma logging curve,the thick sandstone is characterized by low-amplitude,blocky pattern in the lower part,gradually changing to high-amplitude,jagged pattern in the top part. On the seismic profile,mound reflections with flat bottom and convex-up top show the medium amplitude and continuity,indicating the large-scale longshore bar systems. Stacked pattern and spatial distribution were depicted combined with well-logging patterns and matching of seismic attributes and sand content. The longshore bar systems of the Huagang Formation in the Xihu sag were mainly developed at edge of the delta front in the eastern part of the study area,which is formed from the earlier braided-river delta front reworked by wave action during base level fall.