东海陆架盆地西湖凹陷西部斜坡带平湖组沉积环境特征研究

Study on Sedimentary Environment Characteristics of Pinghu Formation in Western Slope Zone of Xihu Sag, East China Sea Shelf Basin

摘要: 东海陆架盆地西湖凹陷西部斜坡带始新统平湖组不仅发育良好的烃源岩具备良好的生烃能力，同时也为优质的储层，一直是东海地区油气勘探研究的主要层段，但当前该层系的勘探突破相较于渐新统花港组而言稍显滞后，其中西部斜坡带平湖组沉积环境特征认识不深是制约勘探突破的重要因素之一。本文以西部斜坡带4口重点井为研究对象，精细描述了渐新统平湖组，分析了上百块岩心、岩屑样品，并优选了其中的41块开展地球化学测试分析。明确了：1) 研究区内X4井始新统平湖组的沉积期整体处于温暖、潮湿气候背景，除部分沉积期遭遇海侵外整体为淡水环境，绝对古水深介于0~7 m之间，具有氧化特征；2) 研究区主要发育受潮汐影响的河控三角洲相，进一步划分为三角洲平原、三角洲前缘两种亚相和泛滥平原、分流河道、河口坝、水下分流间湾、水下分流河道等微相；3) 总结出研究区平湖组发育的两种沉积模式，在低水位时期主要为河控三角洲沉积，水体整体为淡水环境，在发生海侵时期，水体盐度增加，潮汐对研究区的沉积物分配产生一定影响，在三角洲前缘发育沿潮流方向的潮汐沙坝。厘清平湖组沉积相与沉积环境特征，可以为西湖凹陷西部斜坡带平湖组下一步勘探部署提供参考依据。

Abstract: The Eocene Pinghu Formation is not only a well-developed source rock with good hydrocarbon generation capacity, but also a high-quality reservoir in the western slope zone of Xihu Sag in the East China Sea shelf Basin. It has always been the main oil and gas exploration intervals in the East China Sea. However, the current exploration breakthrough of the Pinghu Formation is slightly behind that of the Oligocene Huagang Formation. The lack of understanding of the sedimentary environment characteristics of Pinghu Formation in the western slope is one of the important factors restricting exploration breakthrough. In this paper, four key Wells in the western slope zone are taken as the research objects, and the Oligocene Pinghu Formation is described in detail. Hundreds of core and cuttings samples are analyzed, and 41 of them are selected for geochemical testing and analysis. It is clear that: (1) The sedimentary period of the Eocene Pinghu Formation of Well X4 in the study area was in a warm and humid climate background, and the sedimentary period was in a freshwater environment except that some sedimentary periods encountered transgression. The absolute paleo-water depth was between 0 and 7m, with the characteristics of oxidation. (2) The tidal river controlled delta facies are mainly developed in the study area, which can be further divided into delta plain and delta front subfacies, flood plain, distributary channel, estuary bar, underwater distributary bay, underwater distributary channel and other microfacies. 3) Two sedimentary models of Pinghu Formation in the study area are summarized. In the period of low water level, the sediment is mainly in the river controlled delta, and the water body is a freshwater environment as a whole. In the period of transgression, the salinity of the water body increases, and the tides have a certain influence on the sediment distribution in the study area. Clarifying the sedimentary facies and sedimentary environment characteristics of Pinghu Formation can provide reference for the further exploration and deployment of Pinghu Formation in the western slope belt of Xihu Depression.