南海北部深水区晚渐新世-中新世海相微体古生物地层及古环境

Marine late Oligocene to Neogene strata are only distributed in the continental shelf and the slope of the northern South China Sea, the Leiqiong Basin and Taiwan in China. As we all know, drilling samples are very limited and expensive. In recent years, significant breakthroughs have been made in the oil and gas exploration of the deepwater region of the northern South China Sea, in which the resolution of stratigraphic division and correlation and research on palaeoenvironment of sedimentary basins are in urgent need. Foraminifera, calcareous nannofossils and dinoflagellates are irreplacably useful in the researches on drilling stratgraphy because of their great amount, rapid evolution and wide distribution. This project will systematically study on calcareous nannofossils, dinoflagellates and foraminifera from samples of more than 10 drills in Liwan and Liuhua structures in the deepwater region of the northern South China Sea. Biostratigraphic and chronostratigraphic frameworks in the northern South China Sea will be built, important microfossils will be described and photographed, and an atlas of microfossils will be compiled. This project will also discuss the palaeoenvironment indicated by calcareous nannofossils, dinoflagellates and foraminifera, and study the sequence stratigraphy integrating data of earthquake and well logging as well. The productions of this project will not only enrich the research content of microfossils in the oil- and gas-bearing basins in the northern South China Sea and make up the shortage of microfossil research in this period, but also provide significant guidance to the oil and gas exploration of the deepwater region of the northern South China Sea.

我国海相晚渐新世至新近纪地层仅分布于南海北部大陆架及陆坡、雷琼盆地和台湾等地区。由于缺乏露头剖面，研究材料主要来自钻井样品。近年来，我国南海北部深水区油气勘探取得重大突破，迫切需要解决沉积盆地的地层划分对比及古环境研究等问题。由于有孔虫、钙质超微及沟鞭藻化石数量众多，演化迅速，分布广泛，在钻井地层研究中具无可替代的作用。本项目将以南海北部深水区荔湾及流花构造十余个钻孔样品为研究材料，通过对钙质超微化石、沟鞭藻及有孔虫的系统研究，建立南海北部深水区生物地层及年代地层框架，并对重要化石属种进行描述、照相，并编制微体古生物化石图册。探讨钙质超微化石、沟鞭藻及有孔虫所指示的古环境，并结合地震、测井等资料，进行层序地层学研究。成果不仅可以丰富南海北部深水含油气盆地微体古生物地层的研究内容，弥补该时期微体古生物研究之不足，而且对南海北部深水区油气勘探具有重要的指导意义。

本项目以南海北部深水区典型钻井剖面岩屑和岩芯样品为研究材料，系统开展了有孔虫、钙质超微及沟鞭藻等多门类、高精度生物地层及沉积环境研究。建立了晚渐新世至中新世有孔虫、钙质超微化石生物带及沟鞭藻组合序列，解决该地区井下地层的精确划分问题，弥补了南海北部含油气盆地海相微体化石研究之不足，丰富了海相晚渐新世至中新世地层古生物研究成果。依据有孔虫、沟鞭藻和钙质超微化石的定量统计结果和某些特征生态种，对典型剖面所在区域晚新世至上新世的沉积环境演化特征进行了深入研究。此外，本项目首次对南海西沙群岛全取芯钻井剖面-西科1井开展了系统的多门类生物地层研究。该剖面是目前南海地区钻井深度最深、获取岩芯最全的第一口全井段取芯钻井。根据生物组合面貌及标志化石属种，识别出4个钙质超微化石带，2个底栖有孔虫化石带、1个浮游有孔虫化石带及4个底栖有孔虫组合，划分了4个钙藻组合，在此基础上，对西科1井生物地层进行了划分，建立了该井所在区域生物地层及年代地层格架，首次建立了南海地区早中新世以来的碳酸盐岩―生物礁生物地层标准剖面。依据多门类古生物化石分布、丰度和群落面貌，运用个体生态学和综合生态学相结合的研究方法，阐明了西科1井早中新世至第四纪古生态环境演化特征。发现有位于生物礁礁核部分的礁骨架和礁顶环境、生物礁礁前的礁前滩环境、生物礁礁后的礁后内侧滩与外侧滩以及澙湖环境等完整的生物礁、滩沉积环境。其中，早中新世早期矿物分析表明有陆源碎屑物质发育，推测海水刚刚淹没西沙台地，为岸礁体系发育阶段；至早中新世晚期开始，基本未见陆源碎屑物质，推测西沙台地全部被淹没，环礁体系开始形成。该成果为南海西沙群岛碳酸盐岩地层古生物研究奠定了基础，首次在南海地区建立的早中新世至第四纪碳酸盐岩生物礁生物地层标准剖面，对南海海洋地质学的发展具有重要推动作用。