南海西科1井上新世以来礁滩体系内部构成及其沉积模式

通过对西科1井薄片进行鉴定, 对上新世以来莺歌海组和乐东组礁滩沉积体系的古生物种类、岩石微相类型及成因相类型进行了系统剖析.研究表明: 乐东组造礁生物以珊瑚和红藻为主, 莺歌海组以红藻为主.附礁生物主要包括有孔虫、棘皮类和绿藻, 其中有孔虫为最重要的附礁生物, 绿藻只在乐东组的个别层段极发育.根据生物种类及含量、泥晶、亮晶及粒间孔含量的大小关系, 识别出10种岩石微相.在此基础上, 将礁滩体系划分为生物礁、生屑滩和泻湖3种成因相组合.生物礁包括礁基、礁核和礁盖3种成因相, 其中礁盖可见显著的溶蚀现象, 形成粒内溶孔、铸模孔等次生孔隙.在研究井段仅发育礁后滩, 靠近礁核的礁后内侧滩生屑及粒间孔含量高, 而泥晶少; 靠近泻湖的礁后外侧滩则情况相反.泻湖主要沉积大量灰泥, 生屑含量少.礁滩体系仅在海侵体系域和高位体系域发育, 各成因相之间具有相对固定的成因相垂向叠置关系.海侵体系域主要发育退积型成因相组合序列, 准层序以代表海泛面的泥晶含量较高的成因相结束; 高位体系域主要发育进积型成因相组合序列, 多以对应暴露面的礁盖结束. 

Internal Architecture and Depositional Model of Reef-Bank System since Pliocene in Well Xike-1, the South China Sea

The fossil species, lithofacies types and genetic facies types of the reef-bank depositional system of Yinggehai Formation and Ledong Formation since the Pliocene have been systematically analyzed through the observation of microscopic thin slice of well Xike-1 in Xisha Islands. The results show that coral and red algae are the main reef-building organisms in the Ledong Formation, while red algae is the dominant reef-forming organism in the Yinggehai Formation. Reef-dwelling organisms in the Yinggehai and Ledong formations include primarily foraminifer, echinoderm and green algae, of which foraminifer is the most important, and green algae only develops prosperously at intervals. Ten lithofacies types have been identified according to the fossil species and their contents, and content relationship of micrite, calcsparite and intergranular pore. On this basis, reef-bank system could be divided into three genetic facies groups including reef, biodetritus beach and lagoon. Reef includes three genetic facies types, namely, reef base, reef core and reef cap, among which significant exposures can be observed at reef cap, leading to the formation of secondary pores, such as intragranular dissolved pore, moldic pore, etc.. The backreef bank only develops in well Xike-1, in which there are more content of bioclast and intergranular pore, and minor micrite in the backreef inner bank close to reef core, whereas the opposite is observed in backreef outer bank near the lagoon. Large amounts of micrite limestones with little bioclast are mainly deposited in lagoon environment. Reef-bank system only develops in transgressive system tract and highstand system tract, in which genetic facies have relatively stable vertical stack relationships. The transgressive system tract commonly forms the retrogradational genetic facies group series in which parasequence ends with genetic facies having more micrite representing the flooding surface, but highstand system tract mainly develops the progradational genetic facies group series whose parasequence ends with reef cap corresponding to exposed surface.